The role of water channel proteins in facilitating recovery of leaf hydraulic conductance from water stress in Populus trichocarpa

The Soil-Plant Continuity of Rare Earth Elements: Insights into an Enigmatic Class of Xenobiotics and Their Interactions with Plant Structures and Processes

Rare earth elements (REEs) are increasingly present in the environment owing to their extensive use in modern industries, yet their interactions with plants remain poorly understood. This review explores the soil-plant continuum of REEs, focusing on their geochemical behavior in soil, the mechanisms of plant uptake, and fractionation processes. While REEs are not essential for plant metabolism, they interact with plant structures and interfere with the normal functioning of biological macromolecules. Accordingly, the influence of REEs on the fundamental physiological functions of plants is reviewed, including calcium-mediated signalling and plant morphogenesis. Special attention is paid to the interaction of REEs with photosynthetic machinery and, particularly, the thylakoid membrane. By examining both the beneficial effects at low concentrations and toxicity at higher levels, this review provides some mechanistic insights into the hormetic action of REEs. It is recommended that future research should address knowledge gaps related to the bioavailability of REEs to plants, as well as the short- and long-range transport mechanisms responsible for REE fractionation. A better understanding of REE-plant interactions will be critical in regard to assessing their ecological impact and the potential risks in terms of agricultural and natural ecosystems, to ensure that the benefits of using REEs are not at the expense of environmental integrity or human health.

Stomatal response to VPD is not triggered by changes in soil-leaf hydraulic conductance in Arabidopsis or Callitris

Stomatal closure under high VPDL (leaf to air vapour pressure deficit) is a primary means by which plants prevent large excursions in transpiration rate and leaf water potential (Ψleaf) that could lead to tissue damage. Yet, the drivers of this response remain controversial. Changes in Ψleaf appear to drive stomatal VPDL response, but many argue that dynamic changes in soil-to-leaf hydraulic conductance (Ks-l) make an important contribution to this response pathway, even in well-hydrated soils. Here, we examined whether the regulation of whole plant stomatal conductance (gc) in response to typical changes in daytime VPDL is influenced by dynamic changes in Ks-l. We use well-watered plants of two species with contrasting ecological and physiological features: the herbaceous Arabidopsis thaliana (ecotype Columbia-0) and the dry forest conifer Callitris rhomboidea. The dynamics of Ks-l and gc were continuously monitored by combining concurrent in situ measurements of Ψleaf using an open optical dendrometer and whole plant transpiration using a balance. Large changes in VPDL were imposed to induce stomatal closure and observe the impact on Ks-l. In both species, gc was observed to decline substantially as VPDL increased, while Ks-l remained stable. Our finding suggests that stomatal regulation of transpiration is not contingent on a decrease in Ks-l. Static Ks-l provides a much simpler explanation for transpiration control in hydrated plants and enables simplified modelling and new methods for monitoring plant water use in the field.

Molecular insights into rare earth element (REE)-mediated phytotoxicity and its impact on human health

Rare earth elements (REEs) that include 15 lanthanides, scandium, and yttrium are a special class of elements due to their remarkable qualities such as magnetism, corrosion resistance, luminescence, and electroconductivity. Over the last few decades, the implication of REEs in agriculture has increased substantially, which was driven by rare earth element (REE)-based fertilizers to increase crop growth and yield. REEs regulate different physiological processes by modulating the cellular Ca2+ level, chlorophyll activities, and photosynthetic rate, promote the protective role of cell membranes, and increase the plant's ability to withstand various stresses and other environmental factors. However, the use of REEs in agriculture is not always beneficial because REEs regulate plant growth and development in dose-dependent manner and excessive usage of them negatively affects plants and agricultural yield. Moreover, increasing applications of REEs together with technological advancement is also a rising concern as they adversely impact all living organisms and disturb different ecosystems. Several animals, plants, microbes, and aquatic and terrestrial organisms are subject to acute and long-term ecotoxicological impacts of various REEs. This concise overview of REEs' phytotoxic effects and implications on human health offers a context for continuing to sew fabric scraps to this incomplete quilt's many layers and colors. This review deals with the applications of REEs in different fields, specifically agriculture, the molecular basis of REE-mediated phytotoxicity, and the consequences for human health.

Responses to Drought Stress in Poplar: What Do We Know and What Can We Learn?

Poplar (Populus spp.) is a high-value crop for wood and biomass production and a model organism for tree physiology and genomics. The early release, in 2006, of the complete genome sequence of P. trichocarpa was followed by a wealth of studies that significantly enriched our knowledge of complex pathways inherent to woody plants, such as lignin biosynthesis and secondary cell wall deposition. Recently, in the attempt to cope with the challenges posed by ongoing climate change, fundamental studies and breeding programs with poplar have gradually shifted their focus to address the responses to abiotic stresses, particularly drought. Taking advantage from a set of modern genomic and phenotyping tools, these studies are now shedding light on important processes, including embolism formation (the entry and expansion of air bubbles in the xylem) and repair, the impact of drought stress on biomass yield and quality, and the long-term effects of drought events. In this review, we summarize the status of the research on the molecular bases of the responses to drought in poplar. We highlight how this knowledge can be exploited to select more tolerant genotypes and how it can be translated to other tree species to improve our understanding of forest dynamics under rapidly changing environmental conditions.

