Barley Mutants Lacking Chloroplast Glutamine Synthetase—Biochemical and Genetic Analysis

Abstract

Eight mutants of barley (Hordeum vulgare cv Maris Mink) lacking the chloroplast isozyme of glutamine synthetase (EC 6.3.1.2.) were isolated by their inability to grow under photorespiratory conditions. The cytoplasmic isozyme of glutamine synthetase was present in the leaves of all the mutants, with activities comparable to the wild-type (10-12 nanokatals per gram fresh weight). The mutant plants developed normally and were fully fertile under conditions that minimize photorespiration. In 1% O₂ the rate of CO₂ fixation in leaves of one of the mutants, RPr 83/32, was the same as the wild-type, but in air this rate declined to 60% of the wild-type after 30 minutes. During this time the ammonia concentration in leaves of the mutant rose from 1 to 50 micromoles per gram fresh weight. Such ammonia accumulation in air was found in all the mutant lines. In back-crosses with the parent line, F₁ plants were viable in air. In the F₂ generation, nonviability in air and the lack of chloroplast glutamine synthetase co-segregated, in both the lines tested. These two lines and four others proved to be allelic; we designate them gln 2a-f. The characteristics of these mutants conclusively demonstrate the major role of chloroplast glutamine synthetase in photorespiration and its associated nitrogen recycling.