Abstract
Summary Ustilago maydis is a model organism for the study of biotrophic plant–pathogen interactions. The sexual and pathogenic development of the fungus are tightly connected since fusion of compatible haploid sporidia is prerequisite for infection of the host plant, maize (Zea mays). After plant penetration, the unfolded protein response (UPR) is activated and required for biotrophic growth. The UPR is continuously active throughout all stages of pathogenic development in planta. However, since development of UPR deletion mutants stops directly after plant penetration, the role of an active UPR at later stages of development remained to be determined. Here, we established a gene expression system for U. maydis that uses endogenous, conditionally active promoters to either induce or repress expression of a gene of interest during different stages of plant infection. Integration of the expression constructs into the native genomic locus and removal of resistance cassettes were required to obtain a wild‐type‐like expression pattern. This indicates that genomic localization and chromatin structure are important for correct promoter activity and gene expression. By conditional expression of the central UPR regulator, Cib1, in U. maydis, we show that a functional UPR is required for continuous plant defence suppression after host infection and that U. maydis relies on a robust control system to prevent deleterious UPR hyperactivation.
Highlights
Since the virulence of strain SG200Pmig2_1:cib1s was severely attenuated in plant infection experiments (Fig. 1A), we investigated at which stage pathogenic development was blocked
Suitable tools to address this problem are restricted to the introduction of a gatekeeper mutation in kinases that can be chemically inhibited by non-hydrolysable adenosine triphosphate (ATP) analogues
We report a conditional gene expression system for U. maydis that enables the study of gene functions at different stages of pathogenic development in the plant
Summary
Ustilago maydis is a model organism for the study of biotrophic plant–pathogen interactions. The unfolded protein response (UPR) is activated and required for biotrophic growth. The UPR is continuously active throughout all stages of pathogenic development in planta. We established a gene expression system for U. maydis that uses endogenous, conditionally active promoters to either induce or repress expression of a gene of interest during different stages of plant infection. Integration of the expression constructs into the native genomic locus and removal of resistance cassettes were required to obtain a wild-type-like expression pattern. This indicates that genomic localization and chromatin structure are important for correct promoter activity and gene expression.
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