| In natural environment, feeding-bacteria are taken to be simple passive victims, which are heavily consumed by bacterivorous nematodes. However, it is an important scientific problem in microbiological ecology that the bacteria can survive and maintain balance between prey and predator; further promote the species stability of ecosystem. In this paper, we discover nematode-trapping fungi can change life stage from saprophytes into predacious stage regulated by feeding-bacteria, further help bacteria kill nematode predator, and in turn, nematodes are become to be prey. So, the relationship among bacteria, nematodes, and nematode-trapping fungi achieve dynamic balance.Here, fresh cow dung samples could induce6-fold higher trap activity in Arthrobotrys oligospora than that of autoclaved samples, suggesting that bacteria in cow dung might serve as inducers of trap formation. We identified126isolates in dung sample.55isolates could induce trap formation, and in which Stenotrophomonas maltophilia were the most efficient inducers. A candidate compound was obtained based on column chromatograph, further identified to be urea by the mass spectrometry, nuclear magnetic resonance and elemental analysis. In addition, urea-production levels favored the trap activities in certain bacteria, and urea could induce different trap structures. So, urea was a common signal in responsiveness to trap formation of nematode-trapping fungi.RT-PCR assay showed that putative arcA genes encoding arginase existed in the mRNA of the urea-producing bacteria. And, the expression of arcA in S. maltophila CD52was significantly up-regulated after exposure to the nematode C. elegans using real-time PCR assay. In addition, we blocked the gene of urea production in CD52and obtained SmAarcA mutant, and we could not detect any urea in the fermentation supernatant from the SmAarcA mutant by LC-MS assay. The deletion of arcA significantly suppressed the ability of S. maltophila CD52to induce trap formation. These results suggested that urea produced from bacteria could induce trap formation in A. oligospora.Then, urea transporters and urease in A. oligospora were deleted,[13C,15N2]-urea label indicated [13C,15N2]-urea uptake was completely abolished when transporter Utp79was disrupted, while heavily accumulated when urease was disrupted. Meanwhile, urea could not induce trap formation of AoAutp79and AoAurel mutants. These results proved that urea uptake and metabolite are crucial for trap formation among nematode-trapping fungi.Generally, urea is converted to ammonia and carbon dioxide. We discovered carbon dioxide at different concentrations failed to induce trap formation, whereas, ammonia effectively elicited trap formation in A. oligospora. So, urea breakdown product ammonia is the signal to induce trap formation.Combined to food reference, development, brood size, and lifespan of nematodes, it showed there is indistinguishable influence comparing to worms fed E. coli OP50, indicating the urea-producing bacteria are high-quality foods. Moreover, urea significantly increased the number of traps formation in the AoAurel mutant induced by nematodes, suggesting nematode and urea exhibited synergistic effect on trap formation.Urea, an underground short-range signal molecule, could diffuse readily in soil. We analyzed the dynamics among bacteria, fungi and nematodes, and the data indicated that urea-producing bacteria promote increase of trap number gradually, further elimination of nematodes by A. oligospora. Importantly, A. oligospora rescued the numbers of urea-producting bacteria. These results proved that feeding-bacteria mobilize fungi to help themselves killing namatodes.The present research documented that feeding-bacteria can regulate nematode-trapping fungi killing nematode predator by producing urea, so as to maintain their communities’stability. The study reveals that the relationships between prey and predator play an important role in maintaining the balance of ecology system. |
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