Role of Aquaporins in a Composite Model of Water Transport in the Leaf

Leaf hydraulic maze: Abscisic acid effects on bundle sheath, palisade, and spongy mesophyll conductance

Leaf hydraulic conductance (Kleaf) facilitates the supply of water, enabling continual CO2 uptake while maintaining plant water status. We hypothesized that bundle sheath and mesophyll cells play key roles in regulating the radial flow of water out of the xylem by responding to abscisic acid (ABA). Thus, we generated transgenic Arabidopsis thaliana plants that are insensitive to ABA in their bundle sheath (BSabi) and mesophyll (MCabi) cells. We also introduced tissue-specific fluorescent markers to distinguish between cells of the palisade mesophyll, spongy mesophyll, and bundle sheath. Both BSabi and MCabi plants showed greater Kleaf and transpiration under optimal conditions. MCabi plants had larger stomatal apertures, higher stomatal index, and greater vascular diameter and biomass relative to the wild-type (WT) and BSabi plants. In response to xylem-fed ABA, both transgenic and WT plants reduced their Kleaf and transpiration. The membrane osmotic water permeability (Pf) of the WT's spongy mesophyll was higher than that of the WT's palisade mesophyll. While the palisade mesophyll maintained a low Pf in response to high ABA, the spongy mesophyll Pf was reduced. Compared to the WT, BSabi bundle sheath cells had a higher Pf, but MCabi spongy mesophyll had an unexpected lower Pf. These results suggest that tissue-specific regulation of Pf by ABA may be confounded by whole-leaf hydraulics and transpiration. ABA increased the symplastic permeability, but its contribution to Kleaf was negligible. We suggest that the bundle sheath spongy mesophyll pathway dynamically responds to the fluctuations in water availability, while the palisade mesophyll serves as a hydraulic buffer.

Transcriptome, Biochemical and Phenotypic Analysis of the Effects of a Precision Engineered Biostimulant for Inducing Salinity Stress Tolerance in Tomato

Salinity stress is a major problem affecting plant growth and crop productivity. While plant biostimulants have been reported to be an effective solution to tackle salinity stress in different crops, the key genes and metabolic pathways involved in these tolerance processes remain unclear. This study focused on integrating phenotypic, physiological, biochemical and transcriptome data obtained from different tissues of Solanum lycopersicum L. plants (cv. Micro-Tom) subjected to a saline irrigation water program for 61 days (EC: 5.8 dS/m) and treated with a combination of protein hydrolysate and Ascophyllum nodosum-derived biostimulant, namely PSI-475. The biostimulant application was associated with the maintenance of higher K⁺/Na⁺ ratios in both young leaf and root tissue and the overexpression of transporter genes related to ion homeostasis (e.g., NHX4, HKT1;2). A more efficient osmotic adjustment was characterized by a significant increase in relative water content (RWC), which most likely was associated with osmolyte accumulation and upregulation of genes related to aquaporins (e.g., PIP2.1, TIP2.1). A higher content of photosynthetic pigments (+19.8% to +27.5%), increased expression of genes involved in photosynthetic efficiency and chlorophyll biosynthesis (e.g., LHC, PORC) and enhanced primary carbon and nitrogen metabolic mechanisms were observed, leading to a higher fruit yield and fruit number (47.5% and 32.5%, respectively). Overall, it can be concluded that the precision engineered PSI-475 biostimulant can provide long-term protective effects on salinity stressed tomato plants through a well-defined mode of action in different plant tissues.

Expression patterns of maize PIP aquaporins in middle or upper leaves correlate with their different physiological responses to drought and mycorrhiza

Here we report the effect of Rhizophagus irregularis on maize leaf expression of six plasma membrane aquaporin isoforms from PIP1 and PIP2 subfamilies under severe drought development and recovery. The novelty of our study is the finding that leaf-specific mycorrhizal regulation of aquaporins is dependent on the position of the leaf on the shoot and changes in parallel with the rate of photosynthesis and the stomatal response to drought. The transcripts were isolated from the upper third (L3) or ear (L5) leaf, which differed greatly in physiological response to stress within each symbiotic variant. Aquaporins expression in upper L3 leaves appeared to be largely not sensitive to drought, regardless of symbiotic status. In contrast, L5 leaf of non-mycorrhizal plants, showed strong down-regulation of all PIPs. Mycorrhiza, however, protected L5 leaf from such limitation, which under maximal stress was manifested by 6-fold and circa 4-fold higher transcripts level for PIP1s and PIP2s, respectively. Distinct expression patterns of L3 and L5 leaves corresponded to differences in key parameters of leaf homeostasis - stomatal conductance, photosynthetic rates, and accumulation of ABA and SA as phytohormonal indicators of drought stress. In result symbiotic plants showed faster restoration of photosynthetic capability, regardless of leaf position, which we recognize as the hallmark of better stress tolerance. In summary, arbuscular mycorrhiza alleviates short-term drought effects on maize by preventing the down-regulation of plasma membrane aquaporins within middle leaves, thereby affecting stomatal conductance.

