Water deficit as a regulatory switch for legume root responses

Plant roots perceive declining soil water potential as an initial signal which further triggers an array of physiological, morphological and molecular responses in the whole plant. Understanding the root responses with parallel insights on protein level changes has always been an area of interest for stress biologists. In a recent study, we reported drought stress-induced changes among certain structural and functional root proteins involved in reactive oxygen species (ROS) detoxification, primary and secondary metabolite biosynthetic pathways as well as proteins associated with cell signalling in an economically important legume crop Vigna radiata (L.) Wilczek. We also demonstrated photosynthetic gas exchange characteristics and root physiology under varying levels of water-deficit and recovery. In this report, we depict a closer analysis of the expression patterns of the identified proteins which were categorized into five major functional groups. These proteins represent a unique coherence and networking with each other as well as with the overall physiological and metabolic machinery in the plant cell.

The role of bundle sheath extensions and life form in stomatal responses to leaf water status

Bundle sheath extensions (BSEs) are key features of leaf structure with currently little-understood functions. To test the hypothesis that BSEs reduce the hydraulic resistance from the bundle sheath to the epidermis (r(be)) and thereby accelerate hydropassive stomatal movements, we compared stomatal responses with reduced humidity and leaf excision among 20 species with heterobaric or homobaric leaves and herbaceous or woody life forms. We hypothesized that low r(be) due to the presence of BSEs would increase the rate of stomatal opening (V) during transient wrong-way responses, but more so during wrong-way responses to excision (V(e)) than humidity (V(h)), thus increasing the ratio of V(e) to V(h). We predicted the same trends for herbaceous relative to woody species given greater hydraulic resistance in woody species. We found that V(e), V(h), and their ratio were 2.3 to 4.4 times greater in heterobaric than homobaric leaves and 2.0 to 3.1 times greater in herbaceous than woody species. To assess possible causes for these differences, we simulated these experiments in a dynamic compartment/resistance model, which predicted larger V(e) and V(e)/V(h) in leaves with smaller r(be). These results support the hypothesis that BSEs reduce r(be). Comparison of our data and simulations suggested that r(be) is approximately 4 to 16 times larger in homobaric than heterobaric leaves. Our study provides new evidence that variations in the distribution of hydraulic resistance within the leaf and plant are central to understanding dynamic stomatal responses to water status and their ecological correlates and that BSEs play several key roles in the functional ecology of heterobaric leaves.

Analysis of cytokinin mutants and regulation of cytokinin metabolic genes reveals important regulatory roles of cytokinins in drought, salt and abscisic acid responses, and abscisic acid biosynthesis

Cytokinins (CKs) regulate plant growth and development via a complex network of CK signaling. Here, we perform functional analyses with CK-deficient plants to provide direct evidence that CKs negatively regulate salt and drought stress signaling. All CK-deficient plants with reduced levels of various CKs exhibited a strong stress-tolerant phenotype that was associated with increased cell membrane integrity and abscisic acid (ABA) hypersensitivity rather than stomatal density and ABA-mediated stomatal closure. Expression of the Arabidopsis thaliana ISOPENTENYL-TRANSFERASE genes involved in the biosynthesis of bioactive CKs and the majority of the Arabidopsis CYTOKININ OXIDASES/DEHYDROGENASES genes was repressed by stress and ABA treatments, leading to a decrease in biologically active CK contents. These results demonstrate a novel mechanism for survival under abiotic stress conditions via the homeostatic regulation of steady state CK levels. Additionally, under normal conditions, although CK deficiency increased the sensitivity of plants to exogenous ABA, it caused a downregulation of key ABA biosynthetic genes, leading to a significant reduction in endogenous ABA levels in CK-deficient plants relative to the wild type. Taken together, this study provides direct evidence that mutual regulation mechanisms exist between the CK and ABA metabolism and signals underlying different processes regulating plant adaptation to stressors as well as plant growth and development.

Partial rootzone drying improves almond tree leaf-level water use efficiency and afternoon water status compared with regulated deficit irrigation

To determine whether partial rootzone drying (PRD) optimised leaf gas exchange and soil-plant water relations in almond (Prunus dulcis (Mill.) D.A. Webb) compared with regulated deficit irrigation (RDI), a 2 year trial was conducted on field-grown trees in a semiarid climate. Five irrigation treatments were established: full irrigation (FI) where the trees were irrigated at 100% of the standard crop evapotranspiration (ETc); three PRD treatments (PRD70, PRD50 and PRD30) that applied 70, 50 and 30% ETc, respectively; and a commercially practiced RDI treatment that applied 50% ETc during the kernel-filling stage and 100% ETc during the remainder of the growth season. Measurements of volumetric soil moisture content in the soil profile (0-100cm), predawn leaf water potential (Ψpd), midday stem water potential (Ψms), midday leaf gas exchange and trunk diameter fluctuations (TDF) were made during two growing seasons. The diurnal patterns of leaf gas exchange and stem water potential (Ψs) were appraised during the kernel-filling stage in all irrigation regimes. When tree water relations were assessed at solar noon, PRD did not show differences in either leaf gas exchange or tree water status compared with RDI. At similar average soil moisture status (adjudged by similar Ψpd), PRD50 trees had higher water status than RDI trees in the afternoon, as confirmed by Ψs and TDF. Although irrigation placement showed no effects on diurnal stomatal regulation, diurnal leaf net photosynthesis (Al) was substantially less limited in PRD50 than in RDI trees, indicating that PRD improved leaf-level water use efficiency.

A root proteomics-based insight reveals dynamic regulation of root proteins under progressive drought stress and recovery in Vigna radiata (L.) Wilczek

To understand the complex drought response mechanism in crop plants, a systematic root proteomics approach was adopted to identify and analyze the expression patterns of differentially expressed major root proteins of Vigna radiata during short-term (3 days) and consecutive long-term water-deficit (6 days) as well as during recovery (6 days after re-watering). Photosynthetic gas exchange parameters of the plant were measured simultaneously during the stress treatment and recovery period. A total of 26 major protein spots were successfully identified by mass spectrometry, which were grouped according to their expression pattern during short-term stress as significantly up-regulated (9), down-regulated (10), highly down-regulated, beyond detection level of the software (2) and unchanged (5). The subsequent changes in the expression patterns of these proteins during long-term stress treatment and recovery period was analyzed to focus on the dynamic regulation of these functionally important proteins during progressive drought and recovery period. Cytoskeleton-related proteins were down-regulated initially (3d) but regained their expression levels during subsequent water-deficit (6d) while glycoprotein like lectins, which were primarily known to be involved in legume-rhizobia symbiosis, maintained their enhanced expression levels during both short and long-term drought treatment indicating their possible role in drought stress response of legumes. Oxidative stress-related proteins including Cu/Zn superoxide dismutase, oxidoreductase and aldehyde reductase were also up-regulated. The analyses of the dynamic regulation of these root proteins during short- and long-term water-deficit as well as recovery period may prove crucial for further understanding of drought response mechanisms in food legumes.

Microarray analysis and scale-free gene networks identify candidate regulators in drought-stressed roots of loblolly pine (P. taeda L.)

