Surviving a Dry Future: Abscisic Acid (ABA)-Mediated Plant Mechanisms for Conserving Water under Low Humidity

ROOT PHOTOTROPISM 2 (RPT2) is a KAT1 potassium channel regulator required for its accumulation

In plants, inward rectifying potassium channels regulate potassium entry into guard cells and are a key factor controlling stomatal opening. KAT1 is a major inward rectifying potassium channel present in Arabidopsis thaliana guard cell membranes. The identification of regulators of channels like KAT1 is a promising approach for the development of strategies to improve plant drought tolerance. Using a high-throughput Split-ubiquitin screening in yeast, we identified RPT2 (ROOT PHOTOTROPISM 2) as a KAT1 interactor. Here, we present the results of the characterization of this interaction in yeast and plants. Importantly, we also observe increased KAT1-mediated currents in oocytes co-expressing RPT2, suggesting a functional link between the two proteins. Moreover, using stably transformed KAT1-YFP lines, we show that RPT2 is necessary for KAT1 protein accumulation in A. thaliana. Our data suggest an unexpected role for RPT2 in KAT1 post-translational regulation that may represent a novel connection between light signaling and potassium channel activity.

Dehydration rapidly induces expression of NCED genes from a single subclade in diverse eudicots

MAIN CONCLUSION

A gene within a single subclade of NCED genes is triggered in response to both, short- and long-term dehydration treatments, in three model dicot species. During dehydration, some plants can rapidly synthesise the stress hormone abscisic acid (ABA) in leaves within 20 min, triggering the closure of stomata and limiting further water loss. This response is associated with significant transcriptional upregulation of Nine-cis-Epoxycarotenoid Dioxygenase (NCED) genes, which encode the enzyme considered to be rate-limiting in ABA biosynthesis. However, most embryophyte species possess multiple NCED genes, and it is not currently known whether there is any phylogenetic pattern to which NCED genes are involved in this response. We tested transcriptional responses to dehydration for all NCED genes present in three diverse eudicot species-Arabidopsis thaliana (Arabidopsis), pea and tomato-over both the timeframe of stomatal responses (< 20 min) and in response to sustained dehydration (hours). We found that there is a single NCED gene per species, AtNCED3, PsNCED2, and SlNCED1, respectively, that is rapidly upregulated by dehydration. Using a null mutant, we confirmed that the rapidly responsive gene identified in Arabidopsis is important for physiological responses to a sudden drop in humidity. Analysis of the relationships and the evolutionary history of NCED genes using available sequence data from diverse land plant species revealed that the identified genes in each species all belong to the same subclade within the gene family, suggesting a conserved role for this subclade in rapid dehydration responses in eudicots. These findings enable future phylogenetically-informed prediction of genes of interest for rapid dehydration responses within this important multigene family in eudicot species.

Alpine and subalpine plant microbiome mediated plants adapt to the cold environment: A systematic review

With global climate change, ecosystems are affected, some of which are more vulnerable than others, such as alpine ecosystems. Microbes play an important role in environmental change in global ecosystems. Plants and microbes are tightly associated, and symbiotic or commensal microorganisms are crucial for plants to respond to stress, particularly for alpine plants. The current study of alpine and subalpine plant microbiome only stays at the community structure scale, but its ecological function and mechanism to help plants to adapt to the harsh environments have not received enough attention. Therefore, it is essential to systematically understand the structure, functions and mechanisms of the microbial community of alpine and subalpine plants, which will be helpful for the conservation of alpine and subalpine plants using synthetic microbial communities in the future. This review mainly summarizes the research progress of the alpine plant microbiome and its mediating mechanism of plant cold adaptation from the following three perspectives: (1) Microbiome community structure and their unique taxa of alpine and subalpine plants; (2) The role of alpine and subalpine plant microbiome in plant adaptation to cold stress; (3) Mechanisms by which the microbiome of alpine and subalpine plants promotes plant adaptation to low-temperature environments. Finally, we also discussed the future application of high-throughput technologies in the development of microbial communities for alpine and subalpine plants. The existing knowledge could improve our understanding of the important role of microbes in plant adaptation to harsh environments. In addition, perspective further studies on microbes' function confirmation and microbial manipulations in microbiome engineering were also discussed.

