Physiological and transcriptional memory in guard cells during repetitive dehydration stress

Tree post-drought recovery: scenarios, regulatory mechanisms and ways to improve

Efficient post-drought recovery of growth and assimilation enables a plant to return to its undisturbed state and functioning. Unlike annual plants, trees suffer not only from the current drought, but also from cumulative impacts of consecutive water stresses which cause adverse legacy effects on survival and performance. This review provides an integrated assessment of ecological, physiological and molecular evidence on the recovery of growth and photosynthesis in trees, with a view to informing the breeding of trees with a better ability to recover from water stress. Suppression of recovery processes can result not only from stress damage but also from a controlled downshift of recovery as part of tree acclimation to water-limited conditions. In the latter case, recovery processes could potentially be activated by turning off the controlling mechanisms, but several obstacles make this unlikely. Tree phenology, and specifically photoperiodic constraints, can limit post-drought recovery of growth and photosynthesis, and targeting these constraints may represent a promising way to breed trees with an enhanced ability to recover post-drought. The mechanisms of photoperiod-dependent regulation of shoot, secondary and root growth and of assimilation processes are reviewed. Finally, the limitations and trade-offs of altering the photoperiodic regulation of growth and assimilation processes are discussed.

Abscisic acid-mediated guard cell metabolism regulation

Abscisic acid (ABA) is crucial for plant water deficit (WD) acclimation, but how the interplay between ABA and guard cell (GC) metabolism aids plant WD acclimation remains unclear. Here, we investigated how ABA regulates GC metabolism and how this contributes to plant WD acclimation using tomato wild type (WT) and the ABA-deficient sitiens mutant. These genotypes were characterized at physiological, metabolic, and transcriptional levels under recurring WD periods and were used to perform a13C-glucose labelling experiment using isolated guard cells following exogenously applied ABA. ABA deficiency altered the level of sugars and organic acids in GCs in both irrigated and WD plants and the dynamic of accumulation/degradation of these compounds in GCs during the dark-to-light transition. WD-induced metabolic changes were more pronounced in sitiens than WT GCs. Results from the 13C-labelling experiment indicate that ABA is required for the glycolytic fluxes toward malate and acts as a negative regulator of a putative sucrose substrate cycle. The expression of key ABA-biosynthetic genes was higher in WT than in sitiens GCs after two cycles of WD. Additionally, the intrinsic leaf water use efficiency increased only in WT after the second WD cycle, compared to sitiens. Our results highlight that ABA deficiency disrupts the homeostasis of GC primary metabolism and the WD memory, negatively affecting plant WD acclimation. Our study demonstrates which metabolic pathways are activated by WD and/or regulated by ABA in GCs, which improves our understanding of plant WD acclimation, with clear consequences for plant metabolic engineering in the future.

Abscisic acid induces stomatal closure in horsetails

See also the Commentary on this article by Chater, 243: 503–505.

Crop-Specific Responses to Cold Stress and Priming: Insights from Chlorophyll Fluorescence and Spectral Reflectance Analysis in Maize and Soybean

This study aimed to investigate the impact of cold stress and priming on photosynthesis in the early development of maize and soybean, crops with diverse photosynthetic pathways. The main objectives were to determine the effect of cold stress on chlorophyll a fluorescence parameters and spectral reflectance indices, to determine the effect of cold stress priming and possible stress memory and to determine the relationship between different parameters used in determining the stress response. Fourteen maize inbred lines and twelve soybean cultivars were subjected to control, cold stress, and priming followed by cold stress in a walk-in growth chamber. Measurements were conducted using a portable fluorometer and a handheld reflectance instrument. Cold stress induced an overall downregulation of PSII-related specific energy fluxes and efficiencies, the inactivation of RCs resulting in higher energy dissipation, and electron transport chain impairment in both crops. Spectral reflectance indices suggested cold stress resulted in pigment differences between crops. The effect of priming was more pronounced in maize than in soybean with mostly a cumulatively negative effect. However, priming stabilized the electron trapping efficiency and upregulated the electron transfer system in maize, indicating an adaptive response. Overall, this comprehensive analysis provides insights into the complex physiological responses of maize and soybean to cold stress, emphasizing the need for further genotype-specific cold stress response and priming effect research.

Understanding plant stress memory traits can provide a way for sustainable agriculture

Being sessile, plants encounter various biotic and abiotic threats in their life cycle. To minimize the damages caused by such threats, plants have acquired sophisticated response mechanisms. One major such response includes memorizing the encountered stimuli in the form of a metabolite, hormone, protein, or epigenetic marks. All of these individually as well as together, facilitate effective transcriptional and post-transcriptional responses upon encountering the stress episode for a second time during the life cycle and in some instances even in the future generations. This review attempts to highlight the recent advances in the area of plant memory. A detailed understanding of plant memory has the potential to offer solutions for developing climate-resilient crops for sustainable agriculture.

Mechanisms of Plant Epigenetic Regulation in Response to Plant Stress: Recent Discoveries and Implications

Plant stress is a significant challenge that affects the development, growth, and productivity of plants and causes an adverse environmental condition that disrupts normal physiological processes and hampers plant survival. Epigenetic regulation is a crucial mechanism for plants to respond and adapt to stress. Several studies have investigated the role of DNA methylation (DM), non-coding RNAs, and histone modifications in plant stress responses. However, there are various limitations or challenges in translating the research findings into practical applications. Hence, this review delves into the recent recovery, implications, and applications of epigenetic regulation in response to plant stress. To better understand plant epigenetic regulation under stress, we reviewed recent studies published in the last 5-10 years that made significant contributions, and we analyzed the novel techniques and technologies that have advanced the field, such as next-generation sequencing and genome-wide profiling of epigenetic modifications. We emphasized the breakthrough findings that have uncovered specific genes or pathways and the potential implications of understanding plant epigenetic regulation in response to stress for agriculture, crop improvement, and environmental sustainability. Finally, we concluded that plant epigenetic regulation in response to stress holds immense significance in agriculture, and understanding its mechanisms in stress tolerance can revolutionize crop breeding and genetic engineering strategies, leading to the evolution of stress-tolerant crops and ensuring sustainable food production in the face of climate change and other environmental challenges. Future research in this field will continue to unveil the intricacies of epigenetic regulation and its potential applications in crop improvement.

Physiological and transcriptional analyses reveal formation of memory under recurring drought stresses in seedlings of cotton (Gossypium hirsutum)

Plants are frequently subjected to a range of environmental stresses, including drought, salinity, cold, pathogens, and herbivore attacks. To survive in such conditions, plants have evolved a novel adaptive mechanism known as 'stress memory'. The formation of stress memories necessitates coordinated responses at the cellular, genetic/genomic, and epigenetic levels, involving altered physiological responses, gene activation, hyper-induction and chromatin modification. Cotton (Gossypium spp.) is an important economic crop with numerous applications and high economic value. In this study, we establish G. hirsutum drought memory following cycles of mild drought and re-watering treatments and analyzed memory gene expression patterns. Our findings reveal the physiological, biochemical, and molecular mechanisms underlying drought stress memory formation in G. hirsutum. Specifically, H3K4me3, a histone modification, plays a crucial role in regulating [+ /+ ] transcriptional memory. Moreover, we investigated the intergenerational inheritance of drought stress memory in G. hirsutum. Collectively, our data provides theoretical guidance for cotton breeding.

