Biotechnology approaches to developing herbicide tolerance/selectivity in crops

Abstract

Effective weed control in any crop production system is a prerequisite if high yields and good quality are to be achieved. Over the millennia that humans have practiced crop agriculture, weed control has been the most significant part of cropping operations in ensuring good quality harvests. Initially, hand weeding dominated most weeding practices but it was gradually replaced with mechanical control in the developed world. Mechanical weed control practices are now viewed to be unsatisfactory due to the high-energy requirements and other associated costs, plus the perceived facilitation of soil erosion and compaction. Mechanical weed control has now been largely replaced by chemical weed control using herbicides that can eliminate weeds from crop plantings with minimal soil disturbance. Herbicide treatment in crop plantings has allowed economically viable weed control and increased productivity. The most preferred herbicides today are those that combine weed killing potency with low- or no- environmental persistence. However, the very effective broad spectrum herbicides available also lack selectivity, thus limiting their use in some cropping operations. On the other hand, the continuous use of the few available selective herbicides is speeding up the development of herbicide resistance in weeds; hence making it difficult to achieve effective control in some crops. Herbicides generally function by disrupting unique and essential processes in plants e.g. photosynthesis, mitosis, pigment biosynthesis or essential amino acid biosynthesis. Both crops and weeds share these processes. Consequently, at present, selectivity is mostly based on differential herbicide uptake between weeds and crops, controlled timing and site of application or rapid detoxification of the herbicide by the crop plants. Reliance on these natural selective processes limits the effective use of potent herbicides; hence mechanisms to impart better herbicide selectivity in crops need to be investigated.