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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 phytohormone abscisic acid (ABA) is involved in various plant processes. In response to drought stress, plants quickly accumulate ABA, but the regulatory mechanism of ABA accumulation is largely unknown, especially in woody plants. In this study, we report that MdMYB88 and MdMYB124 are myeloblastosis (MYB) transcription factors critical for ABA accumulation in apple trees (Malus x domestica) following drought, and this regulation is negatively controlled by ABA. MdMYB88 and MdMYB124 positively regulate leaf water transpiration, photosynthetic capacity, and stress endurance in apple trees under drought conditions. MdMYB88 and MdMYB124 regulate the expression of biosynthetic and catabolic genes of ABA, as well as drought- and ABA- responsive genes. MdMYB88 associates with promoter regions of the ABA biosynthetic gene 9-cis-epoxycarotenoid dioxygenase 3 (NCED3). Finally, expression of MdMYB88 and MdMYB124 is repressed by ABA. Our results identify a feedback regulation of MdMYB88 and MdMYB124 in modulating ABA homeostasis in apple trees., (© The Author(s) 2020. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

MhYTP1 is involved in post-transcriptional regulation as a member of YT521-homology (YTH) domain-containing RNA-binding proteins. We previously cloned MhYTP1 and found it participated in various biotic and abiotic stress responses. However, its function in long-term moderate drought has not been verified. Thus, we explored its biological role in response to drought. Under drought condition, the net photosynthesis rate (P n ) and water use efficiency (WUE) were significantly elevated in MhYTP1-overexpressing (OE) apple plants when compared with the non-transgenic (NT) controls. Further analysis indicated MhYTP1 expression was associated with elevated ABA content, increased stomatal density and reduced stomatal aperture. In addition, to gain insight into the function of stem-specific expression of MhYTP1, grafting experiments were performed. Interestingly, lower transpiration rate (T r ) and higher WUE were observed when transgenic plants were used as scions as opposed to rootstocks and when transgenic rather than NT plants were used as rootstocks, indicating MhYTP1 plays crucial roles in grafted plants. These results define a function for MhYTP1 in promoting tolerance to drought conditions, and suggest that MhYTP1 can serve as a candidate gene for future apple drought resistance breeding with the help of biotechnology., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Drought is a major abiotic stress limiting the growth and production of apple trees worldwide. The receptor-like kinase FERONIA is involved in plant growth, development and stress responses; however, the function of FERONIA in apple under drought stress remains unclear. Here, the FERONIA receptor kinase gene MdMRLK2 from apple (Malus domestica) was shown to encode a plasma membrane-localized transmembrane protein and was significantly induced by abscisic acid and drought treatments. 35S::MdMRLK2 apple plants showed less photosystem damage and higher photosynthetic rates compared with wild-type (WT) plants, after withholding water for 7 days. 35S::MdMRLK2 apple plants also had enhanced energy levels, activated caspase activity and more free amino acids, than the WT, under drought conditions. By performing yeast two-hybrid screening, glyceraldehyde-3-phosphate dehydrogenase and MdCYS4, a member of cystatin, were identified as MdMRLK2 interaction partners. Moreover, under drought conditions, the 35S::MdMRLK2 apple plants were characterized by higher abscisic acid (ABA) content. Overall, these findings demonstrated that MdMRLK2 regulates apple drought tolerance, probably via regulating levels of energetic matters, free amino acids and ABA. [ABSTRACT FROM AUTHOR]

Valsa canker, caused by the fungus Valsa mali, is one of the most destructive diseases of apple trees in China and other East Asian countries. The plant receptor‐like kinase FERONIA is involved in plant cell growth, development, and immunity. However, little is known about the function of FERONIA in apple defence against V. mali. In this study, we found that MdMRLK2 was highly induced by V. mali in twigs of V. mali‐susceptible Malus mellana but not in those of the resistant species Malus yunnaensis. 35S:MdMRLK2 apple plants showed compromised resistance relative to wild‐type (WT) plants. Further analyses indicated that 35S:MdMRLK2 apple plants had enhanced abscisic acid (ABA) levels and reduced salicylic acid (SA) levels relative to the WT on V. mali infection. MdMRLK2 overexpression also suppressed polyphenol accumulation and inhibited the activities of phenylalanine ammonia‐lyase (PAL), β‐1,3‐glucanase (GLU), and chitinase (CHT) during V. mali infection. Moreover, MdMRLK2 interacted with MdHIR1, a hypersensitive‐induced response protein, and suppressed the MdHIR1‐mediated hypersensitive reaction (HR), probably by impairing MdHIR1 self‐interaction. Collectively, these findings demonstrate that overexpression of MdMRLK2 compromises Valsa canker resistance, probably by (a) altering ABA and SA levels, (b) suppressing polyphenol accumulation, (c) inhibiting PAL, GLU, and CHT activities, and (d) blocking MdHIR1‐mediated HR by disrupting MdHIR1 self‐interaction. [ABSTRACT FROM AUTHOR]

