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Title: Stage-specific Suppression of Basal Defense Discriminates Barley Plants Containing Fast- and Delayed-acting Mla Powdery Mildew Resistance Alleles

item Wise, Roger

Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/4/2006
Publication Date: 9/1/2006
Citation: Caldo, R.A., Nettleton, D., Peng, J., Wise, R.P. 2006. Stage-specific Suppression of Basal Defense Discriminates Barley Plants Containing Fast- and Delayed-acting Mla Powdery Mildew Resistance Alleles. Molecular Plant-Microbe Interactions. 19(9):939-947.

Interpretive Summary: Non-self recognition is an indispensable system for eukaryotic organisms to defend against microbes. In plants, perception of pathogen-derived general elicitors triggers non-specific basal defense, while recognition of pathogen avirulence effectors initiates gene-specific resistance. Recent evidence indicates that these two modes of disease resistance are linked. Powdery mildew disease of barley is an ideal system to explore the interactions of obligate fungal pathogens with their cereal hosts. In this manuscript, we describe, for the first time, a meta-analysis of transcript accumulation of barley genes involved in basal defense and determine how this differentiates plant responses undergoing host-specific rapid and delayed resistance. Large-scale RNA profiling data sets involving thousands of genes were used to perform comparative meta-profiling to draw conclusions that spanned multiple experiments. In plants containing fast-acting resistance alleles, hyper-accumulation of most basal defense-related transcripts occurs early, which accompanies rapid single-cell death. By contrast, in plants containing the delayed-acting resistance alleles, marked suppression of these same genes at early time points is correlated with further growth of the fungal pathogen. This study provides insights on how pathogens respond to early plant defense mechanisms. Knowledge of the basic mechanisms of disease resistance will provide breeders with better tools, resulting in less damage to crops, therefore, increasing sustainability and profitability.

Technical Abstract: Non-specific recognition of pathogen-derived general elicitors triggers the first line of plant basal defense which, in turn, preconditions the host towards resistance or susceptibility. In order to compare the transcriptional mechanisms in plants undergoing rapid vs. delayed host innate immunity, we performed a meta-analysis of GeneChip mRNA expression for 155 basal defense-related genes of barley (Hordeum vulgare) challenged with Blumeria graminis f. sp. hordei (Bgh), the causal agent of powdery mildew disease. In plants undergoing rapid resistance conferred by Mla1, Mla6, or Mla13, transcripts hyper-accumulated from 0 to 16 hai. By contrast, in plants undergoing delayed resistance controlled by Mla12, an early hyper-induction of transcripts from 0 to 8 hai was observed but the expression of many of these genes is markedly suppressed from 8 to 16 hai. These results suggest that the inhibition of basal defense may precondition the host cells towards the formation of haustoria and secondary hyphae, triggering cell death surrounding interaction sites to inhibit further Bgh growth. Thus, we hypothesize that accumulation of basal defense influences the timing and extent of Mla-mediated hypersensitive response (HR), and that the pathogen’s suppression of general defense mechanisms can drive allelic diversification of gene-specific resistance phenotypes.