Phytohormones selectively affect plant parasitic nematodes associated with Arabidopsis roots.

Phytohormones may affect plant-nematode interactions directly as chemo-attractants or -repellents, or indirectly through the root-associated microbiome or through host defense mechanisms. However, the exact roles of phytohormones in these complex plant-soil-nematode interactions are not well understood. We used Arabidopsis thaliana mutants impaired in phytohormone synthesis or sensitivity to elucidate their role in root-nematode interactions. As root-associated microorganisms may modulate these interactions, we explored correlations between the relative abundances of root-associated nematodes, and bacteria and fungi using amplicon sequencing. We found distinct shifts in relative abundances of a range of nematode taxa in the A. thaliana phytohormone mutants. The root knot nematode Meloidogyne hapla, a sedentary endoparasitic species that is in intimate contact with the host, was highly enriched in JA-, SA- and SL-impaired lines, and in an ET-insensitive line. Positive or negative correlations between specific microbial and nematode taxa were observed, but, as the inference of causal relationships between microbiome responses and effects on nematode communities is premature, this should be studied in detail in future studies. In conclusion, genetic derailment of hormonal balances generally rendered plants vulnerable to endoparasitic nematode attack. Furthermore, preliminary data suggest that this effect may be partially modulated by the associated microbiome.

Benzoxazinoids selectively affect maize root-associated nematode taxa.

Although the effects of plant secondary metabolites on plant defence have been studied for decades, the exact roles of secondary metabolites in shaping plant-associated microbial and nematode communities remain elusive. We evaluated the effects of benzoxazinoids, a group of secondary metabolites present in several cereals, on root-associated nematodes. We employed 18S rRNA metabarcoding to compare maize root-associated nematode communities in a bx1 knockout maize line impaired in benzoxazinoid synthesis and in its parental wild type. Both genotype and plant age affected the composition of the nematode community in the roots, and the effects of benzoxazinoids on nematode communities were stronger in the roots than in the rhizosphere. Differential abundance analysis and quantitative PCR showed that the root lesion nematode Pratylenchus neglectus was enriched in the bx1 mutant line, while another root lesion nematode, Pratylenchus crenatus, was reduced. Correlation analysis showed that benzoxazinoid concentrations in maize roots mostly correlated negatively with the relative abundance of nematode sequence reads. However, positive correlations between benzoxazinoids and nematode taxa, including several plant-parasitic nematodes, were also identified. Our detailed nematode community analysis suggests differential and selective effects of benzoxazinoids on soil nematodes depending on both the nematode species and the benzoxazinoid compound.

Activity of root-knot nematodes associated with composition of a nematode-attached microbiome and the surrounding soil microbiota.

We investigated if activity of the pre-infective juveniles (J2s) of root-knot nematodes is linked to the recruitment of a specific microbiome on the nematode surface and/or to the composition of the surrounding microbiota. For this, we determined the J2 activity (active vs. non-motile, which referred to dead and immobile J2s) upon a 3-day incubation in soil suspensions and studied the composition of bacteria, protists, and fungi present on the nematode surface and in the suspensions using amplicon sequencing of the 16S/18S rRNA genes, and ITS region. We also amended suspensions with Pseudomonas protegens strain CHA0 to study its effects on J2 activity and microbial composition. The J2 activity was suppressed in soil suspensions, but increased when suspensions were amended with P. protegens CHA0. The active and non-motile J2s differed in the composition of surface-attached bacteria, which was altered by the presence of P. protegens CHA0 in the soil suspensions. The bacterial genera Algoriphagus, Pedobacter, and Bdellovibrio were enriched on active J2s and may have protected the J2s against antagonists. The incubation time appeared short for attachment of fungi and protists. Altogether, our study is a step forward in disentangling the complex nematode-microbe interactions in soil for more successful nematode control.

Genetic disruption of Arabidopsis secondary metabolite synthesis leads to microbiome-mediated modulation of nematode invasion.

