AREB1 is a transcription activator of novel ABRE-dependent ABA signaling that enhances drought stress tolerance in Arabidopsis

Wheat TabZIP8, 9, 13 participate in ABA biosynthesis in NaCl-stressed roots regulated by TaCDPK9-1

In plants, basic leucine zipper (bZIP) transcription factors (TFs) participate in various biological processes such as development and stress responses. But the molecular mechanism of wheat bZIP TFs modulating abscisic acid (ABA) biosynthesis is unknown. In this study, we demonstrated the expressions of three bZIP TF genes TabZIP8, 9, 13, were regulated by Triticum aestivum calcium (Ca²⁺)-dependent protein kinase 9-1 (TaCDPK9-1) and they took part in ABA biosynthesis in wheat roots under salt stress. We first isolated TabZIP8, 9, 13 and TaCDPK9-1 from wheat. TabZIP8, 9, 13 genes transcripts were strongly induced by salt stress, but salt-induced TabZIP8, 9, 13 transcriptions were drastically impaired by Ca²⁺ channel blocker LaCl₃. TaCDPK9-1 kinase could interact with TabZIP8, 9, 13 TFs through yeast two-hybrid assay. Next, the expression levels of salt-induced wheat 9-cis-epoxycarotenoid dioxygenase1, 2 (TaNCED1,2) encoding a key enzyme controlling ABA production and salt-induced ABA content were found to be decreased under LaCl₃ treatment, and yeast one-hybrid experiment revealed TabZIP8, 9, 13 could bind to the ABA-responsive elements (ABREs) and the promoter sequence of TaNCED2 gene. Together, our results suggest that salt stress-induced ABA accumulation is mediated by TabZIP8, 9, 13, which are adjusted by TaCDPK9-1 in wheat roots.

Genome-Wide Analysis, Characterization, and Expression Profile of the Basic Leucine Zipper Transcription Factor Family in Pineapple

This study identified 57 basic leucine zipper (bZIP) genes from the pineapple genome, and the analysis of these bZIP genes was focused on the evolution and divergence after multiple duplication events in relation to the pineapple genome fusion. According to bioinformatics analysis of a phylogenetic tree, the bZIP gene family was divided into 11 subgroups in pineapple, Arabidopsis, and rice; gene structure and conserved motif analyses showed that bZIP genes within the same subgroup shared similar intron-exon organizations and motif composition. Further synteny analysis showed 17 segmental duplication events with 27 bZIP genes. The study also analyzed the pineapple gene expression of bZIP genes in different tissues, organs, and developmental stages, as well as in abiotic stress responses. The RNA-sequencing data showed that AcobZIP57 was upregulated in all tissues, including vegetative and reproductive tissues. AcobZIP28 and AcobZIP43 together with the other 25 bZIP genes did not show high expression levels in any tissue. Six bZIP genes were exposed to abiotic stress, and the relative expression levels were detected by quantitative real-time PCR. A significant response was observed for AcobZIP24 against all kinds of abiotic stresses at 24 and 48 h in pineapple root tissues. Our study provides a perspective for the evolutionary history and general biological involvement of the bZIP gene family of pineapple, which laid the foundation for future functional characterization of the bZIP genes in pineapple.

Over-expression of a γ-tocopherol methyltransferase gene in vitamin E pathway confers PEG-simulated drought tolerance in alfalfa

BACKGROUND

α-Tocopherol is one of the most important vitamin E components present in plant. α-Tocopherol is a potent antioxidant, which can deactivate photoproduced reactive oxygen species (ROS) and prevent lipids from oxidation when plants suffer drought stress. γ-Tocopherol methyltransferase (γ-TMT) catalyzes the formation of α-tocopherol in the tocopherol biosynthetic pathway. Our previous studies showed that over-expression of γ-TMT gene can increase the accumulation of α-tocopherol in alfalfa (Medicago sativa). However, whether these transgenic plants confer increased drought tolerance and the underlying mechanism are still unknown.

RESULTS

In the present study, we further evaluate transgenic alfalfa lines, and found that over-expression of MsTMT led to an increase in α-tocopherol and total tocopherol level in the transgenic lines compared with the control plant. It was revealed that drought tolerance of the transgenic alfalfa was remarkably increased, with alleviated oxidative damage and accumulation of more osmolytic substances. The stomatal development in transgenic plants was significantly inhibited on both sides of leaves, which may be resulted from the repression of MsSPCHLESS (MsSPCH) gene. The reduced stomatal density of transgenic plants contributes to a lower stomatal conductance and higher water use efficiency (WUE). Moreover, both RNA-seq and qRT-PCR analyses indicate that regulatory mechanism of MsTMT in drought involved in both ABA-dependent and ABA-independent pathways.

CONCLUSION

Our results suggest that MsTMT gene plays a positive role in regulating alfalfa response to PEG-simulated drought stress, which might involve complex mechanisms, including ROS scavenging system, stomatal development and multiple phytohormone signaling pathways. This study will broaden our view on the function of γ-TMT gene and provide new strategy for genetic engineering in alfalfa breeding.

Roles of the Brassica napus DELLA Protein BnaA6.RGA, in Modulating Drought Tolerance by Interacting With the ABA Signaling Component BnaA10.ABF2

Drought is a major threat to plant growth and crop productivity. Reduced level of the gibberellin would result in increased drought tolerance, but the underlying mechanism is still unclear. In Brassica napus, there are four BnaRGA genes that code for DELLA proteins, negative regulators of GA signaling. Among them, expression of BnaA6.RGA was greatly induced by drought and abscisic acid (ABA). Previously, we created the gain-of-function mutant of BnaA6.RGA, bnaa6.rga-D, and the loss-of-function quadruple mutant, bnarga by CRISPR/Cas9, respectively. Here we show that bnaa6.rga-D displayed enhanced drought tolerance, and its stomatal closure was hypersensitive to ABA treatment. By contrast, bnarga displayed reduced drought tolerance and was less sensitive to ABA treatment, but there is no difference in drought tolerance between single BnaRGA mutant and WT, suggesting a functional redundancy between the BnaRGA genes in this process. Furthermore, we found that BnaRGAs were able to interact physically with BnaA10.ABF2, an essential transcription factor in ABA signaling. The BnaA10.ABF2-BnaA6.RGA protein complex greatly increased the expression level of the drought responsive gene BnaC9.RAB18. Taken together, this work highlighted the fundamental roles of DELLA proteins in drought tolerance in B. napus, and provide desirable germplasm for further breeding of drought tolerance in rapeseed.

Early Drought-Responsive Genes Are Variable and Relevant to Drought Tolerance

Drought stress is an important crop yield limiting factor worldwide. Plant physiological responses to drought stress are driven by changes in gene expression. While drought-responsive genes (DRGs) have been identified in maize, regulation patterns of gene expression during progressive water deficits remain to be elucidated. In this study, we generated time-series transcriptomic data from the maize inbred line B73 under well-watered and drought conditions. Comparisons between the two conditions identified 8,626 DRGs and the stages (early, middle, and late drought) at which DRGs occurred. Different functional groups of genes were regulated at the three stages. Specifically, early and middle DRGs display higher copy number variation among diverse Zea mays lines, and they exhibited stronger associations with drought tolerance as compared to late DRGs. In addition, correlation of expression between small RNAs (sRNAs) and DRGs from the same samples identified 201 negatively sRNA/DRG correlated pairs, including genes showing high levels of association with drought tolerance, such as two glutamine synthetase genes, gln2 and gln6 The characterization of dynamic gene responses to progressive drought stresses indicates important adaptive roles of early and middle DRGs, as well as roles played by sRNAs in gene expression regulation upon drought stress.

Overexpression of CsSnRK2.5 increases tolerance to drought stress in transgenic Arabidopsis

Drought is a major factor limiting crop productivity and quality. Sucrose non-fermenting-1 (SNF1)-related protein kinase 2s (SnRK2s) play critical roles in plant abiotic stress responses, especially in drought stress. However, knowledge regarding the functional roles of SnRK2s in drought stress and their underlying mechanisms is relatively limited in tea plant. In this study, CsSnRK2.5, a PEG 6000- and ABA-induced SnRK2 gene from tea plant, was overexpressed in Arabidopsis to investigate its potential function in drought stress response. The results showed that overexpression of CsSnRK2.5 resulted in enhanced drought tolerance, as indicated by an amelioration of the changes in various physiological indexes, including a decreased rate of water loss and decreased accumulation of ROS and MDA. In addition, CsSnRK2.5 overexpression conferred hypersensitivity to exogenous ABA, and transgenic plants exhibited improved ABA-mediated stomatal closure compared to WT plants. Moreover, the expression of some stress response genes, including AtRAB18 and AtRD29b, was more strongly induced in transgenic plants than in the WT when subjected to ABA and drought treatments. Taken together, our results indicate that CsSnRK2.5 is a positive regulator of ABA-regulated drought stress responses.

