The Verticillium effector TRADE induces host plant cell identity switches by co-opting the osmotic stress and wound response pathways
Cumulative thesis
Date of Examination:2024-09-10
Date of issue:2024-10-08
Advisor:Prof. Dr. Volker Lipka
Referee:Prof. Dr. Volker Lipka
Referee:Prof. Dr. Kai Heimel
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Abstract
English
Climate change not only increases the abiotic stress of our crops but also favors the spread of their pathogens. Thus, understanding the molecular mechanisms of plant-pathogen interactions is more important than ever. Soil-borne fungi of the Verticillium genus are vascular pathogens that infect a broad range of important crop plants. While most Verticillium strains induce a wilting phenotype, V. longisporum and certain V. dahliae isolates induce a phenotype that is rather characterized by stunting and chlorosis. This phenotype is accompanied by transdifferentiation of bundle sheath and xylem parenchyma cells into new functional treachery elements. The transdifferentiation correlates with the activation of VASCULAR-RELATED NAC DOMAIN (VND) 6 and VND7, which both encode key regulators of xylem formation in Arabidopsis. Recent research identified that a single lineage-specific secreted effector, TRANSDIFFERENTIATION EFFECTOR (TRADE), is causative for these phenotypes (Subieta et al., in review). TRADE interacts with the cytoplasmic VARICOSE (VCS) protein and induces its subclass I SUCROSE NON-FERMENTING RELATED KINASE 2 (SRK2s) dependent phosphorylation. VCS is the central scaffolding protein for mRNA decapping in processing bodies (P bodies) and, therefore, an essential component of 5’ - 3’ mRNA decay. TRADE-induced phosphorylation of VCS affects the turnover of VND6 and VND7 transcripts, which accumulate and can induce xylem formation. Still, the mechanisms of host plant-dependent uptake of TRADE into the plant cell, TRADE-mediated and SRK2-dependent phosphorylation of VCS, and how this phosphorylation ultimately results in vein clearing, chlorosis, and bundle sheath transdifferentiation remain obscure. Therefore, this study aimed to elucidate further chlorosis class symptom induction by the Verticillium effector TRADE. As heterologously expressed and purified TRADE protein is sufficient to induce striking chlorosis phenotypes, it was used to screen Arabidopsis mutant collections for TRADE-insensitive plants. This forward genetic screen identified TRADE-insensitive plants with mutations in RAF20 and WIND1. RAF20 encodes a B4 RAF-like kinase that acts upstream of SRK2 kinases in the abscisic acid (ABA)-independent osmotic stress response. Confocal laser scanning microscopy (CLSM) revealed that TRADE treatment induces localization of RAF20 and the subclass I SRK2 into P bodies. The same effect was observed for key components of the decapping machinery, showing how the TRADE effector co-opts the ABA-independent osmotic stress response to manipulate the plant's mRNA turnover. WIND1 encodes a wound-induced transcription factor that induces regulators of tissue differentiation, including the xylogenesis master regulators VND6 and VND7. This study shows that WIND1 transcript accumulates during infection, likely resulting from transcriptional activation rather than mRNA decay inhibition. Furthermore, it was demonstrated that RAF20 and WIND1 are required compatibility factors for chlorosis-inducing Verticillium despite having functionally redundant homologs in the osmotic stress response and wound response, respectively.
Keywords: effector biology; transdifferentiation; wounding response; osmotic stress response; processing bodies; mRNA decay