A key gene involved in plant senescence, mutations of which partially disable chlorophyll catabolism and confer stay-green leaf and cotyledon phenotypes, has been identified in Pisum sativum, Arabidopsis thaliana, and Festuca pratensis by using classical and molecular genetics and comparative genomics. A stay-green locus in F. pratensis is syntenically equivalent to a similar stay-green locus on rice chromosome 9. Functional testing in Arabidopsis of a homolog of the rice candidate gene revealed (i) senescence-associated gene expression and (ii) a stay-green phenotype after RNA interference silencing. Genetic mapping in pea demonstrated cosegregation with the yellow/green cotyledon polymorphism (I/i) first reported by Gregor Mendel in 1866.

The role of sucrose synthase (SS) in the fixation of N was examined in the rug4 mutant of pea (Pisum sativum L.) plants in which SS activity was severely reduced. When dependent on nodules for their N supply, the mutant plants were not viable and appeared to be incapable of effective N fixation, although nodule formation was essentially normal. In fact, N and C resources invested in nodules were much greater in mutant plants than in the wild-type (WT) plants. Low SS activity in nodules (present at only 10% of WT levels) resulted in lower amounts of total soluble protein and leghemoglobin and lower activities of several enzymes compared with WT nodules. Alkaline invertase activity was not increased to compensate for reduced SS activity. Leghemoglobin was present at less than 20% of WT values, so O 2 flux may have been compromised. The two components of nitrogenase were present at normal levels in mutant nodules. However, only a trace of nitrogenase activity was detected in intact plants and none was found in isolated bacteroids. The results are discussed in relation to the role of SS in the provision of C substrates for N fixation and in the development of functional nodules.

No abstract