Down Regulation of Nodule Sucrose Synthase by Mutation and Antisense

Abstract

Legume nodules are entirely dependent on photosynthetic sucrose to fuel N2 fixation. Nodule cells have two enzymes capable of metabolizing sucrose, alkaline invertase (AI) and sucrose synthase (SS). The fact that SS gene expression is greatly enhanced in nodules and that soybean nodule SS is down regulated in response to stress, in line with the decline in N2 fixation (Gordon et al, 1997), suggests that SS is very important for nodule function. Studies using mutant peas (Pisum sativum), in which nodule SS activity in the sucrose hydrolytic direction is greatly reduced (Wang, Hedley, 1993), and Lotus japonicus where SS activity is reduced using an antisense strategy, have confirmed this. In the absence of mineral nitrogen, the mutant pea lines were not viable. Nodule SS activity was only 5% of wild type rates, resulting in more than a 90% reduction in N2 fixation. As a consequence, growth was severely impaired (13% of WT dry weight at 6 weeks) and the plant invested increasing proportions of its resources in nodules (19% compared with 6.5% for WT plants at 6 weeks). Many key nodule enzymes were lower in the mutant with no compensatory increase in invertase activity. Invertase, therefore is not an effective alternative to SS. Sucrose and glucose levels in mutant nodules were higher than in WT nodules, fructose levels were not affected, and starch levels were reduced to one third of WT levels. Total amino acids were lower in mutant nodules. The mutated SS proteins were present in nodules but the cause of their inactivity is not yet known.KeywordsPisum SativumNodule CellNodule FunctionInvertase ActivityLotus JaponicusThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.