Upstream open reading frames of the barley Mla13 powdery mildew resistance gene function co-operatively to down-regulate translation

ABSTRACT

To protect themselves against pathogen infection, plants have evolved multifarious defence mechanisms that include the production of antimicrobial compounds and hypersensitive cell death. This defence includes the regulation of expression of resistance proteins, which function to detect pathogen effectors and control the response. Barley Mla is one of several Ml genes that confer specific resistance to the powdery mildew fungus, Blumeria graminis f. sp. hordei. The Mla13 allele contains a long (523 nt) transcript leader region with three upstream open reading frames (uORFs) and two alternatively spliced introns. The lengths of the Mla13 uORFs, which encode up to 70 amino acid peptides, are directly influenced by the splicing of these introns. We have used in vivo and in vitro translation assays to show that Mla13 uORFs function co-operatively to down-regulate translation of the parent cistron up to 13-fold. However, resistance conferred by constitutively transcribed Mla appears to be unaffected by removal of these uORFs, signifying that a potential increase in MLA13 protein is innocuous to barley cells. In addition, cells that possess a predicted increase in MLA13 translation via over-expression and uORF mutation retain their requirement for RAR1, suggesting that this increase is not sufficient to overcome the concentration threshold required for activation of RAR1-independent resistance.