The introduction of semidwarf rice (Oryza sativa L.) led to record yield increases throughout Asia in the 1960s. The major semidwarfing allele, sd-1, is still extensively used in modern rice cultivars. The phenotype of sd-1 is consistent with dwarfism that results from a deficiency in gibberellin (GA) plant growth hormones. We propose that the semidwarf (sd-1) phenotype is the result of a deficiency of active GAs in the elongating stem arising from a defective 20-oxidase GA biosynthetic enzyme. Sequence data from the rice genome was combined with previous mapping studies to locate a putative GA 20-oxidase gene (Os20ox2) at the predicted map location of sd-1 on chromosome 1. Two independent sd-1 alleles contained alterations within Os20ox2: a deletion of 280 bp within the coding region of Os20ox2 was predicted to encode a nonfunctional protein in an indica type semidwarf (Doongara), whereas a substitution in an amino acid residue (Leu-266) that is highly conserved among dioxygenases could explain loss of function of Os20ox2 in a japonica semidwarf (Calrose76). The quantification of GAs in elongating stems by GC-MS showed that the initial substrate of GA 20-oxidase activity (GA53) accumulated, whereas the content of the major product (GA20) and of bioactive GA1 was lower in semidwarf compared with tall lines. We propose that the Os20ox2 gene corresponds to the sd-1 locus.
半矮水稻的推廣引起了上世紀60年代亞洲水稻產(chǎn)量的新紀錄。主效半矮基因sd-1在現(xiàn)代的水稻品種中仍然廣泛使用。sd-1的半矮表型是由于植物生長激素赤霉素的缺乏引起的。我們認為sd-1的半矮表型是由于赤霉素生物合成酶赤霉素20氧化酶發(fā)生突變導(dǎo)致伸長的莖中活性GA的缺乏引起的。對水稻基因組序列進行分析以及結(jié)合之前的定位結(jié)果將1號染色體上的sd-1確定為赤霉素20氧化酶基因(Os20ox2)。兩個獨立的sd-1等位基因位點在Os20ox2基因上都發(fā)生了突變：秈稻半矮稈品種Doongara中Os20ox2基因的編碼區(qū)中存在280bp的缺失，形成一個沒有功能的蛋白，而粳稻半矮稈品種Calrose76中則在雙加氧酶高度保守的氨基酸殘基（第266位亮氨酸）上發(fā)生了氨基酸置換。氣相色譜質(zhì)譜聯(lián)用儀測定了伸長的莖中GA含量，結(jié)果表明與高桿植株相比半矮稈植株中積累了赤霉素20氧化酶的起始底物GA53，而主要產(chǎn)物GA20和有活性的GA1的含量則降低。因此我們認為Os20ox2基因?qū)?yīng)著sd-1位點。
結(jié)果：sd-1位點的遺傳和物理分析；sd-1基因區(qū)中Os20ox2基因的鑒定；sd-1株系中Os20ox2基因突變的特征；伸長的莖中GA含量分析。