Targeting of plant pattern recognition receptor-triggered immunity by bacterial type-III secretion system effectors.

Available online at www.sciencedirect.com

ScienceDirect Targeting of plant pattern recognition receptortriggered immunity by bacterial type-III secretion system effectors Alberto P Macho and Cyril Zipfel During infection, microbes are detected by surface-localized pattern recognition receptors (PRRs), leading to an innate immune response that prevents microbial ingress. Therefore, successful pathogens must evade or inhibit PRR-triggered immunity to cause disease. In the past decade, a number of type-III secretion system effector (T3Es) proteins from plant pathogenic bacteria have been shown to suppress this layer of innate immunity. More recently, the detailed mechanisms of action have been defined for several of these effectors. Interestingly, effectors display a wide array of virulence targets, being able to prevent activation of immune receptors and to hijack immune signaling pathways. Besides being a fascinating example of pathogen-host co-evolution, effectors have also emerged as valuable tools to dissect important biological processes in host cells. Addresses The Sainsbury Laboratory, Norwich Research Park, Norwich NR4 7UH, United Kingdom Corresponding author: Zipfel, Cyril ([email protected])

Current Opinion in Microbiology 2015, 23:14–22 This review comes from a themed issue on Host–microbe interactions Edited by David Holden and Dana Philpott

http://dx.doi.org/10.1016/j.mib.2014.10.009 1369-5274/# 2014 Elsevier Ltd. All right reserved.

Introduction Major bacterial pathogens of plants include members of the Pseudomonas, Xanthomonas, Ralstonia, Agrobacterium and Pectobacterium genera, amongst others. As all would-be pathogens, they face efficient innate immune responses when first coming into contact with plant cells. Surface-localized pattern recognition receptors (PRRs) ensure initial perception of this infectious non-self [1,2]. Plant PRRs can perceive conserved bacterial molecules, usually termed pathogen- or microbe-associated molecular patterns (PAMPs/MAMPs), such as bacterial flagellin, elongation factor Tu (EF-Tu), peptidoglycan (PGN) or lipopolysaccharides [3,4]. Current Opinion in Microbiology 2015, 23:14–22

Plant PRRs are either plasma membrane-resident receptor kinases (RKs) or receptor-like proteins (RLPs) [3,4]. Plant PRRs exist within dynamic protein complexes composed of PRRs themselves, co-receptors, regulatory proteins, and components directly involved in intracellular signaling [3,5]. Upon PAMP perception, the activation of different PRR complexes leads to the phosphorylation and activation of cytoplasmic receptor-like cytoplasmic kinases (RLCKs) [5,6]. Downstream immune responses include the activation of mitogen-activated protein kinases (MAPKs) and calcium-dependent protein kinases (CDPKs), the rapid generation of reactive oxygen species (ROS), the expression of immune-related genes, and the deposition of callose at the cell wall [7,8]. Collectively, these responses culminate in PRR- or PAMP-triggered immunity (PTI), which is sufficient to halt the ingress or growth of most microbes. Therefore, successful pathogens must either evade or actively suppress this important layer of plant innate immunity in order to cause disease. Several Gramnegative bacterial pathogens do so by using the type III-secretion system (T3SS), which resembles a molecular syringe that injects proteins directly inside the host cell [9]. These proteins are usually referred to as type III effectors (T3Es), and promote bacterial infection by manipulating host cell functions, including immunity. The relevance of this secretion system in the overall virulence of plant pathogenic bacteria is best proven by the near-complete loss of infectivity of strains unable to secrete T3Es [10]. However, resistant plants employ intracellular immune receptors, most often of the nucleotide-binding domain-LRR receptor (NLR) type, which directly or indirectly recognize T3Es, thereby inducing effector-triggered immunity (ETI; [2]. Plant pathogenic bacteria also employ additional virulence strategies, such as evasion of PAMP detection or production of toxins, among others [7,11]. Here, we will review the different strategies evolved by T3Es from plant pathogenic bacteria to impede PTI by directly targeting PRR themselves, their biogenesis, or key signaling components acting downstream of PRR activation. We will mainly focus on T3Es for which molecular plant targets are known. Therefore, we apologize to colleagues whose work on other effectors is not covered. www.sciencedirect.com

Inhibition of plant PTI by bacterial type-III effectors Macho and Zipfel 15

Effectors that target de novo PRR biogenesis In response to PAMP perception, transcripts of a large number of PRRs are rapidly up-regulated [12–14]. This may represent an amplification mechanism, but also could serve to replenish the plasma membrane with ligand-free PRRs for subsequent waves of PAMP recognition and PRR activation. The targeting of this de novo PRR biogenesis could therefore represent a virulence strategy to block the establishment of sustained PTI during infection (Figure 1). Interestingly, the T3E HopU1 from Pseudomonas syringae pv. tomato (Pto) DC3000 is an mono-ADP-ribosyltransferase that specifically modifies several plant RNA binding proteins, such as GRP7 [15,16]. The action of HopU1 impedes their RNA-binding function and inhibits PTI [15,16]. Notably, GRP7 can bind mRNAs encoding the leucine-rich repeat (LRR)-RKs FLS2 and EFR (which

are the PRRs for bacterial flagellin and EF-Tu, respectively), potentially contributing to the pathogen-induced translation of these mRNAs. HopU1 affects this binding, causing reduced PRR protein levels in planta upon infection with Pto DC3000 [17] (Figure 1). Notably, the T3E HopQ1 from Pto DC3000 activates the production of the plant hormone cytokinin, which results in reduced FLS2 expression [18] (Figure 1). HopQ1 exhibits similarities with nucleoside hydrolases and might contribute to enhanced cytokinin production by catalysing the conversion of its precursor N6-(2-isopentenyl)adenine-9-riboside-5’-monophosphate (iPRMP) [18]. It is worth mentioning that other studies on HopQ1 from Pto DC3000 or P. syringae pv. phaseolicola (Pph) 1448A identified plant 14-3-3 proteins as interactors for this effector, but have failed to observe alterations in early

Figure 1

(a)

(c)

