Growing Out of Stress: The Role of Cell- and Organ-Scale Growth Control in Plant Water-Stress Responses

ABA-Dependent Salt Stress Tolerance Attenuates Botrytis Immunity in Arabidopsis

Plants have evolved adaptive measures to cope with abiotic and biotic challenges simultaneously. Combinatorial stress responses require environmental signal integration and response prioritization to balance stress adaptation and growth. We have investigated the impact of salt, an important environmental factor in arid regions, on the Arabidopsis innate immune response. Activation of a classical salt stress response resulted in increased susceptibility to infection with hemibiotrophic Pseudomonas syringae or necrotrophic Alternaria brassicicola, and Botrytis cinerea, respectively. Surprisingly, pattern-triggered immunity (PTI)-associated responses were largely unaffected upon salt pre-treatment. However, we further observed a strong increase in phytohormone levels. Particularly, abscisic acid (ABA) levels were already elevated before pathogen infection, and application of exogenous ABA substituted for salt-watering in increasing Arabidopsis susceptibility toward B. cinerea infection. We propose a regulatory role of ABA in attenuating Botrytis immunity in this plant under salt stress conditions.

Root Adaptation via Common Genetic Factors Conditioning Tolerance to Multiple Stresses for Crops Cultivated on Acidic Tropical Soils

Crop tolerance to multiple abiotic stresses has long been pursued as a Holy Grail in plant breeding efforts that target crop adaptation to tropical soils. On tropical, acidic soils, aluminum (Al) toxicity, low phosphorus (P) availability and drought stress are the major limitations to yield stability. Molecular breeding based on a small suite of pleiotropic genes, particularly those with moderate to major phenotypic effects, could help circumvent the need for complex breeding designs and large population sizes aimed at selecting transgressive progeny accumulating favorable alleles controlling polygenic traits. The underlying question is twofold: do common tolerance mechanisms to Al toxicity, P deficiency and drought exist? And if they do, will they be useful in a plant breeding program that targets stress-prone environments. The selective environments in tropical regions are such that multiple, co-existing regulatory networks may drive the fixation of either distinctly different or a smaller number of pleiotropic abiotic stress tolerance genes. Recent studies suggest that genes contributing to crop adaptation to acidic soils, such as the major Arabidopsis Al tolerance protein, AtALMT1, which encodes an aluminum-activated root malate transporter, may influence both Al tolerance and P acquisition via changes in root system morphology and architecture. However, trans-acting elements such as transcription factors (TFs) may be the best option for pleiotropic control of multiple abiotic stress genes, due to their small and often multiple binding sequences in the genome. One such example is the C2H2-type zinc finger, AtSTOP1, which is a transcriptional regulator of a number of Arabidopsis Al tolerance genes, including AtMATE and AtALMT1, and has been shown to activate AtALMT1, not only in response to Al but also low soil P. The large WRKY family of transcription factors are also known to affect a broad spectrum of phenotypes, some of which are related to acidic soil abiotic stress responses. Hence, we focus here on signaling proteins such as TFs and protein kinases to identify, from the literature, evidence for unifying regulatory networks controlling Al tolerance, P efficiency and, also possibly drought tolerance. Particular emphasis will be given to modification of root system morphology and architecture, which could be an important physiological "hub" leading to crop adaptation to multiple soil-based abiotic stress factors.

Rice plants respond to ammonium stress by adopting a helical root growth pattern

High levels of ammonium nutrition reduce plant growth and different plant species have developed distinct strategies to maximize ammonium acquisition while alleviating ammonium toxicity through modulating root growth. To date, the mechanisms underlying plant tolerance or sensitivity towards ammonium remain unclear. Rice (Oryza sativa) uses ammonium as its main N source. Here we show that ammonium supply restricts rice root elongation and induces a helical growth pattern, which is attributed to root acidification resulting from ammonium uptake. Ammonium-induced low pH triggers the asymmetric distribution of auxin in rice root tips through changes in auxin signaling, thereby inducing a helical growth response. Blocking auxin signaling completely inhibited this root response. In contrast, this root response is not activated in ammonium-treated Arabidopsis. Acidification of Arabidopsis roots leads to the protonation of indole-3-acetic acid and dampening of the intracellular auxin signaling levels that are required for maintaining root growth. Our study suggests a different mode of action by ammonium on the root pattern and auxin response machinery in rice versus Arabidopsis, and the rice-specific helical root response towards ammonium is an expression of the ability of rice to moderate auxin signaling and root growth to utilize ammonium while confronting acidic stress.

Drought Disrupts Auxin Localization in Abscission Zone and Modifies Cell Wall Structure Leading to Flower Separation in Yellow Lupine

Drought causes the excessive abscission of flowers in yellow lupine, leading to yield loss and serious economic consequences in agriculture. The structure that determines the time of flower shedding is the abscission zone (AZ). Its functioning depends on the undisturbed auxin movement from the flower to the stem. However, little is known about the mechanism guiding cell-cell adhesion directly in an AZ under water deficit. Therefore, here, we seek a fuller understanding of drought-dependent reactions and check the hypothesis that water limitation in soil disturbs the natural auxin balance within the AZ and, in this way, modifies the cell wall structure, leading to flower separation. Our strategy combined microscopic, biochemical, and chromatography approaches. We show that drought affects indole-3-acetic acid (IAA) distribution and evokes cellular changes, indicating AZ activation and flower abortion. Drought action was manifested by the accumulation of proline in the AZ. Moreover, cell wall-related modifications in response to drought are associated with reorganization of methylated homogalacturonans (HG) in the AZ, and upregulation of pectin methylesterase (PME) and polygalacturonase (PG)-enzymes responsible for pectin remodeling. Another symptom of stress action is the accumulation of hemicelluloses. Our data provide new insights into cell wall remodeling events during drought-induced flower abscission, which is relevant to control plant production.

