Regulation Of Stomatal Aperture Research Articles

Precision phenotyping of a barley diversity set reveals distinct drought response strategies.

Plants exhibit an array of drought responses and adaptations, where the trade-off between water loss and CO2 uptake for growth is mediated by regulation of stomatal aperture in response to soil water content (SWC), among other factors. For crop yield stability, the question is how drought timing and response patterns relate to post-drought growth resilience and vigor. We earlier identified, in a few reference varieties of barley that differed by the SWC at which transpiration was curtailed, two divergent water use strategies: water-saving ("isohydric") and water-spending ("anisohydric"). We proposed that an isohydric strategy may reduce risk from spring droughts in climates where the probability of precipitation increases during the growing season, whereas the anisohydric is consistent with environments having terminal droughts, or with those where dry periods are short and not seasonally progressive. Here, we have examined drought response physiology in an 81-line barley (Hordeum vulgare L.) diversity set that spans 20th century European breeding and identified several lines with a third, dynamic strategy. We found a strong positive correlation between vigor and transpiration, the dynamic group being highest for both. However, these lines curtailed daily transpiration at a higher SWC than the isohydric group. While the dynamic lines, particularly cv Hydrogen and Baronesse, were not the most resilient in terms of restoring initial growth rates, their strong initial vigor and high return to initial transpiration rates meant that their growth nevertheless surpassed more resilient lines during recovery from drought. The results will be of use for defining barley physiological ideotypes suited to future climate scenarios.

Direct Observation and Automated Measurement of Stomatal Responses to Pseudomonas syringae pv. tomato DC3000 in Arabidopsis thaliana.

Stomata are microscopic pores found in the plant leaf epidermis. Regulation of stomatal aperture is pivotal not only for balancing carbon dioxide uptake for photosynthesis and transpirational water loss but also for restricting bacterial invasion. While plants close stomata upon recognition of microbes, pathogenic bacteria, such as Pseudomonas syringae pv. tomato DC3000 (Pto), reopen the closed stomata to gain access into the leaf interior. In conventional assays for assessing stomatal responses to bacterial invasion, leaf epidermal peels, leaf discs, or detached leaves are floated on bacterial suspension, and then stomata are observed under a microscope followed by manual measurement of stomatal aperture. However, these assays are cumbersome and may not reflect stomatal responses to natural bacterial invasion in a leaf attached to the plant. Recently, a portable imaging device was developed that can observe stomata by pinching a leaf without detaching it from the plant, together with a deep learning-based image analysis pipeline designed to automatically measure stomatal aperture from leaf images captured by the device. Here, building on these technical advances, a new method to assess stomatal responses to bacterial invasion in Arabidopsis thaliana is introduced. This method consists of three simple steps: spray inoculation of Pto mimicking natural infection processes, direct observation of stomata on a leaf of the Pto-inoculated plant using the portable imaging device, and automated measurement of stomatal aperture by the image analysis pipeline. This method was successfully used to demonstrate stomatal closure and reopening during Pto invasion under conditions that closely mimic the natural plant-bacteria interaction.

AIM1-dependent high basal salicylic acid accumulation modulates stomatal aperture in rice.

The basal levels of salicylic acid (SA) vary dramatically among plant species. In the shoot, for example, rice contains almost 100 times higher SA levels than Arabidopsis. Despite its high basal levels, neither the biosynthetic pathway nor the biological functions of SA are well understood in rice. Combining with metabolite analysis, physiological, and genetic approaches, we found that the synthesis of basal SA in rice shoot is dependent on OsAIM1, which encodes a beta-oxidation enzyme in the phenylalanine ammonia-lyase (PAL) pathway. Compromised SA accumulation in the Osaim1 mutant led to a lower shoot temperature than wild-type plants. However, this shoot temperature defect resulted from increased transpiration due to elevated steady-state stomatal aperture in the mutant. Furthermore, the high basal SA level is required for sustained expression of OsWRKY45 to modulate the steady-state stomatal aperture and shoot temperature in rice. Taken together, these results provide the direct genetic evidence for the critical role of the PAL pathway in the biosynthesis of high basal level SA in rice, which plays an important role in the regulation of steady-state stomatal aperture to promote fitness under stress conditions.