Foliar water uptake as a source of hydrogen and oxygen in plant biomass

Introductory biology lessons around the world typically teach that plants absorb water through their roots, but, unfortunately, absorption of water through leaves and subsequent transport and use of this water for biomass formation remains a field limited mostly to specialists. Recent studies have identified foliar water uptake as a significant net water source for terrestrial plants. The growing interest in the development of a new model that includes both foliar water uptake (in liquid form) and root water uptake to explain hydrogen and oxygen isotope ratios in leaf water and tree rings demands a method for distinguishing between these two water sources. Therefore, in this study, I have devised a new labelling method that utilizes two different water sources, one enriched in deuterium (HDO + D2O; δD = 7.0 × 10 4‰, δ18O = 4.1‰) and one enriched in oxygen-18 (H218O; δD = -85‰, δ18O = 1.1 × 104‰), to simultaneously label both foliar-absorbed and root-absorbed water and quantify their relative contributions to plant biomass. Using this new method, I here present evidence that, in the case of well-watered Cryptomeria japonica D. Don, hydrogen and oxygen incorporated into new leaf cellulose in the rainy season derives mostly from foliar-absorbed water (69% from foliar-absorbed water and 31% from root-absorbed water), while that of new root cellulose derives mostly from root-absorbed water (20% from foliar-absorbed water and 80% from root-absorbed water), and new branch xylem is somewhere in between (55% from foliar-absorbed water and 45% from root-absorbed water). The dual-labelling method first implemented in this study enables separate and simultaneous labelling of foliar-absorbed and root-absorbed water and offers a new tool to study the uptake, transport and assimilation processes of these waters in terrestrial plants.

Foliar water uptake as a source of hydrogen and oxygen in plant biomass

Introductory biology lessons around the world typically teach that plants absorb water through their roots, but, unfortunately, absorption of water through leaves and subsequent transport and use of this water for biomass formation remains a field limited mostly to specialists. Recent studies have identified foliar water uptake as a significant net water source for terrestrial plants. The growing interest in the development of a new model that includes both foliar water uptake (in liquid form) and root water uptake to explain hydrogen and oxygen isotope ratios in leaf water and tree rings demands a method for distinguishing between these two water sources. Therefore, in this study, I have devised a new labelling method that utilizes two different water sources, one enriched in deuterium (HDO + D2O; δD = 7.0 × 10 4‰, δ18O = 4.1‰) and one enriched in oxygen-18 (H218O; δD = -85‰, δ18O = 1.1 × 104‰), to simultaneously label both foliar-absorbed and root-absorbed water and quantify their relative contributions to plant biomass. Using this new method, I here present evidence that, in the case of well-watered Cryptomeria japonica D. Don, hydrogen and oxygen incorporated into new leaf cellulose in the rainy season derives mostly from foliar-absorbed water (69% from foliar-absorbed water and 31% from root-absorbed water), while that of new root cellulose derives mostly from root-absorbed water (20% from foliar-absorbed water and 80% from root-absorbed water), and new branch xylem is somewhere in between (55% from foliar-absorbed water and 45% from root-absorbed water). The dual-labelling method first implemented in this study enables separate and simultaneous labelling of foliar-absorbed and root-absorbed water and offers a new tool to study the uptake, transport and assimilation processes of these waters in terrestrial plants.

Foliar-applied cerium oxide nanomaterials improve maize yield under salinity stress: Reactive oxygen species homeostasis and rhizobacteria regulation

Salinity stress seriously threatens agricultural productivity and food security worldwide. This work reports on the mechanisms of alleviating salinity stress by cerium oxide nanomaterials (CeO2 NMs) in maize (Zea may L.). Soil-grown maize plants were irrigated with deionized water or 100 mM NaCl solution as the control or the salinity stress treatment. CeO2 NMs (1, 5, 10, 20, and 50 mg/L) with antioxidative enzyme mimicking activities were foliarly applied on maize leaves for 7 days. The morphological, physiological, biochemical, and transcriptomic responses of maize were evaluated. Specifically, salinity stress significantly reduced 59.0% and 63.8% in maize fresh and dry biomass, respectively. CeO₂ NMs at 10, 20, and 50 mg/L improved the salt tolerance of maize by 69.5%, 69.1%, and 86.8%, respectively. Also, 10 mg/L CeO₂ NMs maintained Na⁺/K⁺ homeostasis, enhanced photosynthetic efficiency by 30.8%, and decreased reactive oxygen species (ROS) level by 58.5% in salt-stressed maize leaves. Transcriptomic analysis revealed that the antioxidative defense system-related genes recovered to the normal control level after CeO₂ NMs application, indicating that CeO₂ NMs eliminated ROS through their intrinsic antioxidative enzyme properties. The down-regulation of genes related to lignin synthesis in the phenylpropanoid biosynthesis pathway accelerated leaf cell elongation. In addition, CeO₂ NMs increased the rhizobacteria richness and diversity through the increment of carbon source in root exudates and improved the abundance of halotolerant plant growth-promoting rhizobacteria (HT-PGPR). Importantly, the yield of salt-stressed maize was enhanced by 293.3% after 10 mg/L CeO₂ NMs foliar application. These results will provide new insights for the application of CeO₂ NMs in management to reduce the salinity-caused crop loss.

Limiting factors for panicle photosynthesis at the anthesis and grain filling stages in rice (Oryza sativa L.)

Panicle photosynthesis is crucial for grain yield in cereal crops; however, the limiting factors for panicle photosynthesis are poorly understood, greatly impeding improvement in this trait. In the present study, pot experiments were conducted to investigate the limiting factors for panicle photosynthesis at the anthesis stage in seven rice genotypes and to examine the temporal variations in photosynthesis during the grain filling stage in the Liangyou 287 genotype. At the anthesis stage, leaf and panicle photosynthesis was positively correlated with stomatal conductance and maximum carboxylation rate, which were in turn associated with hydraulic conductance and nitrogen content, respectively. Panicle hydraulic conductance was positively correlated with the area of bundle sheaths in the panicle neck. During grain filling, leaf and panicle photosynthesis remained constant at the early stage but dramatically decreased from 8 to 9 days after anthesis. The trends of variations in panicle photosynthesis were consistent with those in stomatal conductance but not with those in maximum carboxylation rate. At first, the maximum carboxylation rate and respiration rate in the panicle increased, through elevated panicle nitrogen content, but then drastically decreased, as a result of dehydration. The present study systematically investigated the limiting factors for panicle photosynthesis, which are vital for improving photosynthesis and crop yield.

