CRR1 encoding callose synthase functions in ovary expansion by affecting vascular cell patterning in rice

The ovary of rice undergoes rapid expansion immediately after fertilization, and this process determines the final sink strength potential of caryopses. To date, work on rice grain development has mainly focused on endosperm filling, whereas information on the essential elements for ovary expansion remains limited. We report here a functional analysis of the ovary expansion retarded mutant crr1 in rice. Map-based cloning revealed that CRR1 encodes a protein homologous to the Arabidopsis callose synthases AtGSL8 and AtGSL10. Point mutation in crr1 resulted in alternative splicing, which led to the formation of the truncated crr1 protein without the -glucan synthase domain. Iodine staining showed that there were few starch granules and these were unevenly distributed in the pericarp of crr1, and a 5,6-carboxyfluorescein diacetate transport assay revealed that carbohydrates were less efficiently unloaded from the lateral vasculature into the developing caryopsis. CRR1 transcripts were detected in all plant organs, with the highest level found in receptacles, which are mainly composed of vascular tissues. Analysis of pCRR1::GUS transgenic plants showed that CRR1 was specifically expressed in vascular bundle cells. Consistently, loss of function of CRR1 led to disordered patterns of vascular cells in the ovaries and receptacles of the mutant. Furthermore, a small portion of cells in the vascular bundles of crr1 showed defective cell wall formation, and callose deposition was specifically reduced at the plasmodesmata (PD) of cells with aberrant walls. Our results suggest that CRR1 performs a pivotal role in determining initial ovary expansion in rice, possibly via the PD-mediated permeability of cell fate determinants for vascular cell differentiation. Significance Statement Elucidating the mechanisms underlying ovary expansion could be beneficial for improving grain yield and quality. Here we characterize a mutant in a callose synthase, and show that callose synthase is important for initial ovary expansion in rice, possibly by affecting the plasmodesmata-mediated permeability of cell fate determinants required for vascular cell differentiation.