THE PETUNIA TRA1 GENE CONTROLS CELL ELONGATION AND PLANT DEVELOPMENT,AND MEDIATES RESPONSES TO CYTOKININS

The molecular control of cell elongation, one of the basic processes o f plant morphogenesis, is still largely not understood. This paper des cribes a Petunia hybrida mutant of dumpy phenotype, trapu, which ident ifies tra1, a gene required for cell elongation and mediating response s to applied cytokinin. This mutant displayed an extreme reduction in length, due to a single recessive mutation which was expressed in ever y part of the plant and during the entire life of the plant, including the mature embryo. The mutant was unable to flower. The mutant roots, as well as the leafy organs, were short and thick, and the root elong ation zone, hypocotyl and petioles were absent. The mutant plantlets r esponded neither to applied auxin nor to gibberellin, indicating that this phenotype was not caused by a deprivation of these phytohormones. However, unlike the wild-type, the mutant growth was stimulated by ap plied cytokinin, even though its morphology remained abnormal. A histo logical study revealed the presence of all tissue types in normal posi tions, including root hairs and vascular bundles. The mutant's cells w ere rounder in every tissue. Both shoot and root meristems were disorg anized, without consistent cell shape and size. The regular cell files , which are typical of a normal root apex organization, were totally a bsent in the mutant root apex. Indirect immunofluorescence of alpha-tu bulin on root apices showed the cortical microtubules in the mutant ce lls to be unable to form the parallel arrays in elongating cells and t he preprophase band in dividing cells. This default resulted in the pr evention of unidirectional cell elongation and formation of regular ce ll files, thus causing the trapu phenotype. This paper discusses the s imilarities and differences of trapu to the Arabidopsis mutants, lass and ton, trapu confirming that the establishment of plant body pattern and differentiation can be dissociated from cell elongation.