Rice caryopses are enclosed by the outside glumes. It is long been acknowledged that the size and dimension of the outer glume is the main determinant that dictates the caryopsis size. However, it is unclear whether the development of caryopsis is completely dependent on the size of the glume, or it can grow and expand autonomously in spite of the constraint of glume enclosure. Here we report the identification of a mutant line that produces normal-sized glumes with smaller mature caryopsis

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... do not fill up the entire glume cavity. The mutant phenotype is caused by ectopic expression of a leucine-rice repeat extensin gene, OsPEX1 . The caryopsis phenotype in pex1 was caused by a reduction in cell size. OsPEX1 is highly expressed in the developing caryopsis. Over expression of the OsPEX1 gene driven by a constitutive promoter recapitulates the mutant phenotype, confirming that the small caryopsis phenotype is caused by ectopic expression of the OsPEX1 gene. Our results suggest that caryopsis development can be genetically uncoupled from maternally controlled glume development.