Positional Cloning Of Rice Panicle Apical Development-Related Gene PAA2

Rice panicle apical abortion(PAA) reduced number of grain per panicle, and caused significant yield loss. The paa2 was a tissue culture-derived mutant showing abortion on apical spikelet,accompanying with pleiotropic phenotype i.e. shortened plant height, delayed heading date, decreased1000-grains weight. We crossed paa2 with normal line IRAT129 to construct a genetic population for the mutated gene isolation, and finally mapped the mutated gene into a 49.7 Kb region on the long arm of chromosome 2. Within the fragment, anunidentified insertion was found at the promoter of ORF4, causing the gene ORF4 over-expression. We further indicated that the ORF4 was the causative gene to cause the phenotype on the mutant paa2.The main results are as follows:1. The paa2 was a tissue culture-derived panicle apical abortion mutant from Japonica cultivar Kitaake. In early stage of panicle development, paa2 did not display any phenotypic differences from its wild-type. However, the spikelets on panicle apical of the mutant ceased development priorto heading. Aborted spikelets of the mutant are whitish and failed to pollinate and fertilization, or visualized as a incomplete filled grain. While spikelets at the bottom of panicle developed normally,the size of the grain still smaller than that of the wild type. The IRAT129 was a conventional japonica cultivar, we crossed the paa2 with IRAT129 to construct a genetic mapping population. F1 hybrid showed panicle apical abortion, indicating that PAA is a dominant character.2. The PAA2 was fine mapped into a 49.7 Kb region on long arm of chromosome 2. Four ORFs are presented in this interval. At about 1 Kb far upstream of ORF4, there was one unidentified fragment insertion, which caused ORF4 high expression in paa2. We then over-expressing the ORF4 in wild-type, and succeeded in imitating the paa2 phenotype in transgenic plants, suggesting ORF4 a causative gene of the paa2 and supported the hypothesis the panicle apicalabortion in paa2 is caused by the over-expression of ORF4.3. PAA2 belongs to NPF gene family.Evolutionary analysis show that two nitrate transportors in Arabidopsis most similarity to PAA2 in amino acid sequences. The content of free amino acid in grain and young panicle were determined, but no significant difference was found between wild-type and paa2. Moreover, total nitrogen content of shoot decreased significantly in paa2, indicating there is unknown relationship between panicle development and nitrate supplement, which is valuable to further investigation.