(b) Time upon contact with bacteria

T3SS T3E T3E T3E

HopU1

Immune signalling

T3Es

T3E T3E

T3E

mRNAbinding proteins

PRR mRNAs

Immune signalling HopQ1

High cytokinin levels

Current Opinion in Microbiology

Targeting of de novo PRR biogenesis during bacterial infection. (A) Basal levels of PRRs perceive the initial bacterial PAMPs present in the apoplast and trigger immune signaling in the plant cell. (B) Immune signaling leads to a first wave of defense-related responses, which include de novo synthesis of immune components. (C) As a consequence, PRRs and other signaling components accumulate at higher levels in order to build up defenses against a potential subsequent bacterial invasion. Pseudomonas syringae T3Es HopU1 and HopG1 are able to suppress this mechanism of plant reinforcement by targeting de novo PRR biosynthesis (B). www.sciencedirect.com

Current Opinion in Microbiology 2015, 23:14–22

16 Host–microbe interactions

PTI outputs triggered by the flagellin-derived immunogenic epitope flg22 upon HopQ1 expression in planta [19– 21]. In addition, HopQ1 orthologs from Pto DC3000, Pph 1448A and Xanthomonas oryzae pv. oryzae (Xoo) KACC10331 do not exhibit nucleoside hydrolase activity [19–21], and a recent structural study on XopQ1Xoo revealed that it can bind adenosine diphosphate ribose (ADPR) [22]. It is therefore possible that HopQ1 has different substrates to modulate PTI via distinct mechanisms.

Effectors that target directly PRRs and their co-receptors Several T3Es have been found to target PRRs directly, causing their degradation and/or the inhibition of their PAMP-induced activation (Figure 2). An example of T3E that exerts both functions is AvrPtoB, which is a multi-domain protein with orthologs in several P. syringae pathovars. AvrPtoB uses a C-terminal E3 ligase

domain to ubiquitinate and induce the proteasomemediated degradation of PRRs, such as FLS2, and the LysM-RK CERK1, which is involved in PGN perception [23–25]. Moreover, AvrPtoB possesses both an N-terminal and a central domain that can act as kinase inhibitors to target important kinases involved in PTI. The Nterminal domain binds the LysM-RK Bti9, which is the tomato ortholog of CERK1 [26]. In contrast, the central domain can interact with FLS2 and BAK1 [27,28], an LRR-RK that acts as an important co-receptor for several LRR-type PRRs [3,5]. Similarly, another P. syringae effector, AvrPto, targets FLS2 and EFR to inhibit their kinase activity and block PTI signaling [29,30]. AvrPto may target additional PRRs, as expression of AvrPto in plants leads to the inhibition of immune responses triggered by multiple PAMPs [28]. Notably, AvrPto can also interact with BAK1, thereby inhibiting its interaction with FLS2 [28]. Despite the fact that AvrPto is a potent inhibitor

Figure 2

PRRs Co-receptors PAMP

AvrPtoB

AvrPto

? AvrPto

AvrPtoB HopF2 HopF2 HopAO1

AvrRpt2

Xoo2875

AvrB AvrPphB

HopF2

MEKK MKK MPK

RLCKs

RIN4

XopAC

Xoo1488 HopAI1

?

AvrRpm1

IMMUNE RESPONSES

Current Opinion in Microbiology

Multiple targeting of early PTI components by T3Es. Simplified diagram of early PTI components targeted by T3Es from Pseudomonas and Xanthomonas strains. Current Opinion in Microbiology 2015, 23:14–22

www.sciencedirect.com


of PRR-BAK1 complex activation, whether PRRs and/or BAK1 represent(s) the true physiological target(s) during infection remains a matter of debate [29,31].

recently revealed for the recognition of HopZ1a (an acetyltransferase from P. syringae pv syringae A2) action on the RLCK ZED1 by the NLR ZAR1 [46].

HopAO1 (formerly HopPtoD2) from Pto DC3000 exhibits tyrosine phosphatase activity that is important for suppression of PAMP-induced responses [32–34]. A recent study revealed that HopAO1 interacts directly with FLS2 and EFR [35]. Interestingly, HopAO1 dephosphorylates tyrosine residues on EFR to inhibit the activation of the EFR-BAK1 complex and therefore subsequent initiation of immune signalling [35].

The T3E Xoo1488 from Xoo MAFF311018 targets several rice RLCKs, including OsRLCK185. These RLCKs act downstream of the rice LysM-RK CERK1 [47], which is involved in the perception of bacterial PGN and fungal chitin [48,49]. The mechanisms by which Xoo1488 impedes OsRLCK185 activation remain to be characterized.

Effectors that target MAP kinases The Pto DC3000 T3E HopF2 is an mono-ADP-ribosyltransferase that also suppresses early PTI outputs [36– 38]. HopF2 contains a potential myristoylation site required for its plasma membrane localization and virulence functions [39]. Recently, it was found that BAK1 is a major virulence target of HopF2 [31], although the underlying mechanisms remain unknown. BAK1 seems to be a conserved target for T3Es from different bacterial species that infect different plant hosts. The T3E Xoo2875 from Xoo KACC10331 interacts with the rice ortholog of BAK1 and increases rice susceptibility to Xoo, although the mechanism of PTI suppression remains unknown [40].

Effectors that target PRR-associated cytoplasmic kinases The T3E AvrPphB from Pph 1448A is a cysteine protease related to Yersinia pestis YopT [41]. It can inhibit PTI signaling by cleaving multiple members of the BIK1 family of RLCKs that are direct substrates of multiple PRR complexes [5,42] (Figure 2). Interestingly, plants harboring the intracellular immune receptor RPS5, an NLR protein, are able to detect AvrPphB-mediated cleavage of one of those RLCKs, PBS1, to initiate ETI [43]. This is good illustration that, while T3Es are key virulence factors, they can also betray the pathogen by revealing their presence to plants that have evolved direct or indirect surveillance mechanisms for these effectors or their action. An additional interesting mechanism to target RLCKs is displayed by the T3E AvrAC/XopAC from X. campestris pv. campestris (Xcc) 8004. This effector displays a previously uncharacterized uridine 5’-monophosphate transferase enzymatic activity to promote bacterial virulence [44]. AvrAC transfers uridine 5’-monophosphate to important phosphorylation sites in the activation loop of several RLCKs, including BIK1, inhibiting its kinase activity, thereby blocking the activation of PTI responses [44]. Notably, AvrAC triggers ETI in specific Arabidopsis accessions in a manner dependent on the RLCKs PBL2 and RIPK [45]. This suggests that their modifications may be sensed by an intracellular NLR, as www.sciencedirect.com

