Evidence for a Mendelian factor controlling the cytokinin requirement of cultured tobacco cells

Cultured leaf tissues of Nicotiana tabacum L. cv. “Havana 425” normally require an exogenous source of cytokinin for rapid growth; stem-cortex tissues do not—ie, they exhibit the cytokinin-habituated phenotype. We found that plants regenerated from cloned cortex and leaf tissues from one particular plant differed in leaf-tissue phenotype: Leaf tissues derived from leaf cells exhibited the normal, nonhabituated phenotype, whereas leaf tissues derived from cortex cells were cytokinin-habituated. This difference in leaf phenotype was not found using leaf and cortex cells from six other donor plants. The inheritance of the habituated leaf trait was studied in tissues from cortex-derived plants and hybrids between these plants and normal plants. F₁ hybrids were intermediate between the parental types in degree of habituation. No differences were found between reciprocal hybrids. These results suggest that the habituated leaf trait is an incompletely dominant, nuclear trait. Both parental and intermediate phenotypes were recovered in the F₂ progeny. The frequency of habituated leaf progeny in the F₂ and backcross populations provide evidence that the trait is regulated at a single genetic locus.