BACKGROUND

Global transcriptional analysis of loblolly pine (Pinus taeda L.) is challenging due to limited molecular tools. PtGen2, a 26,496 feature cDNA microarray, was fabricated and used to assess drought-induced gene expression in loblolly pine propagule roots. Statistical analysis of differential expression and weighted gene correlation network analysis were used to identify drought-responsive genes and further characterize the molecular basis of drought tolerance in loblolly pine.

RESULTS

Microarrays were used to interrogate root cDNA populations obtained from 12 genotype × treatment combinations (four genotypes, three watering regimes). Comparison of drought-stressed roots with roots from the control treatment identified 2445 genes displaying at least a 1.5-fold expression difference (false discovery rate = 0.01). Genes commonly associated with drought response in pine and other plant species, as well as a number of abiotic and biotic stress-related genes, were up-regulated in drought-stressed roots. Only 76 genes were identified as differentially expressed in drought-recovered roots, indicating that the transcript population can return to the pre-drought state within 48 hours. Gene correlation analysis predicts a scale-free network topology and identifies eleven co-expression modules that ranged in size from 34 to 938 members. Network topological parameters identified a number of central nodes (hubs) including those with significant homology (E-values ≤ 2 × 10-30) to 9-cis-epoxycarotenoid dioxygenase, zeatin O-glucosyltransferase, and ABA-responsive protein. Identified hubs also include genes that have been associated previously with osmotic stress, phytohormones, enzymes that detoxify reactive oxygen species, and several genes of unknown function.

CONCLUSION

PtGen2 was used to evaluate transcriptome responses in loblolly pine and was leveraged to identify 2445 differentially expressed genes responding to severe drought stress in roots. Many of the genes identified are known to be up-regulated in response to osmotic stress in pine and other plant species and encode proteins involved in both signal transduction and stress tolerance. Gene expression levels returned to control values within a 48-hour recovery period in all but 76 transcripts. Correlation network analysis indicates a scale-free network topology for the pine root transcriptome and identifies central nodes that may serve as drivers of drought-responsive transcriptome dynamics in the roots of loblolly pine.

The effect of competition from neighbours on stomatal conductance in lettuce and tomato plants

Competition decreased transpiration from young lettuce plants after 2 days, before any reductions in leaf area became apparent, and stomatal conductance (g(s) ) of lettuce and tomato plants was also reduced. Stomatal closure was not due to hydraulic signals or competition for nutrients, as soil water content, leaf water status and leaf nitrate concentrations were unaffected by neighbours. Competition-induced stomatal closure was absent in an abscisic acid (ABA)-deficient tomato mutant, flacca, indicating a fundamental involvement of ABA. Although tomato xylem sap ABA concentrations were unaffected by the presence of neighbours, ABA/pH-based stomatal modulation is still likely to underlie the response to competition, as soil and xylem sap alkalization was observed in competing plants. Competition also modulated leaf ethylene production, and treatment of lettuce plants with an ethylene perception inhibitor (1-methylcyclopropene) diminished the difference in g(s) between single and competing plants grown in a controlled environment room, but increased it in plants grown in the greenhouse: ethylene altered the extent of the stomatal response to competition. Effects of competition on g(s) are discussed in terms of the detection of the absence of neighbours: increases in g(s) and carbon fixation may allow faster initial space occupancy within an emerging community/crop.

Comparison of three approaches to model grapevine organogenesis in conditions of fluctuating temperature, solar radiation and soil water content

BACKGROUND AND AIMS

There is increasing interest in the development of plant growth models representing the complex system of interactions between the different determinants of plant development. These approaches are particularly relevant for grapevine organogenesis, which is a highly plastic process dependent on temperature, solar radiation, soil water deficit and trophic competition.

METHODS

The extent to which three plant growth models were able to deal with the observed plasticity of axis organogenesis was assessed. In the first model, axis organogenesis was dependent solely on temperature, through thermal time. In the second model, axis organogenesis was modelled through functional relationships linking meristem activity and trophic competition. In the last model, the rate of phytomer appearence on each axis was modelled as a function of both the trophic status of the plant and the direct effect of soil water content on potential meristem activity.

KEY RESULTS

The model including relationships between trophic competition and meristem behaviour involved a decrease in the root mean squared error (RMSE) for the simulations of organogenesis by a factor nine compared with the thermal time-based model. Compared with the model in which axis organogenesis was driven only by trophic competition, the implementation of relationships between water deficit and meristem behaviour improved organogenesis simulation results, resulting in a three times divided RMSE. The resulting model can be seen as a first attempt to build a comprehensive complete plant growth model simulating the development of the whole plant in fluctuating conditions of temperature, solar radiation and soil water content.

CONCLUSIONS

We propose a new hypothesis concerning the effects of the different determinants of axis organogenesis. The rate of phytomer appearance according to thermal time was strongly affected by the plant trophic status and soil water deficit. Furthermore, the decrease in meristem activity when soil water is depleted does not result from source/sink imbalances.

Modulation of the root-sourced ABA signal along its way to the shoot in Vitis riparia x Vitis labrusca under water deficit

The intensity of the root-sourced abscisic acid (ABA) signal has long been thought to decrease along its long-distance transport pathway, and hence the shoot responses to the ABA signal would be expected to become less sensitive with the increase in plant height. It is reported here that there is a significant modification of the ABA signal intensity in its pathway to leaves in grapevine (Vitis riparia×Vitis labrusca), but in contrast to the expectation that the ABA signal intensity may decrease along its long-distance transport pathway, it was found that the root-sourced ABA signal is gradually intensified along a vine for as long as 3 m under both water-stressed and non-stressed conditions. Consistent with the alterations in ABA signal intensity, stomatal sensitivity to a root-sourced ABA signal was also gradually increased from the base to the apex. Leaf stomatal conductance near the apex was more severely inhibited than in the leaves at the base of the vine. It was observed that xylem pH was significantly increased from the base to the apex, and that artificially changing the xylem sap pH to be more alkaline by feeding with buffers increased the xylem ABA concentration. Our results suggest that the pH gradient along the stem may play a role in the modification and enhancement of ABA signal intensity such that the stomata at the top of canopy can be more sensitively regulated in response to soil drying.

Progress studies of drought-responsive genes in rice

Rice (Oryza sativa L.), one of the most agronomically important crops, supplies staple food for more than half of the world's population, especially those living in developing countries. The intensively increasing world population has put a great burden on rice production. Drought as one of the major limiting factors for rice productivity has challenged researchers to improve both the water management system and rice characteristics. Biotechnology has assisted researchers to identify genes that are responsive toward drought. This review consolidates the recent studies that expose a number of drought-responsive genes in rice, which are potential candidates for development of improved drought-tolerant transgenic rice cultivars. In addition, examples are provided of how various drought-responsive genes, such as transcription factor and protein kinase encoding genes, were explored to engineer rice plants for enhanced drought tolerance using transgenic approach. Furthermore, the involvement of various phytohormones in regulation of drought response as well as the complexity of drought-responsive networks, which is indicated by the crosstalks with other stress-responsive networks such as cold and salt stresses, will be discussed. It is hoped that by understanding how rice responds to drought, crop performance can be stabilized and protected under water deficit conditions.

