*4.3. Chemical Signals*

In their natural habitats, nematodes and fungi exist as parts of complex multitrophic communities that depend on and communicate through elaborate networks of chemical signaling. A key feature of microbe–microbe interaction is the secretion of chemical mediators that can influence interactions involving both microbial partners and the co-occurring multicellular organisms. Both nematodes and fungi have developed elaborate communication systems that are based on secretion of chemicals, allowing intra- and inter-kingdom interactions.

The most common chemical signals in the evolution of predator–prey relationships are those related to recognitions of specific pathogens or food sources [138]. Recent studies based on the model nematode *Caenorhabditis elegans* identified ascarosides, a group of small molecules, as involved in inter-organismal communications. These chemicals play a central role in regulating nematode development and behavior [139,140]. Under nitrogen starvation, ascaroside-induced morphogens are required for *A. oligospora* to sense and initiate trap formation in response to the availability of nematode prey [24,141]. Recent studies have also characterized several other morphology-regulating arthrosporol metabolites from *A. oligospora* and identified them as important signaling cues for hyphal development, nematode attraction, and trap morphogenesis [29,62,142].

Ammonia is another molecule identified to influence interactions among multiple interacting partners. Specifically, it has been reported as an intracellular signal for altering fungal morphological switch and mediating interspecific interactions among bacteria, fungi, and nematodes [143]. Wang et al. [26] demonstrated that when bacteria were consumed by bacterivorous nematodes, urea production and release were enhanced by upregulating the arginase's expression. The urease within the fungi eventually catabolized urea to ammonia, which initiates formation of predatory structures. Similarly, diketopiperazines (DKPs) were shown to facilitate chemotaxis of *Stenotrophomonas* bacteria towards fungal extracts, leading to bacterial biofilm formation on fungal nematode traps and enhancing fungal trapping activity against nematodes [36].

In the relationship among entomopathogenic nematodes, plant, insects, and nematophagous fungi, behavioral plasticity of entomopathogenic nematodes in response to a plant volatile organic compound (d-limonene) affected nematode–fungi interactions. Two mechanisms were suggested for their interactions. In the first, nematodes' response to d-limonene may make them less likely to respond to other environmental stimuli, such as to attractants released by nematophagous fungi. In the second, the learned response by nematodes to plant volatiles may motivate entomopathogenic nematodes to move faster in the event of being exposed to such volatiles, potentially making it harder to catch them by fungal traps [144,145].
