Fresh perspectives on the roles of arbuscular mycorrhizal fungi in plant nutrition and growth

Abstract

Recent research on arbuscular mycorrhizas has demonstrated that AM fungi play a significant role in plant phosphorus (P) uptake, regardless of whether the plant responds positively to colonization in terms of growth or P content. Here we focus particularly on implications of this finding for consideration of the balance between organic carbon (C) use by the fungi and P delivery (i.e. the C-P trade between the symbionts). Positive growth responses to arbuscular mycorrhizal (AM) colonization are attributed frequently to increased P uptake via the fungus, which results in relief of P deficiency and increased growth. Zero AM responses, compared with non-mycorrhizal (NM) plants, have conventionally been attributed to failure of the fungi to deliver P to the plants. Negative responses, combined with excessive C use, have been attributed to this failure. The fungi were viewed as parasites. Demonstration that the AM pathway of P uptake operates in such plants indicates that direct P uptake by the roots is reduced and that the fungi are not parasites but mutualists because they deliver P as well as using C. We suggest that poor plant growth is the result of P deficiency because AM fungi lower the amount of P taken up directly by roots but the AM uptake of P does compensate for the reduction. The implications of interplay between direct root uptake and AM fungal uptake of P also include increased tolerance of AM plants to toxins such as arsenate and increased success when competing with NM plants. Finally we discuss the new information on C-P trade in the context of control of the symbiosis by the fungus or the plant, including new information (from NM plants) on sugar transport and on the role of sucrose in the signaling network involved in responses of plants to P deprivation.

The extent of mycorrhizal colonization of roots and its influence on plant growth and phosphorus content

“…Indeed, a fair amount of evidence has accumulated indicating that AMF are balanced mutualisms that function by a form of regulated exchange, so that plants should support high levels of PRLC only if the AMF provide benefits (reviewed in Brundrett 2004). Nevertheless, Smith and Smith (2012) reported physiological evidence for large differences in P transfer by different AM fungi that would be expected to influence the size (and possible direction) of AM-mediated growth responses and total P uptake. This area requires more research, given that AM roots contain multiple AM fungal taxa.…”

Ecological relevance of strigolactones in nutrient uptake and other abiotic stresses, and in plant-microbe interactions below-ground

“…However, neutral or even negative effects on plant growth, attributed to P deprivation and an excessive carbon use by the AM fungus, have also been described (Grace et al 2009;Li et al 2008;Smith and Smith 2012). The negative plant response to AM colonisation has been proposed to be associated with the reduced P absorption capacity by the 'direct pathway' induced by the symbiosis and to a lower P uptake capacity by the AM fungus through the 'AM pathway' (Smith and Smith 2012). Therefore, searching for the optimal 'dance partner' is crucial for a mutualistic beneficial association.…”

Duplicate and Conquer: Multiple Homologs ofPHOSPHORUS-STARVATION TOLERANCE1Enhance Phosphorus Acquisition and Sorghum Performance on Low-Phosphorus Soils

“…Plants can mobilize P through the exudation of organic acids, acid phosphatases, and ribonucleases, resulting in enhanced P availability and uptake (Hinsinger, 2001;Ryan et al, 2001;Dakora and Phillips, 2002;Hammond and White, 2008;Ma et al, 2009;Pang et al, 2009). Another strategy to cope with low-P availability is to increase the soil volume accessed by root systems by forming mycorrhizal symbioses Smith and Smith, 2012;Rai et al, 2013). Due to low-P mobility on tropical soils, changes in root architecture and morphology enhance P uptake by facilitating soil exploration (Williamson et al, 2001;Ho et al, 2005;Walk et al, 2006;Svistoonoff et al, 2007;Lynch, 2011;Ingram et al, 2012;Niu et al, 2013).…”