Plant growth under suboptimal water conditions: early responses and methods to study them

Stomatal opening under high temperatures is controlled by the OST1-regulated TOT3-AHA1 module

Plants continuously respond to changing environmental conditions to prevent damage and maintain optimal performance. To regulate gas exchange with the environment and to control abiotic stress relief, plants have pores in their leaf epidermis, called stomata. Multiple environmental signals affect the opening and closing of these stomata. High temperatures promote stomatal opening (to cool down), and drought induces stomatal closing (to prevent water loss). Coinciding stress conditions may evoke conflicting stomatal responses, but the cellular mechanisms to resolve these conflicts are unknown. Here we demonstrate that the high-temperature-associated kinase TARGET OF TEMPERATURE 3 directly controls the activity of plasma membrane H+-ATPases to induce stomatal opening. OPEN STOMATA 1, which regulates stomatal closure to prevent water loss during drought stress, directly inactivates TARGET OF TEMPERATURE 3 through phosphorylation. Taken together, this signalling axis harmonizes stomatal opening and closing under high temperatures and/or drought. In the context of global climate change, understanding how different stress signals converge on stomatal regulation allows the development of climate-change-ready crops.

Diverse functional interactions between ABA and ethylene in plant development and responses to stress

Abscisic acid (ABA) and ethylene are two essential hormones that play crucial roles throughout the entire plant life cycle and in their tolerance to abiotic or biotic stress. In recent decades, increasing research has revealed that, in addition to their individual roles, these two hormones are more likely to function through their interactions, forming a complex regulatory network. More importantly, their functions change and their interactions vary from synergistic to antagonistic depending on the specific plant organ and development stage, which is less focused, compared and systematically summarized. In this review, we first introduce the general synthesis and action signaling pathways of these two plant hormones individually and their interactions in relation to seed dormancy and germination, primary root growth, shoot development, fruit ripening, leaf senescence and abscission, and stomatal movement regulation under both normal and stress conditions. A better understanding of the complex interactions between ABA and ethylene will enhance our knowledge of how plant hormones regulate development and respond to stress and may facilitate the development of crops with higher yields and greater tolerance to stressful environments through tissue-specific genetic modifications in the future.

A cytoplasmic osmosensing mechanism mediated by molecular crowding-sensitive DCP5

Plants are frequently challenged by osmotic stresses. How plant cells sense environmental osmolarity changes is not fully understood. We report that Arabidopsis Decapping 5 (DCP5) functions as a multifunctional cytoplasmic osmosensor that senses and responds to extracellular hyperosmolarity. DCP5 harbors a plant-specific intramolecular crowding sensor (ICS) that undergoes conformational change and drives phase separation in response to osmotically intensified molecular crowding. Upon hyperosmolarity exposure, DCP5 rapidly and reversibly assembles to DCP5-enriched osmotic stress granules (DOSGs), which sequestrate plenty of mRNA and regulatory proteins, and thus adaptively reprograms both the translatome and transcriptome to facilitate plant osmotic stress adaptation. Our findings uncover a cytoplasmic osmosensing mechanism mediated by DCP5 with plant-specific molecular crowding sensitivity and suggest a stress sensory function for hyperosmotically induced stress granules.

Silver nanoparticles enhance the mitigation of osmotic stress in Chenopodium quinoa microshoots grown under in vitro osmo-stressing conditions

Osmotic stress is one of the main destructive abiotic factors that hinder plant growth and development. In this research, the role of silver nanoparticles (Ag NPs) in mitigating the negative impact of osmotic stress on in vitro grown Chenopodium quinoa (Quinoa 6 Line; Q6) was investigated to determine whether Ag NPs were able to reduce the negative effects on the in vitro grown cultures of the Q6 line. The explants were subcultured onto a special osmostressing media containing sucrose, sorbitol, or mannitol at different levels (0.1, 0.2, 0.3, and 0.4 mol/L) to mimic the osmotic stressing environment for four weeks. Then, stress physiological responses of in vitro grown Q6 under the induced osmotic stress were investigated to determine the highest stress level that the microshoots could tolerate. Next, Ag NPs; 25, 50, and 75 mg/L were added to the medium that contained the highest stress level of the induced osmotic stress to determine if their addition improved the physiological performance of the Q6 microshoots under the most severe osmotic agent levels. The results revealed that 0.4 mol/L sucrose, 0.3 mol/L sorbitol, and 0.3 mol/L mannitol were the highest stress levels that the microshoots could tolerate. The addition of 75 mg/L Ag NPs to the previous highest stress levels resulted in a significant increase in the following: stem length (SL), leaves number (LN), fresh weight (FW), dry weight (DW), total chlorophyll, protein, calcium (Ca), and phosphorus (P) contents, while it caused a reduction in proline, sodium (Na) ions, and potassium (K) ions. These results indicate that the negative consequences of osmotic stress on Q6 quinoa microshoots could be mitigated by adding specific concentrations of Ag NPs to the culture medium.

