Microbiota and functional analyses of nitrogen-fixing bacteria in root-knot nematode parasitism of plants.

BACKGROUND: Root-knot nematodes (RKN) are among the most important root-damaging plant-parasitic nematodes, causing severe crop losses worldwide. The plant rhizosphere and root endosphere contain rich and diverse bacterial communities. However, little is known about how RKN and root bacteria interact to impact parasitism and plant health. Determining the keystone microbial taxa and their functional contributions to plant health and RKN development is important for understanding RKN parasitism and developing efficient biological control strategies in agriculture. RESULTS: The analyses of rhizosphere and root endosphere microbiota of plants with and without RKN showed that host species, developmental stage, ecological niche, and nematode parasitism, as well as most of their interactions, contributed significantly to variations in root-associated microbiota. Compared with healthy tomato plants at different developmental stages, significant enrichments of bacteria belonging to Rhizobiales, Betaproteobacteriales, and Rhodobacterales were observed in the endophytic microbiota of nematode-parasitized root samples. Functional pathways related to bacterial pathogenesis and biological nitrogen fixation were significantly enriched in nematode-parasitized plants. In addition, we observed significant enrichments of the nifH gene and NifH protein, the key gene/enzyme involved in biological nitrogen fixation, within nematode-parasitized roots, consistent with a potential functional contribution of nitrogen-fixing bacteria to nematode parasitism. Data from a further assay showed that soil nitrogen amendment could reduce both endophytic nitrogen-fixing bacteria and RKN prevalence and galling in tomato plants. CONCLUSIONS: Results demonstrated that (1) community variation and assembly of root endophytic microbiota were significantly affected by RKN parasitism; (2) a taxonomic and functional association was found for endophytic nitrogen-fixing bacteria and nematode parasitism; and (3) the change of nitrogen-fixing bacterial communities through the addition of nitrogen fertilizers could affect the occurrence of RKN. Our results provide new insights into interactions among endophytic microbiota, RKN, and plants, contributing to the potential development of novel management strategies against RKN. Video Abstract.

Differential proteomics reveals main determinants for the improved pectinase activity in UV-mutagenized Aspergillus niger strain.

OBJECTIVES: To reveal the potential mechanism and key determinants that contributed to the improved pectinase activity in Aspergillus niger mutant EIMU2, which was previously obtained by UV-mutagenesis from the wild-type A. niger EIM-6. RESULTS: Proteomic analysis for Aspergillus niger EIMU2 by two-dimensional electrophoresis demonstrated that mutant EIMU2 harbored a multiple enzyme system for the degradation of pectin, mainly constituting by main-chain-cleaving enzymes polygalacturonase, pectate lyase, pectinesterase, and some accessory enzymes rhamnogalacturonan lyase and arabinofuranosidase. Further quantitatively differential proteomic analysis revealed that the quantities of four proteins, pectinesterase, rhamnogalacturonan lyase A, DNA-directed RNA polymerase A, and a hypothetical protein in strain EIMU2 were much higher than those in EIM-6. PCR amplification, sequencing and alignment analysis of genes for the two main members of pectin-degrading enzymes, pectate lyase and polygalacturonase showed that their sequences were completely consistent in A. niger EIM-6 and mutant EIMU2. CONCLUSIONS: The result demonstrated that the improved pectinase activity by UV-mutagenesis in A. niger EIMU2 was probably contributed to the up-regulated expression of rhamnogalacturonan lyase, or pectinesterase, which resulted in the optimization of synergy amongst different components of pectin-degrading enzymes.