SUMMARY: Fructose (Fru) content is a key determinant of fruit sweetness and quality. An F1 hybrid population of the apple cultivars 'Honeycrisp' × 'Qinguan' was used to investigate the quantitative trait locus (QTL) regions and genes controlling Fru content in fruit. A stable QTL on linkage group (LG) 01 in 'Honeycrisp' was detected on the single nucleotide polymorphism (SNP) genetic linkage maps. In this region, a sorbitol dehydrogenase (SDH) gene, MdSDH2, was detected and showed promoter variations and differential expression patterns between 'Honeycrisp' and 'Qinguan' fruits as well as their hybrids. A SNP variant (A/G) in the MdSDH2 promoter region (SDH2p‐491) affected the binding ability of the transcription factor MdABI3, which can affect the expression of MdSDH2. Promoter sequences with an A nucleotide at SDH2p‐491 had stronger binding affinity for MdABI3 than those with a G. Among 27 domesticated apple cultivars and wild relatives, this SNP (A/G) was associated with Fru content. Our results indicate that MdSDH2 can alter Fru content as the major regulatory gene and that ABA signaling might be involved in Fru content accumulation in apple fruit. Significance Statement: Here, we detected a stable overlapping QTL region of fructose content in apple fruit with a SNP genetic map, and confirmed that A/G SNP variation in the promoter of the sorbitol dehydrogenase gene MdSDH2 in this QTL region affects binding of the transcription factor MdABl3, which regulates MdSDH2 expression and fructose accumulation. Our results demonstrate the major role of the candidate gene MdSDH2 in regulating fructose content of apple fruit. [ABSTRACT FROM AUTHOR]

The function of serrate (SE) in miRNA biogenesis in Arabidopsis is well elucidated, whereas its role in plant drought resistance is largely unknown. In this study, we report that MdSE acts as a negative regulator of apple (Malus × domestica) drought resistance by regulating the expression levels of MdMYB88 and MdMYB124 and miRNAs, including mdm-miR156, mdm-miR166, mdm-miR172, mdm-miR319, and mdm-miR399. MdSE interacts with MdMYB88 and MdMYB124, two positive regulators of apple drought resistance. MdSE decreases the transcript and protein levels of MdMYB88 and MdMYB124, which directly regulate the expression of MdNCED3, a key enzyme in abscisic acid (ABA) biosynthesis. Furthermore, MdSE is enriched in the same region of the MdNECD3 promoter where MdMYB88/MdMYB124 binds. Consistently, MdSE RNAi transgenic plants are more sensitive to ABA-induced stomatal closure, whereas MdSE OE plants are less sensitive. In addition, under drought stress, MdSE is responsible for the biogenesis of mdm-miR399, a negative regulator of drought resistance, and negatively regulates miRNAs, including mdm-miR156, mdm-miR166, mdm-miR172, and mdm-miR319, which are positive regulators of drought resistance. Taken together, by revealing the negative role of MdSE, our results broaden our understanding of the apple drought response and provide a candidate gene for apple drought improvement through molecular breeding. [ABSTRACT FROM AUTHOR]

Drought and nitrogen (N) deficiency are common factors that limit apple production in the Loess Plateau region of China. Different apple cultivars respond to drought and low N differently; however, the mechanism that underlies the difference in nitrogen-use efficiency (NUE) under drought conditions is not well understood. In this study, by comparing the physiological responses of two apple (Malus domestica Borkh.) cultivars with contrasting NUE, 'Qinguan' (higher NUE) and 'Honeycrisp' (lower NUE), under low N and drought conditions, we discovered that, 'Qinguan' had larger stomatal apertures, higher chlorophyll fluorescence levels, more active N metabolism and antioxidant enzymes, higher abscisic acid and auxin concentrations, larger root size and more efficient N uptake mediated by higher expression of MdNRT2.4 in rootstock than that of 'Honeycrisp'. Additionally, we experimentally confirmed that MdNRT2.4 enhanced low N and osmotic stress tolerance in Arabidopsis when being overexpressed. Taken together, our findings shed light on the mechanism that underlies the difference in NUE of apple under drought and N-deficient conditionss and provide MdNRT2.4 as a candidate gene for future genetic engineering. [ABSTRACT FROM AUTHOR]

