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IntroductionSubstantial previous studies have reported that fulvic acid (FA) application plays an important role in Chinese agricultural production. However, little is known about the mechanisms for using FA to increase apple trees resistance to Cd toxicity. In order to clarify the mechanism underlying FA alleviation in Cd-induced growth inhibition in apple seedlings.MethodsHerein, we treated M9T337 seedlings to either 0 or 30 µM/L Cd together with 0 or 0.2 g/L FA and analyzed the root growth, antioxidant enzyme activities, carbon (C) assimilation, nitrogen (N) metabolism, and C and N transport.ResultsThe results presented that, compared with CK (without Cd addition or FA spraying application), Cd poisoning significantly inhibited the root growth of apple seedlings. However, this Cd-induced root growth inhibition was significantly alleviated by FA spraying relative to the Cd treatment (Cd addition alone). On the one hand, the mitigation of inhibition effects was due to the reduced oxidative damage by enhancing antioxdiant enzyme (SOD, POD, and CAT) activities in leaves and roots. On the other hand, this growth advantage demonstrated compared to the Cd treatment was found to be associated with the strengthen of photosynthetic performance and the elevation of C and N metabolism enzymes activities. Meanwhile, we also found that under Cd stress condition, the distribution of C and N nutrients in apple seedlings was optimised by FA spraying application relative to the Cd treatment, according to the results of 13C and 15N tracing.ConclusionConclusively, our results suggested that the inhibitory effect of Cd on apple seedlings root growth was alleviated by FA through regulating antioxdiant capacities and C and N metabolism.

Abstract Background Peach (Prunus persica L. Batsch) is one of the most popular fruits worldwide. Although the reference genome of ‘Lovell’ peach has been released, the diversity of genome-level variations cannot be explored with one genome. To detect these variations, it is necessary to assemble more genomes. Results We sequenced and de novo assembled the genome of ‘Feichenghongli’ (FCHL), a representative landrace with strict self-pollination, which maintained the homozygosity of the genome as much as possible. The chromosome-level genome of FCHL was 239.06 Mb in size with a contig N50 of 26.93 Mb and only 4 gaps at the scaffold level. The alignment of the FCHL genome with the reference ‘Lovell’ genome enabled the identification of 432535 SNPs, 101244 insertions and deletions, and 7299 structural variants. Gene family analysis showed that the expanded genes in FCHL were enriched in sesquiterpenoids and triterpenoid biosynthesis. RNA-seq analyses were carried out to investigate the two distinct traits of late florescence and narrow leaves. Two key genes, PpDAM4 and PpAGL31, were identified candidates for the control of flower bud dormancy, and an F-box gene, PpFBX92, was identified as a good candidate gene in the regulation of leaf size. Conclusions The assembled high-quality genome could deepen our understanding of variations among diverse genomes and provide valuable information for identifying functional genes and improving the molecular breeding process.

Photosynthetic products in most fleshy fruits are unloaded via the apoplasmic pathway. Sugar transporters play an important role in the apoplasmic unloading pathway and are involved in sugar transport for fruit development. The MdSWEET23, cloned from ‘‘Hanfu’’ apple (Malus × domestica Borkh.) fruits, belongs to Clade III of the SWEET family. Subcellular localization revealed that MdSWEET23 is localized on the plasma membrane. β-glucuronidase activity assays showed that MdSWEET23 was primarily expressed in the sepal and carpel vascular bundle of apple fruits. Heterologous expression assays in yeast showed that MdSWEET23 functions in sucrose transport. The overexpression of MdSWEET23 in the ‘‘Orin” calli increased the soluble sugar content. The silencing of MdSWEET23 significantly reduced the contents of sucrose and sorbitol in apple fruits. Ectopic overexpression of MdSWEET23 in tomato altered sugar metabolism and distribution in leaves and fruits, causing a reduction in photosynthetic rates and plant height, enhanced cold stress tolerance, and increased the content of sucrose, fructose, and glucose in breaking color fruits, but did not increase sugar sink potency of tomato fruits.

IntroductionApricot 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. MethodsThe 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.ResultsThe 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.DiscussionThese 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.

IntroductionHigh 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.MethodsWe 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.ResultsThe 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.ConclusionOur 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.

