To examine the potential roles of melatonin in cadmium (Cd) uptake, accumulation and detoxification in Malus plants, we exposed two different apple rootstocks varying greatly in Cd uptake and accumulation to either 0 or 30 μM Cd together with 0 or 100 μM melatonin. Cadmium stress stimulated endogenous melatonin production to a greater extent in the Cd-tolerant Malus baccata Borkh. than in the Cd-susceptible Malus micromalus ‘qingzhoulinqin’. Melatonin application attenuated Cd-induced reductions in growth, photosynthesis and enzyme activity, as well as reactive oxygen species (ROS) and malondialdehyde accumulation. Melatonin treatment more effectively restored photosynthesis, photosynthetic pigments and biomass in Cd-challenged M. micromalus ‘qingzhoulinqin’ than in Cd-stressed M. baccata. Exogenous melatonin lowered root Cd²⁺ uptake, reduced leaf Cd accumulation, decreased Cd translocation factors and increased root, stem and leaf melatonin contents in both Cd-exposed rootstocks. Melatonin application increased both antioxidant concentrations and enzyme activities to scavenge Cd-induced ROS. Exogenous melatonin treatment altered the mRNA levels of several genes regulating Cd uptake, transport and detoxification including HA7, NRAMP1, NRAMP3, HMA4, PCR2, NAS1, MT2, ABCC1 and MHX. Taken together, these results suggest that exogenous melatonin reduced aerial parts Cd accumulation and mitigated Cd toxicity in Malus plants, probably due to the melatonin-mediated Cd allocation in tissues, and induction of antioxidant defense system and transcriptionally regulated key genes involved in detoxification.

The interaction between plant, soil and microorganism plays a crucial role in sustainable development of terrestrial ecosystem function and diversity. However, little information is known about how plant growth, soil organic carbon (C) fractions and microorganism respond to exogenous C addition in soils with low organic C content. Three levels of ¹³C-glucose (equal to 0, 100% and 500% of initial microbial biomass C) were added to non-sterilized (corresponding to treatment abbreviation of CK, Glu-1, Glu-2, respectively) and sterilized soils (corresponding to treatment abbreviation of SS, SS+Glu-1, SS+Glu-2, respectively) planted with apple rootstock (Malus baccata (L.) Borkh.) seedings. The objectives of this study were to analyse the dynamics of soil organic C (SOC) fractions and soil bacterial community diversity with glucose levels and soil sterilization, and to explore the morphology of roots and nitrogen (N) metabolism by plant after glucose addition to sterilized/non-sterilized soils. Results showed that the contents of labile organic C fractions were significantly varied (P<0.05) with the levels of glucose addition and soil sterilization. SS+Glu-2 and Glu-2 treatments increased the contents of labile organic C fractions, on average, by 48.47% and 35.33% compared with no glucose addition, respectively. About 21.42% and 16.17% of glucose-C remained in sterilized and non-sterilized soils, respectively at the end of experiment (day 45). Regardless of soil sterilized or not, the glucose addition increased the richness and diversity indices of soil bacterial community compared with no-glucose addition. The glucose addition optimized root zone conditions, and enhanced root vitality, morphology and biomass. Both SS+Glu-2 and Glu-2 treatments significantly enhanced (P<0.05) the contents of nitrate (NO₃^—N) and nitrite (NO₂^—N), but sharply decreased (P<0.05) the ammonium (NH₄⁺-N) content compared with no glucose addition. Also, these two treatments significantly (P<0.05) increased the enzymic activities and gene transcript levels involved in root N metabolism, which demonstrated that the high level of glucose addition promoted N assimilation and transformation into free amino acids by root. Overall, the addition of exogenous C to not only promotes its fixation and bacterial community diversity in C-poor soils, but also improves root morphology and N absorption by plant.

Hanfu apple is the main cultivar grown in the cool areas of Northeast, Northwest, and North China. Here, we proposed a chromosome-level Hanfu genome assembly using PacBio, Illumina and Hi-C sequencing data. The total contig length was 628.99 Mb, with scaffold and contig N50 sizes of 36.18 Mb and 1.25 Mb, respectively. The Hanfu genome had a total of 39,617 genes, of which we predicted the function for 38,816. Evolutionary analysis showed that Hanfu may have undergone a γ-event, a recent whole-genome duplication. A comparative analysis was conducted on the genomes of Hanfu and homozygous triploid HFTH1, which were cultured using the anthers of diploid Hanfu apples. Three variants were identified, including 2,155,184 single nucleotide polymorphisms (SNPs), 413,108 insertions/deletions (indels), and 7,587 structural variants (SVs).This high-quality genome will provide a reference for the genetic improvement of apples and the breeding of more varieties with high resistance and high quality.

Low temperature is an abiotic stress factor limiting the distribution of fruit tree cultivation areas. As temperate deciduous fruit trees, apple (Malus domestica) trees go dormant in the winter to adapt to or avoid damage caused by low temperatures. The capacity for cold resistance is closely linked to the physiological, biochemical, and structural characteristics of one-year-old branches. In this study, we investigated such changes in the branches of cold-resistant ‘Hanfu’ (HF) and cold-sensitive ‘Naganofuji 2’ (CF) apple varieties. The relative electrical conductivity, malondialdehyde content, and reactive oxygen species content of HF branches were lower than those of CF branches, while the antioxidant enzyme activity was higher in HF. The proline, soluble protein, and soluble sugar contents in both varieties showed an initial increase, followed by a subsequent decrease. Sucrose and sorbitol were the main sugar components, but sucrose and fructose were higher in HF than in CF. The periderm, phloem, and xylem of HF branches were also found to be thicker than those of CF branches, while the vessel diameter was smaller and the density greater. The results of this study provide a theoretical reference for further research on the low temperature adaptability of apple tree branches during dormancy.