Halotropism: Phytohormonal Aspects and Potential Applications

Halotropism is a sodium specific tropic movement of roots in order to obtain the optimal salt concentration for proper growth and development. Numerous results suggest that halotropic events are under the control and regulation of complex plant hormone pathway. This minireview collects some recent evidences about sodium sensing during halotropism and the hormonal regulation of halotropic responses in glycophytes. The precise hormonal mechanisms by which halophytes plant roots perceive salt stress and translate this perception into adaptive, directional growth forward increased salt concentrations are not well understood. This minireview aims to gather recently deciphered information about halotropism focusing potential hormonal aspects both in glycophytes and halophytes. Advances in our understanding of halotropic responses in different plant species could help these plants to be used for sustainable agriculture and other future applications.

How does nitrogen shape plant architecture?

Plant nitrogen (N), acquired mainly in the form of nitrate and ammonium from soil, dominates growth and development, and high-yield crop production relies heavily on N fertilization. The mechanisms of root adaptation to altered supply of N forms and concentrations have been well characterized and reviewed, while reports concerning the effects of N on the architecture of vegetative and reproductive organs are limited and are widely dispersed in the literature. In this review, we summarize the nitrate and amino acid regulation of shoot branching, flowering, and panicle development, as well as the N regulation of cell division and expansion in shaping plant architecture, mainly in cereal crops. The basic regulatory steps involving the control of plant architecture by the N supply are auxin-, cytokinin-, and strigolactone-controlled cell division in shoot apical meristem and gibberellin-controlled inverse regulation of shoot height and tillering. In addition, transport of amino acids has been shown to be involved in the control of shoot branching. The N supply may alter the timing and duration of the transition from the vegetative to the reproductive growth phase, which in turn may affect cereal crop architecture, particularly the structure of panicles for grain yield. Thus, proper manipulation of N-regulated architecture can increase crop yield and N use efficiency.

Multiple Integrated Root Phenotypes Are Associated with Improved Drought Tolerance

To test the hypothesis that multiple integrated root phenotypes would co-optimize drought tolerance, we phenotyped the root anatomy and architecture of 400 mature maize (Zea mays) genotypes under well-watered and water-stressed conditions in the field. We found substantial variation in all 23 root phenes measured. A phenotypic bulked segregant analysis revealed that bulks representing the best and worst performers in the field displayed distinct root phenotypes. In contrast to the worst bulk, the root phenotype of the best bulk under drought consisted of greater cortical aerenchyma formation, more numerous and narrower metaxylem vessels, and thicker nodal roots. Partition-against-medians clustering revealed several clusters of unique root phenotypes related to plant performance under water stress. Clusters associated with improved drought tolerance consisted of phene states that likely enable greater soil exploration by reallocating internal resources to greater root construction (increased aerenchyma content, larger cortical cells, fewer cortical cell files), restrict uptake of water to conserve soil moisture (reduced hydraulic conductance, narrow metaxylem vessels), and improve penetrability of hard, dry soils (thick roots with a larger proportion of stele, and smaller distal cortical cells). We propose that the most drought-tolerant-integrated phenotypes merit consideration as breeding ideotypes.

Dynamic Construction, Perception, and Remodeling of Plant Cell Walls

Plant cell walls are dynamic structures that are synthesized by plants to provide durable coverings for the delicate cells they encase. They are made of polysaccharides, proteins, and other biomolecules and have evolved to withstand large amounts of physical force and to resist external attack by herbivores and pathogens but can in many cases expand, contract, and undergo controlled degradation and reconstruction to facilitate developmental transitions and regulate plant physiology and reproduction. Recent advances in genetics, microscopy, biochemistry, structural biology, and physical characterization methods have revealed a diverse set of mechanisms by which plant cells dynamically monitor and regulate the composition and architecture of their cell walls, but much remains to be discovered about how the nanoscale assembly of these remarkable structures underpins the majestic forms and vital ecological functions achieved by plants.

Salt Tolerance Mechanisms of Plants

Crop loss due to soil salinization is an increasing threat to agriculture worldwide. This review provides an overview of cellular and physiological mechanisms in plant responses to salt. We place cellular responses in a time- and tissue-dependent context in order to link them to observed phases in growth rate that occur in response to stress. Recent advances in phenotyping can now functionally or genetically link cellular signaling responses, ion transport, water management, and gene expression to growth, development, and survival. Halophytes, which are naturally salt-tolerant plants, are highlighted as success stories to learn from. We emphasize that (a) filling the major knowledge gaps in salt-induced signaling pathways, (b) increasing the spatial and temporal resolution of our knowledge of salt stress responses, (c) discovering and considering crop-specific responses, and (d) including halophytes in our comparative studies are all essential in order to take our approaches to increasing crop yields in saline soils to the next level.

How Plants Sense and Respond to Stressful Environments

Plants are exposed to an ever-changing environment to which they have to adjust accordingly. Their response is tightly regulated by complex signaling pathways that all start with stimulus perception. Here, we give an overview of the latest developments in the perception of various abiotic stresses, including drought, salinity, flooding, and temperature stress. We discuss whether proposed perception mechanisms are true sensors, which is well established for some abiotic factors but not yet fully elucidated for others. In addition, we review the downstream cellular responses, many of which are shared by various stresses but result in stress-specific physiological and developmental output. New sensing mechanisms have been identified, including heat sensing by the photoreceptor phytochrome B, salt sensing by glycosylinositol phosphorylceramide sphingolipids, and drought sensing by the specific calcium influx channel OSCA1. The simultaneous occurrence of multiple stress conditions shows characteristic downstream signaling signatures that were previously considered general signaling responses. The integration of sensing of multiple stress conditions and subsequent signaling responses is a promising venue for future research to improve the understanding of plant abiotic stress perception.

Cellular Heterogeneity in Pressure and Growth Emerges from Tissue Topology and Geometry

Cell-to-cell heterogeneity prevails in many systems, as exemplified by cell growth, although the origin and function of such heterogeneity are often unclear. In plants, growth is physically controlled by cell wall mechanics and cell hydrostatic pressure, alias turgor pressure. Whereas cell wall heterogeneity has received extensive attention, the spatial variation of turgor pressure is often overlooked. Here, combining atomic force microscopy and a physical model of pressurized cells, we show that turgor pressure is heterogeneous in the Arabidopsis shoot apical meristem, a population of stem cells that generates all plant aerial organs. In contrast with cell wall mechanical properties that appear to vary stochastically between neighboring cells, turgor pressure anticorrelates with cell size and cell neighbor number (local topology), in agreement with the prediction by our model of tissue expansion, which couples cell wall mechanics and tissue hydraulics. Additionally, our model predicts two types of correlations between pressure and cellular growth rate, where high pressure may lead to faster- or slower-than-average growth, depending on cell wall extensibility, yield threshold, osmotic pressure, and hydraulic conductivity. The meristem exhibits one of these two regimes, depending on conditions, suggesting that, in this tissue, water conductivity may contribute to growth control. Our results unravel cell pressure as a source of patterned heterogeneity and illustrate links between local topology, cell mechanical state, and cell growth, with potential roles in tissue homeostasis.

