Portal de Periódicos da CAPES

Phosphoenolpyruvate carboxylase reduces photorespiration in Panicum milioides, a C3C4 intermediate species

1979; Elsevier BV; Volume: 193; Issue: 2 Linguagem: Inglês

10.1016/0003-9861(79)90039-0

1096-0384

C.K.M. Rathnam, Raymond Chollet,

Plant Stress Responses and Tolerance

Panicum milioides represents the first well-documented example of a higher plant species with reduced photorespiration and O2 inhibition of photosynthesis. We have investigated the biochemical mechanism(s) involved in reducing O2 sensitivity of photosynthesis in this species by parallel enzyme inhibitor experiments with thin leaf slices of P. milioides and C3 and C4Panicum species. The reduced O2 sensitivity of net photosynthesis in P. milioides gradually increased with increasing concentrations of the phosphoenolpyruvate carboxylase (EC 4.1.1.31) inhibitors, maleate and malonate. At saturating levels of inhibitor, photosynthesis in 2% O2 was decreased by about 18%, and the inhibitory effects of both 21% O2 and 49% O2 were identical to those observed with a C3Panicum species in the absence or presence of inhibitor. A significant potential for C4 photosynthesis in P. milioides, compared to its complete absence in a C3Panicum species, was demonstrated on the basis of: (a) a coupling of leaf slice CO2 fixation by phosphoenolpyruvate carboxylase with the C3 cycle; (b) NAD-malic enzyme (EC 1.1.1.39)-dependent aspartate and malate decarboxylation in leaf slices; (c) a full complement of C4 cycle enzymes in leaf extracts, including pyruvate, Pi dikinase (EC 2.7.9.1) and NAD-malic enzyme; and (d) Kranz-like leaf anatomy with numerous plasmodesmata traversing the mesophyll-bundle sheath interfacial cell wall. These data indicate that the reduced photorespiration and O2 inhibition of photosynthesis in P. milioides is due to phosphoenolpyruvate carboxylase participation, possibly by creating a limited C4-like CO2 pump, rather than an altered ribulose 1,5-bisphosphate carboxylase-oxygenase (EC 4.1.1.39).