Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/27/1998
Publication Date: N/A
Citation: N/A Interpretive Summary: Alfalfa plants living in symbiosis with the soil bacterium Rhizobium through a process called nitrogen fixation can convert atmospheric nitrogen gas to nitrogen fertilizer. This process occurs in wart-like structures, termed nodules, that form on roots. The plant gives the bacterium energy compounds derived from sugars produced in the leaves, while the bacterium gives the plant nitrogen fertilizer. Before the nodule bacteria can use the energy from plant sugars, those sugars must be converted to a useful form. The plant enzyme (protein catalyst) sucrose synthase degrades the plant sugars to the correct form for the bacterium. How sucrose synthase in nodules is related to the process of nitrogen fixation is not well understood. In this study we report the isolation and characterization of the alfalfa gene that makes nodule sucrose synthase. Moreover we show that the amount of gene product (sucrose synthase enzyme) is directly correlated dto how well nitrogen fixation is occurring. Nodules that do not fix nitrogen have little to no sucrose synthase. In addition, we show which cells in nodules have the most sucrose synthase. This work is important because it describes how the gene controlling energy flow to root nodules is regulated. It also identifies a target gene for manipulation to improve sugar metabolism and nitrogen fixation.
Technical Abstract: The carbon (C) cost for symbiotic nitrogen (N) fixation is quite high. The ultimate source of C for N2 fixation is sucrose derived from leaves. For use in root nodules, sucrose must be cleaved to glucose and other smaller carbon compounds. Here we report the isolation and characterization of a full length cDNA encoding the enzyme sucrose synthase (SS; EC 126.96.36.199). This SS cDNA shows greatest expression in effective nitrogen-fixing nodules. It is also expressed in stems and roots with little expression in leaves and cotyledons. While maximum expression of SS in nodules appears to require active nitrogenase, the gene is also expressed in ineffective nodules, albeit at reduced levels. Measurement of starch concentrations in nodules shows that SS expression is not coupled to starch biosynthesis. In situ hybridization studies show that expression of SS occurs in both infected and uninfected cells and in the nodule meristem.