Involvement of root ABA and hydraulic conductivity in the control of water relations in wheat plants exposed to increased evaporative demand

Precision phenotyping of a barley diversity set reveals distinct drought response strategies

Plants exhibit an array of drought responses and adaptations, where the trade-off between water loss and CO2 uptake for growth is mediated by regulation of stomatal aperture in response to soil water content (SWC), among other factors. For crop yield stability, the question is how drought timing and response patterns relate to post-drought growth resilience and vigor. We earlier identified, in a few reference varieties of barley that differed by the SWC at which transpiration was curtailed, two divergent water use strategies: water-saving ("isohydric") and water-spending ("anisohydric"). We proposed that an isohydric strategy may reduce risk from spring droughts in climates where the probability of precipitation increases during the growing season, whereas the anisohydric is consistent with environments having terminal droughts, or with those where dry periods are short and not seasonally progressive. Here, we have examined drought response physiology in an 81-line barley (Hordeum vulgare L.) diversity set that spans 20th century European breeding and identified several lines with a third, dynamic strategy. We found a strong positive correlation between vigor and transpiration, the dynamic group being highest for both. However, these lines curtailed daily transpiration at a higher SWC than the isohydric group. While the dynamic lines, particularly cv Hydrogen and Baronesse, were not the most resilient in terms of restoring initial growth rates, their strong initial vigor and high return to initial transpiration rates meant that their growth nevertheless surpassed more resilient lines during recovery from drought. The results will be of use for defining barley physiological ideotypes suited to future climate scenarios.

The Long-Distance Transport of Some Plant Hormones and Possible Involvement of Lipid-Binding and Transfer Proteins in Hormonal Transport

Adaptation to changes in the environment depends, in part, on signaling between plant organs to integrate adaptive response at the level of the whole organism. Changes in the delivery of hormones from one organ to another through the vascular system strongly suggest that hormone transport is involved in the transmission of signals over long distances. However, there is evidence that, alternatively, systemic responses may be brought about by other kinds of signals (e.g., hydraulic or electrical) capable of inducing changes in hormone metabolism in distant organs. Long-distance transport of hormones is therefore a matter of debate. This review summarizes arguments for and against the involvement of the long-distance transport of cytokinins in signaling mineral nutrient availability from roots to the shoot. It also assesses the evidence for the role of abscisic acid (ABA) and jasmonates in long-distance signaling of water deficiency and the possibility that Lipid-Binding and Transfer Proteins (LBTPs) facilitate the long-distance transport of hormones. It is assumed that proteins of this type raise the solubility of hydrophobic substances such as ABA and jasmonates in hydrophilic spaces, thereby enabling their movement in solution throughout the plant. This review collates evidence that LBTPs bind to cytokinins, ABA, and jasmonates and that cytokinins, ABA, and LBTPs are present in xylem and phloem sap and co-localize at sites of loading into vascular tissues and at sites of unloading from the phloem. The available evidence indicates a functional interaction between LBTPs and these hormones.

Plasma membrane aquaporins regulate root hydraulic conductivity in the model plant Setaria viridis

The high rate of productivity observed in panicoid crops is in part due to their extensive root system. Recently, green foxtail (Setaria viridis) has emerged as a genetic model system for panicoid grasses. Natural accessions of S. viridis originating from different parts of the world, with differential leaf physiological behavior, have been identified. This work focused on understanding the physiological and molecular mechanisms controlling root hydraulic conductivity and root-to-shoot gas exchange signaling in S. viridis. We identified 2 accessions, SHA and ZHA, with contrasting behavior at the leaf, root, and whole-plant levels. Our results indicated a role for root aquaporin (AQP) plasma membrane (PM) intrinsic proteins in the differential behavior of SHA and ZHA. Moreover, a different root hydraulic response to low levels of abscisic acid between SHA and ZHA was observed, which was associated with root AQPs. Using cell imaging, biochemical, and reverse genetic approaches, we identified PM intrinsic protein 1;6 (PIP1;6) as a possible PIP1 candidate that regulates radial root hydraulics and root-to-shoot signaling of gas exchange in S. viridis. In heterologous systems, PIP1;6 localized in the endoplasmic reticulum, and upon interaction with PIP2s, relocalization to the PM was observed. PIP1;6 was predominantly expressed at the root endodermis. Generation of knockout PIP1;6 plants (KO-PIP1;6) in S. viridis showed altered root hydraulic conductivity, altered gas exchange, and alteration of root transcriptional patterns. Our results indicate that PIPs are essential in regulating whole-plant water homeostasis in S. viridis. We conclude that root hydraulic conductivity and gas exchange are positively associated and are regulated by AQPs.

Transpiration response to soil drying versus increasing vapor pressure deficit in crops: physical and physiological mechanisms and key plant traits

The water deficit experienced by crops is a function of atmospheric water demand (vapor pressure deficit) and soil water supply over the whole crop cycle. We summarize typical transpiration response patterns to soil and atmospheric drying and the sensitivity to plant hydraulic traits. We explain the transpiration response patterns using a soil-plant hydraulic framework. In both cases of drying, stomatal closure is triggered by limitations in soil-plant hydraulic conductance. However, traits impacting the transpiration response differ between the two drying processes and act at different time scales. A low plant hydraulic conductance triggers an earlier restriction in transpiration during increasing vapor pressure deficit. During soil drying, the impact of the plant hydraulic conductance is less obvious. It is rather a decrease in the belowground hydraulic conductance (related to soil hydraulic properties and root length density) that is involved in transpiration down-regulation. The transpiration response to increasing vapor pressure deficit has a daily time scale. In the case of soil drying, it acts on a seasonal scale. Varieties that are conservative in water use on a daily scale may not be conservative over longer time scales (e.g. during soil drying). This potential independence of strategies needs to be considered in environment-specific breeding for yield-based drought tolerance.

Influence of Plant Growth-Promoting Rhizobacteria on the Formation of Apoplastic Barriers and Uptake of Water and Potassium by Wheat Plants

The formation of apoplastic barriers is important for controlling the uptake of water and ions by plants, thereby influencing plant growth. However, the effects of plant growth-promoting bacteria on the formation of apoplastic barriers, and the relationship between these effects and the ability of bacteria to influence the content of hormones in plants, have not been sufficiently studied. The content of cytokinins, auxins and potassium, characteristics of water relations, deposition of lignin and suberin and the formation of Casparian bands in the root endodermis of durum wheat (Triticum durum Desf.) plants were evaluated after the introduction of the cytokinin-producing bacterium Bacillus subtilis IB-22 or the auxin-producing bacterium Pseudomonas mandelii IB-Ki14 into their rhizosphere. The experiments were carried out in laboratory conditions in pots with agrochernozem at an optimal level of illumination and watering. Both strains increased shoot biomass, leaf area and chlorophyll content in leaves. Bacteria enhanced the formation of apoplastic barriers, which were most pronounced when plants were treated with P. mandelii IB-Ki14. At the same time, P. mandelii IB-Ki14 caused no decrease in the hydraulic conductivity, while inoculation with B. subtilis IB-22, increased hydraulic conductivity. Cell wall lignification reduced the potassium content in the roots, but did not affect its content in the shoots of plants inoculated with P. mandelii IB-Ki14. Inoculation with B. subtilis IB-22 did not change the potassium content in the roots, but increased it in the shoots.

