THE MOLECULAR-BASIS OF C-4 PHOTOSYNTHESIS IN SORGHUM - ISOLATION, CHARACTERIZATION AND RFLP MAPPING OF MESOPHYLL-SPECIFIC AND BUNDLE-SHEATH-SPECIFIC CDNAS OBTAINED BY DIFFERENTIAL SCREENING

C-4 photosynthesis depends upon the strict compartmentalization of the CO2-assimilatory enzymes of the C-4 and Calvin cycle in two different cell types, mesophyll and bundle-sheath cells. A differential accumul ation is also observed for enzymes of other metabolic pathways, and me sophyll and bundle-sheath chloroplasts of NADP-malic enzyme type C-4 p lants differ even in their photosynthetic electron transport chains. A large number of studies indicate that this division of labour between mesophyll and bundle-sheath cells is the result of differential gene expression. To investigate the extent of this differential gene expres sion and thus gain insight into the genetic basis of C-4 photosynthesi s, genes that are differentially expressed in the mesophyll and bundle -sheath cells were catalogued in the NADP-malic enzyme type C-4 grass Sorghum bicolor. A total of 58 cDNAs were isolated by differential scr eening. Using a tenfold difference in transcript abundance between mes ophyll and bundle-sheath cells as a criterion, 25 cDNAs were confirmed to encode mesophyll-specific gene sequences and 8 were found to encod e bundle-sheath-specific sequences. Eight mesophyll-specific cDNAs sho wed no significant similarities within GenBank and may therefore repre sent candidates for the elucidation of hitherto unknown functions in t he differentiation of mesophyll and bundle-sheath cells. The chromosom al location of 50 isolated cDNAs was determined by RFLP mapping using an interspecific sorghum cross.