The soil environment that natural populations of plants are exposed to is quite complex. Bacteria represent major players in the soil with cell numbers thought to approach 1 × 109 –1.5 × 1010 per gram of soil constituting over 4000 unique genomes. These bacteria participate heavily in nutrient cycling, as well as intimate crosstalk with sporophyte roots. The bacteria and roots communicate through both chemical and hormonal signals, which can directly initiate changes in plant behaviour and morphology. The crosstalk between the sporophyte root and bacteria and between sporophyte roots themselves are well studied. Less explored are the potential relationships between free-living gametophytes and soil bacteria.
In a recent article published in AoBP, Ganger et al. identify a soil bacterium and explore its effects on sexual and rhizoid development in the fern Ceratopteris richardii. The multicellular haploid generation of plants (gametophyte) is responsible for sperm and egg production. In C. richardii, gametophytes are free-living in the soil and may develop into either males or hermaphrodites. This developmental decision is not genetically programmed, but instead is environmentally determined. A pheromone released by hermaphrodites called antheridiogen induces individuals to develop as males. The presence of the bacterium Pseudomonas nitroreducens blocks male induction and results in more individuals developing as hermaphrodites. The bacterium also induces longer but fewer rhizoids to develop in both males and hermaphrodites.