The Maize Golden2 Gene Defines a Novel Class of Transcriptional Regulators in Plants

In the C(4) plant maize, three photosynthetic cell types differentiate: C(4) bundle sheath, C(4) mesophyll, and C(3) mesophyll cells. C(3) mesophyll cells represent the ground state, whereas C(4) bundle sheath and C(4) mesophyll cells are specialized cells that differentiate in response to light-induced positional signals. The Golden2 (G2) gene regulates plastid biogenesis in all photosynthetic cells during the C(3) stages of development. However, G2 function is specifically committed to the differentiation of bundle sheath cell chloroplasts in C(4) leaf blades. In this article, we report the isolation of G2-like (Glk) genes from maize and rice, providing evidence for a family of Glk genes in plants. The expression profiles of the rice Glk genes suggest that these genes may act redundantly to promote photosynthetic development in this C(3) species. In maize, G2 and ZmGlk1 transcripts accumulate primarily in C(4) bundle sheath and C(4) mesophyll cells, respectively, suggesting a specific role for each gene in C(4) differentiation. We show that G2 and ZmGLK1 both can transactivate reporter gene transcription and dimerize in yeast, which supports the idea that these proteins act as transcriptional regulators of cell-type differentiation processes.