Induction of Crassulacean acid metabolism by water limitation

Abstract

Crassulacean acid metabolism (CAM), a key adaptation of photosynthetic carbon fixation to limited water availability, is characterized by nocturnal CO 2 fixation and daytime CO 2 re-assimilation, which generally results in improved wateruse efficiency. However, CAM plants display a remarkable degree of photosynthetic plasticity within a continuum of diel gas exchange patterns. Genotypic, ontogenetic and environmental factors combine to govern the extent to which CAM is expressed. The ecological diversity of CAM is mirrored by plasticity in a range of biochemical and physiological attributes. In C 3 /CAM-intermediate plants, limited water availability can induce or enhance the expression of CAM. CAM induction is controlled by a combination of transcriptional, post-transcriptional and post-translational regulatory events. Early events in CAM induction point to a requirement for calcium and calcium-dependent protein kinase activities. Gene discovery efforts, improved transformation technologies and genetic models for CAM plants, coupled with detailed physiological investigations, will lead to new insights into the molecular genetic basis of induction processes and the circadian oscillator that governs carbon flux during CAM. Future integration of genomic, biochemical and physiological approaches in selected CAM models promise to provide a detailed view of the complex regulatory dynamics involved in CAM induction and modulation by water deficit. Such information is expected to have broad significance as the ecological and agricultural importance of CAM species increases in the face of global warming trends and the associated expansion of desertification in semi-arid regions around the world.

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

“…Although the decrease in photorespiratory serine could in part be due to the general decrease in RUBISCO, the possible C3-to-CAM shift is further supported by the induction of genes encoding phosphoenolpyruvate carboxylase (PEPC), the key enzyme for C4-/CAM-type photosynthesis. 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].…”

The effects of salinity, crassulacean acid metabolism and plant age on the carbon isotope composition of Mesembryanthemum crystallinum L., a halophytic C3-CAM species

“…The induction of CAM in M. crystallinum is not salt-specific, drought and other treatments that reduce water availability can also elicit CAM, and some CAM activity may even occur in mature to old plants in the absence of a stress treatment. The latter observation has led some authors to conclude that the switch from C 3 photosynthesis to CAM is primarily a developmental phenomenon, and that salinity merely accelerates an inevitable, essentially irreversible ontogenetic process (von Willert and Kramer 1972;Herppich et al 1992;Adams et al 1998;Cushman and Borland 2002;Lu¨ttge 2004). However, growth of a halophyte under non-saline conditions may eventually become stressful because the plant will lack the salts required for adequate osmotic adjustment and turgor maintenance.…”

Evolution along the crassulacean acid metabolism continuum

“…The degree to which CAM operates can vary greatly depending on the evolutionary history of a given species and its environmental context, resulting in a continuum of differences in the degree to which nocturnal net uptake of CO 2 occurs in relation to day-time net CO 2 uptake (Cushman and Bohnert 1999;Cushman 2001;Cushman and Borland 2002;Dodd et al 2002). For example, many CAM species engage in 'obligate' or 'constitutive' CAM in fully mature photosynthetic organs (i.e.…”

“…Furthermore, environmental conditions can modulate the extent to which each phase is manifested (see also next section) (Cushman 2001;Cushman and Borland 2002). For example, water deficit stress can reduce or eliminate phase IV and light and temperature can regulate the appearance or onset of phases II and III (Griffiths 1988).…”