Plant microbiota affects seed transmission of phytopathogenic microorganisms.

Seeds, defined, in this Opinion piece, as sexually derived structures of spermatophytes, are involved in the vertical transmission of microorganisms from one plant generation to another and consequently act as a primary source of inoculum for crops. A variety of microorganisms, such as plant growth‐promoting agents and plant or animal pathogens, have been isolated from the seed surfaces or the seed tissues of various plant species. These seed‐associated microorganisms could represent transient colonizers of the seed habitat or, alternatively, be transmitted to the plantlet and influence seedling‐associated microbial assemblages. Therefore, we should differentiate between seed‐borne and seed‐transmitted microorganisms. Seed transmission of microorganisms can have various detrimental effects on seed physiological quality, including seed discoloration or decrease in germination rate. Moreover, the sanitary quality of seed can also be impacted by the transmission of pathogens through contamination of seeds with mycotoxins or Shiga toxins. From an epidemiological point of view, seed transmission of phytopathogenic microorganisms represents an important means of pathogen dispersion and is therefore significant in the emergence of disease. Even a low level of seed contamination is sufficient to lead to the efficient colonization of plants by bacterial pathogens (Darrasse et al., 2007). Moreover, seed transmission of plant‐pathogenic agents can occur on non‐host plants. For instance, the bacterial pathogen of Brassicas, Xanthomonas campestris pv. campestris (Xcc), is efficiently transmitted from the mother plant to seeds of bean and from bean seeds to seedlings (Darrasse et al., 2010; Darsonval et al., 2008). This non‐host carriage could serve as a potential reservoir of pathogenic agents in new planting areas and may contribute to an increase in the gene pool available for recombination. For all of these reasons, exploring plant–pathogen interactions during the plant reproductive stage is of interest for the control of plant disease. Vertical transmission guarantees the persistence of a microorganism from parents to offspring. To manage vertically transmitted pathogens, we should either exclude pathogens from the mother plant or treat the seed to eliminate them. Chemical treatments applied to seed‐producing crops or as seed treatments are efficient methods for the control of fungal pathogens. In contrast, these chemical‐based methods are unsatisfactory for bacterial plant pathogens. Therefore, the control of seed‐transmitted bacterial pathogens relies either on alternative seed treatments, such as thermotherapy, or on prophylactic measures performed on crops and seed samples. Nevertheless, none of these strategies guarantees pathogen‐free seeds. Biological control is a promising option, but has been historically hampered by the variation in efficacy of the microbial strains employed, which could partly be explained by the empirical selection of biocontrol agents. The survey of seed‐associated microbial assemblages presented in this Opinion piece should provide novel options for the selection of biocontrol agents.