Effect of low root temperature on hydraulic conductivity of rice plants and the possible role of aquaporins

The poorly-explored stomatal response to temperature at constant evaporative demand

Changes in leaf temperature are known to drive stomatal responses, because the leaf-to-air water vapour gradient (Δw) increases with temperature if ambient vapour pressure is held constant, and stomata respond to changes in Δw. However, the direct response of stomata to temperature (DRST; the response when Δw is held constant by adjusting ambient humidity) has been examined far less extensively. Though the meagre available data suggest the response is usually positive, results differ widely and defy broad generalisation. As a result, little is known about the DRST. This review discusses the current state of knowledge about the DRST, including numerous hypothesised biophysical mechanisms, potential implications of the response for plant adaptation, and possible impacts of the DRST on plant-atmosphere carbon and water exchange in a changing climate.

Genetic variation in the aquaporin TONOPLAST INTRINSIC PROTEIN 4;3 modulates maize cold tolerance

Cold stress is a major abiotic stress that threatens maize (Zea mays L.) production worldwide. Understanding the molecular mechanisms underlying cold tolerance is crucial for breeding resilient maize varieties. Tonoplast intrinsic proteins (TIPs) are a subfamily of aquaporins in plants. Here, we report that TIP family proteins are involved in maize cold tolerance. The expression of most TIP genes was responsive to cold stress. Overexpressing TIP2;1, TIP3;2 or TIP4;3 reduced the cold tolerance of maize seedlings, while loss-of-function mutants of TIP4;3 exhibited enhanced cold tolerance. Candidate gene-based association analysis revealed that a 328-bp transposon insertion in the promoter region of TIP4;3 was strongly associated with maize cold tolerance. This transposon insertion conferred cold tolerance by repressing TIP4;3 expression through increased methylation of its promoter region. Moreover, TIP4;3 was found to suppress stomatal closure and facilitate reactive oxygen species (ROS) accumulation under cold stress, thereby inhibiting the expression of cold-responsive genes, including DEHYDRATION-RESPONSIVE ELEMENT BINDING FACTOR 1 (DREB1) genes and a subset of peroxidase genes, ultimately attenuating maize cold tolerance. This study thus elucidates the mechanism underlying TIP-mediated cold tolerance and identifies a favourable TIP4;3 allele as a potential genetic resource for breeding cold-tolerant maize varieties.

The sensitivity of root water uptake to cold root temperature follows species-specific upper elevational distribution limits of temperate tree species

Physiological water stress induced by low root temperatures might contribute to species-specific climatic limits of tree distribution. We investigated the low temperature sensitivity of root water uptake and transport in seedlings of 16 European tree species which reach their natural upper elevation distribution limits at different distances to the alpine treeline. We used 2H-H2O pulse-labelling to quantify the water uptake and transport velocity from roots to leaves in seedlings exposed to constant 15°C, 7°C or 2°C root temperature, but identical aboveground temperatures between 20°C and 25°C. In all species, low root temperatures reduced the water transport rate, accompanied by reduced stem water potentials and stomatal conductance. At 7°C root temperature, the relative water uptake rates among species correlated positively with the species-specific upper elevation limits, indicating an increasingly higher sensitivity to lower root zone temperatures, the lower a species' natural elevational distribution limit. Conversely, 2°C root temperature severely inhibited water uptake in all species, irrespective of the species' thermal elevational limits. We conclude that low temperature-induced hydraulic constraints contribute to the cold distribution limits of temperate tree species and are a potential physiological cause behind the low temperature limits of plant growth in general.

Seeds Priming with Melatonin Improves Root Hydraulic Conductivity of Wheat Varieties under Drought, Salinity, and Combined Stress

Drought and salinity stress reduce root hydraulic conductivity of plant seedlings, and melatonin application positively mitigates stress-induced damage. However, the underlying effect of melatonin priming on root hydraulic conductivity of seedlings under drought-salinity combined remains greatly unclear. In the current report, we investigated the influence of seeds of three wheat lines' 12 h priming with 100 μM of melatonin on root hydraulic conductivity (Lpr) and relevant physiological indicators of seedlings under PEG, NaCl, and PEG + NaCl combined stress. A previous study found that the combined PEG and NaCl stress remarkably reduced the Lpr of three wheat varieties, and its value could not be detected. Melatonin priming mitigated the adverse effects of combined PEG + NaCl stress on Lpr of H4399, Y1212, and X19 to 0.0071 mL·h-1·MPa-1, 0.2477 mL·h-1·MPa-1, and 0.4444 mL·h-1·MPa-1, respectively, by modulating translation levels of aquaporin genes and contributed root elongation and seedlings growth. The root length of H4399, Y1212, and X19 was increased by 129.07%, 141.64%, and 497.58%, respectively, after seeds pre-treatment with melatonin under PEG + NaCl combined stress. Melatonin -priming appreciably regulated antioxidant enzyme activities, reduced accumulation of osmotic regulators, decreased levels of malondialdehyde (MDA), and increased K+ content in stems and root of H4399, Y1212, and X19 under PEG + NaCl stress. The path investigation displayed that seeds primed with melatonin altered the modification of the path relationship between Lpr and leaf area under stress. The present study suggested that melatonin priming was a strategy as regards the enhancement of root hydraulic conductivity under PEG, NaCl, and PEG + NaCl stress, which efficiently enhanced wheat resistant to drought-salinity stress.

Terahertz spectroscopic monitoring and analysis of citrus leaf water status under low temperature stress

Low temperature stress, in the form of chilling and freezing, is one of the major environmental factors impacting on citrus yield, which changes plant's water state and results in the crops' sub-health or injury. The innovative terahertz (THz) spectroscopy and imaging based sensing technology has been shown to be a suitable tool for plant leaf water status determination, due to THz radiation's innate sensitivity to hydrogen bond vibration in aqueous solutions, which is usually related to plant phenotype change. We demonstrate experimentally that the THz absorption coefficient of leaf could be used for distinguishing plant's physiological stress status, exhibiting clear decreasing or increasing trend under chilling or freezing stress respectively. The underlying rationale might be that membrane damage shows a diverse pattern, changing the intra- or extra-cellular liquid environments, likely being linked to the various THz spectral characteristics. There were different adaptations in leaf morphology, leading to different leaf density, which in turn affects the water volume fraction. Moreover, different patterns of the dynamic equilibrium state of free water and bound water under chilling and freezing treatment were revealed by THz spectroscopy. Here, THz spectroscopic monitoring has shown unique potential for judging citrus's low temperature stress state through bio-water detection and discrimination.

Genome-wide identification of RsGRAS gene family reveals positive role of RsSHRc gene in chilling stress response in radish (Raphanus sativus L.)

