Coordinated attenuation of oxidative stress metabolism and alterations of root exudates in maize (Zea mays L.) induced by Streptomyces rhizobacteria.

Interactions between plants and plant growth-promoting rhizobacteria (PGPR) are critically important for the healthy development of plants. Several Streptomyces species stand out as PGPR and biological control agents. This study evaluated the effects of the interaction between maize plants and four Streptomyces strains on oxidative stress metabolism and root exudate profiles. Plants treated with strains CLV16, CLV95, CLV104, and CLV179 showed a marked reduction in antioxidant enzyme activities. In shoots, ascorbate peroxidase activity decreased by up to 99% at 15 days compared to control, while catalase and guaiacol peroxidase activities in roots were reduced by approximately 70% and 40-65%, respectively. Root exudate profiling revealed a shift from defense-related specialized metabolites toward primary metabolites. Total amino acids increased up to 2.3-fold (reaching 187 µg/mg root dry mass in CLV104-treated plants), whereas total sugars and phenolic compounds decreased by up to 60% and 70%, respectively. Five-day-old plants exuded the highest levels of caffeic acid when treated with Streptomyces isolates, while 15-day-old plants showed higher levels of hydroxybenzoic acid, vanillic acid, and the flavonoid rutin in CLV16 and CLV179 treatments. Notably, the CLV104 strain promoted the lowest exudation of phenolic compounds with potential antimicrobial activity, including caffeic acid (reduced by approximately 92% at 15 days) and undetectable levels of coumarin and cinnamic acid. These findings indicate that Streptomyces may cause a coordinated suppression of oxidative defense metabolism alongside alterations of root exudation patterns, with strain-specific effects that may favor beneficial rhizosphere interactions.