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Red-fleshed apples are receiving increased attention because of their remarkable anthocyanin contents. To provide sufficient experimental evidence about their phenolic composition and strong antioxidant capacity, three red-fleshed varieties from Xinjiang, P.R. China and one from the USA were evaluated. For comparison, two commercial white-fleshed varieties were also included. In samples of the flesh, contents of phenolics (total phenolics, flavonoids, flavanols, and anthocyanins) and antioxidant activity were significantly higher for red-fleshed varieties than for white-fleshed varieties. Flavonoid profiles also differed between the red- and white-fleshed varieties. Among the red-fleshed varieties, ‘Roberts Crab’ had the highest amounts of total phenolics, flavonoids, and anthocyanins, as well as the strongest antioxidant activity, while ‘Xiahongrou’ had the lowest levels of total phenolics and flavonoids and the weakest activity. HPLC analysis revealed that individual phenolic compounds varied significantly among red-fleshed varieties. These results suggest that red-fleshed apples are a promising source of antioxidants for human nutrition. [ABSTRACT FROM AUTHOR]

Abstract: We used 10 simple sequence repeat primers in combination with touchdown PCR to evaluate the genetic relationships among 15 biotypes of Malus prunifolia. Out of 170 detected fragments, 156 (90.74%) were polymorphic. Coefficients of genetic similarity ranged from 0.53 to 0.88, with an average value of 0.66 between pairs of biotypes, which also indicated the high degree of polymorphism within this species. Cluster analysis sorted all biotypes into three groups, which was then confirmed by principal coordinate analysis. A dendrogram showed that some of the clustered biotypes were largely congruent for geographical distribution. However, the DNA patterns for some biotype groups did not demonstrate relative agreement in their origin. Five biotypes arising from the Xinjiang Autonomous Region were dispersed within the dendrogram, reflecting their complex genetic backgrounds. [Copyright &y& Elsevier]

Abstract: Effects of light irradiation on the photosynthetic electron transport chain between P680 and P700 in apple tree leaves was probed with chlorophyll a fluorescence transient and 820 nm transmission measurements during dehydration under different light intensities. The results showed that light accelerated the leaf water-loss rate during dehydration. Leaf dehydration lowered the maximum quantum yield of PSII and the far-red light induced maximal transmission change at 820 nm, but increased the relative variable fluorescence intensity at J-step, especially under increasing irradiation conditions. During leaf dehydration, irradiation lowered the relative variable fluorescence intensity at I-step. At the beginning of leaf dehydration, moderate light accelerated the leaf water-loss rate and then lowered the maximal light-trapping efficiency of P680. Upon further dehydration under moderate light or dehydration under high light, light accelerated the water-loss rate and also directly decreased the maximal light-trapping efficiency of P680. The more significant decrease in the exchange capacity of plastoquinones at the QB site was mainly attributed to the faster water-loss rate under moderate light than in the dark. Under high light, irradiation also directly lowered the capacity. The reoxidation of PQH2 in the dehydrated leaves was enhanced by the light irradiation. The rapidly decreased contents of P700 + plastocyanin were mainly attributed to the faster water-loss rate under light conditions in contrast with that in the dark. The different effects of light irradiations on the photosynthetic electron transport chain might be involved in the acclimation of apple tree leaves to dehydration. [Copyright &y& Elsevier]

Abstract: To elucidate the roles of cell wall-modifying enzymes in apple fruit, we investigated the activity and gene expression of β-galactosidase (β-Gal), α-l-arabinofuranosidase (α-l-Af), polygalacturonase (PG), and pectin methylesterase (PME). Their regulation by ethylene and cold storage (0°C) was also assessed. ‘Golden Delicious’ and ‘Fuji’ fruit showed differences in their rates of respiration and decline of firmness, as well as demonstrating unique regulated effects. Activities of β-Gal and α-l-Af were higher in ‘Golden Delicious’ than in ‘Fuji’ fruit, although both had similar patterns of change. They were dramatically inhibited by 1-methylcyclopropene (1-MCP) and 0°C, and enhanced by exposure to ethephon, with stronger response in ‘Golden Delicious’ fruit. Gene expression of cell wall enzymes also was significantly affected by 0°C, 1-MCP, and ethephon. The difference in α- l-Af expression among treatments in ‘Golden Delicious’ was more significant than in ‘Fuji’ fruit, especially early in storage. In contrast, expression of β-Gal was inhibited by 1-MCP at early stages in ‘Golden Delicious’ fruit and over the entire storage period in ‘Fuji’ fruit, and was significantly enhanced by ethephon treatment in the former but only slightly in the latter. At 0°C, β-Gal mRNA accumulation was inhibited in both cultivars. PG and PME activities increased during softening, and differed at different stages for each cultivar, and were obviously regulated by ethylene and 0°C. PME expression was higher in ‘Golden Delicious’ fruit, with far greater differences between cultivars than that detected for PG. Both PME and PG mRNA were more intensively influenced by ethylene and cold storage in ‘Golden Delicious’ fruit. The results confirm that cell wall enzymes play an important role in fruit softening. Of these, β-Gal and α-l-Af may be more closely related to the storability of apples than PG and PME, especially when fruit ripening and softening begin. [Copyright &y& Elsevier]

