Identification of the ADP-glucose pyrophosphorylase isoforms essential for starch synthesis in the leaf and seed endosperm of rice (Oryza sativa L.).
ADP-glucose pyrophosphorylase (AGP) catalyzes the first committed step of starch biosynthesis in higher plants. To identify AGP isoforms essential for this biosynthetic process in sink and source tissues of rice plants, we analyzed the rice AGP gene family which consists of two genes, OsAGPS1 and OsAGPS2, encoding small subunits (SSU) and four genes, OsAGPL1, OsAGPL2, OsAGPL3 and OsAGPL4, encoding large subunits (LSU) of this enzyme heterotetrameric complex. Subcellular localization studies using green fluorescent protein (GFP) fusion constructs indicate that OsAGPS2a, the product of the leaf-preferential transcript of OsAGPS2, and OsAGPS1, OsAGPL1, OsAGPL3, and OsAGPL4 are plastid-targeted isoforms. In contrast, two isoforms, SSU OsAGPS2b which is a product of a seed-specific transcript of OsAGPS2, and LSU OsAGPL2, are localized in the cytosol. Analysis of osagps2 and osagpl2 mutants revealed that a lesion of one of the two cytosolic isoforms, OsAGPL2 and OsAGPS2b, causes a shrunken endosperm due to a remarkable reduction in starch synthesis. In leaves, however, only the osagps2 mutant appears to severely reduce the transitory starch content. Interestingly, the osagps2 mutant was indistinguishable from wild type during vegetative plant growth. Western blot analysis of the osagp mutants and wild type plants demonstrated that OsAGPS2a is an SSU isoform mainly present in leaves, and that OsAGPS2b and OsAGPL2 are the major SSU and LSU isoforms, respectively, in the endosperm. Finally, we propose a spatiotemporal complex model of OsAGP SSU and LSU isoforms in leaves and in developing endosperm of rice plants.
Lee
Sang-Kyu
SK
Graduate School of Biotechnology & Plant Metabolism Research Center, Kyung Hee University, Yongin 446-701, Korea.
Hwang
Seon-Kap
SK
Han
Muho
M
Eom
Joon-Seob
JS
Kang
Hong-Gyu
HG
Han
Yulyi
Y
Choi
Sang-Bong
SB
Cho
Man-Ho
MH
Bhoo
Seong Hee
SH
An
Gynheung
G
Hahn
Tae-Ryong
TR
Okita
Thomas W
TW
Jeon
Jong-Seong
JS
D001483:Base Sequence; D015153:Blotting, Western; D017931:DNA Primers; D005816:Genetic Complementation Test; D051599:Glucose-1-Phosphate Adenylyltransferase / Q000378:metabolism*; D007527:Isoenzymes / Q000378:metabolism*; D016254:Mutagenesis, Insertional; D012275:Oryza / Q000201:enzymology* / Q000378:metabolism; D018515:Plant Leaves / Q000201:enzymology*; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D012639:Seeds / Q000201:enzymology*; D013213:Starch / Q000096:biosynthesis*; D013347:Subcellular Fractions / Q000201:enzymology / Q000378:metabolism
D017931:DNA Primers; D007527:Isoenzymes; D011993:Recombinant Fusion Proteins; D013213:Starch; D051599:Glucose-1-Phosphate Adenylyltransferase