The soybean cyst nematode (SCN; Heterodera glycines Ichinohe) is a significant agricultural pest that causes extensive damage to soybean production worldwide. Second-stage juveniles (J2s) of the SCN migrate through the soil and infest the roots of host plants in response to certain chemical substances secreted from the host roots. Therefore, controlling SCN chemotaxis could be an effective strategy for its management. In the present study, we identified the Hg-gpa-3d gene, which encodes the G protein alpha subunit, as a key regulator of SCN chemotaxis. Gene silencing of Hg-gpa-3d reduced the attraction of SCN J2s to host roots, as well as to nitrate ions, a chemoattractant recognized through a mechanism different from that of host recognition. However, silencing of Hg-gpa-3d did not affect avoidance behavior towards unpleasant temperatures or stylet protrusion. These results suggest that Hg-gpa-3d is a crucial gene in the regulation of SCN chemotaxis and provide new insights into the chemotactic mechanisms of the SCN.

Soybean (Glycine max [L.] Merr.) is an important oilseed crop with a high economic value. However, three damaging soybean diseases, soybean cyst nematode (SCN; Heterodera glycines Ichinohe), Sclerotinia stem rot caused by the fungus Sclerotinia sclerotiorum (Lid.) de Bary, and soybean root rot caused by Fusarium spp., are major constraints to soybean production in the Great Plains. Current disease management options, including resistant or tolerant varieties, fungicides, nematicides, and agricultural practices (crop rotation and tillage), have limited efficacy for these pathogens or have adverse effects on the ecosystem. Microbes with antagonistic activity are a promising option to control soybean diseases with the advantage of being environmentally friendly and sustainable. In this study, 61 bacterial strains isolated from wheat rhizospheres were used to examine their antagonistic abilities against three soybean pathogens. Six bacterial strains significantly inhibited the growth of Fusarium graminearum in the dual-culture assay. These bacterial strains were identified as Chryseobacterium ginsengisoli, C. indologenes, Pseudomonas poae, two Pseudomonas spp., and Delftia acidovorans by 16S rRNA gene sequencing. Moreover, C. ginsengisoli, C. indologenes, and P. poae significantly increased the mortality of SCN second-stage juveniles (J2), and two Pseudomonas spp. inhibited the growth of S. sclerotiorum in vitro. Further growth chamber tests found that C. ginsengisoli and C. indologenes reduced soybean Fusarium root rot disease. C. ginsengisoli and P. poae dramatically decreased SCN egg number on SCN-susceptible soybean 'Williams 82'. Two Pseudomonas spp. protected soybean plants from leaf damage and collapse after being infected by S. sclerotiorum. These bacteria exhibit versatile antagonistic potential. This work lays the foundation for further research on the field control of soybean pathogens.