Are mycorrhizal fungi our sustainable saviours? Considerations for achieving food security

Abstract

Summary The 20th century saw dramatic increases in agricultural productivity, largely through the development and application of pesticides, fertilisers rich in nitrogen and phosphorus, and advances in plant breeding and genetic technologies. In the last 15 years, however, many key crop yields have plateaued. Climate change, an ever‐increasing human population, depletion of global rock‐phosphorus and growing energy prices make current fertiliser production unsustainable and represent sizeable challenges to global food security. Many important crops form symbioses with arbuscular mycorrhizal fungi (AMF), and this has motivated the development of novel approaches in crop breeding and agricultural practices to support and promote AMF in agroecosystems. Arbuscular mycorrhizal fungal symbiosis can be high beneficial in crops and wider agroecosystems in many ways, including improved soil structure and resistance to pests. However, AMF colonisation does not necessarily translate directly into enhanced plant performance or crop yield, while land management practices that would encourage mycorrhiza–crop associations, such as low‐till or minimal chemical input often incur yield‐reducing trade‐offs. Synthesis. We draw on ecological knowledge of AMF to inform their role in agroecosystems, providing a balanced look at mycorrhiza–crop symbioses in terms of plant ecophysiology and the wider role of AMF in agroecosystems and ask the question: are AMF our sustainable saviours?

Trade-Offs in Arbuscular Mycorrhizal Symbiosis: Disease Resistance, Growth Responses and Perspectives for Crop Breeding

“…AMF have also been shown to reduce symptoms of arsenic toxicity by improved phosphate nutrition and increased activity of compounds, such as glutathione-S-transferase, which transform arsenic into non-toxic forms [72,73]. AM symbiosis has been shown to reduce assimilation of other adverse heavy metals, such as copper, chromium, cadmium and cobalt [19,36,77]. It has been suggested that altered phosphate transport during AM colonization alleviates symptoms of phosphate toxicity in highly-fertilized soils [76].…”

“…AMF may also have potential for phytoremediation [36][37][38]; however, this review focusses on approaches that could be taken by crop breeding rather that adding AMF to soil. [4,[60][61][62][63][64][65][66] Water acquisition [67,68] Zinc acquisition [5,6] Drought tolerance [67,68] Salinity tolerance [29,[69][70][71] Reduced arsenic toxicity [72,73] Reduced sulphur starvation [74,75] Reduced phosphate toxicity [76] Reduced assimilation of adverse heavy metals [19,36,77] Disease resistance Increased resistance to root [31,[78][79][80] and foliar [81][82][83][84][85] pathogens Reduced arbuscular mycorrhizal colonization in mlo mutants [86] Jasmonic acid production [87][88][89][90] Components required for AM colonization used by pathogens, Golovinomyces cichoracerum (DC.) V.P.…”

Options for Improved Phosphorus Cycling and Use in Agriculture at the Field and Regional Scales

“…As a result of the immobility of P in the soil (especially in soils low in available Pi), the ability of a crop to access sufficient P depends on its rooting characteristics and the ability to form relationships with AMF, which effectively act as extensions of the root system (Liu et al, 2003). The AMF symbiosis is well known to assist plants with P uptake (Landry et al, 2008; Thirkell et al, 2017) by reaching beyond the P depletion zone that occurs immediately surrounding roots, allowing greater access to soil P reserves (Bolan, 1991). Arbuscular mycorrhizal fungi have been shown to have a greater P uptake efficiency than plant roots by accessing P at lower soil solution P i concentrations and by taking it up at increased rates (Liu et al, 2007).…”

“…Arbuscular mycorrhizal fungi, like plant roots, can take up orthophosphate anions from the soil solution but have also recently been shown to be able to access Po stores via the secretion of acid phosphatases (Sato et al, 2015). Because of AMF's P scavenging and uptake efficiency abilities, there is an interest in managing soil AMF populations to reduce chemical fertilizer use while maintaining crop yield (Bender et al, 2016; Thirkell et al, 2017). …”

“…For example, in forage soils in Ontario, Canada, Schneider et al (2017) showed an increase in alfalfa root colonization by AMF as STP concentrations decreased below 10 mg kg −1 Olsen P, which is consistent with findings reported by Jansa et al (2006), Jansa et al (2011), and Deng et al (2014). The exact STP concentration where AMF benefits start to increase will vary by crop and soil factors, but in general, greater benefits are to be expected when STP concentrations are less than the agronomically optimal range (Thirkell et al, 2017). This raises the question: To reap the benefits of AMF, a lower STP concentration is desirable, but will the P supplied to the crop be enough to maintain crop yields?…”