Radish (Raphanus sativus L.) is an important worldwide root vegetable crop. Little information of the GRAS gene family was available in radish. Herein, a total of 51 GRAS family members were firstly identified from radish genome, and unevenly located onto nine radish chromosomes. Expression analysis of RsGRAS genes in taproot displayed that RsSCL15a and RsSHRc were highly expressed in the radish cambium, and its expression level was increased with the taproot thickening. Comparative transcriptome analysis revealed that the expression patterns of RsGRAS genes varied upon exposure to different abiotic stresses including heavy metals, salt and heat. The expression level of six RsGRAS genes including RsSHRc was increased under chilling stress in two radish genotypes with different cold tolerance. Further analysis indicated that RsGRAS genes could respond to cold stress rapidly and the expression of RsSHRc was up-regulated at different development stages (cortex splitting and thickening stages) under long-term cold treatment. Transient expression of RsSHRc gene in radish showed that RsSHRc possessed the reliable function of eliminating reactive oxygen species (ROS), inhibiting the formation of malondialdehyde (MDA) and promoting to accumulate proline under cold stress. Together, these findings provided insights into the function of RsGRAS genes in taproot development and chilling stress response in radish.

A model bridging waterlogging, stomatal behavior and water use in trees in drained peatland

Waterlogging causes hypoxic or anoxic conditions in soils, which lead to decreases in root and stomatal hydraulic conductance. Although these effects have been observed in a variety of plant species, they have not been quantified continuously over a range of water table depths (WTD) or soil water contents (SWC). To provide a quantitative theoretical framework for tackling this issue, we hypothesized similar mathematical descriptions of waterlogging and drought effects on whole-tree hydraulics and constructed a hierarchical model by connecting optimal stomata and soil-to-leaf hydraulic conductance models. In the model, the soil-to-root conductance is non-monotonic with WTD to reflect both the limitations by water under low SWC and by hypoxic effects associated with inhibited oxygen diffusion under high SWC. The model was parameterized using priors from literature and data collected over four growing seasons from Scots pine (Pinus sylvestris L.) trees grown in a drained peatland in Finland. Two reference models (RMs) were compared with the new model, RM1 with no belowground hydraulics and RM2 with no waterlogging effects. The new model was more accurate than the RMs in predicting transpiration rate (fitted slope of measured against modeled transpiration rate = 0.991 vs 0.979 (RM1) and 0.984 (RM2), R2 = 0.801 vs 0.665 (RM1) and 0.776 (RM2)). Particularly, RM2's overestimation of transpiration rate under shallow water table conditions (fitted slope = 0.908, R2 = 0.697) was considerably reduced by the new model (fitted slope = 0.956, R2 = 0.711). The limits and potential improvements of the model are discussed.

Stomatal conductance, not biochemistry, drives low temperature acclimation of photosynthesis in Populus balsamifera, regardless of nitrogen availability

Low-temperature thermal acclimation may require adjustments to N and water use to sustain photosynthesis because of slow enzyme functioning and high water viscosity. However, understanding of photosynthetic acclimation to temperatures below 11 °C is limited. We acclimated Populus balsamifera to 6 °C and 10 °C (6A and 10A, respectively) and provided the trees with either high or low N fertilizer. We measured net CO₂ assimilation (A_net ), stomatal conductance (g_s ), maximum rates of Rubisco carboxylation (V_cmax ), electron transport (J_max ) and dark respiration (R_d ) at leaf temperatures of 2, 6, 10, 14 and 18 °C, along with leaf N concentrations. The 10A trees had higher A_net than the 6A trees at warmer leaf temperatures, which was correlated with higher g_s in the 10A trees. The instantaneous temperature responses of V_cmax , J_max and R_d were similar for trees from both acclimation temperatures. While soil N availability increased leaf N concentrations, this had no effect on acclimation of photosynthesis or respiration. Our results indicate that acclimation below 11 °C occurred primarily through changes in stomatal conductance, not photosynthetic biochemistry, and was unaffected by short-term N supply. Thermal acclimation of stomatal conductance should therefore be a priority for future carbon cycle model development.

Low-temperature acclimation and legacy effects of summer water deficits in olive freezing resistance

Low temperatures and drought are the main environmental factors affecting plant growth and productivity across most of the terrestrial biomes. The objective of this study was to analyze the effects of water deficits before the onset of low temperatures in winter to enhance freezing resistance in olive trees. The study was carried out near the coast of Chubut, Argentina. Plants of five olive cultivars were grown outdoor in pots and exposed to different water deficit treatments. We assessed leaf water relations, ice nucleation temperature (INT), cell damage (LT50), plant growth and leaf nitrogen content during summer and winter in all cultivars and across water deficit treatments. Leaf INT and LT50 decreased significantly from summer to winter within each cultivar and between treatments. We observed a trade-off between resources allocation to freezing resistance and vegetative growth, such that an improvement in resistance to sub-zero temperatures was associated with lower growth in tree height. Water deficit applied during summer increased the amount of osmotically active solutes and decreased the leaf water potentials. This type of legacy effect persists during the winter after the water deficit even when treatment was removed by natural rainfalls.

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.

Proofing Direct-Seeded Rice with Better Root Plasticity and Architecture

The underground reserve (root) has been an uncharted research territory with its untapped genetic variation yet to be exploited. Identifying ideal traits and breeding new rice varieties with efficient root system architecture (RSA) has great potential to increase resource-use efficiency and grain yield, especially under direct-seeded rice, by adapting to aerobic soil conditions. In this review, we tried to mine the available research information on the direct-seeded rice (DSR) root system to highlight the requirements of different root traits such as root architecture, length, number, density, thickness, diameter, and angle that play a pivotal role in determining the uptake of nutrients and moisture at different stages of plant growth. RSA also faces several stresses, due to excess or deficiency of moisture and nutrients, low or high temperature, or saline conditions. To counteract these hindrances, adaptation in response to stress becomes essential. Candidate genes such as early root growth enhancer PSTOL1, surface rooting QTL qSOR1, deep rooting gene DRO1, and numerous transporters for their respective nutrients and stress-responsive factors have been identified and validated under different circumstances. Identifying the desired QTLs and transporters underlying these traits and then designing an ideal root architecture can help in developing a suitable DSR cultivar and aid in further advancement in this direction.

Interrelations of nutrient and water transporters in plants under abiotic stress

Environmental changes cause abiotic stress in plants, primarily through alterations in the uptake of the nutrients and water they require for their metabolism and growth and to maintain their cellular homeostasis. The plasma membranes of cells contain transporter proteins, encoded by their specific genes, responsible for the uptake of nutrients and water (aquaporins). However, their interregulation has rarely been taken into account. Therefore, in this review we identify how the plant genome responds to abiotic stresses such as nutrient deficiency, drought, salinity and low temperature, in relation to both nutrient transporters and aquaporins. Some general responses or regulation mechanisms can be observed under each abiotic stress such as the induction of plasma membrane transporter expression during macronutrient deficiency, the induction of tonoplast transporters and reduction of aquaporins during micronutrients deficiency. However, drought, salinity and low temperatures generally cause an increase in expression of nutrient transporters and aquaporins in tolerant plants. We propose that both types of transporters (nutrients and water) should be considered jointly in order to better understand plant tolerance of stresses.

