Submitted to: Molecular Plant Pathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/28/2006
Publication Date: 5/1/2006
Citation: Halterman, D.A., Wise, R.P. 2006. Upstream open reading frames of the barley Mla13 powdery mildew resistance gene function cooperatively to down-regulate translation. Molecular Plant Pathology. 7:167-176. Interpretive Summary: Powdery mildew of barley is an ideal system for investigating fungal diseases in cereal crops. At least 30 resistance variants have been identified at a major resistance determinant to this disease, designated Mla. However, the mechanisms of regulation of plant disease resistance genes and resistance gene products are largely unknown. We have used the previously cloned Mla13 resistance gene as a model to study translational regulation of resistance. We have characterized three small genes upstream of Mla13 that control the regulation of downstream protein synthesis. The regulation of protein synthesis is not affected by structural variations within the Mla13 gene. Removal of each upstream gene results in an increase in downstream proteins, but combined removal results in much higher production of protein, suggesting they work collectively. Our results also indicate that a predicted increase in protein accumulation through the removal of these additional genes does not alleviate the dependency on Rar1, another gene required for resistance. This is the first description of translational regulation of a resistance protein in plants. The results described in this manuscript will impact scientists that are working to understand to intricacies of disease defense responses in plants.
Technical Abstract: To protect themselves against pathogen infection, plants have evolved multifarious defense mechanisms that include the production of antimicrobial compounds and hypersensitive cell death. This includes the regulation of expression of resistance proteins, which function to detect pathogen effectors and initiate the defense response. Barley Mla is one of several Ml genes that confer specific resistance to the powdery mildew fungus, Blumeria graminis f. sp. hordei. Mla13 contains a long (523 nt) transcript leader region with three upstream open reading frames (uORFs) and two alternatively spliced introns. The length 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 cooperatively 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 overexpression and uORF mutation retain their requirement for RAR1, suggesting that this increase is not sufficient to overcome the concentration threshold required for activation of resistance in the absence of RAR1.