BYPASS1: synthesis of the mobile root-derived signal requires active root growth and arrests early leaf development

BACKGROUND

The Arabidopsis bypass1 (bps1) mutant root produces a biologically active mobile compound that induces shoot growth arrest. However it is unknown whether the root retains the capacity to synthesize the mobile compound, or if only shoots of young seedlings are sensitive. It is also unknown how this compound induces arrest of shoot growth. This study investigated both of these questions using genetic, inhibitor, reporter gene, and morphological approaches.

RESULTS

Production of the bps1 root-synthesized mobile compound was found to require active root growth. Inhibition of postembryonic root growth, by depleting glutathione either genetically or chemically, allowed seedlings to escape shoot arrest. However, the treatments were not completely effective, as the first leaf pair remained radialized, but elongated. This result indicated that the embryonic root transiently synthesized a small amount of the mobile substance. In addition, providing glutathione later in vegetative development caused shoot growth arrest to be reinstated, revealing that these late-arising roots were still capable of producing the mobile substance, and that the older vegetative leaves were still responsive. To gain insight into how leaf development responds to the mobile signal, leaf development was followed morphologically and using the CYCB1,1::GUS marker for G2/M phase cells. We found that arrest of leaf growth is a fully penetrant phenotype, and a dramatic decrease in G2/M phase cells was coincident with arrest. Analyses of stress phenotypes found that late in development, bps1 cotyledons produced necrotic lesions, however neither hydrogen peroxide nor superoxide were abundant as leaves underwent arrest.

CONCLUSIONS

bps1 roots appear to require active growth in order to produce the mobile bps1 signal, but the potential for this compound's synthesis is present both early and late during vegetative development. This prolonged capacity to synthesize and respond to the mobile compound is consistent with a possible role for the mobile compound in linking shoot growth to subterranean conditions. The specific growth-related responses in the shoot indicated that the mobile substance prevents full activation of cell division in leaves, although whether cell division is a direct response remains to be determined.

Root-synthesized cytokinins improve shoot growth and fruit yield in salinized tomato (Solanum lycopersicum L.) plants

Salinity limits crop productivity, in part by decreasing shoot concentrations of the growth-promoting and senescence-delaying hormones cytokinins. Since constitutive cytokinin overproduction may have pleiotropic effects on plant development, two approaches assessed whether specific root-localized transgenic IPT (a key enzyme for cytokinin biosynthesis) gene expression could substantially improve tomato plant growth and yield under salinity: transient root IPT induction (HSP70::IPT) and grafting wild-type (WT) shoots onto a constitutive IPT-expressing rootstock (WT/35S::IPT). Transient root IPT induction increased root, xylem sap, and leaf bioactive cytokinin concentrations 2- to 3-fold without shoot IPT gene expression. Although IPT induction reduced root biomass (by 15%) in control (non-salinized) plants, in salinized plants (100 mM NaCl for 22 d), increased cytokinin concentrations delayed stomatal closure and leaf senescence and almost doubled shoot growth (compared with WT plants), with concomitant increases in the essential nutrient K(+) (20%) and decreases in the toxic ion Na(+) (by 30%) and abscisic acid (by 20-40%) concentrations in transpiring mature leaves. Similarly, WT/35S::IPT plants (scion/rootstock) grown with 75 mM NaCl for 90 d had higher fruit trans-zeatin concentrations (1.5- to 2-fold) and yielded 30% more than WT/non-transformed plants. Enhancing root cytokinin synthesis modified both shoot hormonal and ionic status, thus ameliorating salinity-induced decreases in growth and yield.

Enhanced cytokinin synthesis in tobacco plants expressing PSARK::IPT prevents the degradation of photosynthetic protein complexes during drought

To identify genes associated with the cytokinin-induced enhanced drought tolerance, we analyzed the transcriptome of wild-type and transgenic tobacco (Nicotiana tabacum 'SR1') plants expressing P(SARK)::IPT (for senescence-associated receptor kinase::isopentenyltransferase) grown under well-watered and prolonged water deficit conditions using the tomato GeneChip. During water deficit, the expression of genes encoding components of the carotenoid pathway leading to ABA biosynthesis was enhanced in the wild-type plants, but repressed in the transgenic plants. On the other hand, transgenic plants displayed higher transcript abundance of genes involved in the brassinosteroid biosynthetic pathways. Several genes coding for proteins associated with Chl synthesis, light reactions, the Calvin-Benson cycle and photorespiration were induced in the transgenic plants. Notably, increased transcript abundance of genes associated with PSII, the cytochrome b(6)/f complex, PSI, NADH oxidoreductase and the ATP complex was found in the P(SARK)::IPT plants. The increased transcript abundance was assessed by quantitative PCR and the increased protein levels were confirmed by Western blots. Our results indicated that while the photosynthetic apparatus in the wild-type plants was degraded, photosynthesis in the transgenic plants was not affected and photosynthetic proteins were not degraded. During water deficit, wild-type plants displayed a significant reduction in electron transfer and photochemical quenching, with a marked increase in non-photochemical quenching, suggesting a decrease in energy transfer to the PSII core complexes and an increase in cyclic electron transfer reactions.

Proteomic analysis of rice leaves shows the different regulations to osmotic stress and stress signals

Following the idea of partial root-zone drying (PRD) in crop cultivation, the morphological and physiological responses to partial root osmotic stress (PROS) and whole root osmotic stress (WROS) were investigated in rice. WROS caused stress symptoms like leaf rolling and membrane leakage. PROS stimulated stress signals, but did not cause severe leaf damage. By proteomic analysis, a total of 58 proteins showed differential expression after one or both treatments, and functional classification of these proteins suggests that stress signals regulate photosynthesis, carbohydrate and energy metabolism. Two other proteins (anthranilate synthase and submergence-induced nickel-binding protein) were upregulated only in the PROS plants, indicating their important roles in stress resistance. Additionally, more enzymes were involved in stress defense, redox homeostasis, lignin and ethylene synthesis in WROS leaves, suggesting a more comprehensive regulatory mechanism induced by osmotic stress. This study provides new insights into the complex molecular networks within plant leaves involved in the adaptation to osmotic stress and stress signals.

Antioxidant response of wheat roots to drought acclimation

Wheat (Triticum aestivum L.) seedlings of a drought-resistant cv. C306 were subjected to severe water deficit directly or through stress cycles of increasing intensity with intermittent recovery periods. The antioxidant defense in terms of redox metabolites and enzymes in root cells and mitochondria was examined in relation to membrane damage. Acclimated seedlings exhibited higher relative water content and were able to limit the accumulation of H(2)O(2) and membrane damage during subsequent severe water stress conditions. This was due to systematic up-regulation of superoxide dismutase, ascorbate peroxidase (APX), catalase, peroxidases, and ascorbate-glutathione cycle components at both the whole cell level as well as in mitochondria. In contrast, direct exposure of severe water stress to non-acclimated seedlings caused greater water loss, excessive accumulation of H(2)O(2) followed by elevated lipid peroxidation due to the poor antioxidant enzyme response particularly of APX, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase, and ascorbate-glutathione redox balance. Mitochondrial antioxidant defense was found to be better than the cellular defense in non-acclimated roots. Termination of stress followed by rewatering leads to a rapid enhancement in all the antioxidant defense components in non-acclimated roots, which suggested that the excess levels of H(2)O(2) during severe water stress conditions might have inhibited or down-regulated the antioxidant enzymes. Hence, drought acclimation conferred enhanced tolerance toward oxidative stress in the root tissue of wheat seedlings due to both reactive oxygen species restriction and well-coordinated induction of antioxidant defense.