Distinct guard cell-specific remodeling of chromatin accessibility during abscisic acid- and CO2-dependent stomatal regulation

In plants, epidermal guard cells integrate and respond to numerous environmental signals to control stomatal pore apertures, thereby regulating gas exchange. Chromatin structure controls transcription factor (TF) access to the genome, but whether large-scale chromatin remodeling occurs in guard cells during stomatal movements, and in response to the hormone abscisic acid (ABA) in general, remains unknown. Here, we isolate guard cell nuclei from Arabidopsis thaliana plants to examine whether the physiological signals, ABA and CO2 (carbon dioxide), regulate guard cell chromatin during stomatal movements. Our cell type-specific analyses uncover patterns of chromatin accessibility specific to guard cells and define cis-regulatory sequences supporting guard cell-specific gene expression. We find that ABA triggers extensive and dynamic chromatin remodeling in guard cells, roots, and mesophyll cells with clear patterns of cell type specificity. DNA motif analyses uncover binding sites for distinct TFs enriched in ABA-induced and ABA-repressed chromatin. We identify the Abscisic Acid Response Element (ABRE) Binding Factor (ABF) bZIP-type TFs that are required for ABA-triggered chromatin opening in guard cells and roots and implicate the inhibition of a clade of bHLH-type TFs in controlling ABA-repressed chromatin. Moreover, we demonstrate that ABA and CO2 induce distinct programs of chromatin remodeling, whereby elevated atmospheric CO2 had only minimal impact on chromatin dynamics. We provide insight into the control of guard cell chromatin dynamics and propose that ABA-induced chromatin remodeling primes the genome for abiotic stress resistance.

Dynamic physiological and transcriptomic changes reveal memory effects of salt stress in maize

BACKGROUND

Pre-exposing plants to abiotic stresses can induce stress memory, which is crucial for adapting to subsequent stress exposure. Although numerous genes involved in salt stress response have been identified, the understanding of memory responses to salt stress remains limited.

RESULTS

In this study, we conducted physiological and transcriptional assays on maize plants subjected to recurrent salt stress to characterize salt stress memory. During the second exposure to salt stress, the plants exhibited enhanced salt resistance, as evidenced by increased proline content and higher POD and SOD activity, along with decreased MDA content, indicative of physiological memory behavior. Transcriptional analysis revealed fewer differentially expressed genes and variations in response processes during the second exposure compared to the first, indicative of transcriptional memory behavior. A total of 2,213 salt stress memory genes (SMGs) were identified and categorized into four response patterns. The most prominent group of SMGs consisted of genes with elevated expression during the first exposure to salt stress but reduced expression after recurrent exposure to salt stress, or vice versa ([+ / -] or [- / +]), indicating that a revised response is a crucial process in plant stress memory. Furthermore, nine transcription factors (TFs) (WRKY40, WRKY46, WRKY53, WRKY18, WRKY33, WRKY70, MYB15, KNAT7, and WRKY54) were identified as crucial factors related to salt stress memory. These TFs regulate over 53% of SMGs, underscoring their potential significance in salt stress memory.

CONCLUSIONS

Our study demonstrates that maize can develop salt stress memory, and the genes identified here will aid in the genetic improvement of maize and other crops.

Distinct guard cell specific remodeling of chromatin accessibility during abscisic acid and CO2 dependent stomatal regulation

In plants, epidermal guard cells integrate and respond to numerous environmental signals to control stomatal pore apertures thereby regulating gas exchange. Chromatin structure controls transcription factor access to the genome, but whether large-scale chromatin remodeling occurs in guard cells during stomatal movements, and in response to the hormone abscisic acid (ABA) in general, remain unknown. Here we isolate guard cell nuclei from Arabidopsis thaliana plants to examine whether the physiological signals, ABA and CO2, regulate guard cell chromatin during stomatal movements. Our cell type specific analyses uncover patterns of chromatin accessibility specific to guard cells and define novel cis-regulatory sequences supporting guard cell specific gene expression. We find that ABA triggers extensive and dynamic chromatin remodeling in guard cells, roots, and mesophyll cells with clear patterns of cell-type specificity. DNA motif analyses uncover binding sites for distinct transcription factors enriched in ABA-induced and ABA-repressed chromatin. We identify the ABF/AREB bZIP-type transcription factors that are required for ABA-triggered chromatin opening in guard cells and implicate the inhibition of a set of bHLH-type transcription factors in controlling ABA-repressed chromatin. Moreover, we demonstrate that ABA and CO2 induce distinct programs of chromatin remodeling. We provide insight into the control of guard cell chromatin dynamics and propose that ABA-induced chromatin remodeling primes the genome for abiotic stress resistance.

FlbZIP12 gene enhances drought tolerance via modulating flavonoid biosynthesis in Fagopyrum leptopodum

Karst lands provide a poor substrate to support plant growth, as they are low in nutrients and water content. Common buckwheat (Fagopyrum esculentum) is becoming a popular crop for its gluten-free grains and their high levels of phenolic compounds, but buckwheat yields are affected by high water requirements during grain filling. Here, we describe a wild population of drought-tolerant Fagopyrum leptopodum and its potential for enhancing drought tolerance in cultivated buckwheat. We determined that the expression of a gene encoding a Basic leucine zipper (bZIP) transcription factor, FlbZIP12, from F. leptopodum is induced by abiotic stresses, including treatment with the phytohormone abscisic acid, salt, and polyethylene glycol. In addition, we show that overexpressing FlbZIP12 in Tartary buckwheat (Fagopyrum tataricum) root hairs promoted drought tolerance by increasing the activities of the enzymes superoxide dismutase and catalase, decreasing malondialdehyde content, and upregulating the expression of stress-related genes. Notably, FlbZIP12 overexpression induced the expression of key genes involved in flavonoid biosynthesis. We also determined that FlbZIP12 interacts with protein kinases from the FlSnRK2 family in vitro and in vivo. Taken together, our results provide a theoretical basis for improving drought tolerance in buckwheat via modulating the expression of FlbZIP12 and flavonoid contents.

Plant transcriptional memory and associated mechanism of abiotic stress tolerance

Plants face various adverse environmental conditions, particularly with the ongoing changes in global climate, which drastically affect the growth, development and productivity of crops. To cope with these stresses, plants have evolved complex mechanisms, and one of the crucial ways is to develop transcriptional memories from stress exposure. This induced learning enables plants to better and more strongly restart the response and adaptation mechanism to stress when similar or dissimilar stresses reoccur. Understanding the molecular mechanism behind plant transcriptional memory of stress can provide a theoretical basis for breeding stress-tolerant crops with resilience to future climates. Here we review the recent research progress on the transcriptional memory of plants under various stresses and the applications of underlying mechanisms for sustainable agricultural production. We propose that a thorough understanding of plant transcriptional memory is crucial for both agronomic management and resistant breeding, and thus may help to improve agricultural yield and quality under changing climatic conditions.

Biochemical and Epigenetic Modulations under Drought: Remembering the Stress Tolerance Mechanism in Rice

A plant, being a sessile organism, needs to modulate biochemical, physiological, and molecular responses to the environment in a quick and efficient manner to be protected. Drought stress is a frequently occurring abiotic stress that severely affects plant growth, development, and productivity. Short- and long-term memories are well-known phenomena in animals; however, the existence of such remembrance in plants is still being discovered. In this investigation, different rice genotypes were imposed with drought stress just before flowering and the plants were re-watered for recovery from the stress. Seeds collected from the stress-treated (stress-primed) plants were used to raise plants for the subsequent two generations under a similar experimental setup. Modulations in physio-biochemical (chlorophyll, total phenolics and proline contents, antioxidant potential, lipid peroxidation) and epigenetic [5-methylcytosine (5-mC)] parameters were analyzed in the leaves of the plants grown under stress as well as after recovery. There was an increase in proline (>25%) and total phenolic (>19%) contents, antioxidant activity (>7%), and genome-wide 5-mC level (>56%), while a decrease (>9%) in chlorophyll content was recorded to be significant under the stress. Interestingly, a part of the increased proline content, total phenolics content, antioxidant activity, and 5-mC level was retained even after the withdrawal of the stress. Moreover, the increased levels of biochemical and epigenetic parameters were observed to be transmitted/inherited to the subsequent generations. These might help in developing stress-tolerant crops and improving crop productivity under the changing global climate for sustainable food production and global food security.