Tolerance to drought stress is significantly enhanced following pre-treatment with melatonin in plants of two Malus species. Mechanisms involve regulation of ABA metabolism, free-radical detoxification, and improved functioning of stomata.Melatonin pre-treatment significantly increases the tolerance of both drought-tolerant Malus prunifolia and drought-sensitive M. hupehensis plants. Its beneficial effects include better water conservation in leaves, less electrolyte leakage, steady chlorophyll contents, and greater photosynthetic performance under stress conditions. Melatonin selectively down-regulates MdNCED3, an abscisic acid (ABA) synthesis gene, and up-regulates its catabolic genes, MdCYP707A1 and MdCYP707A2, thereby reducing ABA contents in drought-stressed plants. Melatonin also directly scavenges H2O2 and enhances the activities of antioxidant enzymes to detoxify H2O2 indirectly. These two mechanisms work synergistically to improve the functions of stomata, i.e. causing them to re-open. Plants can effectively regulate their water balance under drought conditions by up-regulating the expression of melatonin synthesis genes MdTDC1, MdAANAT2, MdT5H4, and MdASMT1. Therefore, inducing melatonin production is an important mechanism by which plants can counteract the influence of this abiotic stressor. [ABSTRACT FROM AUTHOR]

Melatonin, a widely known indoleamine molecule that mediates various animal and plant physiological processes, is formed from N-acetyl serotonin via N-acetylserotonin methyltransferase (ASMT). ASMT is an enzyme that catalyzes melatonin synthesis in plants in the rate-determining step and is homologous to hydroxyindole-O-methyltransferase (HIOMT) melatonin synthase in animals. To date, little is known about the effect of HIOMT on salinity in apple plants. Here, we explored the melatonin physiological function in the salinity condition response by heterologous expressing the homologous human HIOMT gene in apple plants. We discovered that the expression of melatonin-related gene (MdASMT) in apple plants was induced by salinity. Most notably, compared with the wild type, three transgenic lines indicated higher melatonin levels, and the heterologous expression of HIOMT enhanced the expression of melatonin synthesis genes. The transgenic lines showed reduced salt damage symptoms, lower relative electrolyte leakage, and less total chlorophyll loss from leaves under salt stress. Meanwhile, through enhanced activity of antioxidant enzymes, transgenic lines decreased the reactive oxygen species accumulation, downregulated the expression of the abscisic acid synthesis gene (MdNCED3), accordingly reducing the accumulation of abscisic acid under salt stress. Both mechanisms regulated morphological changes in the stomata synergistically, thereby mitigating damage to the plants' photosynthetic ability. In addition, transgenic plants also effectively stabilized their ion balance, raised the expression of salt stress–related genes, as well as alleviated osmotic stress through changes in amino acid metabolism. In summary, heterologous expression of HIOMT improved the adaptation of apple leaves to salt stress, primarily by increasing melatonin concentration, maintaining a high photosynthetic capacity, reducing reactive oxygen species accumulation, and maintaining normal ion homeostasis. [ABSTRACT FROM AUTHOR]

Plant hormones play important roles in regulating developmental processes and signaling networks involved in plant responses to biotic and abiotic stresses. We comparatively studied the growth and endogenous hormonal levels in leaves and roots in two Malus species ( M. sieversii and M. hupehensis) differing in hypoxia tolerance under normoxic and hypoxia stress. The results showed that hypoxia stress inhibited growth of seedlings of both Malus species, but with significant differences in intensity . Exposure to hypoxia altered the levels of endogenous hormones in leaves and roots in both Malus seedlings. Leaf and root abscisic acid (ABA) contents increased in response to hypoxia stress in both genotypes despite different extents. Compared with M. hupehensis, M. sieversii was more responsive to hypoxia stress, resulting in larger increases in leaf and root ABA contents. The changes in leaf and root ABA contents correlating with the different tolerance levels of the genotypes confirm the involvement of this hormone in plant responses to hypoxia stress. Gibberellins (GAs; GA + GA) continuously increased in leaves and roots during the whole period of stress, whereas indole-3-acetic acid (IAA) showed a sharp increase at the early stage in both Malus seedlings. In addition, zeatin riboside (ZR), dihydrozeatin riboside (DHZR), and isopentenyl adenine (IPA) differed in their pattern of changes in both Malus seedlings under hypoxia stress. Based on variations in endogenous hormonal levels in both Malus species that differ in their ability to tolerate hypoxia, we conclude that not a single hormone but multiple hormones and their interplay are responsible for hypoxia tolerance. [ABSTRACT FROM AUTHOR]

