Regulation of plant immune receptors by ubiquitination

Abstract

From pathogen perception and the activation of signal transduction cascades to the deployment of defense responses, protein ubiquitination plays a key role in the modulation of plant immunity. Ubiquitination is mediated by three enzymes, of which the E3 ubiquitin ligases, the substrate determinants, have been the major focus of attention. Accumulating evidence suggests that ubiquitination modulates signaling mediated by pattern recognition receptors and is important for the accumulation of nucleotide-binding leucine-rich repeat type intracellular immune sensors. Recent studies also indicate that ubiquitination directs vesicle trafficking, a function that has been clearly established for immune signaling in animals. In this mini review, we discuss these and other recent advances and highlight important open questions.

Keywords: E3 ubiquitin ligases; PTI; effectors; protein degradation; receptor-like kinases; ubiquitination; vesicle trafficking.

FIGURE 1

Ubiquitin ligases that interact with receptor kinases. (A) The effector protein AvrPtoB from P. syringae pv. tomato binds to the co-receptor BAK1, the LRR-RLK FLS2 and the LysM RLK CERK1. AvrPtoB is able to ubiquitinate FLS2 and CERK1 and mediate their degradation. AvrPtoB can ubiquitinate BAK1 weakly in vitro. The mechanism leading to reduced RLK levels by AvrPtoB activity in vivo requires further clarification. (B) PUB12 and PUB13 constitutively interact with the co-receptor kinase BAK1. Constitutive phosphorylation of PUB12 and PUB13 by BAK1 is enhanced by flg22 which induces the interaction with FLS2. PUB12 and PUB13 ubiquitinate FLS2 and mediate its degradation. (C) The rice XB3 ligase interacts with the LRR-RLK XA21. XA21 phosphorylates XB3 in vitro. Whether ligand binding is required for the phosphorylation is not known. XB3 contributes to XA21 accumulation and is therefore unlikely to ubiquitinate XA21. XB3 conceivably targets a protein that affects XA21 accumulation. (D) The L. japonicus SINA4 was shown to interact with and negatively regulate the levels of the LRR-RLK SYMRK, which mediates symbiotic signaling. (E) The B. napus ARC1 interacts and is phosphorylated by the S-domain SRK, which mediates SI reaction. ARC1 was proposed to regulate SI through the degradation of Exo70A1. Further experimental clarification is needed to determine whether ARC1 affects SRK levels. (F) The M. truncatula PUB1 interacts with and is phosphorylated by LYK3, a LysM type RLK involved in nodulation. PUB1, a negative regulator of nodulation, does not ubiquitinate LYK3 in vitro. PUB1 might therefore target an alternate protein required for symbiosis. Shapes with dotted lines denote potential involvement (e.g., ligand) or a hypothetical target.