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

Abstract

In the C₄ plant maize, three photosynthetic cell types differentiate: C₄ bundle sheath, C₄ mesophyll, and C₃ mesophyll cells. C₃ mesophyll cells represent the ground state, whereas C₄ bundle sheath and C₄ 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 C₃ stages of development. However, G2 function is specifically committed to the differentiation of bundle sheath cell chloroplasts in C₄ 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 C₃ species. In maize, G2 and ZmGlk1 transcripts accumulate primarily in C₄ bundle sheath and C₄ mesophyll cells, respectively, suggesting a specific role for each gene in C₄ 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.