Role of RIN4 in Regulating PAMP-Triggered Immunity and Effector-Triggered Immunity: Current Status and Future Perspectives

Abstract

As sessile organisms, plants have developed sophisticated system to defend themselves against microbial attack. Since plants do not have specialized immune cells, all plant cells appear to have the innate ability to recognize pathogens and turn on an appropriate defense response. The plant innate immune system has two major branches: PAMPs (pathogen associated molecular patterns)-triggered immunity (PTI) and effector-triggered immunity (ETI). The ability to discriminate between self and non-self is a fundamental feature of living organisms, and it is a prerequisite for the activation of plant defenses specific to microbial infection. Arabidopsis cells express receptors that detect extracellular molecules or structures of the microbes, which are called collectively PAMPs and activate PTI. However, nucleotidebinding site leucine-rich repeats (NB-LRR) proteins mediated ETI is induced by direct or indirect recognition of effector molecules encoded by avr genes. In Arabidopsis, plasmamembrane localized multifunctional protein RIN4 (RPM1interacting protein 4) plays important role in both PTI and ETI. Previous studies have suggested that RIN4 functions as a negative regulator of PTI. In addition, many different bacterial effector proteins modify RIN4 to destabilize plant immunity and several NB-LRR proteins, including RPM1 (resistance to Pseudomonas syringae pv. maculicola 1), RPS2 (resistance to P. syringae 2) guard RIN4. This review summarizes the current studies that have described signaling mechanism of RIN4 function, modification of RIN4 by bacterial effectors and different interacting partner of RIN4 in defense related pathway. In addition, the emerging role of the RIN4 in plant physiology and intercellular signaling as it presents in exosomes will be discussed.

Keywords: AvrB; AvrRpm1; AvrRpt2; PAMP-triggered immunity; RIN4; effector-triggered immunity.

Fig. 1. Effector-mediated RIN4 modification inhibits PTI

(A) RIN4 negatively regulates PAMP-triggered immunity. (B) PAMP perception by PRR and PRPs increases FLS2-dependent RIN4 phosphorylation at the S141 residue and contributes to PTI. (C) In the absence of RPM1, two effector proteins, AvrB and AvrRpm1, induce RIN4^T166 phosphorylation, which is epistatic to RIN4^S141 phosphorylation, and repress PTI activation. (D) Activation and inactivation of AHA1 and AHA2 control stomata opening and closing. (E) RIN4 binds to the C-terminal region of H⁺-ATPase and activates it. The effectors AvrRpm1 and AvrB phosphorylate RIN4^T166, resulting in increased association of RIN4 with H⁺-ATPase and activation of H⁺-ATPase. AvrRpt2: , AvrB: , AvrRpm1: , RIPK: .

Fig. 2. Type III effectors modify RIN4 to promote bacterial growth and activate resistance proteins

(A) In rpm1 mutant plants, the type III effectors, AvrRpm1 and AvrB, phosphorylate RIN4 to promote bacterial growth. ROC1 isomerizes RIN4^P149 and inhibits RPS2. (B) In resistant plants, ROC1 suppresses RPM1 and RPS2. RIN4 phosphorylation at the T166 residue removes ROC1 suppression and activates RPM1. RPM1 activation initiates a hypersensitive response and decreases bacterial growth. AvrPphB inhibits AvrB-induced RPM1 activation. (C) The type III effector AvrRpt2 cleaves RIN4 upon delivery and processing. In rps2 mutant plants, AvrRpt2 cleaves RIN4 to promote bacterial growth. RIN4 cleavage subsequently decreases the accumulation of RPM1. (D) In resistant plants, RIN4 degradation activates RPS2, which initiates hypersensitive response and decreases bacterial growth. The RIN4–NDR1 association is important for RPS2 activation. RIN4 degradation activates the resistance protein RPS2, and HopF2 inhibits AvrRpt2-mediated RIN4 cleavage. AvrRpt2: , AvrB: , AvrRpm1: , RIPK: , ROC1: , NDR1: .

Fig. 3. A model depicting the role of RIN4 in PTI and ETI

RIN4 negatively regulates PTI response upon PAMP recognition (orange). RIN4 strongly binds to the C-terminal inhibitory region of H⁺-ATPase and activates H⁺-ATPase to control stomatal immunity (orange). The effector proteins AvrB and AvrRpm1 induce RIN4 phosphorylation and activate RPM1, if present (green). Another effector, AvrRpt2, degrades RIN4 and activates RPS2, if present (blue). RIN4 degradation also interfere with RPM1 accumulation and function. Expression of the effector protein HopF2 halts AvrRpt2-mediated RIN4 degradation (black). AvrPto and AvrPtoB also degrade RIN4 (purple).