The natural resistance of plants to disease is based not only on preformed mechanisms, but also on induced mechanisms. The defense mechanisms present in resistant plants may also be found in susceptible ones. This study attempted to analyze the metabolic alterations in plants of the potato Solanum tuberosum L. cv. Agata that were inoculated with the incompatible plant-pathogenic bacteria X. axonopodis and R. solanacearum, and the compatible bacterium E. carotovora. Levels of total phenolic compounds, including the flavonoid group, and the activities of polyphenol oxidase (PPO) and peroxidase (POX) were evaluated. Bacteria compatibility was evaluated by means of infiltration of tubers. The defense response was evaluated in the leaves of the potato plants. Leaves were inoculated depending on their number and location on the stem. Multiple-leaf inoculation was carried out on basal, intermediate, and apical leaves, and single inoculations on intermediate leaves. Leaves inoculated with X. axonopodis and with R. solanacearum showed hypersensitive responses within 24 hours post-inoculation, whereas leaves inoculated with E. carotovora showed disease symptoms. Therefore, the R. solanacearum isolate used in the experiments did not exhibit virulence to this potato cultivar. Regardless of the bacterial treatments, the basal leaves showed higher PPO and POX activities and lower levels of total phenolic compounds and flavonoids, compared to the apical leaves. However, basal and intermediate leaves inoculated with R. solanacearum and X. axonopodis showed increases in total phenolic compounds and flavonoid levels. In general, multiple-leaf inoculation showed the highest levels of total phenolics and flavonoids, whereas the single inoculations resulted in the highest increase in PPO activity. The POX activity showed no significant difference between single-and multiple-leaf inoculations. Plants inoculated with E. carotovora showed no significant increase in defense mechanisms such as enzyme activity and phenolic compounds. Therefore, resistance or susceptibility in S. tuberosum cv. Agata might depend on leaf age, type of inoculation performed (single or multiple), and the interaction between plant and pathogen.

BackgroundAraucariaceae are important forest trees of the southern hemisphere. Life expectancy of their seedlings can largely be reduced by fungal infections. In this study we have isolated and characterized such a fungus and investigated the potential of Streptomyces Actinobacteria from the respective rhizosphere to act as antagonists.ResultsThe pathogenic fungus from Araucaria angustifolia seeds was identified by morphological markers (pore-associated Woronin-bodies) as belonging to the Pezizomycotina. Molecular data identified the fungus as Neofusicoccum parvum (Botryosphaeriaceae). Co-cultures on agar of this fungus with certain streptomycete isolates from the rhizosphere, and from the surface of Araucaria roots significantly reduced the growth of the fungus. HPLC analysis of the agar yielded streptomycete-specific exudate compounds which were partly identified. There were differences in compounds between single (bacteria, fungus) and dual cultures (bacteria + fungus).ConclusionStreptomycetes from the rhizosphere of Araucariaceae produce exudates which can suppress the development of pathogenic fungi in their seeds.

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.