A novel sucrose transporter gene IbSUT4 involves in plant growth and response to abiotic stress through the ABF-dependent ABA signaling pathway in Sweetpotato

BACKGROUND

To maintain sweetpotato (Ipomoea batatas (L.) Lam) growth and yield, sucrose must be transported from the leaves to the roots. Sucrose transporters or carriers (SUTs or SUCs) transport sucrose and are involved in plant growth and response to abiotic stress. However, the mechanisms of SUTs in sweetpotato abiotic stress resistance remains to be determined.

RESULTS

In the present study, we cloned a novel IbSUT4 gene; the protein encoded by this gene is localized in the tonoplast and plasma membrane. The plant growth was promoted in the IbSUT4 transgenic Arabidopsis thaliana lines, with increased expression of AtFT, a regulator of flowering time in plants. Over-expression of IbSUT4 in Arabidopsis thaliana resulted in higher sucrose content in the roots and lower sucrose content in the leaves, as compared to the wild-type (WT) plants, leading to improved stress tolerance during seedling growth. Moreover, we systematically analyzed the mechanisms of IbSUT4 in response to abiotic stress. The results suggest that the ABRE-motif was localized in the IbSUT4 promoter region, and the expression of the ABA signaling pathway genes (i.e., ABF2, ABF4, SnRK2.2, SnRK2.3, and PYL8/RCAR3) were induced, and the expression of ABI1 was inhibited.

CONCLUSIONS

Our dates provide evidence that IbSUT4 is not only involved in plant growth but also is an important positive regulator in plant stress tolerance through the ABF-dependent ABA signaling pathway.

Negative Roles of Strigolactone-Related SMXL6, 7 and 8 Proteins in Drought Resistance in Arabidopsis

Previous investigations have shown that the SUPPRESSORS OF MAX2 1-LIKE6, 7 and 8 (SMXL6, 7 and 8) proteins redundantly repress strigolactone (SL) signaling in plant growth and development. Recently, a growing body of evidence indicated that SLs positively regulate plant drought resistance through functional analyses of genes involved in SL biosynthesis and positive regulation of SL signaling. However, the functions of the SL-signaling negative regulators SMXL6, 7 and 8 in drought resistance and the associated mechanisms remain elusive. To reveal the functions of these SMXL proteins, we analyzed the drought-resistant phenotype of the triple smxl6,7,8 mutant plants and studied several drought resistance-related traits. Our results showed that the smxl6,7,8 mutant plants were more resistant to drought than wild-type plants. Physiological investigations indicated that the smxl6,7,8 mutant plants exhibited higher leaf surface temperature, reduced cuticle permeability, as well as decreases in drought-induced water loss and cell membrane damage in comparison with wild-type plants. Additionally, smxl6,7,8 mutant plants displayed an increase in anthocyanin biosynthesis during drought, enhanced detoxification capacity and increased sensitivity to abscisic acid in cotyledon opening and growth inhibition assays. A good correlation between the expression levels of some relevant genes and the examined physiological and biochemical traits was observed. Our findings together indicate that the SMXL6, 7 and 8 act as negative regulators of drought resistance, and that disruption of these SMXL genes in crops may provide a novel way to improve their drought resistance.

Plant abiotic stress response and nutrient use efficiency

Abiotic stresses and soil nutrient limitations are major environmental conditions that reduce plant growth, productivity and quality. Plants have evolved mechanisms to perceive these environmental challenges, transmit the stress signals within cells as well as between cells and tissues, and make appropriate adjustments in their growth and development in order to survive and reproduce. In recent years, significant progress has been made on many fronts of the stress signaling research, particularly in understanding the downstream signaling events that culminate at the activation of stress- and nutrient limitation-responsive genes, cellular ion homeostasis, and growth adjustment. However, the revelation of the early events of stress signaling, particularly the identification of primary stress sensors, still lags behind. In this review, we summarize recent work on the genetic and molecular mechanisms of plant abiotic stress and nutrient limitation sensing and signaling and discuss new directions for future studies.

Effects of Rhizome Integration on the Water Physiology of Phyllostachys edulis Clones Under Heterogeneous Water Stress

Water is crucial to plant growth and development. Under heterogeneous environmental water deficiency, physiological integration of the rhizomatous clonal plant triggers a series of physiological cascades, which induces both signaling and physiological responses. It is known that the rhizome of Phyllostachys edulis, which connects associated clonal ramets, has important significance in this physiological integration. This significance is attributed to the sharing of water and nutrients in the vascular bundle of clonal ramets under heterogeneous water conditions. However, the physiological characteristics of physiological integration under heterogeneous water stress remain unclear. To investigate these physiological characteristics, particularly second messenger Ca²⁺ signaling characteristics, long-distance hormone signaling molecules, antioxidant enzyme activity, osmotic adjustment substance, and nitrogen metabolism, ramets with a connected (where integration was allowed to take place) and severed rhizome (with no integration) were compared in this study. The vascular bundle structure of the rhizome was also observed using laser confocal microscopy. Overall, the results suggest that interconnected rhizome of P. edulis can enhance its physiological function in response to drought-induced stress under heterogeneous water deficiency. These measured changes in physiological indices serve to improve the clonal ramets’ drought adaptivity through the interconnected rhizome.

Comprehensive Analysis of the TIFY Gene Family and Its Expression Profiles under Phytohormone Treatment and Abiotic Stresses in Roots of Populus trichocarpa

The TIFY gene family is specific to land plants, exerting immense influence on plant growth and development as well as responses to biotic and abiotic stresses. Here, we identify 25 TIFY genes in the poplar (Populus trichocarpa) genome. Phylogenetic tree analysis revealed these PtrTIFY genes were divided into four subfamilies within two groups. Promoter cis-element analysis indicated most PtrTIFY genes possess stress- and phytohormone-related cis-elements. Quantitative real-time reverse transcription polymerase chain reaction (qRT–PCR) analysis showed that PtrTIFY genes displayed different expression patterns in roots under abscisic acid, methyl jasmonate, and salicylic acid treatments, and drought, heat, and cold stresses. The protein interaction network indicated that members of the PtrTIFY family may interact with COI1, MYC2/3, and NINJA. Our results provide important information and new insights into the evolution and functions of TIFY genes in P. trichocarpa.

Abscisic acid-triggered guard cell l-cysteine desulfhydrase function and in situ hydrogen sulfide production contributes to heme oxygenase-modulated stomatal closure

Recent studies have demonstrated that hydrogen sulfide (H₂ S) produced through the activity of l-cysteine desulfhydrase (DES1) is an important gaseous signaling molecule in plants that could participate in abscisic acid (ABA)-induced stomatal closure. However, the coupling of the DES1/H₂ S signaling pathways to guard cell movement has not been thoroughly elucidated. The results presented here provide genetic evidence for a physiologically relevant signaling pathway that governs guard cell in situ DES1/H₂ S function in stomatal closure. We discovered that ABA-activated DES1 produces H₂ S in guard cells. The impaired guard cell ABA phenotype of the des1 mutant can be fully complemented when DES1/H₂ S function has been specifically rescued in guard cells and epidermal cells, but not mesophyll cells. This research further characterized DES1/H₂ S function in the regulation of LONG HYPOCOTYL1 (HY1, a member of the heme oxygenase family) signaling. ABA-induced DES1 expression and H₂ S production are hyper-activated in the hy1 mutant, both of which can be fully abolished by the addition of H₂ S scavenger. Impaired guard cell ABA phenotype of des1/hy1 can be restored by H₂ S donors. Taken together, this research indicated that guard cell in situ DES1 function is involved in ABA-induced stomatal closure, which also acts as a pivotal hub in regulating HY1 signaling.

