A Biochemical Study of the Resistance of Apples to Several Pathogens
Abstract
A study has been undertaken to elucidate the nature of the resistance of immature apple fruit to the rots caused by Botryosphaeria ribis Gross and Dug., Physalospora obtusa (Schw.) Cke., Glomerella singulata (Atk.) Spauld. and von Schrenk, and a Physalospora species (isolated by the author but not completely characterized). The resistance of the fruit is lost at a certain stage in the fruit's development. This study is part of an overall investigation of disease resistance mechanisms found in plants.The four pathogens were investigated for their ability to produce extracellular enzymes in mineral media containing 1% water- insoluble material obtained from the pulp of apples at varying stages of maturity. Each of the pathogens produced a Type III polygalacturonase (exo-glycosidase). The fungi produced detectable amounts of the enzyme after 2 days of growth in media containing water-insoluble matter from susceptible fruit (I), and after 3-4 days in media containing water-insoluble material from resistant fruit (II). Culture filtrates also contained pectin methylesterase (PME). Two differences from polygalacturonase production were noted. 9. cingulata produced only trace levels of PME on media containing I or II. The inability of G. cingulata to rapidly parasitize unharvested fruits, as compared to the three other fungi, may be associated with its low level of PME production. PME production by B. ribis, P. species, and P. obtusa increased rapidly, after an initial lag, in media containing II and after 6-12 days of growth far surpassed the level produced in media containing I. PME was detected after 2 days of growth in media containing I and after 4-6 days in media containing II. Growth of the four fungi in mineral media containing I was consistently greater than in the media containing II, and the level of PME associated with good growth of B. ribis, R. obtusa, and P. species on II was far higher than that associated with good growth on I. The methoxyl content of I and II was relatively constant. These observations suggest that the methoxyl ester groups of the pectins constituting II are more sterically hindered than those in I. The steric hinderence would decrease the availability of the groups for enzymatic cleavage. To overcome this metabolic stress, B. ribis, P. obtusa, and P. species greatly increase their production of PME.
Degree
Ph.D.
Subject Area
Biochemistry
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