Protoporphyrinogen oxidase (PPO)-inhibiting herbicides are used to control weeds
in a variety of crops. These herbicides inhibit heme and photosynthesis in plants.
PPO-inhibiting herbicides are used to control Amaranthus palmeri (Palmer amaranth)
especially those with resistance to glyphosate and acetolactate synthase (ALS) inhibiting
herbicides. While investigating the basis of high fomesafen-resistance in A. palmeri, we
identified a new amino acid substitution of glycine to alanine in the catalytic domain
of PPO2 at position 399 (G399A) (numbered according to the protein sequence of
A. palmeri). G399 is highly conserved in the PPO protein family across eukaryotic
species. Through combined molecular, computational, and biochemical approaches,
we established that PPO2 with G399A mutation has reduced affinity for several PPOinhibiting
herbicides, possibly due to steric hindrance induced by the mutation. This is
the first report of a PPO2 amino acid substitution at G399 position in a field-selected
weed population of A. palmeri. The mutant A. palmeri PPO2 showed high-level in vitro
resistance to different PPO inhibitors relative to the wild type. The G399A mutation is
very likely to confer resistance to other weed species under selection imposed by the
extensive agricultural use of PPO-inhibiting herbicides.

https://doi.org/10.3389/fpls.2019.00568