HopAI1 from Pto DC3000 inactivates MPK3, MPK4 and MPK6 in an irreversible manner [50,51]. This is achieved through a phosphothreonine lyase activity, similar to what is observed in the Shigella flexneri T3E OspF, which targets animal MAPKs [50,52]. However, in plants carrying the NLR SUMM2, inactivation of MPK4 by HopAI1 results in ETI [51]. The fact that both RLCKs and MAPKs are guarded by resistance proteins support the notion that they constitute key components of plant immune signaling targeted by pathogen virulence factors. In addition to targeting BAK1 (see above), HopF2 also targets MKK5 through ADP-ribosylation to block its phosphorylation and consequent activation of downstream MAPKs [53]. It is thus apparent that HopF2 has multiple host targets. It also seems to regulate post-transcriptional gene silencing, via yet unknown mechanisms [54]. Furthermore, HopF2, as well as the orthologous HopF1 from Pph 1449b, also directly interact with RIN4, an important regulator of plant immunity [11,55,56]. Notably, several other T3Es from P. syringae (e.g. AvrRpm1, AvrRpt2 and AvrB) target RIN4 and suppress flg22-induced responses [57,58]. Interestingly, RIN4 cleavage by AvrRpt2 leads to ETI via activation of the NLR RPS2, while RIN4 phosphorylation promoted by AvrRmp1 or AvrB activities leads to ETI via the NLR RPM1 [59–61]. It is however still unclear how perturbation of RIN4 by AvrRpm1, AvrRpt2 or AvrB regulates PTI.

Effectors that target vesicle trafficking to suppress PTI Vesicle trafficking is an important cellular function. Regarding plant immune functions, vesicle trafficking is necessary for the transport of immune receptors and associated proteins, and for the secretion of immune related molecules and antimicrobial compounds upon pathogen detection [62]. HopM1 from Pto DC3000 interacts with and induces the degradation of the ADP ribosylation factor (ARF) guanine nucleotide exchange factor (GEF) protein AtMIN7 in a Current Opinion in Microbiology 2015, 23:14–22


proteasome-dependent manner [63]. AtMIN7 localizes to the trans-Golgi/early endosome network and is important for PTI, potentially mediating immune-related vesicle trafficking [63–65]. Several T3Es from X. campestris pv. vesicatoria (Xcv) target plant protein secretion pathways to suppress PTI. As an example, XopJ, a member of the YopJ family of SUMO

peptidases and acetyltransferases, interferes with protein secretion to inhibit immune-associated callose deposition at the cell wall [66,67]. Additionally, XopB and XopS suppress PAMP-triggered gene expression [68]. XopB localizes to the Golgi apparatus and cytoplasm and interferes with eukaryotic vesicle trafficking [68]. However, specific PTI-related targets are still unknown for these Xcv effectors.

Table 1 Examples of bacterial type III effectors that suppress PTI. Effector name

Bacterial species

HopU1

Pseudomonas syringae pv. tomato

HopQ1

Pseudomonas syringae pathovars

AvrPtoB

Pseudomonas syringae pathovars

AvrPto HopAO1 HopF1/F2

Pseudomonas syringae pv. tomato Pseudomonas syringae pv. tomato Pseudomonas syringae pathovars

Xoo2875 AvrPphB

Xanthomonas oryzae pv. oryzae Pseudomonas syringae pv. phaseolicola Xanthomonas campestris pv. campestris

AvrAC/XopAC

Activity

PTI-related plant target

Plant species

Mono-ADPribosyltransferase Putative nucleoside hydrolase

GRP7, other RNAbinding proteins Cytokinin biosynthesis/ 14-3-3 proteins

Arabidopsis

[15,16,17] [18,19,21]

E3-ubiquitin ligase/Kinase inhibitor Kinase inhibitor Tyrosine phosphatase ADP-ribosyltransferase

PRRs

Arabidopsis Tomato Bean Arabidopsis

[28–30,31] [32–34,35] [31,36–39,53]

Unknown Cysteine protease

OsSERK2 RLCKs

Arabidopsis Arabidopsis Nicotiana benthamiana Arabidopsis Bean Rice Arabidopsis

Uridine 5’-monophosphate transferase Unknown Phosphothreonine lyase Unknown

RLCKs

Arabidopsis

[44]

RLCKs MAPKs

Rice Arabidopsis

[47] [36,50,51]

Vesicle trafficking/ 14-3-3 proteins

Arabidopsis Nicotiana benthamiana Arabidopsis

[37,63,64,73]

Arabidopsis Tomato Arabidopsis Arabidopsis Arabidopsis

[68] [74,75] [83] [84] [85]

Arabidopsis Arabidopsis Arabidopsis Arabidopsis Nicotiana benthamiana Arabidopsis Nicotiana benthamiana Arabidopsis Arabidopsis Arabidopsis Arabidopsis Arabidopsis Arabidopsis Nicotiana benthamiana

[86] [57] [57] [58] [37]

PRRs EFR and FLS2 BAK1/MAPKs

Xoo1488 HopAI1

Xanthomonas oryzae pv. oryzae Pseudomonas syringae pv. tomato

HopM1


XopJ

X. campestris pv. vesicatoria

XopB/XopS XopN XopL XopR AvrBsT

X. campestris X. campestris X. campestris Xanthomonas X. campestris

pv. vesicatoria pv. vesicatoria pv. vesicatoria oryzae pv. oryzae pv. vesicatoria

Putative SUMO peptidase Unknown Unknown E3 Ligase Unknown Acetyltransferase

HopK1 AvrRpt2 AvrRpm1 AvrB AvrE

Pseudomonas Pseudomonas Pseudomonas Pseudomonas Pseudomonas

syringae syringae syringae syringae syringae

Unknown Cysteine protease Unknown Unknown Unknown

Protein secretion/ callose deposition Vesicle trafficking TFT1 (14-3-3 protein) Unknown Unknown Microtubuleassociated proteins Chroloplasts RIN4 RIN4 RAR1/RIN4 Unknown

HopG1


Unknown

Mitochondria

HopS1 HopAF1 HopT1-1 HopT1-2 HopAA1-1 HopC1 NopM

Pseudomonas syringae Pseudomonas syringae Pseudomonas syringae Pseudomonas syringae Pseudomonas syringae Pseudomonas syringae Rhizobium sp.

