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DNA methylation is an important epigenetic mark involved in many biological processes. The genome of the climacteric tomato fruit undergoes a global loss of DNA methylation due to active DNA demethylation during the ripening process. It is unclear whether the ripening of other fruits is also associated with global DNA demethylation. We characterized the single-base resolution DNA methylomes of sweet orange fruits. Compared with immature orange fruits, ripe orange fruits gained DNA methylation at over 30,000 genomic regions and lost DNA methylation at about 1,000 genomic regions, suggesting a global increase in DNA methylation during orange fruit ripening. This increase in DNA methylation was correlated with decreased expression of DNA demethylase genes. The application of a DNA methylation inhibitor interfered with ripening, indicating that the DNA hypermethylation is critical for the proper ripening of orange fruits. We found that ripening-associated DNA hypermethylation was associated with the repression of several hundred genes, such as photosynthesis genes, and with the activation of hundreds of genes, including genes involved in abscisic acid responses. Our results suggest important roles of DNA methylation in orange fruit ripening.

Seedlings of ‘Shatian pummelo’ ( Citrus grandis ) and ‘Xuegan’ ( Citrus sinensis ) were supplied daily with nutrient solution at a concentration of 0.5 (control), 100, 200, 300, 400, or 500 μM CuCl 2 for 6 months. Thereafter, seedling growth; leaf, root, and stem levels of nutrients; leaf gas exchange; levels of pigments; chlorophyll a fluorescence (OJIP) transients and related parameters; leaf and root relative water content; levels of nonstructural carbohydrates; H 2 O 2 production rate; and electrolyte leakage were comprehensively examined ( a ) to test the hypothesis that Cu directly damages root growth and function, thus impairing water and nutrient uptake and hence inhibiting shoot growth; ( b ) to establish whether the Cu-induced preferential accumulation of Cu in the roots is involved in Cu tolerance of Citrus ; and ( c ) to elucidate the possible causes for the Cu-induced decrease in photosynthesis. Most of the growth and physiological parameters were greatly altered only at 300–500 μM (excess) Cu-treated seedlings. Cu supply increased the level of Cu in the roots, stems, and leaves, with a greater increase in the roots than that in the stems and leaves. Many of the fibrous roots became rotten and died under excess Cu. These findings support the hypothesis that Cu directly damages root growth and function, thus impairing water and nutrient uptake and hence inhibiting shoot growth, and the conclusion that the preferential accumulation of Cu in the roots under excess Cu is involved in the tolerance of Citrus to Cu toxicity. The lower CO 2 assimilation in excess Cu-treated leaves was caused mainly by nonstomatal factors, including structural damage to thylakoids, feedback inhibition due to increased accumulation of nonstructural carbohydrates, decreased uptake of water and nutrients, increased production of reactive oxygen species, and impaired photosynthetic electron transport chain. Also, we discussed the possible causes for the excess Cu-induced decrease in leaf pigments and accumulation of nonstructural carbohydrates in the roots and leaves.

Background Glyphosate is widely used in citrus production, but its overuse can cause oxidative stress and reduced growth in young orange trees. Silicon (Si), a beneficial element, strengthens antioxidant defense pathways and attenuates oxidative damage. However, its role in alleviating glyphosate-induced stress, particularly through the non-enzymatic antioxidant systems, remains unclear. This study examined whether Si application can reduce oxidative stress in young “Valencia” orange trees by enhancing non-enzymatic defenses, reducing oxidative stress indicators, and improving plant growth. Results In an 8-month greenhouse experiment using a 4 × 2 factorial design, four glyphosate rates (0, 576, 1008 and 1440 g acid equivalent (a.e.) ha −1 ) and two Si treatments (0- and 2-mM Si), were applied to trees via fertigation and foliar sprays. Key parameters were measured nine and sixteen days after the fourth Si application in older and younger leaves, respectively. Trees treated with Si exhibited a 66% Si increase in young leaves and 44% in old leaves. Oxidative stress, measured by malondialdehyde (MDA) levels, was significantly lower in trees treated with Si across all glyphosate rates in old leaves, and in young leaves at higher glyphosate rates (1008 and 1440 g a.e. ha −1 ). Proline levels were elevated in control trees exposed to glyphosate, whereas Si treatment increased carotenoid accumulation, particularly in old leaves. Phenolic compounds increased in old leaves where Si was applied across all glyphosate rates, while in young leaves, increases occurred only at lower glyphosate rates (0 and 576 g a.e. ha −1 ). Trees treated with Si retained more leaves across most glyphosate rates and showed increased dry matter production, except at 1440 g a.e. ha −1 . Conclusion Si application effectively mitigates glyphosate-induced oxidative stress in young orange trees by enhancing non-enzymatic antioxidant defenses, particularly carotenoids and phenolic compounds, while lowering MDA levels. These findings suggest Si as a sustainable strategy to improve herbicide tolerance and strengthen the citrus tree resilience.