Transcription factor TCP20 regulates peach bud endodormancy by inhibiting DAM5/DAM6 and interacting with ABF2

The dormancy-associated MADS-box (DAM) genes PpDAM5 and PpDAM6 have been shown to play important roles in bud endodormancy; however, their molecular regulatory mechanism in peach is unclear. In this study, by use of yeast one-hybrid screening, we isolated a TEOSINTE BRANCHED1/CYCLOIDEA/PROLIFERATING CELL FACTOR transcription factor, PpTCP20, in the peach cultivar 'Zhongyou 4' (Prunus persica var. nectarina). The protein was localized in the nucleus and was capable of forming a homodimer. Electrophoretic mobility shift assays demonstrated that PpTCP20 binds to a GCCCR element in the promoters of PpDAM5 and PpDAM6, and transient dual luciferase experiments showed that PpTCP20 inhibited the expression of PpDAM5 and PpDAM6 as the period of the release of flower bud endodormancy approached. In addition, PpTCP20 interacted with PpABF2 to form heterodimers to regulate bud endodormancy, and the content of abscisic acid decreased with the release of endodormancy. PpTCP20 also inhibited expression of PpABF2 to regulate endodormancy. Taken together, our results suggest that PpTCP20 regulates peach flower bud endodormancy by negatively regulating the expression of PpDAM5 and PpDAM6, and by interacting with PpABF2, thus revealing a novel regulatory mechanism in a perennial deciduous tree.

Maize NCP1 negatively regulates drought and ABA responses through interacting with and inhibiting the activity of transcription factor ABP9

NCP1, a NINJA family protein lacking EAR motif, acts as a negative regulator of ABA signaling by interacting with and inhibiting the activity of transcriptional activator ABP9. The phytohormone abscisic acid plays a pivotal role in regulating plant responses to a variety of abiotic stresses including drought and salinity. Maize ABP9 is an ABRE-binding bZIP transcription activator that enhances plant tolerance to multiple stresses by positively regulating ABA signaling, but the molecular mechanism by which ABP9 is regulated in mediating ABA responses remains unknown. Here, we report the identification of an ABP9-interacting protein, named ABP Nine Complex Protein 1 (NCP1) and its functional characterization. NCP1 belongs to the recently identified NINJA family proteins, but lacks the conserved EAR motif, which is a hallmark of this class of transcriptional repressors. In vitro and in vivo assays confirmed that NCP1 physically interacts with ABP9 and that they are co-localized in the nucleus. In addition, NCP1 and ABP9 are similarly induced with similar patterns by ABA treatment and osmotic stress. Interestingly, NCP1 over-expressing Arabidopsis plants exhibited a reduced sensitivity to ABA and decreased drought tolerance. Transient assay in maize protoplasts showed that NCP1 inhibits the activity of ABP9 in activating ABRE-mediated reporter gene expression, a notion further supported by genetic analysis of drought and ABA responses in the transgenic plants over-expressing both ABP9 and NCP1. These data together suggest that NCP1 is a novel negative regulator of ABA signaling via interacting with and inhibiting the activity of ABP9.

A Role for Plant KASH Proteins in Regulating Stomatal Dynamics

Stomatal movement, which regulates gas exchange in plants, is controlled by a variety of environmental factors, including biotic and abiotic stresses. The stress hormone abscisic acid (ABA) initiates a signaling cascade, which leads to increased H₂O₂ and Ca²⁺ levels and F-actin reorganization, but the mechanism of, and connection between, these events is unclear. SINE1, an outer nuclear envelope component of a plant Linker of Nucleoskeleton and Cytoskeleton complex, associates with F-actin and is, along with its putative paralog SINE2, expressed in guard cells. Here, we have determined that Arabidopsis (Arabidopsis thaliana) SINE1 and SINE2 play an important role in stomatal opening and closing. Loss of SINE1 or SINE2 results in ABA hyposensitivity and impaired stomatal dynamics but does not affect stomatal closure induced by the bacterial elicitor flg22. The ABA-induced stomatal closure phenotype is, in part, attributed to impairments in Ca²⁺ and F-actin regulation. Together, the data suggest that SINE1 and SINE2 act downstream of ABA but upstream of Ca²⁺ and F-actin. While there is a large degree of functional overlap between the two proteins, there are also critical differences. Our study makes an unanticipated connection between stomatal regulation and nuclear envelope-associated proteins, and adds two new players to the increasingly complex system of guard cell regulation.

GAI Functions in the Plant Response to Dehydration Stress in Arabidopsis thaliana

DELLA (GAI/RGA/RGL1/RGL2/RGL3) proteins are key negative regulators in GA (gibberellin) signaling and are involved in regulating plant growth as a response to environmental stresses. It has been shown that the DELLA protein PROCERA (PRO) in tomato promotes drought tolerance, but its molecular mechanism remains unknown. Here, we showed that the gai-1 (gibberellin insensitive 1) mutant (generated from the gai-1 (Ler) allele (with a 17 amino acid deletion within the DELLA domain of GAI) by backcrossing gai-1 (Ler) with Col-0 three times), the gain-of-function mutant of GAI (GA INSENSITIVE) in Arabidopsis, increases drought tolerance. The stomatal density of the gai-1 mutant was increased but its stomatal aperture was decreased under abscisic acid (ABA) treatment conditions, suggesting that the drought tolerance of the gai-1 mutant is a complex trait. We further tested the interactions between DELLA proteins and ABF2 (abscisic acid (ABA)-responsive element (ABRE)-binding transcription factors) and found that there was a strong interaction between DELLA proteins and ABF2. Our results provide new insight into DELLA proteins and their role in drought stress tolerance.

Genome-wide analysis of expression quantitative trait loci (eQTLs) reveals the regulatory architecture of gene expression variation in the storage roots of sweet potato

Dissecting the genetic regulation of gene expression is critical for understanding phenotypic variation and species evolution. However, our understanding of the transcriptional variability in sweet potato remains limited. Here, we analyzed two publicly available datasets to explore the landscape of transcriptomic variations and its genetic basis in the storage roots of sweet potato. The comprehensive analysis identified a total of 724,438 high-confidence single nucleotide polymorphisms (SNPs) and 26,026 expressed genes. Expression quantitative trait locus (eQTL) analysis revealed 4408 eQTLs regulating the expression of 3646 genes, including 2261 local eQTLs and 2147 distant eQTLs. Two distant eQTL hotspots were found with target genes significantly enriched in specific functional classifications. By combining the information from regulatory network analyses, eQTLs and association mapping, we found that IbMYB1-2 acts as a master regulator and is the major gene responsible for the activation of anthocyanin biosynthesis in the storage roots of sweet potato. Our study provides the first insight into the genetic architecture of genome-wide expression variation in sweet potato and can be used to investigate the potential effects of genetic variants on key agronomic traits in sweet potato.

Characterization of the bZIP Transcription Factor Family in Pepper (Capsicum annuum L.): CabZIP25 Positively Modulates the Salt Tolerance

The basic leucine zipper (bZIP) proteins compose a family of transcription factors (TFs), which play a crucial role in plant growth, development, and abiotic and biotic stress responses. However, no comprehensive analysis of bZIP family has been reported in pepper (Capsicum annuum L.). In this study, we identified and characterized 60 bZIP TF-encoding genes from two pepper genomes. These genes were divided into 10 groups based on their phylogenetic relationships with bZIP genes from Arabidopsis. Six introns/exons structural patterns within the basic and hinge regions and the conserved motifs were identified among all the pepper bZIP proteins, on the basis of which, we classify them into different subfamilies. Based on the transcriptomic data of Zunla-1 genome, expression analyses of 59 pepper bZIP genes (not including CabZIP25 of CM334 genome), indicated that the pepper bZIP genes were differentially expressed in the pepper tissues and developmental stages, and many of the pepper bZIP genes might be involved in responses to various abiotic stresses and phytohormones. Further, gene expression analysis, using quantitative real-time PCR (qRT-PCR), showed that the CabZIP25 gene was expressed at relatively higher levels in vegetative tissues, and was strongly induced by abiotic stresses and phytohormones. In comparing with wild type Arabidopsis, germination rate, fresh weight, chlorophyll content, and root lengths increased in the CabZIP25-overexpressing Arabidopsis under salt stress. Additionally, CabZIP25-silenced pepper showed lower chlorophyll content than the control plants under salt stress. These results suggested that CabZIP25 improved salt tolerance in plants. Taken together, our results provide new opportunities for the functional characterization of bZIP TFs in pepper.

Abscisic acid dynamics, signaling, and functions in plants

Abscisic acid (ABA) is an important phytohormone regulating plant growth, development, and stress responses. It has an essential role in multiple physiological processes of plants, such as stomatal closure, cuticular wax accumulation, leaf senescence, bud dormancy, seed germination, osmotic regulation, and growth inhibition among many others. Abscisic acid controls downstream responses to abiotic and biotic environmental changes through both transcriptional and posttranscriptional mechanisms. During the past 20 years, ABA biosynthesis and many of its signaling pathways have been well characterized. Here we review the dynamics of ABA metabolic pools and signaling that affects many of its physiological functions.

