The architecture of the rice inflorescence, which is determined mainly by the number and length of primary and secondary inflorescence branches, is of importance in both agronomy and developmental biology. The position and number of primary branches are established during the phase transition from vegetative to reproductive growth, and several of the genes identified as participating in this process do so by regulating the meristemic activities of inflorescence. However, little is known about the molecular mechanism that controls inflorescence branch elongation. Here, we report on a novel rice mutant, short panicle1 (sp1), which is defective in rice panicle elongation, and thus leads to the short-panicle phenotype. Gene cloning and characterization indicate that SP1 encodes a putative transporter that belongs to the peptide transporter (PTR) family. This conclusion is based on the findings that SP1 contains a conserved PTR2 domain consisting of 12 transmembrane domains, and that the SP1-GFP fusion protein is localized in the plasma membrane. The SP1 gene is highly expressed in the phloem of the branches of young panicles, which is consistent with the predicted function of SP1 and the sp1 phenotype. Phylogenetic analysis implies that SP1 might be a nitrate transporter. However, neither nitrate transporter activity nor any other compounds transported by known PTR proteins could be detected in either a Xenopus oocyte or yeast system, in our study, suggesting that SP1 may need other component(s) to be able to function as a transporter, or that it transports unknown substrates in the monocotyledonous rice plant.
水稻花序的建成在農(nóng)學(xué)上和發(fā)育生物學(xué)上都有重要意義，并主要取決于一級(jí)花序枝梗枝和二級(jí)花序枝梗的數(shù)目和長(zhǎng)度。其中一級(jí)枝梗的位置和數(shù)量在植物從營(yíng)養(yǎng)生長(zhǎng)到生殖生長(zhǎng)的轉(zhuǎn)變過(guò)程中就已經(jīng)建立了，一些調(diào)控花分生組織活性的基因參與了這一過(guò)程。然而，控制花分枝伸長(zhǎng)的的分子機(jī)制還知之甚少。這里，我們報(bào)道一個(gè)新型水稻突變體-短穗1（sp1），它在水稻圓錐化學(xué)伸長(zhǎng)過(guò)程中有缺陷，并導(dǎo)致短穗表型。基因克隆和分析暗示SP1編碼一個(gè)屬于多肽轉(zhuǎn)運(yùn)體(PTR)家族的多肽轉(zhuǎn)運(yùn)體。支持這一結(jié)論的證據(jù)是，SP1包含一個(gè)保守的由12個(gè)跨膜結(jié)構(gòu)域組成的PTR2域，還有，SP1-GFP融合蛋白定位在質(zhì)膜上。SP1基因在幼嫩的短穗的韌皮部高表達(dá)，與預(yù)測(cè)的該基因的功能和突變體表型一致。系統(tǒng)發(fā)生分析暗示SP1可能是一個(gè)硝酸鹽轉(zhuǎn)運(yùn)體。但是，在非洲爪蟾卵母細(xì)胞和酵母系統(tǒng)中，沒(méi)有發(fā)現(xiàn)已知的PTR蛋白有硝酸鹽或者其他化合物的轉(zhuǎn)運(yùn)體活性。而我們的研究表明SP1可能需要其他組分才能完成轉(zhuǎn)運(yùn)體功能，或者它轉(zhuǎn)運(yùn)的是水稻中未知的底物。