COS 33-4
Down-regulation of PIP1 aquaporins interferes with plant’s ability to recover from drought-induced embolism and reduced stomatal conductance
Plants are in danger of embolism formation in xylem vessels when the balance between water transport capacity and transpirational demand is compromised. However, as they are exposed to variable micro-environment under drought stress conditions, they might experience short time periods when evolved safety measures to maintain this balance inadequately protect them from embolism formation and reduction of stomatal conductance. While embolism formation is a purely physical process, embolism removal requires that empty vessels fill with water against the existing energy gradients under reduced transpirational demand (low stomatal conductance). The recovery process requires movement of water across cellular membranes, either in xylem parenchyma cells or in leaf epidermis, and it may involve plasma intrinsic proteins (PIP, aquaporin) as facilitators of the water transport. Thus, the role of PIPs must be considered when investigating how plants recover from embolism formation. Using a reverse genetic approach several lines of transgenic Populus tremula x albatrees were generated with significant down regulation of expression PIP1 subfamily.
Results/Conclusions
While no morphological phenotypes were found in transformed trees, physiology of response to water stress was significantly affected by down regulation of PIP1 genes. Antisense plants were ‘apparently’ less tolerant to embolism formation. This effect most likely resulted from reduced capacity of vessels to refill, while embolism formation was most likely unaffected. Interestingly, down-regulated PIP1 trees operated at a narrower safety margin due to their impaired capacity to control stomatal conductance (gs) in response to drought. A significant delay in gs recovery during the stress relief period was also observed. Stomatal behavior could be attributed to either damage in the photosynthetic system and/or to different abscissic acid content (ABA). Down regulation of PIPs can indeed reduce the ABA concentration and consequently delay stomatal closure in response to drought stress. Thus, delayed gs recovery suggests that the regulation of stomatal conductance in transgenic lines depends on factors beyond water supply via the xylem and stem water pressure. Our results confirm i) the role of PIP1 genes in maintaining stem hydraulic capacity via an adequate safety margin that protects the plant from variable micro-environment under stress conditions and ii) their contribution to the hydraulic recovery from embolism under water stress.