RESUMEN
C4 plants can more efficiently fix carbon in drought, high temperatures, and limitations of nitrogen or CO2. Primary carboxylation is mediated by phosphoenolpyruvate carboxylase (PEPC, 4.1.1.31) in mesophyll cytosol of C4 plants. Studies on hormonal regulation of C4 PEPC have been quite limited. We have examined the activity/regulation of PEPC by abscisic acid (ABA), a plant hormone, in the leaves of Amaranthus hypochondriacus. PEPC activity was enhanced upon 1-h incubation with 20 µM ABA by about 30% in dark and more than 2-fold in light. Glucose-6-phosphate activation of PEPC was enhanced, and sensitivity to L-malate was decreased after ABA treatment. Butyric acid (a weak acid) decreased PEPC activity and restricted the stimulation by ABA. In contrast, methylamine (an alkalinizing agent) increased the PEPC activity and enhanced the effect of ABA. ABA increased the levels of PEPC protein as well as its mRNA. Butyric acid/methylamine modulated the changes induced by ABA of PEPC protein and mRNA levels, indicating that acidification/alkalinization of leaf disks was very important. Our results emphasize the marked modulation of PEPC in C4 plants, by ABA. Such modulation by ABA could be significant when C4 plants are under water stress, when ABA is known to accumulate. When present, cycloheximide decreased the PEPC protein levels and restricted the extent of activation by ABA. We conclude that the enhancement by ABA of PEPC activity depends on cellular alkalinization as well as elevated PEPC protein levels.