Effect of Severe Water Stress on Aspects of Crassulacean Acid Metabolism in Xerosicyos

Abstract

Xerosicyos danguyi H. Humb. (Cucurbitaceae) is a Crassulacean acid metabolism (CAM) species native to Madagascar. Previously, it was shown that when grown under good water conditions, it is a typical CAM plant, but when water stressed, it shifts to a dampened form of CAM, termed CAM-idling, in which stomata are closed day and night but with a continued, low diurnal organic acid fluctuation. We have now studied the kinetics of some metabolic features of the shift from CAM to CAM-idling under severe water stress and the recovery upon rewatering. When water is withheld, there is a steady decrease in relative water content (RWC), reaching about 50%, at which point the water potential decreases precipitously from about -2 or -3 bars to -12 bars.Abscisic acid (ABA) increases sharply at about 75% RWC. Stomata close, which limits COz uptake, and there is a dampened diurnal organic acid fluctuation typical of CAM-idling. Throughout an extended stress period to 50% RWC, there is no change in chlorophyll, protein, and ribulose bisphosphate carboxylase activity compared with the well-watered plants. Despite the fact that the tissue was already in CAM, the stress is accompanied by an increase in phosphoenolpyruvate carboxylase (PEPc) mRNA, extractable PEPc activity, and PEPc protein (such that the specific activity remained approximately constant) and a decrease in the apparent KJPEP). It is not known if the changes in KJPEP) in response to drought are related to or are separate from the increases in PEPc protein and mRNA. The changes in Km(PEP) could be in response to the decreased endogenous levels of organic acids, but evidently are not an assay artifact. The increases in PEPc protein and mRNA appear to be related to the water-stress treatment and may result from the increased concentration of ABA or the decreased levels of endogenous organic acids. When rewatered, the metabolism quickly returns to the well-watered control typical of CAM.

Inducibility of crassulacean acid metabolism (CAM) in Clusia species; physiological/biochemical characterisation and intercellular localization of carboxylation and decarboxylation processes in three species which exhibit different degrees of CAM

“…Therefore, in theory, the rapid induction of CAM which has been demonstrated in some Clusia species (Zotz and Winter 1993) could be achieved through post-translational modi®cation of existing PEPCase and PEPCK enzymes. Given the potential excess carboxylation capacity in well-watered plants, the proportional increases in organic acid accumulation at night and the activity/amount of PEPCase elicited by drought stress are somewhat surprising, and contrast with ®ndings on several other C 3 -CAM intermediates where drought induces an increase in PEPCase capacity with little or even a decrease in acid¯uctuations (Brulfert et al 1991;Bastide et al 1993). The decarboxylation of malate, and citrate in particular, which increased substantially in all three Clusia species in response to drought stress, appeared to be directly related to the capacity of the decarboxylase, PEPCK.…”

What are the evolutionary origins of stomatal responses to abscisic acid in land plants?

“…ABA has also been implicated in inducing crassulacean acid metabolism (CAM), whereby stomata remain closed during the day and open only at night, in facultative CAM plants, which can naturally be induced to shift from the typical C 3 carbon metabolism to CAM under water stress (Ting ). In addition, ABA is thought to be involved in inducing a shift from CAM to CAM‐idling in CAM plants, whereby stomata remain closed day and night, during conditions of extreme water stress (Bastide et al ). The molecular mechanisms for these specialized responses remain to be determined.…”

Molecular Genetics of Crassulacean Acid Metabolism

“…By refixing respiratory CO, at night, plants that perform CAM cycling are thought to be poised to enter full CAM rapidly when necessary due to environmental stress conditions. Another variation, "CAM idling," occurs under extreme environmental stress conditions when stomata remain closed day and night, yet diurnal fluctuations in organic acids continue as a result of refixation of respiratory CO, (Bastide et al, 1993). CAM idling may help to preserve photosynthetic enzyme activities until favorable growth conditions return.…”