A 2-Mb Chromosome Inversion Interrupted Transcription of LAX2-4 and Generated Pleiotropic Phenotypes in Rice

The number of grains per panicle is one of the most important factors of rice yield and determinated by branch and spikelet number. Genetic and molecular mechanisms responsible for panicle branching and spikelet development remain unclear in rice. Here, we report a mutant lax2-4 mainly showing lax panicle, longer grains, a little abnormal spikelet, lower setting rate and abaxial curled leaf. Genetic analysis indicated that the defective panicle phenotype of lax2-4 was controlled by single recessive gene. Further mapping and re-sequencing revealed that the fourth exon of LAX2 was lost caused by an approximately 2-Mb INV in lax2-4, thus LAX2-4 could be a new allele of LAX2. Overexpression assays showed that lax panicle was rescued in correspondence with the gradually increased expression level of wild-type LAX2-4 transcript. CRISPR/Cas9-mediated knockout of the fourth exon verified that LAX2-4 participates the regulation of sterile lemma and lemma development in rice. LAX2-4 could interact with an E-class gene, OsMADS1, and the fourth exon was necessary for its function. Collectively, we proposed that LAX2-4 plays a dual role in lateral spikelet and lemma development.