NOM as Natural Xenobiotics. In: Rosario-ortiz F, editor. Advances in the Physicochemical Characterization of Dissolved Organic Matter: Impact on Natural and Engineered Systems. Washington: American Chemical Society; 2014. pp. 115-44. [DOI: 10.1021/bk-2014-1160.ch006]

Molecular level changes during suppression of Rhizoctonia solani growth by humic substances and relationships with chemical structure

Elucidation of the inhibitory effects of humic substances (HSs) on phytopathogenic fungi and the underlying molecular mechanisms are highly important for improved biocontrol. In this study, we investigated the growth suppression, morphological characteristics, transcriptomic sequence, and radical signals of Rhizoctonia solani following HS addition (50 mg/L). Through mycelial cultured experiment, mycelia growth of R. solani had been suppressed with HS addition, and the inhibition rate was 24.88 ± 0.11% compared to the control. Field emission-scanning electron microscopy showed increased and superimposed branching mycelial growth, with a shriveled appearance. RNA samples of R. solani cultured with or without HSs were both extracted to examine the sequence on molecular level by Illumina HiSeq sequencing platform. RNA sequencing analysis revealed 175 differentially expressed genes (DEGs; 111 upregulated and 64 downregulated) between the HSs treatment and control. The upregulated unigenes were annotated and significantly enriched to three molecular processes: vitamin B6 metabolism, ABC transporters, and glutathione metabolism, while the downregulated unigenes were annotated to carbohydrate metabolism, but not significantly enriched. Real time-quantitative polymerase chain reaction analysis showed that the unigenes related to hexokinase, glucose-6-phosphate isomerase, glutathione synthase, and glutathione reductase were significantly decreased (by 60.03%, 70.70%, 60.33%, and 57.59%, respectively), while those related to glutathione S-transferase were significantly increased (2.66-fold). The electron paramagnetic resonance spectra showed that HSs induced increased the intensity of radical signals of R. solani in a cultured system increased by 59.56% compared to CK (without HSs addition). Network analysis based on DEGs expression and the chemical structure of HSs revealed that the carbonyl moiety in HSs formed the most links with nodes of the DEGs (sum of the links of positive and negative effects = 70), implicating this structure as the active fraction responsible for the inhibitory effect. This study provides molecular and chemical evidence of the biofungicidal activity of HSs with the potential for practical application.

Contrasting cellular stress responses of Baikalian and Palearctic amphipods upon exposure to humic substances: environmental implications

The species-rich, endemic amphipod fauna of Lake Baikal does not overlap with the common Palearctic fauna; however, the underlying mechanisms for this are poorly understood. Considering that Palearctic lakes have a higher relative input of natural organic compounds with a dominance of humic substances (HSs) than Lake Baikal, we addressed the question whether HSs are candidate factors that affect the different species compositions in these water bodies. We hypothesized that interspecies differences in stress defense might reveal that Baikalian amphipods are inferior to Palearctic amphipods in dealing with HS-mediated stress. In this study, two key mechanisms of general stress response were examined: heat-shock protein 70 (HSP70) and multixenobiotic resistance-associated transporters (ABCB1). The results of quantitative polymerase chain reaction (qPCR) showed that the basal levels (in 3-day acclimated animals) of hsp70 and abcb1 transcripts were lower in Baikalian species (Eulimnogammarus cyaneus, Eulimnogammarus verrucosus, Eulimnogammarus vittatus-the most typical littoral species) than in the Palearctic amphipod (Gammarus lacustris-the only Palearctic species distributed in the Baikalian region). In the amphipods, the stress response was induced using HSs at 10 mg L(-1) dissolved organic carbon, which was higher than in sampling sites of the studied species, but well within the range (3-10 mg L(-1)) in the surrounding water bodies populated by G. lacustris. The results of qPCR and western blotting (n = 5) showed that HS exposure led to increased hsp70/abcb1 transcripts and HSP70 protein levels in G. lacustris, whereas these transcript levels remained constant or decreased in the Baikalian species. The decreased level of stress transcripts is probably not able to confer an effective tolerance to Baikalian species against further environmental stressors in conditions with elevated HS levels. Thus, our results suggest a greater robustness of Palearctic amphipods and a higher sensitivity of Baikalian amphipods to HS challenge, which might prevent most endemic species from migrating to habitats outside Lake Baikal.