Evolution along the crassulacean acid metabolism continuum

Abstract

Abstract. Crassulacean acid metabolism (CAM) is a specialised mode of photosynthesis that improves atmospheric CO 2 assimilation in water-limited terrestrial and epiphytic habitats and in CO 2 -limited aquatic environments. In contrast with C 3 and C 4 plants, CAM plants take up CO 2 from the atmosphere partially or predominantly at night. CAM is taxonomically widespread among vascular plants and is present in many succulent species that occupy semiarid regions, as well as in tropical epiphytes and in some aquatic macrophytes. This water-conserving photosynthetic pathway has evolved multiple times and is found in close to 6% of vascular plant species from at least 35 families. Although many aspects of CAM molecular biology, biochemistry and ecophysiology are well understood, relatively little is known about the evolutionary origins of CAM. This review focuses on five main topics: (1) the permutations and plasticity of CAM, (2) the requirements for CAM evolution, (3) the drivers of CAM evolution, (4) the prevalence and taxonomic distribution of CAM among vascular plants with emphasis on the Orchidaceae and (5) the molecular underpinnings of CAM evolution including circadian clock regulation of gene expression.

Molecular mechanisms of desiccation tolerance in the resurrection glacial relic Haberlea rhodopensis

“…Drought stress can induce CAM metabolism in C3/CAM-intermediate plants [55]. This strategy is particularly important under severe drought stress, when stomata remain closed and CO 2 is recycled through respiratory release followed by assimilation, a phenomenon known as 'CAM idling' [56]. Although no carbon is being gained during CAM idling, this could be an important ecophysiological adaptation to maintain photosystem stability and survive prolonged periods without water.…”

Biochemical and biophysical CO2 concentrating mechanisms in two species of freshwater macrophyte within the genus Ottelia (Hydrocharitaceae)

“…This C 4 acid is then decarboxylated during the day to produce CO 2 that is fixed by RuBisCO. This is primarily a water conserving mechanism minimising gaseous exchange during the day but it also serves to conserve carbon by reducing respiratory carbon loss (Cushman and Bohnert 1999;Silvera et al 2010). …”

“…In terrestrial plants, the global frequency of C 4 is about 3% (Edwards et al 2004) and that of CAM about 6%, (Silvera et al 2010) with the remainder (91%) being C 3 and so lacking CCMs. In contrast, about 55% of aquatic angiosperms have a biophysical CCM based on HCO 3 -use and others have a biochemical CCM based on CAM (4%) or C 4 (3% ; Maberly and Madsen 2002 …”

Stomatal Biology of CAM Plants

“…CAM cycling involves the nocturnal operation of respiratory recycling and diurnal stomatal opening for direct Rubisco-mediated assimilation and most often occurs as a facultative trait in C 3 -CAM or "weak CAM" species (Silvera et al, 2010). Meanwhile, under CAM idling, stomata remain closed throughout the day and night, with a proportion of respiratory CO 2 being refixed.…”

“…Meanwhile, under CAM idling, stomata remain closed throughout the day and night, with a proportion of respiratory CO 2 being refixed. CAM idling is often induced under extreme seasonal drought stress in "strong CAM" species, maximizing water retention (Silvera et al, 2010). This capacity for close environmental tracking on both diurnal and seasonal bases maximizes integrated water-use efficiency and is therefore an important contributor to the ecological success of CAM plants in stressful habitats (Fig.…”