Characterization of endogenous electron donor systems to Photosystem I in green potato tuber chloroplasts

The existence of a novel C-1 pathway for CO 2 fixation was established in chloroplasts isolated from greening potato tubers (Ramaswamy and Nair, Plant Sci. Lett., 34 (1984) 261–267). The light dependent reduction of CO 2 to formate was the same either in presence or absence of DCMU. From this finding it was inferred that these chloroplasts were capable of CO 2 reduction to formate through an endogenous electron donor system for Photosystem I (PSI) to function in the absence of Photosystem II (PSII). In these studies two endogenous electron donor systems for PSI were identified namely pyruvate oxidase and chlorogenic acid oxidase. The electron donation by chlorogenic acid and pyruvate was confirmed by using cyanide treated chloroplasts and it was found to occur at a site between PC and PSI. Reconstitution studies with PSI and illuminated chloroplasts protein fraction (ICPF) gave additional proof for the above observation. The demonstration of the presence of pyruvate oxidase and chlorogenic acid oxidase activity in PSI and ICPF gave further support for this contention.