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 initi ation in stem explants from a variety of species, including tobacco. A dominant, monogenic mutation previously identified in tobacco (Nicoti ana tabacum cv. Xanthii), rac, confers tenfold auxin resistance to mes ophyll-derived cell suspensions and an impaired primary root developme nt phenotype to seedlings. Results presented here demonstrate that adv entitious root formation does not occur when heterozygous and homozygo us rac stem cuttings are treated in vitro with indole-3-butyric acid ( IBA) concentrations ranging from 0.5 mu M to 500 mu M. Histological an alysis showed that some phloem parenchyma or inner cortical parenchyma cells in wild-type stem cuttings undergo adventitious root morphogene sis when they are treated with 5 mu M IBA. The same cell types in hete rozygous and homozygous rac stem cuttings undergo mitoses in response to auxin, but never form adventitious root meristems. The lack of adve ntitious root initiation in rac stem cuttings is phenotypically distin ct from the aberrant primary root development in me seedlings. The rac mutation appears to block an essential process for auxin induction of adventitious root initiation but not cell division in phloem parenchy ma or inner cortical parenchyma cells. Comparisons of rac heterozygous and homozygous seedling primary root length and callus formation in r esponse to auxin in stem cuttings indicate that rac copy number is cor related to the degree of expression of these two phenotypes.