Dual Inoculation with Rhizophagus irregularis and Bacillus megaterium Improves Maize Tolerance to Combined Drought and High Temperature Stress by Enhancing Root Hydraulics, Photosynthesis and Hormonal Responses

Climate change is leading to combined drought and high temperature stress in many areas, drastically reducing crop production, especially for high-water-consuming crops such as maize. This study aimed to determine how the co-inoculation of an arbuscular mycorrhizal (AM) fungus (Rhizophagus irregularis) and the PGPR Bacillus megaterium (Bm) alters the radial water movement and physiology in maize plants in order to cope with combined drought and high temperature stress. Thus, maize plants were kept uninoculated or inoculated with R. irregularis (AM), with B. megaterium (Bm) or with both microorganisms (AM + Bm) and subjected or not to combined drought and high temperature stress (D + T). We measured plant physiological responses, root hydraulic parameters, aquaporin gene expression and protein abundances and sap hormonal content. The results showed that dual AM + Bm inoculation was more effective against combined D + T stress than single inoculation. This was related to a synergistic enhancement of efficiency of the phytosystem II, stomatal conductance and photosynthetic activity. Moreover, dually inoculated plants maintained higher root hydraulic conductivity, which was related to regulation of the aquaporins ZmPIP1;3, ZmTIP1.1, ZmPIP2;2 and GintAQPF1 and levels of plant sap hormones. This study demonstrates the usefulness of combining beneficial soil microorganisms to improve crop productivity under the current climate-change scenario.

Exploring plant responses to abiotic stress by contrasting spectral signature changes

In this study, daily changes over a short period and diurnal progression of spectral reflectance at the leaf level were used to identify spring wheat genotypes (Triticum aestivum L.) susceptible to adverse conditions. Four genotypes were grown in pots experiments under semi-controlled conditions in Chile and Spain. Three treatments were applied: i) control (C), ii) water stress (WS), and iii) combined water and heat shock (WS+T). Spectral reflectance, gas exchange and chlorophyll fluorescence measurements were performed on flag leaves for three consecutive days at anthesis. High canopy temperature ( H _CT ) genotypes showed less variability in their mean spectral reflectance signature and chlorophyll fluorescence, which was related to weaker responses to environmental fluctuations. While low canopy temperature ( L _CT ) genotypes showed greater variability. The genotypes spectral signature changes, in accordance with environmental fluctuation, were associated with variations in their stomatal conductance under both stress conditions (WS and WS+T); L _CT genotypes showed an anisohydric response compared that of H _CT , which was isohydric. This approach could be used in breeding programs for screening a large number of genotypes through proximal or remote sensing tools and be a novel but simple way to identify groups of genotypes with contrasting performances.

Root ABA Accumulation Delays Lateral Root Emergence in Osmotically Stressed Barley Plants by Decreasing Root Primordial IAA Accumulation

Increased auxin levels in root primordia are important in controlling root branching, while their interaction with abscisic acid (ABA) likely regulates lateral root development in water-deficient plants. The role of ABA accumulation in regulating root branching was investigated using immunolocalization to detect auxin (indoleacetic acid, IAA) and ABA (abscisic acid) in root primordia of the ABA-deficient barley mutant Az34 and its parental genotype (cv. Steptoe) barley plants. Osmotic stress strongly inhibited lateral root branching in Steptoe plants, but hardly affected Az34. Root primordial cells of Steptoe plants had increased immunostaining for ABA but diminished staining for IAA. ABA did not accumulate in root primordia of the Az34, and IAA levels and distribution were unaltered. Treating Az34 plants with exogenous ABA decreased root IAA concentration, while increasing root primordial ABA accumulation and decreasing root primordial IAA concentration. Although ABA treatment of Az34 plants increased the root primordial number, it decreased the number of visible emerged lateral roots. These effects were qualitatively similar to that of osmotic stress on the number of lateral root primordia and emerged lateral roots in Steptoe. Thus ABA accumulation (and its crosstalk with auxin) in root primordia seems important in regulating lateral root branching in response to water stress.

Identification of ABF/AREB gene family in tomato (Solanum lycopersicum L.) and functional analysis of ABF/AREB in response to ABA and abiotic stresses

Abscisic acid (ABA) is a plant hormone that plays an important regulatory role in plant growth and stress response. The AREB (ABA-responsive element binding protein)/ABF (ABRE-binding factor) are important ABA-signaling components that participate in abiotic stress response. However, genome-scale analysis of ABF/AREB has not been systemically investigated in tomato. This study was conducted to identify tomato ABF/AREB family members and analyze their response to ABA and abiotic stresses. The results show that a total of 10 ABF/AREB members were identified in tomato, which are randomly distributed on five chromosomes. Domain analysis showed that these members exhibit high protein similarity, especially in the basic leucine zipper (bZIP) domain region. Subcellular localization analysis indicated that all 10 ABF/AREB members are localized in the nucleus. Phylogenetic tree analysis showed that tomato ABF/AREB genes are divided into two groups, and they are similar with the orthologs of other plants. The analysis of cis-acting elements showed that most tomato ABF/AREB genes contain a variety of hormones and stress-related elements. Expression profiles of different tissues indicated that SlABF2 and SlABF10 play an important role in fruit ripening. Finally, qRT-PCR analysis revealed that 10 tomato ABF/AREB genes respond to ABA, with SlABF3 being the most sensitive. SlABF3, SlABF5 and SlABF10 positively respond to salt and cold stresses. SlABF1, SlABF3 and SlABF10 are significantly induced under UV radiation treatment. SlABF3 and SlABF5 are significantly induced in osmotic stress. Overall, this study may provide insight into the role of tomato ABF/AREB homologues in plant response to abiotic stresses, which laid a foundation for future functional study of ABF/AREB in tomato.

The genetic basis of transpiration sensitivity to vapor pressure deficit in wheat

Genetic manipulation of whole-plant transpiration rate (TR) response to increasing atmospheric vapor pressure deficit (VPD) is a promising approach for crop adaptation to various drought regimes under current and future climates. Genotypes with a non-linear TR response to VPD are expected to achieve yield gains under terminal drought, thanks to a water conservation strategy, while those with a linear response exhibit a consumptive strategy that is more adequate for well-watered or transient-drought environments. In wheat, previous efforts indicated that TR has a genetic basis under naturally fluctuating conditions, but because TR is responsive to variation in temperature, photosynthetically active radiation, and evaporative demand, the genetic basis of its response VPD per se has never been isolated. To address this, we developed a controlled-environment gravimetric phenotyping approach where we imposed VPD regimes independent from other confounding environmental variables. We screened three nested association mapping populations totaling 150 lines, three times over a 3-year period. The resulting dataset, based on phenotyping nearly 1400 plants, enabled constructing 63-point response curves for each genotype, which were subjected to a genome-wide association study. The analysis revealed a hotspot for TR response to VPD on chromosome 5A, with SNPs explaining up to 17% of the phenotypic variance. The key SNPs were found in haploblocks that are enriched in membrane-associated genes, consistent with the hypothesized physiological determinants of the trait. These results indicate a promising potential for identifying new alleles and designing next-gen wheat cultivars that are better adapted to current and future drought regimes.

Growth-Promoting Effect of Rhizobacterium (Bacillus subtilis IB22) in Salt-Stressed Barley Depends on Abscisic Acid Accumulation in the Roots

An ABA-deficient barley mutant (Az34) and its parental cultivar (Steptoe) were compared. Plants of salt-stressed Az34 (100 mmol m⁻³ NaCl for 10 days) grown in sand were 40% smaller than those of “Steptoe”, exhibited a lower leaf relative water content and lower ABA concentrations. Rhizosphere inoculation with IB22 increased plant growth of both genotypes. IB22 inoculation raised ABA in roots of salt-stressed plants by supplying ABA exogenously and by up-regulating ABA synthesis gene HvNCED2 and down-regulating ABA catabolic gene HvCYP707A1. Inoculation partially compensated for the inherent ABA deficiency of the mutant. Transcript abundance of HvNCED2 and related HvNCED1 in the absence of inoculation was 10 times higher in roots than in shoots of both mutant and parent, indicating that ABA was mainly synthesized in roots. Under salt stress, accumulation of ABA in the roots of bacteria-treated plants was accompanied by a decline in shoot ABA suggesting bacterial inhibition of ABA transport from roots to shoots. ABA accumulation in the roots of bacteria-treated Az34 was accompanied by increased leaf hydration, the probable outcome of increased root hydraulic conductance. Thereby, we tested the hypothesis that the ability of rhizobacterium (Bacillus subtilis IB22) to modify responses of plants to salt stress depends on abscisic acid (ABA) accumulating in roots.

