Ectomycorrhiza: phosphorus source specific economy and potential in resource partitioning

Many natural and anthropogenic soils are phosphorus (P) limited, as P is largely immobilized in forms of low bioavailability affecting the plant productivity. One of the strategies to overcome this shortage lies in the symbiosis of plants with mycorrhizal fungi that increases the plant P uptake of these hardly accessible sources in exchange for photoassimilates. Nevertheless, the required investment to acquire the free phosphate from various P sources increases with decreasing P source bioavailability, indicating a different carbon (C) sink potential. Ectomycorrhizae were shown to be able to acquire various P sources, but experimental evidence for their P source dependent C sink potential as well as for their role in resource partitioning for P is missing. The present study aimed to address following objectives: (i) to design a system to observe mycorrhizal mediated nutrient exchange (Study I); and to investigate (ii) whether the C investment from the host plant for the mycorrhizal mediated P derived from different P species is P source dependent (Study II); as well as the (iii) preferences of mycorrhizal plant to specific P sources from a mixed P pool (Study III). Following P sources were tested for the mycorrhizal mediated plant P uptake: PO43- (oP), the primary mineral apatite (AP; Study II) or hydroxyapatite (HAP; Study III), the organic P species (Porg) phytic acid (Phy; Study II) or adenosine monophosphate (AMP; Study III), and goethitebound PO43- (gP) as secondary mineral-P adsorption complex. The practical experience made in Study I revealed that, compared with the in vitro culture system, the construction and maintenance of the axenic rhizotrone and the mesocosm culture systems are less complicated and time consuming and at the same time more robust and very versatile systems that are also suitable for greenhouse conditions. In Study II a P source dependent trend in exchange of host C for ectomycorrhizal mediated P in roots was determined. The exchange of C for P in ectomycorrhizal roots of AP and gP treatments were of similar magnitude and could result from the applying the same mechanism, the exudation of LMWOAs and hyphal growth to acquire mineral P sources (mineral P effect). In contrast, the readily available P source oP, the soluble Phy, as well as the complete P limitation turn the ectomycorrhizal plant into a system of C retention in the mycorrhizal root interface. These results indicate a trend that the C allocation into the belowground is P source specific, which can have an impact on the C balance in soils of natural and anthropogenic ecosystems. The results of Study III show that an ectomycorrhizal plant is able to utilize all provided P sources via its mycorrhizal fungal associate. The acquisition timing was determined by the most bioavailable P sources, with oP and AMP over HAP and gP, and a mixed P pool over single P source. In contrast, the magnitude was defined by the amount of supplied P source and provision of additional nitrogen, hence AMP over oP and gP, as well as by P source complexity, with gP as the least favourable P form. Nevertheless, these results provide evidence that an ectomycorrhiza has the potential to occupy fundamental niches of various P sources differing in their bioavailability, indicating that being a generalist in P nutrition can facilitate adaptation to various nutritional settings in soil

Designing a Robust and Versatile System to Investigate Nutrient Exchange in, and Partitioning by, Mycorrhiza (Populus x canesces x Paxillus involutus) Under Axenic or Greenhouse Conditions

Phosphorus (P) bioavailability affects plant nutrition. P can be present in soils in different chemical forms that are not available for direct plant uptake and have to be acquired by different mechanisms, representing different resource niches. These mechanisms, of which many seem to be attributed to mycorrhiza, likely influence the diversity and stability of plant communities in natural ecosystems, as they also might help to overcome a future shortage of P supply in agro-ecosystems. In order to understand the mechanisms of P acquisition, the associated carbon costs, and the resource partitioning by mycorrhizal fungi, the ecosystem situation has to be mimicked in smaller scaled experiments. Here, different experimental setups are evaluated using plantlets of Populus x canescens and its functional ectomycorrhizal (ECM) fungus Paxillus involututs strain MAJ. To investigate resource partitioning involving mycorrhizae, the protocols of this study describe preparation of an in vitro and a rhizotrone culture systems for studies under axenic conditions as well as a mesocosm culture system for greenhouse conditions. We also describe the construction of separate compartments containing nutrients and excluding plant roots as well as the progress that has been made in in vitro propagation of plant and ECM fungal material. The practical experience made in our study shows that the in vitro culture system is prone to desiccation and its construction and maintenance are more time consuming and complicated. In contrast, with the axenic rhizotrone culture system and the mesocosms we have created more robust and very versatile systems that are also suitable for greenhouse conditions

Mycorrhizal Mediated Partitioning of Phosphorus: Ectomycorrhizal (Populus x canescens x Paxillus involutus) Potential to Exploit Simultaneously Organic and Mineral Phosphorus Sources