The Arabidopsis kinase-associated protein phosphatase KAPP, interacting with protein kinases SnRK2.2/2.3/2.6, negatively regulates abscisic acid signaling

KEY MESSAGE

The kinase-associated protein phosphatase, KAPP, is negatively involved in abscisic acid (ABA) signaling. KAPP interacts physically with SnRK2.2, SnRK2.3 and SnRK2.6, and functionally acts upstream of SnRK2.2 and SnRK2.3. The kinase-associated protein phosphatase (KAPP) has been reported to be involved in the regulation of many developmental and signaling events, but it remains unknown whether KAPP is involved in ABA signaling. Here, we report that KAPP is negatively involved in ABA-mediated seed germination and early seedling growth in Arabidopsis thaliana. The two loss-of-function mutants of KAPP, kapp-1 and kapp-2, exhibit increased ABA sensitivity in ABA-induced seed germination inhibition and post-germination growth arrest. The three closely-related protein kinase, (SNF1)-related protein kinase SnRK2.2, SnRK2.3 and SnRK2.6, which play critical roles in ABA signaling, interact and co-localize with KAPP. Genetic evidence showed that the ABA-hypersensitive phenotypes caused by KAPP mutation were suppressed by the double mutation of SnRK2.2 and SnRK2.3, indicating that KAPP functions upstream of SnRK2.2 and SnRK2.3 in ABA signaling. RNA-sequencing analysis revealed that KAPP mutation affects expression of multiple ABA-responsive genes. These results demonstrated that KAPP is negatively involved in plant response to ABA, which help to understand the complicated ABA signaling mechanism.

The poplar R2R3 MYB transcription factor PtrMYB94 coordinates with abscisic acid signaling to improve drought tolerance in plants

In plants, R2R3 MYB transcription factors (TFs) consist of one large gene family and are involved in the regulation of many developmental processes and various stresses. However, the functions of most of MYB TFs in woody plants remain unknown. Here, PtrMYB94, an R2R3 MYB TF from Populus trichocarpa, is characterized to be involved in the regulation of drought responses and abscisic acid (ABA) signaling. PtrMYB94 encodes a nuclear-localized R2R3 MYB TF. RT-PCR results showed that the PtrMYB94 transcripts were relatively abundant in leaves and stems, and were induced rapidly in response to dehydration stress. Overexpression of PtrMYB94 improved plant drought responses, suggesting that this MYB TF may functionally regulate poplar adaptability to drought stress. Furthermore, the analysis of transcriptional expression and PtrMYB94 promoter: GUS activity showed that PtrMYB94 responded to ABA induction. PtrMYB94-overexpressing plants exhibited the inhibition of seed germination compared with the wild-type (WT) control under ABA exposure condition. The ABA content was evidently increased in the PtrMYB94-overexpressing plants relative to the WT plants. In addition, transcript levels of several ABA- and drought-responsive genes, such as ABA1 and DREB2B, were up-regulated. Taken together, our results suggest that PtrMYB94 is involved in an ABA-dependent drought stress regulation in Populus.

Barley ABI5 (Abscisic Acid INSENSITIVE 5) Is Involved in Abscisic Acid-Dependent Drought Response

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.

Arabidopsis CPK6 positively regulates ABA signaling and drought tolerance through phosphorylating ABA-responsive element-binding factors

Abscisic acid (ABA) regulates numerous developmental processes and drought tolerance in plants. Calcium-dependent protein kinases (CPKs) are important Ca2+ sensors playing crucial roles in plant growth and development as well as responses to stresses. However, the molecular mechanisms of many CPKs in ABA signaling and drought tolerance remain largely unknown. Here we combined protein interaction studies, and biochemical and genetic approaches to identify and characterize substrates that were phosphorylated by CPK6 and elucidated the mechanism that underlines the role of CPK6 in ABA signaling and drought tolerance. The expression of CPK6 is induced by ABA and dehydration. Two cpk6 T-DNA insertion mutants are insensitive to ABA during seed germination and root elongation of seedlings; in contrast, overexpression of CPK6 showed the opposite phenotype. Moreover, CPK6-overexpressing lines showed enhanced drought tolerance. CPK6 interacts with and phosphorylates a subset of core ABA signaling-related transcription factors, ABA-responsive element-binding factors (ABFs/AREBs), and enhances their transcriptional activities. The phosphorylation sites in ABF3 and ABI5 were also identified through MS and mutational analyses. Taken together, we present evidence that CPK6 mediates ABA signaling and drought tolerance through phosphorylating ABFs/AREBs. This work thus uncovers a rather conserved mechanism of calcium-dependent Ser/Thr kinases in ABA signaling.

Overexpression of a pectin methylesterase gene PtoPME35 from Populus tomentosa influences stomatal function and drought tolerance in Arabidopsis thaliana

Poplar is a superior forestation species with high adaptability. The woody tissue of poplar is mainly derived from cell wall. Cell wall formation determines cell shape and woody growth. Pectin is rich in primary cell wall, but it is also involved in the regulation of wood formation. In our study, we cloned a gene from poplar (Populus tomentos), designed as PtoPME35, which encodes a putative pectin methylesterase. PtoPME35 has higher sequence similarity with Arabidopsis AtPME35. Gene expression analysis shows that PtoPME35 has a constitutive expression pattern in multiple tissues, with the highest expression in stem. Subcellular localization result indicates that PtoPME35 is localized to the cell wall. To elucidate the biological function of PtoPME35 in vivo, we generated overexpression plants in poplar and Arabidopsis. The degree of pectin methylesterification is decreased in PtoPME35-overexpressing transgenic poplar, although no obvious phenotypes were displayed. In PtoPME35-overexpressing Arabidopsis plants, stomatal opening is inhibited and water loss rate is decreased under the drought condition. Moreover, the expression levels of drought-stress responsive genes were higher with mannitol treatment in PtoPME35-overexpressing Arabidopsis plants than in wild type controls. Accordingly, these results suggest that PtoPME35 may regulate osmotic stress responses by modulating stomatal functions.

Integration of Abscisic Acid Signaling with Other Signaling Pathways in Plant Stress Responses and Development

Plants are immobile and, to overcome harsh environmental conditions such as drought, salt, and cold, they have evolved complex signaling pathways. Abscisic acid (ABA), an isoprenoid phytohormone, is a critical signaling mediator that regulates diverse biological processes in various organisms. Significant progress has been made in the determination and characterization of key ABA-mediated molecular factors involved in different stress responses, including stomatal closure and developmental processes, such as seed germination and bud dormancy. Since ABA signaling is a complex signaling network that integrates with other signaling pathways, the dissection of its intricate regulatory network is necessary to understand the function of essential regulatory genes involved in ABA signaling. In the present review, we focus on two aspects of ABA signaling. First, we examine the perception of the stress signal (abiotic and biotic) and the response network of ABA signaling components that transduce the signal to the downstream pathway to respond to stress tolerance, regulation of stomata, and ABA signaling component ubiquitination. Second, ABA signaling in plant development processes, such as lateral root growth regulation, seed germination, and flowering time regulation is investigated. Examining such diverse signal integration dynamics could enhance our understanding of the underlying genetic, biochemical, and molecular mechanisms of ABA signaling networks in plants.

Natural Root Cellular Variation in Responses to Osmotic Stress in Arabidopsis thaliana Accessions

Arabidopsis naturally occurring populations have allowed for the identification of considerable genetic variation remodeled by adaptation to different environments and stress conditions. Water is a key resource that limits plant growth, and its availability is initially sensed by root tissues. The root’s ability to adjust its physiology and morphology under water deficit makes this organ a useful model to understand how plants respond to water stress. Here, we used hyperosmotic shock stress treatments in different Arabidopsis accessions to analyze the root cell morphological responses. We found that osmotic stress conditions reduced root growth and root apical meristem (RAM) size, promoting premature cell differentiation without affecting the stem cell niche morphology. This phenotype was accompanied by a cluster of small epidermal and cortex cells with radial expansion and root hairs at the transition to the elongation zone. We also found this radial expansion with root hairs when plants are grown under hypoosmotic conditions. Finally, root growth was less affected by osmotic stress in the Sg-2 accession followed by Ws, Cvi-0, and Col-0; however, after a strong osmotic stress, Sg-2 and Cvi-0 were the most resilience accessions. The sensitivity differences among these accessions were not explained by stress-related gene expression. This work provides new cellular insights on the Arabidopsis root phenotypic variability and plasticity to osmotic stress.

