Bacteria on leaves

Earth’s green foliage serves as a monumental energy source, generating oxygen and organic matter essential for all life, including microbes. A single square centimeter of leaf surface can harbor millions of bacterial cells, amounting to a septillion globally. These bacteria have significant potential to influence plant health, growth, and ecosystem functioning.

Unraveling the factors that shape microbial communities is a crucial pursuit in microbial ecology. However, our understanding of leaf-associated bacteria has been predominantly derived from studies conducted at small spatial scales, leaving us with limited knowledge about the geographic patterns and drivers that influence leaf-bacteria associations.

Based on a biogeographic survey of leaf bacteria on 329 tree species across 10 forest sites along a broad latitudinal gradient in China (thanks to many collaborators), we for the first time provided quantitative and process-oriented understanding on the biogeography of leaf-associated bacteria.

We found that, similar to macroorganisms, leaf bacteria exhibit biogeographic patterns, with diversity and composition varying across latitudes. These patterns are likely driven by the selective forces imposed by hosts and climates, while dispersal limitation appears to play a less significant role.

Of particular interest, we observed an intriguing trend: the relative importance of host selection in shaping local bacterial assemblages increased with latitude, suggesting that bacterial responses to host changes may differ across latitudes.

Additionally, our investigation revealed that leaf bacteria specializing in specific hosts tend to possess smaller genome sizes and lower GC content compared to host-generalized bacteria, indicative of genome streamlining in host specialization.

Furthermore, we discovered that bacteria associated with locally abundant host species exhibit wider distribution ranges and lower levels of host specialization compared to those associated with rare host species. This implies that bacterial horizontal transmission may hinder the development of host specialization.

The importance of genome evolution and ecological dispersal in driving host specialization has captured my attention, motivating me to further explore the genetic basis of host specialization and how genome evolution intertwines with ecological processes to shape plant-bacteria interactions.