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Ferrous (Fe(II)) is readily oxidized and precipitated in alkaline conditions. The development of a highly efficient, eco-friendly and alkali-tolerant ferrous complex has been one of the most important and challenging goals in application of iron. In this study, a facile method of synthesis antioxidative ferrous complex by introducing iron nanoparticles into Maillard reaction was conducted. The complex was constructed by taking advantage of reduction reaction and chelating interaction of Maillard reaction. The relationship between iron content and synthetic conditions was studied by quadratic regression orthogonal design. Both of the products made from inorganic iron (M-FS) and iron hydroxide (M-IH) could prevent ferrous from oxidation, but exhibited different structural characteristics and stabilities. The results revealed that M-IH could interact with Maillard reaction products (MRPs) more closely based on constructing more Fe–N bonds, which improved its stability and made it survive in the strong alkaline environment for long-term periods. Furthermore, unlike the precipitation in M-FS, the iron ions in M-IH were driven to coordinate with the polymerized fraction of MRPs to turn low binding affinity to high binding affinity under strong alkaline conditions. This work provides a new way to prepare antioxidative ferrous complex and potential applications in agrochemicals.

In apple (Malus) cultivation, the adaptability of rootstocks has a fundamental influence on multiple abiotic-stress resistance properties of their scions. Here, we investigated the high-salt resistance of apple rootstocks, by treating 2-year-old apple trees consisting of the ‘Fuji’ cultivar grafted onto three different dwarfing rootstocks (SH6, M9-T337 and G935) with a high-salt solution for 30 days. We found that the high-salt stress caused leaf yellowing and growth inhibition to all apple plants, and that the most serious reduction in relative chlorophyll content, plant height and biomass accumulation was associated with the SH6 rootstock. In addition, based on the salt injury index, the high-salt resistance of the ‘Fuji’ scion on the three dwarfing rootstocks was ranked as G935 > M9-T337 > SH6. In addition, analysis of mineral elemental composition showed that the Na+ content in ‘Fuji’ leaves on the G935 rootstock increased less than twofold compared to the control, whereas those on the SH6 rootstock increased nearly 20-fold, accompanied with reduced K and Fe element absorption under high-salt treatments. Expression analysis of nine Na+ transport-related genes indicated that in the salt overly sensitive (SOS) pathway, the SOS1, SOS2 and SOS3 genes showed considerably greater induction by high-salt stress in G935 roots than in SH6 roots. Given the Na+ content detected in roots of different rootstocks, we propose that more efficient Na+ extracellular efflux in G935 roots, rather than intracellular Na+ compartmentalization, contributed to the improved characteristics of the G935 rootstock under high-salt conditions.

Additional file 1: Table S1. Information on the bisulfite sequencing libraries. Table S2. The coverage of the intergenic region. Table S3. The DMRs distributed on each chromosome. Table S4. RNA-seq sequencing data and mapping of the genome. Table S5. Differentially expressed genes involved in the flavonoid, chlorophyll and carotenoid biosynthesis pathways in WM/NL, PM/NL and LM/NL. Table S6. DMR-associated DEGs involved in the carotenoid and flavonoid biosynthesis pathways. Table S7. The average methylation levels in NL, WM, PM and LM. Table S8. The primers used in qRT-PCR and BS-PCR. Fig. S1. Information of the sequencing and mapping. Fig. S2. The global methylome in NL, WM, PM and LM. Fig. S3. The chromosomal mCs distributions in NL, WM, PM and LM. Fig. S4. The KEGG analysis of DMRs in WM/NL, PM/NL and LM/NL. Fig. S5. The principal component analysis (PCA) of expressed genes. Fig. S6. DGEs in WM/NL, PM/NL and LM/NL. Fig. S7. GO and KEGG pathway analyses of the DEGs in WM/NL, PM/NL and LM/NL. Fig. S8. Number of differentially methylated genes of CG, CHG and CHH between DMRs and DEGs in WM/NL, PM/NL and LM/NL. Fig. S9. Relationship between differential methylation and gene expression. Fig. S10. DNA methylation validated in WM/NL, PM/NL and LM/NL. Fig. S11. Gene expression validation involved in the chlorophyll, carotenoid and flavonoid biosynthesis pathways.