A novel TF molecular switch-mechanism found in two contrasting ecotypes of a psammophyte, Agriophyllum squarrosum, in regulating transcriptional drought memory

BACKGROUND

Prior drought stress may change plants response patterns and subsequently increase their tolerance to the same condition, which can be referred to as "drought memory" and proved essential for plants well-being. However, the mechanism of transcriptional drought memory in psammophytes remains unclear. Agriophyllum squarrosum, a pioneer species on mobile dunes, is widely spread in Northern China's vast desert areas with outstanding ability of water use efficiency. Here we conducted dehydration-rehydration treatment on A. squarrosum semi-arid land ecotype AEX and arid land ecotype WW to dissect the drought memory mechanism of A. squarrosum, and to determine the discrepancy in drought memory of two contrasting ecotypes that had long adapted to water heterogeneity.

RESULT

Physiological traits monitoring unveiled the stronger ability and longer duration in drought memory of WW than that of AEX. A total of 1,642 and 1,339 drought memory genes (DMGs) were identified in ecotype AEX and WW, respectively. Furthermore, shared DMGs among A. squarrosum and the previously studied species depicted that drought memory commonalities in higher plants embraced pathways like primary and secondary metabolisms; while drought memory characteristics in A. squarrosum were mainly related to response to heat, high light intensity, hydrogen peroxide, and dehydration, which might be due to local adaptation to desert circumstances. Heat shock proteins (HSPs) occupied the center of the protein-protein interaction (PPI) network in drought memory transcription factors (TF), thus playing a key regulatory role in A. squarrosum drought memory. Co-expression analysis of drought memory TFs and DMGs uncovered a novel regulating module, whereby pairs of TFs might function as molecular switches in regulating DMG transforming between high and low expression levels, thus promoting drought memory reset.

CONCLUSION

Based on the co-expression analysis, protein-protein interaction prediction, and drought memory metabolic network construction, a novel regulatory module of transcriptional drought memory in A. squarrosum was hypothesized here, whereby recurrent drought signal is activated by primary TF switches, then amplified by secondary amplifiers, and thus regulates downstream complicated metabolic networks. The present research provided valuable molecular resources on plants' stress-resistance basis and shed light on drought memory in A. squarrosum.

Abscisic Acid and Cytokinins Are Not Involved in the Regulation of Stomatal Conductance of Scots Pine Saplings during Post-Drought Recovery

Delayed or incomplete recovery of gas exchange after water stress relief limits assimilation in the post-drought period and can thus negatively affect the processes of post-drought recovery. Abscisic acid (ABA) accumulation and antagonistic action between ABA and cytokinins (CKs) play an important role in regulation of stomatal conductance under water deficit. Specifically, in pine species, sustained ABA accumulation is thought to be the main cause of delayed post-drought gas exchange recovery, although the role of CKs is not yet known. Therefore, we aimed to study the effects of ABA and CKs on recovery of stomatal conductance in greenhouse-grown 3-year-old Scots pine saplings recovering from water stress. We analysed both changes in endogenous ABA and CK contents and the effects of treatment with exogenous CK on stomatal conductance. Drought stress suppressed stomatal conductance, and post-drought stomatal conductance remained suppressed for 2 weeks after plant rewatering. ABA accumulated during water stress, but ABA levels decreased rapidly after rewatering. Additionally, trans-zeatin/ABA and isopentenyladenine/ABA ratios, which were decreased in water-stressed plants, recovered rapidly in rewatered plants. Spraying plants with 6-benzylaminopurine (0.1–100 µM) did not influence recovery of either stomatal conductance or needle water status. It can be concluded that the delayed recovery of stomatal conductance in Scots pine needles was not due to sustained ABA accumulation or a sustained decrease in the CK/ABA ratio, and CK supplementation was unable to overcome this delayed recovery.

Drought priming mechanisms in wheat elucidated by in-situ determination of dynamic stomatal behavior

Stomata play a critical role in balancing photosynthesis and transpiration, which are essential processes for plant growth, especially in response to abiotic stress. Drought priming has been shown to improve drought tolerance. Lots of studies have been done with the response of stomatal behavior to drought stress. However, how the stomatal dynamic movement in intact wheat plants response to drought priming process is not known. Here, a portable microscope was used to take microphotographs in order to in-stiu determination of stomatal behavior. Non-invasive micro-test technology was used for measurements of guard cell K⁺, H⁺ and Ca²⁺ fluxes. Surprisingly, the results found that primed plants close stomatal much faster under drought stress, and reopening the stomatal much quicker under recovery, in relation to non-primed plants. Compared with non-primed plants, primed plants showed higher accumulation of ABA and Ca²⁺ influx rate in guard cells under drought stress. Furthermore, genes encoding anion channels were higher expressed and K⁺ outward channels activated, leading to enhanced K⁺ efflux, resulting in faster stomatal closure in primed plants than non-primed plants. During recovery, both guard cell ABA and Ca²⁺ influx of primed plants were found to be significantly reducing K⁺ efflux and accelerating stomatal reopening. Collectively, a portable non-invasive stomatal observation of wheat found that priming promoted faster stomatal closure under drought stress and faster reopening during post-drought recovery in relation to non-primed plants, thereby enhancing overall drought tolerance.

Stress memory and its regulation in plants experiencing recurrent drought conditions

Developing stress-tolerant plants continues to be the goal of breeders due to their realized yields and stability. Plant responses to drought have been studied in many different plant species, but the occurrence of stress memory as well as the potential mechanisms for memory regulation is not yet well described. It has been observed that plants hold on to past events in a way that adjusts their response to new challenges without altering their genetic constitution. This ability could enable training of plants to face future challenges that increase in frequency and intensity. A better understanding of stress memory-associated mechanisms leading to alteration in gene expression and how they link to physiological, biochemical, metabolomic and morphological changes would initiate diverse opportunities to breed stress-tolerant genotypes through molecular breeding or biotechnological approaches. In this perspective, this review discusses different stress memory types and gives an overall view using general examples. Further, focusing on drought stress, we demonstrate coordinated changes in epigenetic and molecular gene expression control mechanisms, the associated transcription memory responses at the genome level and integrated biochemical and physiological responses at cellular level following recurrent drought stress exposures. Indeed, coordinated epigenetic and molecular alterations of expression of specific gene networks link to biochemical and physiological responses that facilitate acclimation and survival of an individual plant during repeated stress.

Transcriptome Analysis Points to BES1 as a Transducer of Strigolactone Effects on Drought Memory in Arabidopsis thaliana

Strigolactones (SLs) are carotenoid-derived phytohormones governing a wide range of physiological processes, including drought-associated stomatal closure. We have previously shown in tomato that SLs regulate the so-called after-effect of drought, whereby stomatal conductance is not completely restored for some time during recovery after a drought spell, irrespective of the water potential. To ease the elucidation of its molecular underpinnings, we investigated whether this SL effect is conserved in Arabidopsis thaliana by contrasting the physiological performances of the wild-type with SL-depleted (more axillary growth 4, max4) and insensitive (dwarf 14, d14) mutants in a drought and recovery protocol. Physiological analyses showed that SLs are important to achieve a complete after-effect in A. thaliana, while transcriptome results suggested that the SL-dependent modulation of drought responses extends to a large subset (about 4/5) of genes displaying memory transcription patterns. Among these, we show that the activation of over 30 genes related to abscisic acid metabolism and signaling strongly depends on SL signaling. Furthermore, by using promoter-enrichment tools, we identified putative cis- and trans-acting factors that may be important in the SL-dependent and SL-independent regulation of genes during drought and recovery. Finally, in order to test the accuracy of our bioinformatic prediction, we confirmed one of the most promising transcription factor candidates mediating SL signaling effects on transcriptional drought memory-BRI-EMS SUPPRESSOR1 (BES1). Our findings reveal that SLs are master regulators of Arabidopsis transcriptional memory upon drought and that this role is partially mediated by the BES1 transcription factor.

