Monitoring of in planta gene expression for xylan degradation and assimilation in the maize pathogen Bipolaris maydis.

Abstract Swift and efficient onset of feeding on host tissue by phytopathogenic fungi is a requisite event for their successful infection and propagation. Necrotrophic fungi colonizing host cell walls appear to obtain carbon and energy sources from plant wall degradants, but what they actually utilize for nutrition after host invasion remains unclear. Here we focus on plant wall xylan, the major hemicellulosic polysaccharide in cereal plants, and study its participation in post-invasion nutrition of the maize necrotrophic pathogen Bipolaris maydis (syn: Cochliobolus heterostrophus). Using a fluorescence reporter assay, we demonstrated that a B. maydis β-xylosidase gene, BmXyp1 , is strongly upregulated at the beginning of infection, specifically within invading hyphae. Additionally, our time-course measurements of mRNA expression during maize infection revealed that xylan degradation and assimilation are concomitantly induced during an early infection stage. These findings suggest that this fungus can access xylan degradants as an early in planta nutrient source after host penetration; however, mutant strains deficient in xylan-assimilation ability still retained virulence, although the lesion size was decreased as compared with the wild-type strain. Overall, we conclude that xylan degradation and assimilation by B. maydis are initial post-invasion events but do not play an essential role in fungal nutrient acquisition. Highlights • Role of xylan as a post-invasion nutrient source for Bipolaris maydis was explored. • A gene for xylan degradation is highly induced at the initial phase of invasion. • Genes for xylan hydrolysate assimilation are also upregulated during early infection. • Mutants deficient in xylan assimilation ability have relatively low virulence. • This fungus does not depend on xylan as a major part of nutrient supply in the host. [ABSTRACT FROM AUTHOR]