An overview of models of stomatal conductance at the leaf level

Stomata play a key role in plant adaptation to changing environmental conditions as they control both water losses and CO(2) uptake. Particularly, in the context of global change, simulations of the consequences of drought on crop plants are needed to design more efficient and water-saving cropping systems. However, most of the models of stomatal conductance (g(s)) developed at the leaf level link g(s) to environmental factors or net photosynthesis (A(net)), but do not include satisfactorily the effects of drought, impairing our capacity to simulate plant functioning in conditions of limited water supply. The objective of this review was to draw an up-to-date picture of the g(s) models, from the empirical to the process-based ones, along with their mechanistic or deterministic bases. It focuses on models capable to account for multiple environmental influences with emphasis on drought conditions. We examine how models that have been proposed for well-watered conditions can be combined with those specifically designed to deal with drought conditions. Ideas for future improvements of g(s) models are discussed: the issue of co-regulation of g(s) and A(net); the roles of CO(2), absissic acid and H(2)O(2); and finally, how to better address the new challenges arising from the issue of global change.

Gas exchange and hydraulics in seedlings of Hevea brasiliensis during water stress and recovery

The response of plants to drought has received significant attention, but far less attention has been given to the dynamic response of plants during recovery from drought. Photosynthetic performance and hydraulic capacity were monitored in seedlings of Hevea brasiliensis under water stress and during recovery following rewatering. Leaf water relation, gas exchange rate and hydraulic conductivity decreased gradually after water stress fell below a threshold, whereas instantaneous water use efficiency and osmolytes increased significantly. After 5 days of rewatering, leaf water relation, maximum stomatal conductance (g(s-max)) and plant hydraulic conductivity had recovered to the control levels except for sapwood area-specific hydraulic conductivity, photosynthetic assimilation rate and osmolytes. During the phase of water stress, stomata were almost completely closed before water transport efficiency decreased substantially, and moreover, the leaf hydraulic pathway was more vulnerable to water stress-induced embolism than the stem hydraulic pathway. Meanwhile, g(s-max) was linearly correlated with hydraulic capacity when water stress exceeded a threshold. In addition, a positive relationship was shown to occur between the recovery of g(s-max) and of hydraulic capacity during the phase of rewatering. Our results suggest (i) that stomatal closure effectively reduces the risk of xylem dysfunction in water-stressed plants at the cost of gas exchange, (ii) that the leaf functions as a safety valve to protect the hydraulic pathway from water stress-induced dysfunction to a larger extent than does the stem and (iii) that the full drought recovery of gas exchange is restricted by not only hydraulic factors but also non-hydraulic factors.

Accumulation of xylem transported protein at pit membranes and associated reductions in hydraulic conductance

Proteins and traces of polysaccharide are the only polymeric colloids consistently transported in the xylem sap of plants. The hypothesis that such proteins could have physical inhibitory effects on xylem water transport was investigated. Ovalbumin, with a molecular weight of 45 kDa and a molecular diameter of 5.4 nm, is an inert, water-soluble protein that is midway along the size range of endogenous xylem sap proteins. Solutions of ovalbumin conjugated to a fluorescent marker and supplied to transpiring shoot explants of tobacco (Nicotiana tabacum L.) and olive (Olea europaea L.) were shown by confocal laser scanning microscopy to accumulate specifically at wall-based pit membranes that connect neighbouring xylem conduits. In addition, pressure-induced perfusion of micro-filtered ovalbumin solutions, at concentrations similar to those of endogenous xylem sap proteins, through the xylem of tobacco stem or olive twig segments resulted in the retention of c. 40% of the ovalbumin and reductions in the axial hydraulic conductance of the xylem. Smaller molecules such as Texas Red 3000 (MW 3 kDa) and Alexafluor 488-cadaverin conjugates (MW 0.64 kDa) did not show similar characteristics. The partial reduction in xylem hydraulic conductance appeared to be related to the accumulation of ovalbumin at xylem pit membranes and the consequent fouling of trans-membrane water-conducting pores with smaller diameters than those of the ovalbumin molecules. Potential implications of these novel findings for whole-plant water relations are considered.

Relationships between xylem anatomy, root hydraulic conductivity, leaf/root ratio and transpiration in citrus trees on different rootstocks

The aim of the study was to determine the extent in which leaf and whole plant transpiration (Tp) were influenced by root hydraulic conductance (K(r)), leaf to root ratio and leaf mass. Also, the relationships between the anatomic characteristics of roots and K(r) were investigated. To this end, 9-month-old seedlings of the citrus rootstocks Cleopatra mandarin (CM), Poncirus trifoliata (PT), and their hybrids Forner-Alcaide no 5 (FA-5) and Forner-Alcaide no 13 (FA-13) and 15-month-old trees of Valencia orange budded on these four rootstocks were tested. The hybrid FA-13 and PT had higher values of K(r) and leaf transpiration rates (E) than FA-5 and CM. There was a positive curvilinear correlation between E and K(r). Furthermore, E levels in the different types of plants decreased with increased leaf/root (L/R) ratios. Pruning of the roots and defoliation confirmed that transpiration rates were strongly influenced by the L/R ratio. However, variations in E because of differences in L/R ratios were less pronounced in trees budded on FA-13 and PT than on the other two rootstocks. In addition, there was a positive correlation between Tp and leaf biomass, although differences between rootstocks may be attributed to differences in K(r). The average lumen diameter of xylem vessels was greater in rootstocks with high K(r). Size of epidermal and hypodermal cells of fibrous roots may also restrict K(r).

Physiological and biochemical changes in plants under waterlogging

Waterlogging usually results from overuse and/or poor management of irrigation water and is a serious constraint with damaging effects. The rapidly depleting oxygen from submerged root zone is sensed and plant adjusts expressing anaerobic proteins. Plant cells shift their metabolism towards low energy yielding anaerobic fermentation pathways in the absence of oxygen. Structural modifications are also induced as aerenchyma formation and adventitious rootings, etc. Studies at molecular and biochemical levels to facilitate early perception and subsequent responses have also been worked out to produce resistant transgenic plants. This review explores the sequential changes of plant responses at different levels regarding their defense strategies and efforts made to enhance them, tailoring crucial regulators so that they can withstand waterlogging stress.

Grapevine under deficit irrigation: hints from physiological and molecular data

BACKGROUND

A large proportion of vineyards are located in regions with seasonal drought (e.g. Mediterranean-type climates) where soil and atmospheric water deficits, together with high temperatures, exert large constraints on yield and quality. The increasing demand for vineyard irrigation requires an improvement in the efficiency of water use. Deficit irrigation has emerged as a potential strategy to allow crops to withstand mild water stress with little or no decreases of yield, and potentially a positive impact on fruit quality. Understanding the physiological and molecular bases of grapevine responses to mild to moderate water deficits is fundamental to optimize deficit irrigation management and identify the most suitable varieties to those conditions.

