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.
This paper identifies multiple mechanisms: as well as psbA gene mutation there is a non-target site resistance mechanism of enhanced metabolism. Of course, we expect multiple mechanisms of herbicide resistance as evident here in both target site psbA gene mutation and non-target site enhanced metabolism resistance.