Cyclopiazonic Acid Is a Pathogenicity Factor for Aspergillus flavus and a Promising Target for Screening Germplasm for Ear Rot Resistance

Abstract

Aspergillus flavus, an opportunistic pathogen, contaminates maize and other key crops with carcinogenic aflatoxins (AFs). Besides AFs, 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 in causing ear rot than their wild-type parent in field inoculation assays. Additionally, genes encoding ECAs are expressed in developing maize seeds and are induced by A. flavus infection. Building on these results, we developed a seedling assay in which maize roots were exposed to CPA, and cell death was measured as Evans Blue uptake. Among >40 maize inbreds screened for CPA tolerance, inbreds with proven susceptibility to ear rot were also highly CPA sensitive. The publicly available data on resistance to silk colonization or AF contamination for many of the lines was also broadly correlated with their CPA sensitivity. In summary, our studies show that i) CPA serves as a key pathogenicity factor that enables the saprophytic life style of A. flavus and ii) 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 Aspergillus ear rot.