Genetic control of root anatomical plasticity in maize

Root anatomical phenes have important roles in soil resource capture and plant performance; however, their phenotypic plasticity and genetic architecture is poorly understood. We hypothesized that (a) the responses of root anatomical phenes to water deficit (stress plasticity) and different environmental conditions (environmental plasticity) are genetically controlled and (b) stress and environmental plasticity are associated with different genetic loci than those controlling the expression of phenes under water-stress and well-watered conditions. Root anatomy was phenotyped in a large maize (Zea mays L.) association panel in the field with and without water deficit stress in Arizona and without water deficit stress in South Africa. Anatomical phenes displayed stress and environmental plasticity; many phenotypic responses to water deficit were adaptive, and the magnitude of response varied by genotype. We identified 57 candidate genes associated with stress and environmental plasticity and 64 candidate genes associated with phenes under well-watered and water-stress conditions in Arizona and under well-watered conditions in South Africa. Four candidate genes co-localized between plasticity groups or for phenes expressed under each condition. The genetic architecture of phenotypic plasticity is highly quantitative, and many distinct genes control plasticity in response to water deficit and different environments, which poses a challenge for breeding programs.

A wall with integrity: surveillance and maintenance of the plant cell wall under stress

The structural and functional integrity of the cell wall needs to be constantly monitored and fine-tuned to allow for growth while preventing mechanical failure. Many studies have advanced our understanding of the pathways that contribute to cell wall biosynthesis and how these pathways are regulated by external and internal cues. Recent evidence also supports a model in which certain aspects of the wall itself may act as growth-regulating signals. Molecular components of the signaling pathways that sense and maintain cell wall integrity have begun to be revealed, including signals arising in the wall, sensors that detect changes at the cell surface, and downstream signal transduction modules. Abiotic and biotic stress conditions provide new contexts for the study of cell wall integrity, but the nature and consequences of wall disruptions due to various stressors require further investigation. A deeper understanding of cell wall signaling will provide insights into the growth regulatory mechanisms that allow plants to survive in changing environments.

The quest for osmosensors in plants

Osmotic stress has severe effects on crop productivity. Since climate change is predicted to exacerbate this problem, the development of new crops that are tolerant to osmotic stresses, especially drought and salinity stress, is required. However, only limited success has been achieved to date, primarily because of the lack of a clear understanding of the mechanisms that facilitate osmosensing. Here, we discuss the potential mechanisms of osmosensing in plants. We highlight the roles of proteins such as receptor-like kinases, which sense stress-induced cell wall damage, mechanosensitive calcium channels, which initiate a calcium-induced stress response, and phospholipase C, a membrane-bound enzyme that is integral to osmotic stress perception. We also discuss the roles of aquaporins and membrane-bound histidine kinases, which could potentially detect changes in extracellular osmolarity in plants, as they do in prokaryotes and lower eukaryotes. These putative osmosensors have the potential to serve as master regulators of the osmotic stress response in plants and could prove to be useful targets for the selection of osmotic stress-tolerant crops.

Phytosphinganine Affects Plasmodesmata Permeability via Facilitating PDLP5-Stimulated Callose Accumulation in Arabidopsis

Plant plasmodesmata (PDs) are specialized channels that enable communication between neighboring cells. The intercellular permeability of PDs, which affects plant development, defense, and responses to stimuli, must be tightly regulated. However, the lipid compositions of PD membrane and their impact on PD permeability remain elusive. Here, we report that the Arabidopsis sld1 sld2 double mutant, lacking sphingolipid long-chain base 8 desaturases 1 and 2, displayed decreased PD permeability due to a significant increase in callose accumulation. PD-located protein 5 (PDLP5) was significantly enriched in the leaf epidermal cells of sld1 sld2 and showed specific binding affinity to phytosphinganine (t18:0), suggesting that the enrichment of t18:0-based sphingolipids in sld1 sld2 PDs might facilitate the recruitment of PDLP5 proteins to PDs. The sld1 sld2 double mutant seedlings showed enhanced resistance to the fungal-wilt pathogen Verticillium dahlia and the bacterium Pseudomonas syringae pv. tomato DC3000, which could be fully rescued in sld1 sld2 pdlp5 triple mutant. Taken together, these results indicate that phytosphinganine might regulate PD functions and cell-to-cell communication by modifying the level of PDLP5 in PD membranes.

Abscisic Acid Is Required for Root Elongation Associated With Ca2+ Influx in Response to Water Stress

Abscisic acid (ABA) is a critical hormone for plant survival under water stress. In this study, large-scale mutants of the Arabidopsis ecotype Columbia-0 (Col-0) were generated by ethyl methanesulfonate (EMS)-mutagenesis, and an improved root elongation under water-stress 1 (irew1) mutant showing significantly enhanced root growth was isolated under a water potential gradient assay. Then, irew1 and ABA-related mutants in Arabidopsis or tomato plants were observed under water potential gradient assay or water-deficient conditions. ABA pathway, Ca²⁺ response, and primary root (PR) elongation rate were monitored in addition to DNA- and RNA-Seq analyses. We found that based on phenotyping and transcriptional analyses, irew1 exhibited enhanced PR growth, ABA, and Ca²⁺ responses, compared to wild type subjected to water stress. Interestingly, exogenous Ca²⁺ application enhanced PR growth of irew1, ABA-biosynthesis deficient mutants in Arabidopsis, and tomato plants, in response to water potential gradients or water-deficient conditions. In combination with other ABA-related mutants and pharmacological studies, our results suggest that ABA is required for root elongation associated with Ca²⁺ influx in response to water stress.

