揭示小型調節RNA於阿拉伯芥光形態發育之多重貢獻

Abstract

光形態發生，又稱幼苗去白化現象 (seedling de-etiolation)，對甫出土幼苗之生存，可謂至關重大；而小型調節RNA (small regulatory RNA，sRNA)，及其引起之轉錄後調控，對眾多生物之生長發育，有不可或缺的重要性。於阿拉伯芥中，當參與sRNA合成途徑之基因產生突變，植物會對光過度敏感，且有嚴重之發育缺陷。本實驗室先前之研究指出，依據有穩定sRNA之功能之甲基轉移酶，HUA ENHANCER 1，除藉其甲基轉移酶之功能穩定sRNA外，本身亦為光形態發生之負調節因子。已知miRNAs，miR157 和 miR319，可分別調節光形態發生之正向及負向調控轉錄因子。先前之證據，雖可略窺sRNA對於光形態發生之作用；但若能提供全基因組規模之探討，可使此研究領域有更長足之進步。然而，截至目前為止，並未有此等規模之探討。因此，藉由sRNA及降解組定序分析，吾等發現照光24小時的去白化幼苗，有335件mRNA剪切現象，乃肇因於sRNA之作用；造成這些剪切現象之主因，為sRNA之表現量，而與目標基因 (target mRNA) 之表現量無關。饒富意味的是，無論miRNA本身之表現是否受光調節，與其對應之目標基因表現，多在光照後降低。負責攜帶miRNA並剪切目標基因之ARGONAUTE1，其本身之mRNA 於光中之表現，受miR168a/b負向調節。此外，吾等亦發現miR396a/b，由抑制負調控者GROWTH REGULATING FACTORs mRNA之累積，得以正向調節光形態發生。總結而論，最佳化光形態發生需要sRNA。sRNA藉由調控光形態發生正調控者及負調控者之基因表現，使阿拉伯芥幼苗，可以有即時且穩定之適應機制，以因應光照環境之改變。

Photomorphogenesis, or seedling de-etiolation, is a vital step for seedling survival once they have emerged from soil. It was reported that many small RNA (sRNA) mediated post-transcriptional regulation is required for growth and development of diverse organisms. In Arabidopsis, many mutants of sRNA biogenesis show light hypersensitivity and severe morphological defects. In our previous study, we have also shown that HUA ENHANCER 1, a methyltransferase that stabilizes sRNA duplexes, is a negative regulator of photomorphogenesis. miR157 and miR319 were known to regulate positive and negative regulators of photomorphogenesis, respectively. Although previous evidence gave a glimpse of sRNA-mediated regulation of de-etiolation, a genome-wide profiling of sRNAs and their regulation of target genes during photomorphogenesis has been missing. I aimed to provide a comprehensive view of sRNA-controlled gene expression during photomorphogenesis. By profiling sRNAs and the 5’ ends of degraded mRNAs during the first 24 h of light irradiation, I identified 335 sRNA-mediated mRNA cleavage events in de-etiolating seedlings. These cleavage events are mainly resulted from actions of highly expressed sRNAs and irrelevant to the abundance of target mRNAs. Interestingly, the target mRNAs with cleavage signature identified tend to show down-regulation by light, regardless of the miRNA expression pattern. The expression of the slicer protein gene ARGONAUTE1 in the miRNA functioning pathway could be tuned down by miRNA168a/b. I also found that miR396a/b positively regulates photomorphogenesis by suppressing GROWTH REGULATING FACTORs. Our results suggest that the sRNAs are required to optimize the target mRNAs and regulate photomorphogenesis. With sRNAs controlling both positive and negative regulators of photomorphogenesis, young Arabidopsis seedlings can have a timely but robust development to adapt to light.