Aquaporins in Cereals-Important Players in Maintaining Cell Homeostasis under Abiotic Stress

Cereal productivity is reduced by environmental stresses such as drought, heat, elevated CO₂, salinity, metal toxicity and cold. Sometimes, plants are exposed to multiple stresses simultaneously. Plants must be able to make a rapid and adequate response to these environmental stimuli in order to restore their growing ability. The latest research has shown that aquaporins are important players in maintaining cell homeostasis under abiotic stress. Aquaporins are membrane intrinsic proteins (MIP) that form pores in the cellular membranes, which facilitate the movement of water and many other molecules such as ammonia, urea, CO₂, micronutrients (silicon and boron), glycerol and reactive oxygen species (hydrogen peroxide) across the cell and intercellular compartments. The present review primarily focuses on the diversity of aquaporins in cereal species, their cellular and subcellular localisation, their expression and their functioning under abiotic stresses. Lastly, this review discusses the potential use of mutants and plants that overexpress the aquaporin-encoding genes to improve their tolerance to abiotic stress.

The Wild Rice Locus CTS-12 Mediates ABA-Dependent Stomatal Opening Modulation to Limit Water Loss Under Severe Chilling Stress

A near-isogenic line (NIL) DC90 which was generated by introgressing a wild rice (Oryza rufipogon Griff.) locus CTS-12 into the 9311(Oryza sativa L. ssp. indica) background confers chilling tolerance phenotype. Here, our pilot trials showed that chilling tolerance was positively correlated with abscisic acid (ABA) biosynthesis. To understand how CTS-12 mediated the ABA-dependent multi-levels of regulation, the integration of transcriptomic and metabolomic profiling using the two-way orthogonal projections to latent structures (O2PLS) and discriminant analysis (OPLS-DA) modeling was performed to investigate the mechanisms underlying chilling tolerance. Our results revealed that metabolic shifts, including the activation of stachyose biosynthesis, amino acid metabolism pathways, phenylpropanoid/flavonoid biosynthesis, ABA biosynthesis, and perturbation of glycolysis, occurred under chilling treatment; in the recovery period, glutamate-related pathways, β-alanine biosynthesis and degradation, and serotonin biosynthesis pathways were differentiated between 9311 and DC90. Particularly, the differentially accumulated metabolites (DAMs) and differentially expressed genes (DEGs), including galactinol, β-alanine, glutamate, naringenin, serotonin, ABA, and LOC_Os03g44380 (9-cis-epoxycarotenoid dioxygenase 3, OsNCED3), might be involved in the chilling tolerance variation of 9311 and DC90. CRISPR/Cas9-edited OsNCED3 resulted in chilling sensitive of japonica rice ZH11, demonstrating the involvement of ABA pathway in chilling stress response. In addition, chilling tolerance of rice was associated with the balance of water uptake and loss that was modulated by stomatal movement under chilling stress. Therefore, we speculated that the CTS-12-mediated ABA signaling pathway leads to transcriptional regulation of chilling-responsive genes and, in turn, triggers metabolic shifts to coordinately regulate the stomatal movement of guard cells. The results of this study improve our understanding of the multilevel regulation of wild rice in response to chilling stress.

Photosynthetic activity of indica rice sister lines with contrasting cold tolerance

Incidence of cold during early stages is an obstacle for the growing progress of rice plants. Cold stress has strong negative effects on photosynthetic activity. Previously, our group evaluated plant survival of 90 indica rice genotypes after cold treatment. Two sister lines were characterized as cold-tolerant and cold-sensitive. Transcriptomic analyses of the same genotypes had indicated differential expression of genes related to photosynthesis. Previous work with japonica rice had suggested that cold sensitivity was more related to photosystem II (PSII) than to photosystem I (PSI). Using our previously identified contrasting genotypes, we investigated the role of specific steps of the photosynthetic process in cold tolerance/sensitivity of indica rice plants during and after (recovery period) cold exposure. During both cold treatment and recovery period, the photochemical activity (including PSII and PSI) presented higher levels in the low temperature-tolerant genotype, when compared with the sensitive one. The higher photochemical efficiency during the cold treatment appears to be related to a lower fraction of reduced QA - in PSII. We also observed lower transpiration rates and higher water use efficiency in the cold-tolerant genotype, due to stomatal closure. After the recovery period, the higher efficiency in the cold-tolerant genotype seems to be related to a lower fraction of reduced QA - and a larger pool of final electron acceptors at the PSI. This work uncovered changes in photosynthetic performance including both photosystems and improved water use efficiency which may be important components of cold tolerance mechanisms in indica rice.

Ectopic expression of a rice plasma membrane intrinsic protein (OsPIP1;3) promotes plant growth and water uptake

Plasma membrane intrinsic proteins (PIPs) are known to be major facilitators of the movement of a number of substrates across cell membranes. From a drought-resistant cultivar of Oryza sativa (rice), we isolated an OsPIP1;3 gene single-nucleotide polymorphism (SNP) that is mostly expressed in rice roots and is strongly responsive to drought stress. Immunocytochemistry showed that OsPIP1;3 majorly accumulated on the proximal end of the endodermis and the cell surface around the xylem. Expression of GFP-OsPIP1;3 alone in Xenopus oocytes or rice protoplasts showed OsPIP1;3 mislocalization in the endoplasmic reticulum (ER)-like neighborhood, whereas co-expression of OsPIP2;2 recruited OsPIP1;3 to the plasma membrane and led to a significant enhancement of water permeability in oocytes. Moreover, reconstitution of 10×His-OsPIP1;3 in liposomes demonstrated water channel activity, as revealed by stopped-flow light scattering. Intriguingly, by patch-clamp technique, we detected significant NO₃^- conductance of OsPIP1;3 in mammalian cells. To investigate the physiological functions of OsPIP1;3, we ectopically expressed the OsPIP1;3 gene in Nicotiana benthamiana (tobacco). The transgenic tobacco plants exhibited higher photosynthesis rates, root hydraulic conductivity (Lp_r ) and water-use efficiency, resulting in a greater biomass and a higher resistance to water deficit than the wild-type did. Further experiments suggested that heterologous expression of OsPIP1;3 in cyanobacterium altered bacterial growth under different conditions of CO₂ gas supply. Overall, besides shedding light on the multiple functions played by OsPIP1;3, this work provides insights into the translational value of plant AQPs.

Genotypic variability in radial resistance to water flow in olive roots and its response to temperature variations

As radial root resistance (Rp) represents one of the key components of the soil-plant-atmosphere continuum resistance catena modulating water transport, understanding its control is essential for physiologists, modelers and breeders. Reports of Rp, however, are still scarce and scattered in the scientific literature. In this study, we assessed genetic variability in Rp and its dependence on temperature in five widely used olive cultivars. In a first experiment, cultivar differences in Rp at 25 °C were evaluated from flow-pressure measurements in excised roots and subsequent analysis of root traits. In a second experiment, similar determinations were performed continually over a 5-h period in which temperature was gradually increased from 12 to 32 °C, enabling the assessment of Rp response to changing temperature. Despite some variability, our results did not show statistical differences in Rp among cultivars in the first experiment. In the second, cultivar differences in Rp were not significant at 12 °C, but they became so as temperature increased. Furthermore, the changes in Rp between 12 and 32 °C were higher than those expected by the temperature-driven decrease in water viscosity, with the degree of that change differing among cultivars. Also, Rp at 25 °C reached momentarily in the second experiment was consistently higher than in the first at that same, but fixed, temperature. Overall, our results suggest that there is limited variability in Rp among the studied cultivars when plants have been exposed to a given temperature for sufficient time. Temperature-induced variation in Rp might thus be partly explained by changes in membrane permeability that occur slowly, which explains why our values at 25 °C differed between experiments. The observed cultivar differences in Rp with warming also indicate faster acclimation of Rp to temperature changes in some cultivars than others.