The extra-vascular water pathway regulates dynamic leaf hydraulic decline and recovery in Populus nigra

Leaf hydraulic conductance (K_leaf ) is highly dynamic and typically responds to changes in water status and irradiance. However, the relative contribution of vascular (K_x ) and extra-vascular (K_ox ) water pathways to K_leaf changes in response to water potential decline and recovery in function of light conditions remains poorly investigated. We investigated the dynamic responses of leaf hydraulics in Populus nigra L. by measuring K_leaf , K_x , and K_ox changes under drought and upon recovery. Measurements were done at both low and high irradiance (LI and HI, respectively). K_leaf increased and became more vulnerable to dehydration under HI conditions than LI, due to marked changes of K_ox . After re-watering, K_leaf recovered in parallel with K_ox recovery, but K_leaf response to irradiance remained inhibited. Strong correlations between K_leaf and drought-induced membrane damage demonstrated the relevance of the cell-to-cell water pathway in driving the dynamic responses of K_leaf under drought and recovery. Our findings highlight the importance of coordination between water and light availability in modulating the overall K_leaf response to environmental conditions.

Combined high leaf hydraulic safety and efficiency provides drought tolerance in Caragana species adapted to low mean annual precipitation

Clarifying the coordination of leaf hydraulic traits with gas exchange across closely-related species adapted to varying rainfall can provide insights into plant habitat distribution and drought adaptation. The leaf hydraulic conductance (Kleaf ), stomatal conductance (gs ), net assimilation (A), vein embolism and abscisic acid (ABA) concentration during dehydration were quantified, as well as pressure-volume curve traits and vein anatomy in 10 Caragana species adapted to a range of mean annual precipitation (MAP) conditions and growing in a common garden. We found a positive correlation between Ψleaf at 50% loss of Kleaf (Kleaf P50 ) and maximum Kleaf (Kleaf-max ) across species. Species from low-MAP environments exhibited more negative Kleaf P50 and turgor loss point, and higher Kleaf-max and leaf-specific capacity at full turgor, along with higher vein density and midrib xylem per leaf area, and a higher ratio of Kleaf-max : maximum gs . Tighter stomatal control mediated by higher ABA accumulation during dehydration in these species resulted in an increase in hydraulic safety and intrinsic water use efficiency (WUEi ) during drought. Our results suggest that high hydraulic safety and efficiency combined with greater stomatal sensitivity triggered by ABA production and leading to greater WUEi provides drought tolerance in Caragana species adapted to low-MAP environments.

The Neglected Reverse Water Pathway: Atmosphere-Plant-Soil Continuum

The soil-plant-atmosphere continuum (SPAC) describes the continuous water movement from soil via plants to atmosphere. Here, we propose to name the reverse water pathway, driven by foliar water uptake, the atmosphere-plant-soil continuum (APSC). We highlight the different hydraulic resistances this reverse water movement has to overcome.

Defoliation-induced compensatory transpiration is compromised in SUT4-RNAi Populus

The tonoplast sucrose transporter PtaSUT4 is well expressed in leaves of Populus tremula × Populus alba (INRA 717-IB4), and its inhibition by RNA-interference (RNAi) alters leaf sucrose homeostasis. Whether sucrose partitioning between the vacuole and the cytosol is modulated by PtaSUT4 for specific physiological outcomes in Populus remains unexplored. In this study, partial defoliation was used to elicit compensatory increases in photosynthesis and transpiration by the remaining leaves in greenhouse-grown poplar. Water uptake, leaf gas exchange properties, growth and nonstructural carbohydrate abundance in source and sink organs were then compared between wild-type and SUT4-RNAi lines. Partial defoliation increased maximum photosynthesis rates similarly in all lines. There was no indication that source leaf sugar levels changed differently between wild-type and RNAi plants following partial defoliation. Sink levels of hexose (glucose and fructose) and starch decreased similarly in all lines. Interestingly, plant water uptake after partial defoliation was not as well sustained in RNAi as in wild-type plants. While the compensatory increase in photosynthesis was similar between genotypes, leaf transpiration increased less robustly in RNAi than wild-type plants. SUT4-RNAi and wild-type source leaves differed constitutively in their bulk modulus of elasticity, a measure of leaf turgor, and storage water capacitance. The data demonstrate that reduced sucrose partitioning due to PtaSUT4-RNAi altered turgor control and compensatory transpiration capacity more strikingly than photosynthesis and sugar export. The results are consistent with the interpretation that SUT4 may control vacuolar turgor independently of sink carbon provisioning.

Improving water use efficiency by changing hydraulic and stomatal characteristics in soybean exposed to drought: the involvement of nitric oxide

A variety of cellular responses is needed to ensure the plants survival during drought, but little is known about the signaling mechanisms involved in this process. Soybean cultivars (EMBRAPA 48 and BR 16, tolerant and sensitive to drought, respectively) were exposed to the following treatments: control conditions (plants in field capacity), drought (20% of available water in the soil), sodium nitroprusside (SNP) treatment (plants irrigated and treated with 100-µM SNP [SNP-nitric oxide (NO) donor molecule], and Drought + SNP (plants subjected to drought and SNP treatment). Plants remained in these conditions until the reproductive stage and were evaluated for physiological (photosynthetic pigments, chlorophyll a fluorescence and gas exchange rates), hydraulic (water potential, osmotic potential and leaf hydraulic conductivity) and morpho-anatomical traits (biomass, venation density and stomatal characterization). Exposure to water deficit considerably reduced water potential in both cultivars and resulted in decrease in photosynthesis and biomass accumulation. The addition of the NO donor attenuated these damaging effects of water deficit and increased the tolerance index of both cultivars. The results showed that NO was able to reduce plant's water loss, while maintaining their biomass production through alteration in stomatal characteristics, hydraulic conductivity and the biomass distribution pattern. These hydraulic and morpho-anatomical alterations allowed the plants to obtain, transport and lose less water to the atmosphere, even in water deficit conditions.

