Competition from weeds is a major constraint on crop performance. For over 60 years synthetic auxin herbicides have played an important role in weed management across the globe, primarily in cereal crops where they are used to control broad-leaved weeds. Despite their long history, new synthetic auxins are still being discovered. The recent identification of a family of auxin receptors as their target site has, for the first time, allowed us to use structure-led rational design to help shape the next generation of auxin herbicides.
The synthetic auxin 2,4-D has been used commercially as a selective herbicide since 1945. It remains a potent and effective product and, over the years, more than 20 additional analogues of the natural plant hormone auxin have been marketed as herbicides and plant growth regulators.
“Since the 1950s, Dow AgroSciences has successfully launched a number of synthetic auxin herbicides that provide weed control solutions to farmers globally,” said Daniel R. Kittle, Ph.D., vice president, Research and Development, Dow AgroSciences. “The collaboration with Professor Napier provides important insights into the fundamental interaction between plants and our newest innovations in auxin herbicides.”
In 2005 the protein known as TIR1 and close family members known as AFB proteins were identified as the receptor targets for auxins. Shortly afterwards the structure of TIR1 was solved by protein crystallography, revealing the auxin binding site in atomic detail. Research by Professor Richard Napier makes use of these details along with advanced analytical instruments to evaluate binding of different auxins to different members of the TIR1 family.
“There is one natural auxin which controls plant growth. There are also many synthetic auxins and these work as herbicides by overpowering the natural system. Our recent work with Dow AgroSciences has shown that not all auxins are equal,” said Napier. “The new arylpicolinates prioritise a different target site than both the natural auxin, and other auxin herbicides, and they bind to this site with high affinity.”
The research has highlighted unique properties of two new arylpicolinate herbicides from Dow AgroSciences. Professor Napier compared ArylexTM active and RinskorTM active to other commercial auxin herbicides and other compounds. Arylex and Rinskor are the first members of the new arylpicolinate class of auxin herbicides and are effective new tools for managing weed resistance at low use rates while providing favourable environmental fate, toxicological and ecotoxicological profiles. The second generation of herbicide-tolerant crops combines glyphosate and phenoxy auxin resistances in maize and soybeans. The scope of use for auxin herbicides is widening, and so products with improved field rates and divergent target specificity will be increasingly important.
Structure-led compound discovery and biophysical evaluation of receptor proteins will continue to lead to innovations in auxin herbicides. There is interest in extending the range of selectivity provided by current auxin herbicides, to give broad-spectrum activity, and even inverting selectivity to kill grass weeds. All this work will need to be managed so that resistance does not build up in weed populations.