Activation of the ABA Signal Pathway Mediated by GABA Improves the Drought Resistance of Apple Seedlings

Drought seriously affects the yield and quality of apples. γ-aminobutyric acid (GABA) plays an important role in the responses of plants to various stresses. However, the role and possible mechanism of GABA in the drought response of apple seedlings remain unknown. To explore the effect of GABA on apple seedlings under drought stress, seedlings of Malus hupehensis were treated with seven concentrations of GABA, and the response of seedlings under 15-day drought stress was observed. The results showed that 0.5 mM GABA was the most effective at relieving drought stress. Treatment with GABA reduced the relative electrical conductivity and MDA content of leaves induced by drought stress and significantly increased the relative water content of leaves. Exogenous GABA significantly decreased the stomatal conductance and intercellular carbon dioxide concentration and transpiration rate, and it significantly increased the photosynthetic rate under drought. GABA also reduced the accumulation of superoxide anions and hydrogen peroxide in leaf tissues under drought and increased the activities of POD, SOD, and CAT and the content of GABA. Exogenous treatment with GABA acted through the accumulation of abscisic acid (ABA) in the leaves to significantly decrease stomatal conductance and increase the stomatal closure rate, and the levels of expression of ABA-related genes PYL4, ABI1, ABI2, HAB1, ABF3, and OST1 changed in response to drought. Taken together, exogenous GABA can enhance the drought tolerance of apple seedlings.     

The E3 Ligase GmPUB21 Negatively Regulates Drought and Salinity Stress Response in Soybean

E3-ubiquitin ligases are known to confer abiotic stress responses in plants. In the present study, GmPUB21, a novel U-box E3-ubiquitin ligase-encoding gene, was isolated from soybean and functionally characterized. The expression of GmPUB21, which possesses E3-ubiquitin ligase activity, was found to be significantly up-regulated by drought, salinity, and ABA treatments. The fusion protein GmPUB21-GFP was localized in the cytoplasm, nucleus, and plasma membrane. Transgenic lines of the Nicotiana benthamiana over-expressing GmPUB21 showed more sensitive to osmotic, salinity stress and ABA in seed germination and inhibited mannitol/NaCl-mediated stomatal closure. Moreover, higher reactive oxygen species accumulation was observed in GmPUB21 overexpressing plants after drought and salinity treatment than in wild-type (WT) plants. Contrarily, silencing of GmPUB21 in soybean plants significantly enhanced the tolerance to drought and salinity stresses. Collectively, our results revealed that GmPUB21 negatively regulates the drought and salinity tolerance by increasing the stomatal density and aperture via the ABA signaling pathway. These findings improved our understanding of the role of GmPUB21 under drought and salinity stresses in soybean.     

Alleviation of Osmotic Stress Effects by Exogenous Application of Salicylic or Abscisic Acid on Wheat Seedlings

The aim of the study was to assess the role of salicylic acid (SA) and abscisic acid (ABA) in osmotic stress tolerance of wheat seedlings. This was accomplished by determining the impact of the acids applied exogenously on seedlings grown under osmotic stress in hydroponics. The investigation was unique in its comprehensiveness, examining changes under osmotic stress and other conditions, and testing a number of parameters simultaneously. In both drought susceptible (SQ1) and drought resistant (CS) wheat cultivars, significant physiological and biochemical changes were observed upon the addition of SA (0.05 mM) or ABA (0.1 μM) to solutions containing half-strength Hoagland medium and PEG 6000 (−0.75 MPa). The most noticeable result of supplementing SA or ABA to the medium (PEG + SA and PEG + ABA) was a decrease in the length of leaves and roots in both cultivars. While PEG treatment reduced gas exchange parameters, chlorophyll content in CS, and osmotic potential, and conversely, increased lipid peroxidation, soluble carbohydrates in SQ1, proline content in both cultivars and total antioxidants activity in SQ1, PEG + SA or PEG + ABA did not change the values of these parameters. Furthermore, PEG caused a two-fold increase of endogenous ABA content in SQ1 and a four-fold increase in CS. PEG + ABA increased endogenous ABA only in SQ1, whereas PEG + SA caused a greater increase of ABA content in both cultivars compared to PEG. In PEG-treated plants growing until the harvest, a greater decrease of yield components was observed in SQ1 than in CS. PEG + SA, and particularly PEG + ABA, caused a greater increase of these yield parameters in CS compared to SQ1. In conclusion, SA and ABA ameliorate, particularly in the tolerant wheat cultivar, the harmful effects and after effects of osmotic stress induced by PEG in hydroponics through better osmotic adjustment achieved by an increase in proline and carbohydrate content as well as by an increase in antioxidant activity.     

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.     