Stomatal Sensitivity to Vapor Pressure Deficit and the Loss of Hydraulic Conductivity Are Coordinated in Populus euphratica, a Desert Phreatophyte Species

There are considerable variations in the percentage loss of hydraulic conductivity (PLC) at mid-day minimum water potential among and within species, but the underpinning mechanism(s) are poorly understood. This study tested the hypothesis that plants can regulate leaf specific hydraulic conductance (K _l) via precise control over PLC under variable ΔΨ (water potential differential between soil and leaf) conditions to maintain the -m/b constant (-m: the sensitivity of stomatal conductance to VPD; b: reference stomatal conductance at 1.0 kPa VPD), where VPD is vapor pressure deficit. We used Populus euphratica, a phreatophyte species distributed in the desert of Northwestern China, to test the hypothesis. Field measurements of VPD, stomatal conductance (g _s), g _s responses to VPD, mid-day minimum leaf water potential (Ψ _lmin), and branch hydraulic architecture were taken in late June at four sites along the downstream of Tarim River at the north edge of the Taklamakan desert. We have found that: 1) the -m/b ratio was almost constant (=0.6) across all the sites; 2) the average Ψ ₅₀ (the xylem water potential with 50% loss of hydraulic conductivity) was -1.63 MPa, and mid-day PLC ranged from 62 to 83%; 3) there were tight correlations between Ψ ₅₀ and wood density/leaf specific hydraulic conductivity (k _l) and between specific hydraulic conductance sensitivity to water potential [d(k _s)/dln(-Ψ)] and specific hydraulic conductivity (k _s). A modified hydraulic model was applied to investigate the relationship between g _s and VPD under variable ΔΨ and K _l conditions. It was concluded that P. euphratica was able to control PLC in order to maintain a relatively constant -m/b under different site conditions. This study demonstrated that branchlet hydraulic architecture and stomatal response to VPD were well coordinated in order to maintain relatively water homeostasis of P. euphratica in the desert. Model simulations could explain the wide variations of PLC across and within woody species that are often observed in the field.

Hydraulic conductance, resistance, and resilience: how leaves of a tropical epiphyte respond to drought

PREMISE

Because of its broad range in the neotropical rainforest and within tree canopies, the tank bromeliad Guzmania monostachia was investigated as a model of how varying leaf hydraulic conductance (K_leaf ) could help plants resist and recover from episodic drought. The two pathways of K_leaf , inside and outside the xylem, were also examined to determine the sites and causes of major hydraulic resistances within the leaf.

METHODS

We measured leaf hydraulic conductance for plants in the field and laboratory under wet, dry, and rewetted conditions and applied physiological, anatomical, and gene expression analysis with modeling to investigate changes in K_leaf .

RESULTS

After 7 d with no rain in the field or 14 days with no water in the glasshouse, K_leaf decreased by 50% yet increased to hydrated values within 4 d of tank refilling. Staining to detect embolism combined with modeling indicated that changes outside the xylem were of greater importance to K_leaf than were changes inside the xylem and were associated with changes in intercellular air spaces (aerenchyma), aquaporin expression and inhibition, and cuticular conductance.

CONCLUSIONS

Low values for all conductances during drying, particularly in pathways outside the xylem, lead to hydraulic resilience for this species and may also contribute to its broad environmental tolerances.

Effects of major vein blockage and aquaporin inhibition on leaf hydraulics and stomatal conductance

The density and architecture of leaf veins determine the network and efficiency of water transport within laminae and resultant leaf gas exchange and vary widely among plant species. Leaf hydraulic conductance ( Kleaf) can be regulated by vein architecture in conjunction with the water channel protein aquaporin. However, our understanding of how leaf veins and aquaporins affect leaf hydraulics and stomatal conductance ( gs) remains poor. By inducing blockage of the major veins and inhibition of aquaporin activity using HgCl2, we examined the effects of major veins and aquaporins on Kleaf and gs in species with different venation types. A vine species, with thick first-order veins and low vein density, displayed a rapidly declined gs with high leaf water potential in response to vein blockage and a greatly reduced Kleaf and gs in response to aquaporin inhibition, suggesting that leaf aquaporins are involved in isohydric/anisohydric stomatal behaviour. Across species, the decline in Kleaf and gs due to aquaporin inhibition increased linearly with decreasing major vein density, possibly indicating that a trade-off function between vein architecture (apoplastic pathway) and aquaporin activity (cell-to-cell pathway) affects leaf hydraulics.

Drought and heat stress-related proteins: an update about their functional relevance in imparting stress tolerance in agricultural crops

We describe here the recent developments about the involvement of diverse stress-related proteins in sensing, signaling, and defending the cells in plants in response to drought or/and heat stress. In the current era of global climate drift, plant growth and productivity are often limited by various environmental stresses, especially drought and heat. Adaptation to abiotic stress is a multigenic process involving maintenance of homeostasis for proper survival under adverse environment. It has been widely observed that a series of proteins respond to heat and drought conditions at both transcriptional and translational levels. The proteins are involved in various signaling events, act as key transcriptional activators and saviors of plants under extreme environments. A detailed insight about the functional aspects of diverse stress-responsive proteins may assist in unraveling various stress resilience mechanisms in plants. Furthermore, by identifying the metabolic proteins associated with drought and heat tolerance, tolerant varieties can be produced through transgenic/recombinant technologies. A large number of regulatory and functional stress-associated proteins are reported to participate in response to heat and drought stresses, such as protein kinases, phosphatases, transcription factors, and late embryogenesis abundant proteins, dehydrins, osmotins, and heat shock proteins, which may be similar or unique to stress treatments. Few studies have revealed that cellular response to combined drought and heat stresses is distinctive, compared to their individual treatments. In this review, we would mainly focus on the new developments about various stress sensors and receptors, transcription factors, chaperones, and stress-associated proteins involved in drought or/and heat stresses, and their possible role in augmenting stress tolerance in crops.