Pathogen-induced pH changes regulate the growth-defense balance in plants

Environmental adaptation of organisms relies on fast perception and response to external signals, which lead to developmental changes. Plant cell growth is strongly dependent on cell wall remodeling. However, little is known about cell wall-related sensing of biotic stimuli and the downstream mechanisms that coordinate growth and defense responses. We generated genetically encoded pH sensors to determine absolute pH changes across the plasma membrane in response to biotic stress. A rapid apoplastic acidification by phosphorylation-based proton pump activation in response to the fungus Fusarium oxysporum immediately reduced cellulose synthesis and cell growth and, furthermore, had a direct influence on the pathogenicity of the fungus. In addition, pH seems to influence cellulose structure. All these effects were dependent on the COMPANION OF CELLULOSE SYNTHASE proteins that are thus at the nexus of plant growth and defense. Hence, our discoveries show a remarkable connection between plant biomass production, immunity, and pH control, and advance our ability to investigate the plant growth-defense balance.

Cadmium stress suppresses lateral root formation by interfering with the root clock

A biological clock activated by oscillating signals, known as root clock, has been linked to lateral root (LR) formation and is essential for regular LR spacing along the primary root. However, it remains unclear how this internal mechanism is influenced by environmental factors known to affect the LR pattern. Here, we report that excessive cadmium (Cd) inhibits LR formation by disrupting the lateral root cap (LRC)-programmed cell death (PCD)-regulated root clock. Cd restricts the frequency of the oscillating signal rather than its amplitude. This could be attributed to the inhibition on meristematic activity by Cd, which resulted in decreased LRC cell number and LRC-PCD frequency. Genetic evidence further showed that LRC cell number is positively correlated with root resistance to Cd. Our study reveals root cap dynamics as a novel mechanism mediating root responses to Cd, providing insight into the signalling pathways of the root clock responding to environmental cues.

Genetic Loci Associated with Early Salt Stress Responses of Roots

Salinity is a devastating abiotic stress accounting for major crop losses yearly. Plant roots can strikingly grow away from high-salt patches. This response is termed halotropism and occurs through auxin redistribution in roots in response to a salt gradient. Here, a natural variation screen for the early and NaCl-specific halotropic response of 333 Arabidopsis accessions revealed quantitative differences in the first 24 h. These data were successfully used to identify genetic components associated with the response through Genome-Wide Association Study (GWAS). Follow-up characterization of knockout mutants in Col-0 background confirmed the role of transcription factor WRKY25, cation-proton exchanger CHX13, and a gene of unknown function DOB1 (Double Bending 1) in halotropism. In chx13 and dob1 mutants, ion accumulation and shoot biomass under salt stress were also affected. Thus, our GWAS has identified genetic components contributing to main root halotropism that provide insight into the genetic architecture underlying plant salt responses.

Blue Light-Triggered Chemical Reactions Underlie Phosphate Deficiency-Induced Inhibition of Root Elongation of Arabidopsis Seedlings Grown in Petri Dishes

To tolerate phosphate (Pi) deficiency in the environment, plants alter their developmental and metabolic programs. In the past two decades, researchers have extensively used Petri dish-grown seedlings of the model plant Arabidopsis thaliana to study the molecular mechanisms underlying root developmental responses to Pi deficiency. A typical developmental response of the Petri dish-grown Arabidopsis seedlings to Pi deficiency is the inhibited growth of primary root (PR). This response is generally thought to enhance the production of lateral roots and root hairs, which increases the plant's ability to obtain Pi and is therefore regarded as an active cellular response. Here, we report that direct illumination of root surface with blue light is critical and sufficient for Pi deficiency-induced inhibition of PR growth in Arabidopsis seedlings. We further show that a blue light-triggered malate-mediated photo-Fenton reaction and a canonical Fenton reaction form an Fe redox cycle in the root apoplast. This Fe redox cycle results in the production of hydroxyl radicals that inhibit PR growth. In addition to revealing the molecular mechanism underlying Pi deficiency-induced inhibition of PR growth, our work demonstrated that this developmental change is not an active cellular response; instead, it is a phenotype resulting from root growth in transparent Petri dishes. This finding is significant because illuminated, transparent Petri dishes have been routinely used to study Arabidopsis root responses to environmental changes.

Novel Miscanthus genotypes selected for different drought tolerance phenotypes show enhanced tolerance across combinations of salinity and drought treatments

BACKGROUND AND AIMS

Water deficit and salinity stresses are often experienced by plants concurrently; however, knowledge is limited about the effects of combined salinity and water deficit stress in plants, and especially in C4 bioenergy crops. Here we aim to understand how diverse drought tolerance traits may deliver tolerance to combinations of drought and salinity in C4 crops, and identify key traits that influence the productivity and biomass composition of novel Miscanthus genotypes under such conditions.

METHODS

Novel genotypes used included M. sinensis and M. floridulus species, pre-screened for different drought responses, plus the commercial accession Miscanthus × giganteus (M×g.). Plants were grown under control treatments, single stress or combinations of water deficit and moderate salinity stress. Morphophysiological responses, including growth, yield, gas exchange and leaf water relations and contents of proline, soluble sugars, ash and lignin were tested for significant genotypic and treatment effects.

KEY RESULTS

The results indicated that plants subjected to combined stresses showed more severe responses compared with single stresses. All novel drought-tolerant genotypes and M×g. were tolerant to moderate salinity stress. Biomass production in M. sinensis genotypes was more resilient to co-occurring stresses than that in M×g. and M. floridulus, which, despite the yield penalty produced more biomass overall. A stay-green M. sinensis genotype adopted a conservative growth strategy with few significant treatment effects. Proline biosynthesis was species-specific and was triggered by salinity and co-occurring stress treatments, mainly in M. floridulus. The ash content was compartmentalized differently in leaves and stems in the novel genotypes, indicating different mechanisms of ion accumulation.

CONCLUSIONS

This study highlights the potential to select novel drought-tolerant Miscanthus genotypes that are resilient to combinations of stress and is expected to contribute to a deeper fundamental knowledge of different mechanistic responses identified for further exploitation in developing resilient Miscanthus crops.

