基于稳定同位素示踪结合组学技术研究龙眼果肉“退糖”的分子机理

The harvest period of longan fruit is short due to its maturation period coinciding with the southern summer. If miss the suitable harvest period, a phenomenon named as "sugar decline": rapid decrease of sweetness as well as aril fibrosis, will happen in longan aril of many varieties and caused serious economic losses to farmers. Previous results indicated that "sugar decline" might be mainly due to a rapid decrease of sucrose content in the aril of longan, which lead to a decrease of total sugar content and sweetness. Therefore, understanding the metabolic regulation of sucrose degradation in longan aril is the key to solve the problem "sugar decline". Although the study of sucrose metabolism has made great progress in many kinds of plants, the "sugar decline" phenomenon has not been reported in these plants. Present study of "sugar decline" still focus at physiological and biochemistry level as well as singe metabolic step, but is lack of systematic research at the molecular regulatory level. Therefore, we intend to use fruits of longan cultivars ‘Shixia’ (with a fast decrease of sugar content) and ‘Chuliang’(with a slow decrease of sugar content) as experiment materials and use isotope tracer technique combining with transcriptomics and metabonomics to analyze the metabolic network of sugars in the "sugar decline" process and screen the key genes related to sugar metabolism whose function will be validated by subsequent VIGS experiments. The results of this program are expected to reveal the metabolic basis and regulatory mechnism of "sugar decline" process, and provided new insight in "sugar decline" for the research field of sugar metabolism in plant as well as theoretical guidance for developing effective measures for delaying the "sugar decline".

龙眼果实成熟期适逢南方酷暑，采收期短。很多品种在错过适宜的采收期后发生甜度迅速下降并伴随果肉纤维化的“退糖”现象，给果农带来严重的经济损失。前期研究结果显示龙眼果肉“退糖”主要由于完熟后蔗糖含量快速下降，导致总糖含量和甜度下降，因此，探明龙眼果肉中蔗糖降解的代谢调控是解决“退糖”问题的关键。蔗糖代谢研究已在多种植物中取得深入进展，但是这些植物中未见“退糖”现象的报道。目前对果实“退糖”的认识仍停留在生理生化水平和单个代谢路径，亟需分子调控层面的系统研究。因此，本项目以退糖速度快的‘石硖’和退糖慢的‘储良’龙眼果实为材料，利用稳定同位素示踪技术结合代谢组学和转录组学，解析龙眼果肉“退糖”过程中的糖代谢网络，筛选糖代谢相关的关键基因并验证其功能。该研究有望系统揭示“退糖”的代谢基础和调控机制，为植物糖代谢研究提供“退糖”方面的新思路，为制定延缓龙眼“退糖”的有效措施提供理论依据。

龙眼果实成熟于高温盛夏，果实采收期短，留树过程极易发生“退糖”现象，这极大的限制了龙眼果实留树保鲜技术的开发。目前龙眼果实“退糖”相关研究及其机制至今尚不清楚。本研究结果发现，龙眼果实成熟时TSS含量的高低不决定“退糖”过程中TSS含量下降的多少，即“退糖”与果实TSS含量高低无关。龙眼果实“退糖”主要以总糖含量下降，己糖含量增加为主要表现形式，其中所有品种的果实蔗糖含量都呈下降趋势，因此探究龙眼果实“退糖”机理，应主要关注龙眼果实蔗糖代谢。龙眼果实“退糖”速度的快慢与果实成熟时蔗糖/己糖比例的大小没有直接关系，即不同的糖积累模式不决定后期龙眼果实“退糖”速度。影响不同品种龙眼果实“退糖”特性的酶主要为AI和SS，AI将蔗糖分解为葡萄糖和果糖，而SS将蔗糖分解为UDPG和果糖，果实后期需要消耗大量的UDPG用于合成淀粉和纤维素的大分子物质，相关性分析等研究表明在龙眼果实“退糖”过程中SS所起的作用要大于AI。利用蛋白质组学技术研究龙眼果实“退糖”过程中差异蛋白质的变化，发现转化酶促进了“退糖”期间蔗糖的分解，而下游的糖酵解、发酵、TCA和能量代谢等途径关键蛋白的上调则可能促进了龙眼果肉中促进了己糖的消耗，推测这些途径中关键蛋白的上下联动促进了龙眼果肉的“退糖”。获得了龙眼“退糖”关键酶AI和SS的基因全长序列，并对其进行相关生物信息学分析，功能验证等分析正在进行。使用2,4-D处理龙眼发现，2,4-D处理能够有效维持较高的果实糖含量，一定程度上可以减缓龙眼果实的“退糖”，但是并不能抑制龙眼果实“退糖”的发生。本项目的研究结果丰富和加深了人们对龙眼果实“退糖”的发生的认识及其相关变化规律，为进一步深入研究龙眼果实“退糖”机理，制定出调控龙眼果实“退糖”方案奠定理论基础。