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Role of the ubiquitin-conjugating enzymes in plant innate immunity

Lirong Zeng: University of Nebraska

<div>Ubiquitination has emerged in recent years as a key regulatory mechanism underlying plant immunity against various pathogens. Of the three classes of enzymes involved in ubiquitination, few ubiquitin-conjugating enzymes (E2) have been investigated in plants. Our recent study revealed the tomato UBC13 type E2s, Fni3 and SlUBC13-2 and their cofactor, Suv positively regulate plant immunity. To further characterize the role of E2 in plant innate immunity, we identified and cloned forty tomato genes encoding E2 proteins. Thioester assays indicated the majority of the genes encode enzymatically active E2. Phylogenetic analysis classified the forty tomato E2 into thirteen groups, of which only members of group III were found to act with AvrPtoB, a <em>Pseudomonas syringae </em>pv. <em>tomato</em> <em></em>effector that uses its ubiquitin ligase (E3) activity to suppress host immunity. Knocking-down the expression of group III E2 genes in <em>Nicotiana benthamiana</em> diminished AvrPtoB-promoted degradation of the Fen kinase and AvrPtoB suppression of host immunity-associated PCD. Importantly, silencing group III E2 genes also resulted in reduced pattern-triggered immunity (PTI). By contrast, PCD induced by several effector-triggered immunity (ETI) elicitors was not affected on group-III-silenced plants. Functional characterization suggested redundancy among group III members for their role in the suppression of plant immunity by AvrPtoB and in PTI. These results suggest AvrPtoB has evolved a strategy for suppressing host immunity that is difficult for the plant to thwart. We also uncovered the involvement of non-group III E2 members in plant immunity through functioning at the endoplasmic reticulum (ER) site. Our work builds a foundation and provides critical inroads into understanding the regulation of plant immunity by an often-overlooked group enzymes in ubiquitination.</div>