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Breeding drought resilient crops requires understanding the mechanisms underlying plant physiological responses to different drought patterns and how these vary within species.Two main plant water use strategies are distinguished in the literature, referred to as “risk – taking” (anisohydric) and “conserving” (isohydric). Under well-watered conditions, risk takers exhibit a higher transpiration rate (TR) associated with a greater CO2 assimilation rate, and hence, greater dry matter production than conserving plants which have a tighter stomatal control. Depending on the root traits the transpiration-limiting soil moisture level (θcrit) can differ between plants with similar shoot traits. A high θcrit (e.g. due to shallow roots) entails early stomata closure. This study aimed at examining the drought response of four high-yielding European spring barley cultivars considered to exhibit different response behavior.We collected detailed plant physiological data with a high- throughput functional phenotyping platform (Plantarray®, Plant-Ditech) and analyzed final yield parameters. Around flowering cv. Chanelle (CHAN), RGT Planet (RGT), Formula (FORM), and Baroness (BAR) were exposed to 12 days of drought. Based on higher TR, higher biomass and grain yield under well-watered conditions and the faster TR reduction below θcrit ,CHAN ranked as very risk taking and RGT as risk taking in contrast to conserving FORM and very conserving BAR.Drought effects on final yield are closely linked to the plants recovery potential, i.e. the ability to increase TR to control plant levels upon re-irrigation. The highest yielding cultivar under ample water supply, CHAN, showed a significantly impaired recovery potential and suffered notable yield penalties under drought (24%). The very conserving response behavior of BAR resulted in good recovery, minimal yield loss (-2% yield) and a final grain yield that was almost similar to very risk taking CHAN (Δ 5g/pot). FORM produced the lowest yields under control and stress conditions yet suffered no drought induced yield penalties, probably due to a better adapted root system. The lower θcrit of FORM delayed stomata closure and the breakdown of assimilation. FORM and RGT had similar recovery rates.RGT produced the second highest yield under well-watered conditions and drought did not cause any yield losses. Under ample water supply, RGT behaved like a risk taker, whereby the high TR allowed it to be more productive than the conserving cultivars. RGT switched to a more conserving behavior under drought where it only gradually (same slope as FORM) decreased TR below θcrit and was thereby more productive than very risk taking CHAN. This rather dynamic water use behavior made RGT the best performing cultivar in the here examined drought scenario.A higher number of seeds per spike (along with reduced kernel size) likely contributed to the yield stability observed under drought in FORM and RGT the exact physiological mechanisms of which still require more investigation. Prospective studies will examine different drought patterns and durations and implement the gained knowledge into crop simulation models for upscaling. Keywords: water-response behavior, isohydric; anisohydric; spring barley, drought

Strigolactones (SLs) are a group of plant hormones involved in many aspects of plant development and stress adaptation. Here, we investigated the drought response of a barley (Hordeum vulgare L.) mutant carrying a missense mutation in the gene encoding the SL-specific receptor HvD14. Our results clearly showed that hvd14.d mutant is hyper-sensitive to drought stress. This was illustrated by a lower leaf relative water content (RWC), impaired photosynthesis, disorganization of chloroplast structure, altered stomatal density and slower closure of stomata in response to drought in the mutant compared to the wild type parent cultivar Sebastian. Although the content of abscisic acid (ABA) and its derivatives remained unchanged in the mutant, significant differences in expression of genes related to ABA biosynthesis were observed. Moreover, hvd14.d was insensitive to ABA during seed germination. Analysis of Arabidopsis thaliana mutant atd14-1 also demonstrated that mutation in the SL receptor resulted in increased sensitivity to drought. Our results indicate that the drought-sensitive phenotype of barley SL mutant might be caused by a disturbed ABA metabolism and/or signalling pathways. These results together uncovered a link between SL signalling and ABA-dependent drought stress response in barley.

Many letters of the plant genetic code have been read since the first plant genome was published in 2000 for Arabidopsis. In a recent paper, Marks et al. examined 798 plant genomes and provide a current view of plant genomics in terms of the quality of the assemblies, coupled with their taxonomical and geographical distribution.

