Carbon balance and circadian regulation of hydrolytic and phosphorolytic breakdown of transitory starch

Transitory starch is formed in chloroplasts during the day and broken down at night. Transitory starch degradation could be regulated by light, circadian rhythms, or carbon balance. To test the role of these potential regulators, starch breakdown rates and metabolites were measured in bean (Phaseolus vulgaris) and Arabidopsis (Arabidopsis thaliana) plants. In continuous light, starch and maltose levels oscillated in a circadian manner. Under photorespiratory conditions, transitory starch breakdown occurred in the light faster than at night and glucose-6-P (G6P) was elevated. Nonaqueous fractionation showed that the increase in G6P occurred in the chloroplast. When Arabidopsis plants lacking the plastidic starch phosphorylase enzyme were placed under photorespiratory conditions, G6P levels remained constant, indicating that the increased chloroplastic G6P resulted from phosphorolytic starch degradation. Maltose was increased under photorespiratory conditions in both wild type and plants lacking starch phosphorylase, indicating that regulation of starch breakdown may occur at a point preceding the division of the hydrolytic and phosphorolytic pathways. When bean leaves were held in N-2 to suppress photosynthesis and Suc synthesis without increasing photorespiration, starch breakdown did not occur and maltose and G6P levels remained constant. The redox status of the chloroplasts was found to be oxidized under conditions favoring starch degradation.