Evolution of resistance to HPPD-inhibiting herbicides in a wild radish population via enhanced herbicide metabolism

Herbicides that inhibit HPPD have become very important. In North America, corn-selective HPPD herbicides have long been used and cases of HPPD herbicide-resistant weeds reported. The first reported North American cases of HPPD herbicide-resistant Amaranthus weeds were cross-resistance selected by earlier-used herbicides that endow cross-resistance to HPPD herbicides (with more-recent reports being resistance evolution from direct HPPD herbicide selection).

In wheat-dominated Australian cropping, HPPD herbicides have been in use for about a decade. In this paper, the first case of resistance in Australia to HPPD herbicides is reported. However, this HPPD resistance did not occur from HPPD herbicide selection but rather from selection with previously used herbicides (ALS, PS2, auxinic etc). In this sense, this resistant wild radish Raphanus population is similar to the first North American cases of HPPD resistant Amaranthus weeds where the HPPD resistance is cross-resistance. The metabolic resistance evolved from selection with earlier used herbicides (ALS, auxinic etc) and this is cross-resistance to certain HPPD herbicides.

AHRI PhD student Huan Lu studied this wild radish population and, as expected, he found no evidence of target site changes in the HPPD gene. Sequencing of the HPPD gene between R and S showed no resistance-endowing SNPS mutations and there was not HPPD gene amplification.

Huan Lu then investigated for non-target site mechanisms. He found no difference in rates of 14C mesotrione uptake or translocation. However, 14C labelled mesotrione metabolism studies showed quantitatively faster rates of mesotrione metabolism in R plants compared to S plants. Table 4 shows that malathion could reverse tembotrione resistance providing an indication of cytochrome P450 genes but definitive evidence is required. What is clear is that this wild radish population has metabolic resistance and is resistant to several HPPD herbicides by a capacity for enhanced rates of herbicide metabolism. Given that new HPPD herbicides continue to be commercialised this first evidence of HPPD herbicide resistance in this very prominent dicot weed is a significant concern.

For decades, Lolium, Echinochloa, Alopecurus and other Gramineae grass weeds have demonstrated metabolic herbicide resistance that can endow cross-resistance to a range of existing and even yet-to-be-discovered herbicide chemistries. Now, important dicot weeds like Amaranthus and Raphanus are showing metabolic herbicide resistance. This is a significant challenge!