Suppressor of apical dominance1 of the maize pathogen Sporisorium reilianum induces outgrowth of subapical ears by induction of abiotic stress response

Drechsler, Frank; Schirawski, Jan (Thesis advisor); van Dongen, Joost Thomas (Thesis advisor)

Aachen (2016)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2016

Abstract

The basidiomycete Sporisorium reilianum is a maize pathogen. Upon infection of its host, S. reilianum induces the formation of multiple phenotypes. Along with the formation of spores and phyllody, S. reilianum induces the outgrowth of subapical ears. Earlier studies identified the S. reilianum effector protein SUPPRESSOR OF APICAL DOMINANCE1 (SAD1) to be responsible for the increased outgrowth of subapical ears. This study aimed to understand the function of SAD1 and the mechanism underlying the induced changes in the plant developmental process, known as apical dominance.To study the function of SAD1 transgenic Arabidopsis thaliana plants that expressed GFP-SAD1 were created. GFP-SAD1 promoted early branching in A. thaliana. Thus, SAD1 manipulated branching by a pathway conserved between mono- and dicotyl plants. Localization of SAD1 in maize and A. thaliana was studied by fluorescence microscopy. In maize, SAD1-GFP was secreted from the fungal hyphae, but uptake by the plant cells was not detectable. In A. thaliana GFP-SAD1 and GFP-SAD1phosphorylation site mutants revealed phosphorylation-dependent nuclear accumulation. Additional information about a possible function of SAD1 was produced by the identification of the strongest interaction partner of SAD1. This interaction partner was the E3 ubiquitin ligase RGLG2. Sequencing of RNA, isolated from infected maize ears, revealed that SAD1 changed plant-gene expression. SAD1 induced the expression of stress response genes and inhibited the expression of floral development genes.In the current model for SAD1 function it is hypothesized that SAD1 would be secreted from the fungal hyphae and when taken up by the plant cell, SAD1 would actively enter the nucleus depending on its phosphorylational status. SAD1 would interact with RGLG2, either in the nucleus or before, which would change expression of genes involved in stress response and floral development. These changes would ultimately induce outgrowth of subapical ears in maize and promote branching in A. thaliana.

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