A WD40-Repeat Protein From the Recretohalophyte Limonium bicolor Enhances Trichome Formation and Salt Tolerance in Arabidopsis

The Arabidopsis thaliana WD40-repeat protein TRANSPARENT TESTA GLABRA1 (TTG1) controls epidermis development, playing opposite roles in trichome differentiation and root hair formation. We isolated and characterized LbTTG1 (encoding a WD40-repeat protein with high sequence similarity to TTG1) from the recretohalophyte Limonium bicolor, which actively excretes absorbed salt via a salt gland. The complete open reading frame of LbTTG1 was 1,095 bp, encoding a protein of 364 amino acids, and showed highest expression during the salt gland initiation stage. We heterologously expressed LbTTG1 in wild type and ttg1-13 Arabidopsis plants to verify the protein's function, and the copies of LbTTG1 were identified in transgenic strains using southern blotting. Trichomes were extremely induced on the first true leaves of plants heterologously expressing LbTTG1, whereas no trichomes were produced by ttg1-13 plants. Conversely, plants heterologously expressing LbTTG1 produced fewer root hairs than ttg1-13 plants. In plants heterologously expressing LbTTG1 compared to controls, epidermis differentiation genes (GLABRA1 and GLABRA3) were up-regulated while genes encoding negative regulators of trichome development (TRIPTYCHON and CAPRICE) were down-regulated. Under increased NaCl concentrations, both of the transgenic lines showed enhanced germination and root length, and accumulated less malondialdehyde (MDA) and Na+ and produced more proline, soluble sugar, and higher glutathione S-transferase activity, compared with the ttg1-13 mutant. These results indicate that LbTTG1 participates in epidermis development in Arabidopsis, similarly to other WD40-repeat proteins, and specifically increases salt tolerance of transgenic Arabidopsis by reducing ion accumulation and increasing osmolyte levels.

Tartary buckwheat transcription factor FtbZIP83 improves the drought/salt tolerance of Arabidopsis via an ABA-mediated pathway

Plants are subjected to a variety of abiotic stresses during their lifetime, and drought and salt stress are some of the main causes of reduced crop yields. Previous studies have shown that AREB/ABFs within bZIP transcription factors are involved in plant drought and salt stress responses in an ABA-dependent manner. However, the properties and functions of AREB/ABFs in Fagopyrum tataricum, a cereal with good resistance to abiotic stresses, are poorly understood. In this study, a gene encoding an AREB/ABF, designated FtbZIP83, was first isolated from Tartary buckwheat. Expression analysis in Tartary buckwheat indicated that FtbZIP83 was significantly induced by abscisic acid (ABA), NaCl and polyethylene glycol (PEG). The overexpression of FtbZIP83 in Arabidopsis resulted in increased drought/salt tolerance, which was attributed not only to higher proline (Pro) contents and antioxidant enzyme activity in transgenic lines compared with controls but also to the lower reactive oxygen species (ROS) accumulation and malondialdehyde (MDA) content. In addition, we found that FtbZIP83 was able to respond to drought and salt stress by upregulating the transcript abundance of downstream ABA-inducible gene. Furthermore, promoter sequence analysis showed that ABREs were present, and the activity of the FtbZIP83 promoter in transgenic Arabidopsis after drought stress was significantly higher than that under normal conditions. Based on the potential signalling pathways involved in AREB/ABFs, we also screened for the interaction protein FtSnRK2.6/2.3, which may phosphorylate FtbZIP83. Collectively, these results provide evidence that FtbZIP83, as a positive regulator, responds to drought/salt stress via an ABA-dependent signalling pathway composed of SnRK2-AREB/ABF.

Comprehensive analysis of bZIP transcription factors uncovers their roles during dimorphic floret differentiation and stress response in Cleistogenes songorica

BACKGROUND

Transcription factors act as important regulators of transcription networks. Basic leucine zipper (bZIP) transcription factors have been shown to be involved in multiple biological processes in plants. However, no information is available for the bZIP family in Cleistogenes songorica, which is an important xerophytic and allotetraploid grass in desert grasslands.

RESULTS

In this study, 86 CsbZIPs were identified in the allotetraploid C. songorica genome. For location analysis, CsbZIPs were distributed evenly across two subgenomes of C. songorica. Phylogenetic tree analysis among three species indicated that CsbZIPs were evolutionarily more closely related to OsbZIPs than AtbZIPs. Syntenic and phylogenetic analyses confirmed that the CsbZIPs were mainly expanded by whole-genome duplication events. Furthermore, it was determined that rice and C. songorica might have undergone purified selection during their long evolutionary history by calculating the Ks values and Ka/Ks ratios of orthologous gene pairs. By analysing the expression patterns of CsbZIPs in different tissues and under abiotic stresses, 21 CsbZIP genes were differentially expressed between chasmogamous (CH) and cleistogamous (CL) flowers, including two FLOWERING LOCUS D (FD) genes. In shoots and roots, 79.1 and 87.2% of the CsbZIP genes, respectively, displayed transcript changes under at least one stress treatment, such as heat, cold, drought and salt. Strikingly, 17 common CsbZIP genes showed differential expression under stress response and during CL flowering. Co-expression network, GO annotation and real-time quantitative reverse transcription PCR (qRT-PCR) analyses revealed a close relationship between CL flowering-associated genes and abiotic stress-related genes.

CONCLUSIONS

BZIP TFs were comprehensively analysed and identified in allotetraploid C. songorica. Our results provide insights into the evolutionary history of the bZIP family in C. songorica and provide abiotic stress-responsive and CL-associated candidate CsbZIP genes for potential applications in the genetic improvement of plants.

Wheat F-box Protein TaFBA1 Positively Regulates Plant Drought Tolerance but Negatively Regulates Stomatal Closure

The phytohormone abscisic acid (ABA) regulates plant growth and development, as well as responses to various stresses, such as salt and drought. The wheat TaFBA1 gene, which encodes an F-box protein, was previously identified in our laboratory by homologous cloning. We previously found that TaFBA1 expression was induced by ABA and drought stress. In this study, wild-type (WT), TaFBA1 over-expressing (OEs), TaFBA1 homologous gene mutants, and TaFBA1 recovery (Rs) Arabidopsis plants were used. We found that the germination rate, the cotyledon greening rate, the root length, and the photosynthetic performance of TaFBA1 OE plants were better than those of WT under drought and ABA conditions, but mutant plants showed the opposite trend, and overexpression of TaFBA1 in mutants can recover their phenotype. In addition, TaFBA1 was found to be a negative regulator of ABA-induced stoma movement; mRNA transcription of certain ABA signaling-related genes was lower in TaFBA1 OE plants than in WT plants following ABA treatment. Further, we found that TaFBA1 can interact with RCAR1 (an ABA receptor) and ABI5. BiFC assay showed that TaFBA1 may interact with RCAR1 in the plasma membrane. In addition, accumulation of ROS and MDA in TaFBA1 OE plants was lower than that in the WT plants after ABA and drought treatments. Based on these results, we suggest that TaFBA1-regulated ABA insensitivity may be dependent on regulating ABA-mediated gene expression through interacting with RCAR1 and ABI5. Increased antioxidant competence and decreased ROS accumulation may be an important mechanism that underlies improved drought tolerance in TaFBA1 OE plants.

Genome-Wide Characterization and Expression Profiling of Squamosa Promoter Binding Protein-like (SBP) Transcription Factors in Wheat (Triticum aestivum L.)

Transcription factors (TFs) play fundamental roles in the developmental processes of all living organisms. Squamosa Promoter Binding Protein-like (SBP/SBP-Box) is a major family of plant-specific TFs, which plays important roles in multiple processes involving plant growth and development. While some work has been done, there is a lot more that is yet to be discovered in the hexaploid wheat SBP (TaSBP) family. With the completion of whole genome sequencing, genome-wide analysis of SBPs in common hexaploid wheat is now possible. In this study, we used protein–protein Basic Local Alignment Search Tool (BLASTp) to hunt the newly released reference genome sequence of hexaploid wheat (Chinese spring). Seventy-four TaSBP proteins (belonging to 56 genes) were identified and clustered into five groups. Gene structure and motif analysis indicated that most TaSBPs have relatively conserved exon–intron arrangements and motif composition. Analysis of transcriptional data showed that many TaSBP genes responded to some biological and abiotic stresses with different expression patterns. Moreover, three TaSBP genes were generally expressed in the majority of tissues throughout the wheat growth and also responded to many environmental biotic and abiotic stresses. Collectively, the detailed analyses presented here will help in understanding the roles of the TaSBP and also provide a reference for the further study of its biological function in wheat.

