TPS-b family genes involved in signature aroma terpenes emission in ripe kiwifruit

Aroma is a critical factor influencing consumer acceptability of ripe fruit. When fruit are eaten, the aroma travels retronasally from the mouth into the olfactory receptors located in the nose after exhaling. In kiwifruit (<i>Actinidia</i> spp.), terpene volatiles such as α-terpinolene and 1,8-cineole have been shown to contribute to the characteristic aroma of ripe fruit. Notably, 1,8-cineole contributes a key floral/eucalyptus note to the aroma of ripe <i>A. chinensis</i> ‘Hort16A’ kiwifruit, based on sensory descriptive and discriminant analysis. Emission of α-terpinolene and 1,8-cineole in kiwifruit is induced by ethylene, and production peaks when fruit are at eating ripeness. Two monoterpene synthase TPS-b family genes have been isolated from the fruit of <i>A. arguta</i> and <i>A. chinensis</i> that produce α-terpinolene and 1,8-cineole, respectively. Here we discuss terpene volatiles with respect to fruit aroma and consumer sensory evaluation, analyze the gene structure and conserved motifs of TPS-b genes in published kiwifruit genomes and then construct a transcriptional regulatory network based on <i>Actinidia</i> TPS-b. These data provide further insights into the potential molecular mechanisms underlying signature monoterpene synthesis to improve flavor in kiwifruit.

香气是影响成熟果实消费者接受度的关键因素。当果实被食用时，呼气后香气会通过鼻后嗅觉通路从口腔进入鼻腔内的嗅觉受体。在猕猴桃（Actinidia spp.）中，α-萜品烯（α-terpinolene）与1,8-桉树脑（1,8-cineole）等萜类挥发物（terpene volatiles）已被证实可赋予成熟果实标志性香气。值得注意的是，基于感官描述与判别分析，1,8-桉树脑为成熟中华猕猴桃（A. chinensis）‘Hort16A’品种的香气赋予了关键的花香/桉叶香调。猕猴桃体内α-萜品烯与1,8-桉树脑的释放受乙烯诱导，且其合成量在果实达到食用成熟度时达到峰值。研究人员已从软枣猕猴桃（A. arguta）与中华猕猴桃（A. chinensis）的果实中分离得到两个单萜合酶（monoterpene synthase）TPS-b家族基因，二者分别催化合成α-萜品烯与1,8-桉树脑。本研究围绕萜类挥发物与果实香气及消费者感官评价展开讨论，分析已发表猕猴桃基因组中TPS-b家族基因的基因结构与保守基序，并基于猕猴桃属（Actinidia）TPS-b基因构建转录调控网络（transcriptional regulatory network）。上述研究结果可为解析猕猴桃标志性单萜合成的潜在分子机制、改良果实风味提供新的深入见解。