Phytomers and phases represent different levels of epigenetic control over the process of morphogenesis. As a result the form of particular organs and their repetitious sequence is programmed to function at high fitness in the overall design of the whole individual. The phenotype that is manifest during morphogenesis may change by epimutation just as somatic mutations produce chimeras. But epimutations are usually repaired during or just before gamete formation so that fertilization can reset the clock and repeat the whole sequence of morphogenesis in contrast to the standard mutations that can become fixed within the progeny (review by R. Holliday, Science 238:163, 1987).
In its original expression, A. Gray (Structural Botany, Amer. Book Co., New York, 1879) and W.E. Worsdell (The Principles of Plant Teratology, Ray Soc., London, 1915-16) considered the plant to be formed by a succession of individual units (phytomers or phytons) each consisting of an internode with a leaf at its upper end and a root or potential root(s) at its base. This concept was modified by Galinat (Bot. Mus. Leafl., Harvard Univ., pp.1-19, 1959) with the vegetative phytomer formed by the leaf, its axillary bud and prophyll all at the base of the internode and then extended to the floral axis with homologous transformations in all lateral components. Occasional slippage, stuttering or other imperfections resulting from a non-rigid coupling between components of the phytomer and arrangement within the phytomer do not negate the role of the phytomer as a guideline, if not a framework, for directing morphogenesis.
Control at the level of the phase is higher than that of the phytomer. It is most obvious in the vegetative and floral phases, both of which are subdivided into juvenile and adult stages and in the case of maize and teosinte are further differentiated into separate male and female inflorescences. Nevertheless, underlying control at the level of the phytomer is a basic pattern of construction with the manifestation of its floral phase components being markedly different from their vegetative homologues.
The epimutations of morphogenesis occur by a methylation or demethylation of various genes according to the phytomer and phase programs. In reviewing this process, Holliday (1987) states "the control of gene expression in higher organisms is related to the methylation of cytosine in DNA, and the pattern of methylation is inherited."
A study of inherited defects in the normal methylation process that have been either induced by mutagens or are just of spontaneous origin should be helpful in understanding the epigenetic control of phase change and phytomer manifestation. Such mutations include the Cg (corn-grass) and Tp (teopod) loci that have difficulty in shutting the vegetative phase off after the floral phase is turned on as well as the contrasting loci of is (interrupted spikelets; after starting to use ir for interrupted rachis, I found it was being used by molecular geneticists to mean inverted repeat) and Bif (one of Neuffer's EMS induced mutants on chromosome 8S). The is mutant produces a barren condition in sectors at the base of the rachis while the Bif mutant does a similar thing at the apex of the rachis.
Walton C. Galinat
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