Many natural and anthropogenic soils are phosphorus (P) limited often due to larger P stocks sequestered in forms of low bioavailability. One of the strategies to overcome this shortage lies in the symbiosis of plants with mycorrhizal fungi, increasing the plant P uptake of these hardly accessible sources. However, little is known about mycorrhizal fungal mediated partitioning of differently available P forms, which could contribute to more efficient use of P by plants and, thereby, reduce competition for soil P. This study aimed to investigate the uptake of P from differently bioavailable P sources by ectomycorrhiza. For that, we conducted a rhizotrone study using Populus x canescens and its compatible ectomycorrhizal fungus Paxillus involutus. Four different P sources [ortho-phosphate (oP), adenosine monophosphate (AMP), hydroxyapatite (HAP), and oP bound to goethite (gP)] or only HAP as 1P control were supplied in separate compartments, where only the fungal partner had access to the P sources. The amount of the specific P sources was increased according to their decreasing bioavailability. In order to distinguish between the P sources, we applied 33P to track its incorporation in plants by a non-destructive analysis via digital autoradiography. Our results show that an ectomycorrhizal plant is able to utilize all provided P sources via its mycorrhizal fungal associate. The acquisition timing was determined by the most bioavailable P sources, with oP and AMP over HAP and gP, and a mixed P pool over a single P source. In contrast, the magnitude was defined by the by the amount of supplied P source provision of additional nitrogen, hence AMP over oP and gP, as well as by P source complexity, with gP as the least favorable P form. Nevertheless, the results of the present study provide evidence that an ectomycorrhiza has the potential to occupy fundamental niches of various P sources differing in their bioavailability, indicating that being a generalist in P nutrition can facilitate adaptation to various nutritional settings in soil

Ectomycorrhizal potential in occupying fundamental niches of various P sources differing in bioavailability

P is an essential element for the plant net primary productivity, but it is also limited in soils in many terrestrial ecosystems. The association of plants with mycorrhizal fungi can increase the bioavailability of P. Moreover, resource partitioning of P associated with mycorrhizal fungi could contribute to more efficient use of different P forms by plants, reducing competition for soil P. Ectomycorrhizae were shown to mine different chemical forms of P. Nevertheless, experimental evidence for the ectomycorrhizal (Populus x canescens x Paxillus involutus) contribution in resource partitioning for P is missing. In order to understand such ecosystem situations, we aimed to investigate the ectomycorrhizal preferences to specific P sources from a mixed P pool (PO43- (oP), adenosine-monophosphate (AMP), hydroxyapatite (HAP), PO43- bound to goethite (gP)). Our results show that an ectomycorrhizal plant is able to utilize all provided P sources via its mycorrhizal fungal associate. The acquisition timing was determined by the most bioavailable P sources, with oP and AMP over HAP and gP, and a mixed P pool over single P source. In contrast, the magnitude was defined by the amount of supplied P source and provision of additional nitrogen, hence AMP over oP and gP, as well as by P source complexity, with gP as the least favourable P form. Nevertheless, the results of the present study provide evidence that under not competing conditions the ectomycorrhizal fungus P. involutus has the potential to be a generalist, whereas according to the nutrient niching theory a specialization to one or a few specific P forms occurs due to the need to compete for limited resources

Mycorrhizal Mediated Partitioning of Phosphorus: Ectomycorrhizal (Populus x canescens x Paxillus involutus) Potential to Exploit Simultaneously Organic and Mineral Phosphorus Sources

Many natural and anthropogenic soils are phosphorus (P) limited often due to larger Pstocks sequestered in forms of low bioavailability. One of the strategies to overcome thisshortage lies in the symbiosis of plants with mycorrhizal fungi, increasing the plant Puptake of these hardly accessible sources. However, little is known about mycorrhizalfungal mediated partitioning of differently available P forms, which could contributeto more efficient use of P by plants and, thereby, reduce competition for soil P. Thisstudy aimed to investigate the uptake of P from differently bioavailable P sources byectomycorrhiza. For that, we conducted a rhizotrone study using Populus x canescensand its compatible ectomycorrhizal fungus Paxillus involutus. Four different P sources[ortho-phosphate (oP), adenosine monophosphate (AMP), hydroxyapatite (HAP), andoP bound to goethite (gP)] or only HAP as 1P control were supplied in separatecompartments, where only the fungal partner had access to the P sources. The amountof the specific P sources was increased according to their decreasing bioavailability. Inorder to distinguish between the P sources, we applied 33P to track its incorporation inplants by a non-destructive analysis via digital autoradiography. Our results show that anectomycorrhizal plant is able to utilize all provided P sources via its mycorrhizal fungalassociate. The acquisition timing was determined by the most bioavailable P sources,with oP and AMP over HAP and gP, and a mixed P pool over a single P source. Incontrast, the magnitude was defined by the by the amount of supplied P source provisionof additional nitrogen, hence AMP over oP and gP, as well as by P source complexity,with gP as the least favorable P form. Nevertheless, the results of the present studyprovide evidence that an ectomycorrhiza has the potential to occupy fundamental nichesof various P sources differing in their bioavailability, indicating that being a generalist in Pnutrition can facilitate adaptation to various nutritional settings in soil