Plasma Membrane Aquaporin Members PIPs Act in Concert to Regulate Cold Acclimation and Freezing Tolerance Responses in Arabidopsis thaliana

Aquaporins play a major role in plant water uptake at both optimal and environmentally stressed conditions. However, the functional specificity of aquaporins under cold remains obscure. To get a better insight to the role of aquaporins in cold acclimation and freezing tolerance, we took an integrated approach of physiology, transcript profiling and cell biology in Arabidopsis thaliana. Cold acclimation resulted in specific upregulation of PIP1;4 and PIP2;5 aquaporin (plasma membrane intrinsic proteins) expression, and immunoblotting analysis confirmed the increase in amount of PIP2;5 protein and total amount of PIPs during cold acclimation, suggesting that PIP2;5 plays a major role in tackling the cold milieu. Although single mutants of pip1;4 and pip2;5 or their double mutant showed no phenotypic changes in freezing tolerance, they were more sensitive in root elongation and cell survival response under freezing stress conditions compared with the wild type. Consistently, a single mutation in either PIP1;4 or PIP2;5 altered the expression of a number of aquaporins both at the transcriptional and translational levels. Collectively, our results suggest that aquaporin members including PIP1;4 and PIP2;5 function in concert to regulate cold acclimation and freezing tolerance responses.

The influence of soil temperature and water content on belowground hydraulic conductance and leaf gas exchange in mature trees of three boreal species

Understanding stomatal regulation is fundamental to predicting the impact of changing environmental conditions on vegetation. However, the influence of soil temperature (ST) and soil water content (SWC) on canopy conductance (g_s ) through changes in belowground hydraulic conductance (k_bg ) remains poorly understood, because k_bg has seldom been measured in field conditions. Our aim was to (a) examine the dependence of k_bg on ST and SWC, (b) examine the dependence of g_s on k_bg and (c) test a recent stomatal optimization model according to which g_s and soil-to-leaf hydraulic conductance are strongly coupled. We estimated k_bg from continuous sap flow and xylem diameter measurements in three boreal species. k_bg increased strongly with increasing ST when ST was below +8°C, and typically increased with increasing SWC when ST was not limiting. g_s was correlated with k_bg in all three species, and modelled and measured g_s were well correlated in Pinus sylvestris (a model comparison was only possible for this species). These results imply an important role for k_bg in mediating linkages between the soil environment and leaf gas exchange. In particular, our finding that ST strongly influences k_bg in mature trees may help us to better understand tree behaviour in cold environments.

Effects of Root Cooling on Plant Growth and Fruit Quality of Cocktail Tomato during Two Consecutive Seasons

Understanding the effects of root temperature on plant growth and key food components of horticultural crops under greenhouse conditions is important. Here, we assess the impact of root cooling on plant growth and fruit quality of two cocktail tomato cultivars (Lycopersicon esculentum cv “Amoroso” and cv “Delioso”) during the winter of 2017-2018 and the summer of 2018. Plants were grown hydroponically on rockwool under different root temperatures (16–27°C and 10°C) from the 2nd inflorescence to harvest inside the greenhouse. A root temperature of 10°C was controlled independently from air temperature (18–23°C in winter and 21–29°C in summer) by circulating cooling water. Reductions of marketable yield per plant (7.9–20.9%) in both cultivars were observed in response to root cooling in winter, but not significantly in summer. In most cases, root cooling had a positive effect on the functional quality (sugars, vitamin C, and carotenoids levels). In the case of “Delioso,” glucose concentration increased by 7.7–10.3%, vitamin C by 20–21%, and lycopene by 16.9–20.5% in both seasons. “Amoroso” exhibited only higher consistent values in glucose with increments between 6.9 and 7.8% in the two seasons. The levels of elements decreased by root cooling, with statistically significant reduction of N, P, S, and Fe by 12.1–15.7% in “Delioso” in winter and P and Zn by 9.1–22.2% in both cultivars in summer. Thus, manipulation of root temperature could be a feasible method to improve the overall fruit quality of cocktail tomato; however, this effect was also dependent on cultivars and other environmental factors.

Changes in apoplastic peroxidase activity and cell wall composition are associated with cold-induced morpho-anatomical plasticity of wheat leaves

Temperate grasses, such as wheat, become compact plants with small thick leaves after exposure to low temperature. These responses are associated with cold hardiness, but their underlying mechanisms remain largely unknown. Here we analyse the effects of low temperature on leaf morpho-anatomical structure, cell wall composition and activity of extracellular peroxidases, which play key roles in cell elongation and cell wall thickening, in two wheat cultivars with contrasting cold-hardening ability. A combined microscopy and biochemical approach was applied to study actively growing leaves of winter (ProINTA-Pincén) and spring (Buck-Patacón) wheat developed under constant warm (25 °C) or cool (5 °C) temperature. Cold-grown plants had shorter leaves but longer inter-stomatal epidermal cells than warm-grown plants. They had thicker walls in metaxylem vessels and mestome sheath cells, paralleled with accumulation of wall components, predominantly hemicellulose. These effects were more pronounced in the winter cultivar (Pincén). Cold also induced a sharp decrease in apoplastic peroxidase activity within the leaf elongating zone of Pincén, and a three-fold increase in the distal mature zone of the leaf. This was consistent with the enhanced cell length and thicker cell walls in this cultivar at 5 °C. The different response to low temperature of apoplastic peroxidase activity and hemicellulose between leaf zones and cultivar types suggests they might play a central role in the development of cold-induced compact morphology and cold hardening. New insights are presented on the potential temperature-driven role of peroxidases and hemicellulose in cell wall dynamics of grasses.

Effect of aquaporin 3 knockdown by RNA interference on antrum formation in sheep secondary follicles cultured in vitro

SummaryThe objectives were to develop an effective protocol for transfection of ovine secondary follicles and to assess the effect of attenuating aquaporin 3 (AQP3) using a small interfering RNA (siRNA-AQP3) on antrum formation and follicular growth in vitro. Various combinations of Lipofectamine® volumes (0.5, 0.75 or 1.0 µl), fluorescent oligonucleotide (BLOCK-iT ™) concentrations (3.18, 27.12 or 36.16 nM) and exposure times (12, 14, 16, 18 or 20 h) were tested. The BLOCK-iT™ was replaced by siRNA-AQP3 in the transfection complex. Ovine secondary follicles were isolated and cultured in vitro for 6 days using standard protocols. Follicles were transfected on day 0 or 3 or on both days (0 and 3) and then cultured for an additional 3 or 6 days. As revealed by the fluorescence signal, the Lipofectamine®/BLOCK-iT™ complex (0.75 µl + 27.12 nM by 12 h of incubation) crossed the basement membrane and granulosa cell and reached the oocytes. In general, the rate of intact follicles was higher and the rate of antrum formation was lower in transfected follicles compared with control follicles. In conclusion, ovine secondary follicles can be successfully transfected during in vitro culture, and siRNA-mediated attenuation of AQP3 gene reduced antrum formation of secondary follicles.

