The target site of antimicrotubule drugs, such as colchicine and the amiprophosmethyl and dinitroaniline pre-emergence herbicides, is the microtubular cytoskeleton. The primary components of microtubules are the alpha- and beta-tubulins. Therefore, we characterized the genes encoding these tubulins from three biotypes of goosegrass expressing different dinitroaniline herbicide response phenotypes (DRP) [i.e., susceptible (S), intermediately resistant (I) and highly resistant(R)]. By reverse transcriptase polymerase chain reaction and rapid amplification of cDNA ends, three alpha-tubulin cDNA classes and four beta-tubulin cDNA classes were isolated. For the beta-tubulins, sequence identity was high, and the nucleotide substitutions identified in coding regions did not result in amino acid replacements. Thus, missense mutations between the biotypes were not detected. In contrast, missense mutations between the biotypes were detected for two classes of alpha-tubulins. The most significant differences were mutations from the wildtype (Drps) allele that occurred in TUA1 of the I and R biotypes. Such mutations convert Met-268 to Thr in the I biotype (Drpi allele), or convert Thr 239 to Ile in the R biotype (Drpr allele). The presence of these mutations was strictly correlated with the DRP and the known genotype of individual plants -- heterozygous plants being the most informative. These findings suggest that the molecular basis for dinitroaniline resistance in goosegrass differs from those of colchicine/dinitroaniline cross-resistant Chlamydomonas reinhardtii and from benzimidazole resistant fungi. Goosegrass is a compelling system for the study of molecular, biochemical and biophysical mechanisms of antimicrotubule drug resistance. This research was supported by USDA-NRI grants (95-37315-2152/-2154).

Key words: goosegrass, herbicide resistance, microtubules, tubulins