植物因無法自由移動來逃避外在的不利環境，而時時刻刻都面臨環境中的各種影響與刺激。高溫對於植物而言是主要的非生物性逆境之一，高溫逆境會影響植物正常的生長發育，嚴重的話會導致植物的死亡，因此植物必需發展出一套有效的機制來因應高溫所帶來的傷害，提高在高溫逆境下的存活率。為了瞭解植物如何抵禦高溫逆境，我們利用正向式遺傳研究法(forward genetic approach)找尋植物中抵禦高溫相關的耐熱基因。本實驗是以化學突變劑甲基磺酸乙酯(ethylmethane sulfonate, EMS)處理阿拉伯芥 (Arabidopsis thaliana)野生型種子，以高溫處理，挑選出兩株對熱不具耐受性的突變株，分別命名為hs 29 (heat sensitive 29)及et (essential for thermotolerance)。實驗結果顯示hs 29突變基因位於阿拉伯芥第三條染色體底部；et突變點則是阿拉伯芥第五條染色體上一個基因內的一個核苷酸由C變成A，導致其所編碼之胺基酸由絲胺酸(serine)變成酪胺酸(tyrosine)。ET為一個含434個胺基酸、帶有細胞核定位訊號(nuclear localization signal sequence, NLS)，但功能未知的蛋白質。實驗結果得知ET位於細胞核中由異染色質(heterochromatin)所聚集形成的染色質中心(chromocenter)上，推測當植物遭遇高溫逆境時ET可能是藉由調控某些平時表現被抑制但對植物抵禦高溫必需的基因的活化。Plants, being immobile and cannot escape from unfavorable environmental conditions, must develop effective protecting mechanisms for survive when exposed to high temperature. To identify the genetic determinants that are essential for plant to cope with heat stress, we have used a forward genetic approach and isolated the et (for essential for thermotolerance) mutant of EMS-mutagenized Arabidopsis. Incubation at 37℃ for 4 days was lethal for et but not for wild-type plants. Gene mapping showed that the mutated locus of et is a single nucleotide change from C to A within the 6th exon of ET gene, leading to a Ser to Tyr substitution at amino acid 227. BLASTp analysis showed that ET is a novel protein whose biological function has never been characterized. Nevertheless, a typical nuclear localization sequence (NLS) was identified, and fluorescent tagged ET was subnuclearly localized to chromocenters. Chromocenters are compact region of chromosome rich in genes under control of transcriptional gene silencing (TGS). Furthermore, we found that heat can mediate chromocenter decondensation and TGS release in wild-type but not in et mutant. Therefore, we conclude that ET may be involved in the epigenetic regulation which is essential for theromotolerance of plants.
