Developmentally Regulated Expression of the Gene Family for Cytosolic Glutamine Synthetase in Pisum sativum

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Abstract
In Pisum sativum, two classes of genes encode distinct isoforms of cytosolic glutamine synthetase (GS). The first class comprises two nearly identical or "twin" GS genes (GS341 and GS132), while the second comprises a single GS gene (GS299) distinct in both coding and noncoding regions from the "twin" GS genes. Gene-specific analyses were used to monitor the individual contribution of each gene for cytosolic GS during root nodule development and in cotyledons during germination, two contexts where large amounts of ammonia must be assimilated by GS for nitrogen transport. mRNAs corresponding to all three genes for cytosolic GS were shown to accumulate coordinately during a time course of nodule development. All the GS mRNAs also accumulate to wild-type levels in mutant nodules formed by a nifD- strain of Rhizobium leguminosarum indicating that induced GS exprssion in pea root nodules does not depend on the production of ammonia. Distinct patterns of expression for the two classes of GS genes were observed in certain mutant root nodules and most dramatically in cotyledons of germinating seedlings. The different patterns of expression between the two classes of genes for cytosolic GS suggests that their distinct gene products may serve nonoverlapping functions during pea development.