Day/night regulation of aquaporins during the CAM cycle in Mesembryanthemum crystallinum

Abstract

Mesembryanthemum crystallinum exhibits induction of Crassulacean acid metabolism (CAM) after a threshold stage of development, by exposure to long days with high light intensities or by water and salt stress. During the CAM cycle, fluctuations in carbon partitioning within the cell lead to transient drops in osmotic potential, which are likely stabilized/balanced by passive movement of water via aquaporins (AQPs). Protoplast swelling assays were used to detect changes in water permeability during the day/night cycle of CAM. To assess the role of AQPs during the same period, we followed transcript accumulation and protein abundance of four plasma membrane intrinsic proteins (PIPs) and one tonoplast intrinsic protein (TIP). CAM plants showed a persistent rhythm of specific AQP protein abundance changes throughout the day/night cycle, including changes in amount of McPIP2;1, McTIP1;2, McPIP1;4 and McPIP1;5, while the abundance of McPIP1;2 was unchanged. These protein changes did not appear to be coordinated with transcript levels for any of the AQPs analysed; however, they did occur in parrallel to alterations in water permeability, as well as variations in cell osmolarity, pinitol, glucose, fructose and phosphoenolpyruvate carboxylase (PEPc) levels measured throughout the day/night CAM cycle. Results suggest a role for AQPs in maintaining water balance during CAM and highlight the complexity of protein expression during the CAM cycle. During the CAM cycle, fluctuations in cellular carbon partitioning lead to transient drops in osmotic potential which are likely stabilized/balanced by passive movement of water via aquaporins. In M. crystallinum CAM plants measurements of protoplast swelling rates indicate a persistent diurnal rhythm of water permeability changes that are closely correlated to changes in abundance of specific PIP and TIP aquaporin proteins throughout the day/night cycle. This behaviour was linked to CAM-related changes in cell osmolarity, pH, levels of inositol, glucose, and PEPc, photosynthetic rate, CO2 uptake and stomatal conductance. Our results suggest a role for aquaporins in maintaining water balance during CAM and highlight the complexity of the circadian clock orquestration of protein expression. © 2011 Blackwell Publishing Ltd.