Melatonin promotes the recovery of apple plants after waterlogging by shaping the structure and function of the rhizosphere microbiome

The dynamics of the physiological adaptability of plants and the rhizosphere soil environment after waterlogging remain unclear. Here we investigated the mechanisms regulating plant condition and shaping of the rhizosphere microbiome in a pot experiment. In the experiment, we added melatonin to waterlogged plants, which promoted waterlogging relief. The treatment significantly enhanced photosynthesis and the antioxidant capacity of apple plants, and significantly promoted nitrogen (N) utilization efficiency by upregulating genes related to N transport and metabolism. Multiperiod soil microbiome analysis showed the dynamic effects of melatonin on the diversity of the microbial community during waterlogging recovery. Random forest and linear regression analyses were used to screen for potential beneficial bacteria (e.g., Azoarcus, Pseudomonas and Nocardioides) specifically regulated by melatonin and revealed a positive correlation with soil nutrient levels and plant growth. Furthermore, metagenomic analyses revealed the regulatory effects of melatonin on genes involved in N cycling in soil. Melatonin positively contributed to the accumulation of plant dry weight by upregulating the expression of nifD and nifK (N fixation). In summary, melatonin positively regulates physiological functions in plants and the structure and function of the microbial community; it promoted the recovery of apple plants after waterlogging stress.

The Mh-miR393a-TIR1 module regulates Alternaria alternata resistance of Malus hupehensis mainly by modulating the auxin signaling

miRNAs govern gene expression and regulate plant defense. Alternaria alternata is a destructive fungal pathogen that damages apple. The wild apple germplasm Malus hupehensis is highly resistant to leaf spot disease caused by this fungus. Herein, we elucidated the regulatory and functional role of miR393a in apple resistance against A. alternata by targeting Transport Inhibitor Response 1. Mature miR393 accumulation in infected M. hupehensis increased owing to the transcriptional activation of MIR393a, determined to be a positive regulator of A. alternata resistance to either 'Orin' calli or 'Gala' leaves. 5' RLM-RACE and co-transformation assays showed that the target of miR393a was MhTIR1, a gene encoding a putative F-box auxin receptor that compromised apple immunity. RNA-seq analysis of transgenic calli revealed that MhTIR1 upregulated auxin signaling gene transcript levels and influenced phytohormone pathways and plant-pathogen interactions. miR393a compromised the sensitivity of several auxin-signaling genes to A. alternata infection, whereas MhTIR1 had the opposite effect. Using exogenous indole-3-acetic acid or the auxin synthesis inhibitor L-AOPP, we clarified that auxin enhances apple susceptibility to this pathogen. miR393a promotes SA biosynthesis and impedes pathogen-triggered ROS bursts by repressing TIR1-mediated auxin signaling. We uncovered the mechanism underlying the miR393a-TIR1 module, which interferes with apple defense against A. alternata by modulating the auxin signaling pathway.

Host-induced gene silencing in wild apple germplasm Malus hupehensis confers resistance to the fungal pathogen Botryosphaeria dothidea

Host-induced gene silencing (HIGS) is an inherent mechanism of plant resistance to fungal pathogens, resulting from cross-kingdom RNA interference (RNAi) mediated by small RNAs (sRNAs) delivered from plants into invading fungi. Introducing artificial sRNA precursors into crops can trigger HIGS of selected fungal genes, and thus has potential applications in agricultural disease control. To investigate the HIGS of apple (Malus sp.) during the interaction with Botryosphaeria dothidea, the pathogenic fungus causing apple ring rot disease, we evaluated whether apple miRNAs can be transported into and target genes in B. dothidea. Indeed, miR159a from Malus hupehensis, a wild apple germplasm with B. dothidea resistance, silenced the fungal sugar transporter gene BdSTP. The accumulation of miR159a in extracellular vesicles (EVs) of both infected M. hupehensis and invading B. dothidea suggests that this miRNA of the host is transported into the fungus via the EV pathway. Knockout of BdSTP caused defects in fungal growth and proliferation, whereas knockin of a miR159a-insensitive version of BdSTP resulted in increased pathogenicity. Inhibition of miR159a in M. hupehensis substantially enhanced plant sensitivity to B. dothidea, indicating miR159a-mediated HIGS against BdSTP being integral to apple immunity. Introducing artificial sRNA precursors targeting BdSTP and BdALS, an acetolactate synthase gene, into M. hupehensis revealed that double-stranded RNAs were more potent than engineered MIRNAs in triggering HIGS alternative to those natural of apple and inhibiting infection. These results provide preliminary evidence for cross-kingdom RNAi in the apple-B. dothidea interaction and establish HIGS as a potential disease control strategy in apple.