Isolation and characterization of an atypical LEA gene (IpLEA) from Ipomoea pes-caprae conferring salt/drought and oxidative stress tolerance

Late embryogenesis abundant (LEA) proteins belong to a large family that exists widely in plants and is mainly involved in desiccation processes during plant development or in the response to abiotic stresses. Here, we reported on an atypical LEA gene (IpLEA) related to salt tolerance from Ipomoea pes-caprae L. (Convolvulaceae). Sequence analysis revealed that IpLEA belongs to the LEA_2 (PF03168) group. IpLEA was shown to have a cytoplasmic localization pattern. Quantitative reverse transcription PCR analysis showed that IpLEA was widely expressed in different organs of the I. pes-caprae plants, and the expression levels increased following salt, osmotic, oxidative, freezing, and abscisic acid treatments. Analysis of the 1,495 bp promoter of IpLEA identified distinct cis-acting regulatory elements involved in abiotic stress. Induction of IpLEA improved Escherichia coli growth performance compared with the control under abiotic stresses. To further assess the function of IpLEA in plants, transgenic Arabidopsis plants overexpressing IpLEA were generated. The IpLEA-overexpressing Arabidopsis seedlings and adult plants showed higher tolerance to salt and drought stress than the wild-type. The transgenic plants also showed higher oxidative stress tolerance than the wild-type Arabidopsis. Furthermore, the expression patterns of a series of stress-responsive genes were affected. The results indicate that IpLEA is involved in the plant response to salt and drought, probably by mediating water homeostasis or by acting as a reactive oxygen species scavenger, thereby influencing physiological processes under various abiotic stresses in microorganisms and plants.

Elucidating Drought Stress Tolerance in European Oaks Through Cross-Species Transcriptomics

The impact of climate change that comes with a dramatic increase of long periods of extreme summer drought associated with heat is a fundamental challenge for European forests. As a result, forests are expected to shift their distribution patterns toward north-east, which may lead to a dramatic loss in value of European forest land. Consequently, unraveling key processes that underlie drought stress tolerance is not only of great scientific but also of utmost economic importance for forests to withstand future heat and drought wave scenarios. To reveal drought stress-related molecular patterns we applied cross-species comparative transcriptomics of three major European oak species: the less tolerant deciduous pedunculate oak (Quercus robur), the deciduous but quite tolerant pubescent oak (Q. pubescens), and the very tolerant evergreen holm oak (Q. ilex). We found 415, 79, and 222 differentially expressed genes during drought stress in Q. robur, Q. pubescens, and Q. ilex, respectively, indicating species-specific response mechanisms. Further, by comparative orthologous gene family analysis, 517 orthologous genes could be characterized that may play an important role in drought stress adaptation on the genus level. New regulatory candidate pathways and genes in the context of drought stress response were identified, highlighting the importance of the antioxidant capacity, the mitochondrial respiration machinery, the lignification of the water transport system, and the suppression of drought-induced senescence - providing a valuable knowledge base that could be integrated in breeding programs in the face of climate change.

Developmental Responses to Water and Salinity in Root Systems

Roots provide the primary mechanism that plants use to absorb water and nutrients from their environment. These functions are dependent on developmental mechanisms that direct root growth and branching into regions of soil where these resources are relatively abundant. Water is the most limiting factor for plant growth, and its availability is determined by the weather, soil structure, and salinity. In this review, we define the developmental pathways that regulate the direction of growth and branching pattern of the root system, which together determine the expanse of soil from which a plant can access water. The ability of plants to regulate development in response to the spatial distribution of water is a focus of many recent studies and provides a model for understanding how biological systems utilize positional cues to affect signaling and morphogenesis. A better understanding of these processes will inform approaches to improve crop water use efficiency to more sustainably feed a growing population.

Root Plasticity in the Pursuit of Water

One of the greatest challenges of terrestrial vegetation is to acquire water through soil-grown roots. Owing to the scarcity of high-quality water in the soil and the environment's spatial heterogeneity and temporal variability, ranging from extreme flooding to drought, roots have evolutionarily acquired tremendous plasticity regarding their geometric arrangement of individual roots and their three-dimensional organization within the soil. Water deficiency has also become an increasing threat to agriculture and dryland ecosystems due to climate change. As a result, roots have become important targets for genetic selection and modification in an effort to improve crop resilience under water-limiting conditions. This review addresses root plasticity from different angles: Their structures and geometry in response to the environment, potential genetic control of root traits suitable for water-limiting conditions, and contemporary and future studies of the principles underlying root plasticity post-Darwin's 'root-brain' hypothesis. Our increasing knowledge of different disciplines of plant sciences and agriculture should contribute to a sustainable management of natural and agricultural ecosystems for the future of mankind.

Genome-Wide Analysis of TCP Family Genes in Zea mays L. Identified a Role for ZmTCP42 in Drought Tolerance

The Teosinte-branched 1/Cycloidea/Proliferating (TCP) plant-specific transcription factors (TFs) have been demonstrated to play a fundamental role in plant development and organ patterning. However, it remains unknown whether or not the TCP gene family plays a role in conferring a tolerance to drought stress in maize, which is a major constraint to maize production. In this study, we identified 46 ZmTCP genes in the maize genome and systematically analyzed their phylogenetic relationships and synteny with rice, sorghum, and ArabidopsisTCP genes. Expression analysis of the 46 ZmTCP genes in different tissues and under drought conditions, suggests their involvement in maize response to drought stress. Importantly, genetic variations in ZmTCP32 and ZmTCP42 are significantly associated with drought tolerance at the seedling stage. RT-qPCR results suggest that ZmTCP32 and ZmTCP42 RNA levels are both induced by ABA, drought, and polyethylene glycol treatments. Based on the significant association between the genetic variation of ZmTCP42 and drought tolerance, and the inducible expression of ZmTCP42 by drought stress, we selected ZmTCP42, to investigate its function in drought response. We found that overexpression of ZmTCP42 in Arabidopsis led to a hypersensitivity to ABA in seed germination and enhanced drought tolerance, validating its function in drought tolerance. These results suggested that ZmTCP42 functions as an important TCP TF in maize, which plays a positive role in drought tolerance.