Warming Scenarios and Phytophthora cinnamomi Infection in Chestnut (Castanea sativa Mill.)

The main threats to chestnut in Europe are climate change and emerging pathogens. Although many works have separately addressed the impacts on chestnut of elevated temperatures and Phytophthora cinnamomi Rands (Pc) infection, none have studied their combined effect. The objectives of this work were to describe the physiology, secondary metabolism and survival of 6-month-old C. sativa seedlings after plants were exposed to ambient temperature, high ambient temperature and heat wave events, and subsequent infection by Pc. Ten days after the warming scenarios, the biochemistry of plant leaves and roots was quantified and the recovery effect assessed. Plant growth and root biomass under high ambient temperature were significantly higher than in plants under ambient temperature and heat wave event. Seven secondary metabolite compounds in leaves and three in roots were altered significantly with temperature. Phenolic compounds typically decreased in response to increased temperature, whereas ellagic acid in roots was significantly more abundant in plants exposed to ambient and high ambient temperature than in plants subjected to heat waves. At recovery, leaf procyanidin and catechin remained downregulated in plants exposed to high ambient temperature. Mortality by Pc was fastest and highest in plants exposed to ambient temperature and lowest in plants under high ambient temperature. Changes in the secondary metabolite profile of plants in response to Pc were dependent on the warming scenarios plants were exposed to, with five compounds in leaves and three in roots showing a significant 'warming scenario' × 'Pc' interaction. The group of trees that best survived Pc infection was characterised by increased quercetin 3-O-glucuronide, 3-feruloylquinic acid, gallic acid ethyl ester and ellagic acid. To the best of our knowledge, this is the first study addressing the combined effects of global warming and Pc infection in chestnut.

Effect of Recurrent Salt and Drought Stress Treatments on the Endangered Halophyte Limonium angustebracteatum Erben

Limonium angustebracteatum is an endemic halophyte from the Spanish Mediterranean coastal salt marshes. To investigate this species' ability to cope with recurrent drought and salt stress, one-year-old plants were subjected to two salt stress treatments (watering with 0.5 and 1 M NaCl solutions), one water stress treatment (complete irrigation withholding), or watered with non-saline water for the control, across three phases: first stress (30 days), recovery from both stresses (15 days), and second stress (15 days). Growth and biochemical parameters were determined after each period. The plants showed high salt tolerance but were sensitive to water deficit, as shown by the decrease in leaf fresh weight and water content, root water content, and photosynthetic pigments levels in response to the first water stress; then, they were restored to the respective control values upon recovery. Salt tolerance was partly based on the accumulation of Na+, Cl- and Ca2+ in the roots and predominantly in the leaves; ion levels also decreased to control values during recovery. Organic osmolytes (proline and total soluble sugars), oxidative stress markers (malondialdehyde and H2O2), and antioxidant compounds (total phenolic compounds and flavonoids) increased by various degrees under the first salt and water stress treatments, and declined after recovery. The analysed variables increased again, but generally to a lesser extent, during the second stress phase, suggesting the occurrence of stress acclimation acquired by the activation of defence mechanisms during the first stress period.

Maintenance of abiotic stress memory in plants: Lessons learned from heat acclimation

Plants acquire enhanced tolerance to intermittent abiotic stress by employing information obtained during prior exposure to an environmental disturbance, a process known as acclimation or defense priming. The capacity for stress memory is a critical feature in this process. The number of reports related to plant stress memory (PSM) has recently increased, but few studies have focused on the mechanisms that maintain PSM. Identifying the components involved in maintaining PSM is difficult due in part to the lack of clear criteria to recognize these components. In this review, based on what has been learned from genetic studies on heat acclimation memory, we propose criteria for identifying components of the regulatory networks that maintain PSM. We provide examples of the regulatory circuits formed by effectors and regulators of PSM. We also highlight strategies for assessing PSMs, update the progress in understanding the mechanisms of PSM maintenance, and provide perspectives for the further development of this exciting research field.

Strigolactones as a hormonal hub for the acclimation and priming to environmental stress in plants

Strigolactones are phytohormones with many attributed roles in development, and more recently in responses to environmental stress. We will review evidence of the latter in the frame of the classic distinction among the three main stress acclimation strategies (i.e., avoidance, tolerance and escape), by taking osmotic stress in its several facets as a non-exclusive case study. The picture we will sketch is that of a hormonal family playing important roles in each of the mechanisms tested so far, and influencing as well the build-up of environmental memory through priming. Thus, strigolactones appear to be backstage operators rather than frontstage players, setting the tune of acclimation responses by fitting them to the plant individual history of stress experience.

Knockdown of NtCPS2 promotes plant growth and reduces drought tolerance in Nicotiana tabacum

Drought stress is one of the primary environmental stress factors that gravely threaten crop growth, development, and yields. After drought stress, plants can regulate the content and proportion of various hormones to adjust their growth and development, and in some cases to minimize the adverse effects of drought stress. In our previous study, the tobacco cis-abienol synthesis gene (NtCPS2) was found to affect hormone synthesis in tobacco plants. Unfortunately, the role of NtCPS2 genes in the response to abiotic stress has not yet been investigated. Here, we present data supporting the role of NtCPS2 genes in drought stress and the possible underlying molecular mechanisms. NtCPS2 gene expression was induced by polyethylene glycol, high-temperature, and virus treatments. The results of subcellular localization showed that NtCPS2 was localized in the cell membrane. The NtCPS2-knockdown plants exhibited higher levels of gibberellin (GA) content and synthesis pathway genes expression but lower abscisic acid (ABA) content and synthesis pathway genes expression in response to drought stress. In addition, the transgenic tobacco lines showed higher leaf water loss and electrolyte loss, lower soluble protein and reactive oxygen species content (ROS), and lower antioxidant enzyme activity after drought treatment compared to wild type plants (WT). In summary, NtCPS2 positively regulates drought stress tolerance possibly by modulating the ratio of GA to ABA, which was confirmed by evidence of related phenotypic and physiological indicators. This study may provide evidence for the feedback regulation of hormone to abiotic and biotic stresses.

Drought Stress Priming Improved the Drought Tolerance of Soybean

The capability of a plant to protect itself from stress-related damages is termed "adaptability" and the phenomenon of showing better performance in subsequent stress is termed "stress memory". While drought is one of the most serious disasters to result from climate change, the current understanding of drought stress priming in soybean is still inadequate for effective crop improvement. To fill this gap, in this study, the drought memory response was evaluated in cultivated soybean (Glycine max). To determine if a priming stress prior to a drought stress would be beneficial to the survival of soybean, plants were divided into three treatment groups: the unprimed group receiving one cycle of stress (1S), the primed group receiving two cycles of stress (2S), and the unstressed control group not subjected to any stress (US). When compared with the unprimed plants, priming led to a reduction of drought stress index (DSI) by 3, resulting in more than 14% increase in surviving leaves, more than 13% increase in leaf water content, slight increase in shoot water content and a slower rate of loss of water from the detached leaves. Primed plants had less than 60% the transpiration rate and stomatal conductance compared to the unprimed plants, accompanied by a slight drop in photosynthesis rate, and about a 30% increase in water usage efficiency (WUE). Priming also increased the root-to-shoot ratio, potentially improving water uptake. Selected genes encoding late embryogenesis abundant (LEA) proteins and MYB, NAC and PP2C domain-containing transcription factors were shown to be highly induced in primed plants compared to the unprimed group. In conclusion, priming significantly improved the drought stress response in soybean during recurrent drought, partially through the maintenance of water status and stronger expression of stress related genes. In sum, we have identified key physiological parameters for soybean which may be used as indicators for future genetic study to identify the genetic element controlling the drought stress priming.