Melatonin alleviates cadmium toxicity by regulating root endophytic bacteria community structure and metabolite composition in apple

The level of cadmium (Cd) accumulation in orchard soils is increasing, and excess Cd will cause serious damage to plants. Melatonin is a potent natural antioxidant and has a potential role in alleviating Cd stress. This study aimed to investigate the effects of exogenous melatonin on a root endophyte bacteria community and metabolite composition under Cd stress. The results showed that melatonin significantly scavenged the reactive oxygen species and restored the photosynthetic system (manifested by the improved photosynthetic parameters, total chlorophyll content and the chlorophyll fluorescence parameters (Fv/Fm)), increased the activity of antioxidant enzymes (the activities of catalase, superoxide dismutase, peroxidase and ascorbate oxidase) and reduced the concentration of Cd in the roots and leaves of apple plants. High-throughput sequencing showed that melatonin increased the endophytic bacterial community richness significantly and changed the community structure under Cd stress. The abundance of some potentially beneficial endophytic bacteria (Ohtaekwangia, Streptomyces, Tabrizicola and Azovibrio) increased significantly, indicating that the plants may absorb potentially beneficial microorganisms to resist Cd stress. The metabolomics results showed that melatonin significantly changed the composition of root metabolites, and the relative abundance of some metabolites decreased, suggesting that melatonin may resist Cd stress by depleting root metabolites. In addition, co-occurrence network analysis indicated that some potentially beneficial endophytes may be influenced by specific metabolites. These results provide a theoretical basis for studying the effects of melatonin on the endophytic bacterial community and metabolic composition in apple plants.

MhCLC-c1, a Cl channel c homolog from Malus hupehensis, alleviates NaCl-induced cell death by inhibiting intracellular Cl- accumulation

BACKGROUND

Overaccumulation of chloride (Cl) when plants suffer NaCl causes cell damage and death, and is regulated by Cl^- channel protein (CLC). Apple roots are very sensitive to Cl^-, but information associated with CLC is limited in apple crop that widely cultivated in the world.

RESULTS

We identified 9 CLCs from the apple genome and divided them into two subclasses. Among them, MdCLC-c1 promoter contained the largest number of cis-acting elements associated with NaCl stress, and only the MdCLC-c1, MdCLC-d, and MdCLC-g were predicted that may be Cl^- antiporters or channels. Expression analysis of MdCLCs homologs in the roots of Malus hupehensis showed that most of the MhCLCs expression were response to NaCl stress, especially MhCLC-c1 expression was upregulated continuously and rapidly expressed during NaCl treatment. Therefore, we isolated MhCLC-c1 and observed it was a plasma membrane-localized protein. The MhCLC-c1 suppression significantly increased sensitivity, reactive oxygen species content, and cell death of apple calli; while MhCLC-c1 overexpression decreased sensitivity, reactive oxygen species content, and cell death of apple calli and Arabidopsis by inhibiting intracellular Cl^- accumulation under NaCl stress.

CONCLUSIONS

The study selected and isolated a CLC-c gene MhCLC-c1 from Malus hupehensis based on identification of CLCs gene family in apple, and their homologs MhCLCs expression patterns during NaCl treatments, revealing that MhCLC-c1 alleviates NaCl-induced cell death by inhibiting intracellular Cl^- accumulation. Our findings confer the comprehensive and in-depth upstanding of the mechanism that plants resist salt stress, and might also confer genetic improvement of salt tolerance in horticultural crops and the development and utilization of saline-alkali land.

miR164-NAC21/22 module regulates the resistance of Malus hupehensis against Alternaria alternata by controlling jasmonic acid signaling

Apple leaf spot disease caused by Alternaria alternata apple pathotype (A. alternata AP) is one of the most severe fungal diseases affecting apple cultivation. Transcription factors are involved in various disease-resistance responses, and many of them are regulated by miRNAs. Here, we performed RNA-Seq to investigate gene expression changes during the defense response of Malus hupehensis against A. alternata AP. NAC21/22 was induced upon A. alternata AP infection and silenced by miR164 via direct mRNA cleavage. Contrasting expression patterns were noted between mature miR164 and NAC21/22 during infection. Contrary to NAC21/22 silencing, transiently overexpressing NAC21/22 in M. hupehensis alleviated disease symptoms on 'gala' leaves, impeded A. alternata AP growth, and promoted jasmonic acid (JA) signaling-related gene expression. Importantly, transient miR164f overexpression in 'gala' leaves enhanced A. alternata AP sensitivity, due perhaps to NAC21/22 downregulation, whereas miR164 suppression produced an opposite effect. In summary, the miR164-NAC21/22 module plays a pivotal role in apple resistance against A. alternata AP by regulating JA signaling.

Bioactive Substances and Biological Functions in Malus hupehensis: A Review

Malus hupehensis (MH), as a natural resource, contains various active ingredients such as polyphenols, polysaccharides, proteins, amino acids, volatile substances, and other components. Increasingly, studies have indicated that MH showed a variety of biological activities, including antioxidant, hypoglycemic, hypolipidemic, anti-cancer, anti-inflammatory activities, and other activities. Hence, MH has attracted wide interest because of its high medical and nutritional value. It is necessary to review the active components and biological activities of MH. This paper systematically reviewed the chemical substances, biological activities, and potential problems of MH to further promote the related research of MH and provide an important reference for its application and development in medicine and food.

