Rainfall and a Phyllosphere's Microbiota

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.
 

Figure 1. Tomato plant phyllosphere. Source. Fron­tis­piece.

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.