Genetic and physiological characterization of the OsCem mutant in rice: formation of connected embryos with multiple plumules or multiple radicles.

Establishment of the apical-basal axis is a critical event in plant embryogenesis. Two functionally distinct primordia (the plumule and radicle) are meristems that originate from the lower and upper regions, respectively, of the embryo and are arranged along an apical-basal axis. One rice mutant (OsCem), which alters the embryo axis pattern to produce multiple plumules and/or radicles, was characterized. The OsCem mutant plants showed three different phenotypes: a multiple-shoot type, a heart-shaped Siamese embryo having multiple plumules and a common radicle; a multiple-root type, an upside-down Y-shaped Siamese embryo possessing multiple radicles and a common plumule; and a connected twin type, a connected Siamese twin having two sets of plumules and radicles. These diverse phenotypes demonstrated that the upper and lower regions of the embryo axis have a similar potential to complete the full spectrum of the developmental program of multiple plumules and radicles, respectively. Genetic analysis and molecular mapping based on simple sequence repeat markers has revealed that a recessive gene was involved in the control of the connected Siamese embryo formation. By using an F2 mapping population derived from a cross between the OsCem mutant and the variety 95-15, the OsCem locus was mapped primarily to the short arm of chromosome 3 of rice (Oryza sativa) at the interval between markers RM148 (6.4 cM) and RM468 (7.5 cM). Subsequently, the OsCem locus was fine mapped to the interval between markers M5 (1.6 cM) and M6 (1.0 cM).