H(2)S Enhanced the Tolerance of Malus hupehensis to Alkaline Salt Stress through the Expression of Genes Related to Sulfur-Containing Compounds and the Cell Wall in Roots

Malus is an economically important plant that is widely cultivated worldwide, but it often encounters saline-alkali stress. The composition of saline-alkali land is a variety of salt and alkali mixed with the formation of alkaline salt. Hydrogen sulfide (H₂S) has been reported to have positive effects on plant responses to abiotic stresses. Our previous study showed that H₂S pretreatment alleviated the damage caused by alkaline salt stress to Malus hupehensis Rehd. var. pingyiensis Jiang (Pingyi Tiancha, PYTC) roots by regulating Na⁺/K⁺ homeostasis and oxidative stress. In this study, transcriptome analysis was used to investigate the overall mechanism through which H₂S alleviates alkaline salt stress in PYTC roots. Simultaneously, differentially expressed genes (DEGs) were explored. Transcriptional profiling of the Control-H₂S, Control-AS, Control-H₂S + AS, and AS-H₂S + AS comparison groups identified 1618, 18,652, 16,575, and 4314 DEGs, respectively. Further analysis revealed that H₂S could alleviate alkaline salt stress by increasing the energy maintenance capacity and cell wall integrity of M. hupehensis roots and by enhancing the capacity for reactive oxygen species (ROS) metabolism because more upregulated genes involved in ROS metabolism and sulfur-containing compounds were identified in M. hupehensis roots after H₂S pretreatment. qRT-PCR analysis of H₂S-induced and alkaline salt-response genes showed that these genes were consistent with the RNA-seq analysis results, which indicated that H₂S alleviation of alkaline salt stress involves the genes of the cell wall and sulfur-containing compounds in PYTC roots.

Brassinolide improves the tolerance of Malus hupehensis to alkaline stress

Malus hupehensis is one of the most widely used apple rootstocks in china but is severely damaged by alkaline soil. Alkaline stress can cause more serious harmful effects on apple plants than salt stress because it also induces high pH stress except for ion toxicity, osmotic stress, and oxidative damage. Brassinolide (BL) plays important roles in plant responses to salt stress. However, its role and function mechanism in apple plants in response to alkaline stress has never been reported. This study showed that applying exogenous 0.2 mg/L BL significantly enhanced the resistance of M. hupehensis seedlings to alkaline stress. The main functional mechanisms were also explored. First, exogenous BL could decrease the rhizosphere pH and promote Ca²⁺ and Mg²⁺ absorption by regulating malic acid and citric acid contents and increasing H⁺ excretion. Second, exogenous BL could alleviate ion toxicity caused by alkaline stress through enhancing Na⁺ efflux and inhibiting K⁺ expel and vacuole compartmentalization. Last, exogenous BL could balance osmotic stress by accumulating proline and reduce oxidative damage through increasing the activities of antioxidant enzymes and antioxidants contents. This study provides an important theoretical basis for further analyzing the mechanism of exogenous BL in improving alkaline tolerance of apple plants.

Melatonin and dopamine mediate the regulation of nitrogen uptake and metabolism at low ammonium levels in Malus hupehensis

In plants, ammonium (NH₄⁺) is the main nitrogen source and acts as a physiological and morphological response signaling molecule. Melatonin and dopamine are associated with plant responses to abiotic stress. However, previous studies have rarely focused on nutrient stress, and the roles of melatonin and dopamine in the uptake and metabolism of nitrogen in plants remain unclear. In this study, we investigated the regulatory effects of melatonin and dopamine on nitrogen utilization efficiency in apple seedlings under two NH₄⁺ concentrations (2 and 0.1 mM) by measuring plant growth, root system architecture, ¹⁵NH₄⁺ content, and related enzyme activity and gene expression. Under low nitrogen supply, apple seedling growth slowed and showed marked reductions in biomass accumulation, chlorophyll content, and nutrient uptake. However, both melatonin and dopamine significantly improved plant growth, chlorophyll content, and root development and enhanced antioxidant enzyme activity. Exogenous application of melatonin or dopamine also promoted the absorption and accumulation of ¹⁵NH₄⁺ and enhanced nitrogen metabolism-related enzyme activity. At the molecular level, melatonin and dopamine significantly increased the expression levels of nitrogen metabolism genes and transporter genes. Overall, these results suggest that melatonin and dopamine can relieve nutrient stress caused by low concentrations of NH₄⁺ through regulating the absorption and metabolism of nitrogen.

Strigolactone alleviates the salinity-alkalinity stress of Malus hupehensis seedlings

Salinity-alkalinity stress can remarkably affect the growth and yield of apple. Strigolactone (SL) is a class of carotenoid-derived compounds that functions in stress tolerance. However, the effects and mechanism of exogenous SL on the salinity-alkalinity tolerance of apple seedlings remain unclear. Here, we assessed the effect of SL on the salinity-alkalinity stress response of Malus hupehensis seedlings. Results showed that treatment with 100 μM exogenous SL analog (GR24) could effectively alleviate salinity-alkalinity stress with higher chlorophyll content and photosynthetic rate than the apple seedlings without GR24 treatment. The mechanism was also explored: First, exogenous GR24 regulated the expression of Na⁺/K⁺ transporter genes and decreased the ratio of Na⁺/K⁺ in the cytoplasm to maintain ion homeostasis. Second, exogenous GR24 increased the enzyme activities of superoxide, peroxidase and catalase, thereby eliminating reactive oxygen species production. Third, exogenous GR24 alleviated the high pH stress by regulating the expression of H⁺-ATPase genes and inducing the production of organic acid. Last, exogenous GR24 application increased endogenous acetic acid, abscisic acid, zeatin riboside, and GA3 contents for co-responding to salinity-alkalinity stress indirectly. This study will provide important theoretical basis for analyzing the mechanism of exogenous GR24 in improving salinity-alkalinity tolerance of apple.

