| In leaves, transitory starch is formed in the chloroplasts during the day and broken down at night. The aim of my thesis work was to elucidate the form of carbon export from the chloroplast at night and to gain further insight into the regulation of starch breakdown. I examined the form of carbon exported from the chloroplast at night and found at least two-thirds of the carbon exported from dark-adapted, isolated chloroplasts was in the form of maltose. I then developed a technique to distinguish between the alpha and beta anomer and found a substantial gradient of beta-maltose between the chloroplast and cytosol at night. beta-Maltose was present only during starch degradation at night and absent in starchless mutants. When maltose metabolism is blocked, beta-maltose levels are high. From this I conclude that transitory starch degradation for export is hydrolytic and beta-maltose is the metabolically active form of maltose.; I then placed plants in photorespiratory conditions to attempt to force starch breakdown. Under photorespiratory conditions starch was degraded at a high rate, and G6P and maltose levels were significantly elevated. Nonaqueous fractionation has shown that the increase in G6P occurs mainly in the chloroplast. When an Arabidopsis mutant in which the plastidic starch phosphorylase enzyme was knocked out, was put into photorespiratory conditions, starch breakdown still occurred with an increase in maltose but no increase in G6P. From this I concluded that G6P is produced by plastidic starch phosphorylase, and regulation of starch breakdown may precede the division between the hydrolytic and phosphorolytic pathways.; When bean plants were placed in 100% nitrogen in the light, starch breakdown did not occur and maltose and G6P levels remained constant. Under conditions of starch degradation, photorespiration or at night, the stroma was in an oxidized state and in conditions when starch was not being degraded, during the day or 100% N2, the stroma was reduced. These data provide evidence that starch degradation is strongly regulated by carbon status in the chloroplast, and signaling may involve Calvin cycle intermediates or redox status. |