Abstract: Ascorbic acid (AsA), as a unique antioxidant and enzyme cofactor, has multiple roles in plants. However, there is very limited information on the mechanism of AsA accumulation and controlling in leaves. In this study, we determined AsA accumulation levels, analyzed expression patterns of the genes involved in synthesizing via l-galactose pathway and recycling as well as enzyme activities in apple (Malus domestica Borkh) leaves with different age. AsA content was found to increase with leaf development, reaching the highest level in 20-day-old leaves. This level was maintained in mature leaves until the dropping in senescent leaves. Comparing with young and senescent leaves, mature leaves had higher capability for AsA synthesis with high expression levels and activity of l-galactose dehydrogenase and l-galactono-1,4-lactone dehydrogenase. The mRNA expression of genes involved in AsA synthesis also showed highest abundance in 20-day-old leaves, though GDP-mannose-3′,5′-epimerase and l-galactose-1-phosphate phosphatase expression reached the highest levels before 20 days old. These results suggest that AsA accumulation in apple leaves mainly occurs during the transition phase from young to mature leaves with high rates of synthesis and recycling, and that l-galactose-1-phosphate phosphatase could play an important role in regulating AsA biosynthesis via the l-galactose pathway. [Copyright &y& Elsevier]

Approximately 80 days after full bloom, well-exposed fruit on the south part of the canopy of mature Liberty/M9 apple (Malus domestica Borkh.) trees were randomly assigned to one of the following two treatments. Some fruits were turned about 180° to expose the original shaded side to full sun, whereas the rest served as untreated controls. On days 0, 1, 2, 4, 7 and 10 after treatment, fruit peel samples were taken from the original shaded side of the treated fruit and both the sun-exposed side and the shaded side of the control fruit at midday, to determine photosynthetic pigments, enzymatic and non-enzymatic antioxidants. Maximum photosystem II (PSII) efficiency, measured as Fv/Fm (maximum fluorescence) of chlorophyll fluorescence at pre-dawn, was higher in the shaded side than the sun-exposed side of the control fruit. Fv/Fm of the original shaded side decreased sharply after 1 day exposure to full sun, and then gradually recovered to a similar value of the sun-exposed side of the control fruit by day 10. The shaded side of the control fruit had much lower xanthophyll cycle pool size, conversion and antioxidant enzymes and also soluble antioxidants of the ascorbate–glutathione cycle than the sun-exposed side. Xanthophyll cycle pool size of the original shaded side increased in response to full sun exposure, reaching a similar value of the sun-exposed side by day 10. Almost all the xanthophyll cycle pool was converted to zeaxanthin and antheraxanthin starting from the first day of exposure to full sun. In contrast, chlorophyll content decreased whereas lutein or neoxanthin content remained unchanged. In response to full sun exposure, ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase and glutathione reductase and total pool size and reduction state of both ascorbate and glutathione, all increased to the corresponding values found in the sun-exposed side of the control fruit over a 10-day period. It is concluded that both xanthophyll cycle and the ascorbate–glutathione cycle in the original shaded side are up-regulated in response to full sun exposure to minimize photo-oxidative damage and contributes to its acclimation to full sun. [Copyright &y& Elsevier]

The objective of this study was to determine how xanthophyll cycle-dependent thermal dissipation and the antioxidant system in the peel of apple fruit respond to the natural light exposure within the tree canopy. Fruit from exterior and interior canopies of both mature ‘Gala’ and ‘Smoothee’ apple trees were sampled at noon and/or predawn to measure chlorophyll fluorescence, xanthophyll cycle pool size and composition, and enzymatic and non-enzymatic antioxidants. Compared with the shaded side, the sun-exposed peel of the fruit had more excess absorbed photon flux density (PFD) as a result of a lower photosystem II operating efficiency and a higher incident PFD at noon. The efficiency of excitation transfer was lower in the sun-exposed peel than in the shaded peel, indicative of higher thermal dissipation. The sun-exposed peel had a larger xanthophyll cycle pool size and a higher conversion state. It also had higher activities of ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione reductase, and a larger size and a higher reduction state of the ascorbate pool and the glutathione pool. However, catalase activity was lower in the sun-exposed peel than in the shaded peel. Superoxide dismutase did not show significant trend with regard to fruit peel type or position in the canopy. We conclude that both the xanthophyll cycle and the ascorbate–glutathione pathway in the apple fruit peel are acclimated to the prevailing light exposure within the tree canopy to meet the respective needs for dissipating excess absorbed PFD and detoxifying reactive oxygen species. [Copyright &y& Elsevier]

