Pseudomonas Inoculation Stimulates Endophytic Azospira Population and Induces Systemic Resistance to Bacterial Wilt

Abstract

Bacterial communities in the rhizosphere play an important role in sustaining plant growth and the health of diverse soils. Recent studies have demonstrated that microbial keystone taxa in the rhizosphere microbial community are extremely critical for the suppression of diseases. However, the mechanisms involved in disease suppression by keystone species remain unclear. The present study assessed the effects of three <i>Pseudomonas</i> strains, which were identified as keystone species in our previous study, on the growth performance and root-associated bacterial community of tobacco plants. A high relative abundance of <i>Ralstonia</i> was found in the non-inoculated group, while a large <i>Azospira</i> population was observed in all groups inoculated with the three <i>Pseudomonas</i> strains. Correspondingly, the activities of the defense-related enzymes and the expression levels of the defense signaling marker genes of the plant were increased after inoculation with the <i>Pseudomonas</i> strains. Moreover, the correlation analyses showed that the relative abundance of <i>Azospira</i>, the activity of superoxide dismutase, catalase, and polyphenol oxidase, and the expression of <i>H1N1</i>, <i>ACC Oxidase</i>, and <i>PR1 a/c</i> had a significantly negative (<i>p</i><0.05) relationship with the abundance of <i>Ralstonia</i>. This further revealed that the keystone species, such as <i>Pseudomonas</i> spp., can suppress bacterial wilt disease by enhancing the systemic resistance of tobacco plants.