Minisymposium 15: Photosynthesis

Abs # 28003: Daylength effects on starch degradation rate and maltose metabolism

Presenter:	Lu, Yan , ylu7@wisc.edu
Authors	Lu, Yan  (A)   Gehan, Jackson P. (A)   Weise, Sean E. (A)   Sharkey, Thomas D. (A)
Affiliations:	(A): University of Wisconsin-Madison

Transitory starch is stored during the day inside chloroplasts and broken down at night for export. Recent evidence from our lab and others indicates that maltose is the primary form of carbon export from chloroplasts at night. Inhibition of maltose export or metabolism results in a 20 to 90-fold increase in maltose content of leaves and a dwarf phenotype. We are investigating the influence of daylength on maltose production and consumption. When plants were put into darkness in the middle of their physiological light period they accumulated very small levels of maltose but in darkness during their normal dark period they accumulated 2 to 4 times the amount of maltose found in the light. When starch breakdown was induced in the light by imposing photorespiratory conditions, maltose increased by 4-fold. We conclude that both carbon status inside the chloroplasts and endogenous rhythms influence the rate of maltose production during transitory starch breakdown (poster xxx). Maltose is metabolized by cytosolic amylomaltase and starch phosphorylase (poster xxx). Null mutants of amylomaltase (dpe2-1, dpe2-2) accumulated a large amount of maltose and starch. Mutation in cytosolic starch phosphorylase (pho2-1) is embryo-lethal. By using real-time RT-PCR, we found that the transcript level of these two enzymes is higher during the light period and lower during the dark period. The dramatic change of transcript level of Pho2 at the transition point of photoperiod suggests that Pho2 is under light regulation. The transcript level of DPE2 and Pho2 is also affected by the length of the photoperiod. Plants under long days produce more starch than plants under short days. We propose that under long days, more DPE2 and Pho2 transcripts are made during the day causing more maltose metabolism enzyme to accumulate. In short days, less mRNA is made and so less protein is present, slowing the rate of starch metabolism. Plants may sense the length of photoperiod and regulate the mRNA and protein levels of DPE2 and Pho2 to adjust their capacity for starch breakdown. To test this hypothesis, we determined the amount of DPE2 and Pho2 transcripts before and after altering the length of photoperiod. The protein level of DPE2 and Pho2 under different treatments will also be determined.