BUNDLE-SHEATH DEFECTIVE, A MUTATION THAT DISRUPTS CELLULAR-DIFFERENTIATION IN MAIZE LEAVES

Post-primordial differentiation events in developing maize leaves prod uce two photosynthetic cell types (bundle sheath and mesophyll) that a re morphologically and biochemically distinct. We have isolated a muta tion that disrupts the differentiation of one of these cell types in l ight-grown leaves. bundle sheath defective 1-mutable 1 (bsd1-m1) is an unstable allele that was induced by transposon mutagenesis. In the bu ndle sheath cells of bsd1-m1 leaves, chloroplasts differentiate aberra ntly and C4 photosynthetic enzymes are absent. The development of meso phyll cells is unaffected. In dark-grown bsd1-m1 seedlings, morphologi cal differentiation of etioplasts is only disrupted in bundle sheath c ells but photosynthetic enzyme accumulation patterns are altered in bo th cell types. These data suggest that, during normal development, the Bsd1 gene directs the morphological differentiation of chloroplasts i n a light-independent and bundle sheath cell-specific fashion. In cont rast, Bsd1 gene action on photosynthetic gene expression patterns is c ell-type independent in the dark (C3 state) but bundle sheath cell-spe cific in the light (C4 state). Current models hypothesize that C4 phot osynthetic differentiation is achieved through a light-induced interac tion between bundle sheath and mesophyll cells (J. A. Langdale and T. Nelson (1991) Trends in Genetics 7, 191-196). Based on the data shown in this paper, we propose that induction of the C4 state restricts Bsd 1 gene action to bundle sheath cells.