Expression of Distinct Blumeria graminis Effectors in Response to Isogenic Barley Mutant Genotypes

During infection of barley (Hordeum vulgare) by the obligate fungal biotroph Blumeria graminis f. sp. hordei (Bgh), countless number of interactions occur between Bgh secreted effectors and host proteins. Blufensin1 (Bln1) is a knottin-like protein found to be highly induced during Bgh infection of barley (Meng et al., 2009). When Bln1 is silenced, host susceptibility in compatible interactions is reduced. Recently, Bln1 was shown to be secreted into the apoplast and plants silenced for Bln1 down-regulate several genes encoding proteins that function in nuclear import and the secretory pathway, notably Importin α1-b and Sec61 γ; plants silenced for these genes also showed reduced susceptibility in compatible interactions. Fast-neutron mutagenesis of barley lines containing the CC-NB-LRR Mla6 resistance gene led to the isolation of several loss-of-function deletions, including those in Bln1 (bln1-m19089) and Mla6 (mla6-m18982). These mutants were analysed individually and in combination using RNA-Seq as well as fungal infection kinetics. Bgh development was identical for bln1 mutants when compared to those containing wildtype Bln1. However, distinct differences were observed in the number and expression of Bgh effectors depending on the host mutation. This evidence suggests that Bgh is able detect loss-of-function in the different mutant hosts and modify expression of its effectors to compensate. It has been shown recently in the fungal biotroph Ustilago maydis that retrograde signalling from the hyphal tip via early endosomes affects the expression of effectors during infection (Bielska et al., 2014). Thus a similar phenomenon could be occurring in Bgh-barley interactions.

Supported by NSF-PGRP grant 09-22746 to RW & DN