Abstract
Endophytic actinomycetes are a promising source of novel metabolites with diverse biological activities. Tea plants (Camellia sinensis) produce arsenals of phytochemicals, which are linked to a number of medicinal and nutritional properties. However, a systematic investigation into the abundance and diversity of cultivated actinomycetes residing in tea plants has not been performed. In this study, a total of 46 actinobacteria were recovered from leaf, stem, and root samples of 15 tea cultivars collected in Fujian province, China. Their abundance and diversity were shown to be influenced by both the genotypes and tissue types of tea plants. Based on 16S RNA sequence analysis, these isolates were taxonomically grouped into 11 families and 13 genera, including Streptomyces, Actinomadura, Kribbella, Nocardia, Kytococcus, Leifsonia, Microbacterium, Micromonospora, Mobilicoccus, Mycobacterium, Nocardiopsis, Piscicoccus, and Pseudonocardia. The genus Streptomyces was most prevalent whereas rare genera, Mobilicoccus and Piscicoccus, were reported for the first time to occur as plant endophytes. PCR screening of polyketide synthase genes (PKS-I and PKS-II) and nonribosomal peptide synthetase genes (NRPS), along with antimicrobial assays against a set of bacterial and fungal pathogens, showed that endophytic actinomycetes associated with tea plants have a high potential for producing antimicrobial metabolites. Furthermore, indole acetic acid (IAA) production and 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase activities were recorded in 93.5% and 21.7% of all isolates, respectively. Overall, these results indicate that endophytic actinomycetes from tea plants represent a valuable source of bioactive metabolites with antibacterial, antifungal, and plant-growth-promoting properties.
Highlights
Actinobacteria, which are characterized by high G+C DNA content and filamentous growth, constitute one of the largest bacterial phyla and are ubiquitously found in aquatic and terrestrial habitats [1]
A total of 15 tea cultivars were sampled from seven collection sites in Fujian province, China
46 endophytic actinobacteria were isolated from 20 tissue samples of nine cultivars based on colony morphology and were further confirmed by 16S rRNA gene sequencing (Table 2)
Summary
Actinobacteria, which are characterized by high G+C DNA content and filamentous growth, constitute one of the largest bacterial phyla and are ubiquitously found in aquatic and terrestrial habitats [1]. Actinomycetes thrive in diverse ecological systems as a result of physiological versatility [2]. This group of microorganisms, Streptomyces in particular, are renowned for their abilities to produce a multitude of natural products with immense structural and biological diversities, many of which have applications in biotechnology, medicine, and agriculture [1, 3]. In recent decades, exhausted efforts to screen soil actinomycetes for new bioactive metabolites for clinical use have met with only limited success, with repeated isolation of known compounds becoming a major issue [4]. As a result, bioprospecting actinobacteria from previously underexplored territories, such as marine sediments, hydrothermal vents, desert soils, plants, and insects, has been proposed as an important strategy to replenish the drug pipeline [4, 5]
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