Kinase CIPK9 integrates glucose and abscisic acid signaling to regulate seed oil metabolism in rapeseed.

Rapeseed (Brassica napus), an important oil crop worldwide, provides large amounts of lipids for human requirements. Calcineurin B-like (CBL)-interacting protein kinase 9 (CIPK9) was reported to regulate seed oil content in the plant. Here, we generated gene-silenced lines through RNA interference biotechnology and loss-of-function mutant bnacipk9 using CRISPR/Cas9 to further study BnaCIPK9 functions in the seed oil metabolism of rapeseeds. We discovered that compared with wild-type (WT) lines, gene-silenced and bnacipk9 lines had substantially different oil contents and fatty acid compositions: seed oil content was improved by 3%-5% and 1%-6% in bnacipk9 lines and gene-silenced lines, respectively; both lines were with increased levels of monounsaturated fatty acids and decreased levels of polyunsaturated fatty acids. Additionally, hormone and glucose content analyses revealed that compared with WT lines the bnacipk9 lines showed significant differences: in bnacipk9 seeds, indoleacetic acid and abscisic acid (ABA) levels were higher; glucose and sucrose contents were higher with a higher hexose-to-sucrose ratio in bnacipk9 mid-to-late maturation development seeds. Furthermore, the bnacipk9 was less sensitive to glucose and ABA than the WT according to stomatal aperture regulation assays and the expression levels of genes involved in glucose and ABA regulating pathways in rapeseeds. Notably, in Arabidopsis (Arabidopsis thaliana), exogenous ABA and glucose imposed on developing seeds revealed the effects of ABA and glucose signaling on seed oil accumulation. Altogether, our results strongly suggest a role of CIPK9 in mediating the interaction between glucose flux and ABA hormone signaling to regulate seed oil metabolism in rapeseed.

Wheat (Triticum aestivum. L) Plant U-box E3 ligases TaPUB2 and TaPUB3 enhance ABA response and salt stress resistance in Arabidopsis.

Plant U-box E3 ligases (PUBs) are important regulators of responses to various abiotic stress conditions. In this study, we found that wheat (Triticum aestivum. L) PUBs TaPUB2 and TaPUB3 enhanced abscisic acid (ABA) responses and salt tolerance in Arabidopsis. We generated transgenic Arabidopsis lines overexpressing TaPUB2 and TaPUB3 and performed various plant physiological experiments. Overexpression of TaPUB2 and TaPUB3 increased tolerance to salinity stress in an ABA-dependent manner in transgenic plants, as evidenced by germination and survival rates, root length, stomatal aperture regulation, membrane peroxidation, photosynthetic activities, reactive oxygen species scavenging activities and expression of various ABA and salinity stress-related genes. These results demonstrate the functions of PUBs under ABA and salinity stress conditions and provide valuable information for the development of salinity stress-tolerant crop species.

Divergent Roles of CNGC2 and CNGC4 in the Regulation of Disease Resistance, Plant Growth and Heat Tolerance in Arabidopsis

Arabidopsis cyclic nucleotide-gated channels (CNGC) 2 and 4 are shown to negatively regulate disease resistance and heat tolerance and to positively regulate plant growth. Whether or not their functions in these processes are interdependent is largely unknown. Here, using the mutation of phytoalexin deficient 4 (PAD4) to inhibit the enhanced defense response and programmed cell death (PCD), we assessed the contribution of the altered defense response to the heat tolerance and plant growth in the cngc2 and cngc4 single and double mutants. The pad4 mutation reverted the enhanced disease resistance of the cngc2 and cngc4 mutants at the normal temperature (22 °C) but not at the elevated temperature (28 °C). The pad4 mutation slightly alleviated the dwarfism of the cngc2 and cngc4 mutants at 22 °C but not at 28 °C, indicating a small contribution from the defense response to plant growth regulation. The pad4 mutation also reduced the enhanced heat tolerance in the cngc mutants, suggesting an involvement of PCD in heat tolerance. In addition, a higher heat tolerance was correlated with more opened stomata under heat treatment among the wild type and mutants of the PAD4, CNGC2 and CNGC4 genes. In sum, this study suggests that the regulation of heat tolerance and plant growth by CNGC2 and CNGC4 is almost independent of their regulation of disease resistance. It also reveals a PAD4-dependent role of CNGC2 and CNGC4 in stomatal aperture regulation and heat tolerance.

