Light induced changes in the polypeptide composition of Sorghum bicolor

Two dimensional gel electrophoresis resolved total protein extracted from Sorgum bicolor (L.) Moench shoots into 430 polypeptides detectable by silver staining. The relative amunts of 82 polypeptides increased and 192 decreased upon exposure of etiolated shoots to light. Total protein extracted from etiolated mesocotyls was resolved into 360 polypeptides. The relative amounts of 23 mesocotyl polypeptides increased and 5 decreased after light exposure. Of 274 polypeptides whose relative amounts was altered in shoots exposed to light, 189 were found in mesocotyls. The relative amounts of only 6 polypeptides showed the same response to light in mesocotyls as in shoots. Immunoblotting identified 2 subunits of phosphoenolpyruvate carboxylase (PEPCase) differing in isoelectric point and molecular weight. The 81 000 dalton subunit was the major subunit found in mesocotyls while the 95 000 dalton subunit was the major subunit found in green shoots suggesting that these subunits are derived from a non-photosynthetic and photosynthetic isozyme of PEPCase. In etiolated shoots but not in mesocotyls, light induced the 81 000 and 95 000 apparent molecular weight (MW) subunits of PEPCase, the subunit of pyruvate orthophosphate dikinase (PPDK) as well as the large and small subunits of ribulose-1,5-bisphosphate carboxylase (RuBPCase). Specific activity measurements indicated that light induced the accumulation of the peroxisomal enzyme hydroxypyruvate reductase and inhibited the accumulation of the mitochondrial enzyme serine hydroxymethyltransferase in both mesocotyls andshoots. NADP-glyceraldehyde-3-phosphate dehydrogenase activity was light induced in shoots but undetectable in mesocotyls. In shoots and meocotyls, light had little effect on the mitochondrial enzyme fumarase or the cytoplasmic enzyme NAD-glyceraldehyde-3-P-dehydrogenase.