Involvement of Reactive Oxygen Species in ABA-Induced Increase in Hydraulic Conductivity and Aquaporin Abundance

The role of reactive oxygen species (ROS) in ABA-induced increase in hydraulic conductivity was hypothesized to be dependent on an increase in aquaporin water channel (AQP) abundance. Single ABA application or its combination with ROS manipulators (ROS scavenger ascorbic acid and NADPH oxidase inhibitor diphenyleneiodonium chloride (DPI)) were studied on detached roots of barley plants. We measured the osmotically driven flow rate of xylem sap and calculated root hydraulic conductivity. In parallel, immunolocalization of ABA and HvPIP2;2 AQPs was performed with corresponding specific antibodies. ABA treatment increased the flow rate of xylem, root hydraulic conductivity and immunostaining for ABA and HvPIP2;2, while the addition of antioxidants prevented the effects of this hormone. The obtained results confirmed the involvement of ROS in ABA effect on hydraulic conductivity, in particular, the importance of H₂O₂ production by ABA-treated plants for the effect of this hormone on AQP abundance.

Crosstalk between Light- and Temperature-Mediated Processes under Cold and Heat Stress Conditions in Plants

Extreme temperatures are among the most important stressors limiting plant growth and development. Results indicate that light substantially influences the acclimation processes to both low and high temperatures, and it may affect the level of stress injury. The interaction between light and temperature in the regulation of stress acclimation mechanisms is complex, and both light intensity and spectral composition play an important role. Higher light intensities may lead to overexcitation of the photosynthetic electron transport chain; while different wavelengths may act through different photoreceptors. These may induce various stress signalling processes, leading to regulation of stomatal movement, antioxidant and osmoregulation capacities, hormonal actions, and other stress-related pathways. In recent years, we have significantly expanded our knowledge in both light and temperature sensing and signalling. The present review provides a synthesis of results for understanding how light influences the acclimation of plants to extreme low or high temperatures, including the sensing mechanisms and molecular crosstalk processes.

Stomata coordinate with plant hydraulics to regulate transpiration response to vapour pressure deficit in wheat

Genotypic variation in transpiration (Tr) response to vapour pressure deficit (VPD) has been studied in many crop species. There is debate over whether shoots or roots drive these responses. We investigated how stomata coordinate with plant hydraulics to mediate Tr response to VPD and influence leaf water status in wheat (Triticum aestivum L.). We measured Tr and stomatal conductance (gs) responses to VPD in well-watered, water-stressed and de-rooted shoots of eight wheat genotypes. Tr response to VPD was related to stomatal sensitivity to VPD and proportional to gs at low VPD, except in the water-stressed treatment, which induced strong stomatal closure at all VPD levels. Moreover, gs response to VPD was driven by adaxial stomata. A simple linear Tr response to VPD was associated with unresponsive gs to VPD. In contrast, segmented linear Tr to VPD response was mostly a function of gs with the breakpoint depending on the capacity to meet transpirational demand and set by the shoots. However, the magnitude of Tr response to VPD was influenced by roots, soil water content and stomatal sensitivity to VPD. These findings, along with a theoretical model suggest that stomata coordinate with plant hydraulics to regulate Tr response to VPD in wheat.

Differential regulation of drought stress by biological membrane transporters and channels

Stress arising due to abiotic factors affects the plant's growth and productivity. Among several existing abiotic stressors like cold, drought, heat, salinity, heavy metal, etc., drought condition tends to affect the plant's growth by inducing two-point effect, i.e., it disturbs the water balance as well as induces toxicity by disturbing the ion homeostasis, thus hindering the growth and productivity of plants, and to survive under this condition, plants have evolved several transportation systems that are involved in regulating the drought stress. The role of membrane transporters has gained interest since genetic engineering came into existence, and they were found to be the important modulators for tolerance, avoidance, ion movements, stomatal movements, etc. Here in this comprehensive review, we have discussed the role of transporters (ABA, protein, carbohydrates, etc.) and channels that aids in withstanding the drought stress as well as the regulatory role of transporters involved in osmotic adjustments arising due to drought stress. This review also provides a gist of hydraulic conductivity by roots that are involved in regulating the drought stress.

Transpiration increases under high-temperature stress: Potential mechanisms, trade-offs and prospects for crop resilience in a warming world

The frequency and intensity of high-temperature stress events are expected to increase as climate change intensifies. Concomitantly, an increase in evaporative demand, driven in part by global warming, is also taking place worldwide. Despite this, studies examining high-temperature stress impacts on plant productivity seldom consider this interaction to identify traits enhancing yield resilience towards climate change. Further, new evidence documents substantial increases in plant transpiration rate in response to high-temperature stress even under arid environments, which raise a trade-off between the need for latent cooling dictated by excessive temperatures and the need for water conservation dictated by increasing evaporative demand. However, the mechanisms behind those responses, and the potential to design the next generation of crops successfully navigating this trade-off, remain poorly investigated. Here, we review potential mechanisms underlying reported increases in transpiration rate under high-temperature stress, within the broader context of their impact on water conservation needed for crop drought tolerance. We outline three main contributors to this phenomenon, namely stomatal, cuticular and water viscosity-based mechanisms, and we outline research directions aiming at designing new varieties optimized for specific temperature and evaporative demand regimes to enhance crop productivity under a warmer and dryer climate.

Root system architecture, physiological and transcriptional traits of soybean (Glycine max L.) in response to water deficit: A review

Drought stress is the main limiting factor for global soybean growth and production. Genetic improvement for water and nutrient uptake efficiency is critical to advance tolerance and enable more sustainable and resilient production, underpinning yield growth. The identification of quantitative traits and genes related to water and nutrient uptake will enhance our understanding of the mechanisms of drought tolerance in soybean. This review summarizes drought stress in the context of the physiological traits that enable effective acclimation, with a particular focus on roots. Genes controlling root system architecture play an important role in water and nutrient availability, and therefore important targets for breeding strategies to improve drought tolerance. This review highlights the candidate genes that have been identified as regulators of important root traits and responses to water stress. Progress in our understanding of the function of particular genes, including GmACX1, GmMS and GmPEPCK are discussed in the context of developing a system-based platform for genetic improvement of drought tolerance in soybean.

Photosynthetically active radiation impacts significantly on root and cell hydraulics in barley (Hordeum vulgare L.)

Photosynthetically active radiation (PAR) affects transpirational water loss, yet we do not know through which mechanisms root water uptake is adjusted in parallel. Here, we exposed hydroponically grown barley plants to three levels of PAR [Normal (control), Low, High] and focused on the role which aquaporins (AQPs), apoplastic barriers (Casparian bands, suberin lamellae) and root morphology play in the adjustment of root hydraulic conductivity (Lp). Plants were analyzed when they were 14-18 days (d) old. Root and cell Lp, which involves AQP activity, was determined through exudation and cell pressure probe measurements, respectively. Gene expression of AQPs was analyzed through qPCR. The formation of apoplastic barriers was studied through staining of cross-sections. The rate of transpirational water loss per plant and unit leaf area increased in response to high-PAR and decreased in response to low-PAR treatments, both during day and night. Hydraulic conductivity in roots decreased significantly at organ and cell level in response to Low-PAR, and increased (organ) or did not change (cell level) in response to High-PAR. The formation of apoplastic barriers was little affected by PAR. Gene expression of AQPs tended to be highest in the Low-PAR treatment. Lateral roots, showing few apoplastic barriers, contributed the least in Low- and the most to root surface area in High-PAR plants. It is concluded that barley plants which experience changes in shoot transpirational water loss in response to PAR adjust root water uptake through changes in root Lp, and that these changes are mediated through altered AQP activity and root morphology.

