Arbuscular mycorrhizal fungi as a tool to ameliorate the phytoremediation potential of poplar: biochemical and molecular aspects

Abstract

© iForest -Biogeosciences and Forestry IntroductionMany large areas around the world are contaminated with heavy metals (HMs) and/or organic compounds; most of these have not been remediated due to the high cost and technical drawbacks of currently available technologies. HMs tend to accumulate in soils and aquatic sediments and can enter the food chain leading to the biomagnification phenomenon thereby representing a risk to the environment and to human health (Clijsters et al. 1999). Some essential elements, such as copper (Cu) and zinc (Zn), may be present in soils and waters at potentially toxic levels mainly as a result of agricultural and industrial practices (Ali et al. 2004).Alternative techniques for the clean-up of polluted soil and water, such as the cost-effective and less disruptive phytoremediation, have gained acceptance in recent years (Pilon-Smits 2005, Thewys et al. 2010. Trees have been suggested as suitable for phytoremediation due to their high biomass production (Dickinson & Pulford 2005) and because tree plantations can be multi-purpose (Tognetti et al. 2013). Poplar has many characteristics suitable for phytoremediation: a fast rate of growth, a deep and wide-spreading root system and a metal-resistance trait (Aronsson & Perttu 2001, Di Baccio et al. 2011, Punshon & Dickinson 1997, Sebastiani et al. 2004). In Italy, P. alba (white poplar) and P. nigra (black poplar) populations of the Ticino river valley constitute a hotspot of biodiversity (Castiglione et al. 2009, Fossati et al. 2004. Their genetic variability is being exploited for the selection of genotypes having interesting traits, such as tolerance to pollutants. The remarkable clonal variability of poplar allows to identify genotypes with a greater ability to accumulate/tolerate pollutants including heavy metals , Kopponen et al. 2001, Laureysens et al. 2004, Punshon & Dickinson 1997, Zalesny et al. 2005. Finally, poplar offers the advantage that, being the first "model tree species" whose genome has been sequenced (Tuskan et al. 2004), physiological and molecular mechanisms at the basis of metal tolerance can be more easily investigated at the transcriptomic level (Di Baccio et al. 2011).Plant symbiotic fungi, such as mycorrhizae, and soil bacteria can confer increased tolerance to stress (Gamalero et al. 2009). Arbuscular mycorrhizal fungi (AMF) form associations with the roots of the vast majority of land plants; the fungus colonizes the roots and forms arbuscules within root cortical cells thus improving plant nutrient uptake, especially phosphorus (Smith & Read 1997). Moreover, increasing evidence shows that symbiotic fungi contribute to plant adaptation to multiple biotic and abiotic stresses (Gohre & Paszkowski 2006, Lebeau et al. 2008, Lingua et al. 2002, Liu et al. 2007, Rodriguez & Redman 2008, Smith et al. 2010). In the case of HMs, the beneficial ef- Poplar is a suitable species for phytoremediation, able to tolerate high concentrations of heavy metals (HMs). Arbuscular mycorrhizal fungi (AMF) form symbiotic associatio...

Effect of mycorrhizal inoculation on metal accumulation by poplar leaves at phytomanaged sites

“…Levels of PTEs in soil may also affect the role of arbuscular mycorrhizal symbiosis (Audet and Charest, 2007), which can lead to an increase of plant metal uptake at low soil PTE concentrations as well as a reduction of PTE bioavailability via fungal metal-binding processes at high soil PTE levels. The mechanisms underlying metal accumulation may involve metallothioneins, small proteins that bind metals in plant detoxification process, as demonstrated in a greenhouse experiment (Cicatelli et al, 2014). Garg and Aggarwal (2011) also reported that AMF induced the production of chelating agents (phytochelatins) and antioxidants (glutathione) to reduce PTE toxicity and translocation in plant shoots.…”

“…Mycorrhizal fungi can trigger root exudation of organic acids, and solubilize PTEs (Churchland and Grayston, 2014). Conversely, it is likely that the inoculation also decreased PTE mobility in other plots (P11, P23, F9 and F12), via metal-binding processes (Cicatelli et al, 2014). Moreover, the extractable PTE concentrations in soils was not correlated with the effect of inoculation on PTE accumulation by poplars (Fig.…”

“…In fact, specific root exudates released by trees trigger the expression of mycorrhization genes, resulting in symbiotic relationships between plants and mycorrhizal fungi (Churchland and Grayston, 2014 (Baum et al, 2002;Cicatelli et al, 2014;Mrnka et al, 2012) but rarely in field conditions. On a slightly contaminated site, Bissonnette et al (2010) reported that arbuscular mycorrhizal fungi inoculation had no significant effect on Cd accumulation in poplars shoots used for phytoextraction,.…”

“…Indeed, when associated with endomycorrhizal fungi, poplars exhibit enhanced tolerance to biotic and abiotic stress, with better growth and higher biomass production (Cicatelli et al, 2014). When plants are introduced for site restoration, the in-situ success of these plants will likely be depending on the recruitment of appropriate rhizosphere microorganisms from their new environment (Bell et al, 2015).…”

Paternal arbuscular mycorrhizal fungal status affects DNA methylation in seeds

“…There is recent evidence that the relationship with AMF can cause changes in DNA methylation of plants [15]. What is not known is whether AMF can in fact impact DNA methylation trangenerationally.…”