ORGANIZED DEVELOPMENT IN PLANTS
85
ing rather directly a part of the control machinery used by the plant in
normal development. It is assumed that endogenous levels of hormone of
identical structure (in the case of IAA and the gibberellins) or similar
structure (in the case of the kinins) are normally active in the plant and
must be taken into account in achieving a particular effective hormonal
balance. All biochemical or physiological activities that affect the levels
of hormone—such as oxidative enzymes systems which may destroy or
synthetic systems which may increase the hormone levels—therefore also
affect the system by influencing the overall hormonal balance.
In favor of the latter view is the fact that control of morphogenesis
by externally supplied auxin and kinin (and, perhaps, gibberellin) levels
for root, bud, or callus initiation operates only within a very narrow
range of concentrations of these substances which must be provided at
very low concentrations and in very precisely balanced mixtures.
Further, the specific hormones active have rather precise molecular
structures, and the plant tolerates relatively little change in such structure for continued physiological activity. Another most important point
is that these hormones or hormone-like substances do occur in plants
generally, and the evidence that they are naturally occurring and active
in hormonal roles increases steadily. Thus, for example, chemical identification of the natural kinin in maize (Letham et al., 1964) now gives
clear support to earlier claims of naturally occurring kinins.
A discussion such as this leads one directly to the question: "What
are the mechanisms of action of these plant hormones?"—a question
much too big to discuss here. At present we do not know at the molecular level the mechanisms whereby any one of these substances acts.
Therefore, we are left to ponder the question which was posed earlier
concerning the role of hormones provided in nutrient media in culture
systems. Not knowing with any precision how these hormones act, it is
useful, however, to explore two possible alternatives whereby these hormones may act, especially in relation to the question of the ultimate
genetic control of development. One view is that the hormones are the
effector agents of the genes, i.e., that gene control of hormone synthesis
via protein synthesis is the means whereby the plant cells and tissues
control cell activities such as division, enlargement, and differentiation.
The other view is that hormones control gene action directly, that is,
the hormone balance influences gene action—perhaps by turning on or
shutting off specific information at specific sites at different times (or
under different hormone balances) which, in turn, through specific RNA
synthesis, controls specific protein synthesis for a particular direction of
cellular activity. A model for the latter type of hormonal control is
given in the recent work by Beerman (1963) with the insect hormone
85
ing rather directly a part of the control machinery used by the plant in
normal development. It is assumed that endogenous levels of hormone of
identical structure (in the case of IAA and the gibberellins) or similar
structure (in the case of the kinins) are normally active in the plant and
must be taken into account in achieving a particular effective hormonal
balance. All biochemical or physiological activities that affect the levels
of hormone—such as oxidative enzymes systems which may destroy or
synthetic systems which may increase the hormone levels—therefore also
affect the system by influencing the overall hormonal balance.
In favor of the latter view is the fact that control of morphogenesis
by externally supplied auxin and kinin (and, perhaps, gibberellin) levels
for root, bud, or callus initiation operates only within a very narrow
range of concentrations of these substances which must be provided at
very low concentrations and in very precisely balanced mixtures.
Further, the specific hormones active have rather precise molecular
structures, and the plant tolerates relatively little change in such structure for continued physiological activity. Another most important point
is that these hormones or hormone-like substances do occur in plants
generally, and the evidence that they are naturally occurring and active
in hormonal roles increases steadily. Thus, for example, chemical identification of the natural kinin in maize (Letham et al., 1964) now gives
clear support to earlier claims of naturally occurring kinins.
A discussion such as this leads one directly to the question: "What
are the mechanisms of action of these plant hormones?"—a question
much too big to discuss here. At present we do not know at the molecular level the mechanisms whereby any one of these substances acts.
Therefore, we are left to ponder the question which was posed earlier
concerning the role of hormones provided in nutrient media in culture
systems. Not knowing with any precision how these hormones act, it is
useful, however, to explore two possible alternatives whereby these hormones may act, especially in relation to the question of the ultimate
genetic control of development. One view is that the hormones are the
effector agents of the genes, i.e., that gene control of hormone synthesis
via protein synthesis is the means whereby the plant cells and tissues
control cell activities such as division, enlargement, and differentiation.
The other view is that hormones control gene action directly, that is,
the hormone balance influences gene action—perhaps by turning on or
shutting off specific information at specific sites at different times (or
under different hormone balances) which, in turn, through specific RNA
synthesis, controls specific protein synthesis for a particular direction of
cellular activity. A model for the latter type of hormonal control is
given in the recent work by Beerman (1963) with the insect hormone