Resistance to protoporphyrinogen IX oxidase (PPO)-inhibitors in Amaranthus palmeri and Amaranthus tuberculatus is mainly contributed by mutations in the PPO enzyme, which renders herbicide molecules ineffective. The deletion of glycine210 (∆G210) is the most predominant PPO mutation. ∆G210-ppo2 is overexpressed in rice (Oryza sativac. ‘Nipponbare’) and Arabidopsis thaliana (Col-0). A foliar assay was conducted on transgenic T1 rice plants with 2× dose of fomesafen(780 g ha−1), showing less injury than the non-transgenic (WT) plants. A soil-based assay conducted with T2 rice seeds confirmed tolerance to fomesafen applied pre-emergence. In agar medium, root growth of WT rice seedlings was inhibited 90% at 5μM fomesafen, while root growth of T2seedlings was inhibited by 50% at 45 μM fomesafen. The presence and expression of the transgene were confirmed in the T2rice survivors of soil-applied fomesafen. A soil-based assay was also conducted with transgenic A. thaliana expressing ∆G210-ppo2 which confirmed tolerance to the pre-emergence application of fomesafen and saflufenacil. The expression of A. palmeri ∆G210-ppo2 successfully conferred tolerance to soil-applied fomesafen in rice and Arabidopsis. This mutant also confers cross-tolerance to saflufenacil in Arabidopsis. This trait could be introduced into high-value crops that lack chemical options for weed management.

https://doi.org/10.3390/genes13061044

Amaranthus palmeri has recently evolved resistance to glufosinate herbicide. Several A. palmeri populations from Missouri and Mississippi, U.S.A. had survivors when sprayed with glufosinate-ammonium (GFA, 657 g ha−1). One population, MO#2 (fourfold resistant) and its progeny (sixfold resistant), were used to study the resistance mechanism, focusing on the herbicide target glutamine synthetase (GS). We identified four GS genes in A. palmeri; three were transcribed: one coding for the plastidic protein (GS2) and two coding for cytoplasmic isoforms (GS1.1 and GS1.2). These isoforms did not contain mutations associated with resistance. The 17 glufosinate survivors studied showed up to 21-fold increase in GS2 copies. GS2 was expressed up to 190-fold among glufosinate survivors. GS1.1 was overexpressed > twofold in only 3 of 17, and GS1.2 in 2 of 17 survivors. GS inhibition by GFA causes ammonia accumulation in susceptible plants. Ammonia level was analyzed in 12 F1 plants. GS2 expression was negatively correlated with ammonia level (r = – 0.712); therefore, plants with higher GS2 expression are less sensitive to GFA. The operating efficiency of photosystem II (ϕPSII) of Nicotiana benthamiana overexpressing GS2 was four times less inhibited by GFA compared to control plants. Therefore, increased copy and overexpression of GS2 confer resistance to GFA in A. palmeri (or other plants). We present novel understanding of the role of GS2 in resistance evolution to glufosinate.

https://doi.org/10.1007/s00425-022-03968-2

In Arkansas, resistance to protoporphyrinogen IX oxidase (PPO)-inhibiting herbicides in Amaranthus palmeri S.Wats. is mainly due to target site mutations. Although A. palmeri PPO-mutations are well investigated, the cross-resistance that each ppo mutant endows to weed populations is not yet well understood. We aimed to evaluate the response of PPO-resistant A. palmeri accessions, harboring the ppo2 mutations DG210 and G399A, to multiple PPO-inhibiting herbicides. Six resistant and one susceptible field accessions were subjected to a dose–response assay with fomesafen, and selected survivors from different fomesafen doses were genotyped to characterize the mutation profile. The level of resistance to fomesafen was determined and a cross-resistance assay was conducted with 1 and 2 times the labeled doses of selected PPO herbicides. The accession with higher predicted dose to control 50% of the population (ED50) had a higher frequency of DG210-homozygous survivors. Survivors harboring both mutations, and those that were DG210-homozygous, incurred less injury at the highest fomesafen rate tested (1120 g ai ha􀀀1). The populations with a high frequency of DG210-homozygous survivors, and those with individuals harboring DG210 + G399A mutations, exhibited high potential for cross-resistance to other PPO herbicides. The new PPO–herbicide chemistries
(saflufenacil, trifludimoxazin) generally controlled the PPO-resistant populations.

https://doi.org/10.3390/agriculture11080760