Involvement of abscisic acid-responsive element-binding factors in cassava (Manihot esculenta) dehydration stress response

Cassava (Manihot esculenta) is a major staple food, animal feed and energy crop in the tropics and subtropics. It is one of the most drought-tolerant crops, however, the mechanisms of cassava drought tolerance remain unclear. Abscisic acid (ABA)-responsive element (ABRE)-binding factors (ABFs) are transcription factors that regulate expression of target genes involved in plant tolerance to drought, high salinity, and osmotic stress by binding ABRE cis-elements in the promoter regions of these genes. However, there is little information about ABF genes in cassava. A comprehensive analysis of Manihot esculenta ABFs (MeABFs) described the phylogeny, genome location, cis-acting elements, expression profiles, and regulatory relationship between these factors and Manihot esculenta betaine aldehyde dehydrogenase genes (MeBADHs). Here we conducted genome-wide searches and subsequent molecular cloning to identify seven MeABFs that are distributed unevenly across six chromosomes in cassava. These MeABFs can be clustered into three groups according to their phylogenetic relationships to their Arabidopsis (Arabidopsis thaliana) counterparts. Analysis of the 5′-upstream region of MeABFs revealed putative cis-acting elements related to hormone signaling, stress, light, and circadian clock. MeABF expression profiles displayed clear differences among leaf, stem, root, and tuberous root tissues under non-stress and drought, osmotic, or salt stress conditions. Drought stress in cassava leaves and roots, osmotic stress in tuberous roots, and salt stress in stems induced expression of the highest number of MeABFs showing significantly elevated expression. The glycine betaine (GB) content of cassava leaves also was elevated after drought, osmotic, or salt stress treatments. BADH1 is involved in GB synthesis. We show that MeBADH1 promoter sequences contained ABREs and that MeBADH1 expression correlated with MeABF expression profiles in cassava leaves after the three stress treatments. Taken together, these results suggest that in response to various dehydration stresses, MeABFs in cassava may activate transcriptional expression of MeBADH1 by binding the MeBADH1 promoter that in turn promotes GB biosynthesis and accumulation via an increase in MeBADH1 gene expression levels and MeBADH1 enzymatic activity. These responses protect cells against dehydration stresses by preserving an osmotic balance that enhances cassava tolerance to dehydration stresses.

A GmSIN1/GmNCED3s/GmRbohBs Feed-Forward Loop Acts as a Signal Amplifier That Regulates Root Growth in Soybean Exposed to Salt Stress

Abscisic acid (ABA) and reactive oxygen species (ROS) act as key signaling molecules in the plant response to salt stress; however, how these signals are transduced and amplified remains unclear. Here, a soybean (Glycine max) salinity-induced NAM/ATAF1/2/CUC2 (NAC) transcription factor encoded by SALT INDUCED NAC1 (GmSIN1) was shown to be a key component of this process. Overexpression of GmSIN1 in soybean promoted root growth and salt tolerance and increased yield under salt stress; RNA interference-mediated knockdown of GmSIN1 had the opposite effect. The rapid induction of GmSIN1 in response to salinity required ABA and ROS, and the effect of GmSIN1 on root elongation and salt tolerance was achieved by boosting cellular ABA and ROS contents. GmSIN1 upregulated 9-cis-epoxycarotenoid dioxygenase coding genes in soybean (GmNCED3s, associated with ABA synthesis) and Respiratory burst oxidase homolog B genes in soybean (GmRbohBs, associated with ROS generation) by binding to their promoters at a site that has not been described to date. Together, GmSIN1, GmNCED3s, and GmRbohBs constitute a positive feed-forward system that enables the rapid accumulation of ABA and ROS, effectively amplifying the initial salt stress signal. These findings suggest that the combined modulation of ABA and ROS contents enhances soybean salt tolerance.

Nitric oxide molecular targets: reprogramming plant development upon stress

Plants are sessile organisms that need to complete their life cycle by the integration of different abiotic and biotic environmental signals, tailoring developmental cues and defense concomitantly. Commonly, stress responses are detrimental to plant growth and, despite the fact that intensive efforts have been made to understand both plant development and defense separately, most of the molecular basis of this trade-off remains elusive. To cope with such a diverse range of processes, plants have developed several strategies including the precise balance of key plant growth and stress regulators [i.e. phytohormones, reactive nitrogen species (RNS), and reactive oxygen species (ROS)]. Among RNS, nitric oxide (NO) is a ubiquitous gasotransmitter involved in redox homeostasis that regulates specific checkpoints to control the switch between development and stress, mainly by post-translational protein modifications comprising S-nitrosation of cysteine residues and metals, and nitration of tyrosine residues. In this review, we have sought to compile those known NO molecular targets able to balance the crossroads between plant development and stress, with special emphasis on the metabolism, perception, and signaling of the phytohormones abscisic acid and salicylic acid during abiotic and biotic stress responses.

Systematic Analysis of Differentially Expressed Maize ZmbZIP Genes between Drought and Rewatering Transcriptome Reveals bZIP Family Members Involved in Abiotic Stress Responses

The basic leucine zipper (bZIP) family of transcription factors (TFs) regulate diverse phenomena during plant growth and development and are involved in stress responses and hormone signaling. However, only a few bZIPs have been functionally characterized. In this paper, 54 maize bZIP genes were screened from previously published drought and rewatering transcriptomes. These genes were divided into nine groups in a phylogenetic analysis, supported by motif and intron/exon analyses. The 54 genes were unevenly distributed on 10 chromosomes and contained 18 segmental duplications, suggesting that segmental duplication events have contributed to the expansion of the maize bZIP family. Spatio-temporal expression analyses showed that bZIP genes are widely expressed during maize development. We identified 10 core ZmbZIPs involved in protein transport, transcriptional regulation, and cellular metabolism by principal component analysis, gene co-expression network analysis, and Gene Ontology enrichment analysis. In addition, 15 potential stress-responsive ZmbZIPs were identified by expression analyses. Localization analyses showed that ZmbZIP17, -33, -42, and -45 are nuclear proteins. These results provide the basis for future functional genomic studies on bZIP TFs in maize and identify candidate genes with potential applications in breeding/genetic engineering for increased stress resistance. These data represent a high-quality molecular resource for selecting resistant breeding materials.

Functional divergence between soybean FLOWERING LOCUS T orthologues FT2a and FT5a in post-flowering stem growth

Genes in the FLOWERING LOCUS T (FT) family integrate external and internal signals to control various aspects of plant development. In soybean (Glycine max), FT2a and FT5a play a major role in floral induction, but their roles in post-flowering reproductive development remain undetermined. Ectopic overexpression analyses revealed that FT2a and FT5a similarly induced flowering, but FT5a was markedly more effective than FT2a for the post-flowering termination of stem growth. The down-regulation of Dt1, a soybean orthologue of Arabidopsis TERMINAL FLOWER1, in shoot apices in early growing stages of FT5a-overexpressing plants was concomitant with highly up-regulated expression of APETALA1 orthologues. The Dt2 gene, a repressor of Dt1, was up-regulated similarly by the overexpression of FT2a and FT5a, suggesting that it was not involved in the control of stem termination by FT5a. In addition to the previously reported interaction with FDL19, a homologue of the Arabidopsis bZIP protein FD, both FT2a and FT5a interacted with FDL12, but only FT5a interacted with FDL06. Our results suggest that FT2a and FT5a have different functions in the control of post-flowering stem growth. A specific interaction of FT5a with FDL06 may play a key role in determining post-flowering stem growth in soybean.

Abscisic acid signalling mediates biomass trade-off and allocation in poplar

Abscisic acid (ABA) is a well known stress hormone regulating drought adaptation of plants. Here, we hypothesised that genetic engineering of genes involved in ABA stress signalling and photoperiodic regulation affected drought resistance by trade-off with biomass production in perennial poplar trees. We grew Populus tremula × tremuloides wild-type (T89) and various transgenic lines (two transformation events of 35S::abi1-1, 35S::RCAR, RCAR:RNAi, 35S::ABI3, 35S::AREB3, 35S::FDL1, FDL1:RNAi, 35S::FDL2 and FDL2:RNAi) outdoors and exposed them to drought in the second growth period. After the winter, the surviving lines showed a huge variation in stomatal conductance, leaf size, whole-plant leaf area, tree height, stem diameter, and biomass. Whole-plant leaf area was a strong predictor for woody biomass production. The 35S::AREB3 lines were compromised in biomass production under well irrigated conditions compared with wild-type poplars but were resilient to drought. ABA signalling regulated FDL1 and FDL2 expression under stress. Poplar lines overexpressing FDL1 or FDL2 were drought-sensitive; they shed leaves and lost root biomass, whereas the FDL RNAi lines showed higher biomass allocation to roots under drought. These results assign a new function in drought acclimation to FDL genes aside from photoperiodic regulation. Our results imply a critical role for ABA-mediated processes in balancing biomass production and climate adaptation.

