Lupinus albus plants acquire mercury tolerance when inoculated with an Hg-resistant Bradyrhizobium strain

Abstract

One strain of Bradyrhizobium canariense (L-7AH) was selected for its metal-resistance and ability to nodulate white lupin (Lupinus albus L.) plants, from a collection of rhizobial strains previously created from soils of the Almadén mining district (Spain) with varying levels of Hg contamination. Plants were inoculated with either strain L-7AH (Hg-tolerant) or L-3 (Hg-sensitive, used as control), and watered with nutrient solutions supplemented with various concentrations (0-200 μM) of HgCl2 in a growth chamber. L. albus inoculated with L-7AH were able to nodulate even at the highest concentration of Hg while those inoculated with L-3 had virtually no nodules at Hg concentrations above 25 μM. Plants inoculated with L-7AH, but not those with the control strain, were able to accumulate large amounts of Hg in their roots and nodules. Nodulation with L-7AH allowed plants to maintain constant levels of both chlorophylls and carotenoids in their leaves and a high photosynthetic efficiency, whereas in those inoculated with L-3 both pigment content and photosynthetic efficiency decreased significantly as Hg concentration increased. Nitrogenase activity of plants nodulated with L-7AH remained fairly constant at all concentrations of Hg used. Results suggest that this symbiotic pair may be used for rhizoremediation of Hg-contaminated soils.

Possible reasons for tolerance to mercury of Lupinus albus cv. G1 inoculated with Hg-resistant and sensitive Bradyrhizobium canariense strains

“…Seeds were inoculated twice (first at sowing and then 1 week later), according to de Lorenzo et al (1994) with 1 ml of the Hgresistant strain L7AH (Ruiz-Díez et al 2012a) or the Hgsensitive strain L3 (Ruiz-Díez et al 2012b) of Bradyrhizobium canariense. Increasing concentrations of HgCl 2 (0-200 μM) were added together with the nutrient solution three times a week and until the end of the experiment (6 weeks), according to Quiñones et al (2013). The conditions in the growth chamber were: a 16 h light/8 h dark photoperiod, 25°C day/15°C night temperature, 58 % relative humidity and an irradiance level of 200 μmol m −2 s −1 .…”

“…Legumes are well known for their role in maintaining soil fertility due to their capacity to fix atmospheric nitrogen via the legume-Rhizobium symbiosis and, recently, are being considered as a good tool for phytoremediation of Hgcontaminated soils (Quiñones et al 2013). Nitrogen fixation is performed in the root nodules, which are formed by the interaction between rhizobia and the legume root.…”

“…The Lupinus-Bradyrhizobium symbiosis has been described as tolerant to several abiotic stresses (de Lorenzo et al 1994;Fernández-Pascual et al 1996) among them heavy metals, like Zn (Pastor et al 2003), Cd (Zornoza 2002), Cu (Sánchez-Pardo et al 2012) or Hg (Quiñones et al 2013). There are some studies on lupin plants and Hg accumulation (Rodríguez et al 2007), removal of Hg from contaminated soils (Zornoza et al 2010) and on kinetics and stress indicators (Esteban et al 2008), but very few on the LupinusBradyrhizobium symbiosis.…”

“…In relation to the Lupinus-Bradyrhizobium symbiosis Hg has been previously studied with respect to its effect on nodulation, nitrogenase activity, photosynthetic efficiency and chlorophyll and carotenoid contents (Quiñones et al 2013), and it has also been demonstrated that inoculation with a mercury-resistant bradyrhizobial strain (Ruiz-Díez et al 2012a) confers on L. albus the ability to grow under high concentrations of Hg and to accumulate it. Plant growth, photosynthetic efficiency and nitrogenase activity of plants inoculated with the resistant strain were similar to control plants without Hg, while in those inoculated with the sensitive strain all of them decreased significantly.…”

Dynamic change of wheat eco-physiology and implications for establishing high-efficient stable agro-ecosystems under Hg stress

“…In addition, mercury ions can also affect chlorophyll content, activities of root systems, and free proline content in crops (Liu et al, 2004;Deng et al, 2013;Debeljak et al, 2013;Górska-Czekaj and Borucki, 2013;Verbruggen et al, 2009 diverse-defensing mechanisms to cope with heavy metals, such as extrusion, chelation, vacuolar sequestration and regulation of distribution (Verbruggen et al, 2009;Singh et al, 2010;Demim et al, 2013;Huang et al, 2000). Other researches also indicated that mercury ions can bring about severe effects on water transportation in individual plants and plant organs through influencing water channel proteins from a molecular perspective (Daniels et al, 1996;Agre et al, 1998;Zhang et al, 2012); and under the existence of sub-mM levels' HgCl 2 concentrations, water conductance in plant root systems can be 60-90% lower than that of controlled group (Martre et al, 2001;Quiñones et al, 2013). The above-mentioned researches revealed to a certain extent that mercury ions affect the physiological characteristics of various types of crops, such as wheat ), Medicago truncatula (García de la Torre et al, 2013), and maize (Gupta et al, 2012;Muddarisna et al, 2013;Liu et al, 2004).…”