Rootstock-induced physiological and biochemical mechanisms of drought tolerance in sweet orange

The poorly understood physiological and biochemical drought responses induced in sweet orange by citrus rootstocks of contrasting drought tolerance were investigated during a drought/rewatering cycle under controlled conditions. Long-term exposure of the grafted trees to a gradually increasing water deficit and subsequent recovery revealed distinct strategies of drought acclimation that were induced by the different rootstocks. Trees grafted onto the drought-tolerant rootstock ‘Cravo’ rangpur lime were less water conservative, exhibiting an increased cell-wall elasticity that contributes to turgor maintenance and its related processes of growth and photosynthesis over a wider range of soil–water potentials. On the other hand, the drought-tolerant ‘Sunki Tropical’ mandarin and drought-sensitive ‘Flying Dragon’ trifoliate orange rootstocks induced a water conservation strategy by increasing tissue rigidity under drought. ‘Sunki Tropical’ was also able to induce osmotic adjustment, conferring thereby a more efficient water conservation strategy than ‘Flying Dragon’ by allowing for turgor maintenance at lower soil–water potentials while attenuating cell dehydration and shrinkage. In contrast to ‘Cravo’ and ‘Sunki Tropical’, trees grafted onto ‘Flying Dragon’ exhibited a significant photoinhibition of the photosystem II reaction centers, as well as an increased H₂O₂ production and lipid peroxidation under drought treatment. A significantly higher activity of the antioxidant enzyme GPX was also observed in drought stressed trees grafted onto ‘Flying Dragon’. Collectively, these results support the involvement of elastic and osmotic adjustments, as well as the control of oxidative stress, as functional leaf traits associated with the rootstock-induced drought tolerance in sweet orange.     

Improving plant drought tolerance and growth under water limitation through combinatorial engineering of signalling networks

Agriculture is by far the biggest water consumer on our planet, accounting for 70 per cent of all freshwater withdrawals. Climate change and a growing world population increase pressure on agriculture to use water more efficiently (‘more crop per drop’). Water‐use efficiency (WUE) and drought tolerance of crops are complex traits that are determined by many physiological processes whose interplay is not well understood. Here, we describe a combinatorial engineering approach to optimize signalling networks involved in the control of stress tolerance. Screening a large population of combinatorially transformed plant lines, we identified a combination of calcium‐dependent protein kinase genes that confers enhanced drought stress tolerance and improved growth under water‐limiting conditions. Targeted introduction of this gene combination into plants increased plant survival under drought and enhanced growth under water‐limited conditions. Our work provides an efficient strategy for engineering complex signalling networks to improve plant performance under adverse environmental conditions, which does not depend on prior understanding of network function.     

Rethinking the conceptual foundations of habitat fragmentation research

The conceptual foundations of habitat fragmentation research have not kept pace with empirical advances in our understanding of species responses to landscape change, nor with theoretical advances in the wider disciplines of ecology. There is now real debate whether explicit recognition of ‘habitat fragmentation’ as an over‐arching conceptual domain will stimulate or hinder further progress toward understanding and mitigating the effects of landscape change. In this paper, we critically challenge the conceptual foundations of the discipline, and attempt to derive an integrated perspective on the best way to advance mechanistic understanding of fragmentation processes. We depict the inherent assumptions underlying the discipline as a ‘conceptual phase space’ of contrasting false dichotomies in fragmentation ‘problem space’. In our opinion, the key determinant of whether ‘habitat fragmentation’ can remain a cohesive framework lies in the concept of ‘interdependence’: 1) interdependence of landscape effects on species and 2) interdependence of species responses to landscape change. If there is non‐trivial interdependence among the various sub‐components of habitat fragmentation, or non‐trivial interdependence among species responses to landscape change, then there will be real heuristic value in ‘habitat fragmentation’ as a single conceptual domain. At present, the current paradigms entrenched in the fragmentation literature are implicitly founded on strict independence of landscape effects (e.g. the debate about the independent effects of habitat loss versus fragmentation per se) and strict independence of species responses (e.g. the individualistic species response models underpinning landscape continuum models), despite compelling evidence for interdependence in both effects and responses to fragmentation. We discuss how strong ‘interdependence’ of effects and responses challenges us to rethink long‐held views, and re‐cast the conceptual foundations of habitat fragmentation in terms of spatial context‐dependence in the effects of multiple interacting spatial components of fragmentation, and community context‐dependence in the responses of multiple interacting species to landscape change.     