Effects of Plant Growth Promoting Rhizobacteria on the Content of Abscisic Acid and Salt Resistance of Wheat Plants

Although salinity inhibits plant growth, application of appropriate rhizosphere bacteria can diminish this negative effect. We studied one possible mechanism that may underlie this beneficial response. Wheat plants were inoculated with Bacillus subtilis IB-22 and Pseudomonas mandelii IB-Ki14 and their consequences for growth, water relations, and concentrations of the hormone abscisic acid (ABA) were followed in the presence of soil salinity. Salinity alone increased ABA concentration in wheat leaves and roots and this was associated with decreased stomatal conductance, but also with chlorophyll loss. Bacterial treatment raised ABA concentrations in roots, suppressed accumulation of leaf ABA, decreased chlorophyll loss, and promoted leaf area and transpiration. However, water balance was maintained due to increased water uptake by inoculated plants, brought about in part by a larger root system. The effect may be the outcome of ABA action since the hormone is known to maintain root extension in stressed plants. Root ABA concentration was highest in salt-stressed plants inoculated with B. subtilis and this contributed to greater root hydraulic conductivity. We conclude that bacteria can raise salt resistance in wheat by increasing root ABA, resulting in larger root systems that can also possess enhanced hydraulic conductivity thereby supporting better-hydrated leaves.

Abscisic Acid-Enemy or Savior in the Response of Cereals to Abiotic and Biotic Stresses?

Abscisic acid (ABA) is well-known phytohormone involved in the control of plant natural developmental processes, as well as the stress response. Although in wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) its role in mechanism of the tolerance to most common abiotic stresses, such as drought, salinity, or extreme temperatures seems to be fairly well recognized, not many authors considered that changes in ABA content may also influence the sensitivity of cereals to adverse environmental factors, e.g., by accelerating senescence, lowering pollen fertility, and inducing seed dormancy. Moreover, recently, ABA has also been regarded as an element of the biotic stress response; however, its role is still highly unclear. Many studies connect the susceptibility to various diseases with increased concentration of this phytohormone. Therefore, in contrast to the original assumptions, the role of ABA in response to biotic and abiotic stress does not always have to be associated with survival mechanisms; on the contrary, in some cases, abscisic acid can be one of the factors that increases the susceptibility of plants to adverse biotic and abiotic environmental factors.

Comparing Hydraulics Between Two Grapevine Cultivars Reveals Differences in Stomatal Regulation Under Water Stress and Exogenous ABA Applications

Hydraulics of plants that have different strategies of stomatal regulation under water stress are relatively poorly understood. We explore how root and shoot hydraulics, stomatal conductance (g _s), leaf and root aquaporin (AQP) expression, and abscisic acid (ABA) concentration in leaf xylem sap ([ABA]_xylemsap) may be coordinated under mild water stress and exogenous ABA applications in two Vitis vinifera L. cultivars traditionally classified as near-isohydric (Grenache) and near-anisohydric (Syrah). Under water stress, Grenache exhibited stronger adjustments of plant and root hydraulic conductances and greater stomatal sensitivity to [ABA]_xylemsap than Syrah resulting in greater conservation of soil moisture but not necessarily more isohydric behavior. Correlations between leaf (Ψ_leaf) and predawn (Ψ_PD) water potentials between cultivars suggested a "hydrodynamic" behavior rather than a particular iso-anisohydric classification. A significant decrease of Ψ_leaf in well-watered ABA-fed vines supported a role of ABA in the soil-leaf hydraulic pathway to regulate g _s. Correlations between leaf and root AQPs expression levels under water deficit could explain the response of leaf (K _leaf) and root (Lp _r) hydraulic conductances in both cultivars. Additional studies under a wider range of soil water deficits are required to explore the possible differential regulation of g _s and plant hydraulics in different cultivars and experimental conditions.

Dynamics of intercellular water transfer in the roots of intact Zea mays L. plants under elevated concentrations of atmospheric CO2

In this study, the effect of atmospheric carbon dioxide concentration increase on the dynamics of radial intercellular water transfer in the root suction zone of intact maize plants was evaluated. To this end, a unique growth chamber, associated with ¹H NMR PGMF (proton nuclear magnetic resonance with a pulsed gradient of the magnetic field) equipment, was used. As the atmospheric CO₂ concentration increased up to 800 ppm and 1200 ppm, and the intensity of water transfer in the roots significantly decreased. The average effective water diffusion coefficient (D_ef) and the water permeability in root cells (P) decreased by approximately 30-35% within 5-6 h after the increase in CO₂ concentration. At a higher concentration of CO₂, 1200 ppm, the rate of decrease in water permeability increased. After a day of exposure to elevated CO_2, the intensity of water transfer was partially restored but remained below the control level (before CO₂ enrichment) over the next 7 days. Inhibitory analysis showed that root cell aquaporins (AQPs) made a significant contribution to the observed decrease in the intensity of water transport in the roots. The decrease in water permeability of root cells under elevated CO₂ concentrations possibly occurs due to the regulatory decrease in water conductivity of AQPs via shoot-to-root long-distance signaling.

Radial water transport in arbuscular mycorrhizal maize plants under drought stress conditions is affected by indole-acetic acid (IAA) application

Drought stress is one of the most devastating abiotic stresses, compromising crop growth, reproductive success and yield. The arbuscular mycorrhizal (AM) symbiosis has been demonstrated to be beneficial in helping the plant to bear with water deficit. In plants, development and stress responses are largely regulated by a complex hormonal crosstalk. Auxins play significant roles in plant growth and development, in responses to different abiotic stresses or in the establishment and functioning of the AM symbiosis. Despite these important functions, the role of indole-3acetic acid (IAA) as a regulator of root water transport and stress response is not well understood. In this study, the effect of exogenous application of IAA on the regulation of root radial water transport in AM plants was analyzed under well-watered and drought stress conditions. Exogenous IAA application affected root hydraulic parameters, mainly osmotic root hydraulic conductivity (Lo), which was decreased in both AM and non-AM plants under water deficit conditions. Under drought, the relative apoplastic water flow was differentially regulated by IAA application in non-AM and AM plants. The effect of IAA on the internal cell component of root water conductivity suggests that aquaporins are involved in the IAA-dependent inhibition of this water pathway.

Steady-State Levels of Cytokinins and Their Derivatives May Serve as a Unique Classifier of Arabidopsis Ecotypes

We determined steady-state (basal) endogenous levels of three plant hormones (abscisic acid, cytokinins and indole-3-acetic acid) in a collection of thirty different ecotypes of Arabidopsis that represent a broad genetic variability within this species. Hormone contents were analysed separately in plant shoots and roots after 21 days of cultivation on agar plates in a climate-controlled chamber. Using advanced statistical and machine learning methods, we tested if basal hormonal levels can be considered a unique ecotype-specific classifier. We also explored possible relationships between hormone levels and the prevalent environmental conditions in the site of origin for each ecotype. We found significant variations in basal hormonal levels and their ratios in both root and shoot among the ecotypes. We showed the prominent position of cytokinins (CK) among the other hormones. We found the content of CK and CK metabolites to be a reliable ecotype-specific identifier. Correlation with the mean temperature at the site of origin and the large variation in basal hormonal levels suggest that the high variability may potentially be in response to environmental factors. This study provides a starting point for ecotype-specific genetic maps of the CK metabolic and signalling network to explore its contribution to the adaptation of plants to local environmental conditions.

