Double mutation in Eleusine indica alpha-tubulin increases the resistance of transgenic maize calli to dinitroaniline and phosphorothioamidate herbicides

The repeated use of dinitroaniline herbicides on the cotton and soybean fie lds of the southern United States has resulted in the appearance of resista nt biotypes of one of the world's worst weeds, Eleusine indica. Two biotype s have been characterized, a highly resistant (R) biotype and an intermedia te resistant (I) biotype. In both cases the resistance has been attributed to a mutation in cx-tubulin, a component of the alp tubulin dimer that is t he major constituent of microtubules. We show here that the I-biotype mutat ion, like the R-biotype mutation shown in earlier work, can confer dinitroa niline resistance on transgenic maize calli. The lever or resistance obtain ed is the same as that for E. indica I- or R-biotype seedlings. The combine d I- and R-biotype mutations increase the herbicide tolerance of transgenic maize calli by a value close to the summation of the maximum herbicide tol erances of calli harbouring the single mutations. These data, taken togethe r with the position of the two different mutations within the atomic struct ure of the alpha/beta tubulin dimer, imply that each mutation is likely to exert its effect by a different mechanism. These mechanisms may involve inc reasing the stability of microtubules against the depolymerizing effects of the herbicide or changing the conformation of the alpha/beta dimer so that herbicide binding is less effective, or a combination of both possibilitie s.