The plant stress hormone jasmonic acid evokes defensive responses in streptomycetes
Actinobacteria are prevalent in the rhizosphere and phyllosphere of diverse plant species where they help to enhance tolerance of plants against biotic and abiotic stresses. Here, we show that the plant hormones jasmonic acid (JA) and methyljasmonate (MeJA) alter growth, development and specialized metabolism of Streptomyces. Challenge of Streptomyces coelicolor with JA or MeJA led to strongly enhanced production of the polyketide antibiotic actinorhodin. JA is toxic to Streptomycetaceae, whereby members of the genus Streptacidiphilus are generally more sensitive than streptomycetes. As a defensive response, extensive amino acid conjugation of JA was observed; the most prevalent conjugation was with glutamine (Gln), while conjugates with Val, Tyr, Phe and Leu/Ile were identified after longer exposure to JA. Synthetic JA conjugates failed to activate antibiotic production and had strongly reduced toxicity, strongly suggesting that conjugation inactivates JA and serves to detoxify the hormone. Thus, for the first time we provide evidence that plant hormones modulate growth, development and secondary metabolism of streptomycetes, whereby amino acid conjugation serves as a defense strategy by the bacteria to circumvent plant hormone toxicity. IMPORTANCEMicroorganisms that live on or inside plants greatly influence plant health. Streptomycetes are considered to have an important role in defense against plant diseases, but the mechanisms through which they protect plants are currently not fully understood. It has been suggested that streptomycetes respond to changes in the plants physiology, among others by producing protective molecules; however, little is known of the signal transduction from plant to bacterium. We here demonstrate that the plant hormones jasmonic acid (JA) and methyljasmonate (MeJA) directly influence the life cycle of streptomycetes by modulating antibiotic synthesis and promoting faster development. Moreover, the plant hormones specifically stimulate the synthesis of the polyketide antibiotic actinorhodin in Streptomyces coelicolor. Jasmonic acid is then modified in the cell by amino acid conjugation, which reduces the bioactivity of the hormone and thus quenches the signal. To the best of our knowledge, this has not been reported previously. Collectively, these results suggest a relationship between plant physiological changes and the response of streptomycetes in multiple ways.