Required for Mla Resistance 3, a New Player in Barley Powdery Mildew Resistance?

Barley alleles of Mildew resistance locus a (Mla) encode c. 30 variants of CC-NB-LRR proteins.  These proteins recognise corresponding avirulence (AVR) effectors secreted by the obligate fungal pathogen, Blumeria graminis f. sp. hordei (Bgh), triggering defense.  Key factors in this resistance pathway include Required for Mla12 Resistance 1 (Rar1), Suppressor of G-two allele of skp1 (SGT1), Heat-shock protein 90 (HSP90) and several WRKY and MYB transcription factors (TFs).

Fast-neutron mutagenesis of barley CI 16151 (Mla6) generated the mutant m11526, which displayed susceptibility to the avirulent isolate Bgh 5874 (AVR_a6). Complementation tests revealed that the susceptibility was not due to mutations in Rar1 or Mla6.  Sequence-based analysis also ruled out other known members of the Mla resistance pathway, notably SGT1, HSP90 and all WRKY/MYB TFs.  This suggests that the mutation has occurred in a novel gene, which we designated as Required for Mla resistance 3 (Rar3).  Several strategies are being used to isolate Rar3, including transcript based cloning, exome capture, and genome resequencing.

Crosses of m11526 with lines carrying other Mla alleles (Mla1, Mla7, Mla10, Mla12, Mla13 & Mla15) were used to indicate which alleles do and, most intriguingly, do not require Rar3 to function.  This differential requirement of Rar3 by Mla alleles is reminiscent of Rar1 dependence.  These data suggest that RAR3 functions in the Mla pathway at, or upstream of, RAR1.  Lastly, Barley1 GeneChip differential transcriptome analysis revealed that while Rar1 mutants affect expression of a dozen genes, Rar3 influences over one-thousand.

Supported by NSF-PGRP grants 09-22746 and 13-39348.