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Low-affinity nitrate transporter genes have been identified in subfamilies 4–8 of the rice nitrate transporter 1 (NRT1)/peptide transporter family (NPF), but the OsNPF3 subfamily responsible for nitrate and phytohormone transport and rice growth and development remains unknown. In this study, we described OsNPF3.1 as an essential nitrate and phytohormone transporter gene for rice tillering and nitrogen utilization efficiency (NUtE). OsNPF3.1 possesses four major haplotypes of its promoter sequence in 517 cultivars, and its expression is positively associated with tiller number. Its expression was higher in the basal part, culm, and leaf blade than in other parts of the plant, and was strongly induced by nitrate, abscisic acid (ABA) and gibberellin 3 (GA3) in the root and shoot of rice. Electrophysiological experiments demonstrated that OsNPF3.1 is a pH-dependent low-affinity nitrate transporter, with rice protoplast uptake assays showing it to be an ABA and GA3 transporter. OsNPF3.1 overexpression significantly promoted ABA accumulation in the roots and GA accumulation in the basal part of the plant which inhibited axillary bud outgrowth and rice tillering, especially at high nitrate concentrations. The NUtE of OsNPF3.1-overexpressing plants was enhanced under low and medium nitrate concentrations, whereas the NUtE of OsNPF3.1 clustered regularly interspaced short palindromic repeats (CRISPR) plants was increased under high nitrate concentrations. The results indicate that OsNPF3.1 transports nitrate and phytohormones in different rice tissues under different nitrate concentrations. The altered OsNPF3.1 expression improves NUtE in the OsNPF3.1-overexpressing and CRISPR lines at low and high nitrate concentrations, respectively.

Citric acid content is a critical quality determinant in citrus (Citrus spp.) fruits. Although arsenic (As) can effectively reduce citric acid content to improve citrus fruit quality, it can have adverse environmental effects. The discovery of nontoxic substitutes is hampered by the incomplete elucidation of the underlying mechanisms of As action in citrus fruits. Metabolic, transcriptomic, and physiological analyses were employed to investigate As action on citric acid accumulation to discover the mechanisms of As action in citrus. The enzyme activity related to citrate biosynthesis was not inhibited and the content of the involved metabolites was not reduced in As-treated fruits. However, the proton pump genes CitPH5 and CitPH1 control the vacuolar citric acid accumulation and transcription factor genes CitTT8 and CitMYB5, which regulate CitPH5 and CitPH1, were downregulated. The oxidative stress-response genes were upregulated in As-treated fruits. The reactive oxygen species (ROS) treatment also downregulated CitTT8 and CitMYB5 in juice cells. The mitochondrial ROS production rate increased in As-treated fruits. AsIII was more potent in stimulating isolated mitochondria to overproduce ROS compared to AsV. Our results indicate that the As inhibition of citric acid accumulation may be primarily due to the transcriptional downregulation of CitPH5, CitPH1, CitTT8, and CitMYB5. As-induced oxidative stress signaling may operate upstream to downregulate these acid regulator genes. Mitochondrial thiol proteins may be the principal targets of As action in citrus fruits.

Portable and low-cost analytical devices are essential for rapid detection of bioactive substrates in agricultural products. This study presents the first highly integrated microelectrochemical sensor based on pencil graphite for rapid and sensitive detection of hesperidin in Citrus reticulate ‘Chachi’ peel. The surface morphology and characterization as well as the electrochemical property of pencil graphite was investigated and discussed. A high electrocatalytic efficiency of hesperidin has been found at used pencil graphite-based microelectrodes. Kinetic analysis was carried out to further understand the electrochemical process of hesperidin at a pencil graphite microelectrode. Consequently, a portable and highly-integrated microelectrochemical sensor exhibits a sensitivity of 0.7251 μA cm−2 μM−1 and a detection limit as low as 25 nM (S/N = 3), and high selectivity was fabricated. Proposed microelectrochemical sensors were applied to electrochemically determinate the hesperidin content in the extract of Citrus reticulata “chachi” peel. As a result, the concentration of hesperidin in the actual real sample detected electrochemically with the proposed portable and low-cost microelectrochemical sensors is highly consistent to that obtained with a common chromatographic method, thus indicating the good reliability and that it can be used in practical applications.

The plant cuticle, as a lipid membrane covering aerial plant surfaces, functions primarily against uncontrolled water loss. Herein, the cuticle chemical composition and the transpiration of wampee fruit (Clausena lansium (Lour.) Skeels) at the green, turning, and yellow stages in cultivars of “Jixin” and “Tianhuangpi” were comprehensively studied. The coverage of wax and cutin monomers per unit of fruit surface area at the green stage was lower in “Jixin” than in “Tianhuangpi” and increased gradually during development. Cutin monomers accumulated ranging from 22.5 μg cm−2 (green) to 52.5 μg cm−2 (turning) in “Jixin” and from 36.5 μg cm−2 (green) to 81.7 μg cm−2 (yellow) in “Tianhuangpi.” The total composition of waxes ranged between 6.0 μg cm−2 (green) and 11.1 μg cm−2 (turning) in “Jixin,” while they increased from 7.4 μg cm−2 (green) to 16.7 μg cm−2 (yellow) in “Tianhuangpi.” Cutin monomers were dominated by ω-, mid-dihydroxy fatty acids (over 40%), followed by multiple monomers of α,ω-dicarboxylic acids with or without added groups, α-monocarboxylic acids with or without ω- or mid-chain hydroxy or mid-epoxy groups, primary alcohols, and phenolics. The very-long-chain (VLC) aliphatic pattern of cuticular waxes was prominently composed of n-alkanes (ranging from 21.4% to 39.3% of total wax content), fatty acids, primary alcohols, and aldehydes. The cyclic waxes were dominated by triterpenoids (between 23.9 and 51.2%), sterols, and phenolics. Water loss in wampee fruit exhibited linear changes over time, indicating an overall monofunctional barrier to transpiration. Permeance for water in wampee fruit was higher at the green stage than at the yellow stage in both “Jixin” and “Tianhuangpi,” which showed a negative correlation with the changes of VLC n-alkanes. The results showed the cuticular chemicals, including cutin monomers and waxes, in wampee fruit and further indicated the potential contributions of the cuticular chemical composition to the physiological functions in fruits.