Osmotic Stress Activates Two Reactive Oxygen Species Pathways with Distinct Effects on Protein Nanodomains and Diffusion

Physiological acclimation of plants to an everchanging environment is governed by complex combinatorial signaling networks that perceive and transduce various abiotic and biotic stimuli. Reactive oxygen species (ROS) serve as one of the second messengers in plant responses to hyperosmotic stress. The molecular bases of ROS production and the primary cellular processes that they target were investigated in the Arabidopsis (Arabidopsis thaliana) root. Combined pharmacological and genetic approaches showed that the RESPIRATORY BURST OXIDASE HOMOLOG (RBOH) pathway and an additional pathway involving apoplastic ascorbate and iron can account for ROS production upon hyperosmotic stimulation. The two pathways determine synergistically the rate of membrane internalization, within minutes after activation. Live superresolution microscopy revealed at single-molecule scale how ROS control specific diffusion and nano-organization of membrane cargo proteins. In particular, ROS generated by RBOHs initiated clustering of the PLASMA MEMBRANE INTRINSIC PROTEIN2;1 aquaporin and its removal from the plasma membrane. This process is contributed to by clathrin-mediated endocytosis, with a positive role of RBOH-dependent ROS, specifically under hyperosmotic stress.

Brassinosteroid signaling in plant development and adaptation to stress

Brassinosteroids (BRs) are steroid hormones that are essential for plant growth and development. These hormones control the division, elongation and differentiation of various cell types throughout the entire plant life cycle. Our current understanding of the BR signaling pathway has mostly been obtained from studies using Arabidopsis thaliana as a model. In this context, the membrane steroid receptor BRI1 (BRASSINOSTEROID INSENSITIVE 1) binds directly to the BR ligand, triggering a signal cascade in the cytoplasm that leads to the transcription of BR-responsive genes that drive cellular growth. However, recent studies of the primary root have revealed distinct BR signaling pathways in different cell types and have highlighted cell-specific roles for BR signaling in controlling adaptation to stress. In this Review, we summarize our current knowledge of the spatiotemporal control of BR action in plant growth and development, focusing on BR functions in primary root development and growth, in stem cell self-renewal and death, and in plant adaption to environmental stress.

Summary: This Review summarizes current knowledge of the spatiotemporal control of brassinosteroid function in plants, focusing on primary root development and growth, stem cell self-renewal and death, and adaptation to environmental stress.

Cytokinin action in response to abiotic and biotic stresses in plants

The phytohormone cytokinin was originally discovered as a regulator of cell division. Later, it was described to be involved in regulating numerous processes in plant growth and development including meristem activity, tissue patterning, and organ size. More recently, diverse functions for cytokinin in the response to abiotic and biotic stresses have been reported. Cytokinin is required for the defence against high light stress and to protect plants from a novel type of abiotic stress caused by an altered photoperiod. Additionally, cytokinin has a role in the response to temperature, drought, osmotic, salt, and nutrient stress. Similarly, the full response to certain plant pathogens and herbivores requires a functional cytokinin signalling pathway. Conversely, different types of stress impact cytokinin homeostasis. The diverse functions of cytokinin in responses to stress and crosstalk with other hormones are described. Its emerging roles as a priming agent and as a regulator of growth-defence trade-offs are discussed.

The battle of two ions: Ca2+ signalling against Na+ stress

Soil salinity adversely affects plant growth, crop yield and the composition of ecosystems. Salinity stress impacts plants by combined effects of Na⁺ toxicity and osmotic perturbation. Plants have evolved elaborate mechanisms to counteract the detrimental consequences of salinity. Here we reflect on recent advances in our understanding of plant salt tolerance mechanisms. We discuss the embedding of the salt tolerance-mediating SOS pathway in plant hormonal and developmental adaptation. Moreover, we review newly accumulating evidence indicating a crucial role of a transpiration-dependent salinity tolerance pathway, that is centred around the function of the NADPH oxidase RBOHF and its role in endodermal and Casparian strip differentiation. Together, these data suggest a unifying and coordinating role for Ca²⁺ signalling in combating salinity stress at the cellular and organismal level.

PYL8 mediates ABA perception in the root through non-cell-autonomous and ligand-stabilization-based mechanisms

The phytohormone abscisic acid (ABA) plays a key role regulating root growth, root system architecture, and root adaptive responses, such as hydrotropism. The molecular and cellular mechanisms that regulate the action of core ABA signaling components in roots are not fully understood. ABA is perceived through receptors from the PYR/PYL/RCAR family and PP2C coreceptors. PYL8/RCAR3 plays a nonredundant role in regulating primary and lateral root growth. Here we demonstrate that ABA specifically stabilizes PYL8 compared with other ABA receptors and induces accumulation of PYL8 in root nuclei. This requires ABA perception by PYL8 and leads to diminished ubiquitination of PYL8 in roots. The ABA agonist quinabactin, which promotes root ABA signaling through dimeric receptors, fails to stabilize the monomeric receptor PYL8. Moreover, a PYL8 mutant unable to bind ABA and inhibit PP2C is not stabilized by the ligand, whereas a PYL8^5KR mutant is more stable than PYL8 at endogenous ABA concentrations. The PYL8 transcript was detected in the epidermis and stele of the root meristem; however, the PYL8 protein was also detected in adjacent tissues. Expression of PYL8 driven by tissue-specific promoters revealed movement to adjacent tissues. Hence both inter- and intracellular trafficking of PYL8 appears to occur in the root apical meristem. Our findings reveal a non-cell-autonomous mechanism for hormone receptors and help explain the nonredundant role of PYL8-mediated root ABA signaling.

Biochemical responses of the desiccation-tolerant resurrection fern Pleopeltis polypodioides to dehydration and rehydration

The epiphytic fern Pleopeltis polypodioides can tolerate repeated drying and rehydration events without conspicuous damage. To understand the biochemical principles of drought-tolerance, we analyzed the effect of dehydration and rehydration at 25 °C on hydroperoxide and lipid hydroperoxide, the activities of antioxidative (catalase and glutathione-oxidizing) enzymes and evaluated changes in fatty acid composition and saturation levels. Dehydration increased peroxide concentration and the activity of glutathione oxidases, but reduced catalase activity. During dehydration, the biosynthesis of palmitic (C16:0), linoleic (C18:2), linolenic (C18:3) and stearic acid (C18:0) increased 18, 12, 20, and 8-fold, respectively. In contrast, rehydration lowered levels of peroxides, the activity of glutathione-oxidizing enzymes, and fatty acids but increased catalase activity. The coordinated changes during de- and rehydration suggest that lipids and oxidative and antioxidative enzymes are components of the drought-resistance system.

