Response of antioxidant enzymes and lignin synthesis system in the flesh of Tainong No. 1 mango fruit to enhanced UV-B radiation treatment

【Objective】Environmental damage causes an increase in UV-B radiation content, posing a threat to crops. According to the research conducted by the experimental group, it was found that under artificial simulation of enhanced UV-B radiation, the fruit trees of Tainong No. 1 showed a decrease in yield and deterioration in fruit quality. Our previous research has shown that enhancing UV-B radiation leads to a significant increase in reactive oxygen species (ROS) in fruits, causing oxidative stress damage and resulting in a decrease in fruit quality. In addition, the response of antioxidant enzymes to enhanced UV-B radiation has been widely reported. Therefore, the purpose of this study is to investigate the effect of enhanced UV-B radiation on the quality of mango fruit, and to elucidate the gene expression and response patterns of antioxidant enzymes in mango fruit under UV-B stress.【Methods】Using the Tainong No. 1 mango tree as the experimental material, ten 16-year-old robust mango trees with a high-yield were selected, five plants in the treatment group were supplemented with 96 kJ·m²·d- ¹ UV-Blamp irradiation under natural light, while five plants in the control group received natural light irradiation. Enhanced UV-B radiation treatment began 30 days after flowering and continued until fruit harvesting. During this period, at 10-day intervals, the changes in fruit size were observed and fruit pulp was extracted for measuring physiological indicators [ROS, hydrogen peroxide (H2O2), malondialdehyde (MDA) content, and relative conductivity (RC)], fruit quality (soluble sugar, titratable acid, and sugar/acid ratio) and changes in the activity of antioxidant enzyme systems [superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT)]. RNA was extracted from fruit pulp for transcriptome sequencing and data analysis, and significantly differentially expressed genes (DEGs) were validated using real-time fluorescence quantitative PCR.【Results】Under enhanced UV-B radiation, the fruit expansion rate was reduced compared with that of the control group, and the fruit size was smaller, which damaged the appearance quality of the fruit. However, at harvest (91 d after flowering), soluble sugar content in the treated group was significantly increased, while the titratable acid content significantly decreased and the sugar to acid ratio significantly increased, resulting in a significant improvement in the main nutritional flavor quality of the fruit. The results indicate that enhancing UV-B radiation promotes early fruit ripening, thereby improving fruit flavor quality. The treatment also led to an increase in the accumulation of ROS in the fruit. At 40 and 91 days after flowering, the H2O2 content in the treatment group was significantly higher than that in the control group; at 77 days after flowering, the content of ROS was significantly higher than that of the control group, but at 91 days after flowering, the treatment was significantly lower than the control, and the damage of ROS in the flesh was basically consistent with the changes in ROS content. The activities of SOD and POD enzymes in the treatment group were lower than those in the control group during the early stage of fruit growth, but when oxidative damage occurred in the treated fruit (50 days after flowering), their activity significantly increased. At harvest, the RC and H2O2 content in the treated pulp were significantly higher than the control, while the SOD activity and ROS content were significantly lower than the control. Interestingly, CAT activity did not show significant changes throughout the experiment. The results indicate that H2O2 might be the major ROS causing early ripening of fruit in the treatment group. They also proved that SOD and POD are important antioxidant enzymes in mango fruit to alleviate oxidative stress and respond to enhanced UV-B radiation. Based on the measured physiological data, transcriptome analysis was performed on samples at 50 days after flowering and 91 days after flowering, when the flesh of mango fruit showed significant oxidative stress. At least 40820438 clean reads were obtained from each sample, with an error rate of 0.01%, indicating high data quality. At 50 days after flowering, treatment and control groups had 804 DEGs, of which 669 genes were upregulated and 135 genes were downregulated by the UV-B treatment. At 91 days after flowering, there were 189 DEGs, of which 118 were upregulated and 71 were downregulated by the treatment. Through KOG enrichment analysis of the DEGs from two periods, it was found that enhanced UV-B radiation increased the expression of secondary metabolism related genes in mango flesh; KEGG enrichment analysis showed that the DEGs in both periods were mainly enriched in the two pathways of biosynthesis of secondary metabolites and phenylalanine biosynthesis. The genes related to the antioxidant enzyme, POD, were enriched in the lignin synthesis pathway of Phenylpropanoid biosynthesis (ko00940). At 50 days after flowering, compared with the control group, the treatment group had a total of 12 DEGs enriched in this pathway, including 7 PODs, 2 HCTs, 1 F5H, 1 COMT and 1 4CL. Except for the downregulated LOC123227833 (F5H) and LOC123194338 (POD), all the other genes were upregulated. At 91 days after flowering treatment, the difference was small, with only downregulation of LOC123194338 (POD) and upregulation of LOC123209447 (4CL)and LOC123225298 (POD). The highly differentially expressed POD family member LOC123195912, validated through real- time fluorescence quantitative PCR analysis, is a key gene responsive to enhanced UV-B stress.【Conclusion】Comprehensive analysis shows that enhanced UV-B radiation treatment hinders fruit growth and improves the intrinsic quality of fruit of Tainong No. 1 mango. The treatment induced upregulation of POD genes in the lignin synthesis pathway in the pulp; it enhances the ability to clear ROS and dissipate UV-B radiation stress, thereby increasing the resistance to UV-B radiation stress.