Phosphorus (P) is an important element in numerous metabolic reactions and signalling pathways, but the molecular details of these pathways remain largely unknown. In this study, physiological, transcriptome and metabolite analyses of apple leaves and roots were compared under different P conditions. The results showed that different P stresses influenced phenotypic characteristics, soil plant analytical development (SPAD) values and the contents of flavonoids and anthocyanins in apple seedlings. The contents of hydrogen peroxide (H2O2) and malondialdehyde (MDA) and the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), acid phosphatase (ACP) and purple acid phosphatase (PAP) were also affected by different P stresses. In addition, RNA sequencing (RNA-seq) was used to characterize the influence of different P stresses on apple seedlings. Compared with control apple plants, there were 1246 and 1183 differentially expressed genes (DEGs) in leaves and roots under the low-P treatment and 60 and 1030 DEGs in leaves and roots under the high-P treatment, respectively. Gene Ontology (GO) analysis indicated that apple trees might change their responses to metabolic processes, cell proliferation, regulation of biological processes, reactive oxygen species metabolic processes and flavonoid metabolic processes under P stress. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis further indicated that DEGs act on the mitogen-activated protein kinase (MAPK) signalling pathway, flavonoid biosynthesis, phenylpropanoid biosynthesis, and ATP-binding cassette (ABC) transporters. The metabolome analysis revealed that the levels of most amino acids and their derivatives, organic acids and flavonoids in roots treated with low-P stress were higher than those in roots of apple seedlings under control growth conditions. Apple seedlings regulate the flavonoid pathway to respond to different phosphorus environments. The results provide a framework for understanding the metabolic processes underlying different P responses and provide a foundation for improving the utilization efficiency of P in apple trees.

Drought, cold, and high salinity are three major abiotic stresses effecting apple tree growth and fruit production. Understanding the genetic mechanisms of crosstalk between stress responses signalling networks and identifying the genes involved in apple has potential importance for crop improvement and breeding strategies. Here, the transcriptome profiling analysis of in vitro-grown apple plants subjected to drought, cold and high salinity stress, showed a total of 377 upregulated and 211 downregulated common differentially expressed genes (DEGs) to all 3 stress treatments compared with the control. Gene Ontology (GO) analysis indicated that these common DEGs were enriched in ‘metabolic process’ under the ‘biological process’ category, as well as in ‘binding’ and ‘catalytic activity’ under the ‘molecular function’ category. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed that common DEGs were mainly belong to the ‘biological functions’ category and 17 DEGs were identified in ‘environmental information processing’ sub-category which may act as signal transduction components in response crosstalk regulation. Overexpression of 5 upregulated genes individually, out of these 17 common DEGs in apple calli promoted the consistent upregulation of DREB6, CBF1 and ZAT10 and increased the mass weight and antioxidase ability, implying these five common DEGs involved in multiple pathways and improved comprehensive resistance to stress.

Novel saccharide/Fe(II) complexes, sucrose/Fe(II) (1), glucose/Fe(II) (2), and fructose/Fe(II) (3) were synthesized. XRD, FTIR, NMR, TEM, XPS, UV–vis and DSC were applied for characterization. The amorphous complex is confirmed as nanoscale. It shows that the complex has ultraviolet absorption characteristics and high stability in acid and weak alkaline environments. The exothermic property at high temperature implies that it could be kept and used in normal environment. An analysis of in vitro bioactivity on apple dwarfing rootstock G. 935 reveals that complex 1 is able to provide iron for plantlets. Compared with EDTA/Fe(II), complex 1 has higher uptake efficiency and harmlessness in tissue culture. The above results suggest that the saccharide-based Fe(II) nano-complex could be applied for plant growth as iron source.

Nitrogen (N) is an essential element for plant growth and development. In this study, physiological, transcriptome and metabolite analyses of apple leaves and roots were compared under different N conditions. The results indicated that different N stresses influenced plant growth, the content of soil plant analytical development (SPAD) value and photosynthesis in apple seedlings. The content of hydrogen peroxide (H2O2), and the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS) and glutamine antinotransferase (GOGAT) were also affected by different N stresses. RNA-Seq was used to analysis the influence of different N tolerances in apple seedlings. Gene Ontology (GO) analysis of differentially expressed genes (DEGs) indicated that apple trees might affect the responses to photosynthesis, organic hydroxy compound biosynthetic process, and secondary metabolite biosynthetic process under N stress. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed that DEGs focused on photosynthesis and secondary metabolite biosynthetic process. There were 527 differential metabolites in LNL/CKL, 157 in HNL/CKL, 477 in LNR/CKR, and 232 in HNR/CKR. The metabolome analysis indicated that total of 1004 and 389 significant differential metabolites were identified under low N and high N conditions in apple. Apple seedlings regulate the carbon metabolism, nitrogen metabolism and flavonoid pathway to adapt the different nitrogen environments. This research provide understanding for the metabolic processes underlying different N responses and provide a foundation for improving the efficiency of N use in apple trees.