A review on strategies for crop improvement against drought stress through molecular insights

The demand for food goods is rising along with the world population growth, which is directly related to the yield of agricultural crops around the world. However, a number of environmental factors, including floods, salinity, moisture, and drought, have a detrimental effect on agricultural production around the world. Among all of these stresses, drought stress (DS) poses a constant threat to agricultural crops and is a significant impediment to global agricultural productivity. Its potency and severity are expected to increase in the future years. A variety of techniques have been used to generate drought-resistant plants in order to get around this restriction. Different crop plants exhibit specific traits that contribute to drought resistance (DR), such as early flowering, drought escape (DE), and leaf traits. We are highlighting numerous methods that can be used to overcome the effects of DS in this review. Agronomic methods, transgenic methods, the use of sufficient fertilizers, and molecular methods such as clustered regularly interspaced short palindromic repeats (CRISPRs)-associated nuclease 9 (Cas9), virus-induced gene silencing (VIGS), quantitative trait loci (QTL) mapping, microRNA (miRNA) technology, and OMICS-based approaches make up the majority of these techniques. CRISPR technology has rapidly become an increasingly popular choice among researchers exploring natural tolerance to abiotic stresses although, only a few plants have been produced so far using this technique. In order to address the difficulties imposed by DS, new plants utilizing the CRISPR technology must be developed.

Review: Research progress on seasonal succession of phyllosphere microorganisms

Phyllosphere microorganisms have recently attracted the attention of scientists studying plant microbiomes. The origin, diversity, functions, and interactions of phyllosphere microorganisms have been extensively explored. Many experiments have demonstrated seasonal cycles of phyllosphere microbes. However, a comprehensive comparison of these separate investigations to characterize seasonal trends in phyllosphere microbes of woody and herbaceous plants has not been conducted. In this review, we explored the dynamic changes of phyllosphere microorganisms in woody and non-woody plants with the passage of the season, sought to find the driving factors, summarized these texts, and thought about future research trends regarding the application of phyllosphere microorganisms in agricultural production. Seasonal trends in phyllosphere microorganisms of herbaceous and woody plants have similarities and differences, but extensive experimental validation is needed. Climate, insects, hosts, microbial interactions, and anthropogenic activities are the diverse factors that influence seasonal variation in phyllosphere microorganisms.

Expression pattern of candidate genes and their correlation with various metabolites of abscisic acid biosynthetic pathway under drought stress in rice

Drought hampers global rice production. Abscisic acid (ABA) plays versatile roles under different environmental stresses. While the link between drought and ABA is known, its effect on ABA biosynthesis genes and metabolites is unclear. This study explored the impact of drought on various metabolites, namely beta-carotene, zeaxanthin, antheraxanthin, violaxanthin, neoxanthin, and candidate genes viz. zeaxanthin epoxidase (ZEP) and 9-cis epoxycarotenoid dioxygenase (NCED) of ABA biosynthesis pathway in rice cultivars (N22 and IR64) at anthesis {65 DAT (Days after transplanting)} with different stress levels. In stressed plants, zeaxanthin significantly increased (92%), while the concentration of beta-carotene, antheraxanthin, violaxanthin and neoxanthin decreased as drought stress progressed. The concentration of metabolites in roots was notably lower than in leaves in both genotypes. The ZEP expression was upregulated in roots (8.24-fold) under drought stress. Among five NCED isoforms, NCED3 showed significant upregulation (7.29-fold) in leaf and root tissue. NCED1 was significantly downregulated as stress progressed and was negatively correlated with ABA accumulation. NCED2, NCED4 and NCED5 showed no significant change in their expression. Drying and rolling of rice leaves was observed after imparting drought stress. The findings revealed that drought stress significantly influenced the expression of candidate genes and the concentration of metabolites of the ABA biosynthesis pathway. There was a significantly higher accumulation of ABA in N22 leaves (47%) and roots (30%) compared to IR64. The N22, a drought-tolerant genotype, exhibited significantly higher concentrations of intermediates and demonstrated increased expression of ZEP and NCED3, potentially contributing to its resilience against drought.

