Brassinolide may control aquaporin activities in Arabidopsis thaliana

It is usually assumed that aquaporins present in the cellular membranes could be an important route in the control of water flux in plants, but evidence for this hypothesis is scarce. In this paper, we report measurements of the osmotic permeability (P(os) of protoplasts isolated from hypocotyls of wild-type and mutant Arabidopsis thaliana (L.) Heynh. Mutants were affected in their growth and exhibited different sensitivities to the phytohormone, brassinolide. For the two mutants studied (cpd: constitutive photomorphogenesis and dwarfism; bril: brassinosteroid insensitive), hypocotyl length was correlated to P(os) for the protoplasts. Under experimental conditions where hypocotyl growth had ceased, restoration of root, hypocotyl and petiole growth by brassinolide was correlated with an increase in P(os) of the hypocotyl protoplasts. We consider that the increase in Pos of the hypocotyl cells was needed because these cells were part of the transcellular water pathway of the plant. This is the first time, to our knowledge, that brassinolide has been shown to be involved in the modification of the water-transport properties of cell membranes. Our results also emphasize the importance of aquaporins and the transcellular pathway in water transport under normal growth conditions.