Introduction: High salinity significantly hampers global agricultural productivity. Plants typically undergo lower nitrogen utilization efficiency (NUE) under salt stress. As an active byproduct from brassinolide biosynthesis, 24-epibrassinolide (EBR) is involved in regulating the stress-treated plant N absorption and assimilation. However, the exogenous EBR application effects’ on N absorption and assimilation in apple exposed to the salt-stressed condition remains unclear.Methods: We sprayed exogenous EBR (0.2 mg L⁻¹) on apple dwarf rootstock (M9T337) seedlings (growing hydroponically) under salt (NaCl) stress in a growth chamber. We analyzed the seedling development, photosynthesis and its-mediated C fixation, N (NO3−) absorption and assimilation in reponse to exogenous EBR application under salt stress.Results: The findings demonstrated that NaCl stress greatly hampered seedlings’ root growth and that exogenous EBR application obviously alleviated this growth suppression. Exogenous EBR-treated plants under NaCl stress displayed the more ideal root morphology and root activity, stronger salt stress tolerance and photosynthetic capacity as well as higher C- and N-assimilation enzyme activities, NO3− ion flow rate and nitrate transporter gene expression level than did untreated plants. Furthermore, the results of isotope labeling noted that exogenous EBR application also enhanced ¹³C-photoassimilate transport from leaves to roots and ¹⁵NO3− transport from roots to leaves under NaCl stress.Conclusion: Our findings imply that exogenous EBR application, through strengthening photosynthesis, C- and N-assimilation enzyme activities, nitrate absorption and transport as well as synchronized optimizing the distribution of seedlings’ C and N, has a fundamental role in improving NUE in apple rootstock seedlings under salt stress.

Both fulvic acid (FA) and nitrogen (N) play important roles in agricultural production in China. Plants typically show a higher nitrogen utilization efficiency (NUE) under FA application. However, the role of FA application in apple growth and NUE remains unclear. A hydroponic culture experiment was performed, and M9T337 seedlings (a dwarf apple rootstock) were used as the experimental subjects. The biomass, photosynthesis, accumulation, and distribution of photosynthates, N absorption and assimilation, and relative gene expression in the seedlings were examined after treatment with five different concentrations of FA (0, 60, 120, 180, and 240 mg·L⁻¹, represented by CK, FA1, FA2, FA3, FA4, respectively). The results showed that the seedling dry weight and ¹⁵NUE were enhanced by FA, and both were highest under the FA2 (the concentration of fulvic acid is 120 mg·L⁻¹) treatment. Further analysis revealed that under the FA2 treatment, the root morphology was optimized, and the root activity was relatively high. Compared with CK (control, the concentration of fulvic acid is 0 mg·L⁻¹), the FA2 treatment strengthened photosynthesis, elevated the key enzyme activities related to C metabolism, upregulated the gene expression of sugar transport proteins, and increased the root sorbitol and sucrose contents, which suggested that the FA2 treatment optimally affected the root growth and N absorption because it enhanced photosynthate synthesis and the leaf-to-root translocation of photoassimilates. The seedlings in the FA2 treatment group also showed a significantly higher NO₃⁻ influx rate and NRT (nitrate transporter) gene expression in the roots. Moreover, relatively high N metabolism-related enzyme activities in the leaves and roots were also observed under the FA2 treatment. The isotope labeling results showed that the optimal FA2 supply not only promoted seedling ¹⁵N absorption but also optimized the distribution of C and N in the seedlings. These results suggested that an optimal FA supply (120 mg·L⁻¹) enhanced seedling NUE by strengthening photoassimilate synthesis and transport from leaves to roots, regulating N absorption, assimilation, and distribution.

