CAM photosynthesis: the acid test

There is currently considerable interest in the prospects for bioengineering crassulacean acid metabolism (CAM) photosynthesis – or key elements associated with it, such as increased water‐use efficiency – into C(3) plants. Resolving how CAM photosynthesis evolved from the ancestral C(3) pathway could provide valuable insights into the targets for such bioengineering efforts. It has been proposed that the ability to accumulate organic acids at night may be common among C(3) plants, and that the transition to CAM might simply require enhancement of pre‐existing fluxes, without the need for changes in circadian or diurnal regulation. We show, in a survey encompassing 40 families of vascular plants, that nocturnal acidification is a feature entirely restricted to CAM species. Although many C(3) species can synthesize malate during the light period, we argue that the switch to night‐time malic acid accumulation requires a fundamental metabolic reprogramming that couples glycolytic breakdown of storage carbohydrate to the process of net dark CO(2) fixation. This central element of the CAM pathway, even when expressed at a low level, represents a biochemical capability not seen in C(3) plants, and so is better regarded as a discrete evolutionary innovation than as part of a metabolic continuum between C(3) and CAM.