Survival of all living organisms requires the ability to detect attack and swiftly counter with protective immune responses, which are tightly regulated by complex layers of dynamic checks and balances. Rapid and reversible modulation of the signaling components requisite for a dynamic regulation commonly occurs through posttranscriptional, translational and posttranslational mechanisms without requiring de novo transcription. Such mechanisms have emerged to amplify initial inputs and rapidly coordinate immune outputs. To discover new key regulators that modulate plant immunity, we profiled rapid changes in the phosphoproteome of Arabidopsis and maize following treatment with Plant Elicitor Peptides (Peps), conserved signals regulating innate immunity in higher plants. We characterized a novel RNA-binding protein, named IMMUNOREGULATORY RNA-BINDING PROTEIN (IRR), that regulates alternative splicing of transcripts encoding defense signaling proteins to dynamically increase immune signaling. We defined a novel dynamic regulatory mechanism acting on the CPK28 buffering system mediated by IRR. In the absence of immune challenge, IRR associates with CPK28 transcripts to promote canonical splicing into mRNA encoding full-length, functional proteins. Upon activation of PEPRs, IRR is transiently dephosphorylated, causing dissociation from CPK28 transcripts. Disruption of IRR interaction with CPK28 transcripts leads to increased levels of a retained-intron variant encoding a truncated protein that lacks EF-hand domains required for calcium-induced stimulation of kinase activity and exhibiting reduced functionality. Altered ratios of canonical versus retained-intron CPK28 transcripts modulate PEPR signaling sensitivity, with proportional increases in the retained intron variant resulting in amplified immune signaling and defense. This novel mechanism directly links PEPR-induced dephosphorylation of IRR with post-transcriptionally-mediated attenuation of CPK28 function, providing a rapid mechanism to modulate PEPR signaling capacity and immune outputs.
Coauthors: Philipp Weckwerth – UC San Diego;Elly Poretsky – UC San Diego;Yohei Takahashi – UC San Diego;Zhouxin Shen – UC San Diego;Carleen Villarreal – UC San Diego;Julian Schroeder – UC San Diego;Steven Briggs – UC San Diego;Alisa Huffaker – UC San Diego