Programmed proteome response for drought avoidance/tolerance in the root of a C(3) xerophyte (wild watermelon) under water deficits

Water availability is a critical determinant for the growth and ecological distribution of terrestrial plants. Although some xerophytes are unique regarding their highly developed root architecture and the successful adaptation to arid environments, virtually nothing is known about the molecular mechanisms underlying this adaptation. Here, we report physiological and molecular responses of wild watermelon (Citrullus lanatus sp.), which exhibits extraordinarily high drought resistance. At the early stage of drought stress, root development of wild watermelon was significantly enhanced compared with that of the irrigated plants, indicating the activation of a drought avoidance mechanism for absorbing water from deep soil layers. Consistent with this observation, comparative proteome analysis revealed that many proteins induced in the early stage of drought stress are involved in root morphogenesis and carbon/nitrogen metabolism, which may contribute to the drought avoidance via the enhancement of root growth. On the other hand, lignin synthesis-related proteins and molecular chaperones, which may function in the enhancement of physical desiccation tolerance and maintenance of protein integrity, respectively, were induced mostly at the later stage of drought stress. Our findings suggest that this xerophyte switches survival strategies from drought avoidance to drought tolerance during the progression of drought stress, by regulating its root proteome in a temporally programmed manner. This study provides new insights into the complex molecular networks within plant roots involved in the adaptation to adverse environments.     

Fast growth involves high dependence on stored resources in seedlings of Mediterranean evergreen trees

Background and Aims Extreme climatic events such as severe droughts are expected to increase with climate change and to limit grassland perennity. The present study aimed to characterize the adaptive responses by which temperate herbaceous grassland species resist, survive and recover from a severe drought and to explore the relationships between plant resource use and drought resistance strategies.Methods Monocultures of six native perennial species from upland grasslands and one Mediterranean drought-resistant cultivar were compared under semi-controlled and non-limiting rooting depth conditions. Above- and below-ground traits were measured under irrigation in spring and during drought in summer (50 d of withholding water) in order to characterize resource use and drought resistance strategies. Plants were then rehydrated and assessed for survival (after 15 d) and recovery (after 1 year).Key Results Dehydration avoidance through water uptake was associated with species that had deep roots (>1·2 m) and high root mass (>4 kg m⁻³). Cell membrane stability ensuring dehydration tolerance of roots and meristems was positively correlated with fructan content and negatively correlated with sucrose content. Species that survived and recovered best combined high resource acquisition in spring (leaf elongation rate >9 mm d⁻¹ and rooting depth >1·2 m) with both high dehydration avoidance and tolerance strategies.Conclusions Most of the native forage species, dominant in upland grassland, were able to survive and recover from extreme drought, but with various time lags. Overall the results suggest that the wide range of interspecific functional strategies for coping with drought may enhance the resilience of upland grassland plant communities under extreme drought events.     

Interact to Survive: Phyllobacterium brassicacearum Improves Arabidopsis Tolerance to Severe Water Deficit and Growth Recovery

Mutualistic bacteria can alter plant phenotypes and confer new abilities to plants. Some plant growth-promoting rhizobacteria (PGPR) are known to improve both plant growth and tolerance to multiple stresses, including drought, but reports on their effects on plant survival under severe water deficits are scarce. We investigated the effect of Phyllobacterium brassicacearum STM196 strain, a PGPR isolated from the rhizosphere of oilseed rape, on survival, growth and physiological responses of Arabidopsis thaliana to severe water deficits combining destructive and non-destructive high-throughput phenotyping. Soil inoculation with STM196 greatly increased the survival rate of A. thaliana under several scenarios of severe water deficit. Photosystem II efficiency, assessed at the whole-plant level by high-throughput fluorescence imaging (F _v/F _m), was related to the probability of survival and revealed that STM196 delayed plant mortality. Inoculated surviving plants tolerated more damages to the photosynthetic tissues through a delayed dehydration and a better tolerance to low water status. Importantly, STM196 allowed a better recovery of plant growth after rewatering and stressed plants reached a similar biomass at flowering than non-stressed plants. Our results highlight the importance of plant-bacteria interactions in plant responses to severe drought and provide a new avenue of investigations to improve drought tolerance in agriculture.     

Plant water uptake modelling: added value of cross-disciplinary approaches

In recent years, research interest in plant water uptake strategies has rapidly increased in many disciplines, such as hydrology, plant ecology and ecophysiology. Quantitative modelling approaches to estimate plant water uptake and spatiotemporal dynamics have significantly advanced through different disciplines across scales. Despite this progress, major limitations, for example, predicting plant water uptake under drought or drought impact at large scales, remain. These are less attributed to limitations in process understanding, but rather to a lack of implementation of cross-disciplinary insights into plant water uptake model structure. The main goal of this review is to highlight how the four dominant model approaches, that is, Feddes approach, hydrodynamic approach, optimality and statistical approaches, can be and have been used to create interdisciplinary hybrid models enabling a holistic system understanding that, among other things, embeds plant water uptake plasticity into a broader conceptual view of soil–plant feedbacks of water, nutrient and carbon cycling, or reflects observed drought responses of plant–soil feedbacks and their dynamics under, that is, drought. Specifically, we provide examples of how integration of Bayesian and hydrodynamic approaches might overcome challenges in interpreting plant water uptake related to different travel and residence times of different plant water sources or trade-offs between root system optimization to forage for water and nutrients during different seasons and phenological stages.     

