Submitted to: Molecular Plant Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 5, 2001
Publication Date: N/A
Interpretive Summary: Decay caused by postharvest pathogens can result in a substantial loss of apple fruit in storage. Although many of these diseases can be controlled by fungicides, the consumers concern about the association of these chemicals with human maladies may limit their future application. To reduce storage losses and provide a continued source of food for the consumer, alternate sources of postharvest disease control must be investigated. Postharvest pathogens can produce pectate lyase, a cell wall degrading enzyme, to decay fruit. Our research has resulted in a newly described mechanism by which nitrogen metabolism can affect pathogenicity. We have found that various postharvest fungal pathogens can metabolize nitrogen in fruit to accumulate ammonia. This ammonia accumulation results in an increase in fruit pH at the infection site, leading to an increase in pectate lyase production and greater decay. The elucidation of this mechanism may be used in plant breeding programs to incorporate factors which would retard nitrogen metabolism by fungi and reduce decay. Fruit packing and storage facilities may then be able to reduce the amount of fungicides used to maintain fruit quality in storage.
The phytopathogenic fungus C. gloeosporioides produces one pectate lyase (PL) that is a key virulence factor in disease development. During growth of C. gloeosporioides, C. acutatum and C. coccodes in acidified yeast extract medium, the fungus secretes ammonia and increases the medium pH. Ammonia accumulation and the consequent pH change increased as a function of initial pH and buffer capacity of the medium. PL secretion by C. gloeosporioides correspondingly increased as pH of the medium increased. C. gloeosporioides in avocado, C. coccodes in tomatoes and C. acutatum in apples showed ammonia accumulation in the infected area where pH increased to 7.5-8 and PL activity is optima. In non-host interactions where C. gloeosporioides was inoculated in apples, the addition of ammonia releasing compounds significantly enhanced pathogenicity to levels similar to those caused by the compatible C. acutatum apple interaction. The results therefore suggest the importance of ammonia secretion as a virulence factor enhancing environmental pH and pathogenicity of Colletotrichum sp.