Asymmetric coalescence of seed and soil microbiota

Seed is one primary habitat of plant microbiota but soil is considered until now as the main diversified reservoir for root microbiota that potentially affects plant growth. How seed microbiota persits and influences microbial communities when seeds encounter soil and during plant emergence and early growth is barely documented. Thus, the interchange events or coalescence of seed and soil microbiota and the influence of factors driving the microbial diversity in initial habitats on the structure of root and stem bacterial and fungal communities were explored. Four seed lots of two Brassica napus genotypes harbouring contrasted microbiota were sown in two soils with contrasted diversity. The seed microbiota diversity did not have significant effect whereas soil microbial diversity influenced the taxonomic structure of root and stem microbiota in both 7- and 14-day sampling. The main recruitment of microbiota came from soil but few microbial taxa were shared between seeds, roots and stems. Roots and stems favored the emergence of some rare taxa not detected in seeds and soil. The outcome of seed and soil microbiota coalescence is therefore strongly asymmetrical with a clear dominance of soil microbiota. The high transmission rate of rare seed-borne and sub-dominant soil-borne taxa to seedling also characterizes this coalescence.