Photosynthetic Formation of Glycerate in Isolated Bundle Sheath Cells and Its Metabolism in Mesophyll Cells of the C₄ Plant Panicum capillare L

Abstract

In isolated bundle sheath cells of P. capillare, glycerate is a major product of ¹⁴CO₂ fixation at low bicarbonate concentrations. After 6 min of CO₂ fixation at 0.34 mM NaHCO₃, about 20% of the labelled products was in glycerate and about 45% in glycine plus serine. Inhibitors of the glycollate pathway prevented incorporation of label into glycerate. After 6 min of CO₂ fixation at high bicarbonate concentration (20 mM NaH¹⁴CO₃), a large percentage of the label was incorporated in phosphate esters, insolubles, and sucrose, and little label was observed in metabolites of the glycollate pathway. The results indicate glycerate is primarily formed through the glycollate pathway in bundle sheath cells rather than through 3-phosphoglycerate phosphatase.

Glycerate-dependent oxygen evolution in the light and ¹⁴C incorporation into phosphate esters from [1-¹⁴C]glycerate was observed with mesophyll chloroplasts and mesophyll protoplasts, respectively, but not with bundle sheath cells. This is consistent with our previously published results that glycerate kinase is localized only in C₄ mesophyll chloroplasts. Both isolated bundle sheath cells and mesophyll chloroplasts exhibited 3-phosphoglycerate dependent O₂ evolution in the light indicating that the reductive phase of photosynthesis is functional in both cells. The results suggest that, during C₄ photosynthesis, glycerate is synthesized by the glycollate pathway in bundle sheath cells and subsequently metabolized in mesophyll cells without direct linkage to the reductive pentose phosphate pathway.