PARENT SESSION
Posters P2B Light, redox and metabolic regulation: Light Reactions. Abstracts (444-478)

Differing roles for hydrolytic and phosphorolytic starch break down. Sean Weise^*,1, Thomas Sharkey¹, ¹ University of Wisconsin-Madison, Madison, Wisconsin, USA

ABSTRACT- Transitory starch is formed in chloroplasts during the day and broken down at night. Two pathways are proposed for starch degradation: (1) hydrolytic, which converts starch into sugars for export, and (2) phosphorolytic, which provides phosphorylated intermediates for chloroplast metabolism. In order to elucidate the regulation of these two pathways during starch degradation, bean plants were placed in photorespiratory conditions to force starch breakdown. The amount of starch degraded was significantly higher than the amount of CO2 given off by photorespiration revealing that starch degradation can provide more than enough carbon for photorespiration. We suggest that phosphorolytic starch degradation is the source of photorespired CO2. Maltose is a transitory sugar produced by the hydrolytic breakdown of starch. The maltose concentration in leaves increased by 47 % in photorespiratory conditions, double those found at night, when maltose is normally produced. We also grew bean plants for 3 weeks under a 12 hr photoperiod. The plants were then exposed to a continuous photoperiod and starch and maltose levels were monitored. Starch levels were observed to oscillate in a circadian fashion. Taken together this data provides evidence that starch degradation is regulated by circadian control but this control can be over ridden by signals within the chloroplast. This also provides evidence that regulation of starch breakdown occurs at a point preceding the division between the hydrolytic and phosphorolytic pathways.

KEY WORDS: starch, maltose, hydrolytic, phosphorolytic