Facilitation and Antagonism in Mycorrhizal Networks. In: Horton TR, editor. Mycorrhizal Networks. Dordrecht: Springer Netherlands; 2015. pp. 203-26. [DOI: 10.1007/978-94-017-7395-9_7]

Susceptibility and plant immune control-a case of mycorrhizal strategy for plant colonization, symbiosis, and plant immune suppression

Plants and microbes (mycorrhizal fungi to be precise) have evolved together over the past millions of years into an association that is mutualist. The plants supply the fungi with photosynthates and shelter, while the fungi reciprocate by enhancing nutrient and water uptake by the plants as well as, in some cases, control of soil-borne pathogens, but this fungi-plant association is not always beneficial. We argue that mycorrhizal fungi, despite contributing to plant nutrition, equally increase plant susceptibility to pathogens and herbivorous pests' infestation. Understanding of mycorrhizal fungi strategies for suppressing plant immunity, the phytohormones involved and the signaling pathways that aid them will enable the harnessing of tripartite (consisting of three biological systems)-plant-mycorrhizal fungi-microbe interactions for promoting sustainable production of crops.

Crop domestication disrupts intercropping benefits: A case study from barley-faba bean mixture under contrasting P inputs

How crop domestication mediates root functional traits and trait plasticity in response to neighboring plants is unclear, but it is important for selecting potential species to be grown together to facilitate P uptake. We grew two barley accessions representing a two-stage domestication process as a sole crop or mixed with faba bean under low and high P inputs. We analyzed six root functional traits associated with P acquisition and plant P uptake in five cropping treatments in two pot experiments. The spatial and temporal patterns of root acid phosphatase activity were characterized in situ with zymography at 7, 14, 21, and 28 days after sowing in a rhizobox. Under low P supply, wild barley had higher total root length (TRL), specific root length (SRL), and root branching intensity (RootBr) as well as higher activity of acid phosphatase (APase) in the rhizosphere, but lower root exudation of carboxylates and mycorrhizal colonization (MC), relative to domesticated barley. In response to neighboring faba bean, wild barley exhibited larger plasticity in all root morphological traits (TRL, SRL, and RootBr), while domesticated barley showed greater plasticity in root exudates of carboxylates and colonization by mycorrhiza. Wild barley with greater root morphology-related trait plasticity was a better match with faba bean than domesticated barley, indicated by higher P uptake benefits in wild barley/faba bean than domesticated barley/faba bean mixtures under low P supply. Our findings indicated that the domestication of barley disrupts the intercropping benefits with faba bean through the shifts of root morphological traits and their plasticity in barley. Such findings provide valuable information for barley genotype breeding and the selection of species combinations to enhance P uptake.

Common Mycorrhizae Network: A Review of the Theories and Mechanisms Behind Underground Interactions

Most terrestrial plants establish symbiotic associations with mycorrhizal fungi for accessing essential plant nutrients. Mycorrhizal fungi have been frequently reported to interconnect plants via a common mycelial network (CMN), in which nutrients and signaling compounds can be exchanged between the connected plants. Several studies have been performed to demonstrate the potential effects of the CMN mediating resource transfer and its importance for plant fitness. Due to several contrasting results, different theories have been developed to predict benefits or disadvantages for host plants involved in the network and how it might affect plant communities. However, the importance of the mycelium connections for resources translocation compared to other indirect pathways, such as leakage of fungi hyphae and subsequent uptake by neighboring plant roots, is hard to distinguish and quantify. If resources can be translocated via mycelial connections in significant amounts that could affect plant fitness, it would represent an important tactic for plants co-existence and it could shape community composition and dynamics. Here, we report and critically discuss the most recent findings on studies aiming to evaluate and quantify resources translocation between plants sharing a CMN and predict the pattern that drives the movement of such resources into the CMN. We aim to point gaps and define open questions to guide upcoming studies in the area for a prospect better understanding of possible plant-to-plant interactions via CMN and its effect in shaping plants communities. We also propose new experiment set-ups and technologies that could be used to improve previous experiments. For example, the use of mutant lines plants with manipulation of genes involved in the symbiotic associations, coupled with labeling techniques to track resources translocation between connected plants, could provide a more accurate idea about resource allocation and plant physiological responses that are truly accountable to CMN.

Symbiosis and the Anthropocene

Recent human activity has profoundly transformed Earth biomes on a scale and at rates that are unprecedented. Given the central role of symbioses in ecosystem processes, functions, and services throughout the Earth biosphere, the impacts of human-driven change on symbioses are critical to understand. Symbioses are not merely collections of organisms, but co-evolved partners that arise from the synergistic combination and action of different genetic programs. They function with varying degrees of permanence and selection as emergent units with substantial potential for combinatorial and evolutionary innovation in both structure and function. Following an articulation of operational definitions of symbiosis and related concepts and characteristics of the Anthropocene, we outline a basic typology of anthropogenic change (AC) and a conceptual framework for how AC might mechanistically impact symbioses with select case examples to highlight our perspective. We discuss surprising connections between symbiosis and the Anthropocene, suggesting ways in which new symbioses could arise due to AC, how symbioses could be agents of ecosystem change, and how symbioses, broadly defined, of humans and "farmed" organisms may have launched the Anthropocene. We conclude with reflections on the robustness of symbioses to AC and our perspective on the importance of symbioses as ecosystem keystones and the need to tackle anthropogenic challenges as wise and humble stewards embedded within the system.

Phosphorus facilitation and covariation of root traits in steppe species

Different phosphorus (P)-acquisition strategies may be relevant for species coexistence and plant performance in terrestrial communities on P-deficient soils. However, how interspecific P facilitation functions in natural systems is largely unknown. We investigated the root physiological activities for P mobilization across 19 coexisting plant species in steppe vegetation, and then grew plants with various abilities to mobilize sorbed P in a microcosm in a glasshouse. We show that P facilitation mediated by rhizosphere processes of P-mobilizing species promoted growth and increased P content of neighbors in a species-specific manner. When roots interacted with a facilitating neighbor, Cleistogenes squarrosa and Bromus inermis tended to show greater plasticity of root proliferation or rhizosheath acid phosphatase activity compared with other non-P-mobilizing species. Greater variation in these root traits was strongly correlated with increased performance in the presence of a facilitator. The results also show, for the first time, that P facilitation was an important mechanism underlying a positive complementarity effect. Our study highlights that interspecific P-acquisition facilitation requires that facilitated neighbors exhibit a better match of root traits with a facilitating species. It provides a better understanding of species coexistence in P-limited communities.

How mycorrhizal associations drive plant population and community biology

Mycorrhizal fungi provide plants with a range of benefits, including mineral nutrients and protection from stress and pathogens. Here we synthesize current information about how the presence and type of mycorrhizal association affect plant communities. We argue that mycorrhizal fungi regulate seedling establishment and species coexistence through stabilizing and equalizing mechanisms such as soil nutrient partitioning, feedback to soil antagonists, differential mycorrhizal benefits, and nutrient trade. Mycorrhizal fungi have strong effects on plant population and community biology, with mycorrhizal type-specific effects on seed dispersal, seedling establishment, and soil niche differentiation, as well as interspecific and intraspecific competition and hence plant diversity.