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

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 expression in pea root nodules does not depend on the production of ammonia. Distinct pattems 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 pattems of expression between the two classes of genes for cytosolic GS suggests that their distinct gene products may serve nonoverlapping functions during pea development.

Cell-specific expression in transgenic plants reveals nonoverlapping roles for chloroplast and cytosolic glutamine synthetase.

“…The high-level expression of the gene for cytosolic GS3A in the vasculature is particularly intense in the cotyledons of germinating seedlings, where glutamine serves as the transport form of nitrogen from seed storage reserves to the developing plant. These findings in transgenic tobacco correlate well with the abundant accumulation of GS3A mRNA in germinating pea cotyledons and in nitrogen-fixing root nodules (12,18). Since expression of the pea GS3A gene in tobacco cotyledons is confined to the vasculature, it will be of interest to determine whether induced chlorophyll a/b binding protein).…”

“…"Sparkle" (Rogers Brothers Seed, Twin Falls, ID) constructed in "Lambda Dash" (Stratagene). Complete sequence analysis of each genomic clone (unpublished data) revealed that the genomic clone for chloroplast GS2 (GS2ct) corresponds to the GS185 cDNA (13), whereas the genomic clone for cytosolic GS (GS3ACY) corresponds to the GS341 cDNA (12,18).…”

“…Early biochemical data revealed that GS functions in the assimilation of ammonia generated by numerous plant processes, which include seed germination (4,5), photorespiration (6,7), nitrite reduction (8), nitrogen-fixation in root nodules (9,10), and primary ammonia assimilation from the soil (11). An analysis of the GS genes in several species has revealed a strong correlation of individual GS gene expression with specific aspects of plant development (12)(13)(14)(15)(16)(17)(18). Recent sequence analysis of GS cDNAs from Pisum sativum and Phaseolus vulgaris has shown that chloroplast and cytosolic GS are encoded by separate but similar nuclear genes (12,13,19).…”

“…In pea it has been shown that two classes of genes encode homologous but distinct cytosolic GS isoforms (13). One class comprises two nearly identical GS genes [GS3A (also known as GS341) and GS3B (also known as GS132)] whose expression is specifically induced in two organs where large amounts of ammonia are mobilized for plant growth, in cotyledons of germinating seedlings, and in nitrogen-fixing root nodules (18).…”

Aspartate Aminotransferase in Effective and Ineffective Alfalfa Nodules

“…Ineffective root nodules have been used to evaluate the role of NH4+ availability in nodule gene expression (1,2,12,37). In most studies, ineffective nodules were induced by mutant strains of rhizobia.…”