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

The biochemical and molecular basis of the wrinkled-seeded phenotype of rug4 mutants of pea (Pisum sativum L.) 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 the rug4 locus, 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 in Rhizobium-infected root nodules is reduced by 85%. Although the nodules of mutant plants contain metabolically active bacteroids, the N content and delta(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 via Rhizobium. Sucrose synthase thus appears to be essential for the supply of carbon for bacteroid metabolism and/or ammonia assimilation during nitrogen assimilation.