INCREASED EXPRESSION OF VACUOLAR AQUAPORIN AND H-ATPASE RELATED TO MOTOR CELL-FUNCTION IN MIMOSA-PUDICA L()

Mature motor cells of Mimosa pudica that exhibit large and rapid turgo r variations in response to external stimuli are characterized by two distinct types of vacuoles, one containing large amounts of tannins (t annin vacuole) and one without tannins (colloidal or aqueous vacuole). In these highly specialized cells we measured the abundance of two to noplast proteins, a putative water-channel protein (aquaporin belongin g to the gamma-TIPs [tonoplast intrinsic proteins]) and the catalytic A-subunit of H+-ATPase, using either high-pressure freezing or chemica l fixation and immunolocalization. gamma-TIP aquaporin was detected al most exclusively in the tonoplast of the colloidal vacuole, and the H-ATPase was also mainly localized in the membrane of the same vacuole. Cortex cells of young pulvini cannot change shape rapidly. Developmen t of the pulvinus into a motor organ was accompanied by a more than 3- fold increase per length unit of membrane in the abundance of both aqu aporin and H+-ATPase cross-reacting protein. These results indicate th at facilitated water fluxes across the vacuolar membrane and energizat ion of the vacuole play a central role in these motor cells.