Nodulated White Lupin Plants Growing in Contaminated Soils Accumulate Unusually High Mercury Concentrations in Their Nodules, Roots and Especially Cluster Roots

Abstract

Two white lupin (Lupinus albus L.) cultivars were tested for their capacity to accumulate mercury when grown in Hg-contaminated soils. Plants inoculated with a Bradyrhizobium canariense Hg-tolerant strain or non-inoculated were grown in two highly Hg-contaminated soils. All plants were nodulated and presented a large number of cluster roots. They accumulated up to 600 μg Hg g−1 DW in nodules, 1400 μg Hg g−1 DW in roots and 2550 μg Hg g−1 DW in cluster roots. Soil, and not cultivar or inoculation, was accountable for statistically significant differences. No Hg translocation to leaves or seeds took place. Inoculated L. albus cv. G1 plants were grown hydroponically under cluster root-promoting conditions in the presence of Hg. They accumulated about 500 μg Hg g−1 DW in nodules and roots and up to 1300 μg Hg g−1 DW in cluster roots. No translocation to the aerial parts occurred. Bioaccumulation factors were also extremely high, especially in soils and particularly in cluster roots. To our knowledge, Hg accumulation in cluster roots has not been reported to date. Our results suggest that inoculated white lupin might represent a powerful phytoremediation tool through rhizosequestration of Hg in contaminated soils. Potential uptake and immobilization mechanisms are discussed.

Adaptive Mechanisms Make Lupin a Choice Crop for Acidic Soils Affected by Aluminum Toxicity

“…It has been proposed as a suitable candidate for phytoremediation. Recently, white lupin has been reported as an Hg-resistant plant capable of Hg rhizosequestration, especially in CR, with null translocation of the metal to the aerial parts of the plant ( Quiñones et al, 2021 ). It appears that CR have a role not only in elevated secretion of metal chelators, but also in the immobilization of toxic metals, a subject that certainly requires further investigation for its practical application in avoiding translocation to the aerial parts of the plant, a topic that requires further analyses, and for the elucidation of metal sequestration mechanisms in plant roots.…”

“…High exopolysaccharide production by lupin-nodulating bradyrhizobia has been proposed to prevent Hg toxicity (Arregui et al, 2021;Quiñones et al, 2021). Physical sequestration, exclusion and complexation mechanisms contribute to reduce metal availability (Rajkumar et al, 2010).…”

“…Several mechanisms have been proposed for acid tolerance, including changes in lipopolysaccharide composition of the outer membrane or polyamine accumulation ( Chen et al, 1993 ; Ferreira et al, 2012 ; Jaiswal et al, 2018 ). High exopolysaccharide production by lupin-nodulating bradyrhizobia has been proposed to prevent Hg toxicity ( Arregui et al, 2021 ; Quiñones et al, 2021 ). Physical sequestration, exclusion and complexation mechanisms contribute to reduce metal availability ( Rajkumar et al, 2010 ).…”

“…As a horticultural crop, white lupins play a role in ecosystem services as honey plants and annual ornamentals (Jansen, 2006). White lupin has a notable tolerance to abiotic stresses (Fernández-Pascual et al, 2007), and a considerable potential as a tool for the recovery of degraded soils and phytoremediation of toxic metal-polluted soils (Coba de la Peña and Pueyo, 2012;Quiñones et al, 2013Quiñones et al, , 2021. Lupins establish symbiosis with soil bacteria leading to the formation of nitrogen-fixing root nodules.…”

Evaluation of the oxidative stress alleviation in Lupinus albus var. orden Dorado by the inoculation of four plant growth-promoting bacteria and their mixtures in mercury-polluted soils

“…Whether these transporters could play a similar role in Hg absorption remains to be demonstrated. Quiñones et al (2013Quiñones et al ( , 2021 have used this plant species to demonstrate its ability to accumulate significant amounts of Hg in roots and nodules. This fact can induce a reduction in biomass production.…”