Control of Photosynthesis in Amaranthus edulis Mutants with Reduced Amounts of PEP Carboxylase

Abstract

Mutants of the NAD-ME plant have been created using sodium azide. These contain reduced activities of PEP carboxylase ranging from 5 to 100% of wild-type. Those with greater than 50% of the wild-type PEPC content showed a lower maximum rate of photosynthesis and reduced carboxylation efficiency compared to the wild-type plants. The PEPC from these heterozygotes was less sensitive to malate inhibition than the PEPC of the wild-type plants and also had an increased phosphorylation state.

Mutants containing 45 and 49% of wild-type PEPC exhibited a greater A_max than was observed for the 55, 66 and 70% mutants. We postulate that there is a compensatory mechanism that activates PEPC when the PEPC protein is less than 55% of the wild-type.

Control coefficients were measured for PEP carboxylase from wild-type and heterozygous (55%) plants. Results suggest that activation of PEPC by phosphorylation and metabolites may be more an ‘on-off’ switch than a means of fine adjustment of PEPC activity in response to varying factors such as PEPC content, CO₂ or temperature changes.