Deciphering bacteria associated with a pre-parasitic stage of the root-knot nematode Meloidogyne hapla in nemato-suppressive and nemato-conducive soils

Nematode-suppressive soils are characterized by the ability of soil microbial communities to reduce populations of plant-parasitic nematodes (PPN) either directly or by inducing systemic resistance in plants. Various microorganisms have been recognized as antagonists of PPN in suppressive soils using culture-dependent and culture-independent methods. However, the associations that PPN form with microorganisms in nematode-conducive soils have been poorly studied. Here we drenched tomato rhizospheres with microbial suspensions from nine different soils and followed their effects on plant growth and root invasion of the infective second-stage juveniles (J2) of the northern root-knot nematode species Meloidogyne hapla. Based on the number of invaded J2, four soils were determined as nematode-suppressive, while five soils were categorized as nematode-conducive. To reveal bacteria attached to the J2 cuticle in soils with varying suppressiveness, we incubated J2 in suspensions from three suppressive and three conducive soils, and analyzed J2-attached bacteria using amplicon sequencing of the V3-V4 region of 16S rRNA gene. Our results suggest that the soil origin had a major effect on the composition of J2-attached bacteria, while the highest bacterial abundance and richness were observed on J2 from two suppressive soils. In addition, the highest number of indicator amplicon sequence variants (ASVs) was associated with J2 in two suppressive soils, but they had a very distinct bacterial profile. Further studies are needed to resolve the complexity of nemato-microbial interactions in soil and to determine the exact function of nematode-attached microorganisms in suppressive and conducive soils and their role in nematode suppression and protection.