In vitro and genetic investigation of auxinic herbicide resistance in wild mustard (Sinapis arvensis L.)
Auxinic herbicides, that mimic several physiological and biochemical responses of the natural plant hormone, indole acetic acid (IAA), also known as auxin, are widely used to selectively control broadleaf weeds in cereal crops. However, the precise mode of action of both auxin and auxinic herbicides remains unknown 60 years after discovery of these compounds. Prolonged use of auxinic herbicides has resulted in development of resistance to these compounds in several weed species including wild mustard ('Sinapis arvensis' L.) a member of the 'Brassicaceae' family. Earlier studies indicate that the resistance to auxinic herbicides in wild mustard is not due to differential herbicide absorption, translocation or metabolism. Previous analysis of binding of 3H-IAA to auxin-binding protein (ABP) preparations suggests a low and a high affinity binding sites in the susceptible (S) and only a low affinity binding site in the resistant (R) biotype. This suggests that the herbicide resistance may be associated with altered binding of auxinic herbicides to ABP in wild mustard. This thesis was primarily focused toward understanding the basis of auxinic herbicide resistance using wild mustard as a model system. The specific objectives were: (a) to further elucidate the role of ABP(s) in auxinic herbicide resistance by comparing auxinic herbicide-induced 'in vitro' responses (e.g. cell elongation and lateral root formation) of R and S wild mustard with ABP1 antisense tobacco and ABP1 over expressing 'Arabidopsis', (b) to determine inheritance of cross-resistance to auxinic herbicides in wild mustard and (c) to develop genetically uniform homozygous lines of R and S wild mustard through microspore culture. The results indicate that in wild mustard, differences in ABP between R and S biotypes may explain resistance to auxinic herbicides. Furthermore, a single dominant nuclear gene determines the resistance to several groups of auxinic herbicides and 'in vitro ' production of homozygous R line was successful although the frequency of microspore-derived embryo regeneration was low. These results provide several new insights toward understanding the physiological basis of auxinic herbicide resistance in wild mustard and also open avenues for the molecular characterization of auxinic herbicide resistance gene.