Transpiration response to soil drying versus increasing vapor pressure deficit in crops: physical and physiological mechanisms and key plant traits

The water deficit experienced by crops is a function of atmospheric water demand (vapor pressure deficit) and soil water supply over the whole crop cycle. We summarize typical transpiration response patterns to soil and atmospheric drying and the sensitivity to plant hydraulic traits. We explain the transpiration response patterns using a soil-plant hydraulic framework. In both cases of drying, stomatal closure is triggered by limitations in soil-plant hydraulic conductance. However, traits impacting the transpiration response differ between the two drying processes and act at different time scales. A low plant hydraulic conductance triggers an earlier restriction in transpiration during increasing vapor pressure deficit. During soil drying, the impact of the plant hydraulic conductance is less obvious. It is rather a decrease in the belowground hydraulic conductance (related to soil hydraulic properties and root length density) that is involved in transpiration down-regulation. The transpiration response to increasing vapor pressure deficit has a daily time scale. In the case of soil drying, it acts on a seasonal scale. Varieties that are conservative in water use on a daily scale may not be conservative over longer time scales (e.g. during soil drying). This potential independence of strategies needs to be considered in environment-specific breeding for yield-based drought tolerance.

Mild and severe salt stress responses are age-dependently regulated by abscisic acid in tomato

Salt stress hampers plant growth and development through both osmotic and ionic imbalances. One of the key players in modulating physiological responses towards salinity is the plant hormone abscisic acid (ABA). How plants cope with salinity largely depends on the magnitude of the soil salt content (stress severity), but also on age-related developmental processes (ontogeny). Here we studied how ABA directs salt stress responses in tomato plants for both mild and severe salt stress in leaves of different ages. We used the ABA-deficient mutant notabilis, which contains a null-mutation in the gene of a rate-limiting ABA biosynthesis enzyme 9-cis-epoxycarotenoid dioxygenase (NCED1), leading to impaired stomatal closure. We showed that both old and young leaves of notabilis plants keep a steady-state transpiration and photosynthesis rate during salt stress, probably due to their dysfunctional stomatal closure. At the whole plant level, transpiration declined similar to the wild-type, impacting final growth. Notabilis leaves were able to produce osmolytes and accumulate ions in a similar way as wild-type plants, but accumulated more proline, indicating that osmotic responses were not impaired by the NCED1 mutation. Besides NCED1, also NCED2 and NCED6 are strongly upregulated under salt stress, which could explain why the notabilis mutant did not show a lower ABA content upon salt stress, except in young leaves. This might be indicative of a salt-mediated feedback mechanism on NCED2/6 in notabilis and might explain why notabilis plants seem to perform better under salt stress compared to wild-type plants with respect to biomass and water content accumulation.

Metabolic adjustment and regulation of gene expression are essential for increased resistance to severe water deficit and resilience post-stress in soybean

Background

Soybean is the main oilseed crop grown in the world; however, drought stress affects its growth and physiology, reducing its yield. The objective of this study was to characterize the physiological, metabolic, and genetic aspects that determine differential resistance to water deficit in soybean genotypes.

Methods

Three soybean genotypes were used in this study, two lineages (L11644 and L13241), and one cultivar (EMBRAPA 48-C48). Plants were grown in pots containing 8 kg of a mixture of soil and sand (2:1) in a greenhouse under sunlight. Soil moisture in the pots was maintained at field capacity until the plants reached the stage of development V4 (third fully expanded leaf). At this time, plants were subjected to three water treatments: Well-Watered (WW) (plants kept under daily irrigation); Water Deficit (WD) (withholding irrigation until plants reached the leaf water potential at predawn of -1.5 ± 0.2 MPa); Rewatered (RW) (plants rehydrated for three days after reached the water deficit). The WW and WD water treatments were evaluated on the eighth day for genotypes L11644 and C48, and on the tenth day for L13241, after interruption of irrigation. For the three genotypes, the treatment RW was evaluated after three days of resumption of irrigation. Physiological, metabolic and gene expression analyses were performed.

Results

Water deficit inhibited growth and gas exchange in all genotypes. The accumulation of osmolytes and the concentrations of chlorophylls and abscisic acid (ABA) were higher in L13241 under stress. The metabolic adjustment of lineages in response to WD occurred in order to accumulate amino acids, carbohydrates, and polyamines in leaves. The expression of genes involved in drought resistance responses was more strongly induced in L13241. In general, rehydration provided recovery of plants to similar conditions of control treatment. Although the C48 and L11644 genotypes have shown some tolerance and resilience responses to severe water deficit, greater efficiency was observed in the L13241 genotype through adjustments in morphological, physiological, genetic and metabolic characteristics that are combined in the same plant. This study contributes to the advancement in the knowledge about the resistance to drought in cultivated plants and provides bases for the genetic improvement of the soybean culture.