SCOPE

How the whole plant acclimatizes to water scarcity and how short- and long-distance chemical and hydraulic signals intervene are reviewed. Chemical compounds synthesized in drying roots are shown to act as long-distance signals inducing leaf stomatal closure and/or restricting leaf growth. This explains why some plants endure soil drying without significant changes in shoot water status. The control of plant water potential by stomatal aperture via feed-forward mechanisms is associated with 'isohydric' behaviour in contrast to 'anysohydric' behaviour in which lower plant water potentials are attained. This review discusses differences in this respect between grapevines varieties and experimental conditions. Mild water deficits also exert direct and/or indirect (via the light environment around grape clusters) effects on berry development and composition; a higher content of skin-based constituents (e.g. tannins and anthocyanins) has generally being reported. Regulation under water deficit of genes and proteins of the various metabolic pathways responsible for berry composition and therefore wine quality are reviewed.

More from less: plant growth under limited water

When subjected to abiotic stresses, plants actively re-program their growth by modulating both cell division and cell expansion. Growth decreases rapidly upon stress onset but it recovers and adapts once stress conditions become stable. Here, we review recent advances in understanding the mechanisms underlying both stress-induced growth repression and adaptation with an emphasis on drought and leaf growth and we briefly discuss how this knowledge can be translated into crops. It is now clear that stress response of growing and mature leaves is distinct and should be studied separately. Both cell proliferation and expansion are regulated by common signaling pathways involving gibberellins and DELLA proteins while down stream effector genes are stage specific.

Abscisic acid, ethylene and gibberellic acid act at different developmental stages to instruct the adaptation of young leaves to stress

Drought stress represents a particularly great environmental challenge for plants. A decreased water availability can severely limit growth, and this jeopardizes the organism's primary goal-to survive and sustain growth long enough to ensure the plentiful production of viable seeds within the favorable growth season. It is therefore vital for a growing plant to sense oncoming drought as early as possible, and to respond to it rapidly and appropriately in all organs. A typical, fast energy-saving response is the arrest of growth in young organs, which is likely mediated by root-derived signals. A recent publication indicates that three plant hormones (abscisic acid, ethylene and gibberellic acid) mediate the adaptation of leaf growth in response to drought, and that they act at different developmental stages. Abscisic acid mainly acts in mature cells, while ethylene and gibberellic acid function in expanding and dividing leaf cells. This provides the plant with a means to differentially control the developmental zones of a growing leaf, and to integrate environmental signals differently in sink and source tissues. Here we discuss the biological implications of this discovery in the context of long-distance xylem and phloem transport.

Reactive oxygen species homeostasis and signalling during drought and salinity stresses

Water deficit and salinity, especially under high light intensity or in combination with other stresses, disrupt photosynthesis and increase photorespiration, altering the normal homeostasis of cells and cause an increased production of reactive oxygen species (ROS). ROS play a dual role in the response of plants to abiotic stresses functioning as toxic by-products of stress metabolism, as well as important signal transduction molecules. In this review, we provide an overview of ROS homeostasis and signalling in response to drought and salt stresses and discuss the current understanding of ROS involvement in stress sensing, stress signalling and regulation of acclimation responses.

Control of leaf growth by abscisic acid: hydraulic or non-hydraulic processes?

Abscisic acid (ABA) affects plant metabolism and water transfers via multiple mechanisms at cell, organ and whole plant levels. These mechanisms translate into contradictory effects on leaf growth, so the literature reports positive, null or negative effects of ABA on leaf growth upon water deficit. We review evidences based on genetic manipulations of ABA biosynthesis, feeding the plant with artificial ABA or partial root drying and provide elements to avoid confusions of effects. We propose that ABA has mainly three effects on growth. (i) Via its controlling effect on stomatal aperture and transpiration rate, an increased concentration of ABA tends to buffer the day-night alternations of leaf growth rate and the negative effect of evaporative demand. (ii) ABA tends to improve leaf growth via an increase in the conductance to water transfer in the plant as a result of increased tissue hydraulic conductivity. (iii) ABA has also a modest non-hydraulic effect which is negative in plants subjected to water deficit, either manipulated for ABA synthesis or fed with artificial ABA, but can be positive in well watered plants deficient of ABA. The overall effect of increasing ABA biosynthesis depends on the relative weight of each of these effects under different environmental scenarios.

The evolution of water transport in plants: an integrated approach

This review examines the evolution of the plant vascular system from its beginnings in the green algae to modern arborescent plants, highlighting the recent advances in developmental, organismal, geochemical and climatological research that have contributed to our understanding of the evolution of xylem. Hydraulic trade-offs in vascular structure-function are discussed in the context of canopy support and drought and freeze-thaw stress resistance. This qualitative and quantitative neontological approach to palaeobotany may be useful for interpreting the water-transport efficiencies and hydraulic limits in fossil plants. Large variations in atmospheric carbon dioxide levels are recorded in leaf stomatal densities, and may have had profound impacts on the water conservation strategies of ancient plants. A hypothesis that links vascular function with stomatal density is presented and examined in the context of the evolution of wood and/or vessels. A discussion of the broader impacts of plant transport on hydrology and climate concludes this review.

Expression of the ethylene biosynthetic machinery in maize roots is regulated in response to hypoxia

Ethylene regulates plant growth in response to many adverse environmental conditions, including the induction of aerenchyma, i.e. the formation of air spaces, in flooded roots in an effort to maintain oxygen levels. In this work, quantitative RT-PCR and in situ RNA hybridization were used to determine how the expression of the ethylene biosynthetic machinery in maize roots is spatially and temporally regulated following exposure to 4% oxygen (i.e. hypoxia) for up to 24 h, conditions that induced aerenchyma formation in the fully-expanded region of the root and reduced cytoplasmic density throughout the root. Expression of ACC oxidase, the ethylene forming enzyme, was observed in the root cap, protophloem sieve elements, and companion cells associated with metaphloem sieve elements. Exposure to 4% oxygen induced ACC oxidase expression in these cell types as well as in the root cortex. ACC synthase, which generates the ethylene precursor, was expressed in the root cap and the cortex and its expression was induced in cortical cells following low oxygen treatment. The induction of expression of the ethylene biosynthetic machinery was accompanied by an induction of ethylene evolution and a reduced rate of root growth. These results suggest that maize roots respond to conditions of hypoxia by inducing the spatially restricted expression of the ethylene biosynthetic machinery, resulting in increased ethylene production.