Plant proline-rich proteins (PRPs) are cell wall proteins that occur in the plant kingdom and are involved in plant development and stress response. In this study, 9 PRP genes were identified from the apple genome and a comprehensive analysis of the PRP family was conducted, including gene structures, phylogenetic analysis, chromosome mapping, and so on. The expression of MdPRPs varied among tissues and in response to different types of stresses. MdPRP4 and MdPRP7 were induced by five detected stress treatments, including heat, drought, abscisic acid, cold, and salt; the expression patterns of the others varied under different types of stress. Subcellular localization showed that MdPRPs mainly functioned in the cytoplasm, except for MdPRP1 and MdPRP5, which also functioned in the nucleus. When MdPRP6 was overexpressed in tobacco, the transgenic plants showed higher tolerance to high temperature (48 °C) compared with wild-type (WT) plants. The transgenic plants showed milder wilting, a lower accumulation of electrolyte leakage, MDA and ROS, and a higher level of chlorophyll and SOD and POD activity, indicating that MdPRP6 may be an important gene in apples for heat stress tolerance. Overall, this study suggested that MdPRPs are critically important for the ability of apple responses to stresses. [ABSTRACT FROM AUTHOR]

Plant height is an important trait for fruit trees. The dwarf characteristic is commonly associated with highly efficient fruit production, a major objective when breeding for apple (Malus domestica). We studied the function of MdNAC1, a novel NAC transcription factor (TF) gene in apple related to plant dwarfing. Localized primarily to the nucleus, MdNAC1 has transcriptional activity in yeast cells. Overexpression of the gene results in a dwarf phenotype in transgenic apple plants. Their reduction in size is manifested by shorter, thinner stems and roots, and a smaller leaf area. The transgenics also have shorter internodes and fewer cells in the stems. Levels of endogenous abscisic acid (ABA) and brassinosteroid (BR) are lower in the transgenic plants, and expression is decreased for genes involved in the biosynthesis of those phytohormones. All of these findings demonstrate that MdNAC1 has a role in plants dwarfism, probably by regulating ABA and BR production. [ABSTRACT FROM AUTHOR]

The Loess Plateau is the main apple production area in China; low precipitation is one of the most important factors limiting apple production here. Autophagy is a conserved process in eukaryotes that recycles cell contents or damaged macromolecules. Previously, we identified an autophagy-related gene MdATG10 from apple plants, which was involved in the responses to stressed conditions. In this study, we found that MdATG10 improved the drought tolerance and water use efficiency (WUE) of transgenic apple plants. MdATG10 -overexpressing (OE) apple plants were more tolerant of short-term drought stress, as evidenced by their fewer drought-related injuries, compared with wild-type (WT) apple plants. In addition, the WUE of OE plants was higher than that of WT plants under long-term moderate water deficit conditions. The growth rate, biomass accumulation, photosynthetic efficiency, and stomatal aperture were higher in OE plants than in WT plants under long-term moderate drought conditions. During the process of adapting to drought, the expressions of genes involved in the abscisic acid (ABA) pathway were reduced in OE plants to decrease the synthesis of ABA, which helped maintain the stomatal opening for gas exchange. Furthermore, autophagic activity was higher in OE plants than in WT plants, as evidenced by the higher expressions of ATG genes and the greater number of autophagy bodies. In sum, our results suggested that overexpression of MdATG10 improved drought tolerance and WUE in apple plants, possibly by regulating stomatal movement and enhancing autophagic activity, which then enhanced the photosynthetic efficiency and reduced damage, as well as the reactive oxygen species (ROS) accumulation in apple plants. • MdATG10 -mediated autophagy positively correlated with WUE regulation, as well as drought tolerance. • Autophagy enhanced the photosynthetic efficiency in apple plants and reduced ROS damage. • Autophagy may regulate stomatal movement by affecting the ABA signaling pathway. [ABSTRACT FROM AUTHOR]

