Molecular basis for differential host response of sorghum against different formae speciales of Sporisorium reilianum

Dutra, Deiziane da Consolacao; Schirawski, Jan (Thesis advisor); Blank, Lars M. (Thesis advisor)

Aachen : RWTH Aachen University (2021, 2022)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2021

Abstract

The biotrophic pathogenic smut fungus Sporisorium reilianum causes head smut in maize and sorghum and threatens the worldwide cultivation of infected plants. The fungus exists in two formae speciales, S. reilianum f. sp. zeae (SRZ) and S. reilianum f. sp. reilianum (SRS), that can infect maize and sorghum, respectively. Prior to plant infection, compatible haploid sporidia mate, forming infectious dikaryotic filaments. Hundreds of effector proteins are predicted to be secreted during S. reilianum infection that may help the fungus to colonize the host or lead to recognition by non-host plants, triggering defense response. When infecting its preferred host, S. reilianum does not induce visual signs of defense response on leaves other than weak chlorosis and minor cell death. Differently, sorghum infection with SRZ is challenged by several plant defense responses that prevent pathogen systemic spread. The strongest visual defense response of sorghum against SRZ is the production of phytoalexins that lead to a red coloration of infected leaf tissues. To better understand the molecular events that take place during SRZ infection of sorghum, we followed several experimental approaches. In approach one, we used fractionated cellular components of SRZ and infiltrated sorghum leaves to test whether cell wall components play a role in phytoalexin induction. Commercial compounds known to induce phytoalexins in other systems were also tested. However, no clear indication of the involvement of cell wall components in the induction of phytoalexins could be proved. In approach two, we tested whether SRZ-specific secreted effectors are responsible for phytoalexin induction. Candidate effectors were tested for their ability to induce phytoalexin formation by particle bombardment of sorghum leaves. None of the tested effectors could consistently induce phytoalexins in the targeted or neighboring cells. In approach three, the most highly differentially expressed selected candidate effectors were individually deleted in SRZ. Deletion of one candidate, sr14274, led to highly reduced phytoalexin induction as a result of greatly impaired virulence. In approach four, we aimed to test the effect of individual proteins of SRZ on the induction of phytoalexins or cell death in sorghum. To this end, 46 selected candidates were cloned for heterologous protein production in E. coli, using the pTSGATE1 plasmid. Due to time constraints, protein expression and leaf infiltration were not tested. Finally, in approach five, we tested selected effector candidates for their ability to suppress hypersensitive cell death triggered by the elicitin INF1 of Phytophthora infestans by agroinfiltration in Nicotiana benthamiana. A predicted cytoplasmic effector, sr16441ΔSP, was able to reliably suppress INF1-induced cell death. This study revealed a putative function for a new effector candidate that may help us to understand the molecular processes happening upon sorghum infection with SRZ.