Phyllosphere microbiome: Diversity and functions

Phyllosphere or aerial surface of plants represents the globally largest and peculiar microbial habitat that inhabits diverse and rich communities of bacteria, fungi, viruses, cyanobacteria, actinobacteria, nematodes, and protozoans. These hyperdiverse microbial communities are related to the host's specific functional traits and influence the host's physiology and the ecosystem's functioning. In the last few years, significant advances have been made in unravelling several aspects of phyllosphere microbiology, including diversity and microbial community composition, dynamics, and functional interactions. This review highlights the current knowledge about the assembly, structure, and composition of phyllosphere microbial communities across spatio-temporal scales, besides functional significance of different microbial communities to the plant host and the surrounding environment. The knowledge will help develop strategies for modelling and manipulating these highly beneficial microbial consortia for furthering scientific inquiry into their interactions with the host plants and also for their useful and economic utilization.

Transpiration increases under high-temperature stress: Potential mechanisms, trade-offs and prospects for crop resilience in a warming world

The frequency and intensity of high-temperature stress events are expected to increase as climate change intensifies. Concomitantly, an increase in evaporative demand, driven in part by global warming, is also taking place worldwide. Despite this, studies examining high-temperature stress impacts on plant productivity seldom consider this interaction to identify traits enhancing yield resilience towards climate change. Further, new evidence documents substantial increases in plant transpiration rate in response to high-temperature stress even under arid environments, which raise a trade-off between the need for latent cooling dictated by excessive temperatures and the need for water conservation dictated by increasing evaporative demand. However, the mechanisms behind those responses, and the potential to design the next generation of crops successfully navigating this trade-off, remain poorly investigated. Here, we review potential mechanisms underlying reported increases in transpiration rate under high-temperature stress, within the broader context of their impact on water conservation needed for crop drought tolerance. We outline three main contributors to this phenomenon, namely stomatal, cuticular and water viscosity-based mechanisms, and we outline research directions aiming at designing new varieties optimized for specific temperature and evaporative demand regimes to enhance crop productivity under a warmer and dryer climate.

Optimal Irrigation Regime for Woody Species Potentially Suitable for Effective and Sustainable Afforestation in the Desert Region of Mongolia

Long-term studies on plant response mechanisms to different irrigation regimes will provide a better understanding of the survivability and establishment of plant communities in a desert environment. Thus, across 10 years, we regularly investigated the effects of the rainfall (control), rainfall + 4 L h−1, rainfall + 8 L h−1, and rainfall + 12 L h−1 irrigation regimes on the growth and leaf morpho-physiology of Tamarix ramosissima Ledeb., Ulmus pumila L., Elaeagnus moorcroftii Wall. ex Schltdl., and Hippophae rhamnoides L. to suggest an optimal irrigation regime for each woody species for effective and sustainable afforestation in Mongolia. We measured the root collar diameter (RCD), annual height growth, survivability, leaf area (LA), specific leaf area (SLA), leaf biomass (LB), total chlorophyll concentration, and predawn (ψp) and midday (ψm) leaf water potentials across the treatments and species. Results showed that trees grown at 12 L h−1 grew taller per year and generally resulted in a higher SLA, but generally resulted in a lower survival rate compared with those in the other treatments in all species. Total chlorophyll content was higher in trees grown under 4 and/or 8 L h−1, particularly for T. ramosissima and E. moorcroftii. Lastly, leaf water potentials were found more negative for trees subjected to 4 L h−1, especially in T. ramosissima and U. pumila, but still resulted in a higher survival rate and LB compared with 12 L h−1. H. rhamnoides showed higher survivability at 8 and/or 12 L h−1 than at 4 L h−1. Therefore, we suggest 4 L h−1 to be the optimal irrigation regime for irrigating T. ramosissima, U. pumila and E. moorcroftii, and 8 and/or 12 L h−1 for H. rhamnoides. Our findings are relevant to ensuring the sustainability of afforestation programs in arid and semiarid landscapes in Mongolia.

