The influence of phospho-enol pyruvate carboxylase on stomatal conductance

BAROLI, IRENE; COUSINS, ASAPH B; LEA, PETER J; LEEGOOD, RICHARD C; SUSANNE VON CAEMMERER

Snowbird, Utah

Congreso; Special Meeting of the American Society of Plant Biology: The Biology of Transpiration; 2006

American Society of Plant Biologists

Phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) plays a key role during C4 photosynthesis and is involved in anapleroticmetabolism, pH regulation, and stomatal opening. Heterozygous (Pp) and homozygous (pp) forms of a PEPC-deficient mutantof the C4 dicot Amaranthus edulis were used to study the effect of reduced PEPC activity on CO2 assimilation rates, stomatalconductance, and 13CO2 (D13C) and C18OO (D18O) isotope discrimination during leaf gas exchange. PEPC activity was reducedto 42% and 3% and the rates of CO2 assimilation in air dropped to 78% and 10% of the wild-type values in the Pp and ppmutants, respectively. Stomatal conductance in air (531 mbar CO2) was similar in the wild-type and Pp mutant but the ppmutant had only 41% of the wild-type steady-state conductance under white light and the stomata opened more slowly inresponse to increased light or reduced CO2 partial pressure, suggesting that the C4 PEPC isoform plays an essential role instomatal opening. There was little difference in D13C between the Pp mutant (3.0& 6 0.4&) and wild type (3.3& 6 0.4&),indicating that leakiness (f), the ratio of CO2 leak rate out of the bundle sheath to the rate of CO2 supply by the C4 cycle, ameasure of the coordination of C4 photosynthesis, was not affected by a 60% reduction in PEPC activity. In the pp mutant D13Cwas 16& 6 3.2&, indicative of direct CO2 fixation by Rubisco in the bundle sheath at ambient CO2 partial pressure. D18Omeasurements indicated that the extent of isotopic equilibrium between leaf water and the CO2 at the site of oxygen exchange(u) was low (0.6) in the wild-type and Pp mutant but increased to 0.9 in the pp mutant. We conclude that in vitro carbonicanhydrase activity overestimated u as compared to values determined from D18O in wild-type plants.