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Increasing stomata movement is beneficial to improve plant water use efficiency and drought resilience. Contradictory results indicate that aquaporins might regulate stomatal movement. Here, we tested whether the maize plasma membrane PIP2;5 aquaporin affects stomatal closure under water deficit, abscisic acid (ABA) or vapor pressure deficit (VPD) treatment in intact plants, detached leaves, or peeled epidermis. Transpiration, stomatal conductance (gs) and aperture, and reactive oxygen species (ROS) in stomatal complexes were studied in maize lines with increased or knocked down (KD) PIP2;5 gene expression. In well-watered conditions, the PIP2;5 overexpressing (OE) plants transpired more than wild types (WT), while no significant difference in transpiration was observed between pip2;5 KD and WT. Upon mild-water deficit or low ABA concentration treatments, transpiration and gs decreased more in PIP2;5 OE, and less in pip2;5 KD lines, in comparison with WTs. In detached epidermis, ABA treatment induced faster stomatal closing in PIP2;5 OE lines compared to WT, while pip2;5 KD stomata were ABA insensitive. These phenotypes were associated with guard cell ROS accumulation. Additionally, PIP2;5 is involved in the transpiration decrease observed under high VPD. These data indicate that maize PIP2;5 is a key actor increasing the sensitivity of stomatal closure to water deficit

The plasma membrane intrinsic protein PIP2;5 is the most highly expressed aquaporin in maize (Zea mays) roots. Here, we investigated how deregulation of PIP2;5 expression affects water relations and growth using maize overexpression (OE; B104 inbred) or knockout (KO; W22 inbred) lines. The hydraulic conductivity of the cortex cells of roots grown hydroponically was higher in PIP2;5 OE and lower in pip2;5 KO lines compared with the corresponding wild-type plants. While whole-root conductivity decreased in the KO lines compared to the wild type, no difference was observed in OE plants. This paradox was interpreted using the MECHA hydraulic model, which computes the radial flow of water within root sections. The model hints that the plasma membrane permeability of the cells is not radially uniform but that PIP2;5 may be saturated in cell layers with apoplastic barriers, i.e. the endodermis and exodermis, suggesting the presence of posttranslational mechanisms controlling the abundance of PIP in the plasma membrane in these cells. At the leaf level, where the PIP2;5 gene is weakly expressed in wildtype plants, the hydraulic conductance was higher in the PIP2;5 OE lines compared with the wild-type plants, whereas no difference was observed in the pip2;5 KO lines. The temporal trend of leaf elongation rate, used as a proxy for that of xylem water potential, was faster in PIP2;5 OE plants upon mild stress, but not in well-watered conditions, demonstrating that PIP2;5 may play a beneficial role in plant growth under specific conditions.

Aquaporins are channel proteins that facilitate the diffusion of water and small uncharged solutes across cellular membranes. Plant aquaporins form a large family of highly divergent proteins that are involved in many different physiological processes. This book will summarize the recent advances regarding plant aquaporins, their phylogeny, structure, substrate specificity, mechanisms of regulation and roles in various important physiological processes related to the control of water flow and small solute distribution at the cell, tissue and plant level in an ever-changing environment.

Plant aquaporins are highly regulated water channels controlling the water flow across cell membranes. The quantification of the expression of plasma membrane aquaporins (PIPs) in maize roots, leaves, stomatal complexes and reproductive organs showed that the expression was dependent on the organ developmental stage and environmental conditions. This work aimed at investigating the physiological roles of PIP aquaporins using a transgenic approach. We focused our attention on PIP isoforms known to be highly or specifically expressed in some tissues. ZmPIP2;5 is one of most highly expressed aquaporins in roots and its association with apoplastic barriers suggests a role in the radial water movement. ZmPIP1;6 is almost exclusively expressed in stomatal complexes following a strict diurnal pattern and could have a role in stomatal movements in response to environmental stimuli. After transformation and selection of the maize transgenic lines overexpressing these isoforms, physiological parameters were investigated in hydroponic culture and compared to wild-type plants. Interestingly, while no differences of gas exchange were found in normal conditions or upon osmotic stress, both ZmPIP1;6 and ZmPIP2;5 overexpressing lines showed improved root hydraulic properties. The growth of these plants in water deficit conditions was analyzed in phenotyping platforms and revealed different responses to evaporative demand. ZmPIP2;5 overexpressing lines showed a better leaf growth rate under severe water stress. These data demonstrate the role of PIP aquaporins in fine-tuning the hydraulic properties of the plant to control its development and physiology in changing environmental conditions., (SC - Sciences) -- UCL, 2017

Plasma membrane intrinsic proteins (PIPs) are involved in the adjustment of plant water balance in response to changing environmental conditions. In this study, Arabidopsis wild-type (Col-0) and transgenic lines overexpressing PIP2;7 were used to investigate and compare their response to salt stress. Hydraulic conductivity measurements using a high-pressure flowmeter (HPFM) revealed that overexpression of PIP2;7 induced a sixfold increase in root hydraulic conductivity of four week-old Arabidopsis thaliana plants compared to WT. Exposure to a high salt stress (150 mM NaCl) triggered a rapid repression of overall aquaporin activity in both genotypes. Response to salt stress was also investigated in 8 day-old seedlings. Exposure to salt led to a repression of PIP2;7 promoter activity and a significant decrease in PIP2;7 mRNA abundance within 2 h. Concomitantly, a rapid internalization of fluorescently-tagged PIP2;7 proteins was observed but removal from the cell membrane was not accompanied by further degradation of the protein within 4 h of exposure to salinity stress. These data suggest that PIP transcriptional repression and channel internalization act in concert during salt stress conditions to modulate aquaporin activity, thereby significantly altering the plant hydraulic parameters in the short term.