Four Mutant Alleles Elucidate the Role of the G2 Protein in the Development of C₄ and C₃ Photosynthesizing Maize Tissues

Corresponding author: Jane A. Langdale, Department of Plant Sciences, University of Oxford, South Parks Rd., Oxford OX1 3RB, United Kingdom., jane.langdale{at}plants.ox.ac.uk (E-mail)

Communicating editor: K. J. NEWTON

Maize leaf blades differentiate dimorphic photosynthetic cell types, the bundle sheath and mesophyll, between which the reactions of C₄ photosynthesis are partitioned. Leaf-like organs of maize such as husk leaves, however, develop a C₃ pattern of differentiation whereby ribulose bisphosphate carboxylase (RuBPCase) accumulates in all photosynthetic cell types. The Golden2 (G2) gene has previously been shown to play a role in bundle sheath cell differentiation in C₄ leaf blades and to play a less well-defined role in C₃ maize tissues. To further analyze G2 gene function in maize, four g2 mutations 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 inserted in the second intron and in g2-pg14 the element is inserted in the promoter. In the fourth case, g2-R, four amino acid changes and premature polyadenylation of the G2 transcript are observed. The phenotypes conditioned by these four mutations demonstrate that the primary role of G2 in C₄ leaf blades is to promote bundle sheath cell chloroplast development. C₄ photosynthetic enzymes can accumulate in both bundle sheath and mesophyll cells in the absence of G2. In C₃ tissue, however, G2 influences both chloroplast differentiation and photosynthetic enzyme accumulation patterns. On the basis of the phenotypic data obtained, a model that postulates how G2 acts to facilitate C₄ and C₃ patterns of tissue development is proposed.

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