Sugars Will Eventually be Exported Transporters (SWEETs) play important roles during plant growth and development. Bioinformatics revealed 27 SWEET genes in the apple (Malus × domestica Borkh.) genome and classified them into four clades. These genes are unevenly located on 12 chromosomes, and 8 tandem-repeat genes and 18 fragment-repeat genes are present in the MdSWEET family. PlantCARE-based prediction of cis-acting elements of the MdSWEET promoter suggested that most MdSWEETs may be expressed specifically in the phloem and respond to hormones and stresses. qRT-PCR results showed that expression patterns of MdSWEETs displayed pronounced differences in different tissues/organs and different stages of apple fruit development. MdSWEET5, MdSWEET7, and MdSWEET21 were strongly expressed in mature leaves, MdSWEET2, MdSWEET12, MdSWEET13, MdSWEET14, MdSWEET17, and MdSWEET22 were highly expressed in peduncles, MdSWEET4 was highly expressed in young leaves, MdSWEET3, MdSWEET11, MdSWEET15, MdSWEET16, MdSWEET19, MdSWEET24, and MdSWEET25 were highly expressed in different parts of flowers, and MdSWEET1, MdSWEET6, MdSWEET8, MdSWEET9, MdSWEET10, MdSWEET18, MdSWEET20, MdSWEET23, and MdSWEET26 were strongly expressed in fruits. MdSWEET8 showed higher expression in the early stage of fruit development, MdSWEET1, MdSWEET10, and MdSWEET27 were highly expressed in the middle stage of fruit development, and MdSWEET6, MdSWEET9, MdSWEET18, MdSWEET20, MdSWEET23, and MdSWEET26 were sharply upregulated in the late developmental period. Our study could facilitate SWEET functional analysis in different tissue/organs and in sugar accumulation throughout the development and ripening of apple fruits. These findings provide potential opportunities to increase sugar accumulation in fruit, thereby improving fruit quality and yield.

Introduction: Apricot fruits are edible and serve as a source of medicinal compounds. Flavonols are important plant secondary metabolites that have antioxidant and antitumor effects and may promote cardiovascular health. Methods: The flavonoid content in three stages of the ‘Kuijin’ and the ‘Katy’ was observed, followed by the combination of metabolome and transcriptome analysis to explore the metabolic basis of flavonol synthesis.Results: The differences in the metabolite contents between stages (of the same cultivar) and between cultivars (at the same stage) revealed decreases in the flavonoid content as fruits developed (i.e., from 0.28 mg/g to 0.12 mg/g in ‘Kuijin’ and from 0.23 mg/g to 0.05 mg/g in ‘Katy’). To decipher the regulation of flavonol synthesis in apricot (Prunus armeniaca L.), the metabolomes and transcriptomes of fruit pulp at three developmental stages of ‘Kuijin’ and the ‘Katy’ were analyzed. A total of 572 metabolites were detected in ‘Kuijin’ and the ‘Katy’ pulp, including 111 flavonoids. The higher flavonol content young ‘Kuijin’ fruits at 42 days after full bloom is mainly due to 10 types of flavonols. Three pairs of significant differences in flavonol content were identified. From these three comparison groups, three structural genes were strongly correlated with the levels of 10 types of flavonols (Pearson correlation coefficients > 0.8, p value < 0.05), including PARG09190, PARG15135, and PARG17939. The weighted gene co-expression network analysis showed that the turquoise module genes were highly correlated with flavonol contents (P < 0.01). There were 4897 genes in this module. Out of 4897 genes, 28 transcription factors are associated with 3 structural genes based on weight value. Two of the transcription factors are not only associated with PARG09190 but also with PARG15135, indicating their critical importance in the flavonols biosynthesis. The two TFs are PARG27864 and PARG10875.Discussion: These findings provide new insights into the biosynthesis of flavonols and may explain the significant differences in flavonoid content between the ‘Kuijin’ and the ‘Katy’ cultivars. Moreover, it will aid in genetic improvement to enhance the nutritional and health value of apricots.

Leaves require the coordinated regulation of multiple types of cells to accumulate energy for plants. Transcriptome analysis at the tissue level cannot ana