Effects of exogenous calcium on the drought response of the tea plant (Camellia sinensis (L.) Kuntze).

Drought is one of the major factors reducing the yield of many crops worldwide, including the tea crop (Camellia sinensis (L.) Kuntze). Calcium participates in most of cellular signaling processes, and its important role in stress detection and triggering a response has been shown in many crops. The aim of this study was to evaluate possible effects of calcium on the tea plant response to drought. Experiments were conducted using 3-year-old potted tea plants of the best local cultivar Kolkhida. Application of ammonium nitrate (control treatment) or calcium nitrate (Ca treatment) to the soil was performed before drought induction. Next, a 7-day drought was induced in both groups of plants. The following physiological parameters were measured: relative electrical conductivity, pH of cell sap, and concentrations of cations, sugars, and amino acids. In addition, relative expression levels of 40 stress-related and crop quality-related genes were analyzed. Under drought stress, leaf electrolyte leakage differed significantly, indicating greater damage to cell membranes in control plants than in Ca-treated plants. Calcium application resulted in greater pH of cell sap; higher accumulation of tyrosine, methionine, and valine; and a greater Mg2+ content as compared to control plants. Drought stress downregulated most of the quality-related genes in both groups of tea plants. By contrast, significant upregulation of some genes was observed, namely CRK45, NAC26, TPS11, LOX1, LOX6, Hydrolase22, DREB26, SWEET2, GS, ADC, DHN2, GOLS1, GOLS3, and RHL41. Among them, three genes (LOX1, RHL41, and GOLS1) showed 2-3 times greater expression in Ca-treated plants than in control plants. Based on these results, it can be speculated that calcium affects galactinol biosynthesis and participates in the regulation of stomatal aperture not only through activation of abscisic-acid signaling but also through jasmonic-acid pathway activation. These findings clarify calcium-mediated mechanisms of drought defense in tree crops. Thus, calcium improves the drought response in the tea tree.

Improved abiotic stress tolerance in Arabidopsis by constitutive active form of a banana DREB2 type transcription factor, MaDREB20.CA, than its native form, MaDREB20.

Banana is grown as one of the important fruit crops in tropical and subtropical regions of the world. In this study, we report induced expression of a dehydration responsive element binding 2 (DREB2) gene (MaDREB20) under individual heat, drought, and combined drought and heat stress in root of two banana genotypes Grand Nain (GN) and Hill Banana (HB). Motif analysis of MaDREB20 protein demonstrated the presence of a negative regulatory domain (NRD) or PEST motif between 150 and 184 amino acids. Transgenic Arabidopsis overexpressing MaDREB20 gene showed more survival rate, above-ground biomass, seed yield, leaf relative water content, and proline content but less ion leakage and malonaldehyde content, revealing improved tolerance against heat and drought as well as their combination than the wild-type. Overexpression of MaDREB20.CA (constitutive active form of MaDREB20 after removal of PEST region) showed better abiotic stress tolerance in Arabidopsis than its native form (MaDREB20). Transgenic Arabidopsis overexpressing MaDREB20 and MaDREB20.CA genes appeared to be associated with reduced stomatal densities under normal condition, better regulation of stomatal aperture under drought than in wild-type plants, and differential regulation of downstream target (AtTCH4 and AtIAA1) genes under heat, drought, and combined stress. Taken together, our findings revealed important functions of MaDREB20 in abiotic stress responses in transgenic Arabidopsis and could form a basis for CRISPR/Cas9-mediated removal of its NRD to enhance stress tolerance in banana.

Phytochrome interacting factor regulates stomatal aperture by coordinating red light and abscisic acid.

