Summary The transformed phenotype of Nicotiana tabacum cv. Petit Havana «hairy-root» (h-r) regenerants is characterized by morphological changes which include heterostyly, reduction of all floral organs, reduced apical dominance, shorter size, and reduced numbers of internodes. Floral neoformation in vitro in h-r thin cellular layers was higher than in the control, and the floral gradient was altered. The in vitro flowers that reached anthesis showed the same alterations as the in vivoones. Phenotypic alterations in vivo and the response of thin cellular layers in vitro are correlated with the expression of pRi T-DNA auxin genes.