Polymers with strong polar side groups of combined push-pull azobenzene and triphenylamine were synthesized by ring-opening metathesis polymerization, and exhibited excellent dielectric and energy storage properties mainly due to highly dipolar polarization. The dielectric constant of polymer containing one azobenzene group and one triphenylamine group was 13.5, which was enhanced to 15.7 when one azobenzene group and two triphenylamine groups were incorporated into polymer in virtue of its higher dipole density. Besides, the dielectric loss of polymers was lowered to 0.004–0.025, and the stored-energy density of polymer can reach up to 5.09 J cm−3 at electric field strength of 270 MV m−1 with good charge-discharge efficiency of 86.4%. Therefore, polymers are promising for dielectric and electrical energy storage applications. Image 1 • Polymer with azobenzene and triphenylamine side groups was synthesized by ROMP. • Polymer displayed high dielectric constant of 15.7 and low dielectric loss of 0.01. • Polymer exhibited energy storage density of 5.09 J cm−3 with efficiency of 86.4%. [ABSTRACT FROM AUTHOR]

Apples with red fruit flesh are desirable for breeding because they contain more health-beneficial compounds. That color trait is also considered more attractive to consumer. To obtain experimental evidence about the molecular mechanism for this pigmentation, we investigated how the practice of bagging fruit might affect levels of phenolic compounds as well as the expression of anthocyanin biosynthetic and regulatory genes. Two red-fleshed varieties from Xinjiang, P.R. China, were studied – ‘Xiahongrou’ and ‘No.1 Hongxun’. HPLC analysis showed that the contents of anthocyanins in both peel and flesh were decreased by bagging, but were increased again when fruits were re-exposed to sunlight. Except for F3H in ‘No.1 Hongxun’, transcription levels for most anthocyanin biosynthetic genes in the flesh were enhanced after bag removal. MYB10 is a key transcriptional factor in the anthocyanin pathway of these two apple varieties, particularly in the flesh. These results suggest that the development of red coloring when bags are removed could be a result of light-induced coordinated expression by MYB10 and anthocyanin biosynthetic genes in red-fleshed apples. [ABSTRACT FROM AUTHOR]

Strigolactones have multiple roles for plants, including repression of the development of lateral buds and lateral roots, and improvement of stress tolerance, while the knowledge about the regulation of strigolactone synthesis is rare. The Malus genus is important resource of apple’s rootstocks, thus study on strigolactone synthesis in Malus is helpful to realize the signal communication between apple scion and rootstock. Here, we detected the strigolactone contents of seven Malus species and confirmed the synthesis of strigolactones widely existed in Malus . Then the putative strigolactone synthesis gene, carotenoid cleavage dioxygenase 7 ( CCD7 ), and its promoter sequence were cloned. The sequences were quite conserved and some stress responsive cis -acting elements were predicted in the promoters. The CCD7’s expressions and its promoter's activity decreased under drought, waterlogging or methyl jasmonate (MeJA) treated conditions. By yeast one-hybrid screening, it was found that one transcriptional factor (TF) basic pentacysteine 1 ( BPC1 ) had the ability of binding to CCD7’s promoter. Transient overexpression of BPC1 repressed the activity of CCD7 promoter. The expressions of BPC1 were increased when Malus suffered from the stresses, and it was believed that BPC1 was one of the potential regulator of CCD7’s expression and strigolactone synthesis. [ABSTRACT FROM AUTHOR]

Abstract: Apple trees (Malus domestica L.) are often exposed to severe water stress in the summertime. We determined how levels of nonstructural carbohydrates (NC) changed in the leaves of two cultivars – drought-tolerant ‘QinGuan’ and drought-sensitive ‘NaganoFuji’ – when plants were subjected for 10 d to a sudden water deficit. Photosynthetic performance was investigated in terms of gas exchange and antioxidant enzyme activity. Two separate irrigation scenarios were tested with one-year-old plants grafted onto Malus hupehensis (Pamp.) Rehd. rootstock: 1) water applied to 75% field capacity (FC) (Control, CK) and 2) irrigation withheld to achieve <75% FC ranging from 30 to 75% (Treatment, T). At Days 0, 1, 2, 4, 6, and 10 of the experiment, we recorded net CO2 assimilation, stomatal conductance, leaf transpiration rates, and intercellular CO2 concentrations. All of those parameters showed decreases in plants from both stress scenarios, although those declines were not as dramatic in ‘QinGuan’. The photosynthetic rate reduced primarily because of stomatal closure. In both cultivars, water stress induced the accumulation of NC, especially sorbitol, suggesting that this polyol has a role in osmoregulation. The rise in sorbitol levels was much steeper in ‘QinGuan’. Likewise, the interaction between superoxide dismutase and peroxidase activities varied between cultivars. In ‘QinGuan’, the malondialdehyde concentration was much lower and more closely correlated with a high Suc/NC ratio when compared with ‘NaganoFuji’. Scanning electron microscopy revealed a drought-resisting apparent structure in ‘QinGuan’. Overall, our results from both external and internal examinations demonstrated that ‘QinGuan’ is more drought-tolerant than ‘NaganoFuji’. [Copyright &y& Elsevier]

