Phosphoproteins in the primed defense response of plants

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Plants can be primed for more rapid and robust activation of defense which often comes with induced immunity. In Arabidopsis thaliana defense priming requires heat shock transcription coactivator HsfB1, histone modifications on defense gene promoters, and accumulation of mitogen-activated protein kinases MPK3 and MPK6 (at both the mRNA transcript and protein levels). However, MPK3/6 are maintained in an inactive state in primed cells and require pathogen challenge for activation. Upon pathogen challenge more MPK3/6 enzymes are activated in primed than in nonprimed cells. The activation of more MPKs in primed plants correlates with enhanced defense gene expression and induction of immunity. Thus, MPK3/6 are important regulators of priming in Arabidopsis. However, other phosphoproteins important for priming remained elusive. We developed a powerful, tandem metal oxide affinity chromatography approach for the extraction, identification, and quantification by LC/MS of low abundant, transiently phosphorylated proteins with a likely role in priming. In the present proposal we aim to characterize, and functionally analyze selected novel phosphoprotein candidates (e.g. PEARLI4 and TFIIS) that are important for defense priming in Arabidopsis.