Molecular Dissection of the Gene OsGA2ox8 Conferring Osmotic Stress Tolerance in Rice

Gibberellin 2-oxidase (GA2ox) plays an important role in the GA catabolic pathway and the molecular function of the OsGA2ox genes in plant abiotic stress tolerance remains largely unknown. In this study, we functionally characterized the rice gibberellin 2-oxidase 8 (OsGA2ox8) gene. The OsGA2ox8 protein was localized in the nucleus, cell membrane, and cytoplasm, and was induced in response to various abiotic stresses and phytohormones. The overexpression of OsGA2ox8 significantly enhanced the osmotic stress tolerance of transgenic rice plants by increasing the number of osmotic regulators and antioxidants. OsGA2ox8 was differentially expressed in the shoots and roots to cope with osmotic stress. The plants overexpressing OsGA2ox8 showed reduced lengths of shoots and roots at the seedling stage, but no difference in plant height at the heading stage was observed, which may be due to the interaction of OsGA2ox8 and OsGA20ox1, implying a complex feedback regulation between GA biosynthesis and metabolism in rice. Importantly, OsGA2ox8 was able to indirectly regulate several genes associated with the anthocyanin and flavonoid biosynthetic pathway and the jasmonic acid (JA) and abscisic acid (ABA) biosynthetic pathway, and overexpression of OsGA2ox8 activated JA signal transduction by inhibiting the expression of jasmonate ZIM domain-containing proteins. These results provide a basis for a future understanding of the networks and respective phenotypic effects associated with OsGA2ox8.     

Arabidopsis Toxicos en Levadura 12 Modulates Salt Stress and ABA Responses in Arabidopsis thaliana

Salt is one of the most common abiotic stresses, causing ionic and osmotic pressure changes that affect plant growth and development. In this work, we present molecular and genetic evidence that Arabidopsis Toxicos en Levadura 12 (ATL12) is involved in both salt stress and in the abscisic acid response to this stress. We demonstrate that ATL12 is highly induced in response to salt stress and that atl12 mutants have a lower germination rate, decreased root length, and lower survival rate compared to the Col-0 wild-type in response to salt stress. Overexpression of ATL12 increases expression of the salt stress-associated genes SOS1/2, and ABA-responsive gene RD29B. Additionally, higher levels of reactive oxygen species are detected when ATL12 is overexpressed, and qRT-PCR showed that ATL12 is involved in the AtRBOHD/F-mediated signaling. ATL12 expression is also highly induced by ABA treatment. Mutants of atl12 are hypersensitive to ABA and have a shorter root length. A decrease in water loss and reduced stomatal aperture were also observed in atl12 mutants in response to ABA. ABA-responsive genes RD29B and RAB18 were downregulated in atl12 mutants but were upregulated in the overexpression line of ATL12 in response to ABA. Taken together our results suggest that ATL12 modulates the response to salt stress and is involved in the ABA signaling pathway in Arabidopsis thaliana.     

CAND2/PMTR1 Is Required for Melatonin-Conferred Osmotic Stress Tolerance in Arabidopsis

Osmotic stress severely inhibits plant growth and development, causing huge loss of crop quality and quantity worldwide. Melatonin is an important signaling molecule that generally confers plant increased tolerance to various environmental stresses, however, whether and how melatonin participates in plant osmotic stress response remain elusive. Here, we report that melatonin enhances plant osmotic stress tolerance through increasing ROS-scavenging ability, and melatonin receptor CAND2 plays a key role in melatonin-mediated plant response to osmotic stress. Upon osmotic stress treatment, the expression of melatonin biosynthetic genes including SNAT1, COMT1, and ASMT1 and the accumulation of melatonin are increased in the wild-type plants. The snat1 mutant is defective in osmotic stress-induced melatonin accumulation and thus sensitive to osmotic stress, while exogenous melatonin enhances the tolerance of the wild-type plant and rescues the sensitivity of the snat1 mutant to osmotic stress by upregulating the expression and activity of catalase and superoxide dismutase to repress H₂O₂ accumulation. Further study showed that the melatonin receptor mutant cand2 exhibits reduced osmotic stress tolerance with increased ROS accumulation, but exogenous melatonin cannot revert its osmotic stress phenotype. Together, our study reveals that CADN2 functions necessarily in melatonin-conferred osmotic stress tolerance by activating ROS-scavenging ability in Arabidopsis.     

Effect of high pressure on mass transfer kinetics of granny smith apple

The mass transfer kinetics during osmotic dehydration of granny smith apple slices in 60 Brix fructose and sucrose solution was studied at atmospheric pressure and at elevated pressure of 200–600?MPa at 40°C. The moisture and solute fractions in apple slices during osmotic dehydration under high pressure were predicted by Weibull frequency distribution model. The calculated effective moisture diffusivity values of apple slices suspended in fructose and sucrose solution during high-pressure treatment (0.1–600?MPa) were in the range of 6.35?×?10^?10 to 3.60?×?10^?9?m²/s and 7.96?×?10^?10 to 4.32?×?10^?9?m²/s, respectively.     

