Role of oxidative stress in the physiology of sensitive and resistant Amaranthus palmeri populations treated with herbicides inhibiting acetolactate synthase

Abstract

The aim of the present study was to elucidate the role of oxidative stress in the mode of action of acetolactate synthase (ALS) inhibiting herbicides. Two populations of Amaranthus palmeri S. Watson from Spain (sensitive and resistant to nicosulfuron, due to mutated ALS) were grown hydroponically and treated with different rates of the ALS inhibitor nicosulfuron (one time and three times the field recommended rate). Seven days later, various oxidative stress markers were measured in the leaves: H₂O₂, MDA, ascorbate and glutathione contents, antioxidant enzyme activities and gene expression levels. Under control conditions, most of the analysed parameters were very similar between sensitive and resistant plants, meaning that resistance is not accompanied by a different basal oxidative metabolism. Nicosulfuron-treated sensitive plants died after a few weeks, while the resistant ones survived, independently of the rate. Seven days after herbicide application, the sensitive plants that had received the highest nicosulfuron rate showed an increase in H₂O₂ content, lipid peroxidation and antioxidant enzymatic activities, while resistant plants did not show these responses, meaning that oxidative stress is linked to ALS inhibition. A supralethal nicosulfuron rate was needed to induce a significant oxidative stress response in the sensitive population, providing evidence that the lethality elicited by ALS inhibitors is not entirely dependent on oxidative stress.

Keywords: Amaranthus palmeri; acetolactate synthase; herbicide mode of action; nicosulfuron; oxidative stress.

Figure 1

H₂O₂ content (A) and malondialdehyde (MDA) equivalents (B) in Amaranthus palmeri sensitive (S, white) and resistant (R, black) populations treated with different nicosulfuron rates (X axis, times recommended field rate or FR, FR = 0.06 kg ha^-1). Mean ± SE (n = 4). For each population, significant differences between treatments and their respective untreated are highlighted with asterisks (Student’s t-test, p value ≤ 0.05).

Figure 2

Reduced glutathione (GSH) content (A), oxidised glutathione (GSSG) content (B), γ-glutamyl-cysteine (GGC) content (C), Cys content (D), sum of GSH and GSSG (total glutathione) contents (E) and GSH to GSSG ratio (F) in Amaranthus palmeri sensitive (S, white) and resistant (R, black) populations treated with different nicosulfuron rates (X axis, times recommended field rate or FR, FR = 0.06 kg ha^-1). Mean ± SE (n = 4). Significant differences between untreated S and R plants are highlighted with pound symbols (Student’s t-test, p value ≤ 0.05). For each population, significant differences between treatments and their respective untreated are highlighted with asterisks; (Student’s t-test, p value ≤ 0.05). "#" indicates pound.

Figure 3

Ascorbic acid content (A), dehydroascorbic acid content (B), sum of ascorbic acid and dehydroascorbic acid (C) and ascorbic acid to dehydroascorbic acid ratio (D) in Amaranthus palmeri sensitive (S, white) and resistant (R, black) populations treated with different nicosulfuron rates (X axis, times recommended field rate or FR, FR = 0.06 kg ha^-1). Mean ± SE (n = 4). For each population, significant differences between treatments and their respective untreated are highlighted with asterisks (Student’s t test, p value ≤ 0.05).

Figure 4

Superoxide dismutase (SOD) activity (A), catalase (CAT) activity (B), glutathione reductase (GR) activity (C) and peroxidases (POX) activity (D) in Amaranthus palmeri sensitive (S, white) and resistant (R, black) populations treated with different nicosulfuron rates (X axis, times recommended field rate or FR, FR = 0.06 kg ha^-1). Mean ± SE (n = 4). For each population, significant differences between treatments and their respective untreated are highlighted with asterisks (Student’s t-test, p value ≤ 0.05).

Figure 5

CuZn superoxide dismutase (CuZnSOD) gene expression (A), Mn superoxide dismutase (MnSOD) gene expression (B) and catalase (CAT) gene expression (C) in Amaranthus palmeri sensitive (S, white) and resistant (R, black) populations), in relative fold change with respect to untreated plants of each population, treated with different nicosulfuron rates (X axis, times recommended field rate or FR, FR = 0.06 kg ha^-1). Mean ± SE (n = 4). For each population, no significant differences between treatments and their respective untreated were observed (Student’s t test, p value ≤ 0.05).