甘藷sporamin基因在葉片受傷誘導的表現調控網路

Abstract

甘藷為世界上重要的糧食作物之一，易栽種且抗蟲能力強。過去的研究顯示，甘藷所特有的抗蟲基因sporamin會被受傷逆境誘導而大量表現於葉部。本研究之目的為探討甘藷sporamin的受傷訊息傳遞路徑以了解甘藷的抗蟲防禦機制。Sporamin啟動子中具有一段受傷活化的cis-acting element (SWRE)，此DNA片段會被轉錄因子─IbNAC1結合並活化。在甘藷葉片中，IbNAC1轉錄因子會受到傷害逆境刺激而大量表現，IbNAC1大量表現的甘藷轉殖株其抗蟲能力也有明顯的提升。另一方面，IbNAC1也同時參與在茉莉酸反應中，持續性的表現IbNAC1也對植物造成許多負面影響，包括抑制根發育、加速植物老化、大量累積花青素與過氧化氫。大量表現IbNAC1可以提昇植物抗蟲能力，但過度持續表現則會對植物的生理造成負面影響，因此，控制IbNAC1表達的调控機制對於甘藷的受傷反應是相當重要的。進一步研究發現，甘藷中有兩個bHLH轉錄因子─IbbHLH3與IbbHLH4，這兩個轉錄因子會結合IbNAC1啟動子中的G-box cis-element進而調控IbNAC1基因表現。在甘藷葉受傷的初期反應中，IbbHLH3-IbbHLH3蛋白複合體會結合G-box並促進IbNAC1表達以增加植物的抗蟲能力。在受傷反應後期，IbbHLH4會大量表現在甘藷受傷葉片中，因此形成了IbbHLH4-IbbHLH3或IbbHLH4-IbbHLH4的結合形式，任一蛋白複合體若結合在G-box時即抑制IbNAC1表現。此外在受傷逆境中有其他的蛋白質，包括JAZ、EIL和MAPK也會分別與IbbHLH3結合並抑制了IbbHLH3的轉錄活化功能。此bHLH模組的受傷防禦調控機制，經由調控IbNAC1的基因表現，再進一步調控sporamin的表現，不僅提供植物抗蟲防禦能力，並適當地避免植物生長發育受到影響。本論文闡釋了植物在受傷防禦與生長平衡間最適當的調控機制。

Sweet potato is one of important food crops in the world. It grows well and has few herbivore enemies in the field. Previous studies have shown that sporamin, which is induced by wounding in leaves, is an unique anti-herbivore gene in sweet potato. In this study, we further investigated the wounding signaling transduction pathway in stimulation of sporamin expression to validate the defense mechanism against insect feeding in sweet potato. In the promoter region of sporamin, a wound-responsive cis-element promoter (SWRE) was characterized that was bound and activated by IbNAC1 transcription factor. IbNAC1 is a wound-inducible gene as well as sporamin in leaves. Overexpression of IbNAC1 enhanced tolerance against insect feeding in sweet potato. However, constitutive expression of IbNAC1 simultaneously and negatively affected the physiological changes, including inhibition of root formation, accelerated aging, as well as accumulation of anthocyanin and H2O2, by IbNAC1-induced JA responses in sweet potato. These results suggested that IbNAC1 effectively controls the resistance to insect feeding, but causing side-effects to plant physiological status. Thus, a delicate regulatory mechanism controlling IbNAC1 expression is essential for wounding response in sweet potato leaves. In detailed study, two bHLH transcription factors, IbbHLH3 and IbbHLH4, coordinately regulate the expression of IbNAC1 by binding the G-box cis-element in IbNAC1 promoter. In the early wound response, IbbHLH3-IbbHLH3 protein complex binds to and activates the G-box in IbNAC1 promoter to enhance the resistance against herbivory. Until the late wounding, IbbHLH4 is elicited in wounded leaves. Either IbbHLH4-IbbHLH3 or IbbHLH4-IbbHLH4 complex binds to the G-box, and repress the activation of IbNAC1 promoter to avoid hyper-activation of IbNAC1. Additionally, several proteins can also interact with IbbHLH3 to repress the transactivation function of IbbHLH3 during wound stress. The dynamic defense mechanism of wounding response mediated by bHLH modules coordinates the expression of IbNAC1 to confer the resistant ability against herbivory and prevent the injury to growth and development in sweet potato. This research demonstrates an appropriate regulatory mechanism that synergistically couples the tradeoffs between defense and physiology toward wounding response.