阿拉伯芥熱休克轉錄因子HSFA6b作為ABA訊息傳遞和熱休克反應的連結點，扮演正向提升鹽、乾旱以及熱耐受性的角色

Abstract

熱休克反應是一個對抗熱逆境的重要機制，其可以藉由熱休克轉錄因子的調控增加熱休克蛋白的表現。此外藉由植物逆境賀爾蒙離層酸 (ABA) 的訊息傳遞也會參與在後天耐熱性。分析微陣列晶片中的資料顯示，阿拉伯芥熱休克轉錄因子HSFA6b會被鹽、滲透壓和冷逆境誘導表現而不是在熱逆境。在本篇文章中我們確定了HSFA6b會參與在ABA訊息傳遞並且會提升熱耐受性。有趣的是，在先前的研究中顯示在ABA缺失及對ABA不敏感的突變體中，證實ABA相關的訊息傳遞對於HSFA6b的表現是必須的；藉此我們也利用在原生質體中的轉錄活性檢測，確定了HSFA6b啟動子中的ABRE (ABA反應序列) 對ABA和AREB1 (ABA反應結合蛋白1) 的感應是不可或缺的，除此之外DREB2A (乾旱反應序列結合蛋白2A)、HSP18.2 (熱休克蛋白18.2) 和APX2 (抗壞血酸過氧化物酶2) 也會受HSFA6b所調控。而利用HSFA6b的轉植株及其突變體進行實驗，我們也發現HSFA6b會正向調控參與在ABA媒介的鹽和乾旱的耐受性。在耐熱性測試的結果也顯示HSFA6b會參與在後天耐熱性的調控過程中。我們的研究結果說明了一條新的調控途徑，HSFA6b除了會參與在ABA媒介的逆境反應之外也會參與在熱逆境調控的複雜網路之中。

The heat stress (HS) response (HSR) is a conserved mechanism developed to increase the expression of heat shock proteins (HSPs) via a heat shock factor (HSF)-dependent mechanism. As well, signaling by the stress phytohormone abscisic acid (ABA) is involved in acquired thermotolerance. Analysis of Arabidopsis (Arabidopsis thaliana) microarray databases revealed that the expression of HSFA6b, a class-A HSF, was increased with salinity, osmotic and cold but not HS. Here, we show that HSFA6b plays a pivotal role in the response to ABA and in thermotolerance in Arabidopsis. Intriguingly, in previous studies showed salt-inducible HSFA6b expression was downregulated in ABA-insensitive and -deficient mutants, and exogenous ABA application restored its expression in ABA-deficient plants, so the ABA signal is required for proper HSFA6b expression. Consequently, transcriptional activation assay of protoplasts showed that ABA treatment and coexpression of an ABA-signaling master effector, ABRE-binding protein 1 (AREB1), could activate the HSFA6b promoter. In addition, DREB2A, HSP18.2 and APX2 were regulated by heat shock factor A6b that enhanced their expression. Analysis of ABA responses in drought and salt tolerance in HSFA6b-null, -overexpression and -dominant–negative mutants indicated that HSFA6b is a positive regulator participating in ABA-mediated salt and drought resistance. Thermoprotection tests showed that HSFA6b was required for thermotolerance acquisition. Our study reveals a network in which HSFA6b operates as a downstream regulator of the ABA-mediated stress response and is required for HS resistance. This new ABA-signaling pathway is integrated into the complex HSR network in planta.