Functional relationships between hydraulic traits and the timing of diurnal depression of photosynthesis

The hydraulic coordination along the water transport pathway helps trees provide adequate water supply to the canopy, ensuring that water deficits are minimized and that stomata remain open for CO₂ uptake. We evaluated the stem and leaf hydraulic coordination and the linkages between hydraulic traits and the timing of diurnal depression of photosynthesis across seven evergreen tree species in the southern Andes. There was a positive correlation between stem hydraulic conductivity (k_s ) and leaf hydraulic conductance (K_Leaf ) across species. All species had similar maximum photosynthetic rates (A_max ). The species with higher k_s and K_Leaf attained A_max in the morning, whereas the species with lower k_s and K_Leaf exhibited their A_max in the early afternoon concurrently with turgor loss. These latter species had very negative leaf water potentials, but far from the pressure at which the 88% of leaf hydraulic conductance is lost. Our results suggest that diurnal gas exchange dynamics may be determined by leaf hydraulic vulnerability such that a species more vulnerable to drought restrict water loss and carbon assimilation earlier than species less vulnerable. However, under stronger drought, species with earlier CO₂ uptake depression may increase the risk of hydraulic failure, as their safety margins are relatively narrow.

Aquaporins Respond to Chilling in the Phloem by Altering Protein and mRNA Expression

Previous experiments using heat exchangers (liquid cooled blocks) to chill a portion of plant stem have shown a transient stoppage in phloem translocation and an increase in measured phloem pressure. Although a chilled-induced stoppage of phloem transport has been known for over 100 years, the mechanism of this phenomenon is still poorly understood. Recently, work has highlighted that aquaporins occur within the plasma membrane of the sieve tubes along the entire source-to-sink pathway, and that isoforms of these water channel proteins may change dynamically. Aquaporins show regulatory roles in controlling tissue and cellular water status in response to environmental hardships. Thus, we tested if protein localization and mRNA transcript abundance changes occur in response to chilling in balsam poplar (Populus balsamifera) using immunohistochemistry and qrtPCR. The results of the immunolocalization experiments show that the labeling intensity of the sieve elements treated for only 2 min of chill time significantly increased for PIP2. After 10 min of chilling, this signal declined significantly to lower than that of the pre-chilled sieve elements. Overall, the abundance of mRNA transcript increased for the tested PIP2s following cold application. We discuss the implication that aquaporins are responsible for the alleviation of sieve tube pressure and the resumption of flow following a cold-induced blockage event.

Foliar water uptake: Processes, pathways, and integration into plant water budgets

Nearly all plant families, represented across most major biomes, absorb water directly through their leaves. This phenomenon is commonly referred to as foliar water uptake. Recent studies have suggested that foliar water uptake provides a significant water subsidy that can influence both plant water and carbon balance across multiple spatial and temporal scales. Despite this, our mechanistic understanding of when, where, how, and to what end water is absorbed through leaf surfaces remains limited. We first review the evidence for the biophysical conditions necessary for foliar water uptake to occur, focusing on the plant and atmospheric water potentials necessary to create a gradient for water flow. We then consider the different pathways for uptake, as well as the potential fates of the water once inside the leaf. Given that one fate of water from foliar uptake is to increase leaf water potentials and contribute to the demands of transpiration, we also provide a quantitative synthesis of observed rates of change in leaf water potential and total fluxes of water into the leaf. Finally, we identify critical research themes that should be addressed to effectively incorporate foliar water uptake into traditional frameworks of plant water movement.

Engineering Drought Resistance in Forest Trees

Climatic stresses limit plant growth and productivity. In the past decade, tree improvement programs were mainly focused on yield but it is obvious that enhanced stress resistance is also required. In this review we highlight important drought avoidance and tolerance mechanisms in forest trees. Genomes of economically important trees species with divergent resistance mechanisms can now be exploited to uncover the mechanistic basis of long-term drought adaptation at the whole plant level. Molecular tree physiology indicates that osmotic adjustment, antioxidative defense and increased water use efficiency are important targets for enhanced drought tolerance at the cellular and tissue level. Recent biotechnological approaches focused on overexpression of genes involved in stress sensing and signaling, such as the abscisic acid core pathway, and down-stream transcription factors. By this strategy, a suite of defense systems was recruited, generally enhancing drought and salt stress tolerance under laboratory conditions. However, field studies are still scarce. Under field conditions trees are exposed to combinations of stresses that vary in duration and magnitude. Variable stresses may overrule the positive effect achieved by engineering an individual defense pathway. To assess the usability of distinct modifications, large-scale experimental field studies in different environments are necessary. To optimize the balance between growth and defense, the use of stress-inducible promoters may be useful. Future improvement programs for drought resistance will benefit from a better understanding of the intricate networks that ameliorate molecular and ecological traits of forest trees.

