Rice fruit development is associated with an increased IAA content in pollinated ovaries

To analyze fruit set and early fruit (caryopsis) development in rice, we established an in vitro spikelet culture system. The ovary of cultured pollinated spikelets grew rapidly and developed into fruits with an embryo and endosperm. When unpollinated spikelets were cultured on a medium containing 2,4-dichlorophenoxyacetic acid, parthenocarpic fruits lacking an embryo and endosperm developed. The number and size of the cells in the pericarp of parthenocarpic fruits were almost identical to those of fruits induced by pollination, and degeneration of nucellus tissue was observed in both pollinated and parthenocarpic fruits. These results suggested that ovary growth was induced through increased auxin content in the spikelets. Quantitative measurement of indole-3-acetic acid (IAA) content in the spikelets indicated that the IAA level increased after pollination. Further analysis of IAA contents in the ovary and rachilla-pedicel of cultured spikelets suggested that fruit development is associated with IAA synthesis in the ovary following pollination/fertilization and subsequent transport of IAA from the ovary to the rachilla-pedicel. Partial or complete removal of the rachilla and/or pedicel prior to spikelet culture greatly inhibited fruit development. These results indicated that the rachilla and pedicel are essential for rice fruit development. AUX/IAA and ARF genes that might be involved in rice fruit development were identified through transcriptome analysis.