Unknown Unknown Unknown Unknown Unknown Unknown E3 Ubiquitin ligase

Unknown Unknown Unknown Unknown Unknown Unknown Unknown

pv. tomato pv. tomato pv. maculicola pv. tomato

pv. pv. pv. pv. pv. pv.

tomato tomato tomato tomato tomato tomato


Reference(s)

[23,24,26–28]

[40] [42]

[66]

[37,87]

[36] [36] [36] [36] [36] [36] [88]



Effectors that target 14-3-3 proteins to suppress PTI Consistent with the importance of protein phosphorylation for immune signaling [5], 14-3-3 proteins are emerging as important players in plant immunity [69–71]. 14-33 proteins bind to specific motifs containing phosphorylated serine or threonine residues, regulating the function of target proteins by several different mechanisms [72]. Several T3Es interact with 14-3-3 proteins, which could serve as a mechanism to disrupt immunity-associated 143-3 functions, or a way to exploit host 14-3-3 proteins to facilitate other functions of the T3E inside the plant cell. The T3E HopM1 from Pto DC3000 targets 14-3-3 proteins and disrupts several PAMP-triggered responses independently of the previously identified target AtMIN7 [73]. Interestingly, chemical disruption of the 14-3-3 interactions with their client proteins similarly disrupts several PAMP-triggered responses and restores virulence of a HopM1-deficient Pto DC3000 mutant [73]. The Xcv effector XopN interacts with the tomato RK TARK1 and several 14-3-3 isoforms [74]. One of these isoforms, TFT1, is a positive regulator of PTI, and its targeting by XopN induces the formation of a TFT1XopN-TARK1 complex which results in the suppression of PTI in tomato [74,75]. Other T3Es from Xcv (AvrRxc and XopQ), and the XopQ1 homologs in Pto and Pph (HopQ1) interact with 14-3-3 in different plant species to contribute to bacterial virulence [19,21,76]. However, it is unclear whether this virulence activity involves suppression of PTI.

Concluding remarks The detailed study of T3Es, mainly from Pseudomonas and Xanthomonas species, has revealed that one major function of these effectors is to actively suppress PTI by directly targeting key components of this important layer of immune recognition. As such, T3Es (and virulence effectors in general) can be considered as very useful biological tools to dissect key biological processes and may also reveal novel activities evolved to achieve their virulence functions [77,78].

components. In addition to the example listed in this review, these two points are also supported by recent large-scale interactomics studies [79]. Particularly, it is becoming obvious that early PTI components are cellular hubs for suppression by effectors from a single bacterial species or from different bacterial genera or species (Figure 2). Of note, even during a successful infection by a bacterial pathogen, PTI suppression is not completely achieved, as exemplified by the observation that alterations of plant immune components (e.g. PRRs) can render the plant hyper- or hypo-susceptible to pathogens [14,80]. Therefore, redundant targeting of a pathway (e.g. PTI signaling) by multiple effectors from a given pathogen may be important to ensure optimal immune suppression over the course of the infection. Our knowledge on the hierarchy and dynamics of single effector delivery within host cells is still limited [81,82]. Furthermore, most studies on T3Es and their function are based on over-expression in host cells. While this approach is essential to understand what a single effector can do, it will be important to determine the precise amount of effector proteins delivered within single cells during infection and what is their sub-cellular localization at these physiological concentrations. This could be particularly important for effectors that do not exhibit enzymatic activities, as their ability to suppress host proteins may depend on the amount of their respective target proteins. The development of more sensitive cell biological, biochemical and proteomics approaches will be required to address this important question. Importantly, not all T3Es are necessarily involved in the suppression of host immunity. They are likely to have additional roles in nutrient acquisition and in the establishment of a compatible environment for the bacterial replication. Targets for many T3Es are still unknown, and T3Es display a wide range of sub-cellular localization within plant cells, targeting diverse organelles such as chloroplasts or mitochondria (Table 1; [11]), which promises further exciting discoveries in the future about how these fascinating proteins remodel the host’s physiology to the benefit of the pathogen.

Acknowledgements A number of general observations can be drawn based on our current knowledge. Namely, a single effector can have multiple targets. While some effectors may carry different activities (e.g. AvrPtoB), other effectors have a unique activity (e.g. kinase inhibition) that can target host proteins without the need for specificity (e.g. AvrPto is a kinase inhibitor that can globally target receptor kinases, whether they are involved in immunity or not). In this sense, effectors can be seen as weapons of mass destruction delivered inside host cells, being evolutionarily selected based on their final contribution to the infection process. Also, different effectors can target the same www.sciencedirect.com

Research in the Zipfel laboratory is funded by the Gatsby Foundation and the European Research Council. We thank all members of the Zipfel laboratory for helpful discussions. We apologize to colleagues whose work could not be cited due to space limits.

References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: of special interest of outstanding interest 1.

Dangl JL, Horvath DM, Staskawicz BJ: Pivoting the plant immune system from dissection to deployment. Science 2013, 341:746-751. Current Opinion in Microbiology 2015, 23:14–22


2.

Dodds PN, Rathjen JP: Plant immunity: towards an integrated view of plant-pathogen interactions. Nat Rev Genet 2010, 11:539-548.

3.

Bohm H, Albert I, Fan L, Reinhard A, Nurnberger T: Immune receptor complexes at the plant cell surface. Curr Opin Plant Biol 2014, 20C:47-54.

4.

Zipfel C: Plant pattern recognition receptors. Trends Immunol 2014, 35:345-351.

5.

Macho AP, Zipfel C: Plant PRRs and the activation of innate immune signaling. Mol Cell 2014, 54:263-272.

6.

Lin W, Ma X, Shan L, He P: Big roles of small kinases: the complex functions of receptor-like cytoplasmic kinases in plant immunity and development. J Integ Plant Biol 2013, 55:1188-1197.

7.

Boller T, Felix G: A renaissance of elicitors: perception of microbe-associated molecular patterns and danger signals by pattern-recognition receptors. Annu Rev Plant Biol 2009, 60:379-406.

8.

Tena G, Boudsocq M, Sheen J: Protein kinase signaling networks in plant innate immunity. Curr Opin Plant Biol 2011:111.

9.

Galan JE, Lara-Tejero M, Marlovits TC, Wagner S: Bacterial Type III secretion systems: specialized nanomachines for protein delivery into target cells. Annu Rev Microbiol 2014, 68:415-438.

