slender Rice, a Constitutive Gibberellin Response Mutant, Is Caused by a Null Mutation of the SLR1 Gene, an Ortholog of the Height-Regulating Gene GAI/RGA/RHT/D8

The rice slender mutant (slr1-1) is caused by a single recessive mutation and results in a constitutive gibberellin (GA) response phenotype. The mutant elongates as if saturated with GAs. In this mutant, (1) elongation was unaffected by an inhibitor of GA biosynthesis, (2) GA-inducible α-amylase was produced by the aleurone layers without gibberellic acid application, and (3) endogenous GA content was lower than in the wild-type plant. These results indicate that the product of the SLR1 gene is an intermediate of the GA signal transduction pathway. SLR1 maps to OsGAI in rice and has significant homology with height-regulating genes, such as RHT-1Da in wheat, D8 in maize, and GAI and RGA in Arabidopsis. The GAI gene family is likely to encode transcriptional factors belonging to the GRAS gene superfamily. DNA sequence analysis revealed that the slr1-1 mutation is a single basepair deletion of the nuclear localization signal domain, resulting in a frameshift mutation that abolishes protein production. Furthermore, introduction of a 6-kb genomic DNA fragment containing the wild-type SLR1 gene into the slr1-1 mutant restored GA sensitivity to normal. These results indicate that the slr1-1 mutant is caused by a loss-of-function mutation of the SLR1 gene, which is an ortholog of GAI, RGA, RHT, and D8. We also succeeded in producing GA-insensitive dwarf rice by transforming wild-type rice with a modified SLR1 gene construct that has a 17–amino acid deletion affecting the DELLA region. Thus, we demonstrate opposite GA response phenotypes depending on the type of mutations in SLR1.

水稻细秆突变体（slr1-1）由单个隐性突变引发，并呈现组成型赤霉素（gibberellin, GA）响应表型。该突变体的伸长生长仿佛被赤霉素饱和处理一般。在该突变体中：（1）赤霉素生物合成抑制剂无法对其伸长过程产生影响；（2）无需施加赤霉素，糊粉层即可合成赤霉素诱导型α-淀粉酶；（3）内源赤霉素含量低于野生型植株。上述结果表明，SLR1基因的编码产物是赤霉素信号转导通路的中间调控因子。
SLR1基因在水稻中定位于OsGAI位点，且与多个株高调控基因具有显著同源性，包括小麦的RHT-1Da、玉米的D8，以及拟南芥的GAI和RGA。GAI基因家族所编码的蛋白大概率属于GRAS基因超家族（GRAS gene superfamily）的转录因子。
DNA序列分析显示，slr1-1突变属于核定位信号结构域（nuclear localization signal domain）的单碱基缺失，由此引发移码突变（frameshift mutation）并阻断蛋白质的正常合成。此外，将包含野生型SLR1基因的6-kb基因组DNA片段导入slr1-1突变体后，可使其赤霉素敏感性恢复至正常水平。上述结果表明，slr1-1突变体的表型由SLR1基因的功能丧失突变（loss-of-function mutation）引发，该基因是GAI、RGA、RHT及D8的直系同源基因（ortholog）。
我们还通过将携带DELLA结构域（DELLA region）17个氨基酸缺失的改造型SLR1基因载体转化野生型水稻，成功获得了赤霉素不敏感的矮化水稻植株。综上，本研究证实SLR1基因的突变类型不同，可引发相反的赤霉素响应表型。