Inoculation with a Pb-tolerant strain of Paxillus involutus improves growth and Pb tolerance of Populus × canescens under in vitro conditions

Plant Soil (2017) 412:253–266 DOI 10.1007/s11104-016-3062-3 REGULAR ARTICLE Inoculation with a Pb-tolerant strain of Paxillus involutus improves growth and Pb tolerance of Populus × canescens under in vitro conditions Agnieszka Szuba & Leszek Karliński & Magdalena Krzesłowska & Teresa Hazubska-Przybył Received: 10 March 2016 / Accepted: 15 September 2016 / Published online: 27 September 2016 # The Author(s) 2016. This article is published with open access at Springerlink.com Abstract Aims Ectomycorrhizal fungi can improve poplar growth and tolerance to heavy metal stress, and may be useful during the afforestation and phytoremediation of polluted regions with poplar trees. In this study, we determined the effects of the symbiotic interaction between Populus × canescens trees and Paxillus involutus strains different in their tolerance to lead. Methods In vitro inoculated and non-inoculated plants were treated with 0.75 mM Pb(NO3)2. The root colonization rate of the two fungal strains, as well as their impacts on poplar health and lead accumulation were examined. Results Based on the colonization level, the roots were classified into one of three categories: non-mycorrhized, changed (ie, fungal cells were present on the root surface, but the Hartig net did not fully develop), and fully mycorrhized. The lead-tolerant P. involutus strain colonized roots better than the non-tolerant strain (ie, changed and fully mycorrhized roots). Moreover, plants Responsible Editor: Fangjie Zhao. inoculated with the tolerant fungal strain grew better than the control plants (217 % increase in dry weight over the controls), and accumulated lead in the roots and stems. Conclusions Inoculation of P. × canescens trees with a Pb-tolerant strain of P. involutus improves host plant growth and may increase Pb phytostabilization potential. Keywords Ectomycorrhiza . Colonization rate . Heavy metals . Biometrics . Phytoremediation Abbreviations Chl (x + c) Chlorophyll (x + c) total carotenoids Chl a Chlorophyll a Chl b Chlorophyll b DW Dry weight ECM Ectomycorrhizal FW Fresh weight PA Projected area SLA Specific leaf area SRL Specific root length Electronic supplementary material The online version of this article (doi:10.1007/s11104-016-3062-3) contains supplementary material, which is available to authorized users. A. Szuba (*) : L. Karliński : T. Hazubska-Przybył Institute of Dendrology, Polish Academy of Sciences, Parkowa 5, 62-035 Kórnik, Poland e-mail: M. Krzesłowska Laboratory of General Botany, Adam Mickiewicz University, Umultowska 89, 61-614 Poznań, Poland Introduction Ectomycorrhizal (ECM) fungi are obligatory symbionts of vascular plants, including poplar trees (Smith and Read 2008). These fungi belong to various taxonomic groups, with the majority in the phyla Ascomycota and Basidiomycota (Krpata et al. 254 2008). Paxillus involutus is an example of an ECM fungus from the phylum Basidiomycota (Smith and Read 2008). Unlike arbuscular mycorrhizal symbionts, ECM fungi do not penetrate host cells. Instead, they form a mantle that surrounds the roots, and penetrate between the epidermis and cortical cells to produce fully functional ectomycorrhizae (ie, the Hartig net) (Smith and Read 2008). It is here that compounds are exchanged between the plant host and fungi. Plants receive nutrients, especially nitrogen (Willmann et al. 2014) and phosphorus (Vodnik et al. 1996), as well as water (Marjanović et al. 2005) in exchange for carbohydrates. Consequently, nutrient contents increase in plant host tissues (Vodnik et al. 1996; Ma et al. 2014). However, approximately 20 % of the carbon assimilated by the plant is consumed by the fungal symbiont for its external mycelia (Cairney 2012). Nevertheless, in most cases, ECM fungi significantly increase the plant host biomass (Danielsen et al. 2013; Ma et al. 2014). This has economic implications for poplar trees, which serve as valuable sources of biomass (Szuba 2015). Mycorrhizae are important, especially in nutrient-deficient regions (Krpata et al. 2008) and anthropogenically disturbed environments (Krpata et al. 2008; Karliński et al. 2013; Willmann et al. 2014). Moreover, Populus spp., particularly aspens, are early-successional trees (Krpata et al. 2008; Szuba 2015). Because of their potential ability to grow in harsh conditions, poplar trees with mycorrhizal associations may be useful for the phytoremediation of regions polluted with heavy metals, particularly lead (Bhargava et al. 2012; Ali et al. 2013). Lead is one of the biggest threats to the environment. Because lead can precipitate, once an area is affected, it remains polluted (Tangahu et al. 2011; Fahr et al. 2013). Therefore, various methods for soil remediation, including ECM fungi-mediated phytoremediation, are highly sought after (Ali et al. 2013). Lead belongs to a group of non-essential heavy metals, which are toxic to living organisms even at very low concentrations because they accumulate in tissues. In excessive amounts, lead causes abnormal plant cell division, altered nitrogen metabolism, disorders in plant-water relationships, and inhibited growth and enzymatic activities (Fahr et al. 2013). Heavy metals damage cellular membranes, proteins, lipids, and DNA (Michalak 2006; Jiang and Liu 2010). However, plants respond to lead exposure by Plant Soil (2017) 412:253–266 adsorbing Pb2+ ions in the cell wall, predominantly by low-methyl esterified pectins (Rabęda et al. 2015) and other cell wall compounds such as hemicellulose and phenols (Krzesłowska 2011), transporting Pb2+ ions to protoplasts (mainly vacuoles) using thiol-containing groups, and activating antioxidant systems (Bellion et al. 2006). Most Pb2+ ions are bound to cation exchange sites and immobilized in roots (Jentschke and Godbold 2000). Therefore, damages are caused by high cytosolic concentrations of lead (Jiang and Liu 2010). The symbiotic relationship between plants and mycorrhizal fungi results in enhanced host tolerance to various abiotic stresses, including exposure to heavy metals (Jentschke and Godbold 2000; Szuba 2015). This tolerance is associated with the immobilization of lead compounds in the rhizosphere with organic compounds, such as peptides or organic acids, secreted by the fungi (Turnau et al. 2006; Johansson et al. 2008). Consequently, a lower abundance of lead is available for plant roots. Fungal detoxification and storage involve the binding of bioavailable Pb2+ ions to fungal cell wall elements, such as chitin (Marschner et al. 1998; Jentschke and Godbold 2000), or their chelation and transport to vacuoles (Bellion et al. 2006). Hence, fungal mycelia form a barrier that protects plant tissues from the toxic effects of Pb2+ ions (Marschner et al. 1998; Bellion et al. 2006). However, the mechanisms regulating ECM fungimediated responses to heavy metal stress have not been fully characterized. In general, heavy metals inhibit the growth of ECM fungi (Vodnik et al. 1998; Blaudez et al. 2 (...truncated)