The cost of avoiding freezing in stems: trade-off between xylem resistance to cavitation and supercooling capacity in woody plants

Stems and leaves of Olea europaea L. (olive) avoid freezing damage by substantial supercooling during the winter season. Physiological changes during acclimation to low temperatures were studied in five olive cultivars. Water relations and hydraulic traits, ice nucleation temperature (INT) and temperatures resulting in 50% damage (LT50) were determined. All cultivars showed a gradual decrease in INT and LT50 from the dry and warm summer to the wet and cold winter in Patagonia, Argentina. During acclimation to low temperatures there was an increase in leaf cell wall rigidity and stomatal conductance (gs), as well as a decrease in leaf apoplastic water content, leaf water potential (Ψ), sap flow and stem hydraulic conductivity (ks). More negative Ψ as a consequence of high gs and detrimental effects of low temperatures on root activity resulted in a substantial loss of ks due to embolism formation. Seasonal stem INT decrease from summer to winter was directly related to the xylem resistance to cavitation, determined by the loss of ks across cultivars. Thus the loss of freezable water in xylem vessels by embolisms increased stem supercooling capacity and delayed ice propagation from stems to the leaves. For the first time, a trade-off between xylem resistance to cavitation and stem and leaf supercooling capacity was observed in plants that avoid extracellular freezing by permanent supercooling. The substantial loss of hydraulic function in olive cultivar stems by embolism formation with their high repair costs are compensated by avoiding plant damage at very low subzero temperatures.

Changes in root hydraulic conductivity facilitate the overall hydraulic response of rice (Oryza sativa L.) cultivars to salt and osmotic stress

The aim of the present work was to assess the significance of changes in root AQP gene expression and hydraulic conductivity (Lp) in the regulation of water balance in two hydroponically-grown rice cultivars (Azucena, Bala) which differ in root morphology, stomatal regulation and aquaporin (AQP) isoform expression. Plants were exposed to NaCl (25 mM, 50 mM) and osmotic stress (5%, 10% PEG6000). Root Lp was determined for exuding root systems (osmotic forces driving water uptake; 'exudation Lp') and transpiring plants (hydrostatic forces dominating; 'transpiration-Lp'). Gene expression was analysed by qPCR. Stress treatments caused a consistent and significant decrease in plant growth, transpirational water loss, stomatal conductance, shoot-to-root surface area ratio and root Lp. Comparison of exudation-with transpiration-Lp supported a significant contribution of AQP-facilitated water flow to root water uptake. Changes in root Lp in response to treatments were correlated much stronger with root morphological characteristics, such as the number of main and lateral roots, surface area ratio of root to shoot and plant transpiration rate than with AQP gene expression. Changes in root Lp, involving AQP function, form an integral part of the plant hydraulic response to stress and facilitate changes in the root-to-shoot surface area ratio, transpiration and stomatal conductance.

Aquaporin 8 involvement in human cervical cancer SiHa migration via the EGFR-Erk1/2 pathway

Overexpression of aquaporins (AQPs) has been reported in several human cancers. Epidermal growth factor receptor (EGFR)-extracellular signal-regulated kinases 1/2 (Erk1/2) are associated with tumorigenesis and cancer progression and may upregulate AQP expression. In this study, we demonstrated that EGF (epidermal growth factor) induces SiHa cells migration and AQP8 expression. Wound healing results showed that cell migration was increased by 2.79-1.50-fold at 24 h and 48 h after EGF treatment. AQP8 expression was significantly increased (3.33-fold) at 48 h after EGF treatment in SiHa cells. An EGFR kinase inhibitor, PD153035, blocked EGF-induced AQP8 expression and cell migration and AQP8 expression was decreased from 1.59-fold (EGF-treated) to 0.43-fold (PD153035-treated) in SiHa. Furthermore, the MEK (MAPK (mitogen-activated protein kinase)/Erk (extracellular signal regulated kinase)/Erk inhibitor U0126 also inhibited EGF-induced AQP8 expression and cell migration. AQP8 expression was decreased from 1.21-fold (EGF-treated) to 0.43-fold (U0126-treated). Immunofluorescence microscopy further confirmed the results. Collectively, our findings show that EGF induces AQP8 expression and cell migration in human cervical cancer SiHa cells via the EGFR/Erk1/2 signal transduction pathway.

Effects of activation energy and activation volume on the temperature-dependent viscosity of water

Water transport in a leaf is vulnerable to viscosity-induced changes. Recent research has suggested that these changes may be partially due to variation at the molecular scale, e.g., regulations via aquaporins, that induce reductions in leaf hydraulic conductance. What are the quantitative as well as qualitative changes in temperature-dependent viscosity due to the role of aquaporins in tuning activation energy and activation volume? Using the transition-state approach as well as the boundary perturbation method, we investigate temperature-dependent viscosity tuned by activation energy and activation volume. To validate our approach, we compare our numerical results with previous temperature-dependent viscosity measurements. The rather good fit between our calculations and measurements confirms our present approach. We have obtained critical parameters for the temperature-dependent (shear) viscosity of water that might be relevant to the increasing and reducing of leaf hydraulic conductance. These parameters are sensitive to temperature, activation energy, and activation volume. Once the activation energy increases, the (shear) viscosity of water increases. Our results also show that as the activation volume increases (say, 10^{-23}m^{3}), the (shear) viscosity of water decreases significantly and the latter induces the enhancing of leaf hydraulic conductance. Within the room-temperature regime, a small increase in the activation energy will increase the water viscosity or reduce the leaf hydraulic conductance. Our approach and results can be applied to diverse plant or leaf attributes.

Root-Zone Warming Differently Benefits Mature and Newly Unfolded Leaves of Cucumis sativus L. Seedlings under Sub-Optimal Temperature Stress

Sub-optimal temperature extensively suppresses crop growth during cool-season greenhouse production. Root-zone (RZ) warming is considered an economical option to alleviate crop growth reduction. In this study we cultivated cucumber seedlings in nutrient solution under different air-RZ temperature treatments to investigate the effects of RZ warming on seedling growth- and photosynthesis-related parameters in leaves. The air-RZ temperature treatments included sub-optimal RZ temperature 13°C and sub-optimal air temperature 20/12°C (day/night) (S13), RZ warming at 19°C and sub-optimal air temperature (S19), and RZ warming at 19°C and optimal air temperature 26/18°C (day/night) (O19). In addition, for each air-RZ temperature treatment, half of the seedlings were also treated with 2% (m/m) polyethylene glycol (PEG) dissolved in nutrient solution to distinguish the effect of root-sourced water supply from RZ temperature. At the whole-plant level, S19 significantly increased the relative growth rate (RGR) by approximately 18% compared with S13, although the increase was less than in O19 (50%) due to delayed leaf emergence. S19 alleviated both diffusive and metabolic limitation of photosynthesis in mature leaves compared with S13, resulting in a photosynthetic rate similar to that in O19 leaves. In newly unfolded leaves, S19 significantly promoted leaf area expansion and alleviated stomatal limitation of photosynthesis compared with S13. PEG addition had a limited influence on RGR and leaf photosynthesis, but significantly suppressed new leaf expansion. Thus, our results indicate that under sub-optimal temperature conditions, RZ warming promotes cucumber seedling growth by differently benefiting mature and newly unfolded leaves. In addition, RZ warming enhanced root-sourced water supply, mainly promoting new leaf expansion, rather than photosynthesis.