Climate change does not impact the water flow of barley at the vegetative stage, ameliorates at anthesis and worsens after subsequent drought episodes

Climate change will bring the interaction of stresses such as increased temperature and drought under high [CO2] conditions. This is likely to impact on crop growth and productivity. This study aimed to (i) determine the response of barley water relations to vegetative and anthesis drought periods under triple interaction conditions, (ii) test the possibility to prime barley plants for drought, and (iii) analyse the involvement of aquaporins in (i) and (ii). The water status of barley was not affected by drought at the vegetative stage, regardless of the environmental conditions. At the anthesis stage, when the water shortage period was more severe, barley plants growing under combined elevated CO2 and temperature conditions were able to maintain a better water status compared with plants grown under current conditions. Elevated CO2 and temperature conditions reduced the stomatal conductance and slowed down the plant water flow through a root-leaf hydraulic conductivity coordination. Leaf HvPIP2;1 and HvTIP1;1 aquaporins seemed to play a key role regulating barley's water flow, while leaf and root HvPIP2;5 provided basic level of water flow. At anthesis drought and under future combined conditions, plants showed a reduced cell dehydration and decrease in leaf relative water content compared with plants grown under current conditions. Exposure to a previous drought did not prime the water status of barley plants to a subsequent drought, but instead worsened the response under future conditions. This was due to an imbalance between the roots versus shoot development.

Arabidopsis transcriptome responses to low water potential using high-throughput plate assays

Soil-free assays that induce water stress are routinely used to investigate drought responses in the plant Arabidopsis thaliana. Due to their ease of use, the research community often relies on polyethylene glycol (PEG), mannitol, and salt (NaCl) treatments to reduce the water potential of agar media, and thus induce drought conditions in the laboratory. However, while these types of stress can create phenotypes that resemble those of water deficit experienced by soil-grown plants, it remains unclear how these treatments compare at the transcriptional level. Here, we demonstrate that these different methods of lowering water potential elicit both shared and distinct transcriptional responses in Arabidopsis shoot and root tissue. When we compared these transcriptional responses to those found in Arabidopsis roots subject to vermiculite drying, we discovered many genes induced by vermiculite drying were repressed by low water potential treatments on agar plates (and vice versa). Additionally, we also tested another method for lowering water potential of agar media. By increasing the nutrient content and tensile strength of agar, we show the 'hard agar' (HA) treatment can be leveraged as a high-throughput assay to investigate natural variation in Arabidopsis growth responses to low water potential.

Ecotoxicological effects of leachate from e-cigarettes and e-liquid on the performance of perennial ryegrass (Loliumperenne)

Once littered, disposable e-cigarettes present a complex type of waste in the environment. They typically contain a lithium battery, electronics to produce vapour and remnant e-liquid, all of which could leach into the environment. The effects of littered e-cigarettes are not well understood, and they have not been tested in terrestrial ecosystems. To address this, an experiment was set up to assess how leachate from e-cigarettes with or without a battery, but also e-liquid on its own can alter fundamental physical characteristics of Lolium perenne (perennial ryegrass) when irrigated with contaminated water. After 31 days, shoot length of L. perenne was not measurably affected, but the biomass was significantly reduced by 30% when e-liquid, and 24% when leachate from intact e-cigarettes was present compared to control plants. Plants grown with leachate or e-liquid displayed a significant level of early senescence of leaf apices, and the chlorophyll content was increased. Furthermore, root biomass was significantly less (29-46%) compared to the control. Leachate from used disposable e-cigarettes can affect the performance of plants when entering the soil ecosystem, therefore stricter regulations are needed to prevent this new type of electronic litter from becoming more widespread.

Modulation of early gene expression responses to water deprivation stress by the E3 ubiquitin ligase ATL80: implications for retrograde signaling interplay

BACKGROUND

Primary response genes play a pivotal role in translating short-lived stress signals into sustained adaptive responses. In this study, we investigated the involvement of ATL80, an E3 ubiquitin ligase, in the dynamics of gene expression following water deprivation stress. We observed that ATL80 is rapidly activated within minutes of water deprivation stress perception, reaching peak expression around 60 min before gradually declining. ATL80, despite its post-translational regulation role, emerged as a key player in modulating early gene expression responses to water deprivation stress.

RESULTS

The impact of ATL80 on gene expression was assessed using a time-course microarray analysis (0, 15, 30, 60, and 120 min), revealing a burst of differentially expressed genes, many of which were associated with various stress responses. In addition, the diversity of early modulation of gene expression in response to water deprivation stress was significantly abolished in the atl80 mutant compared to wild-type plants. A subset of 73 genes that exhibited a similar expression pattern to ATL80 was identified. Among them, several are linked to stress responses, including ERF/AP2 and WRKY transcription factors, calcium signaling genes, MAP kinases, and signaling peptides. Promoter analysis predicts enrichment of binding sites for CAMTA1 and CAMTA5, which are known regulators of rapid stress responses. Furthermore, we have identified a group of differentially expressed ERF/AP2 transcription factors, proteins associated with folding and refolding, as well as pinpointed core module genes which are known to play roles in retrograde signaling pathways that cross-referenced with the early ATL80 transcriptome.

CONCLUSIONS

Based on these findings, we propose that ATL80 may target one or more components within the retrograde signaling pathways for degradation. In essence, ATL80 serves as a bridge connecting these signaling pathways and effectively functions as an alarm signal.