Knockout of Auxin Response Factor SlARF4 Improves Tomato Resistance to Water Deficit

Auxin response factors (ARFs) play important roles in various plant physiological processes; however, knowledge of the exact role of ARFs in plant responses to water deficit is limited. In this study, SlARF4, a member of the ARF family, was functionally characterized under water deficit. Real-time fluorescence quantitative polymerase chain reaction (PCR) and β-glucuronidase (GUS) staining showed that water deficit and abscisic acid (ABA) treatment reduced the expression of SlARF4. SlARF4 was expressed in the vascular bundles and guard cells of tomato stomata. Loss of function of SlARF4 (arf4) by using Clustered Regularly Interspaced Short Palindromic Repeats/Cas 9 (CRISPR/Cas 9) technology enhanced plant resistance to water stress and rehydration ability. The arf4 mutant plants exhibited curly leaves and a thick stem. Malondialdehyde content was significantly lower in arf4 mutants than in wildtype plants under water stress; furthermore, arf4 mutants showed higher content of antioxidant substances, superoxide dismutase, actual photochemical efficiency of photosystem II (PSII), and catalase activities. Stomatal and vascular bundle morphology was changed in arf4 mutants. We identified 628 differentially expressed genes specifically expressed under water deficit in arf4 mutants; six of these genes, including ABA signaling pathway-related genes, were differentially expressed between the wildtype and arf4 mutants under water deficit and unlimited water supply. Auxin responsive element (AuxRE) elements were found in these genes’ promoters indicating that SlARF4 participates in ABA signaling pathways by regulating the expression of SlABI5/ABF and SCL3, thereby influencing stomatal morphology and vascular bundle development and ultimately improving plant resistance to water deficit.     

Genome-Wide Analysis of the Peptidase M24 Superfamily in Triticum aestivum Demonstrates That TaM24-9 Is Involved in Abiotic Stress Response

The peptidase M24 (Metallopeptidase 24, M24) superfamily is essential for plant growth, stress response, and pathogen defense. At present, there are few systematic reports on the identification and classification of members of the peptidase M24 proteins superfamily in wheat. In this work, we identified 53 putative candidate TaM24 genes. According to the protein sequences characteristics, these members can be roughly divided into three subfamilies: I, II, III. Most TaM24 genes are complex with multiple exons, and the motifs are relatively conserved in each sub-group. Through chromosome mapping analysis, we found that the 53 genes were unevenly distributed on 19 wheat chromosomes (except 3A and 3D), of which 68% were in triads. Analysis of gene duplication events showed that 62% of TaM24 genes in wheat came from fragment duplication events, and there were no tandem duplication events to amplify genes. Analysis of the promoter sequences of TaM24 genes revealed that cis-acting elements were rich in response elements to drought, osmotic stress, ABA, and MeJA. We also studied the expression of TaM24 in wheat tissues at developmental stages and abiotic stress. Then we selected TaM24-9 as the target for further analysis. The results showed that TaM24-9 genes strengthened the drought and salt tolerance of plants. Overall, our analysis showed that members of the peptidase M24 genes may participate in the abiotic stress response and provided potential gene resources for improving wheat resistance.     

Calcium-Dependent Protein Kinase 28 Maintains Potato Photosynthesis and Its Tolerance under Water Deficiency and Osmotic Stress

Calcium-dependent protein kinases (CDPK) are implicated in signaling transduction in eukaryotic organisms. It is largely unknown whether StCDPK28 plays a role in the response to water deficiency and osmotic stress in potato plants (Solanum tuberosum L.). Potato cv. Zihuabai was cultivated under natural, moderate, and severe water deficiency conditions; to induce osmotic stress, potato plants were treated with 10% or 20% PEG. StCDPK28-overexpression and StCDPK28-knockdown plants were constructed. StCDPKs were evaluated by qRT-PCR. The subcellular location of the StCDPK28 protein was observed with confocal scanning laser microscopy. Phenotypic changes were indicated by photosynthetic activity, the contents of H₂O₂, MDA and proline, and the activities of CAT, SOD and POD. Results showed water deficiency and osmotic stress altered StCDPK expression patterns. StCDPK28 exhibited a membrane, cytosolic and nuclear localization. Water deficiency and osmotic stress induced StCDPK28 upregulation. Photosynthetic activity was enhanced by StCDPK28 overexpression, while decreased by StCDPK2 knockdown under water deficiency and osmotic stress. StCDPK28 overexpression decreased H₂O₂ and MDA, and increased proline, while StCDPK28 knockdown showed reverse results, compared with the wild type, in response to water deficiency and osmotic stress. StCDPK28 overexpression increased the activities of CAT, SOD and POD, while StCDPK28-knockdown plants indicated the reverse trend under water deficiency and osmotic stress conditions. Regulation of StCDPK28 expression could be a promising approach to improve the tolerance ability of potato plants in response to drought or high salt media.