Genetic Analysis And Fine Mapping Of A Lax Mutant In Rice

In flowering plants, the transition of the shoot apical meristem (SAM) from vegetative to reproductive development leads to the generation of a more complex structure, the inflorescence. A set of genes have been successfully identified by using Arabidopsis thaliana and Antirrhinum majus as model species, which play critical roles in floral development. Well studies on these genes, especially on the floral identity genes, have greatly improved the understanding of plant floral development. However, the genetic mechanisms of the architecture of inflorescence, which mainly depends on a basic branching pattern and the position of flowers, are still unknown. Identification of the inflorescence mutants will accelerate the studies in this area.Rice (Oryza sativa L.) is one of the most important food crops in the world. The abnormal development of its inflorescence or flowers will greatly reduce the yield. So the studies on rice inflorescence and floral organs have become one of the most interesting fields in rice molecular research.We obtained a rice lax mutant from Dr. Qian Qian (China National Rice Research institute, Hangzhou). An F2 population from the cross of lax/W11 was constructed and used for mapping the corresponding gene. The RT-PCR analysis with the lax mutant was also conducted in this study. The main results are summarized as follows. 1. Phenotype of the lax mutantThe lax mutant exhibits altered panicle architecture: the primary and secondary rachis-branches are normally initiated and each branch ends in a terminal spikelet, but all the lateral spikelets are absent. The microscope analysis and electron micrographs showed thatthe terminal spikelets of the mutant display variegated structures. Based on these observations, the LAX gene identified in this report might be necessary for initiation/maintenance of rachis-branches and lateral spikelets in panicle, moreover it was required for the floral organs development in rice.2. genetic analysis and molecular mapping of the lax mutantTo determine the inheritance mode of the mutation, we made a cross between the lax mutant and the wild-type Wll. All the FI plants displayed wild-type phenotype, while the F2 progenies segregated in a ratio of 3 normal: 1 lax, indicating that a single recessive nuclear gene may control the mutant phenotype. Using this ? population the lax locus was mapped to the long arm of chromosome 1 between CAPS marker HB2 and microsatellite marker MRG4389 with both 0.14cM intervals, and co-segregated with the CAPS marker LZ1.3. RT-PCR analysisSeveral RT-PCR analyses were performed to determine whether the expressions of the floral identity genes were affected in the lax mutant. The result showed that the expression of rice B-function genes (OsMADS2, OsMADS4, OsMADS16 and OsMADS3 ) was significantly reduced, whereas the rice A-function gene RAP1A expressed normally in lax mutant, suggesting that the LAX gene is required for normal expression of some floral identity genes in rice.