Breeding for herbicide tolerance in crops: a review

Weeds compete with crop plants for water, nutrients, sunlight, and space and also harbor insect and disease pests. With continuously increasing labor cost, manual weeding has become an expensive field operation for any crop and farmers are increasingly opting for cultivars tolerant to herbicides. Herbicide-tolerant cultivars offer opportunity of controlling weeds through need-based applications of herbicides. Herbicide tolerant cultivars have been developed in many crops by exploiting already available genetic variability in the germplasm or by creating mutations or by transgenic. A large genetic variation for tolerance to herbicide exists among various crops such as maize, wheat, rice, sunflower, soybean, chickpea, alfalfa etc. Herbicide tolerance in germplasm or in mutant lines may be due to altered binding site of target enzyme for herbicide, improved herbicide metabolism, sequestration of herbicide molecule and overexpression of target protein. Various mutations have been reported which may include SNP or insertion/deletion mutation conferring the herbicide tolerance e.g. a point mutation in the AHAS1 gene at C675 to T675 resulting in an amino acid substitution from Ala205 to Val205 confers resistance to IMI in chickpea. Herbicide resistant crop via transgenic has also been developed by taking resistant genes from various sources such as bacteria or other plant and incorporating them into crop to make them herbicide resistant. Transgenic herbicide tolerant crops occupy 47% of total area under transgenic in world. The disadvantage accompanied with herbicide tolerant crops is super weeds. Super weeds are the plants which are crop related weeds species on which the resistance genes got transferred by natural outcrossing.