Mechanisms involved in the de-acclimation of herbaceous plants caused by warm periods during winter are poorly understood. This study identifies the genes associated with this mechanism in winter barley. Seedlings of eight accessions (four tolerant and four susceptible to de-acclimation cultivars and advanced breeding lines) were cold acclimated for three weeks and de-acclimated at 12 °C/5 °C (day/night) for one week. We performed differential expression analysis using RNA sequencing. In addition, reverse-transcription quantitative real-time PCR and enzyme activity analyses were used to investigate changes in the expression of selected genes. The number of transcripts with accumulation level changed in opposite directions during acclimation and de-acclimation was much lower than the number of transcripts with level changed exclusively during one of these processes. The de-acclimation-susceptible accessions showed changes in the expression of a higher number of functionally diverse genes during de-acclimation. Transcripts associated with stress response, especially oxidoreductases, were the most abundant in this group. The results provide novel evidence for the distinct molecular regulation of cold acclimation and de-acclimation. Upregulation of genes controlling developmental changes, typical for spring de-acclimation, was not observed during mid-winter de-acclimation. Mid-winter de-acclimation seems to be perceived as an opportunity to regenerate after stress. Unfortunately, it is competitive to remain in the cold-acclimated state. This study shows that the response to mid-winter de-acclimation is far more expansive in de-acclimation-susceptible cultivars, suggesting that a reduced response to the rising temperature is crucial for de-acclimation tolerance.

ABA INSENSITIVE 5 (ABI5) is a basic leucine zipper (bZIP) transcription factor which acts in the abscisic acid (ABA) network and is activated in response to abiotic stresses. However, the precise role of barley (Hordeum vulgare) ABI5 in ABA signaling and its function under stress remains elusive. Here, we show that HvABI5 is involved in ABA-dependent regulation of barley response to drought stress. We identified barley TILLING mutants carrying different alleles in the HvABI5 gene and we studied in detail the physiological and molecular response to drought and ABA for one of them. The hvabi5.d mutant, carrying G1751A transition, was insensitive to ABA during seed germination, yet it showed the ability to store more water than its parent cv. "Sebastian" (WT) in response to drought stress. The drought-tolerant phenotype of hvabi5.d was associated with better membrane protection, higher flavonoid content, and faster stomatal closure in the mutant under stress compared to the WT. The microarray transcriptome analysis revealed up-regulation of genes associated with cell protection mechanisms in the mutant. Furthermore, HvABI5 target genes: HVA1 and HVA22 showed higher activity after drought, which may imply better adaptation of hvabi5.d to stress. On the other hand, chlorophyll content in hvabi5.d was lower than in WT, which was associated with decreased photosynthesis efficiency observed in the mutant after drought treatment. To verify that HvABI5 acts in the ABA-dependent manner we analyzed expression of selected genes related to ABA pathway in hvabi5.d and its WT parent after drought and ABA treatments. The expression of key genes involved in ABA metabolism and signaling differed in the mutant and the WT under stress. Drought-induced increase of expression of HvNCED1, HvBG8, HvSnRK2.1, and HvPP2C4 genes was 2-20 times higher in hvabi5.d compared to "Sebastian". We also observed a faster stomatal closure in hvabi5.d and much higher induction of HvNCED1 and HvSnRK2.1 genes after ABA treatment. Together, these findings demonstrate that HvABI5 plays a role in regulation of drought response in barley and suggest that HvABI5 might be engaged in the fine tuning of ABA signaling by a feedback regulation between biosynthetic and signaling events. In addition, they point to different mechanisms of HvABI5 action in regulating drought response and seed germination in barley.