Highlights: [•] Bagging and debagging treatments did not affect the level of total non-structural carbohydrates or total organic acids. [•] Bagging decreased the concentrations of total phenolics in the peel and flesh whereas de-bagging increased them. [•] The expression of anthocyanin biosynthetic genes peaked at Hour 30 after bag removal. [Copyright &y& Elsevier]

Glomerella leaf spot (GLS), caused by Colletotrichum fructicola (Cf), has been one of the main fungal diseases afflicting apple-producing areas across the world for many years, and it has led to substantial reductions in apple output and quality. HD-Zip transcription factors have been identified in several species, and they are involved in the immune response of plants to various types of biotic stress. In this study, inoculation of MdHB-7 overexpressing (MdHB-7- OE) and interference (MdHB-7- RNAi) transgenic plants with Cf revealed that MdHB-7 , which encodes an HD-Zip transcription factor, adversely affects GLS resistance. The SA content and the expression of SA pathway-related genes were lower in MdHB-7- OE plants than in 'GL-3' plants; the content of ABA and the expression of ABA biosynthesis genes were higher in MdHB-7- OE plants than in 'GL-3' plants. Further analysis indicated that the content of phenolics and chitinase and β-1, 3 glucanase activities were lower and H 2 O 2 accumulation was higher in MdHB-7- OE plants than in 'GL-3' plants. The opposite patterns were observed in MdHB-7- RNAi apple plants. Overall, our results indicate that MdHB-7 plays a negative role in regulating defense against GLS in apple, which is likely achieved by altering the content of SA, ABA, polyphenols, the activities of defense-related enzymes, and the content of H 2 O 2. • Colletotrichum fructicola can induce the expression of MdHB-7. • MdHB-7 plays a negative role in regulating apple GLS resistance. • MdHB-7 may function by controlling SA and ABA levels. • MdHB- 7 might affect the polyphenol, H 2 O 2 content, and defense enzyme activities. [ABSTRACT FROM AUTHOR]

Abstract: The shaded peel of ‘Fortune’ (a red cultivar) and ‘Mutsu’ (a yellow/green cultivar) apple (Malus domestica Borkh.) was exposed to full sun by turning fruit 180° at about one week before harvest to determine the expression of key genes involved in anthocyanin synthesis in response to sunlight exposure and their relationships with the levels of anthocyanins and other phenolics. For the unturned (control) fruit, the shaded peel had lower expression levels of MdMYB10 (a transcriptional factor in the regulation of anthocyanin biosynthesis) and seven structural genes in anthocyanin synthesis (MdPAL, MdCHS, MdCHI, MdF3H, MdDFR1, MdLDOX, and MdUFGT), and lower levels of anthocyanins and flavonols than the sun-exposed peel in both cultivars. Exposure of the shaded peel to full sun caused marked up-regulation of the expression of MdMYB10 and all seven structural genes, which peaked between 6 h and 30 h after fruit turning, consequently leading to higher levels of anthocyanins, flavonols, and total phenolics than in the shaded peel and even in the sun-exposed peel of control fruit. Interestingly, the levels of flavonols were higher in the shaded peel of turned fruit (the original sun-exposed peel) than in the sun-exposed peel of both control and turned fruit in both cultivars, suggesting that competition for substrates exists in different branches of the phenylpropanoid pathway. These results indicate that sunlight exposure stimulates the expression of MdMYB10 and structural genes in anthocyanin synthesis, thereby elevating the levels of anthocyanins and other phenolic compounds in both red and yellow/green cultivars. [Copyright &y& Elsevier]

As RNA chaperones, cold shock proteins (CSPs) are essential for cold adaptation. Although the functions of CSPs in cold response have been demonstrated in several species, the roles of CSPs in response to drought are largely unknown. Here, we demonstrated that MdCSP3 , a downstream target gene of MdMYB88 and MdMYB124 , contributes to drought tolerance in apple (Malus × domestica). MdCSP3 responds to various abiotic stresses, including drought, cold, heat, and salt stress. Compared with non-transgenic apple GL-3, the MdCSP3 overexpressing plants exhibit significantly lower drought resistance and a reduced capacity for ROS scavenging by the regulation of antioxidant enzymes SOD, CAT, and POD. Additionally, RNA-seq data shows that MdCSP3 regulates expression of genes involved in oxidative stress response. Taken together, our results demonstrate the functions of MdCSP3 in apple drought tolerance, and this finding provides a new direction for breeding of drought resistant apple. • Apple MdCSP3 is a negative regulator in drought tolerance. • MdCSP3 regulates plant drought tolerance by negatively regulating antioxidant enzyme activities. • MdCSP3 regulates plant drought tolerance by negatively regulating the expression of oxidative-stress responsive genes. [ABSTRACT FROM AUTHOR]