Q&A: How do gene regulatory networks control environmental responses in plants?

A gene regulatory network (GRN) describes the hierarchical relationship between transcription factors, associated proteins, and their target genes. Studying GRNs allows us to understand how a plant's genotype and environment are integrated to regulate downstream physiological responses. Current efforts in plants have focused on defining the GRNs that regulate functions such as development and stress response and have been performed primarily in genetically tractable model plant species such as Arabidopsis thaliana. Future studies will likely focus on how GRNs function in non-model plants and change over evolutionary time to allow for adaptation to extreme environments. This broader understanding will inform efforts to engineer GRNs to create tailored crop traits.

Root cell wall solutions for crop plants in saline soils

The root growth of most crop plants is inhibited by soil salinity. Roots respond by modulating metabolism, gene expression and protein activity, which results in changes in cell wall composition, transport processes, cell size and shape, and root architecture. Here, we focus on the effects of salt stress on cell wall modifying enzymes, cellulose microfibril orientation and non-cellulosic polysaccharide deposition in root elongation zones, as important determinants of inhibition of root elongation, and highlight cell wall changes linked to tolerance to salt stressed and water limited roots. Salt stress induces changes in the wall composition of specific root cell types, including the increased deposition of lignin and suberin in endodermal and exodermal cells. These changes can benefit the plant by preventing water loss and altering ion transport pathways. We suggest that binding of Na⁺ ions to cell wall components might influence the passage of Na⁺ and that Na⁺ can influence the binding of other ions and hinder the function of pectin during cell growth. Naturally occurring differences in cell wall structure may provide new resources for breeding crops that are more salt tolerant.

Pressure-volume (P-V) curves in Atriplex nummularia Lindl. for evaluation of osmotic adjustment and water status under saline conditions

The survival of Atriplex nummularia plants in saline environments is possible mainly due to the presence of salt-accumulating epidermal vesicles. Commonly, destructive methods, such as plant material maceration and subsequent reading in osmometers, are employed in studies on water relations and osmotic adjustment and are inconvenient due to their underestimation of the total water potential inside the cells, which can cause overestimation of an osmotic adjustment that is not present. As a result, methods that preserve leaf structure, such as pressure-volume (P-V) curves, which take into consideration only the salts that compose the symplastic solution, are more adequate. Thus, the main objectives of this study were to evaluate the effect of determination methods of osmotic potential (Ψ_o) in Atriplex nummularia through destructive and leaf structure-preserving techniques and to determine the water relations of the species under increasing NaCl concentrations. Plants were subjected to daily irrigations, maintaining soil moisture at 80% of field capacity, with solutions of increasing NaCl concentration (0, 0.05, 0.1, 0.2, 0.25 and 0.3 M) for 84 days. Water potential, osmotic potential and osmotic adjustment were determined. In addition, P-V curves were constructed using pressure chambers. Water and osmotic potentials decreased linearly with increasing NaCl concentration in the irrigation solution. The main discrepancies observed were related to the osmotic adjustments determined through maceration and P-V curves. Based on the present research, it was possible to conclude that in studies with species that have salt-accumulating vesicles in the epidermis, such as the plants in the genus Atriplex, constructing P-V curves is more adequate than destructive methods.

Growth is required for perception of water availability to pattern root branches in plants

Plant roots activate lateral branching in response to contact with available water, but the mechanism by which this environmental signal is perceived is poorly understood. Through a combination of empirical and mathematical-modeling approaches we discovered a central role of tissue growth in this process. Growth causes water uptake, and the biophysical changes that occur during this process are interpreted by the organism to position new lateral branches. This observation is a significant advancement in our understanding of how the environment shapes plant development and demonstrates that perception of water is intimately tied to a core biological function of the root.

Water availability is a potent regulator of plant development and induces root branching through a process termed hydropatterning. Hydropatterning enables roots to position lateral branches toward regions of high water availability, such as wet soil or agar media, while preventing their emergence where water is less available, such as in air. The mechanism by which roots perceive the spatial distribution of water during hydropatterning is unknown. Using primary roots of Zea mays (maize) we reveal that developmental competence for hydropatterning is limited to the growth zone of the root tip. Past work has shown that growth generates gradients in water potential across an organ when asymmetries exist in the distribution of available water. Using mathematical modeling, we predict that substantial growth-sustained water potential gradients are also generated in the hydropatterning competent zone and that such biophysical cues inform the patterning of lateral roots. Using diverse chemical and environmental treatments we experimentally demonstrate that growth is necessary for normal hydropatterning of lateral roots. Transcriptomic characterization of the local response of tissues to a moist surface or air revealed extensive regulation of signaling and physiological pathways, some of which we show are growth-dependent. Our work supports a “sense-by-growth” mechanism governing hydropatterning, by which water availability cues are rendered interpretable through growth-sustained water movement.

Synergy between root hydrotropic response and root biomass in maize (Zea mays L.) enhances drought avoidance

Roots of higher plants change their growth direction in response to moisture, avoiding drought and gaining maximum advantage for development. This response is termed hydrotropism. There have been few studies of root hydrotropism in grasses, particularly in maize. Our goal was to test whether an enhanced hydrotropic response of maize roots correlates with a better adaptation to drought and partial/lateral irrigation in field studies. We developed a laboratory bioassay for testing hydrotropic response in primary roots of 47 maize elite DTMA (Drought Tolerant Maize for Africa) hybrids. After phenotyping these hybrids in the laboratory, selected lines were tested in the field. Three robust and three weak hybrids were evaluated employing three irrigation procedures: normal irrigation, partial lateral irrigation and drought. Hybrids with a robust hydrotropic response showed growth and developmental patterns, under drought and partial lateral irrigation, that differed from weak hydrotropic responders. A correlation between root crown biomass and grain yield in hybrids with robust hydrotropic response was detected. Hybrids with robust hydrotropic response showed earlier female flowering whereas several root system traits, such as projected root area, median width, maximum width, skeleton width, skeleton nodes, average tip diameter, rooting depth skeleton, thinner aboveground crown roots, as well as stem diameter, were considerably higher than in weak hydrotropic responders in the three irrigation procedures utilized. These results demonstrate the benefit of intensive phenotyping of hydrotropism in primary roots since maize plants that display a robust hydrotropic response grew better under drought and partial lateral irrigation, indicating that a selection for robust hydrotropism might be a promising breeding strategy to improve drought avoidance in maize.

