Characterization of adult plant resistance in the maize-Cochliobolus carbonum race 1 pathosystem revealed a close connection between plant immunity and metabolism
The role if any that host metabolism plays in plant immunity remains an open question. Our research on adult plant resistance (APR) in maize to Cochliobolus carbonum race1 (CCR1), a pathogen that causes a lethal leaf blight disease, seems to provide an interesting glimpse into this question. CCR1 causes disease by using a cyclic tetrapeptide that has been named HC-toxin. This virulence action is countered by the disease resistance gene Hm1 that inactivates HC-toxin by encoding an NADPH-dependent reductase (HCTR). Resistance conferred by Hm1 is absolute and operates in every part of the plant. In contrast, an allele of Hm1 (Hm1A) confers protection only at maturity and not at the seedling stage. The HCTR encoded by Hm1A differs from the wild-type HCTR by five amino acid substitutions, which we predicted only weaken the activity of HCTR and not inactivate it completely. We used targeted mutagenesis with EMS to test this hypothesis. This procedure led to the generation of two new alleles of Hm1 that, like Hm1A , conferred effective resistance to CCR1 only at maturity. These APR alleles of Hm1 were the result of single amino acid substitutions. In one allele the substitution was from Threonine to Methionine at residue 90 (T90M) and the other had a Valine to Methionine conversion at residue 210 (V210M). Five fully susceptible alleles were also found in this study, and they all had undergone drastic mutations such as the formation of premature stop codons. These results demonstrate that the APR alleles of Hm1 are the result of partial loss-of-function mutations of the gene. The question, of course, is why weak alleles behave of Hm1 fail to confer resistance in corn seedlings? One possibility is that it may have something to do with the fluctuating levels of NADPH (a cofactor for HCTR) in corn seedlings. Given that Hm1A seedlings become resistant to CCR1 when grown under extended photoperiod or in the presence of 3% sucrose, are consistent with our hypothesis. During our quest for the mechanistic basis of APR, we identified maize seedlings incubated in continuous dark for 48 hours turned susceptible to CCR1 Tox-, the non-toxin producing isolate. This loss of resistance is described as "DILI", dark-induced loss of immunity. Genetic studies revealed DILI is independent of Hm1 and CCR1 toxin producing ability. In addition to CCR1, Cochliobolus heterostrophus, Magnaporthe grisea and Colletotrichum graminicola were able to infect dark-incubated plants, showing that DILI is not pathogen specific. But what happens in dark that is making maize seedlings susceptible to different pathogens. One possibility could be loss of photosynthesis in the dark. Our results from DCMU herbicide which inhibits photosynthesis showed that loss of photosynthesis is responsible for DILI. Interestingly, by supplementing sugars DILI was suppressed, showing that starvation induced by loss of sugars is indeed responsible for DILI. Supporting these results, transcriptome analysis showed complete shut-down of metabolism. Fascinatingly, this is the first time we are showing a direct correlation between metabolism and immunity in the plant kingdom.
Johal, Purdue University.
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