Hereditary Leaf Symbiosis in Tropical Plants: Evolution and Function of Vertically Transmitted Bacterial Symbionts

The presence of bacteria within leaf galls of some tropical plant species of the Rubiaceae and Primulaceae families was first described in the early 1900s and later recognized as signs of permanent associations, with the presence and morphology of the bacteria-hosting structures even serving as taxonomic markers. Until recently, the inability to culture the bacterial symbionts and the difficulty of obtaining bacteria-free plants reliably have limited the understanding of the functions of these symbioses and the mechanisms enabling transmission across generations. The development of culture-free methods has made it possible to identify bacterial endosymbionts and to realize that these symbioses are highly specific, with each host plant interacting with only one bacterial species. Here, we review the current knowledge of these highly specific leaf symbiotic associations, focusing on recent experimental evidence. We explore hypotheses regarding the mechanisms enabling these highly specific interactions. [Figure: see text] Copyright © 2025 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.