Combining Streptomyces hygroscopicus and phosphite boosts soybean’s defense responses to Phytophthora sojae

To determine the beneficial effect of the combination of phosphite (Phi) and Streptomyces hygroscopicus (S11) on soybean plants infected by Phytophthora sojae, soybean seeds were pre-treated with a suspension of S11, the leaves were sprayed with Phi and the roots were subsequently inoculated with P. sojae zoospores. Seedling growth and colonization of root tissue by the pathogen were monitored and the Phi concentration in the roots was quantified. Finally, the effect of S11 and/or Phi on defense responses and SA and JA production were recorded. Both S11 and Phi inhibited the growth of P. sojae in dual V8 agar cultures, and soybean pre-treatment with S11 and/or Phi reduced further infection in planta. In response to P. sojae, defense genes were upregulated in both tolerant and susceptible soybean varieties, especially in the latter. SA and JA accumulated more in the susceptible variety in response to P. sojae, and the use of S11 and/or Phi prior to inoculation induced a decrease in their production. SA accumulated earlier than JA in soybean roots in response to P. sojae in the tolerant variety, whereas JA accumulated earlier than SA in the susceptible one. As such, S11 combined with Phi significantly reduced P. sojae infection in soybean by delaying and reducing the activation of stress hormones and defense-related genes, offering an efficient mean to manage soybean’s infection by P. sojae.