Cloning and Developmental Expression of a Nodule-Enhanced Sucrose Synthase cDNA from Alfalfa
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 18.104.22.168). 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.
KeywordsSugar Hydrolysis Sucrose Starch Maize
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