Plant genetic bases explaining microbiota diversity shed light into a novel holobiont generalist gene theory

Plants as animals are strictly associated with a cortege of microbial communities influencing their health, fitness and evolution. Therefore, scientists refer to all living organisms as holobionts; complex genetic units that coevolve simultaneously. This is what has been recently proposed as the hologenome theory of evolution. This exciting and attractive theory has important implications on animal and plant health; however, it still needs consistent proof to be validated. Indeed, holobionts are still poorly studied in their natural habitats where coevolution and natural selective processes occur. Compared to animals and crops, wild plant populations are an excellent and unique model to explore the hologenome theory. These sessile holobionts have strictly coevolved with their microbiota for decades and natural selection and adaptive processes acting on wild plants are likely to regulate the plant-microbe interactions. Here we conducted for the first time a microbiota survey, plant genome sequencing and Genome-Environmental Analysis (GEA) of 26 natural populations of the non-model plant species Brassica rapa. We collected plants over two seasons in Italy and France, and analyzed the microbiota on two plant compartments (root and rhizosphere). We identified that plant compartment and season modulate B. rapa microbiota. More importantly, when conducting GEA we evidenced neat peaks of association correlating with both fungal and bacterial microbiota. Surprisingly, we found 13 common genes between fungal and bacterial diversity descriptors that we referred under the name of Holobiont Generalist Genes (HGG). These genes might strongly regulate the diversity and composition of plant microbiota at the inter-kingdom level. Significance StatementThe novel hologenome concept claims that hosts and their associated microbes (considered as holobionts) are a unique evolutionary unit on which natural selection acts. Thus, the hologenome theory assumes that hosts and microbiomes simultaneously coevolve. This novel vision of universal evolutionary entities is promising for both animal and plant health purposes. However, it is still quietly controversial as it suffers from a lack of tangible evidence. How can we enrich the debate on holobionts? How can we translate this concept in discoveries that can change farming practices? Our study is filling the gaps of the hologenome theory by showing that certain genes under natural selection and regulating plant microbiota are generalist in response to fungal and bacterial communities.