Transcriptional Stress Memory and Transgenerational Inheritance of Drought Tolerance in Plants

Plants respond to drought stress by producing abscisic acid, a chemical messenger that regulates gene expression and thereby expedites various physiological and cellular processes including the stomatal operation to mitigate stress and promote tolerance. To trigger or suppress gene transcription under drought stress conditions, the surrounding chromatin architecture must be converted between a repressive and active state by epigenetic remodeling, which is achieved by the dynamic interplay among DNA methylation, histone modifications, loop formation, and non-coding RNA generation. Plants can memorize chromatin status under drought conditions to enable them to deal with recurrent stress. Furthermore, drought tolerance acquired during plant growth can be transmitted to the next generation. The epigenetically modified chromatin architectures of memory genes under stressful conditions can be transmitted to newly developed cells by mitotic cell division, and to germline cells of offspring by overcoming the restraints on meiosis. In mammalian cells, the acquired memory state is completely erased and reset during meiosis. The mechanism by which plant cells overcome this resetting during meiosis to transmit memory is unclear. In this article, we review recent findings on the mechanism underlying transcriptional stress memory and the transgenerational inheritance of drought tolerance in plants.

Learning in Individual Organisms, Genes, Machines, and Groups: A New Way of Defining and Relating Learning in Different Systems

Learning is a central concept in many scientific disciplines. Communication about research on learning is, however, hampered by the fact that different researchers define learning in different ways. In this article, we introduce the extended functional definition of learning that can be used across scientific disciplines. We provide examples of how the definition can be applied to individual organisms, genes, machines, and groups. Using the extended functional definition (a) reveals a heuristic framework for research that can be applied across scientific disciplines, (b) allows researchers to engage in intersystem analyses that relate the behavior and learning of different systems, and (c) clarifies how learning differs from other phenomena such as (changes in) behavior, damaging systems, and programming systems.

Meta-Analysis Reveals Challenges and Gaps for Genome-to-Phenome Research Underpinning Plant Drought Response

Severe drought conditions and extreme weather events are increasing worldwide with climate change, threatening the persistence of native plant communities and ecosystems. Many studies have investigated the genomic basis of plant responses to drought. However, the extent of this research throughout the plant kingdom is unclear, particularly among species critical for the sustainability of natural ecosystems. This study aimed to broaden our understanding of genome-to-phenome (G2P) connections in drought-stressed plants and identify focal taxa for future research. Bioinformatics pipelines were developed to mine and link information from databases and abstracts from 7730 publications. This approach identified 1634 genes involved in drought responses among 497 plant taxa. Most (83.30%) of these species have been classified for human use, and most G2P interactions have been described within model organisms or crop species. Our analysis identifies several gaps in G2P research literature and database connectivity, with 21% of abstracts being linked to gene and taxonomy data in NCBI. Abstract text mining was more successful at identifying potential G2P pathways, with 34% of abstracts containing gene, taxa, and phenotype information. Expanding G2P studies to include non-model plants, especially those that are adapted to drought stress, will help advance our understanding of drought responsive G2P pathways.

Epigenetic marks for mitigating abiotic stresses in plants

Abiotic stressors are one of the major factors affecting agricultural output. Plants have evolved adaptive systems to respond appropriately to various environmental cues. These responses can be accomplished by modulating or fine-tuning genetic and epigenetic regulatory mechanisms. Understanding the response of plants' molecular features to abiotic stress is a priority in the current period of continued environmental changes. Epigenetic modifications are necessary that control gene expression by changing chromatin status and recruiting various transcription regulators. The present study summarized the current knowledge on epigenetic modifications concerning plant responses to various environmental stressors. The functional relevance of epigenetic marks in regulating stress tolerance has been revealed, and epigenetic changes impact the effector genes. This study looks at the epigenetic mechanisms that govern plant abiotic stress responses, especially DNA methylation, histone methylation/acetylation, chromatin remodeling, and various metabolites. Plant breeders will benefit from a thorough understanding of these processes to create alternative crop improvement approaches. Genome editing with clustered regularly interspaced short palindromic repeat/CRISPR-associated proteins (CRISPR/Cas) provides genetic tools to make agricultural genetic engineering more sustainable and publicly acceptable.

Drought tolerance memory transmission by citrus buds

Plants face recurrent drought events, and previous stresses can influence their responses to subsequent stress episodes. Studies on drought stress memory are recent in citriculture, although they show promise as a tool for crop improvement. Here, we investigated whether stress memory mechanisms can be detected in citrus plants grafted with buds from plants subjected to recurrent water deficit. Three rootstock varieties, namely 'Rangpur Santa Cruz' lime, 'Sunki Maravilha' mandarin and 'Sunki Tropical' mandarin, in combination with 'Valencia' orange, were either maintained under full irrigation or subjected to one, two, or three water deficit cycles. Buds from 'Valencia' orange were grafted onto 'Swingle' citrumelo rootstocks and were evaluated. This combination displayed improved physiological and biochemical performance under water limitation, especially 'Valencia' buds grafted onto 'Sunki Maravilha', with better photosynthetic performance under water deficit. These findings indicate that genotype-dependent epigenetic memory is a key factor in restoring citrus plants' capacity to rely on previous stress experiences to restore better photosynthetic and physiological responses when undergoing new water deficit events. Therefore, epigenetic marks can be stored and transmitted to new citrus plants and are a promising alternative to enable increased water deficit tolerance when plants are then challenged by drought-prone environments.

How do plants remember drought?

Plants develop both short-term and transgenerational memory of drought stress through epigenetic regulation of transcription for a better response to subsequent exposure. Recurrent spells of droughts are more common than a single drought, with intermittent moist recovery intervals. While the detrimental effects of the first drought on plant structure and physiology are unavoidable, if survived, plants can memorize the first drought to present a more robust response to the following droughts. This includes a partial stomatal opening in the watered recovery interval, higher levels of osmoprotectants and ABA, and attenuation of photosynthesis in the subsequent exposure. Short-term drought memory is regulated by ABA and other phytohormone signaling with transcriptional memory behavior in various genes. High levels of methylated histones are deposited at the drought-tolerance genes. During the recovery interval, the RNA polymerase is stalled to be activated by a pause-breaking factor in the subsequent drought. Drought leads to DNA demethylation near drought-response genes, with genetic control of the process. Progenies of the drought-exposed plants can better adapt to drought owing to the inheritance of particular methylation patterns. However, a prolonged watered recovery interval leads to loss of drought memory, mediated by certain demethylases and chromatin accessibility factors. Small RNAs act as critical regulators of drought memory by altering transcript levels of drought-responsive target genes. Further studies in the future will throw more light on the genetic control of drought memory and the interplay of genetic and epigenetic factors in its inheritance. Plants from extreme environments can give queues to understanding robust memory responses at the ecosystem level.

Chromatin-Based Transcriptional Reprogramming in Plants under Abiotic Stresses

Plants' stress response machinery is characterized by an intricate network of signaling cascades that receive and transmit environmental cues and ultimately trigger transcriptional reprogramming. The family of epigenetic regulators that are the key players in the stress-induced signaling cascade comprise of chromatin remodelers, histone modifiers, DNA modifiers and regulatory non-coding RNAs. Changes in the histone modification and DNA methylation lead to major alterations in the expression level and pattern of stress-responsive genes to adjust with abiotic stress conditions namely heat, cold, drought and salinity. The spotlight of this review falls primarily on the chromatin restructuring under severe abiotic stresses, crosstalk between epigenetic regulators along with a brief discussion on stress priming in plants.

