CAM photosynthesis: the acid test

What is it about?

The crassulacean acid metabolism (CAM) pathway of photosynthesis represents, along with C4 photosynthesis, one of two alternative modes of primary carbon metabolism found in vascular plants. In recent discussions about evolution of the CAM pathway, and the prospects for bioengineering of this water-use-efficient mode of photosynthesis, it has been proposed this may simply require enhancement of flux pathways already present in C3 plants. However, we show here, in a survey encompassing 40 families of vascular plants, that nocturnal acidification associated with dark CO2 fixation is a feature entirely restricted to CAM species. We argue that the switch from daytime malate accumulation (C3) to nighttime malic-acid accumulation (CAM) requires a fundamental metabolic reprogramming, probably linked to components of the circadian clock, that likely represents one of the earliest steps in the evolution of CAM photosynthesis.

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