2,4-D retards senescence of postharvest citrus fruits by increasing exogenous auxin, endogenous ABA and SA contents, while decreasing ethylene production; and enhancing stress-defense capability through changing epicuticular wax morphology and lignin content in peel

Backround Salinity stress severely limits citrus growth by impairing root development and physiological functions. Therefore, understanding of the mechanisms underlying root system architecture (RSA) adaptation in orange seedlings is crucial for enhancing their resilience to salinity in subtropical regions. The research was conducted to unravel the role of salinity, together with foliar spraying of chitosan/selenium nanoparticles (CS/Se NPs), on nutrient uptake and RSA in Citrus sinensis is examined. Valencia orange seedlings received foliar spraying treatments of distilled water (control), chitosan (CS), selenium nanoparticles (Se NPs), and a CS/Se NPs composite at two levels (10 and 20 mg L − 1 ). Following treatment, seedlings were exposed to three salinity regimes: non-saline or control (0 mM NaCl), moderate (50 mM NaCl), and intense (100 mM NaCl). Results Exposure to intense salinity markedly increased sodium (Na) accumulation while reducing potassium (K), the K/Na ratio, calcium (Ca), the Ca/Na ratio, and magnesium (Mg) levels in roots. Additionally, salinity stress negatively affected RSA traits, including total root length, root surface area, and lateral root formation, relative to non-saline conditions. In contrast, treated seedlings with CS, Se NPs, and the CS/Se NPs composite demonstrated lower Na concentrations, enhanced nutrient accumulation, and improved RSA parameters. Remarkably, foliar spraying of CS/Se NPs at 10 mg L − 1 was the most effective under intense salinity stress, significantly reducing Na accumulation and enhancing K, K/Na, Ca, Ca/Na, and Mg content in roots, while also promoting total root length, root volume, root surface area, lateral root number, root perimeter, and maximum root count per plant compared to controls. Conclusions This study demonstrates that CS/Se NPs, with the 10 mg L − 1 dose being particularly effective, hold potential as a foliar application to mitigate salinity stress, thereby contributing to better crop performance in harsh growing conditions.

It is known that nutrient deprivation following detachment can cause non-chilling peel pitting (NCPP) in citrus fruits when stored under a non-stressful environment and that this damage is reduced by pretreating the fruit with ethylene (ETH) (4 d, 10 µL L−1). The present work investigates the effect of this pretreatment on jasmonate (JA) accumulation and transcriptional regulation in mature Navelate oranges (Citrus sinensis L. Osbeck) stored under non-stressful conditions. ETH increased the expression of abundant genes participating in the synthesis of cis-(+)-12-oxo-phytodienoic acid (OPDA), jasmonic acid (JA), and methyl jasmonate (MeJA). ETH also upregulated genes involved in jasmonoyl–isoleucine (JAIle) synthesis (CsJAR1) and decrease (CsCYP94B3 and CYP94C1), and CsSTA2, related to JA sulfation. The levels of these JA metabolites increased during fruit holding in ETH and after shifting them to air, with MeJA accumulation being especially remarkable. Overall, the beneficial effect of ETH on reducing NCPP appears to be related not only to this redirection of OPDA and JA metabolism towards the formation of JA derivatives but also to the regulation of JA signalling. Indeed, the repression of the receptor CsCOI1 and upregulation of various CsJAZs repressors caused by nutrient deprivation, together with the ETH-mediated induction of CsCOI1, CsTOPLESS, and abundant CsJAZs during long-term storage, suggests the occurrence of an ETH-enhanced negative transcriptional regulatory feedback loop in JA metabolism and signalling, by which the susceptibility of detached Navelate oranges to NCPP might be reduced.

Background Abiotic stress, such as salinity, affects the photosynthetic apparatus of plants. It is reported that the use of selenium nanoparticles (Se NPs), and biochemical compounds such as chitosan (CS) increase the tolerance of plants to stress conditions. Therefore, this study aimed to elucidate the potential of Se NPs, CS, and their composite (CS + Se NPs) in improving the photosynthetic apparatus of C. sinensis under salt stress in greenhouse conditions. The grafted seedlings of C. sinensis cv. Valencia after adapting to the greenhouse condition, were imposed with 0, 50, and 100 mM NaCl. After two weeks, the plants were foliar sprayed with distilled water (control), CS (0.1% w/v), Se NPs (20 mg L − 1 ), and CS + Se NPs (10 and 20 mg L − 1 ). Three months after treatment, the levels of photosynthetic pigments, leaf gas exchange, and chlorophyll fluorescence in the treated plants were evaluated. Results Under salinity stress, total chlorophyll, carotenoid, and SPAD values decreased by 31%, 48%, and 28% respectively, and Fv/Fm also decreased compared to the control, while the ratio of absorption flux (ABS), dissipated energy flux (DI 0 ) and maximal trapping rate of PSII (TR0) to RC (a measure of PSII apparent antenna size) were increased. Under moderate (50 mM NaCl) and intense (100 mM NaCl) salinity stress, the application of CS + Se NPs significantly increased the levels of photosynthetic pigments and the Fv/Fm value compared to plants treated with distilled water. Conclusions It may be inferred that foliar treatment with CS + Se NPs can sustain the photosynthetic ability of C. sinensis under salinity stress and minimize its deleterious effects on photosynthesis.

