A Hypothetical Bottleneck in the Plant Microbiome

Area of Science:

• Plant biology
• Microbiology
• Ecology

Background:

• The plant microbiome, crucial for plant health, may be shaped by microbial interactions within seeds.
• Optimal defense theory predicts strong defenses in developing seeds, potentially leading to microbial exclusion.
• Previous studies suggested single microbial endophytes per seed in *Centaurea stoebe*.

Purpose of the Study:

• To investigate if individual plant seeds act as a bottleneck for microbial endophytes across diverse plant species.
• To test the prevalence of single-culturable endophytes per seed.
• To provide evidence for further investigation into the Primary Symbiont Hypothesis (PSH).

Main Methods:

• Conducted an extensive survey of 98 plant species from 39 families.
• Surface-sterilized individual seeds.
• Cultured for PDA-culturable bacterial and fungal endophytes from each seed.

Main Results:

• Every species surveyed consistently hosted only one PDA-culturable bacterial or fungal endophyte per seed, or none.
• This single-endophyte pattern was observed across all tested plant species.
• The findings suggest a significant bottleneck effect at the seed level for culturable endophytes.

Conclusions:

• Individual plant seeds act as a major bottleneck for microbial endophyte diversity.
• Exclusionary interactions likely govern the microbial communities within seeds.
• This bottleneck supports the Primary Symbiont Hypothesis, indicating dominant seed endophytes may significantly impact seedling traits.