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Screening of Rice Lines Mutated from Cultivar TNG67 for Resisting Sheath Blight Disease
Oryza sativa L.
水稻（主要為Oryza sativa L.）是現今全世界最重要的糧食作物，而紋枯病則為水稻上的重要病害，該病病原菌（Rhizoctonia solani Kühn AG-1 IA）寄主廣泛防治不易。1979年鄧與許提出誘變水稻並篩選抗病品系為一個可行的方向，本實驗即以此為出發點，與行政院農委會農業試驗所農藝系王強生博士等人合作，由王博士提供已經疊氮化鈉（Sodium azide: NaN3）誘變處理並自交十代與十四代的水稻品系以供篩選抗病之用。實驗中的菌株分離自台灣部分縣市，共收集菌株100株，其中菌株TC96已改列為參考菌株。收集所得之菌株經由形態觀察、核染、菌絲融合、菌核切片等步驟以確定其分類，菌株中共有5株立枯絲核菌AG-1 IC，另有3株為雙核的絲核菌，依菌絲與菌核的形態觀察與測量將其歸類為Rhizoctonia fumigata。為了進行有效的抗紋枯病篩選，菌株毒力判定分別依據水稻台農67號的切離葉鞘與植株對接種菌株的反應，並在對台農67號毒力較強的菌株中依據不同溫度下的生長速率區分出一快一慢的菌株用於抗病性的篩選。於第一次篩選時分別得到品系96與286抗Rs1S-1菌株以及品系A227、A232和A242抗Rs17菌株，並且分別有54.5% 與30.2% 的品系維持與原品種台農67號相同的級數，並且也得到極感病的品系共28個，其他品系則分佈於各種級數並具有多種病態的表現，證明誘變可以改變水稻對紋枯病的感受性，並且影響抗感性的因子可能有多個。第一次篩選時各組極抗的品系與少數抗與極感病的品系被選為第二次篩選的待測品系。但意外的是第二次篩選時所有待測品系均出現了病徵，除了品系954以外，其他各品系上的病徵在接種後第五天都出現到第二葉以上的高度，最可能的原因是因為第二次篩選與第一次篩選時的氣候條件大不相同，第一次篩選實驗期間日均溫高於第二次，總降雨量只有3.5公釐也少於第二次篩選期間的百分之一，第一次篩選時除了下雨4天以外每天晴天，第二次則有半數以上的天數下雨，因而在第一次沒有發病或發病輕微的品系，第二次罹病反應都加重。而品系954雖然有明顯抗病的表現，但自第五週到第六週抽穗後，其病徵突然向植株高處發展，但其總病斑數並沒有大幅攀升，每株病斑數平均未超過2.87。當第七週其他品系感染分蘗比均達到100% 時，品系954仍維持在25% 以下。經病徵、植株分蘗數、株型、株高與葉鞘切片的比對後，品系954的抗病因子應可排除物理結構上的變異，而較可能是內部組成份或分泌物的影響。此點仍有待更深入的研究以瞭解真正造成抗病表現的因子。|
Rice (mainly Oryza sativa L.) is the most important food crop recently in the world. However the sheath blight disease is an important disease of rice plants. The pathogen (Rhizoctonia solani Kühn AG-1 IA) with broad host range lets the farmers hardly to control. In 1979, Teng and Hsu suggested that the screening with mutated rice lines is a feasible way. So I requested the cooperation with Dr. Chang-Sheng Wang and Mr. Tung-Hai Tseng (the Department of Agronomy, Taiwan Agricultural Research Institute, Council of Agriculture). Dr. Wang provided the F10 and F14 of mutant lines mutated from rice TNG67 by sodium azide (NaN3). And the total 100 isolates in this study were collected from some counties of Taiwan, and isolate TC96 was classified into a reference isolate. The isolates were identified the classification levels though the observation of morphology, nucleus staining, hyphal fusion, sclerotial sections, and etc. Two isolates are AG-1 IC. Three binucleate isolates were Rhizoctonia fumigata depending on the morphology and measurement of mycelia and sclerotia. For the effectively resistant screening, the virulence analysis of isolates were depending on the response on detached leaf sheaths and plants of rice TNG67 . And then a faster-growing isolate Rs1S-1 and a slower-growing isolate Rs17 with higher virulence were selected for disease-resistant screening by the growth rates at different temperatures. In the first screening, the lines 96 and 286 versus Rs1S-1 and the lines A227, A232, and A242 versus Rs17 were grade 0 (highly resistant). 54.5% versus Rs1S-1 and 30.2% versus Rs17 had the same grade with the original cultivar TNG67. Twenty-eight lines were highly susceptive. The other lines had various disease response grades and morbid states. The results prove that the mutation of rice can change the susceptivity for sheath blight disease and the factors in the resistance or susceptivity of rice may be more than one factor. The highly resistant, resistant, and highly susceptive lines in the first screening were selected as the test lines in the second screening. But unfortunately all test lines in the second screening showed symptoms. The symptoms developed into more than second leaves of plants except line 954 at the 5th day after inoculation. The most possible reason is different climate conditions during these screening. The daily mean temperatures during the first screening were higher than those during the second screening and the total precipitation during the first was less than 1% of that in the second. Every day was sunny during the first except 4 rainy days but the half days during the second were rainy. So the disease response grades of most resistant lines in the first screening were increased in the second screening. Although line 954 were significantly resistant, the symptoms on this line developed suddenly to the higher location of plants at the fifth week after inoculation. But the mean lesions per plant were never more than 2.87. When the infected tiller ratio of other lines increased to 100% after the 7th week, the ratio of line 954 was maintained less than 25%. After comparison with symptoms, tillering numbers, plant shape, plant heights, and frozen sections of leaf sheaths, the mutation of physical structure could be excluded from the factors in the resistance of line 954. It's possible the inner component or secretion. The problem must be studied more in order to understand the real reasons in the resistance.
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