The new plant physiology—molecular approaches to studying hormonal regulation of plant development

In early botany, the study of morphogenesis in higher plants started at the level of the whole plant and its organs, describing their life cycle and the formation of the vegetative and generative organs during plant development. This stage of morphology was followed by that of physiology, in which experimental interference with developmental processes aimed at the analysis of causal relationships in plant morphogenesis. The past century offers beautiful examples of sophisticated experimental analysis of such developmental processes as, e.g., apical dominance and flower induction. It became apparent that plant metabolism, growth and development are genetically determined, and that the expression of the genetic information is dependent upon and modulated by internal and environmental factors such as, e.g., hormones and light. Neither the nature of the genes and the extent of their expression in particular developmental processes, nor the mechanism of action of the endogenous growth-regulating substances mediating internal and external signals were understood, however, although it seemed reasonable to assume that hormones influence developmental processes by bringing about changes in gene expression. Application of biochemical knowledge and methodology allowed for extension of experimentation at the cellular and subcellular levels, leading in several cases to the unravelling of hormonal or light-induced effects on enzyme activities in metabolic pathways. However, the significance of research at the protein level for the explanation of morphogenetic effects at the organ or whole-plant level has been severely limited by the correlative nature of the argumentation. The evidence is largely derived from simultaneously occurring variations at both levels, but apart from such correlations a firm causal argument cannot be obtained from combining physiological and biochemical data alone. Moreover, cell division, extension and differentiation, as well as the formation of organs and the spatial and temporal integration of various meristematic activities all depend on the interplay between various hormonal signals. Different combinations of hormones may act independently, synergistically, or antagonistically, and can influence each other’s metabolism, recognition by cellular receptors, or signal-transduction pathways. These complex interactions stem in part from the stochastic nature of hormonal regulation in plants itself (Trewavas 1991) and give rise to the plasticity of plant development, in which the pattern of growth and development is open to modulation by internal and external constraints and growth of new organs is invariably linked to loss of function of old organs (Woolhouse 1978).