HOW IS EMBRYO SIZE GENETICALLY REGULATED IN RICE

It is unclear how embryo size is genetically regulated in plants. Sinc e cereals have a large persisting endosperm, it is expected that embry o size is affected by endosperm development. Nine single recessive mut ations, four reduced embryo mutations representing three loci, REDUCED EMBRYO1, REDUCED EMBRYO2 and REDUCED EMBRYOS, four giant embryo mutat ions derived from a single locus GIANT EMBRYO, and one endospermless m utation endospermless1-2 were analyzed. Every reduced embryo mutation caused reduction of all the embryonic organs including apical meristem s and the enlargement of the endosperm. The giant embryo mutants have a reduced endosperm and an enlarged scutellum. However, shoot and radi cle sizes were not affected. All the reduced embryo and giant embryo m utations did not largely affect postembryonic development. Accordingly , the expression of genes analyzed are seed-specific. In reduced embry o and giant embryo mutations, abnormalities were detected in both embr yo and endosperm as early as 2 days after pollination. ertdospermless1 -1 resulted in an early loss of endosperm, yielding a giant embryo, su ggesting that embryo growth was physically limited by the endosperm. A double mutant between giant embryo-2 and club-shaped embryo1-1, which has a normal endosperm and a minute undifferentiated embryo, resulted in a club-shaped embryo1-1 embryo and a reduced endosperm of giant em bryo-2, indicating that GIANT EMBRYO regulates the endosperm developme nt. Double mutants between giant embryo-2 and three reduced embryo mut ants exhibited the reduced embryo phenotype in both embryo and endospe rm, suggesting that reduced embryo mutations cause the enlarged endosp erm. Further, a double mutant of reduced emblyo3 and endospermless1-1 showed the enlarged embryo in endospermless seed. This confirms that r educed embryo3 does not regulate embryo size but enlarges endosperm si ze. Together with the results of the other double mutant analysis, RED UCED EMBRYO1 REDUCED EMBRYO2. REDUCED EMBRYO3 and GIANT EMBRYO are con cluded to regulate endosperm development.