Stomata are crucial valves coordinating the fixation of carbon dioxide by photosynthesis and water loss through leaf transpiration. Phytochrome interacting factors (PIFs) are negative regulators of red light responses that belong to the basic helix-loop-helix family of transcription factors. Here, we show that the rice (Oryza sativa) PIF family gene OsPIL15 acts as a negative regulator of stomatal aperture to control transpiration in rice. OsPIL15 reduces stomatal aperture by activating rice ABSCISIC ACID INSENSITIVE 5 (OsABI5), which encodes a critical positive regulator of ABSCISIC ACID (ABA) signaling in rice. Moreover, OsPIL15 interacts with the NIGT1/HRS1/HHOfamily transcription factor rice HRS1 HOMOLOG 3 (OsHHO3) to possibly enhance the regulation of stomatal aperture. Notably, we discovered that the maize (Zea mays) PIF family genes ZmPIF1 and ZmPIF3, which are homologous to OsPIL15, are also involved in the regulation of stomatal aperture in maize, indicating that PIF-mediated regulation of stomatal aperture may be conserved in the plant lineage. Our findings explain the molecular mechanism by which PIFs play a role in red-light-mediated stomatal opening, and demonstrate that PIFs regulate stomatal aperture by coordinating the red light and ABA signaling pathways.

Overexpression of MdMIPS1 enhances drought tolerance and water-use efficiency in apple

Myo-inositol and its derivatives play important roles in the tolerance of higher plants to abiotic stresses, and myo-inositol-1-phosphate synthase (MIPS) is the rate-limiting enzyme in myo-inositol biosynthesis. In this study, we found that increased myo-inositol biosynthesis enhanced drought tolerance in MdMIPS1-overexpressing apple lines under short-term progressive drought stress. The effect of myo-inositol appeared to be mediated by the increased accumulation of osmoprotectants such as glucose, sucrose, and proline, and by the increased activities of antioxidant enzymes that eliminate reactive oxygen species. Moreover, enhanced water-use efficiency (WUE) was observed in MdMIPS1-overexpressing apple lines under long-term moderate water deficit conditions that mimicked the water availability in the soil of the Loess Plateau. Enhanced WUE may have been associated with the synergistic regulation of osmotic balance and stomatal aperture mediated by increased myo-inositol biosynthesis. Taken together, our findings shed light on the positive effects of MdMIPS1-mediated myo-inositol biosynthesis on drought tolerance and WUE in apple.

The CaPti1-CaERF3 module positively regulates resistance of Capsicum annuum to bacterial wilt disease by coupling enhanced immunity and dehydration tolerance.

Bacterial wilt, a severe disease involving vascular system blockade, is caused by Ralstonia solanacearum. Although both plant immunity and dehydration tolerance might contribute to disease resistance, whether and how they are related remains unclear. Herein, we showed that immunity against R. solanacearum and dehydration tolerance are coupled and regulated by the CaPti1-CaERF3 module. CaPti1 and CaERF3 are members of the serine/threonine protein kinase and ethylene-responsive factor families, respectively. Expression profiling revealed that CaPti1 and CaERF3 were upregulated by R. solanacearum inoculation, dehydration stress, and exogenously applied abscisic acid (ABA). They in turn phenocopied each other in promoting resistance of pepper (Capsicum annuum) to bacterial wilt not only by activating salicylic acid-dependent CaPR1, but also by activating dehydration tolerance-related CaOSM1 and CaOSR1 and inducing stomatal closure to reduce water loss in an ABA signaling-dependent manner. Our yeast two hybrid assay showed that CaERF3 interacted with CaPti1, which was confirmed using co-immunoprecipitation, bimolecular fluorescence complementation, and pull-down assays. Chromatin immunoprecipitation and electrophoretic mobility shift assays showed that upon R. solanacearum inoculation, CaPR1, CaOSM1, and CaOSR1 were directly targeted and positively regulated by CaERF3 and potentiated by CaPti1. Additionally, our data indicated that the CaPti1-CaERF3 complex might act downstream of ABA signaling, as exogenously applied ABA did not alter regulation of stomatal aperture by the CaPti1-CaERF3 module. Importantly, the CaPti1-CaERF3 module positively affected pepper growth and the response to dehydration stress. Collectively, the results suggested that immunity and dehydration tolerance are coupled and positively regulated by CaPti1-CaERF3 in pepper plants to enhance resistance against R. solanacearum.

Structure of the Arabidopsis guard cell anion channel SLAC1 suggests activation mechanism by phosphorylation.

