Antagonism between Azospirillum brasilense Az39 and Pseudomonas oryzihabitans, a seed-borne endophyte, in growing rice plants

Abstract

The interactions between native endophytic bacteria and inoculated beneficial bacteria in plant tissues are relevant to guarantee the success of commercial inoculants assuming that benefitial bacteria must persist associated to the plant for a certain period of time. In this study we examine whether Pseudomonas oryzihabitans is able to antagonize Azospirillum brasilense Az39, a Plant Growth Promoting Bacteria widely used in South America. Surface-sterilized rice seeds inoculated with high amounts of P. oryzihabitans G16 or A. brasilense Az39 or with both strains were grown under hydroponic conditions. Shoot and root biomass of 15 days-old inoculated-seedlings were compared to those of uninoculated seedlings. In addition, enumeration and identification of endophytic bacteria were performed on these seedlings. Heterotrophic and siderophore-producing bacteria isolated from seedlings were identified by 16S rRNA gene partial sequencing. A. brasilense and P. oryzihabitans were able to colonize rice plants, being predominant in the roots and shoots of the respective inoculated seedlings. In co-inoculated plants, only P. oryzihabitans was recovered. The siderophore-producing Sphingomonas sp. was predominant in uninoculated plants and was detected in all inoculated plants. The vegetal biomass was the lowest in P. oryzihabitans-inoculated plants and the highest in A. brasilense inoculated plants.This work shows that P. oryzihabitans antagonizes A. brasilense in plant tissues and decreases rice plant yield. Other seed-borne endophytes, particularly siderophore-producer bacteria of the genus Sphingomonas, are not outcompeted by P. oryzihabitans.