Structure and development of somatic embryos formed in Arabidopsis thaliana pt mutant callus cultures derived from seedlings

Seeds of the Arabidopsis thaliana mutant primordia timing (pt) were germina ted in 2,4-dichlorophenoxyacetic acid-containing liquid medium. The seedlin gs formed somatic embryos and nonembryogenic and embryogenic callus in vitr o in a time period of approximately two to three weeks. Embryogenesis and c allus formation were monitored with respect to origin, structure, and devel opment. Ten days after germination globular structures appeared in close vi cinity of and on the shoot apical meristem (SAM). Somatic embryos formed ei ther directly on the SAM region of the seedling or indirectly on embryogeni c callus that developed at the SAM zone. Globular structures developed alon g the vascular tissue of the cotyledons as well, but only incidentally they formed embryos. Upon deterioration, the cotyledons formed callus. Regular subculture of the embryogenic callus gave rise to high numbers of somatic e mbryos. Such primary somatic embryos, grown on callus, originated from meri stematic cell clusters located under the surface of the callus. Embryos at the globular and heart-shape stage were mostly hidden within the callus. Em bryos at torpedo stage appeared at the surface of the callus because their axis elongated. Secondary somatic embryos frequently formed directly on pri mary ones. They preferentially emerged from the SAM region of the primary s omatic embryos, from the edge of the cotyledons, and from the hypocotyl. We conclude that the strong regeneration capacity of the pt mutant is based o n both recurrent and indirect embryogenesis.