Engineering Drought Resistance in Forest Trees

Climatic stresses limit plant growth and productivity. In the past decade, tree improvement programs were mainly focused on yield but it is obvious that enhanced stress resistance is also required. In this review we highlight important drought avoidance and tolerance mechanisms in forest trees. Genomes of economically important trees species with divergent resistance mechanisms can now be exploited to uncover the mechanistic basis of long-term drought adaptation at the whole plant level. Molecular tree physiology indicates that osmotic adjustment, antioxidative defense and increased water use efficiency are important targets for enhanced drought tolerance at the cellular and tissue level. Recent biotechnological approaches focused on overexpression of genes involved in stress sensing and signaling, such as the abscisic acid core pathway, and down-stream transcription factors. By this strategy, a suite of defense systems was recruited, generally enhancing drought and salt stress tolerance under laboratory conditions. However, field studies are still scarce. Under field conditions trees are exposed to combinations of stresses that vary in duration and magnitude. Variable stresses may overrule the positive effect achieved by engineering an individual defense pathway. To assess the usability of distinct modifications, large-scale experimental field studies in different environments are necessary. To optimize the balance between growth and defense, the use of stress-inducible promoters may be useful. Future improvement programs for drought resistance will benefit from a better understanding of the intricate networks that ameliorate molecular and ecological traits of forest trees.

The Causes of Leaf Hydraulic Vulnerability and Its Influence on Gas Exchange in Arabidopsis thaliana

The influence of the dynamics of leaf hydraulic conductance (K leaf) diurnally and during dehydration on stomatal conductance and photosynthesis remains unclear. Using the model species Arabidopsis (Arabidopsis thaliana ecotype Columbia-0), we applied a multitiered approach including physiological measurements, high-resolution x-ray microcomputed tomography, and modeling at a range of scales to characterize (1) K leaf decline during dehydration; (2) its basis in the hydraulic conductances of leaf xylem and outside-xylem pathways (K ox); (3) the dependence of its dynamics on irradiance; (4) its impact on diurnal patterns of stomatal conductance and photosynthetic rate; and (5) its influence on gas exchange and survival under simulated drought regimes. Arabidopsis leaves showed strong vulnerability to dehydration diurnally in both gas exchange and hydraulic conductance, despite lack of xylem embolism or conduit collapse above the turgor loss point, indicating a pronounced sensitivity of K ox to dehydration. K leaf increased under higher irradiance in well-hydrated leaves across the full range of water potential, but no shift in K leaf vulnerability was observed. Modeling indicated that responses to dehydration and irradiance are likely attributable to changes in membrane permeability and that a dynamic K ox would contribute strongly to stomatal closure, improving performance, survival, and efficient water use during drought. These findings for Columbia-0 provide a baseline for assessing variation across genotypes in hydraulic traits and their influence on gas exchange during dehydration.

The value of wet leaves

Contents Summary 1156 I. Introduction 1156 II. How often are leaves wet? 1157 III. The costs of leaf wetting 1157 IV. The real and potential benefits of leaf wetting 1161 V. Wet leaves: costs, benefits and tradeoffs in a changing world 1165 Acknowledgements 1166 References 1166 SUMMARY: An often-overlooked feature of all plants is that their leaf surfaces are wet for significant periods over their lifetimes. Leaf wetting has a number of direct and indirect effects on plant function from the scale of the leaf to that of the ecosystem. The costs of leaf wetting for plant function, such as the growth of pathogens and the leaching of nutrients, have long been recognized. However, an emerging body of research has also begun to demonstrate some very clear benefits. For instance, leaf wetting can improve plant-water relations and lead to increased photosynthesis. Leaf wetting may also lead to synergistic effects on plant function, such as when leaf water potential improvements lead to enhanced growth that does not occur when plant leaves are dry. We identify important reasons why leaf wetting can be critical for plant sciences to not only acknowledge, but also directly address, in future research. To do so, we provide a framework for the consideration of the relative balance of the various costs and benefits resulting from leaf wetting, as well as how this balance may be expected to change given projected scenarios of global climate change in the future.

Leaf hydraulic vulnerability triggers the decline in stomatal and mesophyll conductance during drought in rice

Understanding the physiological responses of crops to drought is important for ensuring sustained crop productivity under climate change, which is expected to exacerbate the frequency and intensity of periods of drought. Drought responses involve multiple traits, and the correlations between these traits are poorly understood. Using a variety of techniques, we estimated the changes in gas exchange, leaf hydraulic conductance, and leaf turgor in rice (Oryza sativa) in response to both short- and long-term soil drought. We performed a photosynthetic limitation analysis to quantify the contributions of each limiting factor to the resultant overall decrease in photosynthesis during drought. Biomass, leaf area, and leaf width significantly decreased during the 2-week drought treatment, but leaf mass per area and leaf vein density increased. Light-saturated photosynthetic rate declined dramatically during soil drought, mainly due to the decrease in stomatal conductance (gs) and mesophyll conductance (gm). Stomatal modeling suggested that the decline in leaf hydraulic conductance explained most of the decrease in stomatal closure during the drought treatment, and may also trigger the drought-related decrease of stomatal conductance and mesophyll conductance. The results of this study provide insight into the regulation of carbon assimilation under drought conditions.

Abiotic stresses influence the transcript abundance of PIP and TIP aquaporins in Festuca species

Festuca arundinacea and F. pratensis are the models in forage grasses to recognize the molecular basis of drought, salt and frost tolerance, respectively. Transcription profiles of plasma membrane intrinsic proteins (PIPs) and tonoplast intrinsic proteins (TIPs) aquaporin genes were obtained for leaves of Festuca species treated with different abiotic stimuli. F. arundinacea plants were exposed to drought and salt stress, whereas F. pratensis plants were cold-hardened. Changes in genes expression measured with use of real time qRT-PCR method were compared between two genotypes characterized with a significantly different level of each stress tolerance. Under drought the transcript level of PIP1;2 and TIP1;1 aquaporin decreased in both analyzed F. arundinacea genotypes, whereas for PIP2;1 only in a high drought tolerant plant. A salt treatment caused a reduction of PIP1;2 transcript level in a high salt tolerant genotype and an increase of TIP1;1 transcript abundance in both F. arundinacea genotypes, but it did not influence the expression of PIP2;1 aquaporin. During cold-hardening a decrease of PIP1;2, PIP2;1, and TIP1;1 aquaporin transcripts was observed, both in high and low frost tolerant genotypes. The obtained results revealed that the selected genotypes responded in a different way to abiotic stresses application. A reduced level of PIP1;2 transcript in F. arundinacea low drought tolerant genotype corresponded with a faster water loss and a lowering of photosynthesis efficiency and gas exchange during drought conditions. In F. pratensis, cold acclimation was associated with a lower level of aquaporin transcripts in both high and low frost tolerant genotypes. This is the first report on aquaporin transcriptional profiling under abiotic stress condition in forage grasses.