The Role of Aquaporins in Plant Growth under Conditions of Oxygen Deficiency

Plants frequently experience hypoxia due to flooding caused by intensive rainfall or irrigation, when they are partially or completely submerged under a layer of water. In the latter case, some resistant plants implement a hypoxia avoidance strategy by accelerating shoot elongation, which allows lifting their leaves above the water surface. This strategy is achieved due to increased water uptake by shoot cells through water channels (aquaporins, AQPs). It remains a puzzle how an increased flow of water through aquaporins into the cells of submerged shoots can be achieved, while it is well known that hypoxia inhibits the activity of aquaporins. In this review, we summarize the literature data on the mechanisms that are likely to compensate for the decline in aquaporin activity under hypoxic conditions, providing increased water entry into cells and accelerated shoot elongation. These mechanisms include changes in the expression of genes encoding aquaporins, as well as processes that occur at the post-transcriptional level. We also discuss the involvement of hormones, whose concentration changes in submerged plants, in the control of aquaporin activity.

Root hydraulics adjustment is governed by a dominant cell-to-cell pathway in Beta vulgaris seedlings exposed to salt stress

Soil salinity reduces root hydraulic conductivity (L_pr) of several plant species. However, how cellular signaling and root hydraulic properties are linked in plants that can cope with water restriction remains unclear. In this work, we exposed the halotolerant species red beet (Beta vulgaris) to increasing concentrations of NaCl to determine the components that might be critical to sustaining the capacity to adjust root hydraulics. Our strategy was to use both hydraulic and cellular approaches in hydroponically grown seedlings during the first osmotic phase of salt stress. Interestingly, L_pr presented a bimodal profile response apart from the magnitude of the imposed salt stress. As well as L_pr, the PIP2-aquaporin profile follows an unphosphorylated/phosphorylated pattern when increasing NaCl concentration while PIP1 aquaporins remain constant. L_pr also shows high sensitivity to cycloheximide. In low NaCl concentrations, L_pr was high and 70 % of its capacity could be attributed to the CHX-inhibited cell-to-cell pathway. More interestingly, roots can maintain a constant spontaneous exudated flow that is independent of the applied NaCl concentration. In conclusion, Beta vulgaris root hydraulic adjustment completely lies in a dominant cell-to-cell pathway that contributes to satisfying plant water demands.

Bio-chemo-electro-mechanical modelling of the rapid movement of Mimosa pudica

A remarkable feature of Mimosa pudica is its ability to deform in response to certain external stimuli. Here, a two-dimensional transient bio-chemo-electro-mechanical model of the rapid movement of the main pulvinus of Mimosa pudica is developed. Based on the laws of mass and momentum conservation, poroelasticity, and representative volume elements, a novel fluid pressure equation is proposed to characterize the cell elasticity. Experiments were conducted to measure the time and amplitude of the rapid movement. After examinations with the published experiments, it is confirmed that the model can predict well the ionic concentrations, petiole bending angle, and membrane potential. The simulation analysis of the biophysical properties provides insights to biomechanics: the hydrostatic pressure in the lowest extensor decreases from 0.35 to 0.05 MPa at t = 0.00 to 3.00 s; fluid pressure increases from 0.00 to 0.11 MPa at t = 0.00 to 0.14 s; and the peak bending angle increases from 57.0° to 70.9° when the reflection coefficient is assigned as 0.10 to 0.20 in the model. The results highlight the biochemical actuation mechanism of the Mimosa pudica movement, and they confirm the importance of ionic and water transports for causing changes in osmotic and hydrostatic pressures.