Supplemental Light-Emitting Diode Inter-Lighting Increases Tomato Fruit Growth Through Enhanced Photosynthetic Light Use Efficiency and Modulated Root Activity

We investigated the effect of supplemental LED inter-lighting (80% red, 20% blue; 70 W m-2; light period 04:00-22:00) on the productivity and physiological traits of tomato plants (Flavance F1) grown in an industrial greenhouse with high pressure sodium (HPS) lamps (235 W m-2, 420 µmol m-2 s-1 at canopy). Physiological trait measurements included diurnal photosynthesis and fruit relative growth rates, fruit weight at specific positions in the truss, root pressure, xylem sap hormone and ion compositions, and fruit quality. In the control treatment with HPS lamps alone, the ratio of far-red to red light (FR:R) was 1.2 at the top of the canopy and increased to 5.4 at the bottom. The supplemental LED inter-lighting decreased the FR:R ratio at the middle and low positions in the canopy and was associated with greener leaves and higher photosynthetic light use efficiency (PLUE) in the leaves in the lower canopy. The use of LED inter-lighting increased the biomass and yield by increasing the fruit weight and enhancing plant growth. The PLUE of plants receiving supplemental LED light decreased at the end of the light period, indicating that photosynthesis of the supplemented plants at the end of the day might be limited by sink capacity. The supplemental LED lighting increased the size of fruits in the middle and distal positions of the truss, resulting in a more even size for each fruit in the truss. Diurnal analysis of fruit growth showed that fruits grew more quickly during the night on the plants receiving LED light than on unsupplemented control plants. This faster fruit growth during the night was related to an increased root pressure. The LED treatment also increased the xylem levels of the phytohormone jasmonate. Supplemental LED inter-lighting increased tomato fruit weight without affecting the total soluble solid contents in fruits by increasing the total assimilates available for fruit growth and by enhancing root activity through an increase in root pressure and water supply to support fruit growth during the night.

Potential involvement of root auxins in drought tolerance by modulating nocturnal and daytime water use in wheat

BACKGROUND AND AIMS

The ability of wheat genotypes to save water by reducing their transpiration rate (TR) at times of the day with high vapour pressure deficit (VPD) has been linked to increasing yields in terminal drought environments. Further, recent evidence shows that reducing nocturnal transpiration (TRN) could amplify water saving. Previous research indicates that such traits involve a root-based hydraulic limitation, but the contribution of hormones, particularly auxin and abscisic acid (ABA), has not been explored to explain the shoot-root link. In this investigation, based on physiological, genetic and molecular evidence gathered on a mapping population, we hypothesized that root auxin accumulation regulates whole-plant water use during both times of the day.

METHODS

Eight double-haploid lines were selected from a mapping population descending from two parents with contrasting water-saving strategies and root hydraulic properties. These spanned the entire range of slopes of TR responses to VPD and TRN encountered in the population. We examined daytime/night-time auxin and ABA contents in the roots and the leaves in relation to hydraulic traits that included whole-plant TR, plant hydraulic conductance (KPlant), slopes of TR responses to VPD and leaf-level anatomical traits.

KEY RESULTS

Root auxin levels were consistently genotype-dependent in this group irrespective of experiments and times of the day. Daytime root auxin concentrations were found to be strongly and negatively correlated with daytime TR, KPlant and the slope of TR response to VPD. Night-time root auxin levels significantly and negatively correlated with TRN. In addition, daytime and night-time leaf auxin and ABA concentrations did not correlate with any of the examined traits.

CONCLUSIONS

The above results indicate that accumulation of auxin in the root system reduces daytime and night-time water use and modulates plant hydraulic properties to enable the expression of water-saving traits that have been associated with enhanced yields under drought.

Local root ABA/cytokinin status and aquaporins regulate poplar responses to mild drought stress independently of the ectomycorrhizal fungus Laccaria bicolor

The relatively better performance of mycorrhizal plants subjected to drought stress has commonly been linked to improved root water uptake through the fungal regulation of plant aquaporins and hormones. In this study, we examined the role of ectomycorrhizal fungi in plant water relations and plant hormonal balance under mild drought using split-root seedlings of Populus trichocarpa × deltoides either with or without inoculation with Laccaria bicolor. The root compartments where the drought treatment was applied had higher ABA and lower cytokinin tZR contents, and greater expression of the plant aquaporins PtPIP1;1, PtPIP1;2, PtPIP2;5, and PtPIP2;7. On the other hand, the presence of L. bicolor within the roots down-regulated PtPIP1;4, PtPIP2;3, and PtPIP2;10, and reduced the abundance of PIP2 proteins. In addition, expression of the fungal aquaporins JQ585595 and JQ585596 were positively correlated with root ABA content, while tZR content was positively correlated with PtPIP1;4 and negatively correlated with PtPIP2;7. The results demonstrate a coordinated plant-fungal system that regulates the different mechanisms involved in water uptake in ectomycorrhizal poplar plants.

Comparative effect of elicitors on the physiology and secondary metabolites in broccoli plants

Elicitation is an economic and sustainable technique for increasing the content of secondary metabolites, mainly bioactive compounds, in plants grown for better human nutrition. The aim of this study was to compare the physiological responses (water relations and mineral nutrition) and the enrichment in glucosinolates (GLSs) and phenolic compounds of broccoli plants (Brassica oleracea L. var. italica) receiving different elicitation treatments. The treatments involved the priming of seeds with KCl and the exposure of plants to elicitors, including K₂SO₄ and NaCl solutions and foliar sprays of methyl jasmonate (MeJA), salicylic acid (SA), and methionine (Met). The physiological response of the plants in terms of root hydraulic conductance was improved by priming with KCl and elicitation with MeJA or Met. Foliar application of Met significantly increased the plant biomass and enhanced mineral nutrition. In general, all treatments increased the accumulation of indole GLSs, but K₂SO₄ and MeJA gave the best response and MeJA also favored the formation of a newly described compound, cinnamic-GLS, in the plants. Also, the use of Met and SA as elicitors and the supply of K₂SO₄ increased the abundance of phenolic compounds; K₂SO₄ also enhanced growth but did not alter the water relations or the accumulation of mineral nutrients. Therefore, although the response to elicitation was positive, with an increased content of bioactive compounds, regulation of the water relations and of the mineral status of the broccoli plants was critical to maintain the yield.

Variability in temperature-independent transpiration responses to evaporative demand correlate with nighttime water use and its circadian control across diverse wheat populations

Nocturnal transpiration, through its circadian control, plays a role in modulating daytime transpiration response to increasing evaporative demand, to potentially enable drought tolerance in wheat. Limiting plant transpiration rate (TR) in response to increasing vapor pressure deficit (VPD) has been suggested to enable drought tolerance through water conservation. However, there is very little information on the extent of diversity of TR response curves to "true" VPD (i.e., independent from temperature). Furthermore, new evidence indicate that water-saving could operate by modulating nocturnal TR (TR_N), and that this response might be coupled to daytime gas exchange. Based on 3 years of experimental data on a diverse group of 77 genotypes from 25 countries and 5 continents, a first goal of this study was to characterize the functional diversity in daytime TR responses to VPD and TR_N in wheat. A second objective was to test the hypothesis that these traits could be coupled through the circadian clock. Using a new gravimetric phenotyping platform that allowed for independent temperature and VPD control, we identified three and fourfold variation in daytime and nighttime responses, respectively. In addition, TR_N was found to be positively correlated with slopes of daytime TR responses to VPD, and we identified pre-dawn variation in TR_N that likely mediated this relationship. Furthermore, pre-dawn increase in TR_N positively correlated with the year of release among drought-tolerant Australian cultivars and with the VPD threshold at which they initiated water-saving. Overall, the study indicates a substantial diversity in TR responses to VPD that could be leveraged to enhance fitness under water-limited environments, and that TR_N and its circadian control may play an important role in the expression of water-saving.

