To understand the physiological function of the pyrenoid, a protein complex in algal chloroplast stroma with surrounding starch sheaths, the effects of environmental conditions on the pyrenoid and pyrenoid starch were investigated in the unicellular green alga Chlamydomonas reinhardtii. Pyrenoid starch was rapidly accumulated within 5 hours when the extracellular CO2 concentration was lowered from 4% to ordinary air level (0.04%). Starting with high-CO2 grown cells containing well-developed stroma starch granules, degradation of stroma starch and accumulation of pyrenoid starch were observed in parallel during the adaptation to low CO2 condition. This pyrenoid-starch accumulation was light dependent and completely inhibited by DCMU. The starch relocalization process was reversible, but the breakdown of pyrenoid starch was slower than its accumulation. The time courses of accumulation (or degradation) of starch around the pyrenoid paralleled increases (or decreases) in carbonic anhydrase (EC 4.2.1.1) activity, and the pyrenoid starch accumulation is thought to be one of the adaptation phenomena to CO2 concentration. When nitrogen assimilation was inhibited, stroma starch and total starch content increased, while that of pyrenoid starch decreased. These results indicate that the synthesis and degradation of the two forms of starch were regulated independently by the environmental conditions. ADP-glucose starch synthase (EC 2.4.1.21) activity was detected at a physiological level, but the change of starch localization could not be explained by total starch synthase activity nor by starch-degrading enzyme activities. We assume that starch metabolism around the pyrenoid is regulated independently from that in other stromal spaces.
No takes yet. Share an insight, caveat, or question.
Kuchitsu et al. (1988) studied this question.