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Ontario Grain Farmer Magazine

Isoflavonoid levels in soybean (Glycine max) cultivars and associated anti-herbivore activity

Principal Investigator: Ian Scott and Sangeeta Dhaubhadel

Research Institution: Agriculture and Agri-Food Canada (AAFC)

Timeline: April 2017 – June 2019

Objectives:

  • Investigate the genetic resistance to important herbivore pests in Ontario soybean: the soybean aphid (key insect pest) and the two spotted spider mite (emerging pest).
  • Measure the isoflavonoid concentration in leaves of several soybean cultivars.
  • Screen resistance to insects, soybean aphid and spider mites, in soybean cultivars that differ in leaf isoflavonoid level and level of susceptibility.
  • Correlate isoflavonoid content with anti-insect/mite activity and damage in soybean cultivars.
  • Recommend resistant cultivars for planting or breeding.

Impacts:

  • The increased knowledge of soybean cultivar resistance to soybean aphids and spider mites may allow growers to better manage these pests through selecting soybean cultivars for Ontario based on resistance and pest forecasts.
  • Understanding the correlation between isoflavonoid concentrations and herbivore resistance in soybeans may lead to improved soybean breeding insect resistance and assess new cultivars that have recently been bred for other desirable traits.

Scientific Summary:

Two important soybean pests are the soybean aphid (Aphis glycines) and the two-spotted spider mite (Tetranychus urticae). When populations of these two herbivores are high, there can be a severe reduction in soybean yield. The current strategy for aphid management involves monitoring and reacting by applying insecticides to reduce the aphid pressure. Spider mites are an emerging pest due to the increasing incidence of warmer, drier weather conditions in Ontario. Insecticides are applied to control high populations of aphids and mites; however, the over-use of insecticides may cause mite populations to flare up by reducing native beneficial enemies (lady bird beetles and predatory mites). Isoflavonoids are legume-specific plant natural products abundant in soybean. Their production is induced by herbivores, including hemipterans (e.g., stink bugs and aphids) and lepidopterans (e.g., armyworms and leaf worms), and are characterized by feeding inhibitory activity and growth inhibitory activity on herbivorous insects. Reducing the number of insecticide applications or delaying applications until later in the growing season by slowing aphid and mite population growth is the goal of developing more resistant soybean cultivars to these pests.

This project examined the levels of isoflavonoids in leaves of several Ontario grown soybean cultivars to determine which compounds are most active and which cultivars are important for managing aphid and mite populations to provide an additional tool for soybean IPM. The project screened resistance of several Ontario soybean cultivars from different maturity groups to the two pests by measuring plant damage as well as growth and reproduction of aphids and mites. Chromatographic techniques were used for analysis of isoflavonoids in the resistant cultivars. Statistics were then used to correlate the biological and chemical data for isoflavonoids identified in the cultivars and the corresponding pest damage ratings. The findings can be used by growers to select cultivars with increased herbivore resistance when early season predictions indicate conditions preferable for aphid and/or mite infestations. The evidence will also provide direction for breeding or metabolic engineering of specific isoflavonoids into currently registered cultivars to improve their resistance.

The overall goals of the project were met, primarily investigating genetic resistance to the soybean aphid and the two spotted spider mite in selected Ontario soybean cultivars. The objectives were 1) measuring isoflavonoid concentration in leaves of 18 soybean cultivars; 2) screening resistance to soybean aphid and spider mites in soybean cultivars that differ in leaf isoflavonoid level and composition; and 3) correlating the isoflavonoid content with anti-insect/mite activity and damage in soybean cultivars.

Objective 1 was met by measuring the total isoflavonoid concentrations at the V1, V3 and R3 stages for 18 Ontario grown soybean cultivars in herbivore un-infested plants. It was noted that the concentration of total isoflavonoids increased over the growing period, but the cultivars that had the highest levels at early growth stages did not remain consistently the highest. The cultivar that had the highest leaf isoflavonoid concentration at each stage was: Harosoy 63 (V1); OT06-23 (V3); OAC Avatar (R3). The concentration of total isoflavonoids was also measured at the V3 stage after soybean aphid and spider mite herbivory.

Objective 2 was met by assessing the antibiosis and antixenosis of 12 soybean cultivars with 10 day and 4 week trials with aphids and mites. The cultivar with the greatest resistance to aphids was OT06-22 followed next by OT06-23, OAC Strive, Harosoy 63, OAC Avatar and finally AC Colombe. The most resistant cultivars had on average 36-fold fewer aphids on the younger leaves relative to least resistant ones. The most resistant cultivar to spider mites was OAC Avatar followed next by OAC Wallace, Pagoda, OAC Lakeview, OT06-22 and finally OT06-23.

Objective 3 was met by determining the relationship between isoflavonoid leaf content and anti-herbivore activity. There was low correlation between leaf total isoflavonoid concentration and antibiosis resistance for both aphids (r = 0.118) and TSSM (r = 0.213), and between isoflavonoids and tolerance to aphid feeding damage. This may be explained in part by the fact that only genistin was measured as the major peak in the leaf extracts. The levels of daidzein and genistein, previously reported to be responsible for anti-herbivore effects, were low. It was determined that only 3 cultivars were consistently resistant to both herbivores: OAC Avatar, OT06-22 and OT06-23. Conrad was the most susceptible to both herbivores.

These findings enabled the project team to make recommendations regarding resistant Ontario soybean cultivars for growers to plant and that can be used for future breeding programs.

External Funding Partners:

None

Project Related Publications:

Elneihoum, F., T. McDowell, J. Renaud, S. Dhaubhadel, L. Chen, V. Grbic, V. Zhurov, S. Krolikowski and I. Scott. 2020. Two-spotted spider mite Tetranychus urticae adaptations to soybean Glycine max plant defenses. Integrated Control of Plant-Feeding Mites, IOBC-WPRS Bulletin. Vol. 149, pp. 3-9.

Hein, T. 2020. Preventing pest damage in soybean. Ontario Grain Farmer. February. Online.

King, C. 2018. Towards increased pest resistance. Top Crop Manager. December. pp 30-33.

Scott, I., McDowell T., Renaud J., Krolikowski S., Chen L., and Dhaubhadel S. 2021. Soybean (Glycine max L Merr) host-plant defenses and resistance to the two-spotted spider mite (Tetranychus urticae Koch). Plos One. Open Access: October 7, 2021.