Photosynthesis in the Higher Plant Vicia faba

Abstract

Abstract The photosynthetic generation of formate from 14CO2 and an anomalous isotope distribution in the acetate fragment of citrate (i.e. citrate C-1,2) are not compatible with the singular functioning of the Calvin cycle and related processes in the higher plant Vicia faba. Studies of other higher plants have shown that alanine is metabolically related to 3-phosphoglyceric acid by a distinctive isotope distribution in which C1 ≥ C2 = C3; the reasonable assumption has been that these 3-carbon metabolites constitute the major precursors of the tricarboxylic acid cycle during photosynthesis. Inhibitor studies, kinetic studies, and labeled formate assimilation studies demonstrate that formate, rather than alanine, is the precursor carbon source of the methylene carbon atom of acetate; serine is tentatively proposed to be the key 3-carbon precursor of the organic acids. Evidence for a direct reduction of CO2 to formate is based upon (a) the seasonal fixation of as much as 65% of the 14CO2 into formate under arsenite-inhibited conditions of photosynthesis; (b) the specific and mutual competition between CO2 and formate for assimilation; (c) the predominant refixation of respiratory 14CO2 from [U-14C]glutamate or [1,5-14C]citrate into formate as opposed to Calvin cycle refixation; (d) the observation that Calvin-type CO2 fixation but not formate synthesis is inhibited by arsenate or arsenite. A mechanism of de novo formate synthesis is postulated, and a function of formate utilization for the synthesis of organic acids in this higher plant is proposed.