Autochthonous Streptomyces regulate the metabolism of seedlings of Araucaria angustifolia (Coniferales) during root colonisation

Araucaria angustifolia (Bertol.) Kuntze, known as Brazilian pine, is an endangered species of great ecological and economic importance. This species grows slowly and unevenly, with high mortality in commercial plantations. Streptomyces is a genus of soil microorganisms that may have a beneficial effect on plant growth. This study evaluated the effect of three autochthonous Streptomyces spp. isolates (PM1, PM4 and PM9) on the initial metabolism and development of A. angustifolia seedlings. The enzymatic activity of phenylalanine ammonia-lyase, polyphenol oxidase and peroxidase, and the levels of phenolic compounds, flavonoids and chlorophyll were determined in extracts from roots and leaves of the seedlings. Assays were carried out 1, 3 and 9 days after the roots were inoculated with each isolate. Length and fresh mass of shoots and roots as well as the volume and density of roots were evaluated at 100 days after seedling inoculation. All the Streptomyces spp. showed rhizospheric competence and produced auxin. The activities of polyphenol oxidase and peroxidase exhibited a tissue-temporal regulation in the presence of the isolates. Levels of phenolics, flavonoids and chlorophylls did not change in the period analysed. The root system of seedlings inoculated with all isolates was shorter and denser, with a small volume. The PM9 isolate promoted shoot growth and affected plant metabolism, proving to be a promising rhizobacterium with a plant growth-promoting rhizobacteria role.