Stomata play a critical role in the regulation of gas exchange and photosynthesis in plants. Stomatal closure participates in multiple stress responses, and is regulated by a complex network including abscisic acid (ABA) signaling and ion-flux-induced turgor changes. The slow-type anion channel SLAC1 has been identified to be a central controller of stomatal closure and phosphoactivated by several kinases. Here, we report the structure of SLAC1 in Arabidopsis thaliana (AtSLAC1) in an inactivated, closed state. The cytosolic amino (N)-terminus and carboxyl (C)-terminus of AtSLAC1 are partially resolved and form a plug-like structure which packs against the transmembrane domain (TMD). Breaking the interactions between the cytosolic plug and transmembrane domain triggers channel activation. An inhibition-release model is proposed for SLAC1 activation by phosphorylation that the cytosolic plug dissociates from the transmembrane domain upon phosphorylation, and induces conformational changes to open the pore. These findings facilitate our understanding of the regulation of SLAC1 activity and stomatal aperture in plants.

A Transcriptome Analysis Revealing the New Insight of Green Light on Tomato Plant Growth and Drought Stress Tolerance.

Light plays a pivotal role in plant growth, development, and stress responses. Green light has been reported to enhance plant drought tolerance via stomatal regulation. However, the mechanisms of green light-induced drought tolerance in plants remain elusive. To uncover those mechanisms, we investigated the molecular responses of tomato plants under monochromatic red, blue, and green light spectrum with drought and well-water conditions using a comparative transcriptomic approach. The results showed that compared with monochromatic red and blue light treated plants, green light alleviated the drought-induced inhibition of plant growth and photosynthetic capacity, and induced lower stomatal aperture and higher ABA accumulation in tomato leaves after 9 days of drought stress. A total of 3,850 differentially expressed genes (DEGs) was identified in tomato leaves through pairwise comparisons. Functional annotations revealed that those DEGs responses to green light under drought stress were enriched in plant hormone signal transduction, phototransduction, and calcium signaling pathway. The DEGs involved in ABA synthesis and ABA signal transduction both participated in the green light-induced drought tolerance of tomato plants. Compared with ABA signal transduction, more DEGs related to ABA synthesis were detected under different light spectral treatments. The bZIP transcription factor- HY5 was found to play a vital role in green light-induced drought responses. Furthermore, other transcription factors, including WRKY46 and WRKY81 might participate in the regulation of stomatal aperture and ABA accumulation under green light. Taken together, the results of this study might expand our understanding of green light-modulated tomato drought tolerance via regulating ABA accumulation and stomatal aperture.

Identification of Genes Preferentially Expressed in Stomatal Guard Cells of Arabidopsis thaliana and Involvement of the Aluminum-Activated Malate Transporter 6 Vacuolar Malate Channel in Stomatal Opening.

Stomatal guard cells (GCs) are highly specialized cells that respond to various stimuli, such as blue light (BL) and abscisic acid, for the regulation of stomatal aperture. Many signaling components that are involved in the stomatal movement are preferentially expressed in GCs. In this study, we identified four new such genes in addition to an aluminum-activated malate transporter, ALMT6, and GDSL lipase, Occlusion of Stomatal Pore 1 (OSP1), based on the expression analysis using public resources, reverse transcription PCR, and promoter-driven β-glucuronidase assays. Some null mutants of GC-specific genes evidenced altered stomatal movement. We further investigated the role played by ALMT6, a vacuolar malate channel, in stomatal opening. Epidermal strips from an ALMT6-null mutant exhibited defective stomatal opening induced by BL and fusicoccin, a strong plasma membrane H+-ATPase activator. The deficiency was enhanced when the assay buffer [Cl–] was low, suggesting that malate and/or Cl– facilitate efficient opening. The results indicate that the GC-specific genes are frequently involved in stomatal movement. Further detailed analyses of the hitherto uncharacterized GC-specific genes will provide new insights into stomatal regulation.

Populus euphratica Apyrases Increase Drought Tolerance by Modulating Stomatal Aperture in Arabidopsis.

