Molecular and Biochemical Mechanisms of Pathogenesis in the Maize Foliar Pathogen Cercospora zeae-maydis

GLS is a serious foliar disease of maize, a major staple crop grown commercially in the USA for both human and animal feed production, and increasingly, for ethanol production. The disease is caused by two Cercospora species, C. zeae-maydis and C. zeina, both of which infect maize in the USA and in other parts of the world, with yield losses potentially greater than 50%, depending on local conditions. In culture, C. zeae-maydis produces a phytotoxic, host non-specific perylenequinone, cercosporin, and abscisic acid (ABA), for which there is no known pathological or physiological function in the fungus. Experimental evidence indicates that cercosporin, is a virulence factor among some species of Cercospora, although evidence from other studies suggests that the phytotoxin may not be universally significant for all species. The overall goal of this project was to determine the biosynthetic regulation and pathological significance of cercosporin and ABA in C. zeae-maydis. To this end, the ortholog of CTB1, a polyketide synthase gene involved in the biosynthesis of cercosporin, was identified and disrupted. Disruption mutants failed to produce cercosporin but were able to infect maize and induce lesions. Furthermore, upon disruption of the nitrogen responsiveness gene AREA and a photoropin-like gene CzmPhot-24 , cercosporin biosynthesis was significantly reduced. However, these two mutants were still able to infect maize, although the AREA mutants were significantly reduced in virulence. AreA is a global regulator of secondary metabolism, thus suggesting that the reduction in virulence may be the result of global down-regulation of other virulence factors. Similarly, deletion of putative ABA biosynthesis pathway genes, which resulted in the loss of ABA biosynthesis, did not affect stomatal infection nor lead to decreased virulence when maize leaves were inoculated with the mutants. Taken together, these findings suggest that cercosporin and ABA individually are not necessary for pathogenesis, but may instead belong to a suite of as-yet unidentified virulence factors produced by C. zeae-maydis during infection of maize.