The causes and consequences of leaf hydraulic decline with dehydration

Resolving the drivers of hydraulic decline during drought is crucial for understanding drought tolerance in crops and natural ecosystems. In the past 15 years, studies of the decline of leaf hydraulic conductance (Kleaf) have supported a major role in controlling plant drought responses. We analyzed the variation in Kleaf decline with dehydration in a global database of 310 species, providing novel insights into its underlying mechanisms, its co-ordination with stem hydraulics, its influence on gas exchange and drought tolerance, and its linkage with species ecological distributions. Kleaf vulnerability varied strongly within and across lineages, growth forms, and biomes. A critical literature review indicates that changes in hydraulic conductance outside the xylem with dehydration drive the overall decline of Kleaf. We demonstrate a significant leaf hydraulic safety-efficiency trade-off across angiosperm species and discuss the importance of the large variation around this trend. Leaves tend to be more vulnerable than stems, with their vulnerabilities co-ordinated across species, and importantly linked with adaptation across biomes. We hypothesize a novel framework to explain diversity across species in the co-ordination of Kleaf and gas exchange during dehydration. These findings reflect considerable recent progress, yet new tools for measurement, visualization, and modeling will result in ongoing discoveries important across fields in plant biology.

Are phloem sieve tubes leaky conduits supported by numerous aquaporins?

PREMISE OF THE STUDY

Aquaporin membrane water channels have been previously identified in the phloem of angiosperms, but currently their cellular characterization is lacking, especially in tree species. Pinpointing the cellular location will help generate new hypotheses of how membrane water exchange facilitates sugar transport in plants.

METHODS

We studied histological sections of balsam poplar (Populus balsamifera L.) in leaf, petiole, and stem organs. Immuno-labeling techniques were used to characterize the distribution of PIP1 and PIP2 subfamilies of aquaporins along the phloem pathway. Confocal and super resolution microscopy (3D-SIM) was used to identify the localization of aquaporins at the cellular level.

KEY RESULTS

Sieve tubes of the leaf lamina, petiole, and stem were labeled with antibodies directed at PIP1s and PIP2s. While PIP2s were mostly observed in the plasma membrane, PIP1s showed both an internal membrane and plasma membrane labeling pattern.

CONCLUSIONS

The specificity and consistency of PIP2 labeling in sieve element plasma membranes points to high water exchange rates between sieve tubes and adjacent cells. The PIP1s may relocate between internal membranes and the plasma membrane to facilitate dynamic changes in membrane permeability of sieve elements in response to changing internal or environmental conditions. Aquaporin-mediated changes in membrane permeability of sieve tubes would also allow for some control of radial exchange of water between xylem and phloem.

Repression of ARF10 by microRNA160 plays an important role in the mediation of leaf water loss

Solanum lycopersicum auxin response factor 10 (SlARF10) is post-transcriptionally regulated by Sl-miR160. Overexpression of a Sl-miR160-resistant SlARF10 (mSlARF10) resulted in narrower leaflet blades with larger stomata but lower densities. 35S:mSlARF10-6 plants with narrower excised leaves had greater water loss, which was in contrast to the wild type (WT). Further analysis revealed that the actual water loss was not consistent with the calculated stomatal water loss in 35S:mSlARF10-6 and the WT under the dehydration treatment, indicating that there is a difference in hydraulic conductance. Pretreatment with abscisic acid (ABA) and HgCl2 confirmed higher hydraulic conductance in 35S:mSlARF10, which is related to the larger stomatal size and higher activity of aquaporins (AQPs). Under ABA treatment, 35S:mSlARF10-6 showed greater sensitivity, and the stomata closed rapidly. Screening by RNA sequencing revealed that five AQP-related genes, fourteen ABA biosynthesis/signal genes and three stomatal development genes were significantly altered in 35S:mSlARF10-6 plants, and this result was verified by qRT-PCR. The promoter analysis showed that upregulated AQPs contain AuxRE and ABRE, implying that these elements may be responsible for the high expression levels of AQPs in 35S:mSlARF10-6. The three most upregulated AQPs (SlTIP1-1-like, SlPIP2;4 and SlNIP-type-like) were chosen to confirm AuxRE and ABRE function. Promoters transient expression demonstrated that the SlPIP2;4 and SlNIP-type-like AuxREs and SlPIP2;4 and SlTIP1-1-like ABREs could significantly enhance the expression of the GUS reporter in 35S:mSlARF10-6, confirming that AuxRE and ABRE may be the main factors inducing the expression of AQPs. Additionally, two upregulated transcription factors in 35S:mSlARF10-6, SlARF10 and SlABI5-like were shown to directly bind to those elements in an electromobility shift assay and a yeast one-hybrid assay. Furthermore, transient expression of down-regulated ARF10 or up-regulated ABI5 in tomato leaves demonstrated that ARF10 is the direct factor for inducing the water loss in 35S:mSlARF10-6. Here, we show that although SlARF10 increased the ABA synthesis/signal response by regulating stomatal aperture to mitigate water loss, SlARF10 also influenced stomatal development and AQP expression to affect water transport, and both act cooperatively to control the loss of leaf water in tomato. Therefore, this study uncovers a previously unrecognized leaf water loss regulatory factor and a network for coordinating auxin and ABA signalling in this important process. In an evolutionary context, miR160 regulates ARF10 to maintain the water balance in the leaf, thus ensuring normal plant development and environmental adaptation.