Stomatal regulation is crucial to reduce water consumption under drought conditions. Extracellular ATP (eATP) serves as a signaling agent in stomatal regulation; however, it is less known whether the eATP mediation of stomatal aperture is linked to apyrases (APYs), the principal enzymes that control the concentration of eATP. To clarify the role of APYs in stomatal control, PeAPY1 and PeAPY2 were isolated from Populus euphratica and transferred into Arabidopsis. Compared with the wild-type Arabidopsis and loss-of-function mutants (Atapy1 and Atapy2), PeAPY1- and PeAPY2-transgenic plants decreased stomatal aperture under mannitol treatment (200 mM, 2 h) and reduced water loss during air exposure (90 min). The role of apyrase in stomatal regulation resulted from its control in eATP-regulated stomatal movements and increased stomatal sensitivity to ABA. The bi-phasic dose-responses to applied nucleotides, i.e., the low ATP (0.3–1.0 mM)-promoted opening and high ATP (>2.0 mM)-promoted closure, were both restricted by P. euphratica apyrases. It is noteworthy that eATP at a low concentration (0.3 mM) counteracted ABA action in the regulation of stomatal aperture, while overexpression of PeAPY1 or PeAPY2 effectively diminished eATP promotion in opening, and consequently enhanced ABA action in closure. We postulate a speculative model of apyrase signaling in eATP- and ABA-regulated stomatal movements under drought.

Pepper ubiquitin-specific protease, CaUBP12, positively modulates dehydration resistance by enhancing CaSnRK2.6 stability.

Abscisic acid (ABA) is a plant hormone that activates adaptive mechanisms to environmental stress conditions. Plant adaptive mechanisms are complex and highly modulated processes induced by stress-responsive proteins; however, the precise mechanisms by which these processes function under adverse conditions remain unclear. Here, we isolated CaUBP12 (Capsicum annuum ubiquitin-specific protease 12) from pepper (C. annuum) leaves. We show that CaUBP12 expression is significantly induced after exposure to abiotic stress treatments. We conducted loss-of-function and gain-of-function genetic studies to elucidate the biological functions of CaUBP12 in response to ABA and dehydration stress. CaUBP12-silenced pepper plants and CaUBP12-overexpressing Arabidopsis plants displayed dehydration-sensitive and dehydration-tolerant phenotypes, respectively; these phenotypes were characterized by regulation of transpirational water loss and stomatal aperture. Under dehydration stress conditions, CaUBP12-silenced pepper plants and CaUBP12-overexpressing Arabidopsis plants exhibited lower and higher expression levels of stress-related genes, respectively, than the control plants. We isolated a CaUBP12 interaction protein, CaSnRK2.6, which is a homolog of Arabidopsis OST1; degradation of this protein was partially inhibited by CaUBP12. Similar to CaUBP12-silenced pepper plants and CaUBP12-overexpressing Arabidopsis plants, CaSnRK2.6-silenced pepper plants and CaSnRK2.6-overexpressing Arabidopsis displayed dehydration-sensitive and dehydration-tolerant phenotypes, respectively. Our findings suggest that CaUBP12 positively modulates the dehydration stress response by suppressing CaSnRK2.6 protein degradation.

Integrating stomatal physiology and morphology: evolution of stomatal control and development of future crops

Stomata are central players in the hydrological and carbon cycles, regulating the uptake of carbon dioxide (CO2) for photosynthesis and transpirative loss of water (H2O) between plants and the atmosphere. The necessity to balance water-loss and CO2-uptake has played a key role in the evolution of plants, and is increasingly important in a hotter and drier world. The conductance of CO2 and water vapour across the leaf surface is determined by epidermal and stomatal morphology (the number, size, and spacing of stomatal pores) and stomatal physiology (the regulation of stomatal pore aperture in response to environmental conditions). The proportion of the epidermis allocated to stomata and the evolution of amphistomaty are linked to the physiological function of stomata. Moreover, the relationship between stomatal density and [CO2] is mediated by physiological stomatal behaviour; species with less responsive stomata to light and [CO2] are most likely to adjust stomatal initiation. These differences in the sensitivity of the stomatal density—[CO2] relationship between species influence the efficacy of the ‘stomatal method’ that is widely used to infer the palaeo-atmospheric [CO2] in which fossil leaves developed. Many studies have investigated stomatal physiology or morphology in isolation, which may result in the loss of the ‘overall picture’ as these traits operate in a coordinated manner to produce distinct mechanisms for stomatal control. Consideration of the interaction between stomatal morphology and physiology is critical to our understanding of plant evolutionary history, plant responses to on-going climate change and the production of more efficient and climate-resilient food and bio-fuel crops.

