The Effect of Calcium on the Expression of Polygalacturonase Activity by Colletotrichum acutatum in Apple Fruit

The effect of increasing the calcium concentration of Golden Delicious apple cell walls on polygalacturonase (PG) expression by Colletotrichum acutatum, the bitter rot pathogen, was examined. Fungal growth and PG expression were determined by growing C acutatum in Richard's solution supplemented with cell walls extracted from 0 or 4% calcium chloride treated fruit. During the first 18 h PG activity was not detected. However, within 24 to 27 h, the PG activity increased and was approximately 45% greater in medium containing cell walls from 0% calcium treated apples as opposed to medium containing cell walls from 4% calcium treated apples. After 72 h, PG activity in both media was similar. These results correspond with qualitative PG gene expression studies using the Reverse Transcription Polymerase Chain Reaction (RT-PCR) of RNA isolated 24 h post inoculation. Zymograms showed that minor PG isozymes were expressed differently depending on whether the medium was amended with cell walls from calcium treated or non-treated apples, although the four major PG isozymes detected were produced to approximately the same extent under both growth conditions.