Abscisic Acid-Induced Stomatal Closure: An Important Component of Plant Defense Against Abiotic and Biotic Stress

Abscisic acid (ABA) is a stress hormone that accumulates under different abiotic and biotic stresses. A typical effect of ABA on leaves is to reduce transpirational water loss by closing stomata and parallelly defend against microbes by restricting their entry through stomatal pores. ABA can also promote the accumulation of polyamines, sphingolipids, and even proline. Stomatal closure by compounds other than ABA also helps plant defense against both abiotic and biotic stress factors. Further, ABA can interact with other hormones, such as methyl jasmonate (MJ) and salicylic acid (SA). Such cross-talk can be an additional factor in plant adaptations against environmental stresses and microbial pathogens. The present review highlights the recent progress in understanding ABA's multifaceted role under stress conditions, particularly stomatal closure. We point out the importance of reactive oxygen species (ROS), reactive carbonyl species (RCS), nitric oxide (NO), and Ca²⁺ in guard cells as key signaling components during the ABA-mediated short-term plant defense reactions. The rise in ROS, RCS, NO, and intracellular Ca²⁺ triggered by ABA can promote additional events involved in long-term adaptive measures, including gene expression, accumulation of compatible solutes to protect the cell, hypersensitive response (HR), and programmed cell death (PCD). Several pathogens can counteract and try to reopen stomata. Similarly, pathogens attempt to trigger PCD of host tissue to their benefit. Yet, ABA-induced effects independent of stomatal closure can delay the pathogen spread and infection within leaves. Stomatal closure and other ABA influences can be among the early steps of defense and a crucial component of plants' innate immunity response. Stomatal guard cells are quite sensitive to environmental stress and are considered good model systems for signal transduction studies. Further research on the ABA-induced stomatal closure mechanism can help us design strategies for plant/crop adaptations to stress.

Drought Resistance by Engineering Plant Tissue-Specific Responses

Drought is the primary cause of agricultural loss globally, and represents a major threat to food security. Currently, plant biotechnology stands as one of the most promising fields when it comes to developing crops that are able to produce high yields in water-limited conditions. From studies of Arabidopsis thaliana whole plants, the main response mechanisms to drought stress have been uncovered, and multiple drought resistance genes have already been engineered into crops. So far, most plants with enhanced drought resistance have displayed reduced crop yield, meaning that there is still a need to search for novel approaches that can uncouple drought resistance from plant growth. Our laboratory has recently shown that the receptors of brassinosteroid (BR) hormones use tissue-specific pathways to mediate different developmental responses during root growth. In Arabidopsis, we found that increasing BR receptors in the vascular plant tissues confers resistance to drought without penalizing growth, opening up an exceptional opportunity to investigate the mechanisms that confer drought resistance with cellular specificity in plants. In this review, we provide an overview of the most promising phenotypical drought traits that could be improved biotechnologically to obtain drought-tolerant cereals. In addition, we discuss how current genome editing technologies could help to identify and manipulate novel genes that might grant resistance to drought stress. In the upcoming years, we expect that sustainable solutions for enhancing crop production in water-limited environments will be identified through joint efforts.

The adaptation strategies of Herpetospermum pedunculosum (Ser.) Baill at altitude gradient of the Tibetan plateau by physiological and metabolomic methods

BACKGROUND

Herpetospermum pedunculosum (Ser.) Baill is annual scandent herbs. They are used in the treatment of piles, inflammation of the stomach and the intestines. It can survive the extreme environment of the Tibetan Plateau (TP). However, the underlying mechanisms of this adaptation to H. pedunculosum from TP remain unclear. Here, we combined physiological and metabolomics methods to analyze H. pedunculosum response to altitude gradient differences.

RESULTS

At high altitude, increases in the activities of Ascorbate peroxidase (APX), Glutathione reductase (GR), Dehydroascorbate reductase (DHAR), Monodehydroascorbate reductase (MDHAR), Superoxide dismutase (SOD) have been observed in leaves. Total Glutathion content, total Ascorbate content and the ASA (ascorbic acid)/docosahexaenoic acid (DHA) ration were highly elevated from low altitude to high altitude. In addition, high altitude induces decrease of the Anthocyanidin content (ANTH) and increase of abscisic acid content (ABA). The GC-MS analyses identified of 50 metabolites from leaves of H. pedunculosum. In addition, a metabolic network was constructed based on metabolomic datasets using a weighted correlation network analysis (WGCNA) approach. The network analysis uncovered 4 distinguished metabolic modules highly associated with I, II, III and IV respectively. Furthermore, the analysis successfully classified 50 samples into seven groups: carbohydrates, amino acids, organic acids, lipid components, polyamine, secondary metabolism and others.

