Origin and Development of Somatic Embryoids Formed Directly on Immature Embryos of Trifolium repens In Vitro

Abstract

When immature zygotic embryos of Trifolium repens are cultured in vitro in the presence of 0.05 mg 1−1 BAP, the cells of the hypocotyl epidermis proliferate to produce somatic embryoids directly without an intervening callus phase. The young epidermal cells show features of proembryogenic cells, and the first sign of embyroid induction is a shift from regular equational, anticlinal divisions to irregular, periclinal and oblique quantal divisions. Multicellular budding and single-cell initiation apparently both occur, with multicellular budding being the more frequent pattern in the present study. Most early proembryoids resemble globular zygotic proembryos but appear to lack a suspensor. It is suggested that the subtending embryonic tissue fulfils the role of a suspensor or proembryonal complex. Secondary proliferations from the young cotyledon and hypocotyl epidermis of primary embryoids are formed by processes similar to those producing primary embryoids, and also from structures initially resembling epidermal hairs. These hair-like structures arise from single superficial cells which show evidence of cutinisation and callose deposition suggesting some degree of physical separation from neighbouring cells.