The Global Regulatory Protein VelA Is Required for Symbiosis Between the Endophytic FungusEpichloë festucaeandLolium perenne

Abstract

Epichloë species fungi form bioprotective endophytic symbioses with many cool-season grasses, including agriculturally important forage grasses. Despite its importance, relatively little is known about the molecular details of the interaction and the regulatory genes involved. The conserved velvet-domain protein VelA (or VeA) is a global regulator of a number of cellular and developmental functions in fungi. In this study, the E. festucae velA gene was functionally characterized in vitro and during interaction with perennial ryegrass. The velA gene is required in E. festucae for resistance to osmotic and cell wall-damaging stresses, repression of conidiation, and normal hyphal morphology during nutrient-limited in-vitro conditions. Expression of velA in E. festucae is light- and nitrogen-dependent and is tissue-specific in mature infected plants. In-planta studies showed that velA is required in E. festucae for a compatible interaction. Inoculating seedlings with mutant ΔvelA induced callose deposition and HO production, and a high level of seedling death was observed. In surviving plants infected with ΔvelA mutant fungi, plants were stunted and we observed increased biomass and invasion of vascular bundles. Overall, this work characterizes a key fungal regulatory factor in this increasingly important model symbiotic association.

VelA and LaeA are key regulators of theEpichloë festucaetranscriptomic response during symbiosis with perennial ryegrass

“…This suggests that VelA has a stronger regulatory effect when growing in seedlings and mature plants under likely nutrient limited conditions, compared to PDA culture, a nutrient rich condition. This also correlates with our previous report that showed much higher expression of velA under nutrient limited medium (water agar) compared to nutrient rich medium (PDA) [11], which is similar to LaeA [12].…”

“…As we showed before, the highest laeA expression was also at 14 DPI although it was not a gradual increase as was the case with velA [12]. In addition to velA and laeA having the highest expression at 14 DPI, this time point also correlated with when Δ velA and Δ laeA mutant inoculated seedlings began to die [11, 12]. Because of these reasons, 14 DPI was chosen to study the requirement of velA and laeA on the expression profile of E. festucae during infection of perennial ryegrass.…”

“…In conclusion, this study provides a significant understanding on the strong regulatory effects of VelA on the E. festucae transcriptome profile during compatible and incompatible interactions with ryegrass. These regulatory effects could explain some of the observed phenotypes resulting from deleting velA in this fungus [11]. It seems that during the interaction of E. festucae with ryegrass, VelA controls fungal gene expression to supress or activate genes involved in either inducing or reducing the hostplant response.…”

“…Based on the knowledge of regulatory roles of VelA on different fungal transcriptome profiles and our findings of VelA importance in E. festucae metabolism and development and successful symbiosis [11], we hypothesised that VelA may be involved in regulating the E. festucae transcriptome. In this study, a set of comparative transcriptomics analyses of Δ velA mutants versus wild type E. festucae in culture, in infected seedlings and in infected mature plants was performed.…”

“…Although the molecular mechanisms that regulate the mutualistic interaction between E. festucae and perennial ryegrass is still largely unknown a number of studies have shown the importance of genes required for hyphal anastomosis (soft gene [10]), fungal biology and development ( velA gene [11] and laeA gene [12]), localised production of reactive oxygen species (required to maintain hyphal polarity [13-16]) and iron homeostasis ( sidN gene [17, 18]). Although it is not clear how Epichloë suppresses or avoids plant responses to establish and maintain a compatible interaction, recent studies have suggested that reducing the chitin inducible host defence response, by altering fungal cell wall chitin, is a possible mechanism [19, 20].…”