CONCLUSIONS

In the present study, the content of parts of amino acid components increased in samples collected at higher altitudes, and most of metabolites, including carbohydrates and organic acids were assigned to the carbon metabolic pathway comprising reductive pentose phosphate pathway, glycolysis and TCA cycle, indicating the direct relationship between adaptability and the carbon metabolic pathway and amino acids in H. pedunculosum response to high altitude. The results of this study laid the foundation of the molecular mechanism on H. pedunculosum from high altitude.

A Nck-associated protein 1-like protein affects drought sensitivity by its involvement in leaf epidermal development and stomatal closure in rice

Water deficit is a major environmental threat affecting crop yields worldwide. In this study, a drought stress-sensitive mutant drought sensitive 8 (ds8) was identified in rice (Oryza sativa L.). The DS8 gene was cloned using a map-based approach. Further analysis revealed that DS8 encoded a Nck-associated protein 1 (NAP1)-like protein, a component of the SCAR/WAVE complex, which played a vital role in actin filament nucleation activity. The mutant exhibited changes in leaf cuticle development. Functional analysis revealed that the mutation of DS8 increased stomatal density and impaired stomatal closure activity. The distorted actin filaments in the mutant led to a defect in abscisic acid (ABA)-mediated stomatal closure and increased ABA accumulation. All these resulted in excessive water loss in ds8 leaves. Notably, antisense transgenic lines also exhibited increased drought sensitivity, along with impaired stomatal closure and elevated ABA levels. These findings suggest that DS8 affects drought sensitivity by influencing actin filament activity.

MtABCG20 is an ABA exporter influencing root morphology and seed germination of Medicago truncatula

Abscisic acid (ABA) integrates internal and external signals to coordinate plant development, growth and architecture. It plays a central role in stomatal closure, and prevents germination of freshly produced seeds and germination of non-dormant seeds under unfavorable circumstances. Here, we describe a Medicago truncatula ATP-binding cassette (ABC) transporter, MtABCG20, as an ABA exporter present in roots and germinating seeds. In seeds, MtABCG20 was found in the hypocotyl-radicle transition zone of the embryonic axis. Seeds of mtabcg20 plants were more sensitive to ABA upon germination, due to the fact that ABA translocation within mtabcg20 embryos was impaired. Additionally, the mtabcg20 produced fewer lateral roots and formed more nodules compared with wild-type plants in conditions mimicking drought stress. Heterologous expression in Arabidopsis thaliana provided evidence that MtABCG20 is a plasma membrane protein that is likely to form homodimers. Moreover, export of ABA from Nicotiana tabacum BY2 cells expressing MtABCG20 was faster than in the BY2 without MtABCG20. Our results have implications both in legume crop research and determination of the fundamental molecular processes involved in drought response and germination.

Acquiring Control: The Evolution of Stomatal Signalling Pathways

In vascular plants, stomata balance two opposing functions: they open to facilitate CO₂ uptake and close to prevent excessive water loss. Here, we discuss the evolution of three major signalling pathways that are known to control stomatal movements in angiosperms in response to light, CO₂, and abscisic acid (ABA). We examine the evolutionary origins of key signalling genes involved in these pathways, and compare their expression patterns between an angiosperm and moss. We propose that variation in stomatal sensitivity to stimuli between plant groups are rooted in differences in: (i) gene presence/absence, (ii) specificity of gene spatial expression pattern, and (iii) protein characteristics and functional interactions.