Understanding plant stress memory response for abiotic stress resilience: Molecular insights and prospects

As sessile species and without the possibility of escape, plants constantly face numerous environmental stresses. To adapt in the external environmental cues, plants adjust themselves against such stresses by regulating their physiological, metabolic and developmental responses to external environmental cues. Certain environmental stresses rarely occur during plant life, while others, such as heat, drought, salinity, and cold are repetitive. Abiotic stresses are among the foremost environmental variables that have hindered agricultural production globally. Through distinct mechanisms, these stresses induce various morphological, biochemical, physiological, and metabolic changes in plants, directly impacting their growth, development, and productivity. Subsequently, plant's physiological, metabolic, and genetic adjustments to the stress occurrence provide necessary competencies to adapt, survive and nurture a condition known as "memory." This review emphasizes the advancements in various epigenetic-related chromatin modifications, DNA methylation, histone modifications, chromatin remodeling, phytohormones, and microRNAs associated with abiotic stress memory. Plants have the ability to respond quickly to stressful situations and can also improve their defense systems by retaining and sustaining stressful memories, allowing for stronger or faster responses to repeated stressful situations. Although there are relatively few examples of such memories, and no clear understanding of their duration, taking into consideration plenty of stresses in nature. Understanding these mechanisms in depth could aid in the development of genetic tools to improve breeding techniques, resulting in higher agricultural yield and quality under changing environmental conditions.

Stress memory responses and seed priming correlate with drought tolerance in plants: an overview

Environmental-friendly techniques based on plant stress memory, cross-stress tolerance, and seed priming help sustainable agriculture by mitigating negative effects of dehydration stress. The frequently uneven rainfall distribution caused by global warming will lead to more irregular and multiple abiotic stresses, such as heat stress, dehydration stress, cold stress or the combination of these stresses. Dehydration stress is one of the major environmental factors affecting the survival rate and productivity of plants. Hence, there is an urgent need to develop improved resilient varieties. Presently, technologies based on plant stress memory, cross-stress tolerance and priming of seeds represent fruitful and promising areas of future research and applied agricultural science. In this review, we will provide an overview of plant drought stress memory from physiological, biochemical, molecular and epigenetic perspectives. Drought priming-induced cross-stress tolerance to cold and heat stress will be discussed and the application of seed priming will be illustrated for different species.

Transcriptomic Analysis Reveals Regulatory Networks for Osmotic Water Stress and Rewatering Response in the Leaves of Ginkgo biloba

To elucidate the transcriptomic regulation mechanisms that underlie the response of Ginkgo biloba to dehydration and rehydration, we used ginkgo saplings exposed to osmotically driven water stress and subsequent rewatering. When compared with a control group, 137, 1453, 1148, and 679 genes were differentially expressed in ginkgo leaves responding to 2, 6, 12, and 24 h of water deficit, and 796 and 1530 genes were differentially expressed responding to 24 and 48 h of rewatering. Upregulated genes participated in the biosynthesis of abscisic acid, eliminating reactive oxygen species (ROS), and biosynthesis of flavonoids and bilobalide, and downregulated genes were involved in water transport and cell wall enlargement in water stress-treated ginkgo leaves. Under rehydration conditions, the genes associated with water transport and cell wall enlargement were upregulated, and the genes that participated in eliminating ROS and the biosynthesis of flavonoids and bilobalide were downregulated in the leaves of G. biloba. Furthermore, the weighted gene coexpression networks were established and correlated with distinct water stress and rewatering time-point samples. Hub genes that act as key players in the networks were identified. Overall, these results indicate that the gene coexpression networks play essential roles in the transcriptional reconfiguration of ginkgo leaves in response to water stress and rewatering.

Drought stress memory in rice guard cells: Proteome changes and genomic stability of DNA

Drought is one of the major threats for crop plants among them rice, worldwide. The effects of drought vary depending on the plant growth phase and the occurrence of a previous stress, which can leave a memory of the stress. Stomata guard cells perform many essential functions and are highly responsive to hormonal and environmental stimuli. Therefore, information on how guard cells respond to drought might be useful for selecting drought tolerant plants. In this work, physiological analysis, comparative proteomics, gene expression and 5 - methylcytosine (%) analysis were used to elucidate the effects of drought in single stress event at vegetative or reproductive stage or recurrent at both stages in guard cells from rice plants. Photosynthesis and stomatal conductance decreased when drought was applied at reproductive stage in single and recurrent event. Twelve drought-responsive proteins were identified, belonging to photosynthesis pathway, response to oxidative stress, stress signalling and others. The expression of their encoding genes showed a positive relation with the protein abundance. Drought stress increased the total DNA methylation when applied at vegetative stage in single (35%) and recurrent event (18%) and decreased it in plants stressed at reproductive stage (9.8%), with respect to the levels measured in well-watered ones (13.84%). In conclusion, a first drought event seems to induce adaptation to water-deficit conditions through decreasing energy dissipation, increasing ATP energy provision, reducing oxidative damage in GC. Furthermore, the stress memory is associated with epigenetic markers.

Differential Response of Two Tomato Genotypes, Wild Type cv. Ailsa Craig and Its ABA-Deficient Mutant flacca to Short-Termed Drought Cycles

Two tomato genotypes with constitutively different ABA level, flacca mutant and wild type of Ailsa Craig cv. (WT), were subjected to three repeated drought cycles, with the aim to reveal the role of the abscisic acid (ABA) threshold in developing drought tolerance. Differential responses to drought of two genotypes were obtained: more pronounced stomatal closure, ABA biosynthesis and proline accumulation in WT compared to the mutant were compensated by dry weight accumulation accompanied by transient redox disbalance in flacca. Fourier-transform infrared (FTIR) spectra analysis of isolated cell wall material and morphological parameter measurements on tomato leaves indicated changes in dry weight accumulation and carbon re-allocation to cell wall constituents in flacca, but not in WT. A higher proportion of cellulose, pectin and lignin in isolated cell walls from flacca leaves further increased with repeated drought cycles. Different ABA-dependent stomatal closure between drought cycles implies that acquisition of stomatal sensitivity may be a part of stress memory mechanism developed under given conditions. The regulatory role of ABA in the cell wall restructuring and growth regulation under low leaf potential was discussed with emphasis on the beneficial effects of drought priming in developing differential defense strategies against drought.

Metabolic responses of date palm (Phoenix dactylifera L.) leaves to drought differ in summer and winter climate

Drought negatively impacts growth and productivity of plants, particularly in arid and semi-arid regions. Although drought events can take place in summer and winter, differences in the impact of drought on physiological processes between seasons are largely unknown. The aim of this study was to elucidate metabolic strategies of date palms in response to drought in summer and winter season. To identify such differences, we exposed date palm seedlings to a drought-recovery regime, both in simulated summer and winter climate. Leaf hydration, carbon discrimination (${\Delta}$13C), and primary and secondary metabolite composition and contents were analyzed. Depending on season, drought differently affected physiological and biochemical traits of the leaves. In summer, drought induced significantly decreased leaf hydration, concentrations of ascorbate, most sugars, primary and secondary organic acids, as well as phenolic compounds, while thiol, amino acid, raffinose and individual fatty acid contents were increased compared with well-watered plants. In winter, drought had no effect on leaf hydration, ascorbate and fatty acids contents, but resulted in increased foliar thiol and amino acid levels as observed in summer. Compared with winter, foliar traits of plants exposed to drought in summer only partly recovered after re-watering. Memory effects on water relations, and primary and secondary metabolites seem to prepare foliar traits of date palms for repeated drought events in summer. Apparently, a well-orchestrated metabolic network, including the anti-oxidative system, compatible solutes accumulation and osmotic adjustment, and maintenance of cell-membrane stability strongly reduces the susceptibility of date palms to drought. These mechanisms of drought compensation may be more frequently required in summer.

