Altering pH levels to Combat Plant Pathogens

A group of BARD Fund-supported scientists have revealed that fungi alter pH levels in plants as part of their invasion strategy. Their knowledge has been translated into effective postharvest treatments for preventing fungi infection, which have been successfully applied commercially since 2004, and are currently used as the main treatment for all exported mango fruit in Israeli packing houses.

During the process of fruit ripening and senescence (i.e., the growth stage from full maturity to death), pH levels in the fruit change. Pathogens, too, can alter the pH so as to increase either the level of acidity (i.e., pH below 7) or alkalinity (i.e., pH above 7) around an infection site, which assists them in modulating the expression of pathogenic factors. In a series of BARD Fund-supported projects, scientists have been studying several such modulatory processes in various postharvest fungal infections.

The scientists’ results indicate that the pH responsiveness of fungal genes involved in infectious processes are critical for initiating fungal attacks in fruits and vegetables. Fungi that increase the host’s pH levels are described as “alkalinizing fungi,” while those that lower it are known as “acidifying fungi.” Postharvest alkalinizing fungi, such as Colletotrichum and A. alternata, cause an infection by actively secreting ammonia. Their acidifying counterparts Penicillium expansum, P. digitatum, P. italicum, Botrytis cinerea, Phomopsis mangifera, and Sclerotinia sclerotiorum create the desired pH environment by secreting organic acids. The capability of these postharvest pathogens to fine-tune their protein expression according to the host’s ambient pH highlights the importance of this specific regulatory system for their successful colonization of fruit.

The scientists’ research findings have facilitated the rapid development, commercialization, and application of new approaches for reducing postharvest storage diseases. For example, acid treatments for preventing infection by the alkalinizing pathogen A. alternata have been successfully applied commercially since 2004, and are currently used by Israeli packaging houses as the main treatment for all exported mango fruit. Not only reducing the extent of fungal colonization, the pH treatments have also achieved excellent results in improving solubility of synthetic fungicides and reducing their residues that affect human health. The scientists expect that their research findings will lead to additional applications in the postharvest produce industry.

The Core Group of Scientists:

• Dov Prusky, The Volcani Center, Israel
• Lisa Villancourt, University of Kentucky, USA
• Robert Fluhr, Weizmann Institute of Science, Israel
• Marty Dickman, Texas A&M University, USA