Thermal Regulation of Phosphoenolpyruvate Carboxylase and Ribulose-1,5-Bisphosphate Carboxylase in C3 and C4 Plants Native to Hot and Temperate Climates

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Abstract
Exposure of leaf sections from 2-week-old seedlings of sorghum (Sorghum bicolor L.) (C4 plant), corn (Zea mays L.) (C4), peanut (Arachis hypogaea L.) (C3 plant), and soybean (Glycine max L.) (C3) to 40 or 45.degree.C for up to 4 hours resulted in significant increases in the levels of 102 kilodalton (C4), 52 kilodalton (C3 and C4), and 15 kilodalton (C3 and C4) polypeptides. These proteins comigrated, respectively, with authentic phosphoenolpyruvate carboxylase (PEPC) and the large (RLSU) and small (RSSU) subunits of ribulose-1,5-bisphosphate carboxylase (Rubisco) during both one- and two-dimensional SDS-PAGE and reacted with antisera raised against these enzymes. After 4 hours at 50.degree.C, levels of the polypeptides either remained relatively stable (PEPC, RLSU) or increased (RSSU) in sorghum and peanut (plants native to hot climates). In corn and soybean (plants native to temperate climates), levels of the proteins either fell sharply (corn) or showed strong evidence of incomplete processing and/or aggregation (soybean). In addition to changes in levels of the proteins, the activities of PEPC and Rubisco in extracts of leaves exposed to 50.degree.C fell by 84% and 11% of their respective control values in sorghum and by 54% each in peanut. In corn and soybean, the activities of both enzymes were depressed at 40.degree.C, with measured values at 50.degree.C not exceeding 5% of those from the non-stressed controls.