BYPASS1: how a tiny mutant tells a big story about root-to-shoot signaling

Plants coordinate their development using long-distance signaling. The vascular system provides a route for long-distance movement, and specifically the xylem for root-to-shoot signaling. Root-to-shoot signals play roles communicating soil conditions, and these signals are important for agricultural water conservation. Using genetic approaches, the Arabidopsis bypass1 (bps1) mutant, which over-produces a root-derived signal, was identified. Although bps1 mutants have both root and shoot defects, the shoot can develop normally if the roots are removed, and the mutant root is sufficient to induce arrest of the wild-type shoot. BYPASS1 encodes a protein with no functionally characterized domains, and BPS1-like genes are found in plant genomes, but not the genomes of animals. Analyses of hormone pathways indicate that the mobile compound that arises in bps1 roots requires carotenoid biosynthesis, but it is neither abscisic acid nor strigolactone. The current model suggests that BPS1 is required to prevent the synthesis of a novel substance that moves from the root to the shoot, where it modifies shoot growth by interfering with auxin signaling.

Principal component analysis of hormone profiling data suggests an important role for cytokinins in regulating leaf growth and senescence of salinized tomato

High throughput analytical methods allow phytohormonal profiling, but the magnitude of the data generated makes it difficult to draw firm conclusions about the physiological roles of different compounds. Principal component analysis (PCA) was used as a mathematical tool to evaluate relationships between physiological and hormonal variables in two experiments with salinised tomato. When tomato plants (cv Boludo F1) were grafted onto a recombinant inbred line (RIL) population derived from a Solanum lycopersicum x S. cheesmaniae cross and grown under moderate salinity (75 mM NaCl) for 100 days under greenhouse conditions, PCA revealed an important role for leaf xylem cytokinins (CKs) in controlling leaf growth and photosystem II efficiency (Fv/Fm) and thus crop productivity under salinity. PCA analysis from a similar experiment, with ungrafted tomato grown under highly saline (100 mM NaCl) conditions, that evaluated the temporal sequence of leaf growth (as relative growth rate, LRGR) and senescence and hormone concentrations, revealed a similar influence of CKs on both processes, since Fv/Fm and LRGR were strongly loaded along the two principal components and placed in the same cluster as leaf trans-zeatin and/or related to other CK-related parameters. The conservative behaviour of the eigen vectors for Fv/Fm and the analyzed phytohormones in different compartments (xylem, leaf and root) between different experiments suggests an important role for CKs in regulating leaf senescence, while CKs and other hormones seem to regulate leaf growth under salinity.

Cell biological mechanism for triggering of ABA accumulation under water stress in Vicia faba leaves

Water stress-induced ABA accumulation is a cellular signaling process from water stress perception to activation of genes encoding key enzymes of ABA biosynthesis, of which the water stress-signal perception by cells or triggering mechanism of the ABA accumulation is the center in the whole process of ABA related-stress signaling in plants. The cell biological mechanism for triggering of ABA accumulation under water stress was studied in leaves of Vicia faba. Mannitol at 890 mmol * kg(-1) osmotic concentration induced an increase of more than 5 times in ABA concentration in detached leaf tissues, but the same concentration of mannitol only induced an increase of less than 40 % in ABA concentration in protoplasts. Like in detached leaf tissues, ABA concentration in isolated cells increased more than 10 times under the treatment of mannitol at 890 mmol * kg(-1) concentration, suggesting that the interaction between plasmalemma and cell wall was essential to triggering of the water stress-induced ABA accumulation. Neither Ca(2+)-chelating agent EGTA nor Ca(2+)channel activator A23187 nor the two cytoskeleton inhibitors, colchicine and cytochalasin B, had any effect on water stress-induced ABA accumulation. Interestingly water stress-induced ABA accumulation was effectively inhibited by a non-plasmalemma-permeable sulfhydryl-modifier PCMBS (p-chloromercuriphenyl-sulfonic acid), suggesting that plasmalemma protein(s) may be involved in the triggering of water stress-induced ABA accumulation, and the protein may contain sulfhydryl group at its function domain.

Abscisic acid regulates root hydraulic conductance via aquaporin expression modulation in Nicotiana tabacum

Abscisic acid (ABA) modifies the hydraulic properties of roots by increasing root water flux. The effects of ABA on aquaporin content and root hydraulic conductance are controversial. We addressed these effects via a combination of experiments. Tobacco (Nicotiana tabacum) plants were grown hydroponically, and ABA (1 microM) was exogenously applied to the roots. Then, the water transport properties of tobacco roots and expression of PIP-type aquaporins were examined. ABA increased the sap flow rate (Jv) and also the osmotic root hydraulic conductance (Lp(r-o)) of excised tobacco roots after 24h. The expression of three aquaporin PIP-type genes and PIP1s proteins abundance in tobacco roots were analyzed by real-time PCR and protein gel blot analysis, respectively. Interestingly, the accumulation of NtAQP1, NtPIP1;1 and NtPIP2;1 transcripts and NtPIP1;1 and NtAQP1 proteins abundance was significantly increased. Although the antibody used recognize NtPIP1;1 and NtAQP1, most probably it also recognizes other PIP1 proteins present in tobacco. Thus, the increase in the expression of the three PIP-type genes and other PIP1s proteins abundance caused by ABA were correlated with an increase in Lp(r-o) and Jv. ABA therefore facilitated the cell-to-cell component of water transport across the root cylinder. The subcellular localization of NtPIP1;1- and NtPIP2;1-GFP was investigated by protoplast transformation with chimeric gene, showing NtPIP2;1 localization in plasma membrane and NtPIP1;1 retention in the endoplasmic reticulum (ER). However, ABA did not change subcellular localization of NtPIP1;1 from ER to plasma membrane.

Leaf hydraulics and drought stress: response, recovery and survivorship in four woody temperate plant species

Efficient conduction of water inside leaves is essential for leaf function, yet the hydraulic-mediated impact of drought on gas exchange remains poorly understood. Here we examine the decline and subsequent recovery of leaf water potential (Psi(leaf)), leaf hydraulic conductance (K(leaf)), and midday transpiration (E) in four temperate woody species exposed to controlled drought conditions ranging from mild to lethal. During drought the vulnerability of K(leaf) to declining Psi(leaf) varied greatly among the species sampled. Following drought, plants were rewatered and the rate of E and K(leaf) recovery was found to be strongly dependent on the severity of the drought imposed. Gas exchange recovery was strongly correlated with the relatively slow recovery of K(leaf) for three of the four species, indicating conformity to a hydraulic-stomatal limitation model of plant recovery. However, there was also a shift in the sensitivity of stomata to Psi(leaf) suggesting that the plant hormone abscisic acid may be involved in limiting the rate of stomatal reopening. The level of drought tolerance varied among the four species and was correlated with leaf hydraulic vulnerability. These results suggest that species-specific variation in hydraulic properties plays a fundamental role in steering the dynamic response of plants during recovery.