Abstract: The physiological responses and salt tolerance of apple rootstocks were evaluated under high salinity. Seedlings of 15 kinds of apple rootstocks were treated with 200mM NaCl for 10d to assess growth, biomass, and physiological responses. Under NaCl stress, their growth parameters decreased compared with the control, with the greatest changes in leaf number, plant height, and relative water content (RWC). In contrast, electrolyte leakage (EL) and activities of peroxidase (POD) and catalase (CAT) were increased. For most rootstocks, stress treatment decreased malondialdehyde (MDA) and soluble protein contents along with superoxide dismutase (SOD) activity. Their degree of salt tolerance was comprehensively evaluated based on salt injury index (SI), adversity resistance coefficients (ARC), and cluster analysis. Malus prunifolia Borkh cv. dongbeihuanghaitang, M. sieboldii Rehd cv. daguohongsanyehaitang, M. prunifolia Borkh cv. qiuzi, and M. xiaojinensis Cheng et Jia were salt-tolerant rootstocks, whereas M. prunifolia Borkh cv. yingyehaitang, M. micromalus Hemsl, and M. sieboldii Rehd cv. lushihongguo were salt-sensitive rootstocks. These differences in sensitivity were associated with variations in the activities of anti-oxidation enzymes and in the amount of organic osmotica. [ABSTRACT FROM AUTHOR]

Abstract: Seedlings of two Malus species (M. hupehensis and M. toringoides) were hydroponically grown in normoxic and hypoxic nutrient solutions to determine their hypoxia tolerance and adaptation of root anaerobic respiration to hypoxia stress. The growth of both species was inhibited under hypoxia stress. During hypoxia stress, the generation of superoxide anion radical (O2 −) and content of hydrogen peroxide (H2O2) in roots were significantly increased in both species with similar trends, which led to an increase of malondialdehyde (MDA) content and relative membrane permeability (RMP). The degree of growth inhibition and the levels of O2 −, H2O2, MDA and RMP were greater in M. toringoides than in M. hupehensis. Under hypoxia stress, pyruvate decarboxylase (PDC), alcohol dehydrogenase (ADH) and lactate dehydrogenase (LDH) activities in roots were increased during the first 12 days of hypoxia stress and then gradually decreased in both species. The contents of acetaldehyde, alcohol and lactate in roots were also increased and showed similar trends as the activities of anaerobic respiration enzymes. The increases in PDC and ADH activities and lactate content in M. hupehensis under hypoxia conditions were greater than those of M. toringoides, but alcohol and acetaldehyde contents showed opposite trends. These data suggest that M. hupehensis is more tolerant of hypoxia and had less damage from oxidative stress than M. toringoides under hypoxia stress. The capability for anaerobic respiration is up-regulated in roots of Malus in response to hypoxia stress to minimize damage, and the higher hypoxia tolerance of M. hupehensis may be partly due to the higher enzyme activity of PDC, ADH and LDH and lower accumulation of acetaldehyde and alcohol. [Copyright &y& Elsevier]

Abstract: Fruit of eight Actinidia genotypes were evaluated for antioxidant potential by several assays (DPPH, ABTS, ORAC, FRAP, SASR and MCC) and tested for their polyphenol composition and vitamin C contents. The significance analysis demonstrated that the antioxidant capacity of Actinidia eriantha and Actinidia latifolia fruits were significantly higher than that of other genotypes, which was about 3.3–8.7-fold higher than the Actinidia deliciosa cv. Hayward assayed in ABTS, DPPH, ORAC and FRAP methods. The total polyphenols and vitamin C contents showed a great variety amongst Actinidia genotypes and highly correlation with the total antioxidant capacity. It is concluded that significant genotypic difference exists in the total antioxidant capacity of Actinidia fruits. The wild A. eriantha and A. latifolia species have significantly higher antioxidant capacity than the cultivars of A. chinensis and A. deliciosa. Both total polyphenols and vitamin C are major contributors to the total antioxidant capacity in Actinidia fruit. [Copyright &y& Elsevier]

