Effects of gibberellin biosynthesis inhibitor on floral induction and apical bud metabolic profiles in mango

Abstract:【Objective】Mango (Mangifera indica L.) is one of the most important fruit crops in the tropical and subtropical regions, and China is the second-largest producer of mangoes after India. Floral induction (FI) is an important development event in perennial woody fruit trees, which determines the onset of fruit and plays a crucial role in yield. Mango FI starts from dormant bud or growing bud and ends with the beginning of flower bud morphological differentiation (commonly known as brush head in production). Although the mechanisms that control flowering remain primarily unknown in mango, it can be induced by cold temperatures. However, insufficient chilling in winter due to the aggravation of global warming has affected mango FI and results in decreased fruit production of commercial mango orchards in China, mainly in Hainan, Guangdong and Guangxi. In recent decades, soil drenching of paclobutrazol (PBZ) has been used as a major practice to induce mango flowering in many commercial mango orchards, including Guangdong, Guangxi and Hainan in China. PBZ inhibits gibberellin biosyn-thesis and vegetative growth, and thus enhances mango FI. However, inappropriate doses of PBZ over a long period can result in soil residues, new bud and panicle compaction, and increased incidence of disease. In addition to PBZ, there are several safer gibberellin inhibitors such as uniconazole, prohexadione calcium and mepiquat chloride, which play roles in regulating plant growth. More importantly, they have the characteristics of low toxicity and low residue in soil. For example, the activity of uniconazole is 6-10 times higher than that of paclobutrazol, but its residue in soil is only 1/10 of that of paclobutrazol. Prohexadione calcium can be rapidly degraded into water and carbon dioxide by microorganisms in the soil, and has no residual toxicity to rotation plants and no pollution to the environment, whereas, the effects of these agents on mango flowering regulation have not been evaluated. This research aimed to carry out a comprehensive analysis of the mechanism of mango flowering induced by gibberellic acid synthesis inhibitors (mepiquat chloride, prohexadione-calcium and uniconazole) during mango flowering induction. Our findings will provide a potential method for mango flowering management in tropical and subtropical areas, and provide a theoretical basis for applying SPD to regulate flowering of mango. 【Methods】Eighteen randomly selected fifteen- year- old‘Tainong’mango trees (Mangifera indica L.) grown in the experimental orchard of the South Subtropical Crops Research Institute of the Chinese Academy of Tropical Agricultural Science in Zhanjiang, China (110°16′E, 21°10′N) were used in this study from 2019 to 2021. In late September, 2019 and 2020, when the leaves of the second flush were turning green, nine trees were sprayed with 12 L SPD (homemade exogenous gibberellin inhibitors) including 2 g·L^-1 mepiquat chloride, 100 mg·L-1 prohexadione-calcium and 300 mg·L^-1 uniconazole) or 12 L water (as a control), respectively. The experiment was conducted in a randomized block design with 3 repetitions per treatment and 3 trees for each repetition. Terminal buds were collected at 30, 80 and 100 days after SPD/water treatment from October 2020 to January 2021. The experiments were conducted on the buds at three stages with three biological replicates. Each biological replicate included 12 buds from three trees. The collected bud samples were immediately placed in liquid nitrogen and stored in a freezer at −80 ℃. They were used for metabolomics detection. About 140 days after SPD/water treatment, the flower formation rate was evaluated by the ratio between the total number of branches per tree and the total number of panicles emitted per tree. When mangoes reached commercial harvest maturity, the yield per tree was counted.【Results】Ultra-performance liquid chromatography-electrospray ionization-tandem mass spectrometry (UPLC-ESI-MS/MS)-based widely targeted metabolomic analysis was carried out to assess the metabolic differences in the apical buds during different stages of mango floral induction by spraying SPD and water. A total of 582 compounds were annotated and 372 metabolites showed significant change in relative abundance (Variable Importance in Projection, VIP≥1 and Fold change, FC≥2 or ≤0.5). Among them, during 80-100 days after SPD treatment, the dormancy of apical buds was released. Lipids, phenolic acids, amino acids, carbohydrates and vitamins were among metabolites showing significant differences over time after SPD treatment. Here, 18 lipids, including 12 lysophosphatidylethanolamines (LPE), 7 lysophosphatidylcholines (LPC) and free fatty acids (FA), were significantly upregulated from 80 to 100 days after SPD treatment compared to water control. Meanwhile, the dormancy release of mango buds from 30 to 100 days after SPD treatment was accompanied by the accumulation of proline, ascorbic acid, carbohydrates and tannins. In addition, metabolites, such as Lhomocysteine, L- histidine and L- homomethionine, showed more than ten- fold difference in relative abundance from 30 to 100 days after SPD treatment. However, there were no significant changes after water treatment.【Conclusion】We confirmed the positive role of SPD in enhancing mango flowering, and revealed that novel metabolites were involved in mango FI in response to SPD, which would pro-vide a theoretical basis for utilizing SPD to induce mango flowering. However, we need further analysis of these metabolites to clarify their roles in FI of mango in response to SPD treatment.