Phenotypic plasticity and genetic adaptation of functional traits influences intra-specific variation in hydraulic efficiency and safety

Understanding which hydraulic traits are under genetic control and/or are phenotypically plastic is essential in understanding how tree species will respond to rapid shifts in climate. We quantified hydraulic traits in Eucalyptus obliqua L'Her. across a precipitation gradient in the field to describe (i) trait variation in relation to long-term climate and (ii) the short-term (seasonal) ability of traits to adjust (i.e., phenotypic plasticity). Seedlings from each field population were raised under controlled conditions to assess (iii) which traits are under strong genetic control. In the field, drier populations had smaller leaves with anatomically thicker xylem vessel walls, a lower leaf hydraulic vulnerability and a lower water potential at turgor loss point, which likely confers higher hydraulic safety. Traits such as the water potential at turgor loss point and ratio of sapwood to leaf area (Huber value) showed significant adjustment from wet to dry conditions in the field, indicating phenotypic plasticity and importantly, the ability to increase hydraulic safety in the short term. In the nursery, seedlings from drier populations had smaller leaves and a lower leaf hydraulic vulnerability, suggesting that key traits associated with hydraulic safety are under strong genetic control. Overall, our study suggests a strong genetic control over traits associated with hydraulic safety, which may compromise the survival of wet-origin populations in drier future climates. However, phenotypic plasticity in physiological and morphological traits may confer sufficient hydraulic safety to facilitate genetic adaptation.

To Produce or to Survive: How Plastic Is Your Crop Stress Physiology?

Abiotic stress causes major crop losses and is considered a greater challenge than biotic stress. Comparisons of the number of published articles and patents regarding these different types of stresses, and the number of commercially released crops designed to tolerate different types of stresses, revealed a huge gap in the bench-to-field transfer rate of abiotic stress-tolerant crops, as compared to crops designed to tolerate biotic stress. These differences underscore the complexity of abiotic stress-response mechanisms. Here, we suggest that breeding programs favoring yield-related quantitative physiological traits (QPTs; e.g., photosynthesis rate or stomatal conductance) have canalized those QPTs at their highest levels. This has affected the sensitivity of those QPTs to changing environmental conditions and those traits have become less plastic. We also suggest that breeding pressure has had an asymmetric impact on different QPTs, depending on their sensitivity to environmental conditions and their interactions with other QPTs. We demonstrate this asymmetric impact on the regulation of whole-plant water balance, showing how plastic membrane water content, stomatal conductance and leaf hydraulic conductance interact to canalize whole-organ water content. We suggest that a QPT's plasticity is itself an important trait and that understanding this plasticity may help us to develop yield-optimized crops.

Coping with gravity: the foliar water relations of giant sequoia

In tall trees, the mechanisms by which foliage maintains sufficient turgor pressure and water content against height-related constraints remain poorly understood. Pressure-volume curves generated from leafy shoots collected crown-wide from 12 large Sequoiadendron giganteum (Lindley) J. Buchholz (giant sequoia) trees provided mechanistic insights into how the components of water potential vary with height in tree and over time. The turgor loss point (TLP) decreased with height at a rate indistinguishable from the gravitational potential gradient and was controlled by changes in tissue osmotica. For all measured shoots, total relative water content at the TLP remained above 75%. This high value has been suggested to help leaves avoid precipitous declines in leaf-level physiological function, and in giant sequoia was controlled by both tissue elasticity and the balance of water between apoplasm and symplasm. Hydraulic capacitance decreased only slightly with height, but importantly this parameter was nearly double in value to that reported for other tree species. Total water storage capacity also decreased with height, but this trend essentially disappeared when considering only water available within the typical range of water potentials experienced by giant sequoia. From summer to fall measurement periods we did not observe osmotic adjustment that would depress the TLP. Instead we observed a proportional shift of water into less mobile apoplastic compartments leading to a reduction in hydraulic capacitance. This collection of foliar traits allows giant sequoia to routinely, but safely, operate close to its TLP, and suggests that gravity plays a major role in the water relations of Earth's largest tree species.

Pollen Aquaporins: The Solute Factor

In the recent years, the biophysical properties and presumed physiological role of aquaporins (AQPs) have been expanded to specialized cells where water and solute exchange are crucial traits. Complex but unique processes such as stomatal movement or pollen hydration and germination have been addressed not only by identifying the specific AQP involved but also by studying how these proteins integrate and coordinate cellular activities and functions. In this review, we referred specifically to pollen-specific AQPs and analyzed what has been assumed in terms of transport properties and what has been found in terms of their physiological role. Unlike that in many other cells, the AQP machinery in mature pollen lacks plasma membrane intrinsic proteins, which are extensively studied for their high water capacity exchange. Instead, a variety of TIPs and NIPs are expressed in pollen. These findings have altered the initial understanding of AQPs and water exchange to consider specific and diverse solutes that might be critical to sustaining pollen's success. The spatial and temporal distribution of the pollen AQPs also reflects a regulatory mechanism that allowing a properly adjusting water and solute exchange.