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Embryos abound with added auxin
The plasticity of plant development is a well-known phenomenon that has extensive use in non-sexual propagation of plants. One widely used method is the induction of somatic embryos, which go on to develop as if they were normal zygotic embryos. The starting material for somatic embryos may be undifferentiated callus, but there are examples where somatic embryogenesis is induced in cells or groups of cells from within a plant. This is illustrated by the work of Kim et al. (Chunchon and Chungju, Korea, pp. 177–183) on a large deciduous tree, Tilia amurensis. Earlier work (Karkonen, 2000, Plant, Cell, Tissue and Organ Culture 61: 205–214) had shown that somatic embryos arise from the surface of the cotyledons of zygotic embryos under certain culture conditions. The authors have now extended these observations and determined exactly which cells it is that express this embryogenic potential. Embryos were dissected from mature seeds and set-out to germinate on a hormone-free culture medium in vitro. Soon after germination, the seedlings produced both hairy and glandular trichomes, especially on hypocotyls and cotyledons. If the growth medium was supplemented with the synthetic auxin 2,4-D, formation of hairy trichomes was suppressed but formation of glandular trichomes was increased. Further, at the optimum concentration of 2,4-D, approx. 45 % of the glandular trichome initials (at the early filamentous stage) gave rise not to trichomes but to somatic embryos. This resulted in an almost bizarre situation in which the cotyledonary surface became coated in a ‘forest’ of embryos. Despite the general plasticity of plant development, this is a very unusual occurrence, indicating that cells committed to a particular developmental pathway within the young plant still retained an embryogenic potential that is readily elicited with 2,4-D. It is a fascinating system with which to study plant development and likely to be very useful in rapid propagation, particularly of T. amurensis genotypes.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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