Root-exuded specialized metabolites reduce arsenic toxicity in maize

Abstract

By releasing specialized metabolites, plants modify their environment. Whether and how specialized metabolites protect plants against toxic levels of trace elements is not well understood. We evaluated whether benzoxazinoids, which are released into the soil by major cereals, can confer protection against arsenic toxicity. Benzoxazinoid-producing maize plants performed better in arsenic-contaminated soils than benzoxazinoid-deficient mutants in the greenhouse and the field. Adding benzoxazinoids to the soil restored the protective effect, and the effect persisted to the next crop generation via positive plant-soil feedback. Arsenate levels in the soil and total arsenic levels in the roots were lower in the presence of benzoxazinoids. Thus, the protective effect of benzoxazinoids is likely soil-mediated and includes changes in soil arsenic speciation and root accumulation. We conclude that exuded specialized metabolites can enhance protection against toxic trace elements via soil-mediated processes and may thereby stabilize crop productivity in polluted agroecosystems.

Keywords: W22 wild-type; arsenic; arsenic tolerance; bx1 mutant; secondary metabolites.

Fig. 1.

Benzoxazinoid-producing maize plants are less affected by soil arsenic than benzoxazinoid-deficient mutants. (A) Plant height and (B) chlorophyll content of wild-type (W22) and benzoxazinoid-deficient bx1 mutant plants growing in soil without (0 mg kg⁻¹) or with arsenic addition (100 mg kg⁻¹) for 7 wk. (C) Shoot dry biomass sampled at the end of the experiment. Asterisks indicate significant differences between genotypes (Tukey’s pairwise comparisons, or Tukey’s post hoc tests). Levels of significance: n.s. nonsignificant, . = marginally significant, *P < 0.05, **P < 0.01, ***P < 0.001. See Dataset S1 for detailed results of Tukey HSD tests.

Fig. 2.

Soil complementation with benzoxazinoids reduces arsenic toxicity. (A) Plant height and (B) shoot dry biomass of wild-type (W22) and benzoxazinoid-deficient bx1 mutant plants growing without (0 mg kg⁻¹) or with arsenic addition (100 mg kg⁻¹). For a subset of bx1 mutant plants, the soil was complemented with a mixture of purified benzoxazinoids, with DIMBOA-Glc as the dominant compound. Levels of significance (among genotypes): n.s. nonsignificant, . = marginally significant, *P < 0.05, **P < 0.01, ***P < 0.001. See Dataset S1 for detailed results of Tukey HSD tests.

Fig. 3.

Benzoxazinoids and arsenic interact to induce subtle changes in rhizosphere microbiota. The results of a CAP coordinates for bacteria and fungi computing dissimilarities with the model Bray–Curtis distance ~ arsenic * genotype is shown. Rhizosphere samples of wild-type (W22) and benzoxazinoid-deficient bx1 mutant plants growing without (0 mg kg⁻¹) or with arsenic addition (100 mg kg⁻¹) were analyzed. Variances and P-values were computed by a PERMANOVA with 999 permutations. Levels of significance: *P < 0.05, **P < 0.01, ***P < 0.001.

Fig. 4.

Benzoxazinoids reduce root arsenic accumulation. (A) Root and (B) leaf arsenic concentrations of wild-type (W22) and benzoxazinoid-deficient bx1 mutant plants growing without (0 mg kg⁻¹) or with arsenic addition (100 mg kg⁻¹). (C) Correlation between root arsenic concentration and plant dry biomass of wild-type plants growing in soils with different levels of arsenic addition. For comparison, the yellow and green lines correspond to minimal and maximal arsenic concentrations in W22 and bx1 mutant plants growing in soil with arsenic addition from A. Levels of significance (between genotypes): n.s. nonsignificant, . = marginally significant, *P < 0.05, **P < 0.01, ***P < 0.001. See Dataset S1 for detailed results of Tukey HSD tests.

Fig. 5.

Field-grown plants benefit more from benzoxazinoids in a field with high arsenic levels. (A) Plant height was recorded during maize growth season by 4 time points (2, 5, 9, and 14 wk after sowing) and (B) shoot dry biomass was measured after harvesting in wild-type (W22) and benzoxazinoid-deficient bx1 mutant plants. Left-hand panels are from a field with low arsenic (As−) (43 mg kg⁻¹) and the Right-hand panels are from a field with high arsenic contamination (As+) (430 mg kg⁻¹). Levels of significance (between genotypes): n.s. nonsignificant, . = marginally significant, *P < 0.05, **P < 0.01, ***P < 0.001. See Dataset S1 for detailed results of Tukey HSD tests.

Fig. 6.

The reduction of arsenic toxicity by benzoxazinoids persists via plant–soil feedbacks. (A) Plant height and (B) shoot dry biomass of wild-type (W22) plants growing in soils without (0 mg kg⁻¹) or with arsenic addition (100 mg kg⁻¹) that were conditioned by either wild-type (W22) or benzoxazinoid-deficient bx1 mutant plants. Levels of significance (between genotypes): n.s. nonsignificant, . = marginally significant, *P < 0.05, **P < 0.01, ***P < 0.001. See Dataset S1 for detailed results of Tukey HSD tests.