Clonal analysis in plant development

Abstract

Clonal analysis is of value in understanding the cellular dinamics of morphogenetic processes. In their review, Dawe and Freeling (1991) have grouped the methods useful in plants to this type of analysis, under the headings chimeras, spontaneous sectors, sectors induced at specific stages of development and sector boundary analysis. The presentation given at the course has been completed by adding principles and concepts of phenomena described as pattern determination (Spena and Salamini, 1995). Plant chimeras are the visible manifestation of the existence of genetic differences within layered meristems. The chimeric state is lost during sexual reproduction, because plant gametes originate from the LII layer only. Plant chimeras have been widely used to understand development processes, establishing, for example, that 1) cell position has a relevant role in plant development; 2) the layered structure of plant apices; 3) the contribution of cell layers to the development of leaf primordia; 4) the ontogenetic variations of the contribution of cell layers to mature organs; 5) the influence of a cell layer on the neighbour cells. Spontaneous sectors can be induced by several methods, but the excision of transposable elements is the phenomenon more useful to the scope. Spontaneous sectors have been analyzed in maize to follow the radially expanding development of cellular clones in the endosperm, using as marker genetic situations affecting amylose synthesis. In Anthirrium majus similar experiments have established that the determination of petal specific cells is cell autonomous. The excision of the Ac (Activator) element of maize from the P (pericarp color) locus has helped in understanding the contribution of LI and LII layers to the maize gynoecioum. The role of early cell division in maize leaf development has been established by using an epidermal marker and the transposition of the Spm (Suppressor mutator) transposable element (Cerioli et aI., 1994). In plants, sectors can be obtained at specific stages of organ development by using agents which induce mutations or chromasomal aberrations, like X-rays. Genes affecting authocyanin or chlorophyll pigmentation are the markers of use. The method has provided fate maps of cells of the embryo or of meristematic tissues and can estimate the NATO ASI Series, Vol. H 104