Jasmonates modulate many growth and developmental processes and act as stress hormones that play an important role in plant tolerance to biotic and abiotic stresses. Therefore, there is a need to identify the genes that are regulated through the jasmonate signalling pathway. Aquaporins, and among them the Tonoplast Intrinsic Proteins (TIPs), form the channels in cell membranes that are responsible for the precise regulation of the movement of water and other substrates between cell compartments. We identified the cis-regulatory motifs for the methyl jasmonate (MeJA)-induced genes in the promoter regions of all the HvTIP genes, which are active in barley seedlings, and thus we hypothesised that the HvTIP expression could be a response to jasmonate signalling. In the presented study, we determined the effect of methyl jasmonate on the growth parameters and photosynthesis efficiency of barley seedlings that had been exposed to different doses of MeJA (15–1000 µM × 120 h) in a hydroponic solution. All of the applied MeJA concentrations caused a significant reduction of barley seedling growth, which was most evident in the length of the first leaf sheath and dry leaf weight. The observed decrease of the PSII parameters after the exposure to high doses of MeJA (500 µM or higher) was associated with the downregulation of HvPsbR gene encoding one of the extrinsic proteins of the Oxygen Evolving Complex. The reduced expression of HvPsbR might lead to the impairment of the OEC action, manifested by the occurrence of the K-band in an analysis of fluorescence kinetics after MeJA treatment as well as reduced photosynthesis efficiency. Furthermore, methyl jasmonate treatment caused a decrease in the nitrogen content in barley leaves, which was associated with an increased expression the four tonoplast aquaporin genes (HvTIP1;2, HvTIP2;2, HvTIP4;1 and HvTIP4;2) predicted to transport the nitrogen compounds from the vacuole to the cytosol. The upregulation of the nitrogen-transporting HvTIPs might suggest their involvement in the vacuolar unloading of ammonia and urea, which both could be remobilised when the nitrogen content in the leaves decreases. Our research provides tips on physiological role of the individual TIP subfamily members of aquaporins under methyl jasmonate action.

CBP20 (Cap-Binding Protein 20) encodes a small subunit of nuclear Cap-Binding Complex (nCBC) that together with CBP80 binds mRNA cap. We previously described barley hvcbp20.ab mutant that demonstrated higher leaf water content and faster stomatal closure than the WT after drought stress. Hence, we presumed that the better water-saving mechanism in hvcbp20.ab may result from the lower permeability of epidermis that together with stomata action limit the water evaporation under drought stress. We asked whether hvcbp20.ab exhibited any differences in wax load on the leaf surface when subjected to drought in comparison to WT cv. ‘Sebastian’. To address this question, we investigated epicuticular wax structure and chemical composition under drought stress in hvcbp20.ab mutant and its WT. We showed that hvcbp20.ab mutant exhibited the increased deposition of cuticular wax. Moreover, our gene expression results suggested a role of HvCBP20 as a negative regulator of both, the biosynthesis of waxes at the level of alkane-forming, and waxes transportation. Interestingly, we also observed increased wax deposition in Arabidopsis cbp20 mutant exposed to drought, which allowed us to describe the CBP20-regulated epicuticular wax accumulation under drought stress in a wider evolutionarily context.

The aerial surfaces of land plants have a protective layer of cuticular wax. Alkanes are common components of these waxes, and their abundance is affected by a range of stresses. The CER16 protein has been implicated in alkane biosynthesis in the cuticular wax of Arabidopsis (Arabidopsis thaliana). Here, we identified two new mutant alleles of CER16 in Arabidopsis resulting in production of less wax with dramatically fewer alkanes than the wild type. Map-based cloning with genetic analysis revealed that the cer16 phenotype was caused by complete loss of AT5G44150, encoding a protein with no known domains or motifs. Comparative transcriptomic analysis revealed that transcripts of CER3, previously shown to play a principal role in alkane production, were markedly reduced in the cer16 mutants. To define the relationship between CER3 and CER16, we transformed the full CER3 gene into a cer16 mutant. Transgenic CER3 expression was silenced, and levels of small interfering RNAs targeting CER3 were significantly increased. Mutating two major components of the RNA-silencing machinery in a cer16 genetic background restored CER3 transcript levels to wild-type levels, with the stems restored to wild-type glaucousness. We suggest that CER16 deficiency induces post-transcriptional gene silencing of both endogenous and exogenous expression of CER3.

The study examined photosynthetic efficiency of two barley landraces (cvs. Arabi Abiad and Arabi Aswad) through a prompt fluorescence technique under influence of 14 different abiotic stress factors. The difference in the behavior of photosynthetic parameters under the same stress factor in–between cv. Arabi Abiad and cv. Arabi Aswad indicated different mechanisms of tolerance and strategies for the conversion of light energy into chemical energy for both the landraces. This study confirmed the suitability of some chlorophyll fluorescence parameters as reliable biomarkers for screening the plants at the level of photosynthetic apparatus.