Genome-wide analysis of FRF gene family and functional identification of HvFRF9 under drought stress in barley

FHY3 and its homologous protein FAR1 are the founding members of FRS family. They exhibited diverse and powerful physiological functions during evolution, and participated in the response to multiple abiotic stresses. FRF genes are considered to be truncated FRS family proteins. They competed with FRS for DNA binding sites to regulate gene expression. However, only few studies are available on FRF genes in plants participating in the regulation of abiotic stress. With wide adaptability and high stress-resistance, barley is an excellent candidate for the identification of stress-resistance-related genes. In this study, 22 HvFRFs were detected in barley using bioinformatic analysis from whole genome. According to evolution and conserved motif analysis, the 22 HvFRFs could be divided into subfamilies I and II. Most promoters of subfamily I members contained abscisic acid and methyl jasmonate response elements; however, a large number promoters of subfamily II contained gibberellin and salicylic acid response elements. HvFRF9, one of the members of subfamily II, exhibited a expression advantage in different tissues, and it was most significantly upregulated under drought stress. In-situ PCR revealed that HvFRF9 is mainly expressed in the root epidermal cells, as well as xylem and phloem of roots and leaves, indicating that HvFRF9 may be related to absorption and transportation of water and nutrients. The results of subcellular localization indicated that HvFRF9 was mainly expressed in the nuclei of tobacco epidermal cells and protoplast of arabidopsis. Further, transgenic arabidopsis plants with HvFRF9 overexpression were generated to verify the role of HvFRF9 in drought resistance. Under drought stress, leaf chlorosis and wilting, MDA and O2 - contents were significantly lower, meanwhile, fresh weight, root length, PRO content, and SOD, CAT and POD activities were significantly higher in HvFRF9-overexpressing arabidopsis plants than in wild-type plants. Therefore, overexpression of HvFRF9 could significantly enhance the drought resistance in arabidopsis. These results suggested that HvFRF9 may play a key role in drought resistance in barley by increasing the absorption and transportation of water and the activity of antioxidant enzymes. This study provided a theoretical basis for drought resistance in barley and provided new genes for drought resistance breeding.

Transcriptomic analysis reveals the gene regulatory networks involved in leaf and root response to osmotic stress in tomato

Introduction

Tomato (Solanum lycopersicum L.) is a major horticultural crop that is cultivated worldwide and is characteristic of the Mediterranean agricultural system. It represents a key component of the diet of billion people and an important source of vitamins and carotenoids. Tomato cultivation in open field often experiences drought episodes, leading to severe yield losses, since most modern cultivars are sensitive to water deficit. Water stress leads to changes in the expression of stress-responsive genes in different plant tissues, and transcriptomics can support the identification of genes and pathways regulating this response.

Methods

Here, we performed a transcriptomic analysis of two tomato genotypes, M82 and Tondo, in response to a PEG-mediated osmotic treatment. The analysis was conducted separately on leaves and roots to characterize the specific response of these two organs.

Results

A total of 6,267 differentially expressed transcripts related to stress response was detected. The construction of gene co-expression networks defined the molecular pathways of the common and specific responses of leaf and root. The common response was characterized by ABA-dependent and ABA-independent signaling pathways, and by the interconnection between ABA and JA signaling. The root-specific response concerned genes involved in cell wall metabolism and remodeling, whereas the leaf-specific response was principally related to leaf senescence and ethylene signaling. The transcription factors representing the hubs of these regulatory networks were identified. Some of them have not yet been characterized and can represent novel candidates for tolerance.

Discussion

This work shed new light on the regulatory networks occurring in tomato leaf and root under osmotic stress and set the base for an in-depth characterization of novel stress-related genes that may represent potential candidates for improving tolerance to abiotic stress in tomato.

Genome-wide characterization of the U-box gene in Camellia sinensis and functional analysis in transgenic tobacco under abiotic stresses

Plants U-box genes are crucial for plant survival, and they extensively regulate plant growth, reproduction and development as well as coping with stress and other processes. In this study, we identified 92 CsU-box genes through genome-wide analysis in the tea plant (Camellia sinensis), all of them contained the conserved U-box domain and were divided into 5 groups, which supported by the further genes structure analysis. The expression profiles in eight tea plant tissues and under abiotic and hormone stresses were analyzed using the TPIA database. 7 CsU-box genes (CsU-box27/28/39/46/63/70/91) were selected to verify and analyze expression patterns under PEG-induced drought and heat stress in tea plant respectively, the qRT-PCR results showed consistent with transcriptome datasets; and the CsU-box39 were further heterologous expressed in tobacco to perform gene function analysis. Phenotypic analyses of overexpression transgenic tobacco seedlings and physiological experiments revealed that CsU-box39 positively regulated the plant response to drought stress. These results lay a solid foundation for studying the biological function of CsU-box, and will provide breeding strategy basis for tea plant breeders.

