Crassulacean acid metabolism guard cell anion channel activity follows transcript abundance and is suppressed by apoplastic malate.

Plants utilising crassulacean acid metabolism (CAM) concentrate CO around RuBisCO while reducing transpirational water loss associated with photosynthesis. Unlike stomata of C and C species, CAM stomata open at night for the mesophyll to fix CO into malate (Mal) and store it in the vacuole. CAM plants decarboxylate Mal in the light, generating high CO concentrations within the leaf behind closed stomata for refixation by RuBisCO. CO may contribute to stomatal closure but additional mechanisms, plausibly including Mal activation of anion channels, ensure closure in the light. In the CAM species Kalanchoë fedtschenkoi, we found that guard cell anion channel activity, recorded under voltage clamp, follows KfSLAC1 and KfALMT12 transcript abundance, declining to near zero by the end of the light period. Unexpectedly, however, we found that extracellular Mal inhibited the anion current of Kalanchoë guard cells, both in wild-type and RNAi mutants with impaired Mal metabolism. We conclude that the diurnal cycle of anion channel gene transcription, rather than the physiological signal of Mal release, is a key factor in the inverted CAM stomatal cycle.