Abstract
Cadmium (Cd) pollution in soils is an increasing problem worldwide, and it affects crop production and safety. We identified Cd-tolerant and -sensitive cultivars by testing 258 accessions of Medicago truncatula at seedling stage, using the relative root growth (RRG) as an indicator of Cd tolerance. The factorial analysis (principal component analysis method) of the different growth parameters analyzed revealed a clear differentiation between accessions depending on the trait (tolerant or sensitive). We obtained a normalized index of Cd tolerance, which further supported the suitability of RRG to assess Cd tolerance at seedling stage. Cd and elements contents were analyzed, but no correlations with the tolerance trait were found. The responses to Cd stress of two accessions which had similar growth in the absence of Cd, different sensitivity to the metal but similar Cd accumulation capacity, were analyzed during germination, seedling stage, and in mature plants. The results showed that the Cd-tolerant accession (CdT) displayed a higher tolerance than the sensitive cultivar (CdS) in all the studied stages. The increased gene expression of the three main NADPH recycling enzymes in CdT might be key for this tolerance. In CdS, Cd stress produced strong expression of most of the genes that encode enzymes involved in glutathione and phytochelatin biosynthesis (MtCYS, MtγECS, and MtGSHS), as well as GR, but it was not enough to avoid a redox status imbalance and oxidative damages. Our results on gene expression, enzyme activity, antioxidant content, and lipid peroxidation indicate different strategies to cope with Cd stress between CdS and CdT, and provide new insights on Cd tolerance and Cd toxicity mechanisms in M. truncatula.
Highlights
Cadmium (Cd) is one of the most toxic substances for all living organisms, and its accumulation represents an increasing problem in agricultural soils due to anthropogenic activities, such as the extensive use of soil amendments and phytochemicals in agriculture, mining and industrial activities, transportation, uncontrolled dumping, and bad practices in waste treatment (Clemens et al, 2013)
Our results suggest different strategies to cope with Cd stress depending on the Cd tolerance trait, and provide new insights on Cd response mechanisms in M. truncatula
The appropriate Cd treatment for the screening assays was determined based on the root growth response of M. truncatula cv
Summary
Cadmium (Cd) is one of the most toxic substances for all living organisms, and its accumulation represents an increasing problem in agricultural soils due to anthropogenic activities, such as the extensive use of soil amendments and phytochemicals in agriculture, mining and industrial activities, transportation, uncontrolled dumping, and bad practices in waste treatment (Clemens et al, 2013). Cd levels in some areas of China have increased up to 250% in the last 30 years (Rodríguez-Eugenio et al, 2018), and in large cultivated areas, Cd accumulates to healththreatening levels in the so-called “cadmium rice” (Holdaway and Wang, 2018) It is becoming of outmost interest the identification of crops and cultivars that are capable to tolerate moderate Cd stress and do not accumulate this heavy metal in their edible parts. Cd-tolerant M. truncatula cultivars which do not accumulate Cd in the aerial part could be cultivated as a forage crop in moderately Cd-polluted soils, while cultivars with high metal accumulation could be used in phytoremediation of Cd-contaminated soils, due to its high biomass and good soil coverage The identification of such cultivars in M. truncatula germplasm represents a potential tool to remediate the problems derived from Cd accumulation in both, arable lands and wild ecosystems. We have shown before the feasibility of identifying heavy metal-tolerant varieties in a M. truncatula germplasm by phenotyping a limited number of accessions (García de la Torre et al, 2013)
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