The Bro1-like domain-containing protein, AtBro1, modulates growth and abiotic stress responses in Arabidopsis

Abscisic acid (ABA) affects plant physiology by altering gene expression, enabling plants to adapt to a wide range of environments. Plants have evolved protective mechanisms to allow seed germination in harsh conditions. Here, we explore a subset of these mechanisms involving the AtBro1 gene, which encodes one of a small family of poorly characterised Bro1-like domain-containing proteins, in Arabidopsis thaliana plants subjected to multiple abiotic stresses. AtBro1 transcripts were upregulated by salt, ABA and mannitol stress, while AtBro1-overexpression lines demonstrated robust tolerance to drought and salt stress. Furthermore, we found that ABA elicits stress-resistance responses in loss-of-function bro1-1 mutant plants and AtBro1 regulates drought resistance in Arabidopsis. When the AtBro1 promoter was fused to the β-glucuronidase (GUS) gene and introduced into plants, GUS was expressed mainly in rosette leaves and floral clusters, especially in anthers. Using a construct expressing an AtBro1-GFP fusion protein, AtBro1 was found to be localized in the plasma membrane in Arabidopsis protoplasts. A broad RNA-sequencing analysis revealed specific quantitative differences in the early transcriptional responses to ABA treatment between wild-type and loss-of-function bro1-1 mutant plants, suggesting that ABA stimulates stress-resistance responses via AtBro1. Additionally, transcripts levels of MOP9.5, MRD1, HEI10, and MIOX4 were altered in bro1-1 plants exposed to different stress conditions. Collectively, our results show that AtBro1 plays a significant role in the regulation of the plant transcriptional response to ABA and the induction of resistance responses to abiotic stress.

Studying root-environment interactions in structured microdevices

In negotiating with the environment, plant roots integrate sensory information over space and time, as the basis of decision making in roots under non-uniform conditions. The complexity and dynamic properties of soil across spatial and temporal scales pose a significant technical challenge for research on mechanisms that drive metabolism, growth and development in roots, as well as on inter-organismal networks in the rhizosphere. Synthetic environments, combining microscopic access and manipulation capabilities with soil-like heterogeneity, are needed to elucidate the intriguing tug-of-war that characterises subsurface ecosystems. Microdevices have provided opportunities for innovative approaches to observe, analyse and manipulate plant roots and advanced our understanding of their development, physiology and interactions with the environment. Initially conceived as perfusion platforms for root cultivation under hydroponic conditions, microdevice design has, in recent years, increasingly shifted to better reflect the complex growth conditions in soil. Heterogeneous micro-environments have been created through co-cultivation with microbes, laminar flow-based local stimulation and physical obstacles and constraints. As such, structured microdevices provide an experimental entry point to the complex network behaviour of soil communities.

Sunflower Bark Extract as a Biostimulant Suppresses Reactive Oxygen Species in Salt-Stressed Arabidopsis

A survey of plant-based wastes identified sunflower (Helianthus annuus) bark extract (SBE), produced via twin-screw extrusion, as a potential biostimulant. The addition of SBE to Arabidopsis (Arabidopsis thaliana) seedlings cultured in vitro showed a dose-dependent response, with high concentrations causing severe growth inhibition. However, when priming seeds with SBE, a small but significant increase in leaf area was observed at a dose of 0.5 g of lyophilized powder per liter. This optimal concentration of SBE in the culturing medium alleviated the growth inhibition caused by 100 mM NaCl. The recovery in shoot growth was accompanied by a pronounced increase in photosynthetic pigment levels and a stabilization of osmotic homeostasis. SBE-primed leaf discs also showed a similar protective effect. SBE mitigated salt stress by reducing the production of reactive oxygen species (ROS) (e.g., hydrogen peroxide) by about 30% and developing more expanded true leaves. This reduction in ROS levels was due to the presence of antioxidative agents in SBE and by activating ROS-eliminating enzymes. Polyphenols, carbohydrates, proteins, and other bioactive compounds detected in SBE may have contributed to the cellular redox homeostasis in salt-stressed plants, thus promoting early leaf development by relieving shoot apical meristem arrest. Sunflower stalks from which SBE is prepared can therefore potentially be valorized as a source to produce biostimulants for improving salt stress tolerance in crops.

Plants utilise ancient conserved peptide upstream open reading frames in stress-responsive translational regulation

The regulation of protein synthesis plays an important role in the growth and development of all organisms. Upstream open reading frames (uORFs) are commonly found in eukaryotic messenger RNA transcripts and typically attenuate the translation of associated downstream main ORFs (mORFs). Conserved peptide uORFs (CPuORFs) are a rare subset of uORFs, some of which have been shown to conditionally regulate translation by ribosome stalling. Here, we show that Arabidopsis CPuORF19, CPuORF46 and CPuORF47, which are ancient in origin, regulate translation of any downstream ORF, in response to the agriculturally significant environmental signals, heat stress and water limitation. Consequently, these CPuORFs represent a versatile toolkit for inducible gene expression with broad applications. Finally, we note that different classes of CPuORFs may operate during distinct phases of translation, which has implications for the bioengineering of these regulatory factors.