Tradeoffs between functional strategies for resource-use and drought-survival in Mediterranean rangeland species

In environments where light is not a limiting resource such as rangelands and grasslands, there is much disagreement regarding the benefits provided by rapid light capture during the growing season and the species’ ability to withstand drought during the dry period. In this study, we selected four perennial herbaceous species with contrasting resource-use strategies (acquisitive versus conservative), which were transplanted as monocultures into PVC pots to evaluate their species-specific responses to drought. The two main strategies of drought-survival (avoidance versus tolerance) were driven by distinct underlying mechanisms that allow the plant to delay or tolerate water deficit in leaves. On the one hand, plants that produced reduced leaves with lower surface area:mass ratio (lower SLA) exhibited higher values of leaf water potential (LWP) and leaf relative water content (LRWC), which could be associated to a higher ability to delay tissue dehydration in enlarged leaves. Regarding the below-ground compartment, dehydration avoidance was promoted by prolonged elongation rates of thinner roots that allow the plant to increase water uptake and accessibility during the dry period. On the other hand, dehydration tolerance was positively related with progressive foliage senescence under water deficit, which probably favored a longer survival of meristematic basal tissues. The results presented in this study suggest the existence of a trade-off between the traits favoring rapid light-acquisition and those enhancing the ability to delay leaf dehydration. Thus, the species related most closely with a resource-acquisition strategy (Bromus erectus and Potentilla neumanniana) could be considered less efficient to delay leaf dehydration than the others (Carex humilis and Festuca christiani-bernardii), as indicated by their lower values of leaf water potential (LWP) and leaf relative water content (LRWC) under identical conditions of water deficit. Our findings support evidence that there is not a single strategy to effectively cope with drought and reveal the diversity of adaptive mechanisms among coexisting species.     

PdEPF1 regulates water‐use efficiency and drought tolerance by modulating stomatal density in poplar

Water deficiency is a critical environmental condition that is seriously reducing global plant production. Improved water‐use efficiency (WUE) and drought tolerance are effective strategies to address this problem. In this study, PdEPF1, a member of the EPIDERMAL PATTERNING FACTOR (EPF) family, was isolated from the fast‐growing poplar clone NE‐19 [Populus nigra × (Populus deltoides × Populus nigra)]. Significantly, higher PdEPF1 levels were detected after induction by dehydration and abscisic acid. To explore the biological functions of PdEPF1, transgenic triploid white poplars (Populus tomentosa ‘YiXianCiZhu B385’) overexpressing PdEPF1 were constructed. PdEPF1 overexpression resulted in increased water deficit tolerance and greater WUE. We confirmed that the transgenic lines with greater instantaneous WUE had approximately 30% lower transpiration but equivalent CO₂ assimilation. Lower transpiration was associated with a 28% reduction in abaxial stomatal density. PdEPF1 overexpression not only strongly enhanced WUE, but also greatly improved drought tolerance, as measured by the leaf relative water content and water potential, under limited water conditions. In addition, the growth of these oxPdEPF1 plants was less adversely affected by reduced water availability than plants with a higher stomatal density, indicating that plants with a low stomatal density may be well suited to grow in water‐scarce environments. Taken together, our data suggest that PdEPF1 improves WUE and confers drought tolerance in poplar; thus, it could be used to breed drought‐tolerant plants with increased production under conditions of water deficiency.     

Plant cellular responses to water deficit

Water availability and drought limit crop yields worldwide. The responses of plants to drought vary greatly depending on species and stress severity. These responses include changes in plant growth, accumulation of solutes, changes in carbon and nitrogen metabolism, and alterations in gene expression. In this article, we review cellular and molecular responses to water deficit, and their influence on plant dehydration tolerance.     

