Quantitative proteomics revealed partial fungistatic mechanism of ammonia against conidial germination of nematode-trapping fungus Arthrobotrys oligospora ATCC24927

Eco-friendly management of Meloidogyne incognita in cadmium-contaminated soil by using nematophagous fungus Purpureocillium lavendulum YMF1.683: Efficacy and mechanism

Root-knot nematodes (RKNs) are distributed globally, including in agricultural fields contaminated by heavy metals (HM), and can cause serious crop damages. Having a method that could control RKNs in HM-contaminated soil while limit HM accumulation in crops could provide significant benefits to both farmers and consumers. In this study, we showed that the nematophagous fungus Purpureocillium lavendulum YMF1.683 exhibited a high nematocidal activity against the RKN Meloidogyne incognita and a high tolerance to CdCl2. Comparing to the P. lavendulum YMF1.838 which showed low tolerance to Cd2+, strain YMF1.683 effectively suppressed M. incognita infection and significantly reduced the Cd2+ uptake in tomato root and fruit in soils contaminated by 100 mg/kg Cd2+. Transcriptome analyses and validation of gene expression by RT-PCR revealed that the mechanisms contributed to high Cd-resistance in YMF1.683 mainly included activating autophagy pathway, increasing exosome secretion of Cd2+, and activating antioxidation systems. The exosomal secretory inhibitor GW4869 reduced the tolerance of YMF1.683 to Cd2+, which firstly demonstrated that fungal exosome was involved in HM tolerance. The up-regulation of glutathione synthesis pathway, increasing enzyme activities of both catalase and superoxide dismutase also played important roles in Cd2+ tolerance of YMF1.683. In Cd2+-contaminated soil, YMF1.683 limited Cd2+-uptake in tomato by up-regulating the genes of ABCC family in favor of HM sequestration in plant, and down-regulating the genes of ZIP, HMA, NRAMP, YSL families associated with HM absorption, transport, and uptake in plant. Our results demonstrated that YMF1.683 could be a promising bio-agent in eco-friendly management of M. incognita in Cd2+ contaminated soils.

Ammonia and Nematode Ascaroside Are Synergistic in Trap Formation in Arthrobotrys oligospora

Nematode-trapping (NT) fungi are natural predators of the soil living nematodes. Diverse external signals mediate the generation of predatory devices of NT fungi. Among these, broad ascarosides and nitrogenous ammonia are highly efficient inducers for trap structure initiation. However, the overlay effect of ammonia and ascaroside on the trap morphogenesis remains unclear. This study demonstrated that the combination of nitrogenous substances with nematode-derived ascarosides led to higher trap production compared to the single inducing cues; notably, ammonia and Ascr#18 had the most synergistic effect on the trap in A. oligospora. Further, the deletion of ammonia transceptor Amt43 blocked trap formation against ammonia addition in A. oligospora but not for the ascaroside Ascr#18 induction. Moreover, ammonia addition could promote plasma endocytosis in the process of trap formation. In contrast, ascaroside addition would facilitate the stability of intracellular organization away from endocytosis. Therefore, there is a synergistic effect on trap induction from different nitrogenous and ascaroside signals.

Fungistatic Mechanism of Ammonia against Nematode-Trapping Fungus Arthrobotrys oligospora, and Strategy for This Fungus To Survive Ammonia

Soil fungistasis is a phenomenon in which the germination and growth of fungal propagules is widely inhibited in soils. Although fungistatic compounds are known to play important roles in the formation of soil fungistasis, how such compounds act on soil fungi is little studied. In this study, it was found that ammonia (NH₃) induced global protein misfolding marked by increased ubiquitination levels of proteins (ubiquitylome data and Western blot verification). The misfolded proteins should trigger the endoplasmic reticulum (ER) stress, which was indicated by electron microscope image and proteome data. Results from the mutants of BiP and proteasome subunit alpha 7 suggested that ER stress played a mechanistic role in inhibiting conidial germination. Results from proteome data indicated that, to survive ammonia fungistasis, conidia first activated the unfolded protein response (UPR) to decrease ER stress and restore ER protein homeostasis, and the function of UPR in surviving ammonia was confirmed by using mutant strains. Second, ammonia toxicity could be reduced by upregulating carbon metabolism-related proteins, which benefited ammonia fixation. The results that metabolites (especially glutamate) could relieve the ammonia fungistasis confirmed this indirectly. Finally, results from gene knockout mutants also suggested that the fungistatic mechanism of ammonia is common for soil fungistasis. This study increased our knowledge regarding the mechanism of soil fungistasis and provided potential new strategies for manipulating soil fungistasis. IMPORTANCE Soil fungistasis is a phenomenon in which the germination and growth of fungal propagules is widely inhibited in soil. Although fungistatic compounds are known to play important roles in the formation of soil fungistasis, how such compounds act on soil fungi remains little studied. This study revealed an endoplasmic reticulum stress-related fungistatic mechanism with which ammonia acts on Arthrobotrys oligospora and a survival strategy of conidia under ammonia inhibition. Our study provides the first mechanistic explanation of how ammonia impacts fungal spore germination, and the mechanism may be common for soil fungistasis. This study increases our knowledge regarding the mechanism of soil fungistasis in fungal spores and provides potential new strategies for manipulating soil fungistasis.