10. Lindgren PB, Peet RC, Panopoulos NJ: Gene cluster of Pseudomonas syringae pv. phaseolicola controls pathogenicity of bean plants and hypersensitivity of nonhost plants. J. Bacteriol. 1986, 168:512-522. 11. Xin XF, He SY: Pseudomonas syringae pv. tomato DC3000: a model pathogen for probing disease susceptibility and hormone signaling in plants. Annu Rev Phytopathol 2013, 51:473-498. 12. Postel S, Kufner I, Beuter C, Mazzotta S, Schwedt A, Borlotti A, Halter T, Kemmerling B, Nurnberger T: The multifunctional leucine-rich repeat receptor kinase BAK1 is implicated in Arabidopsis development and immunity. Eur J Cell Biol 2010, 89:169-174. 13. Zipfel C, Kunze G, Chinchilla D, Caniard A, Jones JDG, Boller T, Felix G: Perception of the bacterial PAMP EF-Tu by the receptor EFR restricts agrobacterium-mediated transformation. Cell 2006, 125:749-760. 14. Zipfel C, Robatzek S, Navarro L, Oakeley EJ, Jones JD, Felix G, Boller T: Bacterial disease resistance in Arabidopsis through flagellin perception. Nature 2004, 428:764-767. 15. Fu ZQ, Guo M, Jeong BR, Tian F, Elthon TE, Cerny RL, Staiger D, Alfano JR: A type III effector ADP-ribosylates RNA-binding proteins and quells plant immunity. Nature 2007, 447:284-288. 16. Jeong BR, Lin Y, Joe A, Guo M, Korneli C, Yang H, Wang P, Yu M, Cerny RL, Staiger D et al.: Structure function analysis of an ADPribosyltransferase type III effector and its RNA-binding target in plant immunity. J Biol Chem 2011, 286:43272-43281. 17. Nicaise V, Joe A, Jeong BR, Korneli C, Boutrot F, Westedt I, Staiger D, Alfano JR, Zipfel C: Pseudomonas HopU1 modulates plant immune receptor levels by blocking the interaction of their mRNAs with GRP7. EMBO J 2013, 32:701-712. The authors report that HopU1 blocks the interaction between GRP7, an RNA-binding protein, and PRR transcripts in vivo, causing reduced PRR protein levels upon Pseudomonas infection.

effector from Pseudomonas syringae, creates a binding site for host 14-3-3 proteins. Plant Physiol 2013, 161:2049-2061. 20. Li W, Chiang YH, Coaker G: The HopQ1 effector’s nucleoside hydrolase-like domain is required for bacterial virulence in arabidopsis and tomato, but not host recognition in tobacco. PLoS One 2013, 8:e59684. 21. Li W, Yadeta KA, Elmore JM, Coaker G: The Pseudomonas syringae effector HopQ1 promotes bacterial virulence and interacts with tomato 14-3-3 proteins in a phosphorylationdependent manner. Plant Physiol 2013, 161:2062-2074. 22. Yu S, Hwang I, Rhee S: The crystal structure of type III effector protein XopQ from Xanthomonas oryzae complexed with adenosine diphosphate ribose. Proteins 2014. 23. Gimenez-Ibanez S, Hann DR, Ntoukakis V, Petutschnig E, Lipka V, Rathjen JP: AvrPtoB targets the LysM receptor kinase CERK1 to promote bacterial virulence on plants. Curr Biol 2009, 19:423-429. 24. Gohre V, Spallek T, Haweker H, Mersmann S, Mentzel T, Boller T, de Torres M, Mansfield JW, Robatzek S: Plant patternrecognition receptor FLS2 is directed for degradation by the bacterial ubiquitin ligase AvrPtoB. Curr Biol 2008, 18:18241832. 25. Willmann R, Lajunen HM, Erbs G, Newman M-A, Kolb D, Tsuda K, Katagiri F, Fliegmann J, Bono J-J, Cullimore JV et al.: Arabidopsis lysin-motif proteins LYM1 LYM3 CERK1 mediate bacterial peptidoglycan sensing and immunity to bacterial infection. Proc Natl Acad Sci 2011, 108:19824-19829. 26. Zeng L, Velasquez AC, Munkvold KR, Zhang J, Martin GB: A tomato LysM receptor-like kinase promotes immunity and its kinase activity is inhibited by AvrPtoB. Plant J 2012, 69:92-103. 27. Cheng W, Munkvold KR, Gao H, Mathieu J, Schwizer S, Wang S, Yan YB, Wang J, Martin GB, Chai J: Structural analysis of Pseudomonas syringae AvrPtoB bound to host BAK1 reveals two similar kinase-interacting domains in a type III effector. Cell Host Microbe 2011, 10:616-626. 28. Shan L, He P, Li J, Heese A, Peck SC, Nurnberger T, Martin GB, Sheen J: Bacterial effectors target the common signaling partner BAK1 to disrupt multiple MAMP receptor-signaling complexes and impede plant immunity. Cell Host Microbe 2008, 4:17-27. 29. Xiang T, Zong N, Zhang J, Chen J, Chen M, Zhou JM: BAK1 is not a target of the Pseudomonas syringae effector AvrPto. Mol Plant-Microbe Interact: MPMI 2011, 24:100-107. 30. Xiang T, Zong N, Zou Y, Wu Y, Zhang J, Xing W, Li Y, Tang X, Zhu L, Chai J et al.: Pseudomonas syringae effector AvrPto blocks innate immunity by targeting receptor kinases. Curr Biol 2008, 18:74-80. 31. Zhou J, Wu S, Chen X, Liu C, Sheen J, Shan L, He P: The Pseudomonas syringae effector HopF2 suppresses Arabidopsis immunity by targeting BAK1. Plant J 2014, 77:235245. This report highlights a novel virulence factor of HopF2. Besides targeting MAPK cascades, the authors report that HopF2, and other T3Es, interact with BAK1 to suppress initiation of plant immunity at the plasma membrane. 32. Bretz JR, Mock NM, Charity JC, Zeyad S, Baker CJ, Hutcheson SW: A translocated protein tyrosine phosphatase of Pseudomonas syringae pv. tomato DC3000 modulates plant defence response to infection. Mol Microbiol 2003, 49:389-400. 33. Espinosa A, Guo M, Tam VC, Fu ZQ, Alfano JR:: The Pseudomonas syringae type III-secreted protein HopPtoD2 possesses protein tyrosine phosphatase activity and suppresses programmed cell death in plants. Mol Microbiol 2003, 49:377-387.

