A mutant of Nicotiana sylvestris deficient in serine glyoxylate aminotransferase activity

Summary A photorespiration mutant of Nicotiana sylvestris lacking serine: glyoxylate aminotransferase activity was isolated in the M₂ generation following EMS mutagenesis. Mutants showing chlorosis in air and normal growth in 1% CO₂ were fed ¹⁴C]-2-glycolate to examine the distribution of ¹⁴C among photorespiratory intermediates. Mutant strain NS 349 displayed a 9-fold increase in serine accumulation relative to wild-type controls. Enzyme assays revealed an absence of serine: glyoxylate aminotransferase (SGAT) activity in NS 349, whereas other peroxisomal enzymes were recovered at normal levels. Heterozygous siblings of NS 349 segregating air-sensitive M₃ progeny in a 31 ratio were shown to contain one half the normal level of SGAT activity, indicating that air sensitivity in NS 349 results from a single nuclear recessive mutation eliminating SGAT activity. Since toxicity of the mutation depends on photorespiratory activity, callus cultures of the mutant were initiated and maintained under conditions suppressing the formation of functional plastids. Plantlets regenerated from mutant callus were shown to retain the SGAT deficiency and conditional lethality in air. The utility of photorespiration mutants of tobacco as vehicles for genetic manipulation of ribulose bisphosphate carboxylase/oxygenase at the somatic cell level is discussed.