STUDY OF THE STRUCTURE AND FUNCTION OF HETEROTRIMERIC G PROTEINS IN PLANTS

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Transgenic rice containing an antisense cDNA for the α subunit of rice heterotrimeric G protein produced little or no mRNA for the subunit and exhibited abnormal morphology including dwarf traits and the setting of small seeds. In normal rice, the mRNA for the α subunit was abundant in the internodes and florets, the tissues closely related to abnormality in the dwarf transformants. The position of the α-subunit gene was mapped on rice chromosome 5 by mapping with the restriction fragment length polymorphism. The position was closely linked to the locus of a rice dwarf mutant, Daikoku dwarf (d1), which is known to exhibit abnormal phenotypes similar to those of the transformants that suppressed the endogenous mRNA for the α subunit by antisense technology.The chimera gene containing the promoter region of the gene of the α subunit of a heterotrimeric G protein (RGA1) and GUS gene, was introduced to the normal cultivar and the seeds at third generation (T3 seeds) were used for further analysis. In germination stage, the GUS activity was detected in aleurone layer, scutellar epithelium and leaf sheath. In the developing stage, it was largely observed in the internodes. Before heading, GUS activity was localized in the lemma and palea. After heading, it was detected in pollen. The expression of the α subunit in leaf sheath, internode, lemma and palea is accountable for explanation of abnormal phenotypes in Daikoku d1.Rice dwarf mutant, d1, is defective in the α subunit of the heterotrimeric G protein. Compared with the wild type, gibberellin induction of α-amylase activity in aleurone cells of d1 was greatly reduced. This result shows that the heterotrimeric G protein concern the gibberellin signal transduction.