18. Hann DR, Dominguez-Ferreras A, Motyka V, Dobrev PI, Schornack S, Jehle A, Felix G, Chinchilla D, Rathjen JP, Boller T: The Pseudomonas type III effector HopQ1 activates cytokinin signaling and interferes with plant innate immunity. New Phytol 2013, 201:585-598. This report provides a novel virulence strategy of a T3E, HopQ1, which induces plant cytokinin production. Increased cytokinin levels correlates with a reduction on FLS2 gene expression and protein accumulation.

34. Underwood W, Zhang S, He SY: The Pseudomonas syringae type III effector tyrosine phosphatase HopAO1 suppresses innate immunity in Arabidopsis thaliana. Plant J 2007, 52:658672.

19. Giska F, Lichocka M, Piechocki M, Dadlez M, Schmelzer E, Hennig J, Krzymowska M: Phosphorylation of HopQ1, a type III

35. Macho AP, Schwessinger B, Ntoukakis V, Brutus A, Segonzac C, Roy S, Kadota Y, Oh MH, Sklenar J, Derbyshire P et al.: A bacterial




tyrosine phosphatase inhibits plant pattern recognition receptor activation. Science 2014, 343:1509-1512. This article uncovers the importance of tyrosine phosphorylation for the ligand-induced activation of PRRs. Consistently, HopAO1, a bacterial tyrosine phosphatase, targets PRR tyrosine phosphorylation to inhibit plant immunity. 36. Li X, Lin H, Zhang W, Zou Y, Zhang J, Tang X, Zhou JM: Flagellin induces innate immunity in nonhost interactions that is suppressed by Pseudomonas syringae effectors. Proc Natl Acad Sci U S A 2005, 102:12990-12995. 37. Oh HS, Collmer A: Basal resistance against bacteria in Nicotiana benthamiana leaves is accompanied by reduced vascular staining and suppressed by multiple Pseudomonas syringae type III secretion system effector proteins. Plant J 2005, 44:348-359. 38. Wu S, Lu D, Kabbage M, Wei HL, Swingle B, Records AR, Dickman M, He P, Shan L:: Bacterial effector HopF2 suppresses arabidopsis innate immunity at the plasma membrane. Mol Plant-Microbe Interact: MPMI 2011, 24:585-593. 39. Robert-Seilaniantz A, Shan L, Zhou JM, Tang X: The Pseudomonas syringae pv. tomato DC3000 type III effector HopF2 has a putative myristoylation site required for its avirulence and virulence functions. Mol Plant-Microbe Interact: MPMI 2006, 19:130-138. 40. Yamaguchi K, Nakamura Y, Ishikawa K, Yoshimura Y, Tsuge S, Kawasaki T: Suppression of rice immunity by Xanthomonas oryzae type III effector Xoo2875. Biosci Biotechnol Biochem 2013, 77:796-801. 41. Shao F, Merritt PM, Bao Z, Innes RW, Dixon JE: A Yersinia effector and a Pseudomonas avirulence protein define a family of cysteine proteases functioning in bacterial pathogenesis. Cell 2002, 109:575-588. 42. Zhang J, Li W, Xiang T, Liu Z, Laluk K, Ding X, Zou Y, Gao M, Zhang X, Chen S et al.: Receptor-like cytoplasmic kinases integrate signaling from multiple plant immune receptors and are targeted by a Pseudomonas syringae effector. Cell Host Microbe 2010, 7:290-301. 43. Shao F, Golstein C, Ade J, Stoutemyer M, Dixon JE, Innes RW: Cleavage of Arabidopsis PBS1 by a bacterial type III effector. Science 2003, 301:1230-1233. 44. Feng F, Yang F, Rong W, Wu X, Zhang J, Chen S, He C, Zhou JM: A Xanthomonas uridine 5’-monophosphate transferase inhibits plant immune kinases. Nature 2012, 485:114-118. In this report, the authors find that the T3E AvrAC targets several RLCKs to inhibit downstream immune signaling. Interestingly, this inhibition is based on a novel enzymatic activity, uridylyl transferase, which AvrAC employs to transfer uridine 5’-monophosphate to the activation loop of their targets to reduce their kinase activity. 45. Guy E, Lautier M, Chabannes M, Roux B, Lauber E, Arlat M, Noel LD: xopAC-triggered immunity against Xanthomonas depends on Arabidopsis receptor-like cytoplasmic kinase genes PBL2 and RIPK. PLoS One 2013, 8:e73469. 46. Lewis JD, Lee AH, Hassan JA, Wan J, Hurley B, Jhingree JR, Wang PW, Lo T, Youn JY, Guttman DS et al.: The Arabidopsis ZED1 pseudokinase is required for ZAR1-mediated immunity induced by the Pseudomonas syringae type III effector HopZ1a. Proc Natl Acad Sci U S A 2013, 110:18722-18727. 47. Yamaguchi K, Yamada K, Ishikawa K, Yoshimura S, Hayashi N, Uchihashi K, Ishihama N, Kishi-Kaboshi M, Takahashi A, Tsuge S et al.: A receptor-like cytoplasmic kinase targeted by a plant pathogen effector is directly phosphorylated by the chitin receptor and mediates rice immunity. Cell Host Microbe 2013, 13:347-357. This article reports a T3E, Xoo1488, which targets a novel rice RLCK, OsRLCK185, involved in chitin and PGN-triggered immunity. The authors use this discovery to uncover an important role of OsRLCK185 as a direct substrate of the OsCERK1 receptor, constituting a link between PRR activation and the subsequent activation of MAPKs. 48. Kouzai Y, Mochizuki S, Nakajima K, Desaki Y, Hayafune M, Miyazaki H, Yokotani N, Ozawa K, Minami E, Kaku H et al.: Targeted gene disruption of OsCERK1 reveals its indispensable role in chitin perception and involvement in the www.sciencedirect.com