Methylglyoxal Has Different Impacts on the Fungistatic Roles of Ammonia and Benzaldehyde, and Lactoylglutathione Lyase Is Necessary for the Resistance of Arthrobotrys oligospora to Soil Fungistasis

Biocontrol of root-knot nematode has attracted increasing attention over the past two decades. The inconsistent field performance of biocontrol agents, which is caused by soil fungistasis, often restricts their commercial application. There is still a lack of research on the genes involved in biocontrol fungi response to soil fungistasis, which is important for optimizing practical applications of biocontrol fungi. In this study, the lactoylglutathione lyase-encoding AOL_s00004g335 in the nematophagous fungi Arthrobotrys oligospora was knocked out, and three mutant strains were obtained. The hyphal growth of mutants on the three media was almost the same as that of the wild-type strain, but mutants had slightly higher resistance to NaCl, SDS, and H₂O₂. Methylglyoxal (MG) significantly increased the resistance of A. oligospora to ammonia, but decreased the resistance to benzaldehyde. Furthermore, the resistance of the mutants to soil fungistasis was largely weakened and MG could not increase the resistance of A. oligospora to soil fungistasis. Our results revealed that MG has different effects on the fungistatic roles of ammonia and benzaldehyde and that lactoylglutathione lyase is very important for A. oligospora to resist soil fungistasis.

Comparative Transcriptomics Reveals Features and Possible Mechanisms of Glucose-Mediated Soil Fungistasis Relief in Arthrobotrys oligospora

Soil-borne pest diseases result in large annual agricultural losses globally. Fungal bio-control agents are an alternative means of controlling pest diseases; however, soil fungistasis limits the effect of fungal agents. Nutrients can relieve soil fungistasis, but the mechanisms behind this process remain poorly understood. In this study, we determined and quantified the transcriptomes of Arthrobotrys oligospora, a nematode-trapping fungus, derived from samples of fresh conidia, germinated conidia, soil fungistatic conidia, and glucose-relieved conidia. The transcriptomes of fungistatic and glucose-relieved conidia were significantly different from those of the other two conidia samples. KEGG pathway analyses showed that those genes upregulated in fungistatic and glucose-relieved conidia were mainly involved in translation and substance metabolism, and the downregulated genes were mainly involved in MAPK pathway, autophagy, mitophagy, and endocytosis. As being different from the transcriptome of fungistatic conidia, upregulated genes in the transcriptome of glucose-relieved conidia are also related to replication and repair, spliceosome, oxidative phosphorylation, autophagy, and degradation pathway (lysosome, proteasome, and RNA degradation). And the upregulated genes resulted from comparison of glucose-relieved conidia and fungistatic conidia were enriched in metabolic pathways, cycle, DNA replication, and repair. The differentially splicing events in the transcriptome of glucose-relieved conidia are far more than that of other two transcriptomes, and genes regulated by differentially splicing were analyzed through KEGG pathway analysis. Furthermore, autophagy genes were proved to play important role in resisting soil fungistasis and glucose-mediated soil fungistasis relief. These data indicate that, in addition to being a carbon and energy source for conidia germination, glucose may also help to relieve soil fungistasis by activating many cellular processes, including autophagy, DNA replication and repair, RNA alternative splicing, and degradation pathways.

Proteomic changes in Arthrobotrys oligospora conidia in response to benzaldehyde-induced fungistatic stress

Soil fungistasis limits the effect of fungal agents designed to control plant-parasitic nematodes. Benzaldehyde is a fungistatic factor produced by soil microorganisms that can suppress conidial germination, but the molecular mechanism of this suppression is unknown. In this study, three conidial proteomes of Arthrobotrys oligospora ATCC24927, a nematode-trapping fungus, were obtained, quantified, and compared. Under benzaldehyde fungistatic stress, conidial protein expression profile changed significantly. Screening with a twofold selection criterion revealed 164 up-regulated and 110 down-regulated proteins. 17 proteins related to protein translation were down-regulated and gene transcription analysis suggested that the repression of proteins translation might be one mechanism by which benzaldehyde inhibites conidial germination. Benzaldehyde also resulted in the down-regulation of respiratory chain proteins and mitochondrial processes, as well as the repression of conidial DNA synthesis. In addition, the conidia up-regulated several proteins that enable it to resist benzaldehyde-induced fungistatis, and this was confirmed by a functional assessment of two knockout mutants. This study reveals putative mechanisms by which benzaldehyde causes fungistasis as well as the proteomic response of conidia to benzaldehyde. SIGNIFICANCE: Soil fungistasis limits the effect of fungal agents designed to control plant-parasitic nematodes. Benzaldehyde is one of fungistatic factors produced by soil microorganisms that can suppress conidial germination. In this study, we found that conidial protein expression profile changed significantly under benzaldehyde fungistatic stress. This research revealed new mechanistic data that describe how benzaldehyde is responsible for fungiststis by inhibiting conidial germination. Moreover, we also found that conidia can resist benzaldehyde by up-regulating proteins such as benzaldehyde dehydrogenase and heat shock proteins. This study also showed that proteomics methods play important roles in addressing soil fungistatic mechanisms.