by Mechas Zambrano and Roberto
Where do the microbes that colonize newly emerging ecosystems come from? Plants are ideal systems to address this fundamental question in microbial ecology. When a seed germinates, new ecosystems emerge as the plant grows and acquires its microbiota. It is useful to think of a growing plant as generating at least two very different ecosystems, the rhizosphere (the narrow region of soil directly influenced by the root) and the phyllosphere (the total above-ground surface of a plant). For the rhizosphere, its component microbes are almost completely recruited from surrounding soil. In contrast, the phyllosphere – by its very nature of being surrounded by air – appears to be colonized from microbes from many different locations: the seed, the soil, and airborne microbes. Recently, investigators identified yet another source of phyllosphere-colonizing microbes, rain. This might seem like a no-brainer, by now we know that microbes are everywhere, and it should not be a surprise that raindrops are laden with microbes. But to demonstrate that some of the microbes in the rain indeed ended up as phyllosphere residents required moving from field work to lab work.
Marco Mechan‑Llontop and colleagues, from Boris Vinatzer's group at Virginia Tech, used tomato plants as a model system to test whether rain-borne microbes colonized the phyllosphere. Prior work showed that rainfall affected the overall phyllosphere community composition, but there was no direct proof that the rain-borne bacteria colonized the plant's surface. To address this question, Mechan‑Llontop et al. did microbial community analyses, using 16S rRNA amplicon sequencing, to compare microbes present in rain to microbes present in lab-grown plants treated with concentrated rain, filter-sterilized rain and sterile water. They found over one hundred taxa (actually, "OTUs" operational taxonomic units) present in rain that were significantly increased on the plants treated with concentrated rain. There were no OTUs that increased when the plants were treated with filter-sterilized rain or sterile water. Importantly, in field experiments they also showed similar increases in tomatoes grown outdoors and exposed to natural rainfall but not in tomatoes grown in rain-protected commercial greenhouses. In short, some rain-borne microbes did become permanent residents of tomatoes. Something to think about next time you consider growing tomatoes or the next time you get soaked by the rain.
Mechas (María Mercedes) Zambrano is a microbiologist and the Scientific Director of Corpogen Institute in Bogotá, Colombia.