• RNAi of MdRAD23D1 decreased WUE and adaptability of apple to long-term drought. • RNAi of MdRAD23D1 exhibited lower photosynthetic capacity under long-term drought. • RNAi of MdRAD23D1 affected stomatal behavior in apple under long-term drought. • RNAi of MdRAD23D1 affected amino acid accumulation in apple under long-term drought. Water deficit is a challenge for apple (Malus domestica) growth and productivity worldwide, and improving water use efficiency (WUE) is important for apple plants to adapt to arid climates. RAD23 (RADIATION SENSITIVE23) proteins are a group of UBL-UBA (ubiquitin-like-ubiquitin-associated) proteins that shuttle ubiquitylated proteins to the 26S proteasome for breakdown. Here, we explored the biological function of a UBL-UBA gene MdRAD23D1 from apple in regulating WUE under long-term moderate drought. The results showed that MdRAD23D1 -RNAi (suppressing MdRAD23D1 by RNA interference) apple plants were smaller, and had lower WUE under long-term moderate drought. When treated with long-term moderate drought, the RNAi plants exhibited less biomass accumulation and relative water content than WT (wild type) plants. The roots of RNAi plants were smaller, the roots fresh and dry weights of RNAi plants were lower, as well as lower root activity and hydraulic conductivity compared with WT. Photosynthetic capacity decreased and the photosystems were much more damaged in RNAi plants than in WT. The RNAi plants also accumulated more reactive oxygen species and had lower activities of superoxide dismutase, peroxidase and catalase under long-term moderate drought. In addition, MdRAD23D1 -RNAi apple plants showed higher ABA (abscisic acid) contents and stomatal apertures were more affected by drought stress. The free amino acid contents were lower in RNAi plants than in WT. In summary, our results indicated that suppressing MdRAD23D1 expression decreased WUE in apple plants under long-term drought stress, possibly by affecting photosynthetic efficiency, stomatal behavior, and accumulation of amino acids. [ABSTRACT FROM AUTHOR]

• Exogenous DA delays leaf senescence of apple. • DA reduces phytohormone levels and inhibits signal pathways. • DA reduces the degradation rate of soluble sugar. Leaf senescence is the ultimate stage in leaf development, accompanied by the degradation and redistribution of nutrients and biological macromolecules. Dopamine (DA), a type of catecholamine, plays a role in responding to various plant stresses through its strong antioxidant capacity. However, the relationship between DA and leaf senescence is poorly understood. Thus, leaves isolated from Malus hupehensis were treated with DA, and a dark-induced senescence experiment was conducted. We first found that 100 μmol L−1 DA was the optimal treatment for delaying leaf senescence. The leaves treated with 100 μmol L−1 DA showed higher chlorophyll content and F v /F m , and lower relative electrolyte leakage and malondialdehyde content during dark-induced senescence. DA was able to markedly decrease the expression of senescence associated genes (MdSAG12, MdSAG13 , and MdSAG29) and a chlorophyll degradation gene (MdPAO). Compared with untreated leaves, the activity of antioxidant enzymes of leaves under DA treatment was significantly higher, and the level of reactive oxygen species (ROS) was significantly lower. Moreover, DA significantly reduced the content of phytohormones (1-aminocyclopropane-1-carboxylate acid, abscisic acid, salicylic acid, and jasmonic acid) that accelerate senescence during dark treatment and changed the expression of genes involved in phytohormone pathways to inhibit the transmission of signals. In addition, DA treatment significantly reduced the expression of genes related to the degradation of soluble sugar (fructose, glucose, sucrose, and sorbitol), thus significantly increasing the soluble sugar content relative to untreated leaves. Overall, our findings revealed that exogenous dopamine could decrease the expression of SAG s, improve the activity of antioxidant enzymes, reduce ROS levels, and the content of phytohormones that accelerate senescence and their signalling pathways, and decrease the degradation rate of soluble sugar to delay the senescence of apple leaves. [ABSTRACT FROM AUTHOR]

Abstract Members of the sucrose non-fermenting-1-related protein kinase 2 (SnRK2) family play central roles in the abscisic acid (ABA) signaling pathway and in mediating osmotic stress signaling and tolerance in plants. Previously, 12 full-length coding sequences that belong to SnRK2 gene family were identified and cloned in wild apple species Malus prunifolia. In this study, one of the members, MpSnRK2.10 , was overexpressed in Arabidopsis and apple to investigate its potential function in response to drought stress. The results showed that overexpression of this gene did not affect plant growth under normal conditions. However, the transgenic plants showed enhanced tolerance to drought, as indicated by the amelioration in phenotype appearance and physiological indices related to drought stress damage. Additionally, transgenic apple plants overexpressing MpSnRK2.10 exhibited greater sensitivity to ABA compared to wild-type (WT) plants. Moreover, expressions of three stress response genes, MdRAB18 , MdRD22, and MdRD29B , were more strongly induced in transgenic apple plants than in the WT when subjected to ABA and mannitol treatments. Taken as a whole, our study suggests that MpSnRK2.10 has a role in the enhancement of ABA signal transduction in response to stress, and that the manipulation of MpSnRK2.10 expression could be a feasible approach for improving abiotic stress tolerance in apple and other important crops. Highlights • Overexpression of MpSnRK2.10 enhanced drought stress-tolerance in transgenic plants. • Overexpression of MpSnRK2.10 did not affect plant growth under normal conditions. • MpSnRK2.10 -overexpressing apple exhibited greater sensitivity to ABA. • Some stress-responsive genes showed greater induction in transgenic apple under stress. [ABSTRACT FROM AUTHOR]