In-depth understanding of metabolite-mediated plant-nematode interactions can guide us towards novel nematode management strategies. To improve our understanding of the effects of secondary metabolites on soil nematode communities, we grew Arabidopsis thaliana genetically altered in glucosinolate, camalexin, or flavonoid synthesis pathways, and analyzed their root-associated nematode communities using metabarcoding. To test for any modulating effects of the associated microbiota on the nematode responses, we characterized the bacterial and fungal communities. Finally, as a proxy of microbiome-modulating effects on nematode invasion, we isolated the root-associated microbiomes from the mutants and tested their effect on the ability of the plant parasitic nematode Meloidogyne incognita to penetrate tomato roots. Most mutants had altered relative abundances of several nematode taxa with stronger effects on the plant parasitic Meloidogyne hapla than on other root feeding taxa. This probably reflects that M. hapla invades and remains embedded within root tissues and is thus intimately associated with the host. When transferred to tomato, microbiomes from the flavonoid over-producing pap1-D enhanced M. incognita root-invasion, whereas microbiomes from flavonoid-deficient mutants reduced invasion. This suggests microbiome-mediated effect of flavonoids on Meloidogyne infectivity plausibly mediated by the alteration of the abundances of specific microbial taxa in the transferred microbiomes, although we could not conclusively pinpoint such causative microbial taxa.

Pseudomonas fluorescens promote photosynthesis, carbon fixation and cadmium phytoremediation of hyperaccumulator Sedum alfredii.

Plant growth-promoting bacteria (PGPB) can promote photosynthesis and biomass production of hyperaccumulators, achieving enhanced phytoremediation efficiency of cadmium (Cd). A better understanding of the mechanisms controlling photosynthesis of hyperaccumulating plants by PGPB is necessary for developing strategies that promote the practical phytoextraction of Cd-polluted soils. In this study, chlorophyll fluorescence, gas exchange, and transcriptome sequencing were conducted to evaluate the physiological and transcriptional changes on photosynthesis and carbon fixation in hyperaccumulator Sedum alfredii after inoculation with PGPB Pseudomonas fluorescens. The results showed that bacterial inoculation significantly enhanced maximum quantum yield of PS II (Fv/Fm), effective quantum yield of PS II (ΦPSII), photochemical quenching (qP) and chlorophyll concentration, while reduced non-photochemical quenching (NPQ) of S. alfredii. Further, inoculation resulted in an increased net photosynthetic rates (Pn), intercellular CO2 concentration (Ci), transpiration rate (Tr) and stomatal conductance (Gs) of the studied plant. At the transcriptional level, 70 photosynthetic genes and 42 C4-pathway carbon fixation related genes were significantly up-regulated in response to inoculation, which could be the reason for enhanced photosynthesis and dry biomass. To sum up, this P. fluorescens strain can simultaneously promote growth and Cd uptake of S. alfredii, which can be a promising bacterial agent applied to Cd phytoremediation practices.

Impacts of Root Metabolites on Soil Nematodes.

Plant parasitic nematodes cause significant crop damage globally. Currently, many nematicides have been banned or are being phased out in Europe and other parts of the world because of environmental and human health concerns. Therefore, we need to focus on sustainable and alternative methods of nematode control to protect crops. Plant roots contain and release a wide range of bioactive secondary metabolites, many of which are known defense compounds. Hence, profound understanding of the root mediated interactions between plants and plant parasitic nematodes may contribute to efficient control and management of pest nematodes. In this review, we have compiled literature that documents effects of root metabolites on plant parasitic nematodes. These chemical compounds act as either nematode attractants, repellents, hatching stimulants or inhibitors. We have summarized the few studies that describe how root metabolites regulate the expression of nematode genes. As non-herbivorous nematodes contribute to decomposition, nutrient mineralization, microbial community structuring and control of herbivorous insect larvae, we also review the impact of plant metabolites on these non-target organisms.