Plant Photorespiration—An Inevitable Consequence of the Existence of Atmospheric Oxygen

Abstract

THE primary event in the glycollate pathway¹, the metabolic sequence responsible for the phenomenon of photorespiration in plants², is the oxygenation of ribulose 1,5-diphosphate (RuDP) to phosphoglycollate and 3-phosphoglycerate^3–5. This reaction appears to be catalysed by the enzyme RuDP carboxylase⁴. When ¹⁸O₂ is supplied, incorporation of one atom of ¹⁸O into the carboxyl group of one of the products only, namely phosphoglycollate, is observed. The other atom of the oxygen molecule is exchanged with the medium⁵. Similarly, intact spinach leaves incorporate isotopic oxygen, supplied as O₂, into the carboxyl groups of the glycollate-pathway intermediates, glycine and serine⁶. The purpose served by photorespiration has remained an enduring riddle⁷. Superficially, it would seem that oxygenation of RuDP must be deleterious to the photosynthetic process, necessitating the glycollate pathway to recoup as much of the phosphoglycollate carbon as possible with one quarter of it being lost as CO₂ at the glycine→serine conversion. Observations that net photosynthesis and plant growth in general are stimulated in oxygen-depleted atmospheres concomitantly with a suppression of photorespiration^8,9 reinforce this concept.