Hormonal signaling in plant immunity

Abstract

Insect hervivores and pathogens are a major problem in agriculture and therefore, control of these pests and diseases is essential. For this, understanding the plant immune response can be instrumental. The plant hormones salicylic acid (SA) and jasmonic acid (JA) play an essential role in defense against different attackers. To fine-tune plant defenses, interactions exist between these two defense signals. In particular, the SA signaling pathway is known to inhibit activation of the JA signaling pathway, a phenomenon called SA/JA crosstalk. Although knowledge on the molecular players in the SA and JA-signaling pathways is increasing, our understanding of the molecular mechanisms of SA/JA crosstalk is still limited. The main goal of the research described in this thesis was to unravel the molecular mechanisms underlying this process. Therefore, the role of ERF repressor proteins in SA/JA crosstalk was investigated. However, seventeen erf mutants showed no change in SA-mediated suppression of JA-responsive gene expression, and a mutant in corepressor TPL was also not affected in this suppression. These results together show that it is unlikely that ERF repressor proteins or co-repressor TPL play a role in the repression of JA-responsive gene expression by JA, thereby invalidating an important hypothesis in SA/JA crosstalk research. Furthermore, NPR1, the master transcriptional regulator of the SA pathway, is known to be required for the antagonistic action of SA on JA signaling, but the mechanism of NPR1-mediated suppression of JA responses by SA was unclear. This was investigated through extensive mutant analysis. We identified a mutant that was unaffected in SA-induced NPR1-dependent gene expression, but was no longer able to suppress JA-responsive gene expression. This mutant was then subjected to RNA sequencing to identify candidate targets with a role in SA/JA crosstalk. Our data suggest that after SA accumulation, NPR1 translocates to the nucleus where it activates WRKY transcription factor genes, resulting in antagonism of JA-inducible genes. Besides its role in plant defense, JA also influences plant development and growth. Effective control of JA accumulation is necessary to inhibit negative effects when its positive effects on disease resistance are not necessary. Although much is known about biosynthesis of JA and the mode of action via its bioactive form JA-isoleucine (JA-Ile), knowledge on JA metabolism is incomplete. In particular, the active JA compound is known to be hydroxylated and thereby deactivated, but how this works was not known. In this thesis, the enzymes that are responsible for the hydroxylation of JA were identified. Knocking out the four genes that encode these enzymes, resulted in increased defense gene expression and resistance to the necrotrophic fungus Botrytis cinerea and the herbivore Mamestra brassicae. The identification of the enzymes responsible for hydroxylation of JA has revealed a missing step in JA metabolism. Knocking out these genes in crop species may create plants that are more resistant to JA-inducing attackers. Future studies into the mechanism of JOX may help to separate their effects on defense and growth, and this can be used to improve both resistance and yield in crops.