Aquaporins (AQP) are channel proteins belonging to the Major Intrinsic Protein (MIP) superfamily that play an important role in plant water relations. The main role of aquaporins in plants is transport of water and other small neutral molecules across cellular biological membranes. AQPs have remarkable features to provide an efficient and often, specific water flow and enable them to transport water into and out of the cells along the water potential gradient. Plant AQPs are classified into five main subfamilies including the plasma membrane intrinsic proteins (PIPs), tonoplast intrinsic proteins (TIPs), nodulin 26 like intrinsic proteins (NIPs), small basic intrinsic proteins (SIPs) and X intrinsic proteins (XIPs). AQPs are localized in the cell membranes and are found in all living cells. However, most of the AQPs that have been described in plants are localized to the tonoplast and plasma membranes. Regulation of AQP activity and gene expression, are also considered as a part of the adaptation mechanisms to stress conditions and rely on complex processes and signaling pathways as well as complex transcriptional, translational and posttranscriptional factors. Gating of AQPs through different mechanisms, such as phosphorylation, tetramerization, pH, cations, reactive oxygen species, phytohormones and other chemical agents, may play a key role in plant responses to environmental stresses by maintaining the uptake and movement of water in the plant body.

HgCl₂ (0.1 mm) reduced pressure-induced water flux and root hydraulic conductivity in the roots of 1-year-old aspen (Populus tremuloides Michx.) seedlings by about 50%. The inhibition was reversed with 50 mm mercaptoethanol. Mercurial treatment reduced the activation energy of water transport in the roots from 10.82 ± 0.700 kcal mol⁻¹ to 6.67 ± 0.193 kcal mol⁻¹ when measured over the 4°C to 25°C temperature range. An increase in rhodamine B concentration in the xylem sap of mercury-treated roots suggested a decrease in the symplastic transport of water. However, the apoplastic pathway in both control and mercury-treated roots constituted only a small fraction of the total root water transport. Electrical conductivity and osmotic potentials of the expressed xylem sap suggested that 0.1 mmHgCl₂ and temperature changes over the 4°C to 25°C range did not induce cell membrane leakage. The 0.1 mmHgCl₂ solution applied as a root drench severely reduced stomatal conductance in intact plants, and this reduction was partly reversed by 50 mm mercaptoethanol. In excised shoots, 0.1 mm HgCl₂ did not affect stomatal conductance, suggesting that the signal that triggered stomatal closure originated in the roots. We suggest that mercury-sensitive processes in aspen roots play a significant role in regulating plant water balance by their effects on root hydraulic conductivity.

The effects of low root temperature on growth and root cell water transport were compared between wild-type Arabidopsis (Arabidopsis thaliana) and plants overexpressing plasma membrane intrinsic protein 1;4 (PIP1;4) and PIP2;5. Descending root temperature from 25°C to 10°C quickly reduced cell hydraulic conductivity (L_p) in wild-type plants but did not affect L_p in plants overexpressing PIP1;4 and PIP2;5. Similarly, when the roots of wild-type plants were exposed to 10°C for 1 d, L_p was lower compared with 25°C. However, there was no effect of low root temperature on L_p in PIP1;4- and PIP2;5-overexpressing plants after 1 d of treatment. When the roots were exposed to 10°C for 5 d, L_p was reduced in wild-type plants and in plants overexpressing PIP1;4, whereas there was still no effect in PIP2;5-overexpressing plants. These results suggest that the gating mechanism in PIP1;4 may be more sensitive to prolonged low temperature compared with PIP2;5. The reduction of L_p at 10°C in roots of wild-type plants was partly restored to the preexposure level by 5 mm Ca(NO₃)₂ and protein phosphatase inhibitors (75 nm okadaic acid or 1 μm Na₃VO₄), suggesting that aquaporin phosphorylation/dephosphorylation processes were involved in this response. The temperature sensitivity of cell water transport in roots was reflected by a reduction in shoot and root growth rates in the wild-type and PIP1;4-overexpressing plants exposed to 10°C root temperature for 5 d. However, low root temperature had no effect on growth in plants overexpressing PIP2;5. These results provide strong evidence for a link between growth at low root temperature and aquaporin-mediated root water transport in Arabidopsis.

Aquaporins are membrane integral proteins responsible for the transmembrane transport of water and other small neutral molecules. Despite their well-acknowledged importance in water transport, their significance in gas transport processes remains unclear. Growing evidence points to the involvement of plant aquaporins in CO₂ delivery for photosynthesis. The role of these channel proteins in the transport of O₂ and other gases may also be more important than previously envisioned. In this study, we examined O₂ permeability of various human, plant, and fungal aquaporins by co-expressing heterologous aquaporin and myoglobin in yeast. Two of the most promising O₂-transporters (Homo sapiens AQP1 and Nicotiana tabacum PIP1;3) were confirmed to facilitate O₂ transport in the spectrophotometric assay using yeast protoplasts. The over-expression of NtPIP1;3 in yeasts significantly increased their O₂ uptake rates in suspension culture. In N. tabacum roots subjected to hypoxic hydroponic conditions, the transcript levels of the O₂-transporting aquaporin NtPIP1;3 significantly increased after the seven-day hypoxia treatment, which was accompanied by the increase of ATP levels in the apical root segments. Our results suggest that the functional significance of aquaporin-mediated O₂ transport and the possibility of controlling the rate of transmembrane O₂ transport should be further explored.

Low root temperatures significantly reduced root hydraulic conductivity and increased resistance to water flow through the roots of aspen (Populus tremuloides Michx.) seedlings. Increased resistance to water flow could not be fully explained by the corresponding increase in water viscosity at low temperatures. The shapes of Arrhenius plots of root water flow and the activation energies were dependent on the direction, sequence and extent of temperature change. The Arrhenius plots suggested that the effect of low root temperature on root water flow was mediated by an effect on root metabolism. The low root temperatures tested did not induce root electrolyte leakage normally associated with cell membrane injury. Although a decrease in root temperatures to 7 or 4 °C induced a reduction in stomatal conductance, this reduction lagged the decline in root water flow by several hours. In contrast, when soil temperatures were raised from 4 or 7 °C to 25 °C, root water flow presumably increased, and stomatal conductance responded rapidly and was temporarily higher than before the cold treatment was imposed.