Abstract

Architecture of 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 phase transition from vegetative to reproductive growth and several identified genes participate in this process by regulating meristemic activities of inflorescence. However, little is known about the molecular mechanism that controls inflorescence branch elongation. Here we report 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 PTR family. This conclusion was made 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 implicates that SP1 might be a nitrate transporter. However, in our study, neither nitrate transporter activity nor any other compounds that are transported by known PTR proteins could be detected in 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 it transports unknown substrates in the monocotyledonous plant rice.