Characterization of new maize genes putatively involved in cytokinin metabolism and their expression during osmotic stress in relation to cytokinin levels

Plant hormones, cytokinins (CKs), have been for a long time considered to be involved in plant responses to stress. However, their exact roles in processes linked to stress signalization and acclimatization to adverse environmental conditions are unknown. In this study, expression profiles of the entire gene families of CK biosynthetic and degradation genes in maize (Zea mays) during development and stress responses are described. Transcript abundance of particular genes is discussed in relation to the levels of different CK metabolites. Salt and osmotic stresses induce expression of some CK biosynthetic genes in seedlings of maize, leading to a moderate increase of active forms of CKs lasting several days during acclimatization to stress. A direct effect of CKs to mediate activation of stress responses does not seem to be possible due to the slow changes in metabolite levels. However, expression of genes involved in cytokinin signal transduction is uniformly down-regulated within 0.5 h of stress induction by an unknown mechanism. cis-Zeatin and its derivatives were found to be the most abundant CKs in young maize seedlings. We demonstrate that levels of this zeatin isomer are significantly enhanced during early stress response and that it originates independently from de novo biosynthesis in stressed tissues, possibly by elevated specific RNA degradation. By enhancing their CK levels, plants could perhaps undergo a reduction of growth rates maintained by abscisic acid accumulation in stressed tissues. A second role for cytokinin receptors in sensing turgor response is hypothesized besides their documented function in CK signaling.

Ozone suppresses soil drying- and abscisic acid (ABA)-induced stomatal closure via an ethylene-dependent mechanism

Elevated atmospheric ozone concentrations (70 ppb) reduced the sensitivity of stomatal closure to abscisic acid (ABA) in Leontodon hispidus after at least 24 h exposure (1) when detached leaves were fed ABA, and (2) when intact plants were sprayed or injected with ABA. They also reduced the sensitivity of stomatal closure to soil drying around the roots. Such effects could already be occurring under current northern hemisphere peak ambient ozone concentrations. Leaves detached from plants which had been exposed to elevated ozone concentrations generated higher concentrations of ethylene, although leaf tissue ABA concentrations were unaffected. When intact plants were pretreated with the ethylene receptor binding antagonist 1-methylcyclopropene, the stomatal response to both applied ABA and soil drying was fully restored in the presence of elevated ozone. Implications of ethylene's antagonism of the stomatal response to ABA under oxidative stress are discussed. We suggest that this may be one mechanism whereby elevated ozone induces visible injury in sensitive species. We emphasize that drought linked to climate change and tropospheric ozone pollution, are both escalating problems. Ozone will exacerbate the deleterious effects of drought on the many plant species including valuable crops that respond to this pollutant by emitting more ethylene.

Cytokinin-dependent photorespiration and the protection of photosynthesis during water deficit

We investigated the effects of P(SARK)IPT (for Senescence-Associated Receptor KinaseIsopentenyltransferase) expression and cytokinin production on several aspects of photosynthesis in transgenic tobacco (Nicotiana tabacum cv SR1) plants grown under optimal or restricted (30% of optimal) watering regimes. There were no significant differences in stomatal conductance between leaves from wild-type and transgenic P(SARK)-IPT plants grown under optimal or restricted watering. On the other hand, there was a significant reduction in the maximum rate of electron transport as well as the use of triose-phosphates only in wild-type plants during growth under restricted watering, indicating a biochemical control of photosynthesis during growth under water deficit. During water deficit conditions, the transgenic plants displayed an increase in catalase inside peroxisomes, maintained a physical association among chloroplasts, peroxisomes, and mitochondria, and increased the CO(2) compensation point, indicating the cytokinin-mediated occurrence of photorespiration in the transgenic plants. The contribution of photorespiration to the tolerance of transgenic plants to water deficit was also supported by the increase in transcripts coding for enzymes involved in the conversion of glycolate to ribulose-1,5-bisphosphate. Moreover, the increase in transcripts indicated a cytokinin-induced elevation in photorespiration, suggesting the contribution of photorespiration in the protection of photosynthetic processes and its beneficial role during water stress.

Increased abscisic acid levels in transgenic tobacco over-expressing 9 cis-epoxycarotenoid dioxygenase influence H2O2 and NO production and antioxidant defences

Abscisic acid (ABA) regulates the plant's adaptive responses to abiotic stresses. Over-expression of the 9-cis-epoxycarotenoid dioxygenase gene (SgNCED1) in the transgenic tobaccos increased ABA content and tolerance to drought and salt stresses. H2O2 and nitric oxide (NO) contents were enhanced in guard cells and mesophyll cells of the transgenic plants, accompanied with increased transcripts and activities of antioxidant enzymes including superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR). The enhancements of H2O2 and NO and transcripts and activities of antioxidant enzymes in the transgenic plants were blocked by pre-treatments with inhibitor of ABA biosynthesis, scavengers of H2O2 and NO, and inhibitors of NADPH oxidase and NO synthase-like (NOS-like). The elevated production of NO in the transgenic plants was blocked by scavenger of H2O2 and inhibitors of NADPH oxidase, whereas H2O2 level was not affected by scavenger of NO and inhibitor of NOS-like, indicating that H2O2 is essential for the elevated production of NO. The results demonstrate that the increased drought and salt tolerance in the transgenic plants is associated with ABA-induced production of H2O2 via NADPH oxidase and NO via NOS-like, which sequentially induce transcripts and activities of SOD, CAT, APX and GR.

Partial root zone drying: regulation of photosynthetic limitations and antioxidant enzymatic activities in young olive (Olea europaea) saplings

The effect of partial root drying (PRD) irrigation on split-root olive (Olea europaea L. cv Picholine marocaine) saplings was investigated. An irrigated control and two PRD regimes were applied (control: irrigation applied on both sides of the root system to keep the soil water content close to field capacity; PRD(50): irrigation applied at 50% of the control amount on one side of the root system and irrigation withheld from the other side, with irrigation regimes switched between the sides of the root system every 2 weeks; and PRD(100): irrigation applied at 100% of the control amount on one side and irrigation withheld on the other side, with irrigation regimes switched between the sides of the root system every 2 weeks. Only saplings in the PRD(50) regime were subjected to water-deficit irrigation. The PRD treatments significantly affected water relations and vegetative growth throughout the growing season. Predawn leaf water potential and relative water content differed significantly between the PRD(50) and PRD(100) saplings, leading to reduced stomatal conductance, carbon assimilation, shoot length and leaf number in PRD(50) saplings. However, the PRD(50) water-deficit treatment did not affect the capacity of the saplings to assimilate CO(2). Activities of superoxide dismutase, soluble and insoluble peroxidase (POX) and polyphenol oxidase were up-regulated by the PRD(50) and PRD(100) treatments compared with control values. The higher activities of both soluble and insoluble POX observed in PRD(50) saplings may reflect the greater inhibitory effect of this treatment on vegetative growth. Up-regulation of the detoxifying systems in the PRD(100) and PRD(50) saplings may have provided protection mechanisms against irreversible damage to the photosynthetic machinery, thereby allowing the photosynthetic apparatus to function and preventing the development of severe water stress. We also measured CO(2) assimilation rate/internal leaf CO(2) concentration (A/C(i)) after exposing the attached leaves to very low [CO(2)] (approximately 50 micromol mol(-1)) to force stomatal opening. These results confirmed that, under conditions of moderate water stress, the sum of the diffusional resistances (i.e., stomatal and mesophyll resistances) sets the limit to photosynthetic rates. Assessing photosynthetic capacity without removing the diffusional limitations may lead to an overestimation of the biochemical limitations to photosynthesis in sclerophyllous plants.