Resveratrol Alleviates the KCl Salinity Stress of Malus hupehensis Rhed

Applying large amounts of potash fertilizer in apple orchards for high apple quality and yield aggravates KCl stress. As a phytoalexin, resveratrol (Res) participates in plant resistance to biotic stress. However, its role in relation to KCl stress has never been reported. Herein we investigated the role of Res in KCl stress response of Malus hupehensis Rehd., a widely used apple rootstock in China which is sensitive to KCl stress. KCl-stressed apple seedlings showed significant wilting phenotype and decline in photosynthetic rate, and the application of 100 μmol Res alleviated KCl stress and maintained photosynthetic capacity. Exogenous Res can strengthen the activities of peroxidase and catalase, thus eliminating reactive oxygen species production induced by KCl stress. Moreover, exogenous Res can decrease the electrolyte leakage by accumulating proline for osmotic balance under KCl stress. Furthermore, exogenous Res application can affect K⁺/Na⁺ homeostasis in cytoplasm by enhancing K⁺ efflux outside the cells, inhibiting Na⁺ efflux and K⁺ absorption, and compartmentalizing K⁺ into vacuoles through regulating the expression of K⁺ and Na⁺ transporter genes. These findings provide a theoretical basis for the application of exogenous Res to relieve the KCl stress of apples.

Ultrasonic-Assisted Aqueous Two-Phase Extraction and Properties of Water-Soluble Polysaccharides from Malus hupehensis

Malus hupehensis (M. hupehensis), an edible and medicinal plant with significant antioxidant and hypoglycemic activity, has been applied to new resource foods. However, the structural characterization and biological effects of its polysaccharides (MHP) are less known. The optimum extraction parameters to achieve the highest extraction efficiency (47.63%), the yield (1.68%) and purity of MHP (89.6%) by ultrasonic-assisted aqueous two-phase system (ATPS) were obtained under the liquid-to-solid ratio of 23 g/mL, ultrasonic power of 65 W, and ultrasonic time of 33 min. According to the analysis results, MHP was composed of Man, GlcA, Rha, GalA, Glc, Gal, Xyl, Ara, and Fuc, in which Ara and Gal were the main components, and the content of GlcA was the lowest. In in vitro activity analysis, MHP showed a significant antioxidant capacity, and an inhibition activity of α-glucosidase and the advanced glycation end products (AGEs) formation in the BSA/Glc reaction model. MHP interacted with α-glucosidase and changed the internal microenvironment of the enzyme, and inhibited the AGEs formation, which provides more evidence for the antihyperglycemic mechanism of MHP. The results suggest that ATPS is an efficient and environmentally friendly solvent system, and M. hupehensis has broad application prospects in functional foods, healthcare products, and pharmaceuticals.

Rapid enrichment and separation of two novel minor phenols from Malus hupehensis utilizing liquid-liquid extraction with three-phase solvent system and high-speed counter-current chromatography based on the polarity parameter

For a rapid enrichment and separation of minor components from Malus hupehensis, the selection of suitable solvent system is the great challenge for liquid-liquid extraction with a three-phase solvent system and high-speed counter-current chromatography. According to the concept of "like dissolves like," the similarity of the average polarity between solvent system and target compounds was the significant characteristic of liquid-liquid extraction with a three-phase solvent system and high-speed counter-current chromatography separation. The polarity parameter model provides a way to calculate the polarity of unknown compounds. Under the guidance of the polarity, an efficient enrichment and separation approach was established through liquid-liquid extraction and high-speed counter-current chromatography with solvent systems composed of n-hexane-ethyl acetate-acetonitrile-water (5:3:5:7, v/v), n-hexane-ethyl acetate-methanol-water (1:2:1:2, v/v), respectively. Thus, the total content of minor compounds was increased from 2.6% to 17.2%, and two novel compounds (6´´-O-coumaroyl-2´-O-glucopyranosylphloretin and 3´´´-methoxy-6´´-O-feruloy-2´-glucopyranosylphloretin) were obtained. The discovery of the new dihydrochalcones expanded the structural diversity of compounds produced by the genus Malus. The experimental results demonstrated that compound polarity can be described by the polarity parameter model and is an important reference for investigating optimum solvent systems for liquid-liquid extraction with a three-phase solvent system and high-speed counter-current chromatography.

Extraction and separation of flavonoids from Malus hupehensis using high-speed countercurrent chromatography based on deep eutectic solvent

In the present experiment, a green and highly efficient extraction method for flavonoids established on deep eutectic solvents (DESs) was investigated by using the response surface methodology. The DES-based high-speed countercurrent chromatography (HSCCC) solvent systems were developed for the separation of high purity compounds from the DES extract of Malus hupehensis for the first time. Under the optimal conditions (liquid-to-solid ratio of 26.3 mL/g, water content of 25.5%, and extraction temperature of 77.5°C), the yield of flavonoids was 15.3 ± 0.1%, which was superior to that of the methanol extraction method. In accordance with the physical property of DES-based HSCCC solvent systems and K values of target compounds, DES-based HSCCC solvent systems composed of choline chloride/glucose-water-ethyl acetate (ChCl/Glu-H₂O-EAC, 1:1:2, v/v) was selected for the HSCCC separation. Thus, five flavonoids (two novel compounds 1-2, 6´´-O-coumaroyl-2´-O-glucopyranosylphloretin and 3´´´-methoxy-6´´-O-feruloy-2´-O-glucopyranosylphloretin; three know compounds 3-5, namely, avicularin, phloridzin, and sieboldin) were efficiently separated from Malus hupehensis. DESs are the environment friendly and highly efficient solvents as the components of extraction solvent and HSCCC solvent system, and can be re-utilized many times. However, ethyl acetate can be soluble with a few hydrogen bond donors, such as urea, carboxylic acid and polyol, through the shake flask test. It is the great difficulty for the efficient and rapid separation of target compounds from the DESs extract because of the DESs residual in the HSCCC fractions. ChCl and Glu are the great choices of DESs without this problem. In addition, K values increased with the increase of the molar ratio of ChCl/Glu and the content of water, which could effectively guide us to choose the suitable DES-based HSCCC solvent system. The twice HSCCC separation results indicated that DES was the valuable and green solvent for the HSCCC separation of pure compounds from the extract for the first time, and showed the recycle superiority of DES-based HSCCC solvent system.

