Two Cytosolic Glutamine Synthetase Isoforms of Maize Are Specifically Involved in the Control of Grain Production.

Martin, Antoine and Lee, Judy and Kichey, Thomas and Gerentes, Denise and Zivy, Michel and Tatout, Christophe and Dubois, Frédéric and Balliau, Thierry and Valot, Benoît and Davanture, Marlène and Tercé-Laforgue, Thérèse and Quilleré, Isabelle and Coque, Marie and Gallais, André and Gonzalez-Moro, María-Begoña and Bethencourt, Linda and Habash, Dimah Z. and Lea, Peter J. and Charcosset, Alain and Perez, Pascual and Murigneux, Alain and Sakakibara, Hitoshi and Edwards, Keith J. and Hirel, Bertrand (2006) Two Cytosolic Glutamine Synthetase Isoforms of Maize Are Specifically Involved in the Control of Grain Production. Plant Cell, 18 (11). pp. 3252-3274. ISSN 1040-4651

Abstract

The roles of two cytosolic maize glutamine synthetase isoenzymes (GS1), products of the Gln1-3 and Gln1-4 genes, were investigated by examining the impact of knockout mutations on kernel yield. In the gln1-3 and gln1-4 single mutants and the gln1-3 gln1-4 double mutant, GS mRNA expression was impaired, resulting in reduced GS1 protein and activity. The gln1-4 phenotype displayed reduced kernel size and gln1-3 reduced kernel number, with both phenotypes displayed in gln1-3 gln1-4. However, at maturity, shoot biomass production was not modified in either the single mutants or double mutants, suggesting a specific impact on grain production in both mutants. Asn increased in the leaves of the mutants during grain filling, indicating that it probably accumulates to circumvent ammonium buildup resulting from lower GS1 activity. Phloem sap analysis revealed that unlike Gln, Asn is not efficiently transported to developing kernels, apparently causing reduced kernel production. When Gln1-3 was overexpressed constitutively in leaves, kernel number increased by 30%, providing further evidence that GS1-3 plays a major role in kernel yield. Cytoimmunochemistry and in situ hybridization revealed that GS1-3 is present in mesophyll cells, whereas GS1-4 is specifically localized in the bundle sheath cells. The two GS1 isoenzymes play nonredundant roles with respect to their tissue-specific localization.