背景回顾：Citrus, one of the largest fruit crops with global economic and nutritional importance, contains fruit known as hesperidium with unique morphological types. Citrus fruit ripening is accompanied by chlorophyll degradation and carotenoid biosynthesis, which are indispensably linked to color formation and the external appearance of citrus fruits. 

提出问题：However, the transcriptional coordination of these metabolites during citrus fruit ripening remains unknown. 

主要发现：Here, we identified the MADS-box transcription factor CsMADS3in Citrus hesperidium that coordinates chlorophyll and carotenoid pools during fruit ripening. 

结果1-表达模式：CsMADS3 is a nucleus-localized transcriptional activator, and its expression is induced during fruit development and coloration. 

结果2-基因功能：Overexpression of CsMADS3 in citrus calli, tomato (Solanum lycopersicum), and citrus fruits enhanced carotenoid biosynthesis and upregulated carotenogenic genes while accelerating chlorophyll degradation and upregulating chlorophyll degradation genes. Conversely, the interference of CsMADS3 expression in citrus calli and fruits inhibited carotenoid biosynthesis and chlorophyll degradation and down-regulated the transcription of related genes. 

结果3-调控机制：Further assays confirmed that CsMADS3 directly binds and activates the promoters of phytoene synthase 1 (CsPSY1) and chromoplast-specific lycopene β-cyclase(CsLCYb2), two key genes in the carotenoid biosynthetic pathway, and STAY-GREEN (CsSGR), a critical chlorophyll degradation gene, which explained the expression alterations of CsPSY1, CsLCYb2, and CsSGR in the above transgenic lines. 

结论：These findings reveal the transcriptional coordination of chlorophyll and carotenoid pools in the unique hesperidium of Citrus and may contribute to citrus crop improvement.

柑橘是具有重要经济和营养价值的全球性水果，其果实被称为柑果，具有独特的形态特征。柑橘果实成熟通常伴随着叶绿素的降解和类胡萝卜素的合成，两者对于柑果的着色以及果皮外观都是必不可少的。但是，柑果成熟过程中有关叶绿素和类胡萝卜素的转录协调调控机制仍不清楚。本文中，作者鉴定到了一个MADS-box转录因子CsMADS3，其能够在柑果成熟过程中协调叶绿素和类胡萝卜素的含量。CsMADS3是一个核定位的转录激活子，其表达在果实发育和着色过程中被诱导表达。在柑橘愈伤、番茄和柑橘果实中过表达CsMADS3，能够增强类胡萝卜素的合成，并且上调类胡萝卜素合成基因的表达，同时上调叶绿素降解基因的表达，加速叶绿素的降解。相反，在柑橘愈伤和果实中干扰CsMADS3的表达，会抑制类胡萝卜素的合成和叶绿素的降解，并下调相关的基因表达。进一步的试验证实了CsMADS3能够直接结合并激活八氢番茄红素合成酶编码基因CsPSY1和有色体特异性的番茄红素β环化酶编码基因CsLCYb2的表达，这两个基因都是类胡萝卜素生物合成通路上的关键基因；另外，CsMADS3也能够直接结合并激活叶绿素一个关键基因CsSGR的表达。本文的研究结果揭示了柑果中叶绿素和类胡萝卜素协的转录协调调控机制，为未来柑橘的育种改良提供理论基础。

** 邓秀新 **