Highlights • Overexpression of MhYTP2 improves apple water-use efficiency. • MhYTP2 decreases the transpiration rate through activation of ABA and ethylene signaling. • MhYTP2 plays more pivotal roles in rootstock than in scion. Abstract A critical component of crop breeding is the identification of genes involved in improving water-use efficiency (WUE). We characterized MhYTP2 , a gene cloned from Malus hupehensis that encodes an RNA-binding protein, to examine its role in WUE. Under both well-watered and drought conditions, overexpression of this gene enhanced WUE in transgenic plants and they accumulated more biomass than untransformed wild-type plants. This increase in WUE was probably due to enhancement of photosynthetic rate and root water uptake, and a reduction in stomatal aperture that resulted from elevated level of ABA and activated ethylene signaling. MhYTP2 was predominantly expressed in apple roots. We noted with interest that the rate of photosynthesis, instantaneous WUE, and root water potential were higher when 35S:MhYTP2 plants were used as rootstocks rather than as scions in our grafting experiments. Our results indicated that MhYTP2 has a more crucial role in the roots than in the aerial parts of the plant, and it implies that this gene can be used for improving WUE by apple rootstocks and, potentially, rootstocks of other graft-suitable crops. [ABSTRACT FROM AUTHOR]

• 0.5 g·L−1 CHT effectively alleviated Glomerella leaf spot caused by G. cingulata on apple leaves. • Exogenous CHT elevated the SA-mediated defense response, including up-regulated the expression of PRs and PAL. • Exogenous CHT enhanced the activity of antioxidant enzymes, and the accumulation of amino acids and phenolic compounds. • Exogenous CHT reduced the content of ABA and ACC. Glomerella leaf spot (GLS), caused by the fungal phytopathogen Glomerella cingulata , is one of the most devastating premature defoliation diseases affecting apple production. Chitosan (CHT) is a degraded product of chitin and is used for bacteriostasis and postharvest preservation. Herein, we investigated the effects of CHT on improving the apple resistance to GLS. We identified that spraying with 0.5 g·L−1 exogenous CHT was the optimal concentration to reduce the G. cingulata disease severity on detached leaves of apple seedlings. Further study demonstrated that the exogenous CHT application significantly enhanced the superoxide dismutase (SOD) activity, elevated alicylic acid (SA) and jasmonic acid (JA) contents, increased certain kinds of phenolic compound contents and up-regulated the expression of different disease resistance-related genes in inoculated leaves than plants without CHT pretreatment. Furthermore, the exogenous CHT application significantly reduced the contents of abscisic acid (ABA), 1-aminocyclopropanecarboxylic acid (ACC), and auxin (IAA) of G. cingulata infection. Additionally, the CHT spraying treatment reduced the amino acid contents compared with the control plants. The results indicated that exogenous spraying of CHT improved the resistance of apples to GLS through enhancing plant immunity. [ABSTRACT FROM AUTHOR]