Responses of hybrid aspen over-expressing a PIP2;5 aquaporin to low root temperature

Aquaporins mediate the movement of water across cell membranes. Plasma membrane intrinsic protein 2;5 from Populus trichocarpa×deltoides (PtdPIP2;5) was previously demonstrated to be a functionally important water conducting aquaporin. To study the relevance of aquaporin-mediated root water transport at low temperatures, we generated transgenic Populus tremula×alba over-expressing PtdPIP2;5 under control of the maize ubiquitin promoter, and compared the physiological responses and water transport properties of the PtdPIP2;5 over-expressing lines (PtdPIP2;5ox) with wild-type plants. We hypothesized that over-expression of PtdPIP2;5 would reduce temperature sensitivity of root water transport and gas exchange. Decreasing root temperatures to 10 and 5°C significantly decreased hydraulic conductivities (Lp) in wild-type plants, but had no significant effect on Lp in PtdPIP2;5ox plants. Recovery of Lp in the transgenic lines returned to 20°C from 5°C was faster than in the wild-type plants. Low root temperature did not induce major changes in transcript levels for other PIPs. When roots were exposed to 5°C in solution culture and shoots were exposed to 20°C, wild-type plants had significantly lower net photosynthetic and transpiration rates compared to PtdPIP2;5ox plants. Taken together, our results demonstrate that over-expression of PtdPIP2;5 in P. tremula×alba was effective in alleviating the effects of low root temperature on Lp and gas exchange.

Aquaporins in developing rice grains

During rice grain filling, grain moisture content and weight show dynamic changes. We focused on the expression of all 33 rice aquaporins in developing grains. Only two aquaporin genes, OsPIP2;1 and OsTIP3;1, were highly expressed in the period 10–25 days after heading (DAH). High-temperature treatment from 7 to 21 DAH abolished the dynamic up-regulation of OsPIP2;1 in the period 15–20 DAH, whereas OsTIP3;1 expression was not affected. Immunohistochemical analysis revealed that OsPIP2;1 was present in the starchy endosperm, nucellar projection, nucellar epidermis, and dorsal vascular bundles, but not in the aleurone layer. OsTIP3;1 was present in the aleurone layer and starchy endosperm. Water transport activity of recombinant OsTIP3;1 was low, in contrast to the high activity of recombinant OsPIP2;1 we reported previously. Our data suggest that OsPIP2;1 and OsTIP3;1 have distinct roles in developing grains.

Overexpression of Laccaria bicolor aquaporin JQ585595 alters root water transport properties in ectomycorrhizal white spruce (Picea glauca) seedlings

The contribution of hyphae to water transport in ectomycorrhizal (ECM) white spruce (Picea glauca) seedlings was examined by altering expression of a major water-transporting aquaporin in Laccaria bicolor. Picea glauca was inoculated with wild-type (WT), mock transgenic or L. bicolor aquaporin JQ585595-overexpressing (OE) strains and exposed to root temperatures ranging from 5 to 20°C to examine the root water transport properties, physiological responses and plasma membrane intrinsic protein (PIP) expression in colonized plants. Mycorrhization increased shoot water potential, transpiration, net photosynthetic rates, root hydraulic conductivity and root cortical cell hydraulic conductivity in seedlings. At 20°C, OE plants had higher root hydraulic conductivity compared with WT plants and the increases were accompanied by higher expression of P. glauca PIP GQ03401_M18.1 in roots. In contrast to WT L. bicolor, the effects of OE fungi on root and root cortical cell hydraulic conductivities were abolished at 10 and 5°C in the absence of major changes in the examined transcript levels of P. glauca root PIPs. The results provide evidence for the importance of fungal aquaporins in root water transport of mycorrhizal plants. They also demonstrate links between hyphal water transport, root aquaporin expression and root water transport in ECM plants.

The contributions of apoplastic, symplastic and gas phase pathways for water transport outside the bundle sheath in leaves

Water movement from the xylem to stomata is poorly understood. There is still no consensus about whether apoplastic or symplastic pathways are more important, and recent work suggests vapour diffusion may also play a role. The objective of this study was to estimate the proportions of hydraulic conductance outside the bundle sheath contributed by apoplastic, symplastic and gas phase pathways, using a novel analytical framework based on measurable anatomical and biophysical parameters. The calculations presented here suggest that apoplastic pathways provide the majority of conductance outside the bundle sheath under most conditions, whereas symplastic pathways contribute only a small proportion. The contributions of apoplastic and gas phase pathways vary depending on several critical but poorly known or highly variable parameters namely, the effective Poiseuille radius for apoplastic bulk flow, the thickness of cell walls and vertical temperature gradients within the leaf. The gas phase conductance should increase strongly as the leaf centre becomes warmer than the epidermis - providing up to 44% of vertical water transport for a temperature gradient of 0.2 K. These results may help to explain how leaf water transport is influenced by light absorption, temperature and differences in leaf anatomy among species.

De novo assembly and characterization of pericarp transcriptome and identification of candidate genes mediating fruit cracking in Litchi chinensis Sonn

Fruit cracking has long been a topic of great concern for growers and researchers of litchi (Litchi chinensis Sonn.). To understand the molecular mechanisms underlying fruit cracking, high-throughput RNA sequencing (RNA-Seq) was first used for de novo assembly and characterization of the transcriptome of cracking pericarp of litchi. Comparative transcriptomic analyses were performed on non-cracking and cracking fruits. A total of approximately 26 million and 29 million high quality reads were obtained from the two groups of samples, and were assembled into 46,641 unigenes with an average length of 993 bp. These unigenes can be useful resources for future molecular studies of the pericarp in litchi. Furthermore, four genes (LcAQP, 1; LcPIP, 1; LcNIP, 1; LcSIP, 1) involved in water transport, five genes (LcKS, 2; LcGA2ox, 2; LcGID1, 1) involved in GA metabolism, 21 genes (LcCYP707A, 2; LcGT, 9; Lcβ-Glu, 6; LcPP2C, 2; LcABI1, 1; LcABI5, 1) involved in ABA metabolism, 13 genes (LcTPC, 1; Ca²⁺/H⁺ exchanger, 3; Ca²⁺-ATPase, 4; LcCDPK, 2; LcCBL, 3) involved in Ca transport and 24 genes (LcPG, 5; LcEG, 1; LcPE, 3; LcEXP, 5; Lcβ-Gal, 9; LcXET, 1) involved in cell wall metabolism were identified as genes that are differentially expressed in cracked fruits compared to non-cracked fruits. Our results open new doors to further understand the molecular mechanisms behind fruit cracking in litchi and other fruits, especially Sapindaceae plants.

