Four mutant alleles elucidate the role of the G2 protein in the development of C-4 and C-3 photosynthesizing maize tissues

Maize leaf blades differentiate dimorphic photosynthetic cell types, the bu ndle sheath and mesophyll, between which the reactions of C-4 photosynthesi s are partitioned. Leaf-like organs of maize such as husk leaves, however, develop a C-3 pattern of differentiation whereby ribulose bisphosphate carb oxylase (RuBPCase) accumulates in all photosynthetic cell types. The Golden 2 (G2) gene has previously been shown to play a role in bundle sheath cell differentiation in C-4 leaf blades and to play a less well-defined role in Q maize tissues. To further analyze G2 gene function in maize, four g2 muta tions have been characterized. Three of these mutations were induced by the transposable element Spm. In g2-bsd1-m1 and g2-bsd1-s1, the element is ins erted in the second intron and in g2-pg14 the element is inserted in the pr omoter. In the fourth case, g2-R four amino acid changes and premature poly adenylation of the G2 transcript are observed. The phenotypes conditioned b y these four mutations demonstrate that the primary role of G2 in C-4 leaf blades is to promote bundle sheath cell chloroplast development. C-4 photos ynthetic enzymes can accumulate in both bundle sheath and mesophyll cells i n the absence of G2. In C-3 tissue, however, G2 influences both chloroplast differentiation and photosynthetic enzyme accumulation patterns. On the ba sis of the phenotypic data obtained, a model that postulates how G2 acts to facilitate C-4 and C-3 patterns of tissue development is proposed.