The Role of the Wheat ()-DELLA Mutants and Associated Hormones in Infection by , the Causal Agent of Ergot.

Partial resistance to the biotrophic fungal pathogen , causal agent of ergot, has been found that colocates with mutant alleles of the wheat () loci on chromosomes 4B and 4D. These loci represent the wheat orthologs of the Della genes. To investigate the role of the mutant DELLA proteins in ergot resistance, we assessed . infection in wheat near-isogenic lines (NILs) carrying the gibberellic acid (GA)-insensitive semidwarf alleles - and - and the severe dwarf alleles - and -. NILs of the GA-sensitive alleles (chromosome 2D) and (chromosome 5A) were also included. A general trend toward increased resistance to . , with smaller and lighter sclerotia, was observed on the NILs -, -, -, and -, and also on . Levels of the bioactive GA and the auxin indole-3-acetic acid increased after inoculation with . , following similar patterns and implicating a potential auxin-mediated induction of GA biosynthesis. In contrast, jasmonic acid (JA) levels fell in the parental lines 'Mercia' and 'Maris Huntsman' after inoculation with . , but increased in all the -mutant NILs. Inoculation with . did not show any informative changes in the levels of salicylic acid. Our results suggest that GA-mediated degradation of the DELLA proteins and down-regulation of JA-signaling pathways supports infection of wheat by . . As these responses are generally associated with necrotrophic fungal pathogens, we propose that the biotroph . may have a necrotrophic growth stage.