Introgression of two chromosome regions for leaf photosynthesis from an indica rice into the genetic background of a japonica rice

Increases in rates of individual leaf photosynthesis (P n) are critical for future increases of rice yields. A previous study, using introgression lines derived from a cross between indica cultivar Habataki, with one of the highest recorded values of P n, and the Japanese elite cultivar Koshihikari, identified four QTLs (qCAR4, qCAR5, qCAR8, and qCAR11) that affect P n. The present study examined the combined effect of qCAR4 and qCAR8 on P n in the genetic background of Koshihikari. The pyramided near-isogenic line NIL(qCAR4+qCAR8) showed higher P n than both NIL(qCAR4) and NIL(qCAR8), equivalent to that of Habataki despite being due to only two out of the four QTLs. The high P n of NIL(qCAR4+qCAR8) may be attributable to the high leaf nitrogen content, which may have been inherited from NIL(qCAR4), to the large hydraulic conductance due to the large root surface area from NIL(qCAR4), and to the high hydraulic conductivity from NIL(qCAR8). It might be also attributable to high mesophyll conductance, which may have been inherited from NIL(qCAR4). The induction of mesophyll conductance and the high leaf nitrogen content and high hydraulic conductivity could not be explained in isolation from the Koshihikari background. These results suggest that QTL pyramiding is a useful approach in rice breeding aimed at increasing P n.

Aquaporin OsPIP1;1 promotes rice salt resistance and seed germination

OsPIP1;1 is one of the most abundant aquaporins in rice leaves and roots and is highly responsible to environmental stresses. However, its biochemical and physiological functions are still largely unknown. The oocyte assay data showed OsPIP1;1 had lower water channel activity in contrast to OsPIP2;1. EGFP and immunoelectron microscopy studies revealed OsPIP1;1 was predominantly localized in not only plasma membrane but also in some ER-like intracellular compartments in the cells. OsPIP1;1 exhibited low water channel activity in Xenopus oocytes but coexpression of OsPIP2;1 significantly enhanced its water permeability. Stop-flow assay indicated that 10His-OsPIP1;1-reconstituted proteoliposomes had significantly higher water permeability than the control liposomes. Overexpression of OsPIP1;1 greatly altered many physiological features of transgenic plants in a dosage-dependent manner. Moderate expression of OsPIP1;1 increased rice seed yield, salt resistance, root hydraulic conductivity, and seed germination rate. This work suggests OsPIP1;1 functions as an active water channel and plays important physiological roles.

Temperature influences the ability of tall fescue to control transpiration in response to atmospheric vapour pressure deficit

Water availability for turfgrass systems is often limited and is likely to become more so in the future. Here, we conducted experiments that examined the ability of tall fescue (Festuca arundinacea Schreb.) to control transpiration with increasing vapour pressure deficit (VPD) and determined whether control was influenced by temperature. The first study was under steady-state conditions at two temperatures (21 and 27°C) and two VPDs (1.2 and 1.8kPa). At the lower temperature, water use was similar at both VPDs, indicating a restriction of transpiration at high VPD. At 27°C, transpiration control at high VPD was weakened and root growth also declined; both responses increase susceptibility to water-deficit stress. Another series of experiments was used to examine the physiological stability of the transpiration control. Temperature and VPD were adjusted in a stepwise manner and transpiration measured across a range of VPD in the days following environmental shifts. Results indicated that VPD control acclimated to the growth environment, with adjustment to drier conditions becoming evident after ~1 week. Control was again more effective at cool than at hot temperatures. Collectively, the results indicate that transpiration control by this cool season grass is most effective in the temperature range where it is best adapted.

Influence of low air humidity and low root temperature on water uptake, growth and aquaporin expression in rice plants

The effects of low air humidity and low root temperature (LRT) on water uptake, growth and aquaporin gene expression were investigated in rice plants. The daily transpiration of the plants grown at low humidity was 1.5- to 2-fold higher than that at high humidity. LRT at 13°C reduced transpiration, and the extent was larger at lower humidity. LRT also reduced total dry matter production and leaf area expansion, and the extent was again larger at lower humidity. These observations suggest that the suppression of plant growth by LRT is associated with water stress due to decreased water uptake ability of the root. On the other hand, the net assimilation rate was not affected by low humidity and LRT, and water use efficiency was larger for LRT. We found that low humidity induced coordinated up-regulation of many PIP and TIP aquaporin genes in both the leaves and the roots. Expression levels of two root-specific aquaporin genes, OsPIP2;4 and OsPIP2;5, were increased significantly after 6 and 13 d of LRT exposure. Taken together, we discuss the possibility that aquaporins are part of an integrated response of this crop to low air humidity and LRT.

Cold stress-induced acclimation in rice is mediated by root-specific aquaporins

Cold acclimation process plays a vital role in the survival of chilling- and freezing-tolerant plants subjected to cold temperature stress. However, it remains elusive whether a cold acclimation process enhances root water uptake (a component of chilling tolerance) in chilling-sensitive crops such as rice. By analyzing the root hydraulic conductivity under cold stress for a prolonged time, we found that cold stress induced a gradual increase in root osmotic hydraulic conductivity [Lp(r(os))]. Compared with the control treatment (roots and shoots at 25°C), low root temperature (LRT) treatment (roots at 10°C; shoots at 25°C) dramatically reduced Lp(r(os)) within 1 h. However, Lp(r(os)) gradually increased during prolonged LRT treatment and it reached 10-fold higher values at day 5. Moreover, a coordinated up-regulation of root aquaporin gene expression, particularly OsPIP2;5, was observed during LRT treatment. Further, comparison of aquaporin gene expression under root-only chilling (LRT) and whole-plant chilling conditions, and in the roots of intact plants vs. shootless plants, suggests that a shoot to root signal is necessary for inducing the expression of aquaporin genes in the root. Collectively, these results demonstrate that a cold acclimation process for root water uptake functions in rice and is possibly regulated through aquaporins.

