Structure vs. chemistry: Alternate mechanisms for controlling leaf microbiomes

Background

The analysis of phyllosphere microbiomes traditionally relied on DNA extracted from whole leaves. To investigate the microbial communities on the adaxial (upper) and abaxial (lower) leaf surfaces, swabs were collected from both surfaces of two garden plants, Rhapis excelsa and Cordyline fruticosa.

Results

The abaxial surface of Rhapis excelsa and Cordyline fruticosa had fewer bacteria in contrast to their adaxial counterpart. This observation was consistent between noon and midnight and across five different locations. Co-occurrence network analysis further showed that bacteria are found almost exclusively on the adaxial surface while only a small group of leaf blotch fungi survive on the abaxial surface. There are higher densities of stomata on the abaxial surface and these openings are vulnerable ports of entry into the plant host. While one might argue about the settling of dust particles and microorganisms on the adaxial surface, we detected differences in reactive chemical activities and microstructures between the adaxial and abaxial surfaces. Our results further suggest that both plant species deploy different defence strategies to deter invading pathogens on the abaxial surface.

Conclusions

We hypothesize that chemical and mechanical defence strategies evolved independently for harnessing and controlling phyllosphere microbiomes. Our findings have also advanced our understanding that the abaxial leaf surface is distinct from the adaxial surface and that the reduced microbial diversity is likely a consequence of plant-microbe interactions.