Drought Stress Memory at the Plant Cycle Level: A Review

Plants are sessile organisms whose survival depends on their strategy to cope with dynamic, stressful conditions. It is urgent to improve the ability of crops to adapt to recurrent stresses in order to alleviate the negative impacts on their productivity. Although our knowledge of plant adaptation to drought has been extensively enhanced during the last decades, recent studies have tackled plant responses to recurrent stresses. The present review synthesizes the major findings from studies addressing plant responses to multiple drought events, and demonstrates the ability of plants to memorize drought stress. Stress memory is described as a priming effect allowing a different response to a reiterated stress when compared to a single stress event. Here, by specifically focusing on water stress memory at the plant cycle level, we describe the different underlying processes at the molecular, physiological and morphological levels in crops as well as in the model species Arabidopsis thaliana. Moreover, a conceptual analysis framework is proposed to study drought stress memory. Finally, the essential role of interactions between plants and soil microorganisms is emphasized during reiterated stresses because their plasticity can play a key role in supporting overall plant resilience.

Comparative Transcriptomics of Non-Embryogenic and Embryogenic Callus in Semi-Recalcitrant and Non-Recalcitrant Upland Cotton Lines

Somatic embryogenesis-mediated plant regeneration is essential for the genetic manipulation of agronomically important traits in upland cotton. Genotype specific recalcitrance to regeneration is a primary challenge in deploying genome editing and incorporating useful transgenes into elite cotton germplasm. In this study, transcriptomes of a semi-recalcitrant cotton (Gossypium hirsutum L.) genotype ‘Coker312’ were analyzed at two critical stages of somatic embryogenesis that include non-embryogenic callus (NEC) and embryogenic callus (EC) cells, and the results were compared to a non-recalcitrant genotype ‘Jin668’. We discovered 305 differentially expressed genes in Coker312, whereas, in Jin668, about 6-fold more genes (2155) were differentially expressed. A total of 154 differentially expressed genes were common between the two genotypes. Gene enrichment analysis of the upregulated genes identified functional categories, such as lipid transport, embryo development, regulation of transcription, sugar transport, and vitamin biosynthesis, among others. In Coker312 EC cells, five major transcription factors were highly upregulated: LEAFY COTYLEDON 1 (LEC1), WUS-related homeobox 5 (WOX5), ABSCISIC ACID INSENSITIVE3 (ABI3), FUSCA3 (FUS3), and WRKY2. In Jin668, LEC1, BABY BOOM (BBM), FUS3, and AGAMOUS-LIKE15 (AGL15) were highly expressed in EC cells. We also found that gene expression of these embryogenesis genes was typically higher in Jin668 when compared to Coker312. We conclude that significant differences in the expression of the above genes between Coker312 and Jin668 may be a critical factor affecting the regenerative ability of these genotypes.

Rootstock effects on scion gene expression in maritime pine

Pines are the dominant conifers in Mediterranean forests. As long-lived sessile organisms that seasonally have to cope with drought periods, they have developed a variety of adaptive responses. However, during last decades, highly intense and long-lasting drought events could have contributed to decay and mortality of the most susceptible trees. Among conifer species, Pinus pinaster Ait. shows remarkable ability to adapt to different environments. Previous molecular analysis of a full-sib family designed to study drought response led us to find active transcriptional activity of stress-responding genes even without water deprivation in tolerant genotypes. To improve our knowledge about communication between above- and below-ground organs of maritime pine, we have analyzed four graft-type constructions using two siblings as rootstocks and their progenitors, Gal 1056 and Oria 6, as scions. Transcriptomic profiles of needles from both scions were modified by the rootstock they were grafted on. However, the most significant differential gene expression was observed in drought-sensitive Gal 1056, while in drought-tolerant Oria 6, differential gene expression was very much lower. Furthermore, both scions grafted onto drought-tolerant rootstocks showed activation of genes involved in tolerance to abiotic stress, and is most remarkable in Oria 6 grafts where higher accumulation of transcripts involved in phytohormone action, transcriptional regulation, photosynthesis and signaling has been found. Additionally, processes, such as those related to secondary metabolism, were mainly associated with the scion genotype. This study provides pioneering information about rootstock effects on scion gene expression in conifers.

Coping with low moisture stress: Remembering and responding

Low-moisture stress, also referred to as drought, is one of the major factors that negatively impact the agricultural yield. The present scenario of climate change is expected to aggravate it further. Considering the extended time required to develop resistant crops, it is important to prioritize research efforts for coping with low moisture, prevalent in arid and semi-arid regions of the world. While agricultural yield is a tradeoff between many choices, tolerance to biotic and abiotic stresses comes with yield penalties. To balance the tradeoffs and maximize productivity, the use of region-specific cultivars and/or introgression of precise genetic proportions in an elite variety may prove useful. Stress memory is an emerging approach that helps plants to record and respond to repeated stress in an effective manner. In this context, we discuss the role of "stress memory" in imparting drought tolerance in plants. Future research efforts for its effective deployment for "drought hardening" in agricultural settings, along with a discussion on the yield tradeoff involved, is implicated.

A manipulative interplay between positive and negative regulators of phytohormones: A way forward for improving drought tolerance in plants

Among different abiotic stresses, drought stress is the leading cause of impaired plant growth and low productivity worldwide. It is therefore essential to understand the process of drought tolerance in plants and thus to enhance drought resistance. Accumulating evidence indicates that phytohormones are essential signaling molecules that regulate diverse processes of plant growth and development under drought stress. Plants can often respond to drought stress through a cascade of phytohormones signaling as a means of plant growth regulation. Understanding biosynthesis pathways and regulatory crosstalk involved in these vital compounds could pave the way for improving plant drought tolerance while maintaining overall plant health. In recent years, the identification of phytohormones related key regulatory genes and their manipulation through state-of-the-art genome engineering tools have helped to improve drought tolerance plants. To date, several genes linked to phytohormones signaling networks, biosynthesis, and metabolism have been described as a promising contender for engineering drought tolerance. Recent advances in functional genomics have shown that enhanced expression of positive regulators involved in hormone biosynthesis could better equip plants against drought stress. Similarly, knocking down negative regulators of phytohormone biosynthesis can also be very effective to negate the negative effects of drought on plants. This review explained how manipulating positive and negative regulators of phytohormone signaling could be improvised to develop future crop varieties exhibiting higher drought tolerance. In addition, we also discuss the role of a promising genome editing tool, CRISPR/Cas9, on phytohormone mediated plant growth regulation for tackling drought stress.

In Response to Abiotic Stress, DNA Methylation Confers EpiGenetic Changes in Plants

Epigenetics involves the heritable changes in patterns of gene expression determined by developmental and abiotic stresses, i.e., drought, cold, salinity, trace metals, and heat. Gene expression is driven by changes in DNA bases, histone proteins, the biogenesis of ncRNA, and changes in the nucleotide sequence. To cope with abiotic stresses, plants adopt certain changes driven by a sophisticated biological system. DNA methylation is a primary mechanism for epigenetic variation, which can induce phenotypic alterations in plants under stress. Some of the stress-driven changes in plants are temporary, while some modifications may be stable and inheritable to the next generations to allow them to cope with such extreme stress challenges in the future. In this review, we discuss the pivotal role of epigenetically developed phenotypic characteristics in plants as an evolutionary process participating in adaptation and tolerance responses to abiotic and biotic stresses that alter their growth and development. We emphasize the molecular process underlying changes in DNA methylation, differential variation for different species, the roles of non-coding RNAs in epigenetic modification, techniques for studying DNA methylation, and its role in crop improvement in tolerance to abiotic stress (drought, salinity, and heat). We summarize DNA methylation as a significant future research priority for tailoring crops according to various challenging environmental issues.

