Effect of Heterotrophic Growth on the Thioredoxins of Wild Type and y-1 Chlamydomonas reinhardtii

Abstract

Linking carbon metabolism to the presence of light via the ferredoxin-thioredoxin (FT) system enables plants to both synthesize and degrade starch within the chloroplast (1). Under photosynthetic growth conditions the green alga, Chlamydomonas reinhardtii, has a photosynthetic thioredoxin system similar to that of higher plants consisting of two types of thioredoxins, m and f, and a ferredoxin-thioredoxin reductase (FTR) (2). Unlike higher plants, Chlamydomonas is able to grow heterotrophically in prolonged dark periods. Although the cells remain green they use exogenous carbon (acetate, for example) rather than fixing CO2. During heterotrophic growth Chlamydomonas cells can increase their starch reserves if excess acetate is present (i.e., 0.2% acetate) (3,4), a behavior reminiscent of nonphotosynthetic storage tissues such as seeds. The thioredoxins of seeds differ from those of the chloroplast in that they are reduced via NADP-thioredoxin reductase (NTR), and no f-type thioredoxin has been found (5).