近日，华中农业大学园艺林学学院园艺植物生物学教育部重点实验室邓秀新院士团队揭示了新发掘的脐橙棕色突变体“宗橙”的突变机理，并进一步解析了柑果类果实发育过程中叶绿素降解和类胡萝卜素合成的协同调控机制，为未来柑橘色泽品质改良提供了重要理论依据。相关研究成果发表在国际学术期刊Plant Physiology上。
柑橘是世界第一大水果，其栽培面积和产量均居果树之首，具有非常重要的经济价值和营养价值。柑果是芸香科柑橘属植物特有的果实类型，具有区别于其它果实独特的形态学特征。叶绿素降解和类胡萝卜素积累是柑橘类果实成熟的必经过程，对果实的色泽品质和商品价值起决定性作用。柑橘果实中叶绿素和类胡萝卜素含量的差异使其呈现出绿色、黄色、橙色、红色甚至棕色等。近年来，学者们对柑橘叶绿素降解和类胡萝卜素合成途径已经有了较多的研究，但关于这两条代谢途径协同调控的分子机制仍然知之甚少。
图1‘宗橙’和‘伦晚脐橙’果实性状对比
“宗橙”为三峡库区秭归县“伦晚脐橙”果园里发现的一个芽变新品系，其果皮呈现独特的棕色。研究对“宗橙”棕色果皮表型及其主要品质性状进行了精细分析，采用多组学和多种遗传资源整合策略鉴定到了突变基因——滞绿基因STAY-GREEN（SGR），并结合生化手段解析了该突变体果皮叶绿素降解受阻和类胡萝卜素积累增加的突变机制。
研究表明，柑橘中SGR基因存在两个等位基因（CsSGRa和CsSGRb），与CsSGRa相比，CsSGRb由于序列变异引起的选择性剪接而发生提前终止，产生截短型蛋白。“宗橙”中CsSGRa编码区碱基突变形成了终止密码子，使得蛋白编码提前终止（命名为CsSGRaSTOP），CsSGRb基因序列与野生型一致。进一步实验发现，仅CsSGRa具有叶绿素降解活性，CsSGRaSTOP和CsSGRb均丧失了降解叶绿素的功能；CsSGRa和CsSGRb均可与类胡萝卜素合成途径限速酶CsPSY1互作，CsSGRaSTOP不能与CsPSY1互作，其可通过解除对CsPSY1的抑制作用促进类胡萝卜素的合成。
综上，CsSGRa的突变导致了宗橙果皮叶绿素降解受阻和类胡萝卜素含量显著增加，绿色和橙色叠加使其最终呈现出棕色表型。该研究阐明了“宗橙”突变的分子机制，揭示了SGR在调控柑果类果实成熟过程中的重要作用，并首次发现柑橘中SGR等位基因的功能分化及其对叶绿素降解和类胡萝卜素合成的独特而精细的协同调控机制。
图2‘宗橙’的突变机理
华中农业大学园艺林学学院邓秀新院士和美国纽约市立大学Eleanore T. Wurtzel教授为该论文的共同通讯作者，邓秀新院士课题组朱凯杰博士为该论文的第一作者。西班牙瓦伦西亚农业研究所、美国纽约植物园、秭归县柑桔良种繁育中心等单位也参与了此项研究。该研究在国家重点研发计划和国家自然科学基金的资助下完成。
【英文摘要】
Domesticated citrus varieties are woody perennials and interspecific hybrid crops of global economic and nutritional importance. The citrus fruit “hesperidium” is a unique morphological innovation not found in any other plant lineage. Efforts to improve nutritional quality of the fruit are predicated on understanding the underlying regulatory mechanisms responsible for fruit development, including temporal control of chlorophyll degradation and carotenoid biosynthesis. Here we investigated the molecular basis of the navel orange （Citrus sinensis） brown flavedo mutation, which conditions flavedo that is brown instead of orange. To overcome the limitations of using traditional genetic approaches in citrus and other woody perennials, we developed a strategy to elucidate the underlying genetic lesion. We used a multi-omics approach to collect data from several genetic sources and plant chimeras, to successfully decipher this mutation. The multi-omics strategy applied here will be valuable in driving future gene discovery efforts in citrus as well as in other woody perennial plants. The comparison of transcriptomic and genomic data from multiple genotypes and plant sectors revealed an underlying lesion in the gene encoding STAY-GREEN protein （SGR）， which simultaneously regulates carotenoid biosynthesis and chlorophyll degradation. However, unlike SGR of other plant species, we found that the carotenoid and chlorophyll regulatory activities could be uncoupled in the case of certain SGR alleles in citrus and thus we propose a model for the molecular mechanism underlying the brown flavedo phenotype. The economic and nutritional value of citrus make these findings of wide interest. The strategy implemented, and the results obtained, constitute an advance for agro-industry by driving opportunities for citrus crop improvement.
论文链接：
https://academic.oup.com/plphys/advance-article/doi/10.1093/plphys/kiab291/6308950
日期：2021-07-01
