Affixing the O to Rubisco: discovering the source of photorespiratory glycolate and its regulation

The source of glycolate in photorespiration and its control, a particularly active and controversial research topic in the 1970s, was resolved in large part by several discoveries and observations described here. George Bowes discovered that the key carboxylation enzyme Rubisco ( ribulosebisphosphate carboxylase/oxygenase) is competitively inhibited by O-2 and that O-2 substitutes for CO2 in the initial 'dark' reaction of photosynthesis to yield glycolate-P, the substrate for photorespiration. William Laing derived an equation from basic enzyme kinetics that describes the CO2, O-2, and temperature dependence of photosynthesis, photorespiration, and the CO2 compensation point in C-3 plants. Jerome Servaites established that photosynthesis cannot be increased by inhibiting the photorespiratory pathway prior to the release of photorespiratory CO2, and Douglas Jordan discovered substantial natural variation in the Rubisco oxygenase/carboxylase ratio. A mutant Arabidopsis plant with defective glycolate-P phosphatase, isolated by Chris Somerville, definitively established the role of O-2 and Rubisco in providing photorespiratory glycolate. Selection techniques to isolate photorespiration- deficient plants were devised by Jack Widholm and by Somerville, but no plants with reduced photorespiration were found. Somerville's approach, directed mutagenesis of Arabidopsis plants, was subsequently successful in the isolation of numerous other classes of mutants and revolutionized the science of plant biology.