peptidoglycan response and immunity in rice. Mol PlantMicrobe Interact 2014, 27:975-982. 49. Shimizu T, Nakano T, Takamizawa D, Desaki Y, Ishii-Minami N, Nishizawa Y, Minami E, Okada K, Yamane H, Kaku H et al.: Two LysM receptor molecules. CEBiP and OsCERK1, cooperatively regulate chitin elicitor signaling in rice. Plant J 2010, 64:204-214. 50. Zhang J, Shao F, Li Y, Cui H, Chen L, Li H, Zou Y, Long C, Lan L, Chai J et al.: A Pseudomonas syringae effector inactivates MAPKs to suppress PAMP-induced immunity in plants. Cell Host Microbe 2007, 1:175-185. 51. Zhang Z, Wu Y, Gao M, Zhang J, Kong Q, Liu Y, Ba H, Zhou J, Zhang Y: Disruption of PAMP-induced MAP kinase cascade by a Pseudomonas syringae effector activates plant immunity mediated by the NB-LRR protein SUMM2. Cell Host Microbe 2012, 11:253-263. 52. Li H, Xu H, Zhou Y, Zhang J, Long C, Li S, Chen S, Zhou JM, Shao F: The phosphothreonine lyase activity of a bacterial type III effector family. Science 2007, 315:1000-1003. 53. Wang Y, Li J, Hou S, Wang X, Li Y, Ren D, Chen S, Tang X, Zhou JM: A Pseudomonas syringae ADP-ribosyltransferase inhibits Arabidopsis mitogen-activated protein kinase kinases. Plant Cell 2010, 22:2033-2044. 54. Sarris PF, Gao S, Karademiris K, Jin H, Kalantidis K, Panopoulos NJ: Phytobacterial type III effectors HopX1. HopAB1 and HopF2 enhance sense-post-transcriptional gene silencing independently of plant R gene-effector recognition. Mol Plant-Microbe Interact: MPMI 2011, 24:907-917. 55. Hou S, Mu R, Ma G, Xu X, Zhang C, Yang Y, Wu D: Pseudomonas syringae pv. phaseolicola effector HopF1 inhibits pathogenassociated molecular pattern-triggered immunity in a RIN4independent manner in common bean (Phaseolus vulgaris). FEMS Microbiol Lett 2011, 323:35-43. 56. Wilton M, Subramaniam R, Elmore J, Felsensteiner C, Coaker G, Desveaux D: The type III effector HopF2 Pto targets Arabidopsis RIN4 protein to promote Pseudomonas syringae virulence. Proc Natl Acad Sci U S A 2010, 107:2349-2354. 57. Kim MG, da Cunha L, McFall AJ, Belkhadir Y, DebRoy S, Dangl JL, Mackey D: Two Pseudomonas syringae type III effectors inhibit RIN4-regulated basal defense in Arabidopsis. Cell 2005, 121:749-759. 58. Shang Y, Li X, Cui H, He P, Thilmony R, Chintamanani S, ZwieslerVollick J, Gopalan S, Tang X, Zhou JM: RAR1, a central player in plant immunity, is targeted by Pseudomonas syringae effector AvrB. Proc Natl Acad Sci U S A 2006, 103:19200-19205. 59. Axtell MJ, Staskawicz BJ: Initiation of RPS2-specified disease resistance in Arabidopsis is coupled to the AvrRpt2-directed elimination of RIN4. Cell 2003, 112:369-377. 60. Mackey D, Belkhadir Y, Alonso JM, Ecker JR, Dangl JL: Arabidopsis RIN4 is a target of the type III virulence effector AvrRpt2 and modulates RPS2-mediated resistance. Cell 2003, 112:379-389. 61. Mackey D, Holt BF 3rd, Wiig A, Dangl JL: RIN4 interacts with Pseudomonas syringae type III effector molecules and is required for RPM1-mediated resistance in Arabidopsis. Cell 2002, 108:743-754. 62. Inada N, Ueda T:: Membrane trafficking pathways and their roles in plant-microbe interactions. Plant Cell Physiol 2014, 55:672-686. 63. Nomura K, Debroy S, Lee YH, Pumplin N, Jones J, He SY: A bacterial virulence protein suppresses host innate immunity to cause plant disease. Science 2006, 313:220-223. 64. Nomura K, Mecey C, Lee YN, Imboden LA, Chang JH, He SY: Effector-triggered immunity blocks pathogen degradation of an immunity-associated vesicle traffic regulator in Arabidopsis. Proc Natl Acad Sci U S A 2011, 108:10774-10779. 65. Tanaka H, Kitakura S, De Rycke R, De Groodt R, Friml J: Fluorescence imaging-based screen identifies ARF GEF Current Opinion in Microbiology 2015, 23:14–22


component of early endosomal trafficking. Curr Biol: CB 2009, 19:391-397.

with tomato and pepper 14-3-3 isoforms to suppress effectortriggered immunity. Plant J 2014, 77:297-309.

66. Bartetzko V, Sonnewald S, Vogel F, Hartner K, Stadler R, Hammes UZ, Bornke F: The Xanthomonas campestris pv. vesicatoria type III effector protein XopJ inhibits protein secretion: evidence for interference with cell wall-associated defense responses. Mol Plant-Microbe Interact: MPMI 2009, 22:655-664.

77. Speth EB, Lee YN, He SY: Pathogen virulence factors as molecular probes of basic plant cellular functions. Curr Opin Plant Biol 2007, 10:580-586.

67. Ustun S, Bartetzko V, Bornke F: The Xanthomonas campestris type III effector XopJ targets the host cell proteasome to suppress salicylic-acid mediated plant defence. PLoS Pathogens 2013, 9:e1003427. 68. Schulze S, Kay S, Buttner D, Egler M, Eschen-Lippold L, Hause G, Kruger A, Lee J, Muller O, Scheel D et al.: Analysis of new type III effectors from Xanthomonas uncovers XopB and XopS as suppressors of plant immunity. New Phytol 2012, 195:894-911. 69. Chang IF, Curran A, Woolsey R, Quilici D, Cushman JC, Mittler R, Harmon A, Harper JF: Proteomic profiling of tandem affinity purified 14-3-3 protein complexes in Arabidopsis thaliana. Proteomics 2009, 9:2967-2985. 70. Oh CS, Pedley KF, Martin GB: Tomato 14-3-3 protein 7 positively regulates immunity-associated programmed cell death by enhancing protein abundance and signaling ability of MAPKKK {alpha}. Plant Cell 2010, 22:260-272. 71. Yang X, Wang W, Coleman M, Orgil U, Feng J, Ma X, Ferl R, Turner JG, Xiao S: Arabidopsis 14-3-3 lambda is a positive regulator of RPW8-mediated disease resistance. Plant J 2009, 60:539-550. 72. de Boer AH, van Kleeff PJ, Gao J: Plant 14-3-3 proteins as spiders in a web of phosphorylation. Protoplasma 2013, 250:425-440. 73. Lozano-Duran R, Bourdais G, He SY, Robatzek S: The bacterial effector HopM1 suppresses PAMP-triggered oxidative burst and stomatal immunity. New Phytol 2014, 202:259-269. This report shows that HopM1 inhibits several PTI responses. Interestingly, the authors find that HopM1 targets 14-3-3 proteins, and that chemical disruption of 14-3-3 interactions with client proteins mimics HopM1 functions. 74. Kim JG, Li X, Roden JA, Taylor KW, Aakre CD, Su B, Lalonde S, Kirik A, Chen Y, Baranage G et al.: Xanthomonas T3S effector XopN suppresses PAMP-triggered immunity and interacts with a tomato atypical receptor-like kinase and TFT1. Plant Cell 2009, 21:1305-1323. 75. Taylor KW, Kim JG, Su XB, Aakre CD, Roden JA, Adams CM, Mudgett MB: Tomato TFT1 is required for PAMP-triggered immunity and mutations that prevent T3S effector XopN from binding to TFT1 attenuate Xanthomonas virulence. PLoS Pathogens 2012, 8:e1002768. This report shows that the T3E XopN inhibits PTI by acting as a molecular scaffold for the formation of a complex between the tomato 14-3-3 isoform TFT1 and TARK1, a tomato atypical receptor kinase required for PTI. 76. Teper D, Salomon D, Sunitha S, Kim JG, Mudgett MB, Sessa G: Xanthomonas euvesicatoria type III effector XopQ interacts