Wheat flag leaf epicuticular wax morphology and composition in response to moderate drought stress are revealed by SEM,FTIR‐ATR and synchrotron X‐ray spectroscopy

Wheat (Triticum aestivum L.) is the largest cereal crop grown in Western Canada where drought during late vegetative and seed filling stages affects plant development and yield. To identify new physiochemical markers associated with drought tolerance, epidermal characteristics of the flag leaf of two wheat cultivars with contrasting drought tolerance were investigated. The drought resistant ‘Stettler’ had a lower drought susceptibility index, greater harvest index and water‐use efficiency than the susceptible ‘Superb’. Furthermore, flag leaf width, relative water content and leaf roll were significantly greater in Stettler than in Superb at moderate drought stress (MdS). Visible differences in epicuticular wax density on the adaxial flag leaf surfaces and larger bulliform cells were identified in Stettler as opposed to Superb. Mid‐infrared attenuated total internal reflectance spectra revealed that Stettler flag leaves had increased asymmetric and symmetric CH₂ but reduced carbonyl esters on its adaxial leaf surface compared to Superb under MdS. X‐ray fluorescence spectra revealed a significant increase in total flag leaf Zn concentrations in Stettler in response to MdS. Such information on the microstructural and chemical features of flag leaf may have potential as markers for drought tolerance and thereby accelerate the selection and release of more drought‐resistant cultivars.     

Relationship of dehydration rate to drought avoidance,dehydration tolerance and dehydration avoidance of cabbage leaves,and to their acclimation during drought-induced water stress*

Abstract. Drought avoidance due to cuticular control increases with leaf number to a maximum in the intermediate leaves, decreasing to a minimum in the upper leaves. Dehydrated intermediate leaves do not rehydrate detectably when floated on water for several days. Excision of their petioles when submerged, permits full rehydration, presumably via the xylem. Stressing the plant by withholding water for 1–3 weeks fails to increase this already high resistance to water movement through the leaf surface. It does, however, markedly decrease the loss of water from the fully rehydrated (100% RWC) leaves during the first hour of dehydration, presumably due to a more rapid stomatal closure than in the non-stressed leaves. Dehydration tolerance increases with leaf number, without an intermediate maximum. The intermediate and upper leaves were markedly more tolerant of dehydration after drought-induced stress than when non-stressed. Dehydration tolerance in some cases, was inversely proportional to dehydration rate. It was possible, however, to equalize the rates of dehydration of drought-stressed and non-drought-stressed leaves without affecting the greater tolerance of the drought-stressed leaves. Dehydration avoidance by osmotic adjustment was markedly developed in the slowly dehydrated attached leaves of drought-stressed plants, but not in the rapidly dehydrated excised leaves. This is evidence of drought acclimation. It must, therefore, be concluded that the slow dehydration of the drought-stressed plants also leads to the increase in dehydration tolerance by permitting drought-induced acclimation. The overall drought resistance of cabbage leaves depends on the three components: drought avoidance, dehydration avoidance and dehydration tolerance. The latter two increase during acclimation but the cuticular control of drought avoidance does not.     

异源表达CkLEA1基因增强了拟南芥对非生物胁迫的耐受性

植物在生长过程中会受到各种非生物胁迫的伤害,导致生长发育和产量受到严重影响,胚胎晚期丰富蛋白（late embryogenesis abundant proteins,LEA蛋白）在植物抵抗非生物胁迫过程中起着重要的保护作用。在前期的研究基础上,将受多种胁迫诱导的柠条锦鸡儿CkLEA1（GenBank登录号KC309408）基因转入野生型拟南芥,通过实时荧光定量PCR从7株T₃代纯合体中筛选出3个转基因株系做进一步研究。种子萌发率实验发现,在200 mmol/L NaCl和400 mmol/L甘露醇处理下,转基因株系萌发率均高于野生型拟南芥。干旱处理2周大的幼苗后,转基因株系明显比野生型更抗旱,存活率高于野生型,并且失水率低于野生型。同时,转基因株系积累了较少的丙二醛（MDA）,超氧化物歧化酶（SOD）活性和谷胱甘肽（GSH）含量也高于野生型。这些结果表明,柠条锦鸡儿CkLEA1基因在种子萌发阶段提高了拟南芥对盐和渗透胁迫的耐受性,并且提高了转基因拟南芥幼苗生长阶段对干旱胁迫的抵抗能力。     

The diversity of drought adaptation in the wide

Life on the earth is highly dependent on the properties and functions of water. In front of water limitation, herbaceous, woody and epiphyte plants have developed a wide diversity of drought tolerance mechanisms at the molecular, metabolic and physiological levels. The strategies of adaptation to drought have been listed in regard of the level of organization: molecules, cells, whole plant. Root development and water uptake, transpiration and micro- and macromorphological adaptations, and water status and osmotic adjustment have important consequences on drought adaptation. The relationship between these characters and mechanisms and the productivity of cultivated plants are the basis of the breeding for drought tolerance.