Beneficial Effects of Exogenous Melatonin and Dopamine on Low Nitrate Stress in Malus hupehensis

Malus hupehensis, as an apple rootstock, is an economically important tree species popular due to its excellent fruit yield and stress resistance. Nitrogen is one of the critical limiting factors of plant growth and fruit yield, so it is crucial to explore new methods to improve nitrogen use efficiency. Melatonin and dopamine, as multifunctional metabolites, play numerous physiological roles in plants. We analyzed the effects of exogenous melatonin and dopamine treatments on the growth, root system architecture, nitrogen absorption, and metabolism of M. hupehensis when seedlings were exposed to nitrate-deficient conditions. Under low nitrate stress, plant growth slowed, and chlorophyll contents and ¹⁵NO₃ ^- accumulation decreased significantly. However, the application of 0.1 μmol/L melatonin or 100 μmol/L exogenous dopamine significantly reduced the inhibition attributable to low nitrate levels during the ensuing period of stress treatment, and the effect of dopamine was more obvious. In addition to modifying the root system architecture of nitrate-deficient plants, exogenous melatonin and dopamine also changed the uptake, transport, and distribution of ¹⁵NO₃ ^-. Furthermore, both exogenous melatonin and dopamine enhanced tolerance to low nitrate stress by maintaining the activity of enzymes (NR, NiR, GS, Fd-GOGAT, and NADH-GOGAT) and the transcription levels of related genes involved in leaf and root nitrogen metabolism. We also found that exogenous melatonin and dopamine promoted the expression of nitrate transporter genes (NRT1.1, NRT2.4, NRT2.5, and NRT2.7) in nitrate-deficient plant leaves and roots. Our results suggest that both exogenous melatonin and dopamine can mitigate low nitrate stress by changing the root system architecture, promoting the absorption of nitrate, and regulating the expression of genes related to nitrogen transport and metabolism. However, according to a comprehensive analysis of the results, exogenous dopamine plays a more significant role than melatonin in improving plant nitrogen use efficiency.

H2S pretreatment mitigates the alkaline salt stress on Malus hupehensis roots by regulating Na+/K+ homeostasis and oxidative stress

Hydrogen sulfide (H₂S) plays an important role in the plant salt stress response. The main component of salt stress is neutral salt (NaCl); NaHCO₃ and Na₂CO₃ play a key role in soil alkaline due to the influence of pH. Malus hupehensis Rehd. var. pingyiensis Jiang (Pingyi Tiancha, PYTC) is a salt-sensitive apple rootstock. Seedlings of PYTC pretreated with NaHS (an H₂S donor) were exposed to an alkaline salt solution, and then the plant growth, root architecture, oxidative damage, Na⁺/K⁺ homeostasis and gene expression of MhSOS1 and MhSKOR were investigated. The results showed that NaHS pretreatment increased the endogenous H₂S content in seedlings, significantly alleviated the alkaline salt stress-induced growth inhibition and oxidative damage by inducing antioxidant enzymes activities, and sustained the root activity and root architecture of PYTC in the alkaline salt solution. NaHS pretreatment significantly decreased the root Na⁺ content and increased K⁺ content to maintain the homeostasis of Na⁺/K⁺, and effect the expression of MhSOS1 and MhSKOR at the transcription level in the presence of the alkaline salt. Our study reveals that application of H₂S could mitigate the toxic effect of alkaline salt stress on Malus hupehensis seedlings, thus providing a foundation for improved plant tolerance to alkaline salt stress.

[Effects of dazomet fumigation on growth, biological characteristics of Malus hupehensis seedlings and soil environment]

In this study, we examined the effects of dazomet fumigation with different concentrations (0, 0.1, 0.2, 0.4 g·kg^-1) on the microbial characteristics of continuous cropping soil and growth of Malus hupehensis seedling in greenhouse and open-field pot. The results showed that all the treatment of dazomet fumigation could promote the growth of M. hupehensis seedlings in continuous cropping soil, with 0.2 g·kg^-1 treatment showing the strongest effect. Compared to the control, plant height, stem diameter, dry weight of M. hupehensis seedlings in 0.2 g·kg^-1 dazomet fumigation were increased by 192.9% and 91.8%, 72.8% and 60.1%, 196.8% and 195.0%, 138.5% and 130.7%, respectively in greenhouse and open-field. The root related indices (root length, root area, root volume, root respiration rate) were significantly promoted. The activities of SOD, POD, CAT in roots were increased by 114.6% and 118.5%, 123.5% and 107.6%, 164.6% and 175.6% respectively compared with the control, whereas the content of malondialdehyde was significantly lowered. Soil bacterial content, fungal content, copy number of Fusarium oxysporum gene and soil enzyme activity were significantly decreased with the increasing dazomet concentrations. In conclusion, 0.2 g·kg^-1 dazomet fumigation could increase the biomass of M. hupehensis seedlings in continuous cropping, improve soil environment, and effectively alleviate the continuous cropping obstacle. Therefore, 0.2 g ·kg^-1 dazomet fumigation could be given priority during the reconstruction of old apple orchards.