Anisohydric sugar beet rapidly responds to light to optimize leaf water use efficiency utilizing numerous small stomata.

Under conditions of high transpiration and low soil water availability, the demand for water can exceed supply causing a reduction in water potential and a loss of cell turgor (wilting). Regulation of stomatal aperture mediates the loss of water vapour (gs), which in turn is dependent in part on the anatomical characteristics of stomatal density (SD) and stomatal size (SS). Anisohydric sugar beet (Beta vulgaris) is atypical, exhibiting wilting under high soil water availability. Spinach (Spinacia oleracea) belongs to the same family Chenopodiaceae s.s., but demonstrates a more typical wilting response. To investigate the role of stomatal dynamics in such behaviours, sugar beet and spinach leaves were exposed to step-changes in photosynthetic photon flux density (PPFD) from 250 to 2500 µmol m−2 s−1. Using a four log-logistic function, the maximum rate of stomatal opening was estimated. Concurrent measurements of SD and SS were taken for both species. While sugar beet coupled faster opening with smaller, more numerous stomata, spinach showed the converse. After exposure to drought, maximum gs was reduced in sugar beet but still achieved a similar speed of opening. It is concluded that sugar beet stomata respond rapidly to changes in PPFD with a high rate and magnitude of opening under both non-droughted and droughted conditions. Such a response may contribute to wilting, even under high soil water availability, but enables photosynthesis to be better coupled with increasing PPFD.

The bZIP Transcription Factor GmbZIP15 Negatively Regulates Salt- and Drought-Stress Responses in Soybean.

Soybean (Glycine max), as an important oilseed crop, is constantly threatened by abiotic stress, including that caused by salinity and drought. bZIP transcription factors (TFs) are one of the largest TF families and have been shown to be associated with various environmental-stress tolerances among species; however, their function in abiotic-stress response in soybean remains poorly understood. Here, we characterized the roles of soybean transcription factor GmbZIP15 in response to abiotic stresses. The transcript level of GmbZIP15 was suppressed under salt- and drought-stress conditions. Overexpression of GmbZIP15 in soybean resulted in hypersensitivity to abiotic stress compared with wild-type (WT) plants, which was associated with lower transcript levels of stress-responsive genes involved in both abscisic acid (ABA)-dependent and ABA-independent pathways, defective stomatal aperture regulation, and reduced antioxidant enzyme activities. Furthermore, plants expressing a functional repressor form of GmbZIP15 exhibited drought-stress resistance similar to WT. RNA-seq and qRT-PCR analyses revealed that GmbZIP15 positively regulates GmSAHH1 expression and negatively regulates GmWRKY12 and GmABF1 expression in response to abiotic stress. Overall, these data indicate that GmbZIP15 functions as a negative regulator in response to salt and drought stresses.

Atmospheric H2S exposure does not affect stomatal aperture in maize

Main conclusionStomatal aperture in maize is not affected by exposure to a subtoxic concentration of atmospheric H2S. At least in maize, H2S, thus, is not a gaseous signal molecule that controls stomatal aperture.Sulfur is an indispensable element for the physiological functioning of plants with hydrogen sulfide (H2S) potentially acting as gasotransmitter in the regulation of stomatal aperture. It is often assumed that H2S is metabolized into cysteine to stimulate stomatal closure. To study the significance of H2S for the regulation of stomatal closure, maize was exposed to a subtoxic atmospheric H2S level in the presence or absence of a sulfate supply to the root. Similar to other plants, maize could use H2S as a sulfur source for growth. Whereas sulfate-deprived plants had a lower biomass than sulfate-sufficient plants, exposure to H2S alleviated this growth reduction. Shoot sulfate, glutathione, and cysteine levels were significantly higher in H2S-fumigated plants compared to non-fumigated plants. Nevertheless, this was not associated with changes in the leaf area, stomatal density, stomatal resistance, and transpiration rate of plants, meaning that H2S exposure did not affect the transpiration rate per stoma. Hence, it did not affect stomatal aperture, indicating that, at least in maize, H2S is not a gaseous signal molecule controlling this aperture.