A review: what is the spermosphere and how can it be studied?

The spermosphere is the zone surrounding seeds where interactions between the soil, microbial communities and germinating seeds take place. The concept of the spermosphere is usually only applied during germination sensu stricto. Despite the transient nature of this very small zone of soil around the germinating seed, the microbial activities which occur there may have long-lasting impacts on plants. The spermosphere is indirectly characterized by either (i) seed exudates, which could be inhibitors or stimulators of micro-organism growth or (ii) the composition of the microbiome on and around the germinating seeds. The microbial communities present in the spermosphere directly reflect that of the germination medium or are host-dependent and influenced quantitatively and qualitatively by host exudates. Despite its strong impact on the future development of plants, the spermosphere remains little studied. This can be explained by the technical difficulties related to characterizing this concept due to its short duration, small size and biomass, and the number and complexity of the interactions that take place. However, recent technical methods, such as metabolite profiling, combining phenotypic methods with DNA- and RNA-based methods, could be used to investigate seed exudates, microbial communities and their interactions with the soil environment.