Epigenomic Regulatory Mechanism in Vegetative Phase Transition of Malus hupehensis

In woody plants, phase transitions substantially affect growth and development. Although there has been considerable interest in the regulatory mechanisms underlying phase changes, the associated epigenetic modifications remain relatively uncharacterized. We examined the DNA methylation changes and the transcriptional responses in adult and juvenile Malus hupehensis leaves. The DNA methylations were 66.61% and 68.3% in the CG context, 49.12% and 52.44% in the CHG context, and 7.02% and 8.22% in the CHH context for the adult and juvenile leaves, respectively. The number of differentially methylated regions in all contexts distributed in the genic regions varied. Additionally, inhibited DNA methylation in adult leaves activated the transcription of indole-3-acetic acid related genes in the signaling, response, and transport pathways. Moreover, the opposite methylation and expression patterns were observed for the SPL and AP2 family genes between the adult and juvenile leaves. Both gene families contribute to the M. hupehensis vegetative phase transition. Furthermore, the hyper-/hypomethylation of the gene body or promoter of transcription factor genes may lead to up-/downregulated gene expression. The methylation levels of the WRKY (22), NAC (21), ERF (8), WOX (2), KNAT (6), EIN3 (2), SCL (7), ZAT (7), and HSF (4) genes were higher in the adult leaves than in the juvenile leaves, whereas the opposite pattern was observed for the TCP (2), MADS-box (11), and DOF (3) genes. An analysis of the correlation between methylation and transcription indicated the methylation of the gene body in all contexts and the methylation of the promoter in the CG and CHG contexts are negatively correlated with gene expression. However, the methylation of the promoter in the CHH context is positively correlated with gene expression. These findings reflect the diversity in the epigenetic regulation of gene expression and may be useful for elucidating the epigenetic regulatory mechanism underlying the M. hupehensis vegetative phase transition.

MicroRNA397b negatively regulates resistance of Malus hupehensis to Botryosphaeria dothidea by modulating MhLAC7 involved in lignin biosynthesis

MicroRNA (miRNA)-mediated post-transcriptional regulation plays a vital role in the response of plants to pathogens. Although the microRNA397 family has been implicated in physiological processes as an important regulator, little is known about its function in the resistance of plants to pathogens. Here, Malus hupehensis miR397, which was induced by Botryosphaeria dothidea infection, was identified to directly target M. hupehensis Laccase7 (MhLAC7). The expression analysis of mature Mh-miR397 and MhLAC7 revealed their partly opposite expression patterns. The coexpression of Mh-miR397b in MhLAC7 overexpressing Nicotiana benthamiana suppressed the accumulation of exogenous MhLAC7 and endogenous NbLAC7, which led to decreased lignin content and reduced plant resistance to Botrytis cinerea. As reflected by increasing disease severity and pathogen growth, overexpression of miR397b in both the resistant M. hupehensis and susceptible M. domestica 'Gala' resulted in an increased sensitivity to B. dothidea infection, owing to reduced LAC7 expression and lignin content; however, the inhibition of miR397 had opposite effects. MicroRNA397 functions as a negative regulator in the resistance of Malus to B. dothidea by modulating the LAC7 expression and lignin biosynthesis.

MhNRAMP1 From Malus hupehensis Exacerbates Cell Death by Accelerating Cd Uptake in Tobacco and Apple Calli

Excessive cadmium (Cd) damages plants by causing cell death. The present study discusses the function of natural resistance-associated macrophage protein (NRAMP) on cell death caused by Cd in Malus hupehensis. MhNRAMP1 was isolated from M. hupehensis roots, and its protein was located in the cell membrane as a transmembrane protein characterized by hydrophobicity. MhNRAMP1 expression in the roots was induced by Cd stress and calcium (Ca) deficiency. MhNRAMP1 overexpression increased Cd concentration in yeasts and enhanced their sensitivity to Cd. Phenotypic comparisons of plants under Cd stress revealed that the growth of transgenic tobacco and apple calli overexpressing MhNRAMP1 was worse than that of the wild type (WT). The Cd²⁺ influx of transgenic tobacco roots and apple calli was higher, and the recovery time of the Cd²⁺ influx to a stable state in transgenic apple calli was longer than that of the WT. Cd accumulation and the percentage of apoptotic cells in transgenic lines were higher. Correspondingly, the caspase-1-like and vacuolar processing enzyme (VPE) activities and MdVPEγ expression were higher in transgenic apple calli, but the expression levels of genes that inhibit cell death were lower than those in the WT under Cd stress. Moreover, the Cd translocation from the roots to leaves was increased after MhNRAMP1 overexpression, but the Cd translocation from the leaves to seeds was not affected. These results suggest that MhNRMAP1 exacerbated Cd-induced cell death, which was accomplished by mediating Cd²⁺ uptake and accumulation, as well as stimulating VPE.