Soil salinization had become a global ecological problem, which restricts the plant growth, and the quantity and quality of fruits. As a signaling molecule, γ-Aminobutyric acid (GABA) mediates a series of physiological processes and stress responses. Our previous research showed that GABA could alleviate drought, low phosphorus, cadmium stresses in apples, but the further research about its physiological mechanisms under salt stress was even more needed. The present study showed that the inhibition of salt stress on plant growth might be effectively alleviated by the treatment of 0.5 mM GABA, and the osmotic balance and photosynthetic capacity of plants could be maintained. Exogenous GABA could effectively inhibit the enrichment of reactive oxygen species and the uptake of Na+, while maintaining ion homeostasis. The experiment results indicated GABA could markedly promote the expression amount of Na+ and K+ transport-related genes (e.g., HKT1 , AKT1 , NHX1 , SOS1 , SOS2 , and SOS3) in apples under salt stress. Overexpression and interference (RNAi) of MdGAD1 in apple roots, which is a crucial enzyme in the GABA biosynthesis, affected the salt tolerance of plants. Transgenic apple plants with roots of overexpression MdGAD1 showed less relative electrolyte leakage and more expression level of related ion transport genes than CK group, but RNAi MdGAD1 led to the opposite results. These results indicated that GABA accumulation could effectively strengthen the resistance of apple plants to salt stress and alleviate the injury of apple seedlings resulted from salinity. • GABA could alleviate the salt-induced damages in apple seedlings. • GABA could promote chlorophyll synthesis, enhance photosynthetic capacity and inhibit ROS accumulation under salt stress. • GABA could sustain ion homeostasis by promoting the expression of genes related to Na+ and K+ transport under salt stress. • MdGAD1 could act as a positive regulatory under salt stress in apples. [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]

The bHLH family, the second largest transcription factor (TF) family in plants, plays a crucial role in regulating plant growth and development processes. However, the biological functions and mechanisms of most bHLH proteins remain unknown, particularly in apples. In this study, we found that MdbHLH4 positively modulates plant growth and development by enhancing cell expansion. Overexpression (OE) of MdbHLH4 resulted in increased biomass, stem and root length, leaf area, and larger areas of pith, xylem, and cortex with greater cell size compared with wild-type apple plants. Conversely, RNA interference (RNAi)-mediated silencing of MdbHLH4 led to reduced xylem and phloem as well as smaller cell size compared to wild-type apple plants. Ectopic expression of MdbHLH4 in tomatoes resulted in enlarged fruits with impaired color appearance, decreased accumulation of soluble solids, and decreased flesh firmness along with larger seeds. Subsequent investigations have shown that MdbHLH4 directly binds to the promoters of MdARF6b and MdPIF4b , enhancing their expression levels. These findings suggest that MdbHLH4 potentially regulates plant cell expansion through auxin and light signaling pathways. These study results not only provide new insights into the roles of bHLH transcription factors in regulating plant growth and development but also contribute to a deeper understanding of their underlying mechanisms. • MdbHLH4 promotes apple plant growth and cell enlargement. • MdbHLH4 regulates fruit and seed size and fruit quality in transgenic tomatoes. • MdbHLH4 activates the expression of MdARF6b and MdPIF4b by directly binding to their promoters. [ABSTRACT FROM AUTHOR]

Salt is one of the main stresses that limit plant growth, especially at the seedling stage, reducing crop production and severely impacting food security. However, the relationship between salt stress and sugar content regulated by sugar transporters remains unknown. Here, we investigated the salt tolerance of transgenic tomato seedlings ectopically expressing MdHT2.2 , which is a fructose and glucose/H+ symporter located on the plasma membrane in apple. Although the contents of fructose, glucose and sucrose in the leaves of seedlings ectopically expressing MdHT2.2 obviously increased compared with those of WT seedlings, the transgenic seedlings were significantly less tolerance to salt stress. Under salt stress, the SlSOS1/2 and SlNHX1 genes were highly expressed, and the accumulation of Na+ was lower in the transgenic seedlings than in WT, however, ROS accumulated to a greater degree in the former, and the ROS-scavenging-related enzyme activities and AsA content were lower in the transgenic seedlings than WT. Taken together, these results indicated that the relatively low salt tolerance of the MdHT2.2 transgenic seedlings was related with the accumulation of ROS, which was caused by reduced ROS-scavenging ability. Our results offer proof that changes in sugar content caused by sugar transporters are related to salt tolerance, and provide new insight into the regulation of sugar content, quality improvement and stress tolerance. Image 1 • MdHT2.2 transgenic seedlings accumulated more sugar than WT. • MdHT2.2 transgenic seedlings showed lower salt resistance but had less Na+ accumulation. • MdHT2.2 transgenic seedlings accumulated more ROS accompanied with lower ROS scavenging ability. • The sugar content change caused by MdHT2.2 ectopic expression influenced the ROS scavenging system sensing salt stress. [ABSTRACT FROM AUTHOR]

