Plastidial starch phosphorylase is highly associated with starch accumulation process in developing squash (Cucurbita sp.) fruit

We investigated changes in starch content and starch metabolic enzyme activities in developing and postharvest squash of distinct species, Cucurbita maxima and Cucurbita moschata, which accumulate high and low levels of starch, respectively. The total activity of starch phosphorylase in developing fruits significantly correlated (r = 0.99) to the amount of starch among Cucurbita species (C. maxima, C. moschata and C. pepo). Separable activity of a plastidial L-form phosphorylase in C. maxima fruit markedly increased corresponding with starch accumulation. We isolated two genes (CmPhoL1 and CmPhoH1) encoding an L-form and a cytosolic H-form phosphorylase from C. maxima fruit. The expression of CmPhoL1 in the fruit dramatically increased at the beginning of starch accumulation. Recombinant CmPhoL1 enzyme showed similar kinetic parameters in both glucan synthesis and phosphorolysis: this enzyme can catalyze the invertible reaction in vitro depending on the concentration of substrates. These results suggest that CmPhoL1 plays a role in the starch accumulation process during squash development, but the aid of other starch synthetic enzymes may be required for in vivo glucan synthesis reaction by CmPhoL1. An importance of plastidial starch phosphorylase in the starch accumulation in the fruit organ was indicated.