Soybean (Glycine max L Merr) host-plant defenses and resistance to the two-spotted spider mite (Tetranychus urticae Koch)

Abstract

In southern Ontario, Canada, the two-spotted spider mite (Tetranychus urticae) is an emerging pest of soybean (Glycine max) due to the increasing incidence of warmer, drier weather conditions. One key strategy to manage soybean pests is breeding resistant cultivars. Resistance to pathogens and herbivores in soybean has been associated with isoflavonoid phytoalexins, a group of specialized metabolites commonly associated with root, leaf and seed tissues. A survey of 18 Ontario soybean cultivars for spider mite resistance included evaluations of antibiosis and tolerance in relation to isoflavonoid and other metabolites detected in the leaves. Ten-day and 4-week trials beginning with early growth stage plants were used to compare survival, growth, fecundity as well as damage to leaves. Two-spotted spider mite (TSSM) counts were correlated with HPLC measurements of isoflavonoid concentration in the leaves and global metabolite profiling by high resolution LC-MS to identify other metabolites unique to the most resistant (R) and susceptible (S) cultivars. Within 10 days, no significant difference (P>0.05) in resistance to TSSM was determined between cultivars, but after 4 weeks, one cultivar, OAC Avatar, was revealed to have the lowest number of adult TSSMs and their eggs. Other cultivars showing partial resistance included OAC Wallace and OAC Lakeview, while Pagoda was the most tolerant to TSSM feeding. A low, positive correlation between isoflavonoid concentrations and TSSM counts and feeding damage indicated these compounds alone do not explain the range of resistance or tolerance observed. In contrast, other metabolite features were significantly different (P<0.05) in R versus S cultivars. In the presence of TSSM, the R cultivars had significantly greater (P<0.05) concentrations of the free amino acids Trp, Val, Thr, Glu, Asp and His relative to S cultivars. Furthermore, the R cultivar metabolites detected are viable targets for more in-depth analysis of their potential roles in TSSM defense.

Fig 1. Total isoflavonoid concentration in the top trifoliate leaves of 18 soybean cultivars at the V1 and V3 growth stages and the mean concentration of both stages.

The concentration of isoflavonoids in the top leaves was used to categorize the soybean cultivar as low, medium or high.

Fig 2. The performance of two-spotted spider mite on the 2^nd and 3^rd full leaf of 10 soybean cultivars after 10 days feeding.

Data are the mean (± S.E.) TSSM larvae, nymph, adult and egg counts. No significant difference in TSSM larva, nymph and adult (Kruskal-Wallis, P>0.05) or TSSM egg (Kruskal-Wallis, P>0.05) numbers was revealed between the different cultivars tested beginning at the V3 stage.

Fig 3. The effect of two-spotted spider mite feeding on isoflavonoid concentrations in the 2^nd, 3^rd and top full leaves of 10 soybean cultivars after 10 days.

Data are mean (± S.E.) isoflavonoid concentrations (μg/g). Bracketed bars for each cultivar with the same letters are not statistically different (2-way ANOVA, Tukey’s pairwise comparison, P>0.05). Analysis of the mid-leaflet of the trifoliate leaves indicated that there was no significant difference (P>0.05) in isoflavonoid concentrations between the 3 leaf levels, with the exception of 2 cultivars where the 2^nd full leaves had lower (++ = P<0.05) isoflavonoids than the 3^rd full and top leaflets.

Fig 4. The performance of two-spotted spider mite on the top 3 trifoliate leaves of 11 soybean cultivars after 4 weeks.

Data are mean (± S.E.) TSSM larvae, nymph, adult and egg counts. Bars with the same letters are not statistically different (2-way ANOVA, Tukey’s pairwise comparison, P>0.05). Analysis of the 3 youngest trifoliate leaves indicated that 3^rd full leaves generally had higher (** = P<0.05) TSSM counts than the 2^nd and top trifoliate for the most susceptible cultivars.

Fig 5. The effect of two-spotted spider mite feeding on total isoflavonoid concentrations in the top 3 trifoliate leaves of 11 soybean cultivars after 4 weeks.

Data are mean (± S.E.) total isoflavonoid concentrations (μg/g). Bars with the same letters are not statistically different (Kruskal-Wallis, P>0.05).

Fig 6. Leaf damage caused by two-spotted spider mite feeding to the top 3 trifoliate leaves of 8 soybean cultivars after 4 weeks.

Data are mean (± S.E.) percent leaf damage. Bracketed bars with the same lower case letter are not statistically different (Kruskal-Wallis, Dunn’s test, P>0.05).

Fig 7

PCA plots derived from XCMS extracted features of resistant (white) and susceptible cultivars (black). (A) cultivars without mites, ESI⁺; (B) cultivars with mites, ESI⁺; (C) cultivars without mites, ESI^- and (D) cultivars with mites, ESI^-. Each symbol represents one of 3 biological replicates.