Abstract Background Cassava (Manihot esculenta Crantz) efficiently accumulates starch in its storage roots. However, how photosynthates are transported from the leaves to the phloem (especially how they are unloaded into parenchymal cells of storage roots) remains unclear. Results Here, we investigated the sucrose unloading pattern and its impact on cassava storage root development using microstructural and physiological analyses, namely, carboxyfluorescein (CF) and C14 isotope tracing. The expression profiling of genes involved in symplastic and apoplastic transport was performed, which included enzyme activity, protein gel blot analysis, and transcriptome sequencing analyses. These finding showed that carbohydrates are transported mainly in the form of sucrose, and more than 54.6% was present in the stem phloem. Sucrose was predominantly unloaded symplastically from the phloem into storage roots; in addition, there was a shift from apoplastic to symplastic unloading accompanied by the onset of root swelling. Statistical data on the microstructures indicated an enrichment of plasmodesmata within sieve, companion, and parenchyma cells in the developing storage roots of a cultivar but not in a wild ancestor. Tracing tests with CF verified the existence of a symplastic channel, and [14C] Suc demonstrated that sucrose could rapidly diffuse into root parenchyma cells from phloem cells. The relatively high expression of genes encoding sucrose synthase and associated proteins appeared in the middle and late stages of storage roots but not in primary fibrous roots, or secondary fibrous roots. The inverse expression pattern of sucrose transporters, cell wall acid invertase, and soluble acid invertase in these corresponding organs supported the presence of a symplastic sucrose unloading pathway. The transcription profile of genes involved in symplastic unloading and their significantly positive correlation with the starch yield at the population level confirmed that symplastic sucrose transport is vitally important in the development of cassava storage roots. Conclusions In this study, we revealed that the cassava storage root phloem sucrose unloading pattern was predominantly a symplastic unloading pattern. This pattern is essential for efficient starch accumulation in high-yielding varieties compared with low-yielding wild ancestors.

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.

In order to optimize and transform closed mature apple orchards with standard rootstocks and improve the quality of fruit, taking a closed Red Fuji apple orchard as the test object, the effects of different density-reducing methods (deinterlacing, removing every other plant in each row, removing every other plant every other row) on the canopy microenvironment, tree structure, leaf photosynthesis and fruit quality were studied. The results showed that different density-reducing methods significantly reduced the orchard coverage and increased the crown transmittance. Among them, the deinterlacing treatment was the best in improving the population structure of the closed orchard, as it reduced the orchard coverage rate by 55.68 percentage points and the canopy transmittance by 82. 38 percentage points, compared with the control (CK). Different density-reducing methods all could significantly reduce the branch amount in the closed orchard and optimized the liranch composition. The three density-reducing methods decreased the number of branches per plant by 18.96% , 12.41% and 19.58% , respectively, cornered with the CK. And compared with the CK, the proportion of short branches and leafy branches to the total branches was increased by 17. 13% , 14. 27% and 7. 37% , respectively, and the proportion of long branches and developmental branches to the total branches was decreased by 24.47% , 18.04% and 10.79% , respectively. The effects of the different density-reducing methods on the temperature, relative light intensity, SPAD and leaf photosynthetic rate in canopies all followed an order of deinterlacing >removing every other plant in each row > removing every other plant every other row >CK, while those on the relative humidity showed an order of deinterlacing < removing every other plant in each row < removing every other plant every other row < CK. The average single fruit weight (238. 3 g) , coloring index (89.2) , smoothness index (83.2) , soluble solid content ( 15. 1% ) and high quality fruit rate (82.4% ) of the deinterlacing treatment were higher than those of other treatments, and the values were 18. 2% , 11.4% , 5. 85% , 26.9% and 25.2% higher than the CK, respectively. The use of deinterlacing to reduce density is the best for improving the microenvironment of closed apple orchards and improving the photosynthetic efficiency and fruit quality. [ABSTRACT FROM AUTHOR]

Phloem unloading plays a pivotal part in photoassimilate transport and partitioning into sink organs. However, it remains unclear whether the unloading pathway alters to adapt to developmental transitions in sinks, especially in fleshy fruits accumulating a high level of soluble sugars. Using a combination of electron microscopy, transport of the phloem-mobile symplasmic tracer carboxyfluorescein and assays of acid invertase, the pathway of unloading was investigated in different varieties of Chinese jujube fruit (Zizyphus jujuba Mill. cv Dongzao and Lizao). Structural investigation showed that the sieve element–companion-cell complex of bundles feeding the fruit flesh is symplasmically connected with surrounding parenchyma cells at the middle stage, and isolated during the early and late stages. Numerous plasmodesmata are present between phloem parenchyma cells and flesh cells throughout fruit development. Confocal laser scanning images of carboxyfluorescein unloading showed that the dye remained confined to the phloem strands during the early and late stages of fruit development, whereas it was released from the functional phloem at the middle stage. The levels of both the expression and activities of cell wall acid invertase and soluble acid invertase varied in an inverse pattern relative to fruit development. These results provided clear evidence for the predominance of an apoplasmic phloem unloading pathway interrupted with a symplasmic pathway and simultaneous symplasmic and apoplasmic unloading pathways in post-phloem transport during fruit development. Similar unloading pathways were obtained in different varieties of jujube fruit. The mechanisms and significance of the adaptive switch in the phloem-unloading pathway during fruit development were discussed. [ABSTRACT FROM PUBLISHER]