Pearl millet (Pennisetum glaucum) contrasting for the transpiration response to vapour pressure deficit also differ in their dependence on the symplastic and apoplastic water transport pathways

Genotypic differences in transpiration rate responses to high vapour pressure deficit (VPD) was earlier reported. Here we tested the hypothesis that this limitation could relate to different degrees of dependence on the apoplastic (spaces between cells), and symplastic water transport pathways (through cells via aquaporin-facilitated transport), which are known to have different hydraulic conductivities. The low transpiration rate (Tr) genotype PRLT 2/89/33 either restricted its transpiration under high VPD, or was more sensitive to VPD than H77/833-2, when grown hydroponically or in soil. The slope of the transpiration response to an ascending series of VPD was lower in whole plants than in de-rooted shoots. In addition, the transpiration response of detached leaves to moderately high VPD (2.67kPa), normalised against leaves exposed to constant VPD (1.27kPa), was similar in low and high Tr genotypes. This suggested that roots hydraulics were a substantial limitation to water flow in pearl millet, especially under high VPD. The dependence on the apoplastic and symplastic water transport pathways was investigated by assessing the transpiration response of plants treated with inhibitors specific to the AQP-mediated symplastic pathway (AgNO3 and H2O2) and to the apoplastic pathway (precipitates of Cu(Fe(CN)6) or Cu(CuFe(CN)6)). When CuSO4 alone was used, Cu ions caused an inhibition of transpiration in both genotypes and more so in H77/833-2. The transpiration of high Tr H77/833-2 was decreased more by AQP inhibitors under low VPD (1.8kPa) than in PRLT 2/89/33, whereas under high VPD (4.2kPa), the transpiration of PRLT 2/89/33 was decreased more by AQP inhibitors than in H77/833-2. The transpiration rate of detached leaves from H77/833-2 when treated with AgNO3 decreased more than in PRLT 2/89/33. Although the root hydraulic conductivity of both genotypes was similar, it decreased more upon the application of a symplastic inhibitor in H77/833-2. The transpiration of low Tr PRLT 2/89/33 was decreased more by apoplastic inhibitors under both low and high VPD. Then the hydraulic conductivity decreased more upon the application of an apoplastic inhibitor in PRLT 2/89/33. In conclusion, both pathways contributed to water transport, and their contribution varied with environmental conditions and genotypes. Roots were a main source of hydraulic limitation in these genotypes of pearl millet, although a leaf limitation was not excluded. The similarity between genotypes in root hydraulic conductivity under normal conditions also suggests changes in this conductivity upon changes in the evaporative demand. The low Tr genotype depended more on the apoplastic pathway for water transport, whereas the high Tr genotype depended on both pathway, may be by 'tuning-up' the symplastic pathway under high transpiration demand, very likely via the involvement of aquaporins.

Roles of Chloroplast Retrograde Signals and Ion Transport in Plant Drought Tolerance

Worldwide, drought affects crop yields; therefore, understanding plants' strategies to adapt to drought is critical. Chloroplasts are key regulators of plant responses, and signals from chloroplasts also regulate nuclear gene expression during drought. However, the interactions between chloroplast-initiated retrograde signals and ion channels under stress are still not clear. In this review, we summarise the retrograde signals that participate in regulating plant stress tolerance. We compare chloroplastic transporters that modulate retrograde signalling through retrograde biosynthesis or as critical components in retrograde signalling. We also discuss the roles of important plasma membrane and tonoplast ion transporters that are involved in regulating stomatal movement. We propose how retrograde signals interact with ion transporters under stress.

Leaves, not roots or floral tissue, are the main site of rapid, external pressure-induced ABA biosynthesis in angiosperms

Rapid biosynthesis of abscisic acid (ABA) in the leaf, triggered by a decrease in cell volume, is essential for a functional stomatal response. However, it is not known whether rapid biosynthesis of ABA is also triggered in other plant tissues. Through the application of external pressure to flower, root, and leaf tissues, we test whether a reduction in cell volume can trigger rapid increases in ABA levels across the plant body in two species, Solanum lycopersicum and Passiflora tarminiana. Our results show that, in contrast to rapid ABA synthesis in the leaf, flower and root tissue did not show a significant, increase in ABA level in response to a drop in cell volume over a short time frame, suggesting that rapid ABA biosynthesis occurs only in leaf, not in flower or root tissues. A gene encoding the key, rate-limiting carotenoid cleavage enzyme (9-cis-epoxycarotenoid dioxygenase, NCED) in the ABA biosynthetic pathway in S. lycopersicum, NCED1, was upregulated to a lesser degree in flowers and roots compared with leaves in response to applied pressure. In both species, floral tissues contained substantially lower levels of the NCED substrate 9'-cis-neoxanthin than leaves, and this ABA precursor could not be detected in roots. Slow and minimal ABA biosynthesis was detected after 2 h in petals, indicating that floral tissue is capable of synthesizing ABA in response to sustained water deficit. Our results indicate that rapid ABA biosynthesis predominantly occurs in the leaves, and not in other tissues.

Heat and Drought Stresses in Crops and Approaches for Their Mitigation

Drought and heat are major abiotic stresses that reduce crop productivity and weaken global food security, especially given the current and growing impacts of climate change and increases in the occurrence and severity of both stress factors. Plants have developed dynamic responses at the morphological, physiological and biochemical levels allowing them to escape and/or adapt to unfavorable environmental conditions. Nevertheless, even the mildest heat and drought stress negatively affects crop yield. Further, several independent studies have shown that increased temperature and drought can reduce crop yields by as much as 50%. Response to stress is complex and involves several factors including signaling, transcription factors, hormones, and secondary metabolites. The reproductive phase of development, leading to the grain production is shown to be more sensitive to heat stress in several crops. Advances coming from biotechnology including progress in genomics and information technology may mitigate the detrimental effects of heat and drought through the use of agronomic management practices and the development of crop varieties with increased productivity under stress. This review presents recent progress in key areas relevant to plant drought and heat tolerance. Furthermore, an overview and implications of physiological, biochemical and genetic aspects in the context of heat and drought are presented. Potential strategies to improve crop productivity are discussed.

Heat and Drought Stresses in Crops and Approaches for Their Mitigation

Drought and heat are major abiotic stresses that reduce crop productivity and weaken global food security, especially given the current and growing impacts of climate change and increases in the occurrence and severity of both stress factors. Plants have developed dynamic responses at the morphological, physiological and biochemical levels allowing them to escape and/or adapt to unfavorable environmental conditions. Nevertheless, even the mildest heat and drought stress negatively affects crop yield. Further, several independent studies have shown that increased temperature and drought can reduce crop yields by as much as 50%. Response to stress is complex and involves several factors including signaling, transcription factors, hormones, and secondary metabolites. The reproductive phase of development, leading to the grain production is shown to be more sensitive to heat stress in several crops. Advances coming from biotechnology including progress in genomics and information technology may mitigate the detrimental effects of heat and drought through the use of agronomic management practices and the development of crop varieties with increased productivity under stress. This review presents recent progress in key areas relevant to plant drought and heat tolerance. Furthermore, an overview and implications of physiological, biochemical and genetic aspects in the context of heat and drought are presented. Potential strategies to improve crop productivity are discussed.

Rapid changes in root HvPIP2;2 aquaporins abundance and ABA concentration are required to enhance root hydraulic conductivity and maintain leaf water potential in response to increased evaporative demand

To address the involvement of abscisic acid (ABA) in regulating transpiration and root hydraulic conductivity (LpRoot) and their relative importance for maintaining leaf hydration, the ABA-deficient barley mutant Az34 and its parental wild-type (WT) genotype (cv. Steptoe) were grown in hydroponics and exposed to changes in atmospheric vapour pressure deficit (VPD) imposed by air warming. WT plants were capable of maintaining leaf water potential (ψL) that was likely due to increased LpRoot enabling higher water flow from the roots, which increased in response to air warming. The increased LpRoot and immunostaining for HvPIP2;2 aquaporins (AQPs) correlated with increased root ABA content of WT plants when exposed to increased air temperature. The failure of Az34 to maintain ψL during air warming may be due to lower LpRoot than WT plants, and an inability to respond to changes in air temperature. The correlation between root ABA content and LpRoot was further supported by increased root hydraulic conductivity in both genotypes when treated with exogenous ABA (10-5 M). Thus the ability of the root system to rapidly regulate ABA levels (and thence aquaporin abundance and hydraulic conductivity) seems important to maintain leaf hydration.