The contribution of vascular and extra-vascular water pathways to drought-induced decline of leaf hydraulic conductance

Drought stress can impair leaf hydraulic conductance (Kleaf), but the relative contribution of changes in the efficiency of the vein xylem water pathway and in the mesophyll route outside the xylem in driving the decline of Kleaf is still debated. We report direct measurements of dehydration-induced changes in the hydraulic resistance (R=1/K) of whole leaf (Rleaf), as well as of the leaf xylem (Rx) and extra-vascular pathways (Rox) in four Angiosperm species. Rleaf, Rx, and Rox were measured using the vacuum chamber method (VCM). Rleaf values during progressive leaf dehydration were also validated with measurements performed using the rehydration kinetic method (RKM). We analysed correlations between changes in Rx or Rox and Rleaf, as well as between morpho-anatomical traits (including dehydration-induced leaf shrinkage), vulnerability to embolism, and leaf water relation parameters. Measurements revealed that the relative contribution of vascular and extra-vascular hydraulic properties in driving Kleaf decline during dehydration is species-specific. Whilst in two study species the progressive impairment of both vascular and extra-vascular pathways contributed to leaf hydraulic vulnerability, in the other two species the vascular pathway remained substantially unaltered during leaf dehydration, and Kleaf decline was apparently caused only by changes in the hydraulic properties of the extra-vascular compartment.

MzPIP2;1: An Aquaporin Involved in Radial Water Movement in Both Water Uptake and Transportation, Altered the Drought and Salt Tolerance of Transgenic Arabidopsis

BACKGROUND

Plants are unavoidably subjected to various abiotic stressors, including high salinity, drought and low temperature, which results in water deficit and even death. Water uptake and transportation play a critical role in response to these stresses. Many aquaporin proteins, localized at different tissues, function in various transmembrane water movements. We targeted at the key aquaporin in charge of both water uptake in roots and radial water transportation from vascular tissues through the whole plant.

RESULTS

The MzPIP2;1 gene encoding a plasma membrane intrinsic protein was cloned from salt-tolerant apple rootstock Malus zumi Mats. The GUS gene was driven by MzPIP2;1 promoter in transgenic Arabidopsis. It indicated that MzPIP2;1 might function in the epidermal and vascular cells of roots, parenchyma cells around vessels through the stems and vascular tissues of leaves. The ectopically expressed MzPIP2;1 conferred the transgenic Arabidopsis plants enhanced tolerance to slight salt and drought stresses, but sensitive to moderate salt stress, which was indicated by root length, lateral root number, fresh weight and K+/Na+ ratio. In addition, the possible key cis-elements in response to salt, drought and cold stresses were isolated by the promoter deletion experiment.

CONCLUSION

The MzPIP2;1 protein, as a PIP2 aquaporins subgroup member, involved in radial water movement, controls water absorption and usage efficiency and alters transgenic plants drought and salt tolerance.

Aquaporins in Coffea arabica L.: Identification, expression, and impacts on plant water relations and hydraulics

Plant aquaporins (AQPs) are involved in the transport of water and other small solutes across cell membranes, and thus play major roles in the regulation of plant water balance, as well as in growth regulation and response to abiotic stress factors. Limited information is currently available about the presence and role of AQPs in Coffea arabica L., despite the economic importance of the species and its vulnerability to drought stress. We identified candidate AQP genes by screening a proprietary C. arabica transcriptome database, resulting in the identification of nine putative aquaporins. A phylogenetic analysis based on previously characterized AQPs from Arabidopsis thaliana and Solanum tuberosum allowed to assign the putative coffee AQP sequences to the Tonoplast (TIP) and Plasma membrane (PIP) subfamilies. The possible functional role of coffee AQPs was explored by measuring hydraulic conductance and aquaporin gene expression on leaf and root tissues of two-year-old plants (C. arabica cv. Pacamara) subjected to different experimental conditions. In a first experiment, we tested plants for root and leaf hydraulic conductance both before dawn and at mid-day, to check the eventual impact of light on AQP activity and plant hydraulics. In a second experiment, we measured plant hydraulic responses to different water stress levels as eventually affected by changes in AQPs expression levels. Our results shed light on the possible roles of AQPs in the regulation of C. arabica hydraulics and water balance, opening promising research lines to improve the sustainability of coffee cultivation under global climate change scenarios.

Heterologous Expression of Two Jatropha Aquaporins Imparts Drought and Salt Tolerance and Improves Seed Viability in Transgenic Arabidopsis thaliana

Drought and high salinity are environmental conditions that cause adverse effects on the growth and productivity of crops. Aquaporins are small integral membrane proteins that belong to the family of the major intrinsic proteins (MIPs), with members in animals, plants and microbes, where they facilitate the transport of water and/or small neutral solutes thereby affecting water balance. In this study we characterized two aquaporin genes namely, plasma membrane intrinsic protein (PIP2;7) and tonoplast intrinsic protein TIP1;3 from Jatropha curcas that are localised to the plasma membrane and vacuole respectively. Transgenic Arabidopsis thaliana lines over-expressing JcPIP2;7 and JcTIP1;3 under a constitutive promoter show improved germination under high salt and mannitol compared to control seeds. These transgenic plants also show increased root length under abiotic stress conditions compared to wild type Col-0 plants. Transgenic lines exposed to drought conditions by withholding water for 20 days, were able to withstand water stress and attained normal growth after re-watering unlike control plants which could not survive. Transgenic lines also had better seed yield than control under salt stress. Importantly, seed viability of transgenic plants grown under high salt concentration was 35%-45% compared to less than 5% for control seeds obtained from plants growing under salt stress. The effect of JcPIP2;7 and JcTIP1;3 on improving germination and seed viability in drought and salinity make these important candidates for genetic manipulation of plants for growth in saline soils.