The plant cuticle covers almost all the outermost surface of aerial plant organs, which play a primary function in limiting water loss and responding to the environmental interactions. Banana fruit is susceptible to thermal changes with chilling injury below 13°C and green ripening over 25°C. Herein, the changes of surface morphology, chemical compositions of cuticle, and the relative expression of cuticle biosynthesis genes in banana fruit under low-temperature storage were investigated. Banana fruit exhibited chilling injury rapidly with browned peel appearance stored at 4°C for 6 days. The surface altered apparently from the clear plateau with micro-crystals to smooth appearance. As compared to normal ones, the overall coverage of the main cuticle pattern of waxes and cutin monomers increased about 22% and 35%, respectively, in browned banana stored under low temperature at 6 days. Fatty acids (C16–C18) and ω-OH, mid-chain-epoxy fatty acids (C18) dominated cutin monomers. The monomers of fatty acids, the low abundant ω, mid-chain-diOH fatty acids, and 2-hydroxy fatty acids increased remarkably under low temperature. The cuticular waxes were dominated by fatty acids (> C19), n-alkanes, and triterpenoids; and the fatty acids and aldehydes were shifted to increase accompanied by the chilling injury. Furthermore, RNA-seq highlighted 111 cuticle-related genes involved in fatty acid elongation, biosynthesis of very-long-chain (VLC) aliphatics, triterpenoids, and cutin monomers, and lipid-transfer proteins were significantly differentially regulated by low temperature in banana. Results obtained indicate that the cuticle covering on the fruit surface was also involved to respond to the chilling injury of banana fruit after harvest. These findings provide useful insights to link the cuticle on the basis of morphology, chemical composition changes, and their biosynthesis regulations in response to the thermal stress of fruit during storage.

The total phenolic, flavonoid, coumarin, and volatile contents and antioxidant activities in three tissues of six Citrus grandis ‘Tomentosa’ (CGT) fruit varieties were investigated. Two flavones, 3 coumarins and 147 volatiles were identified. The exocarp contained 3 coumarins, however, no coumarins were identified in the mesocarp, and only isoimperatorin was found in the pulp. In the exocarp, mesocarp and pulp, 104, 79 and 71 volatiles were identified, respectively. FuMao (FM) contained the highest total phenolic, naringin and rhoifolin amounts; HouYeZhengMao (HY-ZM) contained the highest meranzin hydrate and bergapten amounts; and XiYangZhengMao (XY-ZM) contained the highest isoimperatorin amount. The ABTS radical scavenging power of HuangLongZhengMao (HL-ZM) was the strongest, and FM had the strongest DPPH radical scavenging effect and ferric ion reducing antioxidant power. According to overall performance, FM and HY-ZM are the top two varieties with the highest total phenolic, flavonoid, and coumarin contents and antioxidant capacities.

Citrus grandis ‘Tomentosa’, commonly known as ‘Huajuhong’ pummelo (HJH), is used in traditional Chinese medicine and can moisten the lungs, resolve phlegm, and relieve coughs. A spontaneous bud mutant, named R-HJH, had a visually attractive phenotype with red albedo tissue and red juice sacs. In this study, the content and composition of carotenoids were investigated and compared between R-HJH and wild-type HJH using HPLC–MS analysis. The total carotenoids in the albedo tissue and juice sacs of R-HJH were 4.03- and 2.89-fold greater than those in HJH, respectively. The massive accumulation of carotenoids, including lycopene, β-carotene and phytoene, led to the attractive red color of R-HJH. However, the contents of flavones, coumarins and most volatile components (mainly D-limonene and γ-terpinene) were clearly reduced in R-HJH compared with wild-type HJH. To identify the molecular basis of carotenoid accumulation in R-HJH, RNA-Seq transcriptome sequencing was performed. Among 3948 differentially expressed genes (DEGs), the increased upstream synthesis genes (phytoene synthase gene, PSY) and decreased downstream genes (β-carotene hydroxylase gene, CHYB and carotenoid cleavage dioxygenase gene, CCD7) might be the key factors that account for the high level of carotenoids in R-HJH. These results will be beneficial for determining the molecular mechanism of carotenoid accumulation and metabolism in pummelo.

The large nitrate transporter 1/peptide transporter family (NPF) has been shown to transport diverse substrates, including nitrate, amino acids, peptides, phytohormones, and glucosinolates. However, the rice (Oryza sativa) root-specific expressed member OsNPF7.2 has not been characterized. Here, our data show that OsNPF7.2 is a tonoplast localized low-affinity nitrate transporter, and affects rice growth under high nitrate supply. The expression analysis showed that OsNPF7.2 was mainly expressed in the elongation and maturation zones of roots, especially in the root sclerenchyma, cortex and stele. It was also induced by high concentrations of nitrate. Subcellular localization analysis showed that OsNPF7.2 was localized on the tonoplast of large and small vacuoles. Heterogenous expression in Xenopus laevis oocytes suggested that OsNPF7.2 was a low-affinity nitrate transporter. Knock-down of OsNPF7.2 retarded rice growth under high concentrations of nitrate. Therefore, we deduce that OsNPF7.2 plays a role in intracellular allocation of nitrate in roots, and thus influences rice growth under high nitrate supply.