Background Many citrus orchards of south China suffer from soil acidification, which induces aluminum (Al) toxicity. The Al-immobilization in vivo is crucial for Al detoxification. However, the distribution and translocation of excess Al in citrus species are not well understood. Results The seedlings of ‘Xuegan’ [ Citrus sinensis (L.) Osbeck] and ‘Shatianyou’ [ Citrus grandis (L.) Osbeck], that differ in Al tolerance, were hydroponically treated with a nutrient solution (Control) or supplemented by 1.0 mM Al 3+ (Al toxicity) for 21 days after three months of pre-culture. The Al distribution at the tissue level of citrus species followed the order: lateral roots > primary roots > leaves > stems. The concentration of Al extracted from the cell wall (CW) of lateral roots was found to be about 8 to 10 times higher than in the lateral roots under Al toxicity, suggesting that the CW was the primary Al-binding site at the subcellular level. Furthermore, the Al distribution in CW components of the lateral roots showed that pectin had the highest affinity for binding Al. The relative expression level of genes directly relevant to Al transport indicated a dominant role of Cs6g03670.1 and Cg1g021320.1 in the Al distribution of two citrus species. Compared to C. grandis , C. sinensis had a significantly higher Al concentration on the CW of lateral roots, whereas remarkably lower Al levels in the leaves and stems. Furthermore, Al translocation revealed by the absorption kinetics of the CW demonstrated that C. sinensis had a higher Al retention and stronger Al affinity on the root CW than C. grandis . According to the FTIR (Fourier transform infrared spectroscopy) analysis, the Al distribution and translocation might be affected by a modification in the structure and components of the citrus lateral root CW. Conclusions A higher Al-retention, mainly attributable to pectin of the root CW, and a lower Al translocation efficiency from roots to shoots contributed to a higher Al tolerance of C. sinensis than C. grandis. The aluminum distribution and translocation of two citrus species differing in aluminum tolerance were associated with the transcriptional regulation of genes related to Al transport and the structural modification of root CW.

Plants use volatile terpene compounds as odor cues for communicating with the environment. Fleshy fruits are particularly rich in volatiles that deter herbivores and attract seed dispersal agents. We have investigated how terpenes in citrus fruit peels affect the interaction between the plant, insects, and microorganisms. Because limonene represents up to 97% of the total volatiles in orange (Citrus sinensis) fruit peel, we chose to down-regulate the expression of a limonene synthase gene in orange plants by introducing an antisense construct of this gene. Transgenic fruits showed reduced accumulation of limonene in the peel. When these fruits were challenged with either the fungus Penicillium digitatum or with the bacterium Xanthomonas citri subsp. citri, they showed marked resistance against these pathogens that were unable to infect the peel tissues. Moreover, males of the citrus pest medfly (Ceratitis capitata) were less attracted to low limonene-expressing fruits than to control fruits. These results indicate that limonene accumulation in the peel of citrus fruit appears to be involved in the successful trophic interaction between fruits, insects, and microorganisms. Terpene down-regulation might be a strategy to generate broad-spectrum resistance against pests and pathogens in fleshy fruits from economically important crops. In addition, terpene engineering may be important for studying the basic ecological interactions between fruits, herbivores, and pathogens.

Seedlings of aluminum-tolerant ‘Xuegan’ (Citrus sinensis) and Al-intolerant ‘sour pummelo’ (Citrus grandis) were fertigated for 18 weeks with nutrient solution containing 0 and 1.2 mM AlCl3·6H2O. Al toxicity-induced inhibition of photosynthesis and the decrease of total soluble protein only occurred in C. grandis leaves, demonstrating that C. sinensis had higher Al tolerance than C. grandis. Using isobaric tags for relative and absolute quantification (iTRAQ), we obtained more Al toxicity-responsive proteins from C. sinensis than from C. grandis leaves, which might be responsible for the higher Al tolerance of C. sinensis. The following aspects might contribute to the Al tolerance of C. sinensis: (a) better maintenance of photosynthesis and energy balance via inducing photosynthesis and energy-related proteins; (b) less increased requirement for the detoxification of reactive oxygen species and other toxic compounds, such as aldehydes, and great improvement of the total ability of detoxification; and (c) upregulation of low-phosphorus-responsive proteins. Al toxicity-responsive proteins related to RNA regulation, protein metabolism, cellular transport and signal transduction might also play key roles in the higher Al tolerance of C. sinensis. We present the global picture of Al toxicity-induced alterations of protein profiles in citrus leaves, and identify some new Al toxicity-responsive proteins related to various biological processes. Our results provide some novel clues about plant Al tolerance.