decreases cell death in tobacco roots by controlling Cd2+ uptake

Cadmium (Cd) induces cell death in plant roots. Mitogen-activated protein kinase (MAPK) plays a role in the regulation of cell death induced by Cd in plant roots. In this study, MhMAPK4 was isolated from the roots of Malus hupehensis. Subcellular localization showed that the MhMAPK4 protein was located in the cell membrane and cytoplasm and is a transmembrane protein that is characterized by hydrophily. The expression of MhMAPK4 in the roots of M. hupehensis was up-regulated by Cd sulfate and Cd chloride. Phenotypic comparison under Cd stress showed that the growth of wild-type (WT) tobacco was lower than the transgenic lines overexpressing MhMAPK4. The fresh weight and the root length of WT also was lower than that of the transgenic tobacco. The net Cd2+ influx in the tobacco roots was decreased by the overexpression of MhMAPK4, as was root Cd accumulation. The recovery time of the Cd2+ influx to stable state in the transgenic tobacco was also shorter than the WT. The expression of iron-regulated transporter 1 (NtIRT1) and natural resistance associated macrophage protein 5 (NtNRAMP5) was relatively low in the transgenic lines under Cd stress. Cell death and apoptosis in the tobacco roots was reduced following the overexpression of MhMAPK4. The activity of vacuolar processing enzyme (VPE) and the transcript level of VPE in the transgenic tobacco was lower than that of WT under Cd stress. In addition, the electrolyte leakage and malondialdehyde and hydrogen peroxide contents in the transgenic tobacco were lower than those of WT, whereas the antioxidant enzyme activity and expression were higher. These results suggest that MhMAPK4 regulates Cd accumulation by mediating Cd2+ uptake by the roots, and controls Cd-caused cell death by adjusting VPE activity.

Complete Chloroplast Genome Sequence of Malus hupehensis: Genome Structure, Comparative Analysis, and Phylogenetic Relationships

Malus hupehensis belongs to the Malus genus (Rosaceae) and is an indigenous wild crabapple of China. This species has received more and more attention, due to its important medicinal, and excellent ornamental and economical, values. In this study, the whole chloroplast (cp) genome of Malus hupehensis, using a Hiseq X Ten sequencing platform, is reported. The M. hupehensis cp genome is 160,065 bp in size, containing a large single copy region (LSC) of 88,166 bp and a small single copy region (SSC) of 19,193 bp, separated by a pair of inverted repeats (IRs) of 26,353 bp. It contains 112 genes, including 78 protein-coding genes (PCGs), 30 transfer RNA genes (tRNAs), and four ribosomal RNA genes (rRNAs). The overall nucleotide composition is 36.6% CG. A total of 96 simple sequence repeats (SSRs) were identified, most of them were found to be mononucleotide repeats composed of A/T. In addition, a total of 49 long repeats were identified, including 24 forward repeats, 21 palindromic repeats, and four reverse repeats. Comparisons of the IR boundaries of nine Malus complete chloroplast genomes presented slight variations at IR/SC boundaries regions. A phylogenetic analysis, based on 26 chloroplast genomes using the maximum likelihood (ML) method, indicates that M. hupehensis clustered closer ties with M. baccata, M. micromalus, and M. prunifolia than with M. tschonoskii. The availability of the complete chloroplast genome using genomics methods is reported here and provides reliable genetic information for future exploration on the taxonomy and phylogenetic evolution of the Malus and related species.

[Effects of hydrogen sulfide on root architecture, leaf reactive oxygen and photosynthetic characteristics of Malus hupehensis under waterlogging.]

The root architecture, leaf reactive oxygen species and photosynthetic characteristics of potted Malus hupehensis seedlings were investigated after 10 days of waterlogging under five concentrations of hydrogen sulfide (H2S) donor sodium hydrosulfide (NaSH, 0, 0.02, 0.05, 0.1, 0.2 mmol·L^-1) and NaSH with scavenger hypotaurine (HT). The results showed that the root length, root surface area, root volume, root diameter, root top number, fractal dimension, first lateral roots and root activity all increased, the leaf superoxide anion (O2^-_·) accumulation and hydrogen pero-xide (H2O2) content both decreased, the leaf P_n, T_r, g_s, WUE and CUE all raised, the plant height and biomass both increased when the 0.02-0.1 mmol·L^-1 NaHS were added to the waterlogging environment. Adding 0.1 mmol·L^-1 NaHS showed the best effects. All these parameters dropped back to the level of waterlogging alone when NaHS concentration was 0.2 mmol·L^-1 or HT was added to the 0.1 mmol·L^-1 NaHS solution. These suggested that applying an appropriate amount of H2S could effectively alleviate the inhibition of soil waterlogging on the root growth of M. hupehensis and relieve the effects of waterlogging stress on photosynthesis and plant growth by reducing the production of reactive oxygen species and increasing CUE.

