Regulation of Chloroplast Development by OsGLK1, of Which Overexpression Confers Greening on Rice Calli

Abstract

To uncover functions of a large population of rice genes efficiently, we are employing Full-length cDNA Over-eXpresser gene (FOX) hunting system. Producing FOX-rice lines, we have observed phenotypes of over 25,000 independent FOX-rice calli. Among them, two calli showed a green color on a 2,4-D-containing medium, on which wild-type rice calli are ivory yellow. In the two calli, OsGLK1 gene, which encodes a GARP-transcription factor, was ectopically overexpressed. After transfer of wild-type calli to the regeneration medium, greening of calli and an increase in the OsGLK1 mRNA level occurred synchronously. Transmission electron microscopy showed that chloroplasts with grana stacks were differentiated in the OsGLK1-FOX calli. However, in darkness, both the OsGLK1-FOX calli and plants did not show the green color and the differentiation of grana stacks. These data strongly suggest that OsGLK1 is a key regulator of chloroplast development under the control of light and phytohormones.