Interplant signalling through hyphal networks

Signals and cues in the evolution of plant-microbe communication

Communication has played a key role in organismal evolution. If sender and receiver have a shared interest in propagating reliable information, such as when they are kin relatives, then effective communication can bring large fitness benefits. However, interspecific communication (among different species) is more prone to dishonesty. Over the last decade, plants and their microbial root symbionts have become a model system for studying interspecific molecular crosstalk. However, less is known about the evolutionary stability of plant-microbe communication. What prevents partners from hijacking or manipulating information to their own benefit? Here, we focus on communication between arbuscular mycorrhizal fungi and their host plants. We ask how partners use directed signals to convey specific information, and highlight research on the problem of dishonest signaling.

Signaling in the Rhizosphere

Signaling studies in the rhizosphere have focused on close interactions between plants and symbiotic microorganisms. However, this focus is likely to expand to other microorganisms because the rhizomicrobiome is important for plant health and is able to influence the structure of the microbial community. We discuss here the shaping of the rhizomicrobiome and define which aspects can be considered signaling. We divide signaling in the rhizosphere into three categories: (i) between microbes, (ii) from plants to microorganisms, and (iii) from microorganisms to plants. Signals act on diverse organisms including the plant. Mycorrhizal and rhizobial interkingdom signaling has revealed its pivotal role in establishing associations, and the recent discovery of signaling with non-symbiotic microorganisms indicates the important role of communication in shaping the rhizomicrobiome.

Soil-on-a-Chip: microfluidic platforms for environmental organismal studies

Soil is the habitat of countless organisms and encompasses an enormous variety of dynamic environmental conditions. While it is evident that a thorough understanding of how organisms interact with the soil environment may have substantial ecological and economical impact, current laboratory-based methods depend on reductionist approaches that are incapable of simulating natural diversity. The application of Lab-on-a-Chip or microfluidic technologies to organismal studies is an emerging field, where the unique benefits afforded by system miniaturisation offer new opportunities for the experimentalist. Indeed, precise spatiotemporal control over the microenvironments of soil organisms in combination with high-resolution imaging has the potential to provide an unprecedented view of biological events at the single-organism or single-cell level, which in turn opens up new avenues for environmental and organismal studies. Herein we review some of the most recent and interesting developments in microfluidic technologies for the study of soil organisms and their interactions with the environment. We discuss how so-called "Soil-on-a-Chip" technology has already contributed significantly to the study of bacteria, nematodes, fungi and plants, as well as inter-organismal interactions, by advancing experimental access and environmental control. Most crucially, we highlight where distinct advantages over traditional approaches exist and where novel biological insights will ensue.

Effects of Common Mycorrhizal Network on Plant Carbohydrates and Soil Properties in Trifoliate Orange-White Clover Association

Common mycorrhizal network (CMN) allows nutrients and signals to pass between two or more plants. In this study, trifoliate orange (Poncirus trifoliata) and white clover (Trifolium repens) were planted in a two-compartmented rootbox, separated by a 37-μm nylon mesh and then inoculated with an arbuscular mycorrhizal fungus (AMF), Diversispora spurca. Inoculation with D. spurca resulted in formation of a CMN between trifoliate orange and white clover, whilst the best AM colonization occurred in the donor trifoliate orange-receptor white clover association. In the trifoliate orange-white clover association, the mycorrhizal colonization of receptor plant by extraradical hyphae originated from the donor plant significantly increased shoot and root fresh weight and chlorophyll concentration of the receptor plant. Enzymatic activity of soil β-glucoside hydrolase, protease, acid and neutral phosphatase, water-stable aggregate percentage at 2-4 and 0.5-1 mm size, and mean weight diameter in the rhizosphere of the receptor plant also increased. The hyphae of CMN released more easily-extractable glomalin-related soil protein and total glomalin-related soil protein into the receptor rhizosphere, which represented a significantly positive correlation with aggregate stability. AMF inoculation exhibited diverse changes in leaf and root sucrose concentration in the donor plant, and AM colonization by CMN conferred a significant increase of root glucose in the receptor plant. These results suggested that CMN formed in the trifoliate orange-white clover association, and root AM colonization by CMN promoted plant growth, root glucose accumulation, and rhizospheric soil properties in the receptor plant.

