Morphological, Physiological, and Taxonomic Characterization of Actinobacterial Isolates Living as Endophytes of Cacao Pods and Cacao Seeds

Abstract

Vascular plants are commonly colonized by endophytic actinobacteria. However, very little is known about the relationship between these microorganisms and cacao fruits. In order to determine the physiological and taxonomic relationships between the members of this community, actinobacteria were isolated from cacao fruits and seeds. Among the 49 isolates recovered, 11 morphologically distinct isolates were selected for further characterization. Sequencing of the 16S rRNA gene allowed the partition of the selected isolates into three phylogenetic clades. Most of the selected endophytic isolates belonged to the Streptomyces violaceusniger clade. Physiological characterization was carried out and a similarity index was used to cluster the isolates. However, clustering based on physiological properties did not match phylogenetic lineages. Isolates were also characterized for traits commonly associated with plant growth-promoting bacteria, including antibiosis and auxin biosynthesis. All isolates exhibited resistance to geldanamycin, whereas only two isolates were shown to produce this antibiotic. Endophytes were inoculated on radish seedlings and most isolates were found to possess plant growth-promoting abilities. These endophytic actinobacteria inhibited the growth of various plant pathogenic fungi and/or bacteria. The present study showed that S. violaceusniger clade members represent a significant part of the actinobacterial community living as endophytes in cacao fruits and seeds. While several members of this clade are known to be geldanamycin producers and efficient biocontrol agents of plant diseases, we herein established the endophytic lifestyle of some of these microorganisms, demonstrating their potential as plant health agents.

<i>Streptomyces cameroonensis</i> sp. nov., a Geldanamycin Producer That Promotes <i>Theobroma cacao</i> Growth

“…1). Similar to several other actinobacteria isolated from the rhizosphere or plant tissues, JJY4 T exhibited the ability to promote plant growth (15, 26). …”

“…The observed systemic protective effect of this soil bacterium on cocoa plantlets suggests that JJY4 T stimulates systemic resistance in plants similar to other soil streptomycetes (24). We cannot exclude the direct effects of JJY4 T on P. megakarya because geldanamycin-producing actinobacterial isolates have been found to live as endophytes within cocoa (26). …”

“…Although actinobacteria displaying the characteristics of biocontrol agents and plant growth-promoting bacteria have been successfully isolated from epigeous plant tissues (26), the rhizosphere represents the main reservoir for these microorganisms (9). A large number of studies have reported the isolation of actinobacteria depicted as potential biocontrol agents in the literature.…”

Expression and Function of Different Guanine-Plus-Cytosine Content 16S rRNA Genes in Haloarcula hispanica at Different Temperatures

“…Bulk DNA was extracted from H. hispanica cells grown in Medium 307 with modifications (Tchinda et al, 2016). Briefly, the pelleted cells were lysed with lysozyme and proteinase K solution.…”

Recent advances in understanding Streptomyces

“…23, 24). Endophytic streptomycetes are also found in diverse non-medicinal dicot plants 25, 26 and in monocots including rice 22 , suggesting (but not proving) that such interactions may date back hundreds of millions of years. Metagenomic evidence of particularly exuberant diversity of soil streptomycetes at low latitudes 27, 28 might reflect greater plant and insect diversity in the tropics (along with some degree of specificity in interactions between Streptomyces and plants or insects).…”