Regulation of Rubisco activity in crassulacean acid metabolism plants: better late than never

Abstract

This paper originates from a presentation at the IIIrd International Congress on Crassulacean Acid Metabolism, Cape Tribulation, Queensland, Australia, August 2001.

The diurnal regulation of Rubisco was compared for a range of crassulacean acid metabolism (CAM) species in the context of high carboxylation and electron transport capacities, which may be an order of magnitude greater than rates of net CO₂ uptake. Early in the light period, Rubisco activity and electron transport were limited when phosphoenolpyruvate carboxylase (PEPC) may have been operating, and maximal extractable activities and activation state for Rubisco were achieved at the end of Phase III, prior to the direct atmospheric uptake of CO₂ during Phase IV. The delayed activation was associated with levels of Rubisco activase protein, which reached a maximum at midday, and may account for this pattern of Rubisco activation. This regulation may be modified by environmental conditions - processes that tend to restrict PEPC activity, such as drought stress or incubation of leaves overnight in an oxygen-free atmosphere, release Rubisco from inhibition early in the light period. The quantum yield of light use also tracks Rubisco carboxylation, being particularly low at dawn when PEPC is active. The plasticity in expression of the CAM cycle is therefore matched by the regulation of key carboxylases, with extractable Rubisco activity maximal when drawdown of atmospheric CO₂ to cells in succulent CAM tissues is most likely to limit photon utilization shortly after midday, during Phase IV.