ABA-Mediated Stomatal Response in Regulating Water Use during the Development of Terminal Drought in Wheat

End-of-season drought or "terminal drought," which occurs after flowering, is considered the most significant abiotic stress affecting crop yields. Wheat crop production in Mediterranean-type environments is often exposed to terminal drought due to decreasing rainfall and rapid increases in temperature and evapotranspiration during spring when wheat crops enter the reproductive stage. Under such conditions, every millimeter of extra soil water extracted by the roots benefits grain filling and yield and improves water use efficiency (WUE). When terminal drought develops, soil dries from the top, exposing the top part of the root system to dry soil while the bottom part is in contact with available soil water. Plant roots sense the drying soil and produce signals, which on transmission to shoots trigger stomatal closure to regulate crop water use through transpiration. However, transpiration is linked to crop growth and productivity and limiting transpiration may reduce potential yield. While an early and high degree of stomatal closure affects photosynthesis and hence biomass production, a late and low degree of stomatal closure exhausts available soil water rapidly which results in yield losses through a reduction in post-anthesis water use. The plant hormone abscisic acid (ABA) is considered the major chemical signal involved in stomatal regulation. Wheat genotypes differ in their ability to produce ABA under drought and also in their stomatal sensitivity to ABA. In this viewpoint article we discuss the possibilities of exploiting genotypic differences in ABA response to soil drying in regulating the use of water under terminal drought. Root density distribution in the upper drying layers of the soil profile is identified as a candidate trait that can affect ABA accumulation and subsequent stomatal closure. We also examine whether leaf ABA can be designated as a surrogate characteristic for improved WUE in wheat to sustain grain yield under terminal drought. Ease of collecting leaf samples to quantify ABA compared to extracting xylem sap will facilitate rapid screening of a large number of germplasm for drought tolerance.

Aquaporins as potential drought tolerance inducing proteins: Towards instigating stress tolerance

Aquaporins (AQPs) are primarily involved in maintaining cellular water homeostasis. Their role in diverse physiological processes has fascinated plant scientists for more than a decade, particularly concerning abiotic stresses. Increasing examples of evidence in various crop plants indicate that the AQPs are responsible for precise regulation of water movement and consequently play a crucial role in the drought stress tolerance. Since drought is one of the major abiotic stresses affecting agricultural production worldwide, it has become a critical agenda to focus research on the development of drought tolerant crop plants. AQPs can act as key candidate molecules to confront this issue. Hence, there is an important need to explore the potential of AQPs by understanding the molecular mechanisms and pathways through which they induce drought tolerance. Moreover, the signalling network/s involved in such pathways needs to be mined and understood correctly, and that may lead to the development of drought tolerance in crop plants. In the present review, opportunity and challenges regarding the efficient utilization of AQP-related information is presented and discussed. The complied information and the discussion will be helpful for designing future experiments and to set the specific goals for the enhancement of drought tolerance in crop plants. Biological Significance Knowledge on the role of AQPs in maintaining cellular water homeostasis has given new hope for developing drought tolerance in crop plants. Since drought is one of the major abiotic stresses affecting agricultural production worldwide, it has become a critical agenda to focus research on the development of drought-tolerant crop plants. AQPs can act as key candidate molecules to solve this problem through genetic engineering. For this, it is important to understand the molecular mechanisms and inter-related pathways through which AQPs induce drought tolerance and to explore the signaling network/s involved in such pathways.

Stomatal acclimation to vapour pressure deficit doubles transpiration of small tree seedlings with warming

Future climate change is expected to increase temperature (T) and atmospheric vapour pressure deficit (VPD) in many regions, but the effect of persistent warming on plant stomatal behaviour is highly uncertain. We investigated the effect of experimental warming of 1.9-5.1 °C and increased VPD of 0.5-1.3 kPa on transpiration and stomatal conductance (gs ) of tree seedlings in the temperate forest understory (Duke Forest, North Carolina, USA). We observed peaked responses of transpiration to VPD in all seedlings, and the optimum VPD for transpiration (Dopt ) shifted proportionally with increasing chamber VPD. Warming increased mean water use of Carya by 140% and Quercus by 150%, but had no significant effect on water use of Acer. Increased water use of ring-porous species was attributed to (1) higher air T and (2) stomatal acclimation to VPD resulting in higher gs and more sensitive stomata, and thereby less efficient water use. Stomatal acclimation maintained homeostasis of leaf T and carbon gain despite increased VPD, revealing that short-term stomatal responses to VPD may not be representative of long-term exposure. Acclimation responses differ from expectations of decreasing gs with increasing VPD and may necessitate revision of current models based on this assumption.

Exogenous application of abscisic acid (ABA) increases root and cell hydraulic conductivity and abundance of some aquaporin isoforms in the ABA-deficient barley mutant Az34

Background and Aims Regulation of water channel aquaporins (AQPs) provides another mechanism by which abscisic acid (ABA) may influence water flow through plants. To the best of our knowledge, no studies have addressed the changes in ABA levels, the abundance of AQPs and root cell hydraulic conductivity (LpCell) in the same tissues. Thus, we followed the mechanisms by which ABA affects root hydraulics in an ABA-deficient barley mutant Az34 and its parental line 'Steptoe'. We compared the abundance of AQPs and ABA in cells to determine spatial correlations between AQP abundance and local ABA concentrations in different root tissues. In addition, abundance of AQPs and ABA in cortex cells was related to LpCell. Methods Root hydraulic conductivity (LpRoot) was measured by means of root exudation analyses and LpCell using a cell pressure probe. The abundance of ABA and AQPs in root tissues was assessed through immunohistochemical analyses. Isoform-specific antibodies raised against HvPIP2;1, HvPIP2;2 and HvPIP2;5 were used. Key Results Immunolocalization revealed lower ABA levels in root tissues of Az34 compared with 'Steptoe'. Root hydraulic conductivity (LpRoot) was lower in Az34, yet the abundance of HvPIPs in root tissues was similar in the two genotypes. Root hair formation occurred closer to the tip, while the length of the root hair zone was shorter in Az34 than in 'Steptoe'. Application of external ABA to the root medium of Az34 and 'Steptoe' increased the immunostaining of root cells for ABA and for HvPIP2;1 and HvPIP2;2 especially in root epidermal cells and the cortical cell layer located beneath, parallel to an increase in LpRoot and LpCell. Treatment of roots with Fenton reagent, which inhibits AQP activity, prevented the ABA-induced increase in root hydraulic conductivity. Conclusion Shortly after (<2 h) ABA application to the roots of ABA-deficient barley, increased tissue ABA concentrations and AQP abundance (especially the plasma-membrane localized isoforms HvPIP2;1 and HvPIP2;2) were spatially correlated in root epidermal cells and the cortical cell layer located beneath, in conjunction with increased LpCell of the cortical cells. In contrast, long-term ABA deficiency throughout seedling development affects root hydraulics through other mechanisms, in particular the developmental timing of the formation of root hairs closer to the root tip and the length of the root hair zone.

Physiological impacts of ABA-JA interactions under water-limitation

Plant responses to drought stress depend on highly regulated signal transduction pathways with multiple interactions. This complex crosstalk can lead to a physiological outcome of drought avoidance or tolerance/resistance. ABA is the principal mediator of these responses due to the regulation of stomatal closure that determines plant growth and survival, but also other strategies of drought resistance such as osmotic adjustment. However, other hormones such as JA seem responsible for regulating a subset of plant responses to drought by regulating ABA biosynthesis and accumulation and ABA-dependent signalling, but also by ABA independent pathways. Here, we review recent reports of ABA-JA hormonal and molecular interactions within a physiological framework of drought tolerance. Understanding the physiological significance of this complex regulation offers opportunities to find strategies of drought tolerance that avoid unwanted side effects that limit growth and yield, and may allow biotechnological crop improvement.

