Global warming threatens crop production. G proteins mediate plant responses to multiple abiotic stresses. Here we identified a natural quantitative trait locus, TT2 (THERMOTOLERANCE 2), encoding a Gγ subunit, that confers thermotolerance in rice during both vegetative and reproductive growth without a yield penalty. A natural allele with loss of TT2 function was associated with greater retention of wax at high temperatures and increased thermotolerance. Mechanistically, we found that a transcription factor, SCT1 (Sensing Ca2+ Transcription factor 1), functions to decode Ca2+ through Ca2+-enhanced interaction with calmodulin and acts as a negative regulator of its target genes (for example, Wax Synthesis Regulatory 2 (OsWR2)). The calmodulin–SCT1 interaction was attenuated by reduced heat-triggered Ca2+ caused by disrupted TT2, thus explaining the observed heat-induced changes in wax content. Beyond establishing a bridge linking G protein, Ca2+ sensing and wax metabolism, our study illustrates innovative approaches for developing potentially yield-penalty-free thermotolerant crop varieties.
全球變暖威脅著農(nóng)作物生產(chǎn)。G蛋白介導(dǎo)植物對多種非生物脅迫的反應(yīng)。本文鑒定了一個天然的數(shù)量性狀位點TT2 (THERMOTOLERANCE 2),它編碼一個G蛋白γ亞基,賦予水稻在營養(yǎng)生長期和生殖生長期的耐熱性,且無產(chǎn)量損失。一個TT2功能喪失的自然等位基因,與高溫下蠟的更多保留和耐熱性增加有關(guān)。機(jī)制上,我們發(fā)現(xiàn)一個轉(zhuǎn)錄因子SCT1 (Sensing Ca2+ transcription factor 1),通過Ca2+增強(qiáng)的與鈣調(diào)素的相互作用來解碼Ca2+,并作為其靶基因的負(fù)調(diào)控因子(例如Wax Synthesis Regulatory 2 (OsWR2))。破壞TT2會引起受熱觸發(fā)的Ca2+減少,造成鈣調(diào)素-SCT1互作減弱,這解釋了觀察到的熱誘導(dǎo)蠟含量的變化。除了建立一個連接G蛋白、Ca2+傳感和蠟代謝的橋梁,該研究闡明了開發(fā)潛在的無產(chǎn)量損失的耐熱品種的創(chuàng)新方法。
該研究通過正向遺傳學(xué)方法從水稻耐熱遺傳資源中定位克隆到了TT2,其編碼一個G蛋白γ亞基,并且負(fù)向調(diào)控水稻的耐熱性;熱帶粳稻來源的TT2存在一個SNP,使其編碼一個提前終止形式的蛋白,獲得較強(qiáng)的耐熱性,而在高溫敏感的溫帶粳稻中,該SNP的占比較低。在熱脅迫下,相較于對照,攜帶耐熱性位點的近等基因系NIL-TT2HPS32苗期成活率顯著提高,并且成熟期的單株產(chǎn)量也顯著提高、增幅達(dá)54.7%,表明該基因位點在農(nóng)業(yè)生產(chǎn)上有重要的應(yīng)用價值。進(jìn)一步的研究發(fā)現(xiàn)TT2的功能有無,影響到熱脅迫后的蠟質(zhì)代謝通路,在高溫敏感的對照株系中,蠟質(zhì)相關(guān)調(diào)控基因呈現(xiàn)出明顯地受熱誘導(dǎo)而下調(diào)的趨勢,而在抗熱的NIL-TT2HPS32株系中,有一部分蠟質(zhì)基因則呈現(xiàn)出不響應(yīng)熱且穩(wěn)定表達(dá)的趨勢,其中包括一個正向調(diào)控蠟質(zhì)合成的重要轉(zhuǎn)錄因子OsWR2。在抗熱的NIL-TT2HPS32株系中敲除OsWR2,發(fā)現(xiàn)其耐高溫的表型消失,證明在高溫脅迫下維持正常的蠟質(zhì)含量對于水稻耐熱是至關(guān)重要的。為了進(jìn)一步建立TT2與OsWR2表達(dá)水平的調(diào)控關(guān)系,研究人員通過對OsWR2上游啟動子的分析,發(fā)現(xiàn)了一類鈣調(diào)素結(jié)合轉(zhuǎn)錄因子(CAMTA)的結(jié)合元件CG1-like motif,并通過同源比對,找到了兩個水稻的CAMTA家族成員,并命名為SCT1和SCT2;進(jìn)一步實驗證明SCT1可以直接結(jié)合OsWR2的啟動子,影響OsWR2的表達(dá),并且負(fù)向調(diào)控水稻的耐熱性。SCT1帶有鈣依賴的鈣調(diào)素(CaM)結(jié)合位點,可以通過與CaM的互作來解碼胞內(nèi)的鈣信號。G蛋白之前被多次報道參與動植物的鈣信號調(diào)控(如RGA1)(Ma et al., 2015),該研究也證實了TT2的功能缺失會導(dǎo)致熱誘導(dǎo)的鈣信號減弱。當(dāng)正常功能的TT2存在時,高溫會誘導(dǎo)鈣信號的產(chǎn)生并使得胞內(nèi)鈣濃度提高,高濃度的鈣離子會被CaM感知,并促進(jìn)CaM與SCT1的互作,從而加強(qiáng)CaM對于SCT1轉(zhuǎn)錄活性的抑制,最終導(dǎo)致OsWR2在高溫條件下表達(dá)量迅速下調(diào),蠟質(zhì)減少并最終無法抵御高溫,呈現(xiàn)出熱敏感的表型。當(dāng)TT2功能缺失時,熱誘導(dǎo)的鈣信號減弱,進(jìn)而減弱了SCT1與CaM的互作,降低了CaM對于SCT1轉(zhuǎn)錄活性的抑制,最終維持了OsWR2在高溫脅迫下的正常表達(dá)和穩(wěn)定的蠟質(zhì)含量,呈現(xiàn)出抗熱表型。