PARENT SESSION
Posters P8C C4 and CAM. Abstracts (685-698)

Antisense reduction of Rubisco activase levels in Flaveria bidentis, a C₄ species, reduces carbamylation and photosynthesis. Luke Hendrickson¹, Vanda Quinn^*,1, Anthony Millgate¹, Bob Furbank², Susanne Caemmerer¹, ¹ Molecular Plant Physiology Group, Canberra, ACT, Australia² Plant Industry, Canberra, ACT, Australia

ABSTRACT- To function, the catalytic sites of ribulose-1, 5-bisphosphate carboxylaseoxygenase (Rubisco) need to be activated by carbamylation followed by rapid binding of magnesium. The activation of Rubisco in vivo requires the presence of the regulatory protein Rubisco activase. This enzyme is thought to facilitate the release of sugar phosphate inhibitors from Rubisco^,s catalytic sites thereby influencing carbamylation. In C₃ species Rubisco operates in a low CO₂ environment, which is suboptimal for both catalysis and carbamylation. In C₄ plants Rubisco, located in the bundle sheath cells, operates in a high CO₂ atmosphere close to saturation. To explore the role of Rubisco activase in C₄ photosynthesis, activase levels were reduced in Flaveria bidentis, a C₄ dicot, by transformation with an antisense gene directed against the mRNA for Rubisco activase. A range of primary and secondary transformants with very low activase contents were recovered and cultivated. These plants required high CO₂ (>1.5%) for growth. They had low rates of CO₂ assimilation at high light and ambient CO₂ compared to wild type. This was due to very low levels of carbamylation of Rubisco sites yet the amount of Rubisco was similar to that of wild type plants. The relationship between CO₂ assimilation rate and activase content was investigated at high light, ambient CO₂ and leaf temperatures of 25 and 40°C. The results revealed a threshold activase content (∼30% of wild type) at both leaf temperatures below which maximum rates of CO₂ assimilation were limited by the ability of activase to maintain Rubisco function.

KEY WORDS: Flaveria bidentis, Rubisco activase, C4 photosynthesis