2,4-D transport and herbicide resistance in weeds

As new herbicide-resistant crops come to market, it can be assumed that there will be a dramatic increase in use of 2,4-D and dicamba. Weed resistance could follow suit, and so a better understanding of the mechanism is urgently needed. In this issue of Journal of Experimental Botany (pages 3223–3235), Goggin et al. report on their analysis of 2,4-D resistance in wild radish. An elegant combination of physiological and biochemical studies led the authors to identify impaired transport of the herbicide as the cause of resistance. Auxinic herbicides such as clopyralid, picloram, dicamba and, above all, 2,4-D – one of the first widely used herbicides – have been used as effective weed control agents since the introduction of 2,4-D herbicides in 1945 (Smith, 1989). The importance of weed control using herbicides cannot be over-estimated. The yield loss potential among all crops without weed control is about 37% (Oerke, 2006); through the use of effective herbicides, these losses have been reduced to about 9% through all cropping systems. Nevertheless, this still means that global agriculture is suffering yield losses worth at least $300 billion each year (calculated using FAOSTAT, 2013: www.faostat.fao.org). Weed control without effective herbicide programs would substantially increase the cost of production, as the input of manual labor would need to be increased. In addition to higher production costs, farmers would also see reduced soil health, greenhouse gas emission and increased soil erosion, as effective no-till agriculture would be nearly impossible. Despite its decades-long worldwide use, resistance against 2,4-D has been found in only 28 different weed species, although the first cases had already been reported in wild carrot (Daucus carota) and spreading dayflower (Commelina diffusa) in 1957 (Switzer, 1957; Hilton, 1957; Heap, 2016). The well-known workhorses of the herbicide world, 2,4-D and dicamba, recently became the ‘talk of the agricultural world’ once again. Glyphosate, which came to dominate world herbicide markets after the introduction of transgenic glyphosate-resistant crops in the US from 1996 to 1998 (soybean, maize and cotton) became the epicenter of a rampant resistance epidemic due to overreliance and repeated use. This prompted farmers and the agricultural industry to search for alternative weed-control strategies. Two herbicide producers developed 2,4-D- and dicamba-resistant crops using bacterial resistance genes (Dow AgroSciences with Enlist® and Monsanto with Roundup Ready2 Xtend®, respectively) (Behrens et al., 2007; Wright et al., 2010). These new herbicide-resistant crops are now market-ready and it can be assumed that their release will lead to a dramatic increase in use of 2,4-D and dicamba. However, concerns have already been voiced that this might also lead to an increase in 2,4-D resistance in weeds (Egan et al., 2011). It can only be hoped that lessons from the rapid spread of glyphosate resistance have been learned and a similar scenario can be avoided with the use of these new herbicide tolerant crops.