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      Disruption of a Novel NADH-Glutamate Synthase2 Gene Caused Marked Reduction in Spikelet Number of Rice

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          Abstract

          Inorganic ammonium ions are assimilated by a coupled reaction of glutamine synthetase and glutamate synthase (GOGAT). In rice, three genes encoding either ferredoxin (Fd)-GOGAT, NADH-GOGAT1, or NADH-GOGAT2, have been identified. OsNADH-GOGAT2, a newly identified gene, was expressed mainly in fully expanded leaf blades and leaf sheaths. Although the distinct expression profile to OsNADH-GOGAT1, which is mainly detected in root tips, developing leaf blades, and grains, was shown in our previous studies, physiological role of NADH-GOGAT2 is not yet known. Here, we isolated retrotransposon mediated-knockout mutants lacking OsNADH-GOGAT2. In rice grown under paddy field conditions, disruption of the OsNADH-GOGAT2 gene caused a remarkable decrease in spikelet number per panicle associated with a reductions in yield and whole plant biomass, when compared with wild-type (WT) plants. The total nitrogen contents in the senescing leaf blade of the mutants were approximately a half of the WT plants. Expression of this gene was mainly detected in phloem companion cells and phloem parenchyma cells associated with large vascular bundles in fully expanded leaf blades, when the promoter region fused with a β-glucuronidase gene was introduced into the WT rice. These results suggest that the NADH-GOGAT2 is important in the process of glutamine generation in senescing leaves for the remobilization of leaf nitrogen through phloem to the panicle during natural senescence. These results also indicate that other GOGATs, i.e., NADH-GOGAT1 and ferredoxin-GOGAT are not able to compensate the function of NADH-GOGAT2.

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            Two cytosolic glutamine synthetase isoforms of maize are specifically involved in the control of grain production.

            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.
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              Retrotransposons of rice involved in mutations induced by tissue culture.

              Five retrotransposon families of rice (Tos1-Tos5) have been reported previously. Here we report 15 new retrotransposon families of rice (Tos6-Tos20). In contrast to yeast and Drosophila retrotransposons, all of the rice retrotransposons examined appear inactive (or almost inactive) under normal growth conditions. Three of the rice retrotransposons (Tos10, Tos17, and Tos19) are activated under tissue culture conditions. The most active one, Tos17, was studied in detail. The copy number of Tos17 increased with prolonged culture period. In all of the plants regenerated from tissue cultures, including transgenic plants, 5 to 30 transposed Tos17 copies were detected. The transcript of Tos17 was only detected under tissue culture conditions, indicating that the transposition of Tos17 is mainly regulated at the transcriptional level. To examine the target-site specificity of Tos17 transposition, sequences flanking transposed Tos17 copies were analyzed. At least four out of eight target sites examined are coding regions. Other target sites may also be in genes because two out of four were transcribed. The regenerated plants with Tos17-insertions in the phytochrome A gene and the S-receptor kinase-related gene were identified. These results indicate that activation of Tos17 is an important cause of tissue culture-induced mutations. Tissue culture-induced activation of Tos17 may be a useful tool for insertional mutagenesis and functional analysis of genes.
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                Author and article information

                Journal
                Front Plant Sci
                Front Plant Sci
                Front. Plant Sci.
                Frontiers in plant science
                Frontiers Research Foundation
                1664-462X
                28 September 2011
                2011
                : 2
                : 57
                Affiliations
                [1] 1simpleGraduate School of Agricultural Science, Tohoku University Sendai, Japan
                Author notes

                Edited by: Lixing Yuan, China Agricultural University, China

                Reviewed by: Miyako Kusano, RIKEN Plant Science Center, Japan; Guohua Xu, Nanjing Agricultural University, China

                *Correspondence: Tomoyuki Yamaya, Graduate School of Agricultural Science, Tohoku University, 1-1 Tsutsumidori-Amamiyamachi, Aoba-ku, Sendai 981-8555, Japan. e-mail: tyamaya@ 123456biochem.tohoku.ac.jp

                This article was submitted to Frontiers in Plant Nutrition, a specialty of Frontiers in Plant Science.

                Article
                10.3389/fpls.2011.00057
                3355815
                22645542
                c8bfa645-6d93-43d9-986e-46aa1699d680
                Copyright © 2011 Tamura, Kojima, Toyokawa, Watanabe, Tabuchi-Kobayashi, Hayakawa and Yamaya.

                This is an open-access article subject to a non-exclusive license between the authors and Frontiers Media SA, which permits use, distribution and reproduction in other forums, provided the original authors and source are credited and other Frontiers conditions are complied with.

                History
                : 04 July 2011
                : 09 September 2011
                Page count
                Figures: 7, Tables: 0, Equations: 0, References: 30, Pages: 9, Words: 6114
                Categories
                Plant Science
                Original Research

                Plant science & Botany
                spikelet number,nitrogen remobilization,retrotransposon,knockout mutant,nadh-glutamate synthase2,rice

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