ORGANIZED DEVELOPMENT IN PLANTS
51
embryonic development in dicotyledonous plants had been viewed as
under the control of the physiological state imposed on the zygote within
the embryo sac. Implicit in many of the studies on embryo culture is the
assumption that development occurred in a precise sequence in response
to the chemical and/or physical gradients imposed upon the zygote
as it began its development, with the initiation first of a filamentous row
of cells forming the suspensor and then at one end a rounded mass of
cells—the globular embryo itself. As the embryo became multicellular,
the heterotrophic character was viewed as changing. It formed its own
centers of synthesis, new enzymes came into action, cotyledons were
formed by oriented cell divisions, and, finally, at late stages when the
available supply of nutrients was used up or stored in the embryo itself,
the embryo attained the potentially autotrophic condition ready for
germination. Thus, embryonic form was viewed as determined by forces
external to the embryo itself—at least up to the heart stage or perhaps
even later. This control of embryonic development by the maternal
environment has been the view derived from morphological studies and
perpetuated by the results from isolated embryo culture up until the
early 1960's.
Recent developments in several areas have raised some questions about
the validity of this point of view, and it is clear that a reassessment of
the nature of the control mechanisms is in order. In the field of embryo
culture itself it was found that the complex nutritional picture which had
been built up was perhaps misleading. Raghavan and Torrey (1963)
found that one could, in fact, culture to maturity even globular embryos
of Capsella in a relatively simple nutrient medium [inorganic salts, 2%
sucrose plus vitamins, but lacking added organic nitrogen compounds or
supplementary osmoticum (high sucrose)] if one provided the tiny
embryos with a properly balanced mixture of very low levels (of the
order of 10~
7 M) of plant hormones or hormone-like materials, viz., an
auxin, indoleacetic acid (IAA) ; a kinin, such as kinetin; and the closely
related purine, adenine sulfate. With the addition of these hormone-like
materials to the medium, the nutritional picture looked much less complex. Even so, it should be admitted that it has still not been found
possible to isolate and culture successfully embryos less than 50 μ in
diameter, i.e., embryos of the early globular stage of perhaps 32 cells or
less. It may or may not be a nutritional problem that limits culture;
manipulation of these embryos is very difficult. In the case of Capsella,
it would appear that the critical limitation on early embryo development in vitro centers not in substrate level metabolites, but rather the
balance of hormones which are generally considered to be acting more or
less directly on cell mechanisms of division and enlargement.
51
embryonic development in dicotyledonous plants had been viewed as
under the control of the physiological state imposed on the zygote within
the embryo sac. Implicit in many of the studies on embryo culture is the
assumption that development occurred in a precise sequence in response
to the chemical and/or physical gradients imposed upon the zygote
as it began its development, with the initiation first of a filamentous row
of cells forming the suspensor and then at one end a rounded mass of
cells—the globular embryo itself. As the embryo became multicellular,
the heterotrophic character was viewed as changing. It formed its own
centers of synthesis, new enzymes came into action, cotyledons were
formed by oriented cell divisions, and, finally, at late stages when the
available supply of nutrients was used up or stored in the embryo itself,
the embryo attained the potentially autotrophic condition ready for
germination. Thus, embryonic form was viewed as determined by forces
external to the embryo itself—at least up to the heart stage or perhaps
even later. This control of embryonic development by the maternal
environment has been the view derived from morphological studies and
perpetuated by the results from isolated embryo culture up until the
early 1960's.
Recent developments in several areas have raised some questions about
the validity of this point of view, and it is clear that a reassessment of
the nature of the control mechanisms is in order. In the field of embryo
culture itself it was found that the complex nutritional picture which had
been built up was perhaps misleading. Raghavan and Torrey (1963)
found that one could, in fact, culture to maturity even globular embryos
of Capsella in a relatively simple nutrient medium [inorganic salts, 2%
sucrose plus vitamins, but lacking added organic nitrogen compounds or
supplementary osmoticum (high sucrose)] if one provided the tiny
embryos with a properly balanced mixture of very low levels (of the
order of 10~
7 M) of plant hormones or hormone-like materials, viz., an
auxin, indoleacetic acid (IAA) ; a kinin, such as kinetin; and the closely
related purine, adenine sulfate. With the addition of these hormone-like
materials to the medium, the nutritional picture looked much less complex. Even so, it should be admitted that it has still not been found
possible to isolate and culture successfully embryos less than 50 μ in
diameter, i.e., embryos of the early globular stage of perhaps 32 cells or
less. It may or may not be a nutritional problem that limits culture;
manipulation of these embryos is very difficult. In the case of Capsella,
it would appear that the critical limitation on early embryo development in vitro centers not in substrate level metabolites, but rather the
balance of hormones which are generally considered to be acting more or
less directly on cell mechanisms of division and enlargement.