Cyclopiazonic acid is a pathogenicity factor for Aspergillus flavus and a promising target for screening germplasm for ear rot resistance

Authors: CHALIVENDRA, SUBBAIAH - Louisiana State University Agcenter; DEROBERTIS, CATHERINE - Louisiana State University Agcenter; Chang, Perng Kuang; DAMANN, KENNETH - Louisiana State University Agcenter

Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2017
Publication Date: 4/24/2017
Citation: Chalivendra, S.C., DeRobertis, C., Chang, P.-K., Damann, K.E. 2017. Cyclopiazonic acid is a pathogenicity factor for Aspergillus flavus and a promising target for screening germplasm for ear rot resistance. Molecular Plant-Microbe Interactions. 30(5):361-373.

Interpretive Summary: Aspergillus flavus is a serious threat to human health and economy due to its contamination of crop commodities with carcinogenic aflatoxin (AF). The direct revenue loss due to AF contamination in maize alone is at least $200 million per year in the US. The incorporation of preharvest resistance in elite maize cultivars is the best strategy for controlling AF. However, AF accumulated in seed is an unreliable indicator of A. flavus resistance in maize because most of the resistance identified has been location and season-dependent. Hence, there is an urgent need for a robust, less labor-intensive and yet economically feasible screen to identify host resistance. In this study, we found that cyclopiazonic acid (CPA), a secondary metabolite that often co-contaminates crops with AF, acts as a pathogenicity factor and that CPA tolerance of maize inbreds is a useful indicator for correlating maize resistance to aflatoxin. We also developed a seedling screening method based on primary root CPA sensitivity. In conclusion, CPA can be used as a surrogate for A. flavus infection to screen maize germplasm for ear rot resistance.

Technical Abstract: Aspergillus flavus, an opportunistic pathogen, contaminates maize and other key crops with carcinogenic aflatoxins (AF). Besides AF, A. flavus makes many more secondary metabolites (SMs), whose toxicity in insects or vertebrates has been studied. However, the role of SMs in the invasion of plant hosts by A. flavus remains to be investigated. Cyclopiazonic acid (CPA), a neurotoxic SM made by A. flavus, is a nanomolar inhibitor of endoplasmic reticulum calcium ATPases (ECAs) and a potent inducer of cell death in plants. We hypothesized that CPA, by virtue of its cytotoxicity, may serve as a key pathogenicity factor that kills plant cells and supports the saprophytic life style of the fungus, while compromising the host defense response. This proposal was tested by two complementary approaches. A comparison of CPA levels among A. flavus isolates indicated that CPA may be a determinant of niche adaptation, i.e., isolates that colonize maize make more CPA than those restricted only to the soil. Further, mutants in the CPA biosynthetic pathway are less virulent than their wild type parent in field inoculation assays. Additionally, genes encoding ECAs are expressed in developing maize seeds and induced by A. flavus infection. Building on these results, we developed a seedling assay where maize roots were exposed to CPA and cell death was measured as Evans Blue uptake. Among >40 maize inbreds screened for CPA tolerance, the publicly available AF contamination data for many of the lines was broadly correlated with their CPA sensitivity. In summary, our studies show that (1) CPA serves as a key pathogenicity factor that enables the saprophytic life style of A. flavus and (2) maize inbreds are diverse in their tolerance to CPA. Taking advantage of this natural variation, we are currently pursuing both genome-wide and candidate gene approaches to identify novel components of maize resistance to AF contamination.