The phytohormone abscisic acid modulates protein carbonylation in Arabidopsis thaliana

Protein carbonylation is a post-translational modification associated with the reactive oxygen species. It results from the direct oxidation of the side chains of Lys, Arg, Pro, and Thr residues by hydroxyl radical HO^• or the addition of reactive carbonyl species including α,β-unsaturated aldehydes and oxylipins to the side chain of Cys, His, and Lys. Recent findings indicated that the phytohormone abscisic acid (ABA) induces the production of α,β-unsaturated aldehydes that modulate the effect of ABA on stomatal closure. This indicated that α,β-unsaturated aldehydes might mediate ABA signaling. In this study, we investigated the ABA-induced protein carbonylation events by profiling the carbonylated proteome extracted from Arabidopsis thaliana leaves after ABA treatment. The carbonylated proteins were enriched by affinity chromatography and subjected to liquid chromatography-tandem mass spectrometry. We identified 180 carbonylated proteins. Of these, 26 proteins became carbonylated upon ABA treatment, whereas 163 proteins that were carbonylated in untreated samples were no longer detected in the ABA-treated samples, which points to dynamic control of protein carbonylation by ABA in A. thaliana. A few regulatory stress-related proteins and enzymes involved in the biosynthesis of the aspartate family of amino acids were overrepresented in the list of proteins, which the carbonylation status changed between untreated and ABA-treated samples. These results indicated that ABA triggers a change in the pattern of protein carbonylation in A. thaliana. This change is independent of the commonly seen increased levels of carbonylated proteins in the plants subjected to deadly stress conditions.

Genome-wide identification of U-box genes and protein ubiquitination under PEG-induced drought stress in potato

Protein ubiquitination is one of the most important posttranslational modifications in eukaryotic cells, and it is involved in a variety of biological processes, including abiotic stress response. The ubiquitination modification is highly specific, which depends on the accurate recognition of substrate proteins by ubiquitin ligase. Plant U-box (PUB) proteins are a class of ubiquitin ligases, multiple members of which have shown to participate in water-deficit stress in Arabidopsis and rice. U-box gene family and large-scale profiling of the ubiquitome in potato has not been reported to date, although it is one of the most important food crops. The identified 66 U-box genes from the potato genome database were unevenly distributed on 10 chromosomes. These StPUBs have a large number of tandem repeat sequences. Analysis of gene expression characteristics revealed that many StPUBs responded to abiotic stress. Three hundred and fourteen lys modification sites were identified under PEG-induced drought stress, which were distributed on 200 proteins, with 25 differential ubiquitination modification sites, most of which were up-regulated. The ubiquitination modification in potato protein was enhanced under PEG-induced drought stress, and U-box ubiquitin ligase was involved. This study provides an overall strategy and rich data set to clarify the effects of ubiquitination on potatoes under PEG-induced drought stress and the ubiquitination modification involved in potato U-box genes in response to PEG-induced drought stress.

Increasing yield on dry fields: molecular pathways with growing potential

Drought stress constitutes one of the major constraints to agriculture all over the world, and its devastating effect is only expected to increase in the following years due to climate change. Concurrently, the increasing food demand in a steadily growing population requires a proportional increase in yield and crop production. In the past, research aimed to increase plant resilience to severe drought stress. However, this often resulted in stunted growth and reduced yield under favorable conditions or moderate drought. Nowadays, drought tolerance research aims to maintain plant growth and yield under drought conditions. Overall, recently deployed strategies to engineer drought tolerance in the lab can be classified into a 'growth-centered' strategy, which focuses on keeping growth unaffected by the drought stress, and a 'drought resilience without growth penalty' strategy, in which the main aim is still to boost drought resilience, while limiting the side effects on plant growth. In this review, we put the scope on these two strategies and some molecular players that were successfully engineered to generate drought-tolerant plants: abscisic acid, brassinosteroids, cytokinins, ethylene, ROS scavenging genes, strigolactones, and aquaporins. We discuss how these pathways participate in growth and stress response regulation under drought. Finally, we present an overview of the current insights and future perspectives in the development of new strategies to improve drought tolerance in the field.

Physiological and Morphological Responses of Okra (Abelmoschus esculentus L.) to Rhizoglomus irregulare Inoculation under Ample Water and Drought Stress Conditions Are Cultivar Dependent

Okra is an important crop species for smallholder farmers in many tropical and subtropical regions of the world. Its interaction with mycorrhiza has been rarely studied, and little is known about its mycorrhizal dependency, especially under drought stress. In a glasshouse experiment, we investigated the effect of Arbuscular Mycorrhiza Fungi (AMF) inoculation on growth, evapotranspiration, mineral nutrition and root morphology of five okra cultivars under ample water and drought stress conditions. ‘Khartoumia’, ‘HSD6719’, ‘HSD7058’, ‘Sarah’ and ‘Clemson Spineless’-cultivars commonly used by farmers in Sudan were chosen for their geographical, morphological and breeding background variations. The plants were either inoculated with R. irregulare or mock-inoculated. Seven weeks after seeding, the soil–water content was either maintained at 20% w/w or reduced to 10% w/w to impose drought stress. Drought stress resulted in plant P deficiency and decreased shoot dry biomass (DB), especially in HSD7058 and Clemson Spineless (69% and 56% decrease in shoot DB, in the respective cultivars). Plant inoculation with AMF greatly enhanced the shoot total content of P and the total DB in all treatments. The mycorrhizal dependency (MD)—the degree of total plant DB change associated with AM colonization—differed among the cultivars, irrespective of the irrigation treatment. Key determinants of MD were the root phenotype traits. Khartoumia (with the highest MD) had the lowest root DB, root-to-shoot ratio, and specific root length (SRL). Meanwhile, HSD6719 (with the lowest MD) had the highest respective root traits. Moreover, our data suggest a relationship between breeding background and MD. The improved cultivar Khartoumia showed the highest MD compared with the wild-type Sarah and the HSD7058 and HSD6719 landraces (higher MD by 46%, 17% and 32%, respectively). Interestingly, the drought-affected HSD7058 and Clemson Spineless exhibited higher MD (by 27% and 15%, respectively) under water-deficiency compared to ample water conditions. In conclusion, the mediation of drought stress in the okra plant species by AMF inoculation is cultivar dependent. The presence of AMF propagules in the field soil might be important for increasing yield production of high MD and drought susceptible cultivars, especially under drought/low P environments.

