Submitted to: Physiological and Molecular Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/27/2012
Publication Date: 1/1/2013
Citation: Bailey, B.A., Crozier, J., Strem, M.D., Melnick, R.L., Zhang, D., Maximova, S., Guiltinan, M., Meinhardt, L.W. 2013. Dynamic changes in pod and fungal physiology associated with the shift from biotrophy to necrotrophy during the infection of Theobroma cacao by Moniliophthora roreri. Physiological and Molecular Plant Pathology. 81:84-96. Interpretive Summary: Cacao products are combined with United States agricultural commodities providing a direct benefit to the American farmer. Frosty pod rot is a destructive pod disease on Theobroma cacao (cacao) where it occurs in South and Central America often causing complete crop loss. Chemical and cultural control measures for frosty pod rot are expensive to employ and are often ineffective. We are studying the interaction between the frosty pod pathogen, Moniliophthora roreri, and cacao with the goal of identifying new control measures and new sources of resistance. The susceptible response to frosty pod rot is characterized by rapid colonization and destruction of the pod within 60 days of infection. The cacao pod responds to infection with a general response to stress and an increase in fruit ripening genes. By understanding the susceptible interaction between cacao and frosty pod rot, scientists may be able to identify new targets for disease control and unique sources of resistance to infection. By providing cacao farmers with new control measures or unique sources of resistance for frosty pod rot, cocoa supplies may be stabilized resulting in increased benefits to the cacao farmer, the cocoa industry, and the American farmer.
Technical Abstract: Where it occurs in South and Central America, M. roreri (Mr) causes a destructive pod disease (frosty pod rot) on Theobroma cacao (cacao). Hand pollinated cacao pods were inoculated with Mr spores in the field and assessed for disease symptoms over a 90 day period. On average, pods showed symptoms of malformation by 30 days post inoculation (DPI) and were necrotic and sporulating by 60 DPI. Pods were harvested 7, 30, 60, and 90 DPI and preserved for RNA extraction. qPCR analysis was carried out using primer sets for 5 Mr ESTs and 72 cacao ESTs. Mr EST expression levels increased rapidly between 7 and 60 DPI. The cacao ESTs consisted of ESTs previously associated with M. perniciosa infection of meristems, seedling colonization by Trichoderma species, and drought in leaves or roots, or putatively encoded pathogenesis-related proteins. Most of the 72 cacao ESTs studied responded to Mr infection although they often failed to respond in the same way as the original associated stress. The patterns of change in EST expression were similar between Mr colonized pods and Trichoderma colonized seedling, a reaction characterized as a general response to fungal invasion and not significant resistance. Many of the ESTs were induced or repressed in response to Mr infection in a pattern suggesting a relationship with gene expression changes associated with fruit development/ripening, an association highlighted by principle coordinate analysis. The discontinuous variables associated with ANOVA (DPI and +/- Inoculation) were replaced with continuous variables (Mr EST expression and pods measurements) allowing regression to consistently identify cacao ESTs responsive to Mr infection. Using these techniques should allow us to further extend our understanding of the M. roreri/cacao interaction using randomly collected field samples incorporating divergent plant and pathogen genetic backgrounds.