Soluble sugars play important roles in plant development and stress response, and the nitrogen supply level can affect the among-organ distribution and metabolism of sugar in plants and, in turn, plant growth. To explore the adaptive response of apple root growth to nitrogen supply and its relationship with sugar metabolism, we used a hydroponic culture system to study how the nitrogen supply affects soluble sugar concentrations and sugar metabolism in apple roots. In hydroponic seedlings of Malus hupehensis , low nitrogen application caused rapid and vigorous proliferation of lateral roots, and the transcript levels of MdSOT1 and MdSUT3, which are involved in photoassimilate unloading in roots, were upregulated. The accumulation of sorbitol and sucrose in the fine roots was higher, and the activities of sucrose synthase, invertase and sorbitol dehydrogenase, which are involved in the degradation of sucrose and sorbitol, were significantly increased under a low nitrogen supply. Genes involved in sugar degradation, such as MdSDH1, MdSuSy5, and MdNINV3, play important roles in the efficient use of sorbitol and sucrose under nitrogen deficiency. Additionally, the activity of fructokinase and hexokinase, which are involved in hexose phosphorylation, and transcript levels of MdFRK2 and MdHK3 were significantly upregulated under nitrogen deficiency, and the hexose phosphate products F6P and G6P accumulated greatly in the roots. These results showed that the sugar metabolism capability and sink strength of the roots increased under low nitrogen, indicating that low nitrogen promotes the utilization of sugar in the roots to meet the demand for sugar under rapid root growth. • Sugar metabolism and gene expression were studied in apple roots with low nitrogen application. • Low nitrogen application caused rapid and vigorous proliferation of lateral roots. • Sugar metabolism and transport in roots were increased under low nitrogen. • MdFRK2 might play a role in the response of low nitrogen in root. [ABSTRACT FROM AUTHOR]

Myo -inositol is a versatile compound and plays a vital role in plant growth and stress tolerance. Previously, we found that exogenous application of myo -inositol enhanced the salinity tolerance in Malus hupehensis Rehd. by enhancing myo -inositol metabolism. In this study, we found that the tonoplast-localized myo -inositol transporter 1 (MdINT1) was involved in myo -inositol accumulation and conferred salinity tolerance in apple. MdINT1 is characterized by the highest transcripts among the four apple INT-like genes and could be induced by salt stress at the transcriptional level. Also, it was shown that myo- inositol level was slightly decreased in the leaves of transgenic apple lines over-expressing MdINT1 , but was significantly increased in the leaves and roots of MdINT1 silencing line. Interestingly, overexpression of MdINT1 enhanced salinity tolerance by promoting Na+ and K+ balance, antioxidant activity, and accumulation of osmoprotectants in transgenic apple lines. In contrast, under salinity conditions, the MdINT1-mediated protective roles in the antioxidant activity, homeostasis of ions and osmosis were compromised, which in turn increased the risk of salt intolerance in the MdINT1 silencing line. • The tonoplast-localized MdINT1 is involved in the apple myo -inositol accumulation. • Salt stress increases the transcript level of MdINT1. • MdINT1 confers salinity tolerance in apple. • MdINT1 could improve the antioxidant system, homeostasis of ions, and osmosis in salt-stressed apple plants. [ABSTRACT FROM AUTHOR]

In higher plants, the light-harvesting chlorophyll a / b -binding (Lhc) proteins function in multiple processes that are critical to plant growth, development, and abiotic stress response. However, the Lhc gene family has not been well characterized in the important fruit crop, apple (Malus × domestica Borkh.). In this study, we identified 27 Lhc genes in the apple genome. Phylogenetic analysis showed that the Lhc gene family could be classified into three major subfamilies, each of whose members shared similar conserved motifs. Evolutionary analysis indicated that duplicated MdLhc genes were primarily under purifying selection. MdLhc s were expressed at varying levels in all tissues examined and showed different expression patterns under drought stress. The overexpression of MdLhcb4.3 in transgenic Arabidopsis and apple callus enhanced their tolerance to drought and osmotic stress. Taken together, these results demonstrate the important role of Lhc proteins in the regulation of plant resistance to drought and osmotic stress and provide valuable information for further study of Lhc functions in apple. • This study revealed the presence of 27 apple light-harvesting chlorophyll a / b -binding (Lhc) family genes. • Expression profiles of MdLhc genes in different tissues and upon drought stress were analyzed. • Over-expression of MdLhcb4.3 in transgenic Arabidopsis and apple callus conferred tolerance to drought and osmotic stress, implying that it may be a candidate for stress resistance breeding in apple or other crops. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal fungi (AMF) can form a symbiotic relationships with most terrestrial plants and play an important role in plant growth and adaptation to various stresses. To study the role of AMF in regulating drought resistance in apple, the effects of drought stress on Malus hupehensis inoculated with AMF were investigated. Inoculation of AMF enhanced apple plants growth. Mycorrhizal plants had higher total chlorophyll concentrations but lower relative electrolyte leakage under drought stress. Mycorrhizal plants increased net photosynthetic rate, stomatal conductance, and transpiration rate under drought stress, however, they showed lower inhibition in the quantum yield of PSII photochemistry. Mycorrhizal plants also had higher superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) enzyme activities under drought conditions. Thus, mycorrhizal plants had lower accumulated MDA, H 2 O 2 , and O 2 − than non-mycorrhizal seedlings. Total sugar and proline concentrations also significantly increased, helping maintain the osmotic balance. Furthermore, mitogen-activated protein kinase (MAPK) cascades, which participate in the regulation of responses of plants and microorganisms to biotic and abiotic stress, were up-regulated in apple plants and AMF during drought. We saw that there were at least two motifs that were identical in MAPK proteins and many elements that responded to hormones and stress from these MAPK genes. In summary, our results showed that mycorrhizal colonization enhanced apple drought tolerance by improving gas exchange capacity, increasing chlorophyll fluorescence parameters, creating a greater osmotic adjustment capacity, increasing scavenging of reactive oxygen species (ROS), and using MAPK signals for interactions between AMF and their apple plant hosts. • a better ability to balance the distribution of light energy in photochemical and non-photochemical processes. • Increased antioxidant enzyme activity to scavenge ROS and alleviate oxidative stress injury. • More soluble substances to improve osmotic regulation. • Higher expressions of fungal and apple MAPK genes between the two symbiotic parties that cooperated to regulate the mycorrhizal apple seedlings under drought stress. [ABSTRACT FROM AUTHOR]