Mechanism of Stomatal Closure in Plants Exposed to Drought and Cold Stress

Drought is one of the abiotic stresses which impairs the plant growth/development and restricts the yield of many crops throughout the world. Stomatal closure is a common adaptation response of plants to the onset of drought condition. Stomata are microscopic pores on the leaf epidermis, which regulate the transpiration/CO₂ uptake by leaves. Stomatal guard cells can sense various abiotic and biotic stress stimuli from the internal and external environment and respond quickly to initiate closure under unfavorable conditions. Stomata also limit the entry of pathogens into leaves, restricting their invasion. Drought is accompanied by the production and/or mobilization of the phytohormone, abscisic acid (ABA), which is well-known for its ability to induce stomatal closure. Apart from the ABA, various other factors that accumulate during drought and affect the stomatal function are plant hormones (auxins, MJ, ethylene, brassinosteroids, and cytokinins), microbial elicitors (salicylic acid, harpin, Flg 22, and chitosan), and polyamines . The role of various signaling components/secondary messengers during stomatal opening or closure has been a matter of intense investigation. Reactive oxygen species (ROS) , nitric oxide (NO) , cytosolic pH, and calcium are some of the well-documented signaling components during stomatal closure. The interrelationship and interactions of these signaling components such as ROS, NO, cytosolic pH, and free Ca²⁺ are quite complex and need further detailed examination.Low temperatures can have deleterious effects on plants. However, plants evolved protection mechanisms to overcome the impact of this stress. Cold temperature inhibits stomatal opening and causes stomatal closure. Cold-acclimated plants often exhibit marked changes in their lipid composition, particularly of the membranes. Cold stress often leads to the accumulation of ABA, besides osmolytes such as glycine betaine and proline. The role of signaling components such as ROS, NO, and Ca²⁺ during cold acclimation is yet to be established, though the effects of cold stress on plant growth and development are studied extensively. The information on the mitigation processes is quite limited. We have attempted to describe consequences of drought and cold stress in plants, emphasizing stomatal closure. Several of these factors trigger signaling components in roots, shoots, and atmosphere, all leading to stomatal closure. A scheme is presented to show the possible signaling events and their convergence and divergence of action during stomatal closure. The possible directions for future research are discussed.

Transcriptomic analysis of drought stress responses of sea buckthorn (Hippophae rhamnoidessubsp. sinensis) by RNA-Seq

Sea buckthorn is one of the most important eco-economic tree species in China due to its ability to grow and produce acceptable yields under limited water and fertilizer availability. In this study, the differentially expressed genes under drought stress (DS) of sea buckthorn were identified and compared with control (CK) by RNA-Seq. A total of 122,803 unigenes were identified in sea buckthorn, and 70,025 unigenes significantly matched a sequence in at least one of the seven databases. A total of 24,060 (19.59%) unigenes can be assigned to 19 KEGG pathways, and 1,644 unigenes were differentially expressed between DS and CK, of which 519 unigenes were up-regulated and 1,125 unigenes down-regulated. Of the 47 significantly enriched GO terms, 14, 7 and 26 items were related to BP, CC and MF, respectively. KEGG enrichment analysis showed 398 DEGs involved in 97 different pathways, of which 119 DEGs were up-regulated and 279 DEGs were down-regulated under drought stress. In addition, we found 4438 transcriptor factors (TFs) in sea buckthorn, of which 100 were differentially expressed between DS and CK. These results lay a first foundation for further investigations of the very specific functions of these unigenes in sea buckthorn in response to drought stress.

Protection of photosynthesis in desiccation-tolerant resurrection plants

Inhibition of photosynthesis is a central, primary response that is observed in both desiccation-tolerant and desiccation-sensitive plants affected by drought stress. Decreased photosynthesis during drought stress can either be due to the limitation of carbon dioxide entry through the stomata and the mesophyll cells, due to increased oxidative stress or due to decreased activity of photosynthetic enzymes. Although the photosynthetic rates decrease in both desiccation-tolerant and sensitive plants during drought, the remarkable difference lies in the complete recovery of photosynthesis after rehydration in desiccation-tolerant plants. Desiccation of sensitive plants leads to irreparable damages of the photosynthetic membranes, in contrast the photosynthetic apparatus is deactivated during desiccation in desiccation-tolerant plants. Desiccation-tolerant plants employ different strategies to protect and/or maintain the structural integrity of the photosynthetic apparatus to reactivate photosynthesis upon water availability. Two major mechanisms are distinguished. Homoiochlorophyllous desiccation-tolerant plants preserve chlorophyll and thylakoid membranes and require active protection mechanisms, while poikilochlorophyllous plants degrade chlorophyll in a regulated manner but then require de novo synthesis during rehydration. Desiccation-tolerant plants, particularly homoiochlorophyllous plants, employ conserved and novel antioxidant enzymes/metabolites to minimize the oxidative damage and to protect the photosynthetic machinery. De novo synthesized, stress-induced proteins in combination with antioxidants are localized in chloroplasts and are important components of the protective network. Genome sequence informations provide some clues on selection of genes involved in protecting photosynthetic structures; e.g. ELIP genes (early light inducible proteins) are enriched in the genomes and more abundantly expressed in homoiochlorophyllous desiccation-tolerant plants. This review focuses on the mechanisms that operate in the desiccation-tolerant plants to protect the photosynthetic apparatus during desiccation.