Nutrient loading alters the performance of key nutrient exchange mutualisms

Nutrient exchange mutualisms between phototrophs and heterotrophs, such as plants and mycorrhizal fungi or symbiotic algae and corals, underpin the functioning of many ecosystems. These relationships structure communities, promote biodiversity and help maintain food security. Nutrient loading may destabilise these mutualisms by altering the costs and benefits each partner incurs from interacting. Using meta-analyses, we show a near ubiquitous decoupling in mutualism performance across terrestrial and marine environments in which phototrophs benefit from enrichment at the expense of their heterotrophic partners. Importantly, heterotroph identity, their dependence on phototroph-derived C and the type of nutrient enrichment (e.g. nitrogen vs. phosphorus) mediated the responses of different mutualisms to enrichment. Nutrient-driven changes in mutualism performance may alter community organisation and ecosystem processes and increase costs of food production. Consequently, the decoupling of nutrient exchange mutualisms via alterations of the world's nitrogen and phosphorus cycles may represent an emerging threat of global change.

Plant-mediated 'apparent effects' between mycorrhiza and insect herbivores

Plants mediate indirect 'apparent' effects between above-ground herbivores and below-ground mutualistic mycorrhizal fungi. The herbivore-plant-mycorrhiza continuum is further complicated because signals produced by plants in response to herbivores can be transmitted to other plants via shared fungal networks below ground. Insect herbivores, such as aphids, probably affect the functioning of mycorrhizal fungi by changing the supply of recent photosynthate from plants to mycorrhizas, whereas there is evidence that mycorrhizas affect aphid fitness by changing plant signalling pathways, rather than only through improved nutrition. New knowledge of the transfer of signals through fungal networks between plant species means we now need a better understanding of how this process occurs in relation to the feeding preferences of herbivores to shape plant community composition and herbivore behaviour in nature.

Compost and biochar alter mycorrhization, tomato root exudation, and development of Fusarium oxysporum f. sp. lycopersici

Soil amendments like compost and biochar are known to affect soil properties, plant growth as well as soil borne plant pathogens. Complex interactions based on microbial activity and abiotic characteristics are supposed to be responsible for suppressive properties of certain substrates, however, the specific mechanisms of action are still widely unknown. In the present study, the main focus was on the development of the soil borne pathogen, Fusarium oxysporum f.sp. lycopersici (Fol) in tomato (Solanum lycopersicum L.) and changes in root exudates of tomato plants grown in different soil substrate compositions, such as compost (Comp) alone at application rate of 20% (v/v), and in combination with wood biochar (WB; made from beech wood chips) or green waste biochar (GWB; made from garden waste residues) at application rate of 3% (v/v), and/or with additional arbuscular mycorrhizal fungi (AMF). The association of GWB and AMF had a positive effect on tomato plants growth unlike to the plants grown in WB containing a soil substrate. The AMF root colonization was not enhanced by the addition of WB or GWB in the soil substrate, though a bio-protective effect of mycorrhization was evident in both biochar amended treatments against Fol. Compost and biochars altered root exudates differently, which is evident from variable response of in vitro growth and development of Fol. The microconidia germination was highest in root exudates from plants grown in the soil containing compost and GWB, whereas root exudates of plants from a substrate containing WB suppressed the mycelial growth and development of Fol. In conclusion, the plant growth response and disease suppression in biochar containing substrates with additional AMF was affected by the feedstock type. Moreover, application of compost and biochars in the soil influence the quality and composition of root exudates with respect to their effects on soil-dwelling fungi.

Inter-plant communication through mycorrhizal networks mediates complex adaptive behaviour in plant communities

Adaptive behaviour of plants, including rapid changes in physiology, gene regulation and defence response, can be altered when linked to neighbouring plants by a mycorrhizal network (MN). Mechanisms underlying the behavioural changes include mycorrhizal fungal colonization by the MN or interplant communication via transfer of nutrients, defence signals or allelochemicals. We focus this review on our new findings in ectomycorrhizal ecosystems, and also review recent advances in arbuscular mycorrhizal systems. We have found that the behavioural changes in ectomycorrhizal plants depend on environmental cues, the identity of the plant neighbour and the characteristics of the MN. The hierarchical integration of this phenomenon with other biological networks at broader scales in forest ecosystems, and the consequences we have observed when it is interrupted, indicate that underground 'tree talk' is a foundational process in the complex adaptive nature of forest ecosystems.