Defense responses and oxidative metabolism of glyphosate-resistant soybean plants infected by Phakopsora pachyrhizi modulated by glyphosate and nickel

Among the many factors affecting soybean resistance against Asian soybean rust (ASR), the nutritional status of plants plays a pivotal role. Even though the beneficial effects of nickel (Ni) and glyphosate (Gl) on ASR control have been reported, the mechanism(s) involved in disease control promoted by Gl or its combination with Ni remains to be elucidated. Therefore, this study investigated the effects of Ni, Gl, and their combination on both defense mechanisms and antioxidative metabolism of soybean plants from a Gl-resistant cultivar infected by Phakopsora pachyrhizi. Severity of ASR was significantly reduced by 37, 68, and 77%, respectively, for Ni, Gl, and Ni + Gl sprayed plants compared to water-sprayed plants (control treatment). Infected and water-sprayed plants showed earlier and higher levels of reactive oxygen species and malondialdehyde than plants sprayed with Ni or Gl. The antioxidative metabolism of infected plants from control, Ni, Gl, and Ni + Gl treatments was not affected. For inoculated plants sprayed with Ni or Gl, β-1,3-glucanase (GLU) activity and phenolics level were greater. Additionally, Ni-sprayed and infected plants showed greater phenylalanine ammonia-lyase (PAL) activity and earlier lignin production than water-sprayed and infected plants. Polyphenoloxidase activity was higher at 5 days after inoculation for Gl-sprayed plants regardless of P. pachyrhizi infection. The findings of this study shed light on an intrinsic interplay between Ni and Gl to decrease ASR symptoms, but changes in host defense responses were barely potentiated by Gl. Independent of Gl, Ni reduced ASR symptoms due to modulation of soybean defense mechanisms (higher GLU and PAL activities and great production of phenolics and lignin) against ASR.