Microtubule organization changes severely after mannitol and n-butanol treatments inducing microspore embryogenesis in bread wheat

BACKGROUND

A mannitol stress treatment and a subsequent application of n-butanol, known as a microtubule-disrupting agent, enhance microspore embryogenesis (ME) induction and plant regeneration in bread wheat. To characterize changes in cortical (CMT) and endoplasmic (EMT) microtubules organization and dynamics, associated with ME induction treatments, immunocytochemistry studies complemented by confocal laser scanning microscopy (CLSM) were accomplished. This technique has allowed us to perform advanced 3- and 4D studies of MT architecture. The degree of MT fragmentation was examined by the relative fluorescence intensity quantification.

RESULTS

In uni-nucleated mannitol-treated microspores, severe CMT and EMT fragmentation occurs, although a complex network of short EMT bundles protected the nucleus. Additional treatment with n-butanol resulted in further depolymerization of both CMT and EMT, simultaneously with the formation of MT aggregates in the perinuclear region. Some aggregates resembled a preprophase band. In addition, a portion of the microspores progressed to the first mitotic division during the treatments. Bi-nucleate pollen-like structures showed a high MT depolymerization after mannitol treatment and numerous EMT bundles around the vegetative and generative nuclei after n-butanol. Interestingly, bi-nucleate symmetric structures showed prominent stabilization of EMT.

CONCLUSIONS

Fragmentation and stabilization of microtubules induced by mannitol- and n-butanol lead to new configurations essential for the induction of microspore embryogenesis in bread wheat. These results provide robust insight into MT dynamics during EM induction and open avenues to address newly targeted treatments to induce ME in recalcitrant species.

Tree Growth Conditions Are Demanded When Optimal, Are Unwanted When Limited, but When Are They Suboptimal?

The recent climate projections predict that the intensity and frequency of extreme events will increase as a result of overall increasing mean temperature and reduced precipitations in the temperate regions of the Northern Hemisphere. How these changes will influence the harshness of the environment and the performances of trees growing under natural conditions remains an open question. In this commentary article, we would like to look at the concept of suboptimal growth conditions, widening its application from the traditional in vitro manipulation to trees growing in open air, addressing the main limitations and strengths of the upscaling results from cell to tree. We believe that the traditional single dose–effect approach is not suitable to explain the complex interactions between genotype and environment, occurring in open field or forest stands, where the intensity and frequency of the events are uncontrolled and unpredictable. As forests provide a wide range of ecosystem services, new parameters should be considered in the definition of the response thresholds in addition to growth. Thus, within this Special Issue, we stimulate the discussion over the development of new approaches and technologies that are able to define suitable threshold responses of trees under suboptimal natural conditions, with the aim to furnish new insights on the acclimation and adaptation processes in woody species under global change.

Robotic Assay for Drought (RoAD): an automated phenotyping system for brassinosteroid and drought responses

Brassinosteroids (BRs) are a group of plant steroid hormones involved in regulating growth, development, and stress responses. Many components of the BR pathway have previously been identified and characterized. However, BR phenotyping experiments are typically performed in a low-throughput manner, such as on Petri plates. Additionally, the BR pathway affects drought responses, but drought experiments are time consuming and difficult to control. To mitigate these issues and increase throughput, we developed the Robotic Assay for Drought (RoAD) system to perform BR and drought response experiments in soil-grown Arabidopsis plants. RoAD is equipped with a robotic arm, a rover, a bench scale, a precisely controlled watering system, an RGB camera, and a laser profilometer. It performs daily weighing, watering, and imaging tasks and is capable of administering BR response assays by watering plants with Propiconazole (PCZ), a BR biosynthesis inhibitor. We developed image processing algorithms for both plant segmentation and phenotypic trait extraction to accurately measure traits including plant area, plant volume, leaf length, and leaf width. We then applied machine learning algorithms that utilize the extracted phenotypic parameters to identify image-derived traits that can distinguish control, drought-treated, and PCZ-treated plants. We carried out PCZ and drought experiments on a set of BR mutants and Arabidopsis accessions with altered BR responses. Finally, we extended the RoAD assays to perform BR response assays using PCZ in Zea mays (maize) plants. This study establishes an automated and non-invasive robotic imaging system as a tool to accurately measure morphological and growth-related traits of Arabidopsis and maize plants in 3D, providing insights into the BR-mediated control of plant growth and stress responses.

