<b>Mutations at the</b><i><b>rug4</b></i><b>locus alter the carbon and nitrogen metabolism of pea plants through an effect on sucrose synthase</b>

Abstract

Summary 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 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 via Rhizobium . Sucrose synthase thus appears to be essential for the supply of carbon for bacteroid metabolism and/or ammonia assimilation during nitrogen assimilation.