The differentiation of emergences into adventitious shoots in peanut, Arachis hypogea (L.)

Abstract

In vitro regeneration of peanut may be useful in gene transfer research to develop transgenic peanut plants. The purpose of this study was to determine the feasibility of developing adventitious shoots from emergences. Three different types of explants, viz: whole embryo, whole leaf and hypocotyl were compared for the regeneration potential of their emergences. Segments of explants, including emergences in various stages of differentiation into adventitious shoots, were studied using brightfield and scanning electron microscopy. Emergences, multicellular structures that resemble multicellular trichomes, were observed at the cotyledonary node, petiole base, and on the rachis of the leaf where folioles are attached. Of these sites, the greatest number of emergences was found at the cotyledonary node. Explants grown on a medium containing 10 mg/l thidiazuron (TDZ) for 3 weeks exhibited progressive morphological changes of emergences when compared to controls. Emergences differentiated into three types of adventitious structures with one type resembling radially concentric shoots. These adventitious shoots had an organized apex region from which flattened leaf-like appendages emerged, a central core of procambium that differentiated into vascular tissue, and stomates borne in the epidermal layer. The greatest extent of shoot differentiation occurred at the cotyledonary node. A total of 11% of these shoots developed roots and grew to maturity in a greenhouse. The ability of emergences to form adventitious shoots has important implications for gene transfer research because of their superficial origin in epidermal and subepidermal tissues that would be directly exposed to particle bombardment.