Biocontrol of Aspergillus flavus: strategy to obtain effective biocontrol yeast and molecular mechanisms

Abstract: The use of chemical fungicides has resulted in the development of pest resistance and resurgence. In addition, use of fungicides in certain agricultural systems is impractical due to the expense, the risk of environmental pollution, and negative effect to human health. Biological control of insect pests, plant pathogens, and weeds is the only major alternative to the use of pesticides in agriculture and forestry. Biological control can reduce the harmful effect of phytopathogenic or mycotoxigenic fungi while having a minimal impact on the environment. Yeast species are promising biocontrol agents because they do not produce allergenic spores and they are usually non-pathogenic. A bioassay has been developed to screen for effective yeasts inhibiting both the growth of A. flavus and aflatoxin production. Pichia anomala WRL-076 was identified and field tested in California almond and pistachio orchards. The transcriptional response of P. anomala grown in medium inoculated with A. flavus was investigated by determining expression rate of the Exo-β-1, 3-glucanase genes, PaEXG1 and PaEXG2 relative to control yeast cells. PaEXG1 and PaEXG2 were increased by 4 and 5 fold respectively. The phylogenetic relationship among selected yeast species shows that there are two clusters of proteins. One cluster is strongly homologous to PAXEG2, the consensus for the PAEXG2 homologous genes was 90.4%. Motif binding sites for stress response and mycoparasitic response were found upstream of EXG2 type of genes. The major volatile compound produced by P. anomala WRL-076 is 2-phenylethanol (2-PE). It inhibited spore germination and aflatoxin production of A. flavus. The inhibition was correlated with significant down regulation of clustering AF biosynthesis genes as evidenced by several to greater than 10,000-fold decrease in gene expression. We found that 2-PE also altered the expression patterns of chromatin modifying genes, MYST1, MYST2, MYST3, gcn5, hdaA and rpdA.