Effects of Rice Heading Date Genes under Natural Day Length in Taiwan

抽穗期對水稻(Oryza sativa)是適應地區和季節的重要性狀，也是一大育種目標。水稻有複雜的開花機制與豐富的抽穗期基因自然變異，使其能夠適應不同的日照環境。過去這一方面的研究很多，但偏重於溫帶地區的日長條件與人工短日照處理，而臺灣地區日照長度介於過去研究定義的長日照與短日照之間，並且有兩期作的差異，各基因在臺灣栽培環境下的效應未必與文獻一致。本文整理多篇水稻抽穗期相關研究，著重於不同日照環境下基因對抽穗期的影響以及對偶基因的功能性和分佈，並且重新分析過去臺灣地區的抽穗期研究、針對臺灣水稻品種的目標抽穗基因分型。研究的結果顯示Ehd1基因是臺灣地區影響抽穗期最大的基因，在各個環境中穩定表現，只在臺灣稉稻中發現的ehd1-T65對偶基因因失去部分功能而延遲抽穗約7天。臺灣秈稉稻雜交後代的抽穗期差異主要來自Ehd1基因與其上游調控基因Ehd4/Dth3和Ghd7基因，此3基因在秈稉稻之間分界明顯且有共同解釋抽穗變異的趨勢，顯示秈稻與與稉稻具有不同的抽穗調控機制。另一方面，日本稉稻品種與臺灣稉稻之間抽穗期的差異來源為Hd1基因與Ehd1基因，此二基因皆會造成日本水稻品種在臺灣過早抽穗。臺灣水稻抽穗期的研究已應用於國外優良品種的引進和國產米的鑑別，本篇舉出利用分子標誌輔助選拔育成之台南16號以及利用ehd1-T65基因之國產蓬萊米鑑別為例。抽穗期研究可應用於探討臺灣水稻耕作制度改變為單期作的可能性，此一制度或可有效配合其他雜糧作物的輪作提高雜糧種植比例和解決水稻生產過剩之問題。Rice (Oryza sativa) is considered a short day plant. World-wide adaptation of rice maybe attributed to complicate mechanism of photoperiodic response and variation of heading date genes. Taiwan is located at the southern border and the northern border of the region where temperate Japonica rice and Indica rice are grown, while the natural day length in Taiwan lies between the typical definition of “short day” and “long day” in previous studies. Since some flowering time genes perform differently under distinct area, heading date gene expression in Taiwan may not conform to these studies. In this study, heading date QTL mapping data in Taiwan was reanalyzed and the genotypes of Taiwan cultivars were investigated, including heading date genes Ehd1, Hd1, Ehd4/Dth3, Ghd7 and Dth2. Hd1 is a bi-functional gene that represses flowering under long day length but promotes flowering under short day length environment. The effect of Hd1 in Taiwan is limited because of the day length coincides with the threshold day length of bi-functional Hd1gene, and most alleles of Taiwan cultivars are non-functional. Ehd1 plays an important role in the flowering pathway and the allele ehd1-T65 is a unique allele of Japonica cultivars in Taiwan. Alleles of Ehd4/Dth3, Ghd7 are separated among rice subspecies, indicating difference of flowering pathways between Japonica and Indica. The understanding of effects and mechanisms of heading date genes has led to a successful development of variety, Tainan 16. The identification of specific alleles among production regions may be used to discriminate the rice imported abroad from local produced rice. In the future, the knowledge of rice heading date may be used to breed for cultivars with late heading dates in order to take the full advantage of the new concept of single crop season for rice in Taiwan, which may be an answer to the problem of rice over-production and water shortage in spring