FROM SEED ENDOPHYTES TO PLANT MICROBIOMES: SEED-BORN BACTERIA THAT COLONIZE AERIAL TISSUES IN ALFALFA PLANTS

ZUBER, N; LAGARES, A.; ERDOZAIN, S; PAGNUTTI, A; LOPEZ, JL.; LOZANO, M.J.

Virtual

Congreso; XV CONGRESO ARGENTINO DE MICROBIOLOGÍA GENERAL SAMIGE; 2020

SAMIGE

Plants are naturally colonized by diverse environmental microorganisms generating what are known as the plant microbiomes. Thus, plant-associated microorganisms colonize the outer and inner structures of roots and shoots (i.e. the endosphere) resulting in many cases in an enhanced plant growth and health. Currently, the vast diversity of microorganisms that colonize the endosphere are focus of many studies to understand the benefits and the biochemical basis of the associative life style. Interestingly, evidences from the last years have demonstrated that microorganisms are also able to colonize the inner seed structures through mechanisms that are not yet fully understood. Such community of seed-associated microorganisms―which are inherited to the progeny and so coevolve with the host plant―provide an initial inoculum to seedlings which is available immediately after germination. In a previous report López et al. 2018 have described the species of culturable seed endophytes that are present in five different varieties of Medicago sativa (alfalfa), and found bacteria which belong to 35 distinct genera from 4 different phyla (Actinobacteria, Proteobacteria, Firmicutes, and Bacteroidetes). The long-lasting coevolution between seeds and their associated microorganisms make them a biosafe source of microbial diversity for practical applications.Aiming at characterizing the initial colonization of young plants by their seed endophytes, we performed experiments where different microbial consortia (bacterial mixes) were inoculated on surface-sterilized seed. The tissue tropism of each of the bacterial species used in the inoculum was then evaluated 14 days post inoculation. Bacterial species were identified by using MALDI-TOF mass spectrometry, supported when necessary by PCR-based DNA fingerprinting. The use of multivalent inocula conformed with isolates from our collection of seed endophytes demonstrated that species that belong to the genera Rhodococcus, Microbacterium, Arthrobacter, Cronobacter, Enterobacter, Exiguobacterium, Pantoea, Pseudomonas, Rhizobium, and Stenotrophomonas were all able to colonize alfalfa stems and leaves. The analysis of individual plants revealed, however, that only between 1-to-6 genera were usually detected within a single plant, with at least one of them belonging to the Enterobacteriaceae family (Cronobacter, Enterobacter, Pantoea). Fourteen of the genera that we had previously found in alfalfa seeds (López et al., 2018) could not be isolated from the aerial plant tissues under the experimental conditions used. The information regarding isolates? activities that are compatible with plant growth promotion phenotypes, together with the colonization capacity described here (i.e. the tissue tropism) constitute both the basis toward a rational design of novel consortial bioinoculants based on the use of biosafe seed-borne microbial germplasm.