The Roles of GmERF135 in Improving Salt Tolerance and Decreasing ABA Sensitivity in Soybean

Abscisic acid (ABA) mediates various abiotic stress responses, and ethylene responsive factors (ERFs) play vital role in resisting stresses, but the interaction of these molecular mechanisms remains elusive. In this study, we identified an ABA-induced soybean ERF gene GmERF135 that was highly up-regulated by ethylene (ET), drought, salt, and low temperature treatments. Subcellular localization assay showed that the GmERF135 protein was targeted to the nucleus. Promoter cis-acting elements analysis suggested that numerous potential stress responsive cis-elements were distributed in the promoter region of GmERF135, including ABA-, light-, ET-, gibberellin (GA)-, and methyl jasmonate (MeJA)-responsive elements. Overexpression of GmERF135 in Arabidopsis enhanced tolerance to drought and salt conditions. In addition, GmERF135 promoted the growth of transgenic hairy roots under salt and exogenous ABA conditions. These results suggest that soybean GmERF135 may participate in both ABA and ET signaling pathways to regulate the responses to multiple stresses.

Deep Sequencing Reveals Early Reprogramming of Arabidopsis Root Transcriptomes Upon Ralstonia solanacearum Infection

Bacterial wilt caused by the bacterial pathogen Ralstonia solanacearum is one of the most devastating crop diseases worldwide. The molecular mechanisms controlling the early stage of R. solanacearum colonization in the root remain unknown. Aiming to better understand the mechanism of the establishment of R. solanacearum infection in root, we established four stages in the early interaction of the pathogen with Arabidopsis roots and determined the transcriptional profiles of these stages of infection. A total 2,698 genes were identified as differentially expressed genes during the initial 96 h after infection, with the majority of changes in gene expression occurring after pathogen-triggered root-hair development observed. Further analysis of differentially expressed genes indicated sequential activation of multiple hormone signaling cascades, including abscisic acid (ABA), auxin, jasmonic acid, and ethylene. Simultaneous impairment of ABA receptor genes promoted plant wilting symptoms after R. solanacearum infection but did not affect primary root growth inhibition or root-hair and lateral root formation caused by R. solanacearum. This indicated that ABA signaling positively regulates root defense to R. solanacearum. Moreover, transcriptional changes of genes involved in primary root, lateral root, and root-hair formation exhibited high temporal dynamics upon infection. Taken together, our results suggest that successful infection of R. solanacearum on roots is a highly programmed process involving in hormone crosstalk.

Distinct leaf transcriptomic response of water deficient Eucalyptus grandis submitted to potassium and sodium fertilization

While potassium fertilization increases growth yield in Brazilian eucalyptus plantations, it could also increase water requirements, making trees more vulnerable to drought. Sodium fertilization, which has been shown to promote eucalyptus growth compared to K-deficient trees, could partially mitigate this adverse effect of potassium. However, little is known about the influence of K and Na fertilization on the tree metabolic response to water deficit. The aim of the present study was thus to analyze the transcriptome of leaves sampled from Eucalyptus grandis trees subjected to 37% rainfall reduction, and fertilized with potassium (K), sodium (Na), compared to control trees (C). The multifactorial experiment was set up in a field with a throughfall exclusion system. Transcriptomic analysis was performed on leaves from two-year-old trees, and data analyzed using multifactorial statistical analysis and weighted gene co-expression network analysis (WGCNA). Significant sets of genes were seen to respond to rainfall reduction, in interaction with K or Na fertilization, or to fertilization only (regardless of the water supply regime). The genes were involved in stress signaling, primary and secondary metabolism, secondary cell wall formation and photosynthetic activity. Our focus on key genes related to cation transporters and aquaporins highlighted specific regulation of ion homeostasis, and plant adjustment to water deficit. While water availability significantly affects the transcriptomic response of eucalyptus species, this study points out that the transcriptomic response is highly dependent on the fertilization regime. Our study is based on the first large-scale field trial in a tropical region, specifically designed to study the interaction between water availability and nutrition in eucalyptus. To our knowledge, this is the first global transcriptomic analysis to compare the influence of K and Na fertilization on tree adaptive traits in water deficit conditions.

A stress-responsive bZIP transcription factor OsbZIP62 improves drought and oxidative tolerance in rice

BACKGROUND

Drought is a major abiotic stress factor that influences the yield of crops. Basic leucine zipper motif (bZIP) transcription factors play an important regulatory role in plant drought stress responses. However, the functions of a number of bZIP transcription factors in rice are still unknown.

RESULTS

In this study, a novel drought stress-related bZIP transcription factor, OsbZIP62, was identified in rice. This gene was selected from a transcriptome analysis of several typical rice varieties with different drought tolerances. OsbZIP62 expression was induced by drought, hydrogen peroxide, and abscisic acid (ABA) treatment. Overexpression of OsbZIP62-VP64 (OsbZIP62V) enhanced the drought tolerance and oxidative stress tolerance of transgenic rice, while osbzip62 mutants exhibited the opposite phenotype. OsbZIP62-GFP was localized to the nucleus, and the N-terminal sequence (amino acids 1-68) was necessary for the transcriptional activation activity of OsbZIP62. RNA-seq analysis showed that the expression of many stress-related genes (e.g., OsGL1, OsNAC10, and DSM2) was upregulated in OsbZIP62V plants. Moreover, OsbZIP62 could bind to the promoters of several putative target genes and could interact with stress/ABA-activated protein kinases (SAPKs).

CONCLUSIONS

OsbZIP62 is involved in ABA signalling pathways and positively regulates rice drought tolerance by regulating the expression of genes associated with stress, and this gene could be used for the genetic modification of crops with improved drought tolerance.

Brachypodium histone deacetylase BdHD1 positively regulates ABA and drought stress responses

Despite recent evidence that HDACs are involved in the environmental stress responses of plants, their roles in the abiotic stress responses of monocot plants remain largely unexplored. We investigated a HDAC gene, Bradi3g08060 (BdHD1), in Brachypodium distachyon. The Brachypodium BdHD1-overexpression plants displayed a hypersensitive phenotype to ABA and exhibited better survival under drought conditions. On the other hand, the RNA-interference plants were insensitive to ABA and showed low survival under drought stress. At the genome-wide level, overexpression of BdHD1 led to lower H3K9 acetylation at the transcriptional start sites of 230 genes than in the wild type plants under the drought treatment. We validated our ChIP-Seq data on 10 selected transcription factor genes from the 230 drought-specific genes. These genes exhibited much lower expression in the BdHD1-overexpression plants compared to the wild type plants under drought stress. We further identified an ABA-inducible transcription factor gene BdWRKY24 that was repressed in BdHD1-OE plants, but highly expressed in RNA-interference plants under drought stress. These results indicate that BdHD1 plays a positive role in ABA sensitivity and drought stress tolerance and they provide a link between the role of BdHD1 and the drought stress response at a genome-wide level in Brachypodium.

Mesophyll Abscisic Acid Restrains Early Growth and Flowering But Does Not Directly Suppress Photosynthesis

Abscisic acid (ABA) levels increase significantly in plants under stress conditions, and ABA is thought to serve as a key stress-response regulator. However, the direct effect of ABA on photosynthesis and the effect of mesophyll ABA on yield under both well-watered and drought conditions are still the subject of debate. Here, we examined this issue using transgenic Arabidopsis (Arabidopsis thaliana) plants carrying a dominant ABA-signaling inhibitor under the control of a mesophyll-specific promoter (FBPase::abi1-1, abbreviated to fa). Under normal conditions, fa plants displayed slightly higher stomatal conductance and carbon assimilation than wild-type plants; however, these parameters were comparable following ABA treatment. These observations suggest that ABA does not directly inhibit photosynthesis in the short term. The fa plants also exhibited a variety of altered phenotypes under optimal conditions, including more vigorous initial growth, earlier flowering, smaller flowers, and delayed chlorophyll degradation. Furthermore, under optimal conditions, fa plant seed production was less than a third of that observed for the wild type. However, under drought conditions, wild-type and fa seed yields were similar due to a significant reduction in wild-type seed and no reduction in fa seed. These findings suggest that endogenous basal ABA inhibits a stress-escape response under nonstressed conditions, allowing plants to accumulate biomass and maximize yield. The lack of a correlation between flowering time and plant biomass combined with delayed chlorophyll degradation suggests that this stress-escape behavior is regulated independently and upstream of other ABA-induced effects such as rapid growth and flowering.