Farnesylation is a post-translational modification that promotes the interaction between the modified signaling protein and membrane lipids and/or other proteins. Farnesyltransferase is the crucial enzyme involved in this process. Strikingly, plant mutants in the ERA1 (Enhanced response to ABA 1) gene, encoding β-subunit of farnesyltransferase, exhibited ABA-hypersensitivity during seed germination and drought tolerance in several species including Arabidopsis, wheat and soybean. However, the mechanism of ERA1 action has not been resolved yet. Here, we present the potential regulatory role of ERA1 in the drought signaling network in barley. With the aim of decoding the role of the ERA1 gene, we developed a unique barley mutant using TILLING analysis. Mutation in HvERA1 confers semi-dwarf phenotype, ABA-sensitivity during seed germination and drought tolerance. Our transcriptomic analysis suggested a role of HvERA1 in regulation of the crosstalk between ABA and ethylene at the onset of drought. Furthermore, analysis of hvera1.b response to prolonged drought stress linked HvERA1 to the metabolism of galactolipids, that build the chloroplast membranes. It might results in the protection of hvera1.b photosystem and thus, in its better photosynthesis performance under water stress. Together, these results indicate the possible mechanism of the primary cause of the observed alterations in the hvera1.b mutant.

Plant nuclear CBC consisted of two subunits (CBP20 and CBP80) is involved in both conserved processes related to RNA metabolism and simultaneously in extremely dynamic plant stress response.

Jasmonates modulate many growth and developmental processes and act as stress hormones that play an important role in plant tolerance to biotic and abiotic stresses. Therefore, there is a need to identify the genes that are regulated through the jasmonate signalling pathway. Aquaporins, and among them the Tonoplast Intrinsic Proteins (TIPs), form the channels in cell membranes that are responsible for the precise regulation of the movement of water and other substrates between cell compartments. We identified the cis-regulatory motifs for the methyl jasmonate (MeJA)-induced genes in the promoter regions of all the HvTIP genes, which are active in barley seedlings, and thus we hypothesised that the HvTIP expression could be a response to jasmonate signalling. In the presented study, we determined the effect of methyl jasmonate on the growth parameters and photosynthesis efficiency of barley seedlings that had been exposed to different doses of MeJA (15&ndash, 1000 &micro, M &times, 120 h) in a hydroponic solution. All of the applied MeJA concentrations caused a significant reduction of barley seedling growth, which was most evident in the length of the first leaf sheath and dry leaf weight. The observed decrease of the PSII parameters after the exposure to high doses of MeJA (500 &micro, M or higher) was associated with the downregulation of HvPsbR gene encoding one of the extrinsic proteins of the Oxygen Evolving Complex. The reduced expression of HvPsbR might lead to the impairment of the OEC action, manifested by the occurrence of the K-band in an analysis of fluorescence kinetics after MeJA treatment as well as reduced photosynthesis efficiency. Furthermore, methyl jasmonate treatment caused a decrease in the nitrogen content in barley leaves, which was associated with an increased expression the four tonoplast aquaporin genes (HvTIP1, 2, HvTIP2, 2, HvTIP4, 1 and HvTIP4, 2) predicted to transport the nitrogen compounds from the vacuole to the cytosol. The upregulation of the nitrogen-transporting HvTIPs might suggest their involvement in the vacuolar unloading of ammonia and urea, which both could be remobilised when the nitrogen content in the leaves decreases. Our research provides tips on physiological role of the individual TIP subfamily members of aquaporins under methyl jasmonate action.