78. Win J, Chaparro-Garcia A, Belhaj K, Saunders DG, Yoshida K, Dong S, Schornack S, Zipfel C, Robatzek S, Hogenhout SA et al.: Effector biology of plant-associated organisms: concepts and perspectives. Cold Spring Harb Symp Quant Biol 2012. 79. Mukhtar MS, Carvunis AR, Dreze M, Epple P, Steinbrenner J, Moore J, Tasan M, Galli M, Hao T, Nishimura MT et al.: Independently evolved virulence effectors converge onto hubs in a plant immune system network. Science 2011, 333:596-601. 80. Lacombe S, Rougon-Cardoso A, Sherwood E, Peeters N, Dahlbeck D, van Esse HP, Smoker M, Rallapalli G, Thomma BPHJ, Staskawicz B et al.: Interfamily transfer of a plant patternrecognition receptor confers broad-spectrum bacterial resistance. Nat Biotechnol 2010, 28:365-369. 81. Buttner D: Protein export according to schedule: architecture, assembly, and regulation of type III secretion systems from plant- and animal-pathogenic bacteria. Microbiol Mol Biol Rev 2012, 76:262-310. 82. Lara-Tejero M, Kato J, Wagner S, Liu X, Galan JE: A sorting platform determines the order of protein secretion in bacterial type III systems. Science 2011, 331:1188-1191. 83. Singer AU, Schulze S, Skarina T, Xu X, Cui H, Eschen-Lippold L, Egler M, Srikumar T, Raught B, Lee J et al.: A pathogen type III effector with a novel E3 ubiquitin ligase architecture. PLoS Pathogens 2013, 9:e1003121. 84. Akimoto-Tomiyama C, Furutani A, Tsuge S, Washington EJ, Nishizawa Y, Minami E, Ochiai H:: XopR, a type III effector secreted by Xanthomonas oryzae pv. oryzae, suppresses microbe-associated molecular pattern-triggered immunity in Arabidopsis thaliana. Mol Plant-Microbe Interact 2012, 25:505514. 85. Cheong MS, Kirik A, Kim JG, Frame K, Kirik V, Mudgett MB: AvrBsT acetylates Arabidopsis ACIP1, a protein that associates with microtubules and is required for immunity. PLoS Pathogens 2014, 10:e1003952. 86. Li G, Froehlich JE, Elowsky C, Msanne J, Ostosh AC, Zhang C, Awada T, Alfano JR: Distinct Pseudomonas type-III effectors use a cleavable transit peptide to target chloroplasts. Plant J 2014, 77:310-321. 87. Block A, Guo M, Li G, Elowsky C, Clemente TE, Alfano JR: The Pseudomonas syringae type III effector HopG1 targets mitochondria, alters plant development and suppresses plant innate immunity. Cell Microbiol 2010, 12:318-330. 88. Xin DW, Liao S, Xie ZP, Hann DR, Steinle L, Boller T, Staehelin C: Functional analysis of NopM, a novel E3 ubiquitin ligase (NEL) domain effector of Rhizobium sp. strain NGR234. PLoS Pathogens 2012, 8:e1002707.


Ubiquitination of pattern recognition receptors in plant innate immunity.

A bacterial tyrosine phosphatase inhibits plant pattern recognition receptor activation.

Plant pattern-recognition receptors.

Substrate recognition by the bacterial type II secretion system: more than a simple interaction.

Intrinsic disorder in plant proteins and phytopathogenic bacterial effectors.

Plant innate immunity against human bacterial pathogens.

Type VI secretion system effectors: poisons with a purpose.

Pattern recognition theory and the launch of modern innate immunity.

Bacterial effectors and their functions in the ubiquitin-proteasome system: insight from the modes of substrate recognition.

Pattern recognition receptors in innate immunity, host defense, and immunopathology.

Molecular weaponry: diverse effectors delivered by the Type VI secretion system.

From pathogen recognition to plant immunity: BIK1 cROSses the divide.

Studying the Mechanism of Plasmopara viticola RxLR Effectors on Suppressing Plant Immunity.

The role of effectors and host immunity in plant-necrotrophic fungal interactions.

Receptor Kinases in Plant-Pathogen Interactions: More Than Pattern Recognition.

Ubiquitination independent of E1 and E2 enzymes by bacterial effectors.

SPRYSEC Effectors: A Versatile Protein-Binding Platform to Disrupt Plant Innate Immunity.

Fungal effectors and plant susceptibility.

The Proteasome Acts as a Hub for Plant Immunity and Is Targeted by Pseudomonas Type III Effectors.

A bacterial model system for chromosomal targeting.

Recognition of human oncogenic viruses by host pattern-recognition receptors.

The T6SSs of Pseudomonas aeruginosa Strain PAO1 and Their Effectors: Beyond Bacterial-Cell Targeting.

Getting to PTI of bacterial RNAs: Triggering plant innate immunity by extracellular RNAs from bacteria.

TDO and Their Downstream Effectors.