Rice aquaporin OsPIP2;2 is a water-transporting facilitator in relevance to drought-tolerant responses

In rice (Oryza sativa), the PLASMA MEMBRANE INTRINSIC PROTEIN (PIP) family of aquaporin has 11 members, OsPIP1;1 to OsPIP1;3, and OsPIP2;1 to OsPIP2;8, which are hypothesized to facilitate the transport of H2O and other small compounds across cell membranes. To date, however, only OsPIP1;2, OsPIP2;1, and OsPIP2;4 have been demonstrated for substrate selectivity in their source plant (rice). In this study, OsPIP2;2 was characterized as the most efficient facilitator of H2O transport across cell membranes in comparison with the other 10 OsPIPs. In concomitant tests of all OsPIPs, four genes (OsPIP1;3, OsPIP2;1, OsPIP2;2, and OsPIP2;4) were induced to express in leaves of rice plants following a physiological drought stress, while OsPIP2;2 was expressed to the highest level. After de novo expression in frog oocytes and yeast cells, the four OsPIP proteins were localized to the plasma membranes in trimer and tetramer and displayed the activity to increase the membrane permeability to H2O. In comparison, OsPIP2;2 was most supportive to H2O import to oocytes and yeast cells. After de novo expression in tobacco protoplasts, OsPIP2;2 exceeded OsPIP1;3, OsPIP2;1, and OsPIP2;4 to support H2O transport across the plasma membranes. OsPIP2;2-mediated H2O transport was accompanied by drought-tolerant responses, including increases in concentrations of proline and polyamines, both of which are physiological markers of drought tolerance. In rice protoplasts, H2O transport and drought-tolerant responses, which included expression of marker genes of drought tolerance pathway, were considerably enhanced by OsPIP2;2 overexpression but strongly inhibited by the gene silencing. Furthermore, OsPIP2;2 played a role in maintenance of the cell membrane integrity and effectively protected rice cells from electrolyte leakage caused by the physiological drought stress. These results suggest that OsPIP2;2 is a predominant facilitator of H2O transport in relevance to drought tolerance in the plant.

The impact of multifactorial stress combination on plant growth and survival

Climate change-driven extreme weather events, combined with increasing temperatures, harsh soil conditions, low water availability and quality, and the introduction of many man-made pollutants, pose a unique challenge to plants. Although our knowledge of the response of plants to each of these individual conditions is vast, we know very little about how a combination of many of these factors, occurring simultaneously, that is multifactorial stress combination, impacts plants. Seedlings of wild-type and different mutants of Arabidopsis thaliana plants were subjected to a multifactorial stress combination of six different stresses, each applied at a low level, and their survival, physiological and molecular responses determined. Our findings reveal that, while each of the different stresses, applied individually, had a negligible effect on plant growth and survival, the accumulated impact of multifactorial stress combination on plants was detrimental. We further show that the response of plants to multifactorial stress combination is unique and that specific pathways and processes play a critical role in the acclimation of plants to multifactorial stress combination. Taken together our findings reveal that further polluting our environment could result in higher complexities of multifactorial stress combinations that in turn could drive a critical decline in plant growth and survival.

Strategies to Apply Water-Deficit Stress: Similarities and Disparities at the Whole Plant Metabolism Level in Medicago truncatula

Water-deficit stresses such as drought and salinity are the most important factors limiting crop productivity. Hence, understanding the plant responses to these stresses is key for the improvement of their tolerance and yield. In this study M. truncatula plants were subjected to 250 mM NaCl as well as reduced irrigation (No-W) and 250 g/L polyethylene glycol (PEG)-6000 to induce salinity and drought stress, respectively, provoking a drop to −1.7 MPa in leaf water potential. The whole plant physiology and metabolism was explored by characterizing the stress responses at root, phloem sap and leaf organ level. PEG treatment led to some typical responses of plants to drought stress, but in addition to PEG uptake, an important impairment of nutrient uptake and a different regulation of carbon metabolism could be observed compared to No-W plants. No-W plants showed an important redistribution of antioxidants and assimilates to the root tissue, with a distinctive increase in root proline degradation and alkaline invertase activity. On the contrary, salinity provoked an increase in leaf starch and isocitrate dehydrogenase activity, suggesting key roles in the plant response to this stress. Overall, results suggest higher protection of salt-stressed shoots and non-irrigated roots through different mechanisms, including the regulation of proline and carbon metabolism, while discarding PEG as safe mimicker of drought. This raises the need to understand the effect at the whole plant level of the different strategies employed to apply water-deficit stress.

Short-term drought triggers defence mechanisms faster than ABA accumulation in the epiphytic bromeliad Acanthostachys strobilacea

Epiphytic bromeliads might experience drought after a few hours without water, which is especially critical during early life stages. Consequently, juvenile epiphytic bromeliads probably rely on short-term activation of drought tolerance strategies, although the biochemical processes involved are still poorly understood. In this study, we aimed to evaluate the short-term drought response of juvenile plants of the epiphytic bromeliad Acanthostachys strobilacea (Schult. & Schult. f.) Klotzsch. We hypothesized that short-term drought would induce the accumulation of abscisic acid (ABA) and secondary messengers such as reactive oxygen and nitrogen species (ROS and RNS, respectively) before the activation of defence mechanisms. Three-month-old plants were transferred from well-watered to dry substrates and stress markers were assessed at 0, 2, 5, 10, 24, 48, and 72 h. Drought caused a 27.3% decrease in relative water content compared to the well-watered control at 72 h. A nearly 5-fold increment in the ABA content occurred at 72 h of stress, which was about two days after the first detection of peaks in RNS levels and defence mechanisms activation. Indeed, ascorbate peroxidase (EC 1.11.1.11) activities and proline content increased after 10 h, whereas after 24 h a higher catalase (EC 1.11.1.6) activity and osmotic adjustment occurred. Oxidative stress markers and photochemical efficiency of photosystem II indicated no significant damage induced by drought. We concluded that defence mechanisms activation during early drought in juvenile A. strobilacea might be regulated initially by ABA-independent pathways and RNS, while ABA-induced responses are triggered at subsequent stages of stress.