Improved drought stress tolerance in Arabidopsis by CRISPR/dCas9 fusion with a Histone AcetylTransferase

Drought episodes decrease plant growth and productivity, which in turn cause high economic losses. Plants naturally sense and respond to water stress by activating specific signalling pathways leading to physiological and developmental adaptations. Genetically engineering genes that belong to these pathways might improve the drought tolerance of plants. The abscisic acid (ABA)-responsive element binding protein 1/ABRE binding factor (AREB1/ABF2) is a key positive regulator of the drought stress response. We investigated whether the CRISPR activation (CRISPRa) system that targets AREB1 might contribute to improve drought stress tolerance in Arabidopsis. Arabidopsis histone acetyltransferase 1 (AtHAT1) promotes gene expression activation by switching chromatin to a relaxed state. Stable transgenic plants expressing chimeric dCas9^HAT were first generated. Then, we showed that the CRISPRa dCas9^HAT mechanism increased the promoter activity controlling the β-glucuronidase (GUS) reporter gene. To activate the endogenous promoter of AREB1, the CRISPRa dCas9^HAT system was set up, and resultant plants showed a dwarf phenotype. Our qRT-PCR experiments indicated that both AREB1 and RD29A, a gene positively regulated by AREB1, exhibited higher gene expression than the control plants. The plants generated here showed higher chlorophyll content and faster stomatal aperture under water deficit, in addition to a better survival rate after drought stress. Altogether, we report that CRISPRa dCas9^HAT is a valuable biotechnological tool to improve drought stress tolerance through the positive regulation of AREB1.

Transcriptional profiling of contrasting genotypes revealed key candidates and nucleotide variations for drought dissection in Camellia sinensis (L.) O. Kuntze

Tea is popular health beverage consumed by millions of people worldwide. Drought is among the acute abiotic stress severely affecting tea cultivation, globally. In current study, transcriptome sequencing of four diverse tea genotypes with inherent contrasting genetic response to drought (tolerant & sensitive) generated more than 140 million reads. De novo and reference-based assembly and functional annotation of 67,093 transcripts with multifarious public protein databases yielded 54,484 (78.2%) transcripts with significant enrichment of GO and KEGG drought responsive pathways in tolerant genotypes. Comparative DGE and qRT analysis revealed key role of ABA dependent & independent pathways, potassium & ABC membrane transporters (AtABCG22, AtABCG11, AtABCC5 & AtABCC4) and antioxidant defence system against oxidative stress in tolerant genotypes, while seems to be failed in sensitive genotypes. Additionally, highly expressed UPL3HECT E3 ligases and RING E3 ligases possibly enhance drought tolerance by actively regulating functional modification of stress related genes. Further, ascertainment of, 80803 high quality putative SNPs with functional validation of key non-synonymous SNPs suggested their implications for developing high-throughput genotyping platform in tea. Futuristically, functionally relevant genomic resources can be potentially utilized for gene discovery, genetic engineering and marker-assisted genetic improvement for better yield and quality in tea under drought conditions.

Sweetpotato bZIP Transcription Factor IbABF4 Confers Tolerance to Multiple Abiotic Stresses

The abscisic acid (ABA)-responsive element binding factors (ABFs) play important regulatory roles in multiple abiotic stresses responses. However, information on the stress tolerance functions of ABF genes in sweetpotato (Ipomoea batatas [L.] Lam) remains limited. In the present study, we isolated and functionally characterized the sweetpotato IbABF4 gene, which encodes an abiotic stress-inducible basic leucine zipper (bZIP) transcription factor. Sequence analysis showed that the IbABF4 protein contains a typical bZIP domain and five conserved Ser/Thr kinase phosphorylation sites (RXXS/T). The IbABF4 gene was constitutively expressed in leaf, petiole, stem, and root, with the highest expression in storage root body. Expression of IbABF4 was induced by ABA and several environmental stresses including drought, salt, and heat shock. The IbABF4 protein localized to the nucleus, exhibited transcriptional activation activity, and showed binding to the cis-acting ABA-responsive element (ABRE) in vitro. Overexpression of IbABF4 in Arabidopsis thaliana not only increased ABA sensitivity but also enhanced drought and salt stress tolerance. Furthermore, transgenic sweetpotato plants (hereafter referred to as SA plants) overexpressing IbABF4, generated in this study, exhibited increased tolerance to drought, salt, and oxidative stresses on the whole plant level. This phenotype was associated with higher photosynthetic efficiency and lower malondialdehyde and hydrogen peroxide content. Levels of endogenous ABA content and ABA/stress-responsive gene expression were significantly upregulated in transgenic Arabidopsis and sweetpotato plants compared with wild-type plants under drought stress. Our results suggest that the expression of IbABF4 in Arabidopsis and sweetpotato enhances tolerance to multiple abiotic stresses through the ABA signaling pathway.

OsbZIP42 is a positive regulator of ABA signaling and confers drought tolerance to rice

OsbZIP42 is a positive regulator of ABA signaling and drought stress tolerance. The activation of OsbZIP42 depends on stress-/ABA-activated protein kinase 4 (SAPK4) and an additional ABA-dependent modification of OsbZIP42. Basic leucine zipper transcription factors (bZIP TFs) play important roles in the ABA signaling pathway in plants. Rice OsbZIP42 is a member of the group E bZIP, which is an ortholog of Arabidopsis group A bZIP. This latter group includes abscisic acid-responsive element (ABRE)-binding factors (ABFs) involved in abiotic stress tolerance. The expression of OsbZIP42 was induced by ABA treatment, although it was not induced by drought and salt stresses. Unlike other bZIP TFs, OsbZIP42 contained two transcriptional activation domains. Although the full-length OsbZIP42 protein did not, the N-terminus of the protein interacted with SAPK4. Our results suggest that the activation of OsbZIP42 by SAPK4 requires another ABA-dependent modification of OsbZIP42. Transgenic rice overexpressing OsbZIP42 (OsbZIP42-OX) exhibited a rapidly elevated expression of the ABA-responsive LEA3 and Rab16 genes and was hypersensitive to ABA. Analyses of the OsbZIP42-OX plants revealed enhanced tolerance to drought stress. These results suggest that OsbZIP42 is a positive regulator of ABA signaling and drought stress tolerance depending on its activation, which is followed by an additional ABA-dependent modification. We propose that OsbZIP42 is an important player in rice for conferring ABA-dependent drought tolerance.

Overexpression of an ABA-dependent grapevine bZIP transcription factor, VvABF2, enhances osmotic stress in Arabidopsis

Overexpression of grapevine VvABF2 gene could enhance osmotic stress tolerance in Arabidopsis thaliana but fully required for ABA signaling. The abscisic acid (ABA)-dependent AREB/ABF-SnRK2 pathway has been demonstrated to play a pivotal role in response to osmotic stress in model plants. However, its function in other specific species, for example grapevine, has not been fully characterized. In this study, grapevine (Vitis vinifera L.) ABSCISIC ACID RESPONSE ELEMENT-BINDING FACTOR2 (VvABF2), a homologous gene of AREB/ABFs form Arabidopsis, was isolated and constitutively expressed in Arabidopsis under the control of the cauliflower mosaic virus 35S promoter. The VvABF2 transgenic Arabidopsis plants showed to be more sensitive to exogenous ABA compared to wild type plants and exhibited significant osmotic tolerance, like polyethylene glycol (PEG) and drought but fully required ABA for signaling. This fact was further confirmed by its downstream gene expression assays. In addition, the determination of ROS antioxidant enzymes (including SOD, POD and CAT) and the MDA of transgenic lines indicated that overexpression of VvABF2 in Arabidopsis significantly increased ROS scavenging ability and thereby reduced the cell membrane damage, which might be ABA-independent. Our results provide evidence that VvABF2 has a similar function to the Arabidopsis homolog in response to osmotic stresses, and that there is a similar ancestral function of this gene in ABA-dependent response to stresses in grapevine.