Heat acclimation of grapevine leaf photosynthesis: mezo- and macroclimatic aspects

Heat sensitivity of grapevine (Vitis vinifera L. cv. Kékfrankos) photosynthesis was studied in two vineyards (Eger-Kőlyuktető, flat; and Eger-Nagyeged hill, steep slope) with different mesoclimates and water supply conditions in two climatically different years. 2007 was drier and warmer, with higher vapour pressure deficit (VPD) than 2005. Pre-dawn water potential measurements indicated mild water deficit at the steep-sloped vineyard. In July 2005 mild water deficit enhanced the thermostability of grapevine photosynthesis, as reflected in the temperature dependence of optimal quantum yield (F_v/F_m) and in the critical temperature of initial fluorescence (F₀T_c). Decreased F_v/F_m and actual quantum yield (ΔF/F_m') was recorded at most temperatures in September at the water-stressed (steep slope) site. This time, F₀T_cs were also lower due to early leaf senescence. In September 2007, heat sensitivity of F_v/F_m was similar to 2005, and ΔF/F_m' indicated higher thermostability at both sites, but keeping the consistent difference between the two vineyards. The critical points of steady-state fluorescence (F_sT_c) were higher by 3-6°C at both vineyards in 2007 than in 2005. Although, in September thermolabile F₀ signals were measured at the water-stressed vineyard, the heat sensitivity was not decreased in light adapted state, assumingly as a result of enhanced xanthophyll cycle pigment pool size. The higher xanthophyll pigments pool size (V + A + Z) in 2007 (compared to 2005) at the unstressed (flat) vineyard suggests that high temperature and VPD play a role in changing (V + A + Z)/(chl a + b), and, thus, results in higher thermostability under high light conditions.

Activation of abscisic acid biosynthesis in the leaves of Arabidopsis thaliana in response to water deficit

It is well known that endogenous abscisic acid (ABA) levels increase rapidly in response to drought stress and that this induces stomatal closure. In Arabidopsis thaliana, ABA levels increased rapidly in the leaves and roots when intact wild-type whole plants were exposed to drought stress. However, if the leaves and roots were separated and exposed to drought independently, the ABA level increased only in the leaves. These results suggest that, under our experimental conditions, ABA is synthesized mainly in the leaves in response to drought stress and that some of the ABA accumulated in the leaves is transported to the roots. Tracer experiments using isotope-labeled ABA indicate that the movement of ABA from leaves to roots is activated by water deficit in the roots. We also demonstrate that the endogenous ABA level in the leaves increased only when the leaves themselves were exposed to drought stress, suggesting that leaves play a major role in the production of ABA in response to acute water shortage.

Hormone interactions in stomatal function

Research in recent years on the biology of guard cells has shown that these specialized cells integrate both extra- and intra-cellular signals in the control of stomatal apertures. Among the phytohormones, abscisic acid (ABA) is one of the key players regulating stomatal function. In addition, auxin, cytokinin, ethylene, brassinosteroids, jasmonates, and salicylic acid also contribute to stomatal aperture regulation. The interaction of multiple hormones can serve to determine the size of stomatal apertures in a condition-specific manner. Here, we discuss the roles of different phytohormones and the effects of their interactions on guard cell physiology and function.

Abscisic acid improves drought tolerance of triploid bermudagrass and involves H2O2- and NO-induced antioxidant enzyme activities

Drought is a major limiting factor for turfgrass growth. Protection of triploid bermudagrass against drought stress by abscisic acid (ABA) and its association with hydrogen peroxide (H(2)O(2)) and nitric oxide (NO) were investigated. ABA treatment increased relative water content, decreased ion leakage and the percentage of dead plants significantly under drought stress. Superoxide dismutase (SOD) and catalase (CAT) activities increased in both ABA-treated and control plants, but more in ABA-treated plants, under drought stress. Malondialdehyde, an indicator of plant lipid peroxidation, was lower in ABA-treated plants than in control plants, indicating that ABA alleviated drought-induced oxidative injury. ABA treatment increased H(2)O(2) and NO contents. ABA-induced SOD and CAT activities could be blocked by scavengers of H(2)O(2) and NO, and inhibitors of H(2)O(2) and NO generation. The results indicated that H(2)O(2) and NO were essential for ABA-induced SOD and CAT activities. Both H(2)O(2) and NO could induce SOD and CAT activities individually. SOD and CAT induced by H(2)O(2) could be blocked by scavenger of NO and inhibitors of NO generation, while SOD and CAT induced by NO could not be blocked by scavenger of H(2)O(2) and inhibitor of H(2)O(2). The results revealed that ABA-induced SOD and CAT activities were mediated sequentially by H(2)O(2) and NO, and NO acted downstream of H(2)O(2).

Rhizosphere bacteria containing 1-aminocyclopropane-1-carboxylate deaminase increase yield of plants grown in drying soil via both local and systemic hormone signalling

Decreased soil water availability can stimulate production of the plant hormone ethylene and inhibit plant growth. Strategies aimed at decreasing stress ethylene evolution might attenuate its negative effects. An environmentally benign (nonchemical) method of modifying crop ethylene relations - soil inoculation with a natural root-associated bacterium Variovorax paradoxus 5C-2 (containing the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase that degrades the ethylene precursor ACC), was assessed with pea (Pisum sativum) plants grown in drying soil. Inoculation with V. paradoxus 5C-2, but not with a transposome mutant with massively decreased ACC deaminase activity, improved growth, yield and water-use efficiency of droughted peas. Systemic effects of V. paradoxus 5C-2 included an amplified soil drying-induced increase of xylem abscisic acid (ABA) concentration, but an attenuated soil drying-induced increase of xylem ACC concentration. A local bacterial effect was increased nodulation by symbiotic nitrogen-fixing bacteria, which prevented a drought-induced decrease in nodulation and seed nitrogen content. Successfully deploying a single bacterial gene in the rhizosphere increased yield and nutritive value of plants grown in drying soil, via both local and systemic hormone signalling. Such bacteria may provide an easily realized, economic means of sustaining crop yields and using irrigation water more efficiently in dryland agriculture.

Drought-induced hydraulic limitations constrain leaf gas exchange recovery after precipitation pulses in the C3 woody legume, Prosopis velutina

The hypothesis that drought intensity constrains the recovery of photosynthesis from drought was tested in the C(3) woody legume Prosopis velutina, and the mechanisms underlying this constraint examined. Hydraulic status and gas exchange were measured the day before a 39 mm precipitation pulse, and up to 7 d afterwards. The experiment was conducted under rainout shelters, established on contrasting soil textures and with different vegetation cover at the Santa Rita Experimental Range in southeastern Arizona, USA. Rates of photosynthesis and stomatal conductance after re-watering, as well as the number of days necessary for photosynthesis to recover after re-watering, were negatively correlated with predawn water potential, a measure of drought intensity (R(2) = 0.83, 0.64 and 0.92, respectively). Photosynthetic recovery was incomplete when the vascular capacity for water transport had been severely impaired (percentage loss of hydraulic conductance > 80%) during the drought, which largely increased stomatal limitations. However, changes in biochemical capacity or in mesophyll conductance did not explain the observed pattern of photosynthesis recovery. Although the control that hydraulic limitations impose on photosynthesis recovery had been previously inferred, the first empirical test of this concept is reported here.