A Benzimidazole Proton Pump Inhibitor Increases Growth and Tolerance to Salt Stress in Tomato

Pre-treatment of tomato plants with micromolar concentrations of omeprazole (OP), a benzimidazole proton pump inhibitor in mammalian systems, improves plant growth in terms of fresh weight of shoot and roots by 49 and 55% and dry weight by 54 and 105% under salt stress conditions (200 mM NaCl), respectively. Assessment of gas exchange, ion distribution, and gene expression profile in different organs strongly indicates that OP interferes with key components of the stress adaptation machinery, including hormonal control of root development (improving length and branching), protection of the photosynthetic system (improving quantum yield of photosystem II) and regulation of ion homeostasis (improving the K+:Na+ ratio in leaves and roots). To our knowledge OP is one of the few known molecules that at micromolar concentrations manifests a dual function as growth enhancer and salt stress protectant. Therefore, OP can be used as new inducer of stress tolerance to better understand molecular and physiological stress adaptation paths in plants and to design new products to improve crop performance under suboptimal growth conditions. Highlight: Omeprazole enhances growth of tomato and increases tolerance to salinity stress through alterations of gene expression and ion uptake and transport.

A Benzimidazole Proton Pump Inhibitor Increases Growth and Tolerance to Salt Stress in Tomato

Pre-treatment of tomato plants with micromolar concentrations of omeprazole (OP), a benzimidazole proton pump inhibitor in mammalian systems, improves plant growth in terms of fresh weight of shoot and roots by 49 and 55% and dry weight by 54 and 105% under salt stress conditions (200 mM NaCl), respectively. Assessment of gas exchange, ion distribution, and gene expression profile in different organs strongly indicates that OP interferes with key components of the stress adaptation machinery, including hormonal control of root development (improving length and branching), protection of the photosynthetic system (improving quantum yield of photosystem II) and regulation of ion homeostasis (improving the K⁺:Na⁺ ratio in leaves and roots). To our knowledge OP is one of the few known molecules that at micromolar concentrations manifests a dual function as growth enhancer and salt stress protectant. Therefore, OP can be used as new inducer of stress tolerance to better understand molecular and physiological stress adaptation paths in plants and to design new products to improve crop performance under suboptimal growth conditions. Highlight: Omeprazole enhances growth of tomato and increases tolerance to salinity stress through alterations of gene expression and ion uptake and transport.

Protein Phosphatase 2Cs and Microtubule-Associated Stress Protein 1 Control Microtubule Stability, Plant Growth, and Drought Response

Plant growth is coordinated with environmental factors, including water availability during times of drought. Microtubules influence cell expansion; however, the mechanisms by which environmental signals impinge upon microtubule organization and whether microtubule-related factors limit growth during drought remains unclear. We found that three Clade E Growth-Regulating (EGR) Type 2C protein phosphatases act as negative growth regulators to restrain growth during drought. Quantitative phosphoproteomics indicated that EGRs target cytoskeleton and plasma membrane-associated proteins. Of these, Microtubule-Associated Stress Protein 1 (MASP1), an uncharacterized protein, increased in abundance during stress treatment and could bind, bundle, and stabilize microtubules in vitro. MASP1 overexpression enhanced growth, in vivo microtubule stability, and recovery of microtubule organization during drought acclimation. These MASP1 functions in vivo were dependent on phosphorylation of a single serine. For all EGR and MASP1 mutants and transgenic lines examined, enhanced microtubule recovery and stability were associated with increased growth during drought stress. The EGR-MASP1 system selectively regulates microtubule recovery and stability to adjust plant growth and cell expansion in response to changing environmental conditions. Modification of EGR-MASP1 signaling may be useful to circumvent negative growth regulation limiting plant productivity. EGRs are likely to regulate additional proteins involved in microtubule stability and stress signaling.

Epigenetics and RNA Processing: Connections to Drought, Salt, and ABA?

There have been great research advances in epigenetics, RNA splicing, and mRNA processing over recent years. In parallel, there have been many advances in abiotic stress and Abscisic Acid (ABA) signaling. Here we overview studies that have examined stress-induced changes in the epigenome and RNA processing as well as cases where disrupting these processes changes the plant response to abiotic stress. We also highlight some examples where specific connections of stress or ABA signaling to epigenetics or RNA processing have been found. By implication, this also points out cases where such mechanistic connections are likely to exist but are yet to be characterized. In the absence of such specific connections to stress signaling, it should be kept in mind that stress sensitivity phenotypes of some epigenetic or RNA processing mutants maybe the result of indirect, pleiotropic effects and thus may perhaps not indicate a direct function in stress acclimation.

Brassinosteroid Mediated Cell Wall Remodeling in Grasses under Abiotic Stress

Unlike animals, plants, being sessile, cannot escape from exposure to severe abiotic stresses such as extreme temperature and water deficit. The dynamic structure of plant cell wall enables them to undergo compensatory changes, as well as maintain physical strength, with changing environments. Plant hormones known as brassinosteroids (BRs) play a key role in determining cell wall expansion during stress responses. Cell wall deposition differs between grasses (Poaceae) and dicots. Grass species include many important food, fiber, and biofuel crops. In this article, we focus on recent advances in BR-regulated cell wall biosynthesis and remodeling in response to stresses, comparing our understanding of the mechanisms in grass species with those in the more studied dicots. A more comprehensive understanding of BR-mediated changes in cell wall integrity in grass species will benefit the development of genetic tools to improve crop productivity, fiber quality and plant biomass recalcitrance.