• Finding that salt stress and ABA treatment induced the expression of MpSnRK2.10. • Finding that overexpression of MpSnRK2.10 alleviated growth restriction caused by salt stress in apple. • Finding that MpSnRK2.10 alleviated the salt stress inhibition of apple plants by altering the Na+ balance and ABA pathway. Sucrose non-fermentation-1 related protein kinase 2 (SnRK2) family members play an important role in the abscisic acid (ABA) signaling pathway and adaptation to various environments in plants. Here, we found that salt stress and ABA treatment induced the expression of MpSnRK2.10 , and overexpression of this gene alleviated growth restriction caused by salt stress in apple. After treated with salt stress for 15 d, the relative electrolyte leakage, malondialdehyde content, and reactive oxygen species production were lower in overexpression (OE) lines, and the chlorophyll content, superoxide dismutase activity, photosynthetic capacity, and accumulation of several amino acids were higher in OE lines than in wild-type (WT) plants. OE plants also accumulated less Na+ in response to salt stress, and several genes involved in Na+ balance under salt stress were up-regulated in OE plants compared with WT plants. Overexpression of MpSnRK2.10 altered the role of the ABA pathway in the response to salt stress. We found that the ABA content and stomatal aperture were greater in OE plants than in WT plants. Exogenous ABA and the ABA inhibitor nordihydroguaiaretic acid (NDGA) altered the morphology and physiological status of plants under salt stress, especially OE plants, which became more sensitive to salt stress treatment. The ABA content and stomatal aperture were also altered in response to ABA and NDGA. In sum, our results showed that MpSnRK2.10 alleviated the salt stress inhibition of apple plants by altering the Na+ balance and ABA pathway. [ABSTRACT FROM AUTHOR]

Sucrose non-fermenting-1-related protein kinase 2 (SnRK2) constitutes a small plant-specific serine/threonine kinase family with essential roles in the abscisic acid (ABA) signal pathway and in responses to osmotic stress. Although a genome-wide analysis of this family has been conducted in some species, little is known about SnRK2 genes in apple ( Malus domestica ). We identified 14 putative sequences encoding 12 deduced SnRK2 proteins within the apple genome. Gene chromosomal location and synteny analysis of the apple SnRK2 genes indicated that tandem and segmental duplications have likely contributed to the expansion and evolution of these genes. All 12 full-length coding sequences were confirmed by cloning from Malus prunifolia . The gene structure and motif compositions of the apple SnRK2 genes were analyzed. Phylogenetic analysis showed that MpSnRK2s could be classified into four groups. Profiling of these genes presented differential patterns of expression in various tissues. Under stress conditions, transcript levels for some family members were up-regulated in the leaves in response to drought, salinity, or ABA treatments. This suggested their possible roles in plant response to abiotic stress. Our findings provide essential information about SnRK2 genes in apple and will contribute to further functional dissection of this gene family. [ABSTRACT FROM AUTHOR]

Abscisic acid (ABA) is an important phytohormone for plant growth, development, and the response to various types of abiotic stress. Pyrabactin Resistance 1 (PYR1)/PYR1-Like (PYL)/Regulatory Component of ABA Receptor (RCAR) family proteins (PYLs) are ABA receptors that have been studied in many plant species. However, no detailed functional studies of these genes in regulating apple drought tolerance have been reported. Here, we isolated a homolog of the ABA receptor PYL9 in Arabidopsis, MdPYL9, from apple (Malus domestica) and analyzed its role in regulating drought tolerance in transgenic apple plants. Gene structure and protein sequence analysis showed that MdPYL9 shares a high similarity with AtPYL9. Protein sequence comparison and 3D structure prediction identified several conserved motifs and residues that are important for ABA binding in MdPYL9, including the helix-grip structure and the gate and latch loops. Expression analysis showed that MdPYL9 was significantly induced by drought treatment. Overexpression of MdPYL9 conferred enhanced tolerance to drought stress in transgenic apple plants, including a higher photosynthetic rate and relative water content and lower MDA content and electrolyte leakage compared with wild-type plants. Lower accumulation of reactive oxygen species (ROS) and higher activity of antioxidant enzymes suggest that MdPYL9 overexpression could promote ROS scavenging and thus reduce oxidative damage in transgenic apple plants under drought stress. Yeast two-hybrid assays indicated that there were both ABA-dependent and ABA-independent interactions between MdPYL9 and the clade A subgroup members of the PP2C family (PP2CAs) in apple. Expression analysis identifed two PP2CA genes, MdHAI1 and MdHAI2 , that were highly expressed under drought treatment by not expressed under normal conditions. These results provide new insight that could be used to aid future studies examining the function of MdPYLs and the mechanism by which they regulate apple drought tolerance. • Overexpression of MdPYL9 enhances the drought tolerance of transgenic apple plants. • MdPYL9 interacts with MdPP2CAs in both ABA-dependent and ABA-independent ways. • MdHAI1 and MdHAI2 may play a key negative role in drought induced ABA signaling. [ABSTRACT FROM AUTHOR]