Water shortage is one of the main limiting factors in apple (Malus domestica Borkh.) production. Although dopamine is produced in plants and has been linked with response to abiotic stress, the underlying mechanism remains unknown. In this study, physiological analyses revealed that pretreatment with 100 μM dopamine alleviated drought stress in apple seedlings. Dopamine inhibited the degradation of photosynthetic pigments and increased net photosynthetic rate under drought stress. Dopamine also reduced H 2 O 2 content, possibly through direct scavenging and by mediating the antioxidant enzyme activity. Seedlings pretreated with dopamine had higher sucrose and malic acid contents but lower starch accumulation in their leaves. RNA-Seq analysis identified 1052 differentially expressed genes (DEGs) between non-treated and dopamine-pretreated plants under drought. An in-depth analysis of these DEGs revealed that dopamine regulated the expression of genes related to metabolism of nitrogen, secondary compounds, and amino acids under drought stress. In addition, dopamine may improve apple drought tolerance by activating Ca2+ signaling pathways through increased expression of CNGC and CAM/CML family genes. Moreover, analysis of transcription factor expression suggested that dopamine affected drought tolerance mainly through the regulation of WRKY, ERF, and NAC transcription factors. • Dopamine alleviated the drought-induced damages in apple. • Dopamine mediated photosynthesis, antioxidant enzyme activity, and carbohydrate metabolism under drought. • Dopamine regulated the expression of genes related to the metabolism of nitrogen, secondary compounds, and amino acids. • Dopamine improved apple drought tolerance by activating Ca2+ signaling pathways. • Dopamine regulated the expression of WRKY, ERF, and NAC transcription factors. [ABSTRACT FROM AUTHOR]

For a comprehensive understanding of gene expression, enzyme activity and sugar concentrations in response to short-term water deficit in apple (Greensleeves), sugar-modulated gene expression and enzyme activities were analyzed. Water stress resulted in the accumulation of sorbitol, glucose, fructose, galactose and starch, accompanied by a significant reduction in photosynthesis and sucrose concentration. In response to short-term water deficits, the activities of aldose-6-phosphate reductase (A6PR; EC 1.1.1.200), sorbitol dehydrogenase (SDH; EC 1.1.1.14), neutral invertase (NINV; EC 3.2.1.26), sucrose synthase (SUSY; EC 2.4.1.13), and fructokinase (FK; EC 2.7.1.4) were higher, whereas cell wall invertase (CWINV; EC 3.2.1.26) and hexokinase (HK; EC 2.7.1.1) activities were lower. In addition, sucrose phosphate synthase (SPS; EC 2.4.1.14) activity increased during the initial stages of dehydration and then decreased as the drought strengthened. Transcript levels of MdA6PR , MdSDH1/2 , MdNINV1/2 , MdSUSY3 , MdFK1/2/4 , MdSOT1/2 , MdSUC1-3 , MdTMT2/3 , MdvGT1 , MdpGlcT1-4 were upregulated, whereas transcript levels of MdCWINV1/2 , MdHK1/2/3/5 , and MdTMT1 were downregulated after 6 days of water stress. These findings suggest that the sorbitol metabolism pathway is induced and high levels of hexose derived from photosynthetic products are transported into vacuoles for adjustment to the water deficit. Our results provide insights into the relationships between sugar levels and sugar-modulated gene and enzyme activity in response to the imposition of short-term water stress. • Sorbitol and hexose accumulated, sucrose levels decreased under drought stress. • Sorbitol metabolism (synthesis and degradation) was upregulated under water deficit. • Drought induced sucrose breakdown to generate more hexose molarity. • Sugar transporters play a regulatory role in sugar levels under water deficit. [ABSTRACT FROM AUTHOR]