Negative regulation of PAMP-triggered immunity by an E3 ubiquitin ligase triplet in Arabidopsis.

The first line of active defense in plants is triggered by invariant microbial epitopes known as pathogen-associated molecular patterns (PAMPs). Perception of PAMPs by receptors activates a plethora of reactions ending in PAMP-triggered immunity (PTI), which contributes to broad-spectrum resistance. Here, we report a homologous triplet of U-box type E3 ubiquitin ligases (PUBs), PUB22, PUB23, and PUB24 in Arabidopsis, that act as negative regulators of PTI in response to several distinct PAMPs. Expression of PUB22/PUB23/PUB24 was induced by PAMPs and infection by pathogens. The pub22/pub23/pub24 triple mutant displayed derepression and impaired downregulation of responses triggered by PAMPs. Immune responses including the oxidative burst, the MPK3 activity, and transcriptional activation of marker genes were increased and/or prolonged. Enhanced activation of PTI responses also resulted in increased resistance against bacterial and oomycete pathogens, which was accompanied by increased production of reactive oxygen species and cell death. Our data provide novel insights into the regulation of immunity in plants and links ubiquitination as a mechanism of negative regulation of PTI.