The aim of the research was to compare the impact of short-term deprivation of selected macronutrients (Ca, K, Mg and P) on the photosynthetic characteristics, transpiration and pigment content in maize. The strongest inhibition of photosynthesis was caused by a deprivation of Mg, which was visible as a decrease in the photosynthetic and transpiration rates, stomatal conductance, photosystem II (PSII) performance, chlorophyll and flavonol content with a simultaneously increased content of anthocyanins. In the K-deprived plants, a decrease in the photosynthetic rate was observed. However, the transpiration rate and stomatal conductance did not differ significantly compared with the control. In the K-deprived plants, a decrease in chlorophyll and an increase in the anthocyanin content were also observed. We showed that Ca starvation resulted in a decrease in the photosynthetic and transpiration rates, stomatal conductance and PSII performance, while the pigment content was not significantly different compared with the control. In the case of P-deprived plants, we observed a decrease in the photosynthetic and transpiration rates. Interestingly, the inhibition of stomatal conductance was the strongest in the P-deprived plants compared with all of the investigated elements. However, the performance of PSII was not significantly affected by P starvation compared with the control. Our results present for the first time a comprehensive analysis of the effect of short-term macronutrient deprivation on photosynthesis and transpiration in maize plants.

Balanced photosynthesis under drought is essential for better survival and for agricultural benefits in terms of biomass and yield. Given the current attempts to improve the photosynthetic efficiency for greater crop yield, the explanation of the genetic basis of that process, together with the phenotypic analysis, is significant in terms of both basic studies and potential agricultural application. Therefore, the main objective of this study was to uncover the molecular basis of the photosynthesis process under drought stress in barley. To address that goal, we conducted transcriptomic examination together with detailed photosynthesis analysis using the JIP-test. Using this approach, we indicated that photosynthesis is a process that is very early affected in barley seedlings treated with severe drought stress. Rather than focusing on individual genes, our strategy was pointed to the identification of groups of genes with similar expression patterns. As such, we identified and annotated almost 150 barley genes as crucial core-components of photosystems, electron transport components, and Calvin cycle enzymes. Moreover, we designated 17 possible regulatory interactions between photosynthesis-related genes and transcription factors in barley. Summarizing, our results provide a list of candidate genes for future genetic research and improvement of barley drought tolerance by targeting photosynthesis.

Physiological assays that facilitate screening for various types of responses to abiotic stresses are well established for model plants such as Arabidopsis; however, there is a need to optimize similar tests for cereal crops, including barley. We have developed a set of stress assays to characterize the response of different barley lines during two stages of development-seed germination and seedling growth. The assays presented, including the response to osmotic, salt, oxidative stresses, and exogenously applied abscisic acid, can be used for forward screening of populations after mutagenesis as well as for phenotyping of already isolated mutants, cultivars, or breeding lines. As well as protocols for stress treatments, we also provide methods for plant stress response evaluation, such as chlorophyll a fluorescence (ChlF) and image analysis.

Highlight The root system is the first plant organ that senses a water deficit. Here we have shown evidence that root hairs play a role as sensors of environmental conditions during water stress., An important part of the root system is the root hairs, which play a role in mineral and water uptake. Here, we present an analysis of the transcriptomic response to water deficiency of the wild-type (WT) barley cultivar ‘Karat’ and its root-hairless mutant rhl1.a. A comparison of the transcriptional changes induced by water stress resulted in the identification of genes whose expression was specifically affected in each genotype. At the onset of water stress, more genes were modulated by water shortage in the roots of the WT plants than in the roots of rhl1.a. The roots of the WT plants, but not of rhl1.a, specifically responded with the induction of genes that are related to the abscisic acid biosynthesis, stomatal closure, and cell wall biogenesis, thus indicating the specific activation of processes that are related to water-stress signalling and protection. On the other hand, the processes involved in the further response to abiotic stimuli, including hydrogen peroxide, heat, and high light intensity, were specifically up-regulated in the leaves of rhl1.a. An extended period of severe stress caused more drastic transcriptome changes in the roots and leaves of the rhl1.a mutant than in those of the WT. These results are in agreement with the much stronger damage to photosystem II in the rhl1.a mutant than in its parent cultivar after 10 d of water stress. Taking into account the putative stress sensing and signalling features of the root hair transcriptome, we discuss the role of root hairs as sensors of environmental conditions.