Environment and genotype controls on mercury accumulation in rice (Oryza sativa L.) cultivated along a contamination gradient in Guizhou, China

Accumulation of total Hg (THg) and methylmercury (MeHg) was investigated among 50 indica rice varieties cultivated in 3 sites in Guizhou, China, classified as highly-contaminated, moderately-contaminated, and background sites. Concentrations of soil and pore water (depth: 0-2 cm) THg and MeHg were poor predictors of THg and MeHg concentrations in polished white rice and bran, with significantly higher accumulation in the moderately-contaminated site compared to the other 2 sites (ANOVA, p<0.0001). Results indicated other environmental factors played a critical role in the translocation of Hg species from paddy soil to rice grain. We hypothesized alkaline conditions at the highly-contaminated site (surface water pH 11, other 2 sites pH 7.6-8.0), mitigated the uptake of Hg species by decreasing the solubility of micronutrients needed for plant growth, or by reducing the bioavailability of Hg species. White rice MeHg concentrations were associated with genotype but not inorganic Hg(II) concentrations (IHg=THG-MeHg) (MeHg: p<0.001, IHG: p=0.44), while bran MeHg and IHg concentrations were not significantly associated with genotype (p>0.05 for both analyses), indicating there may be genetic markers for the translocation of MeHg from the caryopsis to the endosperm, i.e., from the maternal to the filial tissue. Lastly, calculation of daily MeHg ingestion rates confirmed international guidelines for MeHg exposure were exceeded at the moderately-contaminated site (background: 0.013±0.0052 μg kg(-1) d(-1), n=46; highly-contaminated: 0.066±0.034 μg kg(-1) d(-1), n=49; moderately-contaminated: 0.42±0.13 μg kg(-1) d(-1), n=49). However, MeHg exposure at this site may be decreased up to 69% by cultivating low Hg-accumulating rice varieties.

Regulation of root water uptake under abiotic stress conditions

A common effect of several abiotic stresses is to cause tissue dehydration. Such dehydration is caused by the imbalance between root water uptake and leaf transpiration. Under some specific stress conditions, regulation of root water uptake is more crucial to overcome stress injury than regulation of leaf transpiration. This review first describes present knowledge about how water is taken up by roots and then discusses how specific stress situations such as drought, salinity, low temperature, and flooding modify root water uptake. The rate of root water uptake of a given plant is the result of its root hydraulic characteristics, which are ultimately regulated by aquaporin activity and, to some extent, by suberin deposition. Present knowledge about the effects of different stresses on these features is also summarized. Finally, current findings regarding how molecular signals such as the plant hormones abscisic acid, ethylene, and salicylic acid, and how reactive oxygen species may modulate the final response of root water uptake under stress conditions are discussed.

Transpiration from shoots triggers diurnal changes in root aquaporin expression

Root hydraulic conductivity (Lp(r)) and aquaporin amounts change diurnally. Previously, these changes were considered to be spontaneously driven by a circadian rhythm. Here, we evaluated the new hypothesis that diurnal changes could be triggered and enhanced by transpirational demand from shoots. When rice plants were grown under a 12h light/12h dark regime, Lp(r) was low in the dark and high in the light period. Root aquaporin mRNA levels also changed diurnally, but the amplitudes differed among aquaporin isoforms. Aquaporins, such as OsPIP2;1, showed moderate changes, whereas root-specific aquaporins, such as OsPIP2;5, showed temporal and dramatic induction around 2h after light initiation. When darkness was extended for 12h after the usual dark period, no such induction was observed. Furthermore, plants under 100% relative humidity (RH) showed no induction even in the presence of light. These results suggest that transpirational demand triggers a dramatic increase in gene expressions such as OsPIP2;5. Immunocytochemistry showed that OsPIP2;5 accumulated on the proximal end of the endodermis and of the cell surface around xylem. The strong induction by transpirational demand and the polar localization suggest that OsPIP2;5 contributes to fine adjustment of radial water transport in roots to sustain high Lp(r) during the day.

Vacuolar proton pumps and aquaporins involved in rapid internode elongation of deepwater rice

Rapid growth of the submerged shoots of deepwater rice is essential for survival during the rainy season. We investigated changes in the expression of vacuolar H(+)-ATPase (V-ATPase), H(+)-pyrophosphatase (V-PPase), and aquaporins under submerged conditions. The amounts of vacuolar proton pumps, which support the active transport of ions into the vacuoles, were maintained on a membrane protein basis in the developing vacuoles. Among the six isogenes of V-PPase, OsVHP1;3 was markedly enhanced by submersion. The gene expression of efficient water channels, OsTIP1;1, OsTIP2;2, OsPIP1;1, OsPIP2;1, and OsPIP2;2, was markedly enhanced by submersion. The increase in aquaporin expression might support quick elongation of internodes. The mRNA levels of OsNIP2;2 and OsNIP3;1, which transport silicic and boric acids respectively, clearly decreased. The present study indicates that internodes of deepwater rice upregulate vacuolar proton pumps and water channel aquaporins and downregulate aquaporins that allow permeation of the substrates that suppress internode growth.

Water uptake by seminal and adventitious roots in relation to whole-plant water flow in barley (Hordeum vulgare L.)

Prior to an assessment of the role of aquaporins in root water uptake, the main path of water movement in different types of root and driving forces during day and night need to be known. In the present study on hydroponically grown barley (Hordeum vulgare L.) the two main root types of 14- to 17-d-old plants were analysed for hydraulic conductivity in dependence of the main driving force (hydrostatic, osmotic). Seminal roots contributed 92% and adventitious roots 8% to plant water uptake. The lower contribution of adventitious compared with seminal roots was associated with a smaller surface area and number of roots per plant and a lower axial hydraulic conductance, and occurred despite a less-developed endodermis. The radial hydraulic conductivity of the two types of root was similar and depended little on the prevailing driving force, suggesting that water uptake occurred along a pathway that involved crossing of membrane(s). Exudation experiments showed that osmotic forces were sufficient to support night-time transpiration, yet transpiration experiments and cuticle permeance data questioned the significance of osmotic forces. During the day, 90% of water uptake was driven by a tension of about -0.15 MPa.

Effect of overexpression of radish plasma membrane aquaporins on water-use efficiency, photosynthesis and growth of Eucalyptus trees

Eucalyptus is a diverse genus of flowering trees with more than 700 genotypic species which are mostly native to Australia. We selected 19 wild provenances of Eucalyptus camaldulensis grown in Australia, compared their growth rate and drought tolerance and determined the protein levels of plasma membrane aquaporins (PIPs). There was a positive relationship between the drought tolerance and PIP content. PIPs are divided into two subgroups, PIP1 and PIP2. Most members of the PIP2 subgroup, but not PIP1 subgroup, exhibit water channel activity. We introduced two radish (Raphanus sativus L.) PIPs, RsPIP1;1 and RsPIP2;1, into a hybrid clone of Eucalyptus grandis and Eucalyptus urophylla to examine the effect of their overexpression. Expression of these genes was confirmed by real-time polymerase chain reaction (PCR) and the protein accumulation of RsPIP2;1 by immunoblotting. Drought tolerance was not enhanced in transgenic lines of either gene. However, one transgenic line expressing RsPIP2;1 showed high photosynthesis activity and growth rate under normal growth conditions. For RsPIP1;1-transformed lines, the RsPIP1;1 protein did not accumulate, and the abundance of endogenous PIP1 and PIP2 was decreased. The endogenous PIP1 and PIP2 genes were suppressed in these lines. Therefore, the decreased levels of PIP1 and PIP2 protein may be due to co-suppression of the PIP genes and/or high turnover of PIP proteins. RsPIP1;1-expressing lines gave low values of photosynthesis and growth compared with the control. These results suggest that down-regulation of PIP1 and PIP2 causes serious damage and that up-regulation of PIP2 improves the photosynthetic activity and growth of Eucalyptus trees.