Cuttings of a tobacco mutant, rac, undergo cell divisions but do not initiate adventitious roots in response to exogenous auxin

Numerous studies have shown that auxin induces adventitious root initiation in stem explants from a variety of species, including tobacco. A dominant, monogenic mutation previously identified in tobacco (Nicotiana tabacum c. Xanthii), rac, confers tenfold auxin resistance to mesophyll-derived cell suspensions and an impaired primary root development phenotype to seedlings. Results presented here demonstrate that adventitious root formation does not occur when heterozygous and homozygous rac stem cuttings are treated in vitro with indole-3-butyric acid (IBA) concentrations ranging from 0.5 μM to 500 μM. Histological analysis showed that some phloem parenchyma or inner cortical parenchyma cells in wild-type stem cuttings undergo adventitious root morphogenesis when they are treated with 5 μM IBA. The same cell types in heterozygous and homozygous rac stem cuttings undergo mitoses in response to auxin, but never form adventitious root meristems. The lack of adventitious root initiation in rac stem cuttings is phenotypically distinct from the aberrant primary root development in rac seedlings. The rac mutation appears to block an essential process for auxin induction of adventitious root initiation but not cell division in phloem parenchyma or inner cortical parenchyma cells. Comparisons of rac heterozygous and homozygous seedling primary root length and callus formation in response to auxin in stem cuttings indicate that rac copy number is correlated to the degree of expression of these two phenotypes.