Seed dormancy is one of the most important adaptive mechanisms in plants, which protects seeds from precocious germination in the presence of the inappropriate conditions for growth continuation. Numerous environmental and molecular signals regulate seed dormancy. Maintenance or release of seed dormancy is dependent on light, temperature, and water availability. Precise response of seeds to environmental factors is mediated by different phytohormonal pathways. ABA is considered as a main phytohormone regulating seed dormancy induction and maintenance. ABA‐ and GA‐responsive components, ensure crosstalk between the GA and ABA pathways and enable seed response adequate to the environment. Phytohormonal regulation mechanism of seed dormancy is similar in dicot and monocot plants. Recently, it is suggested that other phytohormones, such as auxin, jasmonates, brassinosteroids, and ethylene, also take part in seed dormancy regulation. Auxin regulators, enhance ABA action and positively influence seed dormancy. However, jasmonates, brassinosteroids, and ethylene reduce seed dormancy level. Here, we describe recent advances in understanding the complex process of seed dormancy regulated by many phytohormonal pathways and their components. Seed dormancy studies can help obtain crop varieties producing seeds with the most desirable timing of germination.

Different abiotic stresses inhibit or delay the development and growth of plants. The most crucial step of plant life cycle, which ensures the survival of the next generation, is seed germination. Plants are sessile organisms that need to integrate internal and external signals in order to produce the correct response. Plants have evolved mechanisms that enable seed germination to be arrested under stress conditions and then resumed when conditions are favorable. The complexity of this mechanism was explored in Arabidopsis thaliana using mutants that had defects in their phytohormone metabolism and signaling pathways. These analyses led to the identification of many important components that are involved in these pathways and shed light on the complex crosstalk between phytohormones under abiotic stress. Combined "omics" techniques such as functional genomics, transcriptomics, and proteomics with the support of bioinformatics, physiology, and molecular genetics have greatly expanded the present understanding of the seed germination process. This minireview focuses on the current status of knowledge about seed germination under abiotic stress with a particular emphasis on genetic interactions, hormonal balance, and epigenetic regulation that occur in Arabidopsis thaliana during this process.

ABA Insensitive 5 (ABI5) is a basic leucine zipper (bZIP) transcription factor that plays a key role in the regulation of seed germination and early seedling growth in the presence of ABA and abiotic stresses. ABI5 functions in the core ABA signaling, which is composed of PYR/PYL/RCAR receptors, PP2C phosphatases and SnRK2 kinases, through the regulation of the expression of genes that contain the ABSCISIC ACID RESPONSE ELEMENT (ABRE) motif within their promoter region. The regulated targets include stress adaptation genes, e.g. LEA proteins. However, the expression and activation of ABI5 is not only dependent on the core ABA signaling. Many transcription factors such as ABI3, ABI4, MYB7 and WRKYs play either a positive or a negative role in the regulation of ABI5 expression. Additionally, the stability and activity of ABI5 are also regulated by other proteins through post-translational modifications such as phosphorylation, ubiquitination, sumoylation and S-nitrosylation. Moreover, ABI5 also acts as an ABA and other phytohormone signaling integrator. Components of auxin, cytokinin, gibberellic acid, jasmonate and brassinosteroid signaling and metabolism pathways were shown to take part in ABI5 regulation and/or to be regulated by ABI5. Monocot orthologues of AtABI5 have been identified. Although their role in the molecular and physiological adaptations during abiotic stress have been elucidated, knowledge about their detailed action still remains elusive. Here, we describe the recent advances in understanding the action of ABI5 in early developmental processes and the adaptation of plants to unfavorable environmental conditions. We also focus on ABI5 relation to other phytohormones in the abiotic stress response of plants.

The exploration of plant response to drought stress is a key to understanding the mechanisms of the drought signaling network and further implementing the knowledge in breeding programs of crops. Plant hormones are crucial factors in transducing the stress signal and the main player among them is abscisic acid (ABA). ABA controls plants’ stress response at many layers of regulation. These include (1) transcriptional response including interactions of core transcription factors that are regulated by ABA and other plant hormones, and (2) regulation of ABA metabolism and transport itself, with posttranscriptional and posttranslational regulation which still seems to be a hidden and not fully recognized part of stress signaling. The efficient integration and coordination of ABA metabolism, transport, and regulation of core signaling elements are pivotal for maintaining tissue and cell-type-specific hormone concentration and thus signaling efficiency, to achieve the proper growth and developmental responses. Among lifecycle processes controlled by ABA are: inhibition of germination, restriction of shoot and root growth, and stomatal closure. Here, we describe recent advances in decoding the ABA signaling message transmitted through plant cells under drought stress that builds drought tolerance.

