For billions of years life on Earth was restricted to aquatic environments, the oceans, seas, rivers and lakes. Then 450 million years ago the first plants colonized land, evolving in the process multiple types of beneficial relationships with microbes in the soil.
These relationships, known as symbioses, allow plants to access additional nutrients. The most intimate among them are intracellular symbioses that result in the accommodation of microbes inside plant cells.
A study published in Nature Plants, describes the discovery of a common genetic basis for all these symbioses.
It is hypothesized that the colonization of land by plants was made possible through a type of symbiosis that plants form with a group of fungi called mycorrhizal fungi. Even today 80% of plants we find on land can form this mycorrhizal symbiosis. Plants have also evolved the ability to engage in intracellular symbiosis with a large diversity of other microbes.
Over the past two decades, studies on mycorrhizal symbiosis and another type of symbiosis, formed by legumes such as peas and beans with soil bacteria, have allowed the identification of a dozen plant genes that are required for the recognition of beneficial microbes and their accommodation inside plant cells. By contrast, other types of intracellular symbioses have been poorly studied.
To address this, the team compared the genomes of nearly 400 plant species to understand what is unique to those that can form intracellular symbioses. Analyzing 271 transcriptomes and 116 plant genomes spanning the entire land-plant diversity, the authors demonstrate that a common symbiosis signalling pathway co-evolved with intracellular endosymbioses, from the ancestral arbuscular mycorrhiza to the more recent ericoid and orchid mycorrhizae in angiosperms and ericoid-like associations of bryophytes. By contrast, species forming exclusively extracellular symbioses, such as ectomycorrhizae, and those forming associations with cyanobacteria, have lost this signalling pathway.
Surprisingly, they discovered that three genes are shared exclusively by plants forming intracellular symbiosis and lost in plants unable to form this type of beneficial relationship as published in Nature Plants.
"Our study demonstrates that diverse types of intracellular symbioses that plants form with different symbiotic partners are built on top of a conserved genetic program." said the, lead author of the study.
"By demonstrating that different plant symbioses share a common genetic basis, our ambitious goal has become more realistic," says the lead.
Three genes implicated in plant symbioses
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