Physiological basis of differential zinc and copper tolerance of Verbascum populations from metal-contaminated and uncontaminated areas

Metal contamination represents a strong selective pressure favoring tolerant genotypes and leading to differentiation between plant populations. We investigated the adaptive capacity of early-colonizer species of Verbascum recently exposed to Zn- and Cu-contaminated soils (10-20 years). Two Verbascum thapsus L. populations from uncontaminated sites (NMET1, NMET2), one V. thapsus from a zinc-contaminated site (MET1), and a Verbascum lychnitis population from an open-cast copper mine (MET2) were exposed to elevated Zn or Cu in hydroponic culture under glasshouse conditions. MET populations showed considerably higher tolerance to both Zn and Cu than NMET populations as assessed by measurements of growth and net photosynthesis, yet they accumulated higher tissue Zn concentrations in the shoot. Abscisic acid (ABA) concentration increased with Zn and Cu treatment in the NMET populations, which was correlated to stomatal closure, decrease of net photosynthesis, and nutritional imbalance, indicative of interference with xylem loading and divalent-cation homeostasis. At the cellular level, the sensitivity of NMET2 to Zn and Cu was reflected in significant metal-induced ROS accumulation and ion leakage from roots as well as strong induction of peroxidase activity (POD, EC 1.11.1.7), while Zn had no significant effect on ABA concentration and POD activity in MET1. Interestingly, MET2 had constitutively higher root ABA concentration and POD activity. We propose that ABA distribution between shoots and roots could represent an adaptive mechanism for maintaining low ABA levels and unaffected stomatal conductance. The results show that metal tolerance can occur in Verbascum populations after relatively short time of exposure to metal-contaminated soil, indicating their potential use for phytostabilization.

Pyrabactin regulates root hydraulic properties in maize seedlings by affecting PIP aquaporins in a phosphorylation-dependent manner

Pyrabactin, an agonist of abscisic acid (ABA), has led to the isolation and characterization of pyrabactin resistance 1/pyrabactin resistance 1-like (PYR1/PYLs) ABA receptors in Arabidopsis, which has well explained ABA-mediated stomatal movement and stress-related gene expression. In addition to inducing stomatal closure and inhibiting transpiration, ABA can also enhance root hydraulic conductivity (Lpr), thus maintaining water balance under water deficiency-related stress, but its molecular mechanism remains unclear. In the present study, the root hydraulic properties of maize seedlings in response to pyrabactin were compared to those caused by ABA. Similar to ABA, lower concentration of pyrabactin induced a remarkable increase in Lpr as well as in the gene expression of the plasma membrane intrinsic protein (ZmPIP) aquaporin and in the ZmPIP2; 1/2; 2 protein abundance. The pyrabactin-induced enhancement of Lpr was abolished by H2O2 application, indicating that pyrabactin regulates Lpr by modulating ZmPIP at transcriptional, translational and post-translational (activity) level. Pyrabactin-mediated water transport and ZmPIP gene expression were phosphorylation-dependent, suggesting that ABA-PYR1-(PP2C)-protein kinase-AQP signaling pathway may be involved in this process. As we know this is the first established ABA signaling transduction pathway that mediated water transport in roots. This observation further addressed the importance of PYR1/PYLs ABA receptor in regulating plant water use efficiency from the under ground level. Except inhibiting transpiration in leaves, our result introduces the exciting possibility of application ABA agonists for regulating roots water uptake in field, with a species- and dose dependent manner.

Towards a conceptual ABA ideotype in plant breeding for water limited environments

A huge amount of information had been accumulated on abscisic acid (ABA). Laboratory and some field research with ABA-enhanced transgenic plants generally conclude that ABA is a drought resistance hormone, since it causes stomatal closure, reduces transpiration and results in 'water saving' under drought stress. This recurring conclusion is hard to accept in the agronomic domain considering the many direct and indirect negative effects of ABA on plant growth and reproduction. In order to formulate a conceptual phenotypic ABA ideotype for plant breeding, this paper begins by briefly reviewing the phenomics of ABA relative to plant function and productivity. Consequently, it is recognised that ABA enhancement is important in controlling the isohydric ('water saving') plant model, whereas plant hydraulics are more important in controlling the anisohydric ('water spending') plant model. Subsequently, the respective isohydric and anisohydric ideotypes appropriate to specific dryland crop drought stress scenarios are proposed. It is concluded that ABA can by no means be universally defined as a 'drought resistance hormone'. Its benefit or damage depends on the crop drought stress profile and the dynamics of the seasonal regimen of ABA in the plant. The isohydric ideotype might have an advantage in the harshest environments, whereas the anisohydric one will perform relatively better under more moderate drought conditions.

Common and specific responses to availability of mineral nutrients and water

Changes in resource (mineral nutrients and water) availability, due to their heterogeneous distribution in space and time, affect plant development. Plants need to sense these changes to optimize growth and biomass allocation by integrating root and shoot growth. Since a limited supply of water or nutrients can elicit similar physiological responses (the relative activation of root growth at the expense of shoot growth), similar underlying mechanisms may affect perception and acquisition of either nutrients or water. This review compares root and shoot responses to availability of different macronutrients and water. Attention is given to the roles of root-to-shoot signalling and shoot-to-root signalling, with regard to coordinating changes in root and shoot growth and development. Involvement of plant hormones in regulating physiological responses such as stomatal and hydraulic conductance is revealed by measuring the effects of resource availability on phytohormone concentrations in roots and shoots, and their flow between roots and shoots in xylem and phloem saps. More specific evidence can be obtained by measuring the physiological responses of genotypes with altered hormone responses or concentrations. We discuss the similarity and diversity of changes in shoot growth, allocation to root growth, and root architecture under changes in water, nitrate, and phosphorus availability, and the possible involvement of abscisic acid, indole-acetic acid, and cytokinin in their regulation. A better understanding of these mechanisms may contribute to better crop management for efficient use of these resources and to selecting crops for improved performance under suboptimal soil conditions.

Differential responses of plasma membrane aquaporins in mediating water transport of cucumber seedlings under osmotic and salt stresses

It has long been recognized that inhibition of plant water transport by either osmotic stress or salinity is mediated by aquaporins (AQPs), but the function and regulation of AQPs are highly variable among distinct isoforms and across different species. In this study, cucumber seedlings were subjected to polyethylene glycol (PEG) or NaCl stress for duration of 2 h or 24 h. The 2 h treatment with PEG or NaCl had non-significant effect on the expression of plasma membrane AQP (CsPIPs) in roots, indicating the decrease in hydraulic conductivity of roots (Lpr ) and root cells (Lprc ) measured in these conditions were due to changes in AQP activity. After both 2 h and 24 h PEG or NaCl exposure, the decrease in hydraulic conductivity of leaves (Kleaf ) and leaf cells (Lplc ) could be attributed to a down-regulation of the two most highly expressed isoforms, CsPIP1;2 and CsPIP2;4. In roots, both Lpr and Lprc were further reduced after 24 h PEG exposure, but partially recovered after 24 h NaCl treatment, which were consistent with changes in the expression of CsPIP genes. Overall, the results demonstrated differential responses of CsPIPs in mediating water transport of cucumber seedlings, and the regulatory mechanisms differed according to applied stresses, stress durations and specific organs.

Aquaporins: highly regulated channels controlling plant water relations

Plant growth and development are dependent on tight regulation of water movement. Water diffusion across cell membranes is facilitated by aquaporins that provide plants with the means to rapidly and reversibly modify water permeability. This is done by changing aquaporin density and activity in the membrane, including posttranslational modifications and protein interaction that act on their trafficking and gating. At the whole organ level aquaporins modify water conductance and gradients at key "gatekeeper" cell layers that impact on whole plant water flow and plant water potential. In this way they may act in concert with stomatal regulation to determine the degree of isohydry/anisohydry. Molecular, physiological, and biophysical approaches have demonstrated that variations in root and leaf hydraulic conductivity can be accounted for by aquaporins but this must be integrated with anatomical considerations. This Update integrates these data and emphasizes the central role played by aquaporins in regulating plant water relations.