Mutations at the rug4 locus alter the carbon and nitrogen metabolism of pea plants through an effect on sucrose synthase

Summary The biochemical and molecular basis of the wrinkled‐seeded phenotype ofrug4mutants of pea (Pisum sativumL.) has been investigated. Mutant embryos have reduced starch contents and only 5% of the sucrose synthase activity of wild‐type embryos during development. Activities of other enzymes involved in the conversion of sucrose to starch are unaffected. A gene encoding an isoform of sucrose synthase expressed in the embryo co‐segregates with therug4locus, and one of the three mutant alleles has been show to carry a point mutation in this gene that converts a highly conserved arginine residue to a lysine residue. It is highly likely that the reduced starch content of the mutant embryo is a direct consequence of the loss of sucrose synthase activity. The mutations reduce the activity of sucrose synthase in the testa and the leaf by 50% or less, but activity inRhizobium‐infected root nodules is reduced by 85%. Although the nodules of mutant plants contain metabolically active bacteroids, the N content and δ 15 N values of these plants in the field indicate that, unlike wild‐type plants, they derive little of their N from N 2 fixation viaRhizobium. Sucrose synthase thus appears to be essential for the supply of carbon for bacteroid metabolism and/or ammonia assimilation during nitrogen assimilation.