Metabolic potential of endophytic bacteria

Abstract

The bacterial endophytic microbiome promotes plant growth and health and beneficial effects are in many cases mediated and characterized by metabolic interactions. Recent advances have been made in regard to metabolite production by plant microsymbionts showing that they may produce a range of different types of metabolites. These substances play a role in defense and competition, but may also be needed for specific interaction and communication with the plant host. Furthermore, few examples of bilateral metabolite production are known and endophytes may modulate plant metabolite synthesis as well. We have just started to understand such metabolic interactions between plants and endophytes, however, further research is needed to more efficiently make use of beneficial plant-microbe interactions and to reduce pathogen infestation as well as to reveal novel bioactive substances of commercial interest.

Graphical abstract

Figure 1

Metabolites of plant associated bacteria. Sespenine is derived from indolosesquiterpenes found in an endophyte (Streptomyces sp.) of mangrove trees. Spoxazomicins from an orchid endophyte (Streptosporangium oxazolinicum) with structural similarities to pyochelin, a siderophore from Pseudomonas aeruginosa, serobactin A, a siderophore from the grass endophyte Herbaspirillum seropedicae. Valienamine, as illustration of aminocyclitols, which might be produced by the endophytic C. Burkholderia kirkii. Pavettamine is the active toxic principle of South African Rubiaceae, where endophytic Burkholderia spp. are crucial for the biosynthesis in planta. The partial structure of thanamycine has been elucidated without isolation from bacterial colonies. Coronatine as an example of a plant hormone acting agent from the plant pathogenic Pseudomonas syringae and the structure of the actual plant hormone (+)-7-iso-jasmonoyl-l-isoleucine.

Figure 2

Schematic overview showing the different types of plant-endophyte interactions leading to the synthesis of metabolites, which are in many cases not produced by the macro- or microsymbiont alone or in different quantities. Furthermore, the different known functions of endophyte-associated metabolites are presented.

Figure 3

Average numbers of metabolite gene clusters predicted by antiSmash 2.0 [40^••]. The numbers are the mean of 6 Pseudomonas fluorescens (plant-associated) strains, 5 P. syringae (plant pathogens) and 9 P. putida strains (no association with plants) and contain all fully sequenced and published genomes in the given category.