Dissecting the Multifaceted Relationship between Maize and Cochliobolus carbonum Race 1
Abstract
The maize pathogen Cochliobolus carbonum race 1 (CCR1) utilizes HC-toxin, an inhibitor of histone deacetylases, as a key determinant of virulence. The maize Hm1 gene confers complete resistance to CCR1 at all stages of development by encoding for an NADPH-dependent reductase that inactivates HC-toxin. Hm1A, Hm1-SM1, and Hm1-SM2 are alleles of Hm1 that exhibit an adult plant resistance (APR) phenotype, being fairly susceptible during the seedling stage and gradually increasing in resistance with development. The HM1A protein differs from HM1 by five amino acid substitutions while HM1-SM1 and HM1-SM2 have a single amino acid substitution each in the predicted NADPH binding pocket. Given that gene and protein expression of these APR alleles do not increase with age, the APR phenotype must be dictated post-translationally. In this study we characterize the biochemical basis underlying APR. We show that the pool of the NADPH cofactor is higher during the day in adult leaf tissue compared to juvenile leaves. We also demonstrate that the various APR alleles do in fact display compromised enzymatic activities, while also characterizing recombinant proteins to conclude that the superior resistance conferred by Hm1 is unlikely to be simply due to the stronger affinity of its enzyme for the NADPH substrate. We also investigated the role HC-toxin plays in promoting susceptibility by comparing transcriptomic and metabolic data of plants inoculated with either CCR1 or a non-HC-toxin producing strain. We found that HC-toxin is not globally downregulating defense responses as previously thought but is causing massive deregulation of numerous metabolic pathways, including downregulating the light reactions of photosynthesis and increasing protein turnover. These results indicate that HC-toxin is likely promoting susceptibility by interfering with fundamental metabolic processes rather than by suppression of specific defense pathways.
Degree
Ph.D.
Advisors
Johal, Purdue University.
Subject Area
Plant sciences|Plant Pathology|Biochemistry
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