Dopamine alleviates nutrient deficiency-induced stress in Malus hupehensis

Dopamine mediates many physiological processes in plants. We investigated its role in regulating growth, root system architecture, nutrient uptake, and responses to nutrient deficiencies in Malus hupehensis Rehd. Under a nutrient deficiency, plants showed significant reductions in growth, chlorophyll concentrations, and net photosynthesis, along with disruptions in nutrient uptake, transport, and distribution. However, pretreatment with 100 μM dopamine markedly alleviated such inhibitions. Supplementation with that compound enabled plants to maintain their photosynthetic capacity and development of the root system while promoting the uptake of N, P, K, Ca, Mg, Fe, Mn, Cu, Zn, and B, altering the way in which those nutrients were partitioned throughout the plant. The addition of dopamine up-regulated genes for antioxidant enzymes involved in the ascorbate-glutathione cycle (MdcAPX, MdcGR, MdMDHAR, MdDHAR-1, and MdDHAR-2) but down-regulated genes for senescence (SAG12, PAO, and MdHXK). These results indicate that exogenous dopamine has an important antioxidant and anti-senescence effect that might be helpful for improving nutrient uptake. Our findings demonstrate that dopamine offers new opportunities for its use in agriculture, especially when addressing the problem of nutrient deficiencies.

Polyphenolic compounds from Malus hupehensis and their free radical scavenging effects

One new 4-chromanone glycoside, 5-O-β-d-glucopyranoside-4-chromanone (1), together with 21 known polyphenols, was isolated from the leaves of Malus hupehensis. Their structures were elucidated on the basis of extensive spectroscopic methods including NMR (1D and 2D), mass (ESIMS and HRESIMS), IR, and by comparison with the data reported in the literature. Some of the isolated compounds were screened for antioxidant activity. Compounds 18 and 14 exhibited significant antioxidant activities with SC₅₀ values 2.73 and 2.91 μg/mL, respectively, while 17, 19, 11, 7, 20, 22, 12 and 13 exhibited moderate activities with SC₅₀ values ranging from 5.24-11.86 μg/mL. The HPLC fingerprint profiles of the leaves and fruits extracts were also analysed, which showed that the constituents were almost the same in both the extracts except for the content of phlorizin which was present in higher amount in the leaves.

Malus hupehensis miR168 Targets to ARGONAUTE1 and Contributes to the Resistance against Botryosphaeria dothidea Infection by Altering Defense Responses

MicroRNA (miRNA)-mediated post-transcriptional regulation plays a fundamental role in various plant physiological processes, including responses to pathogens. MicroRNA168 has been implicated as an essential factor of miRNA pathways by targeting ARGONAUTE1 (AGO1), the core component of the RNA-induced silencing complex (RISC). A fluctuation in AGO1 expression influences various plant-pathogen interactions, and the homeostasis of AGO1 and miR168 accumulation is maintained by a complicated feedback regulatory loop. In this study, the connection between miR168 and the resistance of Malus hupehensis to Botryosphaeria dothidea is revealed. The induction of both the mature miR168 and its precursor in plants subjected to B. dothidea infection indicate the transcriptional activation of MIR168a. MIR168a promoter analysis demonstrates that the promoter can be activated by B. dothidea and salicylic acid (SA). However, the direct target of miR168, M. hupehensis ARGONAUTE1 (MhAGO1), is shown to be induced under the infection. Expression and transcription activity analysis demonstrate the transcriptional activation and the post-transcriptional suppression of MhAGO1 in response to B. dothidea infection. By inhibiting reactive oxygen species (ROS) production and enhancing SA-mediated defense responses, miR168a delays the symptom development of leaves inoculated with B. dothidea and impedes the pathogen growth, while MhAGO1 is found to have the opposite effects. Collectively, these findings suggest that the expression of miR168 and MhAGO1 in M. hupehensis in response to B. dothidea infection is regulated by a complicated mechanism. Targeting to MhAGO1, a negative regulator, miR168 plays a positive role in the resistance by alterations in diverse defense responses.

[Effects of different potassium levels on growth and NO3- uptake and utilization of Malus hupehensis seedlings]

In order to explore the effects of potassium fertilizer on the absorption and utilization of nitrogen fertilizer, and provide theoretical basis for scientific fertilization in apple orchard, Malus hupehensis seedlings were used to study the effects of different potassium levels (K₀, K₁, K₂, K₃, K₄, K₅, and K₆ equivalent to 0, 2, 4, 6, 8, 10, and 12 mmol·L^-1 K⁺, respectively) on growth and NO₃^- uptake and utilization using the track technology of ¹⁵N and non-damage microtest technique. The results showed that root vigor, nitrate reductase activity and root morphological index of seedlings in K₃ treatment were significantly higher than those in the other treatments. The ¹⁵N derived from fertilizer (Ndff) in different organs of M. hupehensis seedlings was significantly different among treatments, and the Ndff values of root, stem and leaf appeared in K₃ treatment were the highest, being 1.36, 1.33 and 1.47 times as those of the K₀ treatment, respectively. With the increase of potassium application rate, the ¹⁵N utilization rate of seedlings were firstly increased and then decreased, and reached the highest value (23.3%) in K₃ treatment, which was 3.04 times as that of K₀ treatment. The results of non-damage microtest technique showed that there was a strong absorption of NO₃^- in K₃ treatment (19.34 pmol·cm^-2·s^-1), while a clear trend of outflow was observed in the potassium deficiency (K₀) and high potassium (K₆) treatments. Therefore, the deficit or excess of potassium could inhibit the uptake and utilization of nitrogen, while appropriate potassium could promote root growth, enhance the activity of nitrate reductase, and then increase the absorption of nitrogen by seedlings.