• The Pn and PSII system capacity were higher in apple overexpressing MdTyDC. • Overexpression of MdTyDC increased the water use efficiency in apple. • Amino acids changed in transgenic apple in response to long-term moderate drought. • Dopamine content was increased in transgenic apple under drought. • Antioxidant enzymes were increased in transgenic apple under drought. Drought stress affects the apple yield and quality. Tyrosine decarboxylase (TyDC) plays a fundamental role in many secondary metabolite reactions in higher plants (including those involving dopamine). Our aims of this study are: 1) to identify the role of TyDC in dopamine derivative biosynthesis and its function in long-term moderate drought conditions; and 2) to explore the role of MdTyDC in plant growth and development as well as the drought stress response. Wild type and three independently apple plants overexpression of MdTyDC were treated for long-term moderate drought stress. The growth and physiological parameters of apple plant, photosynthetic capacity, antioxidant enzymes activity, water use efficiency (WUE), stomatal behavior, amino acid content and dopamine content were detected under long-term moderate drought stress. Overexpression of MdTyDC (OE) in apple showed better growth performance, higher photosynthetic capacity and higher capacity for photochemical reactions than wild type lines (WT). Under long-term moderate drought stress, OE lines showed higher WUE, increased ABA content, decreased stomatal aperture, higher antioxidant activity, lower accumulation of ROS and increases in amino acids, such as proline, phenylalanine and lysine. In addition, qRT-PCR revealed higher gene expression of MdTyDC and dopamine content in OE compared with WT lines under long-term moderate drought stress. These results indicate that MdTyDC confers long-term moderate drought tolerance by improving photosynthetic capacity, WUE, antioxidant activity, dopamine content and changing the contents of amino acids (such as proline accumulation). [ABSTRACT FROM AUTHOR]

• Overexpressed MdTYDC enhanced drought tolerance by increasing cellular ABA levels in apple plants. • MdTYDC overexpression improved ASA/DHA and GSH/GSSG ratios under drought. • MdTYDC overexpression modulated stomatal movement by increasing ABA levels and ABA sensitivity. • Transcriptomic analyses indicated that MdTYDC overexpression regulated the expression of drought-related transcription factors. Dopamine is produced in plants and plays a vital role in improving plant responses to abiotic stress. Tyrosine decarboxylase (TYDC) synthesizes tyramine from tyrosine, thereby providing the substrate for dopamine production. However, the function of TYDC in response to drought stress in apple plants remains unclear. In this study, transgenic apple plants that overexpressed MdTYDC were exposed to drought, in order to explore the biological function of TYDC in drought stress. Overexpression of MdTYDC significantly increased apple dopamine content. Transgenic apple plants that overexpressed MdTYDC exhibited less electrolyte leakage, a lower water loss rate, higher photosynthetic pigment contents, and enhanced photosynthetic performance under drought conditions. MdTYDC overexpression also improved reducing power by increasing the ratios of ascorbate to dehydroascorbate and reduced to oxidized glutathione. It influenced leaf water retention capacity and stomatal movement by modulating cellular abscisic acid (ABA) levels and ABA sensitivity. Transcriptomic analyses indicated that MdTYDC overexpression enhanced drought tolerance by regulating the expression of ABA- and drought-related transcription factors, including those from the NAC, ERF, and WRKY families. These results provide evidence for the involvement of MdTYDC and dopamine in enhancing the drought tolerance of apple plants. [ABSTRACT FROM AUTHOR]

• This study revealed the HD-Zip I transcription factor MdHB-7 contributes to the water deficit response in transgenic apple. • Through drought stress treatments and phenotypic assessments, we found that MdHB-7 expression enhanced the drought tolerance of apple plants. • Our results lay a foundation for further research on the mechanisms by which HD-Zip I proteins promote drought tolerance. Drought severely restricts global crop yield and quality. These limitations may be partially addressed through genetic engineering to improve crop drought tolerance. The HD-Zip I transcription factors play important roles in the regulation of plant drought tolerance, but their specific functions in the regulation of apple drought response remain unclear. In this study, we found that MdHB-7 was expressed at different levels in a variety of apple tissues and was significantly induced by abscisic acid (ABA) and drought stress treatments. To investigate the functions of MdHB-7 , we created transgenic apple plants that exhibited either MdHB-7 overexpression or RNAi-induced MdHB-7 suppression. Through drought stress treatments and phenotypic assessments, we found that MdHB-7 expression enhanced the drought tolerance of apple plants. Overexpression of MdHB-7 promoted endogenous ABA accumulation, stomatal closure, and reactive oxygen species (ROS) detoxification in response to drought, whereas suppression of MdHB-7 expression had the opposite effect. Our results lay a foundation for further research on the mechanisms by which HD-Zip I proteins promote drought tolerance. [ABSTRACT FROM AUTHOR]