Expression of AhATL1, an ABA Transport Factor Gene from Peanut, Is Affected by Altered Memory Gene Expression Patterns and Increased Tolerance to Drought Stress in Arabidopsis

Arachis hypogaea abscisic acid transporter like-1 (AhATL1) modulates abscisic acid (ABA) sensitivity by specifically influencing the importing of ABA into cells, and is a key player in plant stress responses. However, there is limited information on ABA transporters in crops. In this study, we found that the level of AhATL1 expression and AhATL1 distribution increased more rapidly in the second drought (D2) compared with in the first drought (D1). Compared with the first recovery (R1), the AhATL1 expression level and ABA content remained at a higher level during the second recovery (R2). The heterologous overexpression of AhATL1 in Arabidopsis changed the expression pattern of certain memory genes and changed the post response gene type into the memory gene type. Regarding the proline and water content of Col (Arabidopsis thaliana L. Heynh., Col-0), atabcg22, and AhATL1-OX during drought training, the second drought (D2) was more severe than the first drought (D1), which was more conducive to maintaining the cell osmotic balance and resisting drought. In summary, drought stress memory resulted in a rapid increase in the AhATL1 expression and AhATL1 distribution level, and then raised the endogenous ABA content and changed the post response gene type into the memory gene type, which enhanced the drought resistance and recovery ability.

Impacts of Nitrogen Deficiency on Wheat (Triticum aestivum L.) Grain During the Medium Filling Stage: Transcriptomic and Metabolomic Comparisons

Nitrogen (N) supplementation is essential to the yield and quality of bread wheat (Triticum aestivum L.). The impact of N-deficiency on wheat at the seedling stage has been previously reported, but the impact of distinct N regimes applied at the seedling stage with continuous application on filling and maturing wheat grains is lesser known, despite the filling stage being critical for final grain yield and flour quality. Here, we compared phenotype characteristics such as grain yield, grain protein and sugar quality, plant growth, leaf photosynthesis of wheat under N-deficient and N-sufficient conditions imposed prior to sowing (120 kg/hm²) and in the jointing stage (120 kg/hm²), and then evaluated the effects of this continued stress through RNA-seq and GC-MS metabolomics profiling of grain at the mid-filling stage. The results showed that except for an increase in grain size and weight, and in the content of total sugar, starch, and fiber in bran fraction and white flour, the other metrics were all decreased under N-deficiency conditions. A total of 761 differentially expressed genes (DEGs) and 77 differentially accumulated metabolites (DAMs) were identified. Under N-deficiency, 51 down-regulated DEGs were involved in the process of impeding chlorophyll synthesis, chloroplast development, light harvesting, and electron transfer functions of photosystem, which resulted in the SPAD and Pn value decreased by 32 and 15.2% compared with N-sufficiency, inhibited photosynthesis. Twenty-four DEGs implicated the inhibition of amino acids synthesis and protein transport, in agreement with a 17-42% reduction in ornithine, cysteine, aspartate, and tyrosine from metabolome, and an 18.6% reduction in grain protein content. However, 14 DEGs were implicated in promoting sugar accumulation in the cell wall and another six DEGs also enhanced cell wall synthesis, which significantly increased fiber content in the endosperm and likely contributed to increasing the thousands-grain weight (TGW). Moreover, RNA-seq profiling suggested that wheat grain can improve the capacity of DNA repair, iron uptake, disease and abiotic stress resistance, and oxidative stress scavenging through increasing the content levels of anthocyanin, flavonoid, GABA, galactose, and glucose under N-deficiency condition. This study identified candidate genes and metabolites related to low N adaption and tolerance that may provide new insights into a comprehensive understanding of the genotype-specific differences in performance under N-deficiency conditions.

Signaling mechanisms in abscisic acid-mediated stomatal closure

The plant hormone abscisic acid (ABA) plays a central role in the regulation of stomatal movements under water-deficit conditions. The identification of ABA receptors and the ABA signaling core consisting of PYR/PYL/RCAR ABA receptors, PP2C protein phosphatases and SnRK2 protein kinases has led to studies that have greatly advanced our knowledge of the molecular mechanisms mediating ABA-induced stomatal closure in the past decade. This review focuses on recent progress in illuminating the regulatory mechanisms of ABA signal transduction, and the physiological importance of basal ABA signaling in stomatal regulation by CO2 and, as hypothesized here, vapor-pressure deficit. Furthermore, advances in understanding the interactions of ABA and other stomatal signaling pathways are reviewed here. We also review recent studies investigating the use of ABA signaling mechanisms for the manipulation of stomatal conductance and the enhancement of drought tolerance and water-use efficiency of plants.

Transcriptional Regulation of Protein Phosphatase 2C Genes to Modulate Abscisic Acid Signaling

The plant hormone abscisic acid (ABA) triggers cellular tolerance responses to osmotic stress caused by drought and salinity. ABA controls the turgor pressure of guard cells in the plant epidermis, leading to stomatal closure to minimize water loss. However, stomatal apertures open to uptake CO₂ for photosynthesis even under stress conditions. ABA modulates its signaling pathway via negative feedback regulation to maintain plant homeostasis. In the nuclei of guard cells, the clade A type 2C protein phosphatases (PP2Cs) counteract SnRK2 kinases by physical interaction, and thereby inhibit activation of the transcription factors that mediate ABA-responsive gene expression. Under osmotic stress conditions, PP2Cs bind to soluble ABA receptors to capture ABA and release active SnRK2s. Thus, PP2Cs function as a switch at the center of the ABA signaling network. ABA induces the expression of genes encoding repressors or activators of PP2C gene transcription. These regulators mediate the conversion of PP2C chromatins from a repressive to an active state for gene transcription. The stress-induced chromatin remodeling states of ABA-responsive genes could be memorized and transmitted to plant progeny; i.e., transgenerational epigenetic inheritance. This review focuses on the mechanism by which PP2C gene transcription modulates ABA signaling.

Osmotic stress alters circadian cytosolic Ca2+ oscillations and OSCA1 is required in circadian gated stress adaptation

The circadian clock is a universal timing system that involved in plant physical responses to abiotic stresses. Moreover, OSCA1 is an osmosensor responsible for [Ca2+]i increases induced by osmotic stress in plants. However, there is little information on osmosensor involved osmotic stress-triggered circadian clock responses. Using an aequorin-based Ca2+ imaging assay, we found the gradient (0 mM, 200 mM, 500 mM) osmotic stress (induced by sorbitol) both altered the primary circadian parameter of WT and osca1 mutant. This means the plant switch to a fast day/night model to avoid energy consumption. In contrast, the period of WT and osca1 mutant became short since the sorbitol concentration increased from 0 mM to 500 mM. As the sorbitol concentration increased, the phase of the WT becomes more extensive compared with osca1 mutant, which means WT is more capable of coping with the environmental change. Moreover, the amplitude of WT also becomes broader than osca1 mutant, especially in high (500 mM) sorbitol concentration, indicate the WT shows more responses in high osmotic stress. In a word, the WT has much more flexibility to cope with the osmotic stress than osca1 mutant. It implies the OSCA1 might be involved in the circadian gated plant adaptation to the environmental osmotic stress, which opens an avenue to study Ca2+ processes with other circadian signaling pathways.