Two highly specialized cells, the guard cells that surround the stomatal pore, are able to integrate environmental and endogenous signals in order to control the stomatal aperture and thereby the gas exchange. The uptake of CO2 is associated with a loss of water by leaves. Control of the size of the stomatal aperture optimizes the efficiency of water use through dynamic changes in the turgor of the guard cells. The opening and closing of stomata is regulated by the integration of environmental signals and endogenous hormonal stimuli. The various different factors to which the guard cells respond translates into the complexity of the network of signaling pathways that control stomatal movements. The perception of an abiotic stress triggers the activation of signal transduction cascades that interact with or are activated by phytohormones. Among these, abscisic acid (ABA), is the best-known stress hormone that closes the stomata, although other phytohormones, such as jasmonic acid, brassinosteroids, cytokinins, or ethylene are also involved in the stomatal response to stresses. As a part of the drought response, ABA may interact with jasmonic acid and nitric oxide in order to stimulate stomatal closure. In addition, the regulation of gene expression in response to ABA involves genes that are related to ethylene, cytokinins, and auxin signaling. In this paper, recent findings on phytohormone crosstalk, changes in signaling pathways including the expression of specific genes and their impact on modulating stress response through the closing or opening of stomata, together with the highlights of gaps that need to be elucidated in the signaling network of stomatal regulation, are reviewed.

Abscisic acid plays a pivotal role in the abiotic stress response in plants. Although great progress has been achieved explaining the complexity of the stress and ABA signaling cascade, there are still many questions to answer. Mutants are a valuable tool in the identification of new genes or new alleles of already known genes and in elucidating their role in signaling pathways. We applied a suppressor mutation approach in order to find new components of ABA and abiotic stress signaling in Arabidopsis. Using the abh1 (ABA hypersensitive 1) insertional mutant as a parental line for EMS mutagenesis, we selected several mutants with suppressed hypersensitivity to ABA during seed germination. Here, we present the response to ABA and a wide range of abiotic stresses during the seed germination and young seedling development of two suppressor mutants-soa2 (suppressor of abh1 hypersensitivity to ABA 2) and soa3 (suppressor of abh1 hypersensitivity to ABA 3). Generally, both mutants displayed a suppression of the hypersensitivity of abh1 to ABA, NaCl and mannitol during germination. Both mutants showed a higher level of tolerance than Columbia-0 (Col-0-the parental line of abh1) in high concentrations of glucose. Additionally, soa2 exhibited better root growth than Col-0 in the presence of high ABA concentrations. soa2 and soa3 were drought tolerant and both had about 50% fewer stomata per mm2 than the wild-type but the same number as their parental line-abh1. Taking into account that suppressor mutants had the same genetic background as their parental line-abh1, it was necessary to backcross abh1 with Landsberg erecta four times for the map-based cloning approach. Mapping populations, derived from the cross of abh1 in the Landsberg erecta background with each suppressor mutant, were created. Map based cloning in order to identify the suppressor genes is in progress.

‘Drought stress is as complicated and difficult to plant biology as cancer is to mammalian biology’ said Jian-Kang Zhu, a molecular geneticist at the University of California, River‐ side. The capacity of a plant to turn on or turn off a series of genes that further alter plant physiology and morphology allows a plant to tolerate, escape or avoid drought stress. Many countries around the world experience drought stress in different ways but it always leads to a decreased annual yield of crops. Deciphering the basis of the molecular response to stress and the mechanism for the adaptation and acquisition of tolerance can facilitate the creation of cultivars with increased drought tolerance. Drought response is a complex mech‐ anism that has been investigated using a broad spectrum of ‘omics’ techniques, such as mo‐ lecular genetics, functional genomics, transcriptomics, proteomics and metabolomics combined with advanced phenotyping techniques. The response of plants to dehydration stress has been extensively studied in a wide range of species with particular emphasis on model plants such as Arabidopsis. Taking advantage of the knowledge already obtained from Arabidopsis and other model species, it is possible to gain insight into the stress re‐ sponse in crops such as barley or wheat.