Insight of transcriptional regulators reveals the tolerance mechanism of carpet-grass (Axonopus compressus) against drought

BACKGROUND

Carpet grass [Axonopus compressus (L.)] is an important warm-season perennial grass around the world and is known for its adaptability to varied environmental conditions. However, Carpet grass lacks enough data in public data banks, which confined our comprehension of the mechanisms of environmental adaptations, gene discovery, and development of molecular markers. In current study, the DEGs (differentially expressed genes) in Axonopus compressus under drought stress (DS) were identified and compared with CK (control) by RNA-Seq.

RESULTS

A total of 263,835 unigenes were identified in Axonopus compressus, and 201,303 (also added to the numbers of the remaining 2 databases) a sequence of unigenes significantly matched in at least one of the seven databases. A total of 153,697 (58.25%) unigenes classified to 144 KEGG pathways, and 7444 unigenes were expressed differentially between DS and CK, of which 4249 were up-regulated and 3195 were down-regulated unigenes. Of the 50 significantly enriched GO terms, 18, 6, and 14 items were related to BP, CC, and MF respectively. Analysis of KEGG enrichment revealed 2569 DEGs involved in 143 different pathways, under drought stress. 2747 DEGs were up-regulated and 2502 DEGs were down-regulated. Moreover, we identified 352 transcription factors (TFs) in Axonopus compressus, of which 270 were differentially expressed between CK and DS. The qRT-PCR validation experiment also supports the transcriptional response of Axonopus compressus against drought. Accuracy of transcriptome unigenes of Axonopus compressus was assessed with BLAST, which showed 3300 sequences of Axonopus compressus in the NCBI.

CONCLUSION

The 7444 unigenes were found to be between DS and CK treatments, which indicate the existence of a strong mechanism of drought tolerance in Axonopus compressus. The current findings provide the first framework for further investigations for the particular roles of these unigenes in Axonopus compressus in response to drought.

You Want it Sweeter: How Glycosylation Affects Plant Response to Oxidative Stress

Oxidative stress is a cellular threat which puts at risk the productivity of most of crops valorized by humankind in terms of food, feed, biomaterial, or bioenergy. It is therefore of crucial importance to understand the mechanisms by which plants mitigate the deleterious effects of oxidizing agents. Glycosylation of antioxidant molecules and phytohormones modifies their chemical properties as well as their cellular and histological repartition. This review emphasizes the mechanisms and the outcomes of this conjugation reaction on plant ability to face growing conditions favoring oxidative stress, in mirror with the activity of deglycosylating enzymes. Pioneer evidence bridging flavonoid, glycosylation, and redox homeostasis paved the way for numerous functional analyses of UDP-glycosyltransferases (UGTs), such as the identification of their substrates and their role to circumvent oxidative stress resulting from various environmental challenges. (De)glycosylation appears as a simple chemical reaction regulating the biosynthesis and/or the activity of a myriad of specialized metabolites partaking in response to pathogen and abiotic stresses. This outcome underlies the possibility to valorize UGTs potential to upgrade plant adaptation and fitness in a rising context of sub-optimal growing conditions subsequent to climate change.

Screening of Worldwide Barley Collection for Drought Tolerance: The Assessment of Various Physiological Measures as the Selection Criteria

Drought is a devastating environmental constraint affecting the agronomic production of barley. To facilitate the breeding process, abundant germplasm resources and reliable evaluation systems to identify the true drought-tolerant barley genotypes are needed. In this study, 237 cultivated and 190 wild barley genotypes, originating from 28 countries, were screened for drought tolerance under the conditions of both water deficit and polyethylene glycol (PEG)-simulated drought at seedling stage. Drought stress significantly reduced the plant growth of all barley genotypes, but no significant difference in drought-induced reduction in the performance of barley seedlings was observed under these two drought conditions. Both cultivated and wild barley subspecies displayed considerable genotypic variability in drought tolerance, which underpinned the identification of 18 genotypes contrasting in drought tolerance. A comparative analysis of drought effects on biomass, water relation, photosynthesis, and osmotic adjustment was undertaken using these contrasting barley genotypes, in order to verify the reliability of the screening and to obtain the credible traits as screening criteria of drought tolerance in barley. As expected, the selected drought-tolerant genotypes showed much less reduction in shoot biomass than drought-sensitive ones under water deficit, which was significantly positively correlated with the results of large-scale screening, confirming the reliability of the screening for drought tolerance under two drought conditions in this study. Likewise, the traits of water relation, photosynthetic activity, and osmotic adjustment differed greatly between the contrasting genotypes under water deficit stress, and they were highly correlated to the growth of barley seedlings, suggesting the potential of them to be the selection criteria for drought tolerance. The analysis of the variable importance of these traits in drought tolerance indicated that sap osmolality and relative water content in the youngest fully-expanded leaf are the suitable selection criteria of screening for drought tolerance in barley at seedling stage.