Abstract
Endoparasitic root-knot (Meloidogyne spp.) and lesion (Pratylenchus spp.) nematodes cause considerable damage in agriculture. Before they invade roots to complete their life cycle, soil microbes can attach to their cuticle or surface coat and antagonize the nematode directly or by induction of host plant defenses. We investigated whether the nematode-associated microbiome in soil differs between infective stages of Meloidogyne incognita and Pratylenchus penetrans, and whether it is affected by variation in the composition of microbial communities among soils. Nematodes were incubated in suspensions of five organically and two integrated horticultural production soils, recovered by sieving and analyzed for attached bacteria and fungi after washing off loosely adhering microbes. Significant effects of the soil type and nematode species on nematode-associated fungi and bacteria were revealed as analyzed by community profiling using denaturing gradient gel electrophoresis. Attached microbes represented a small specific subset of the soil microbiome. Two organic soils had very similar bacterial and fungal community profiles, but one of them was strongly suppressive towards root-knot nematodes. They were selected for deep amplicon sequencing of bacterial 16S rRNA genes and fungal ITS. Significant differences among the microbiomes associated with the two species in both soils suggested specific surface epitopes. Among the 28 detected bacterial classes, Betaproteobacteria, Bacilli and Actinobacteria were the most abundant. The most frequently detected fungal genera were Malassezia, Aspergillus and Cladosporium. Attached microbiomes did not statistically differ between these two soils. However, Malassezia globosa and four fungal species of the family Plectosphaerellaceae, and the bacterium Neorhizobium galegae were strongly enriched on M. incognita in the suppressive soil. In conclusion, the highly specific attachment of microbes to infective stages of phytonematodes in soil suggested an ecological role of this association and might be involved in soil suppressiveness towards them.
Highlights
Nematodes are exposed to a myriad of microorganisms in the soil
Microbes associated with infective stages of nematodes in seven horticultural soils analyzed by PCR-DGGE
The fungal and bacterial communities associated with M. incognita or P. penetrans in seven soils were analyzed by PCR-DGGE profiling of fungal ITS2 fragments and bacterial 16S rRNA genes
Summary
Among the most devastating plant-parasitic groups are root-knot (Meloidogyne spp.) and root-lesion nematodes (Pratylenchus spp.). Pratylenchus spp. are migratory endoparasitic nematodes that cause the formation of root lesions as all infective stages move and feed inside the roots. J2 of Meloidogyne and all life stages of Pratylenchus migrate through the soil. Many attempts have been engaged to successfully control plant-parasitic nematodes This includes the biological control by soil-borne microorganisms. A soil suppressive to Pratylenchus thornei was reported, albeit no microorganisms contributing to the suppressiveness were identified [2]. Egg parasites of phytonematodes acting against cyst-forming nematodes [3, 4] or Meloidogyne [5,6,7,8,9] have been suggested to contribute to soil suppressiveness. The correlation between the abundance of egg parasites and soil suppressiveness often is rather weak [9, 10]
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