The wheat Mla homologue TmMla1 exhibits an evolutionarily conserved function against powdery mildew in both wheat and barley

Abstract

SUMMARYThe race-specific barley powdery mildew (Blumeria graminis f. sp. hordei) resistance gene Mla occurs as an allelic series and encodes CC-NB-LRR type resistance proteins. Inter-generic allele mining resulted in the isolation and characterisation of an Mla homologue from diploid wheat, designated TmMla1, which shares 78% identity with barley HvMLA1 at the protein level. TmMla1 was found to be a functional resistance gene against Blumeria graminis f. sp. tritici in wheat, hereby providing an example of R gene orthologs controlling the same disease in two different species. TmMLA1 exhibits race-specific resistance activity and its N-terminal coiled-coil domain interacts with the barley transcription factor HvWRKY1. Interestingly, TmMLA1 was not functional in barley transient assays. Replacement of the TmMLA1 LRR domain with that of HvMLA1 revealed that this fusion protein conferred resistance against B. graminis f. sp. hordei isolate K1 in barley. Thus, TmMLA1 not only confers resistance in wheat but possibly also in barley against an as yet unknown barley powdery mildew race. The conservation of functional R gene orthologs over at least 12 million years is surprising given the observed rapid breakdown of Mla-based resistance against barley mildew in agricultural ecosystems. This suggests a high stability of Mla resistance in the natural environment before domestication.

Barley MLA Immune Receptors Directly Interfere with Antagonistically Acting Transcription Factors to Initiate Disease Resistance Signaling

“…Further conditional expression of MLA10_CC wild type or its L18E variant, but not of the CC L11E variant, dramatically reduced reporter activity to ;0.1 unit ( Figure 3A, left), possibly resulting from competitive interactions (i.e., CC binding to WRKY1, MYB6, or both). To discriminate between these possibilities, we repeated the assays using the truncated bait BD-WRKY1DCT, which interacts with MYB6 ( Figure 2) but not the MLA CC module (Jordan et al, 2011;Maekawa et al, 2011a). BD-WRKY1DCT expression led to reduced reporter activity (;1.4 compared with ;2.0 units for BD-WRKY1; Figure 3A), confirming weaker WRKY1DCT interactions with MYB6 than with WRKY1 ( Figure 2A).…”

“…The conserved C-terminal part of WRKY1 (CT, amino acids 260 to 353) and WRKY2 mediates the interaction with the MLA10 CC domain (Jordan et al, 2011;Maekawa et al, 2011a). We therefore investigated potential interactions between MYB6 and WRKY1 or WRKY2.…”

Unlocking the Barley Genome by Chromosomal and Comparative Genomics

“…Even race-specific disease resistance, a paradigm for species-specific genetic control in plants, shares conserved genetic elements in barley and wheat. Recently, a functional allele of the barley gene Mla, which confers resistance to the powdery mildew fungus (Zhou et al, 2001), was isolated from Triticum monococcum (Jordan et al, 2010). Indeed, an increasing body of information supports the notion of treating the Triticeae as a single genetic system.…”

Cytosolic activation of cell death and stem rust resistance by cereal MLA-family CC–NLR proteins

“…A wheat ortholog of the MLA gene family, TmMla1, confers resistance to wheat powdery mildew (14). Likewise, Sr33 and Sr50 occur at the orthologous locus in diploid wheat (Aegilops tauschii) and rye (Secale cereale), respectively, and are the first members of the MLA family known to recognize a pathogen other than powdery mildew (10,11).…”