Abstract
Abiotic stress induces nitrate (NO3-) allocation to roots, which increases stress tolerance in plants. NRT1.1 is broadly involved in abiotic stress tolerance in plants, but the relationship between NRT1.1 and NO3- allocation under stress conditions is unclear. In this study, we found that Arabidopsis wild-type Col-0 was more cadmium (Cd2+)-tolerant than the nrt1.1 mutant at 20 μM CdCl2. Cd2+ exposure repressed NRT1.5 but upregulated NRT1.8 in roots of Col-0 plants, resulting in increased NO3- allocation to roots and higher [NO3-] root-to-shoot (R:S) ratios. Interestingly, NITRATE REGULATORY GENE2 (NRG2) was upregulated by Cd2+ stress in Col-0 but not in nrt1.1. Under Cd2+ stress, nrg2 and nrg2-3chl1-13 mutants exhibited similar phenotypes and NO3- allocation patterns as observed in the nrt1.1 mutant, but overexpression of NRG2 in Col-0 and nrt1.1 increased the [NO3-] R:S ratio and restored Cd2+ stress tolerance. Our results indicated that NRT1.1 and NRG2 regulated Cd2+ stress-induced NO3- allocation to roots and that NRG2 functioned downstream of NRT1.1. Cd2+ uptake did not differ between Col-0 and nrt1.1, but Cd2+ allocation to roots was higher in Col-0 than in nrt1.1. Stressed Col-0 plants increased Cd2+ and NO3- allocation to root vacuoles, which reduced their cytosolic allocation and transport to the shoots. Our results suggest that NRT1.1 regulates NO3- allocation to roots by coordinating Cd2+ accumulation in root vacuoles, which facilitates Cd2+ detoxification.
Highlights
Heavy metal pollution in soil is an important environmental issue worldwide, which gives rise to agricultural and public health concerns (Bertin and Averbeck, 2006; Mohammed et al, 2011; Åkesson et al, 2014)
Cd can be released to the Abbreviations: Jmax, maximum electron transport rate; MDA, malondialdehyde; NR, nitrate reductase; NRG2, NITRATE REGULATORY GENE2; PPED, photosynthetic photon flux intensity; R:S, root-to-shoot; stress-initiated nitrate allocation to roots (SINAR), stress-initiated nitrate allocation in roots; Superoxide Dismutase (SOD), superoxide dismutase; TPU, triose phosphate utilization; Vc,max, maximum carboxylation rate of Rubisco
The [NO3−] R:S ratio under Cd2+ stress in 35S::NRG2/Col-0 and 35S::NRG2/chl15 was significantly increased as compared to that in Col0 (Figure 5D). These data indicated that NRG2 acted as a downstream element of NRT1.1 to regulate NO3− allocation under Cd2+ stress
Summary
Heavy metal pollution in soil is an important environmental issue worldwide, which gives rise to agricultural and public health concerns (Bertin and Averbeck, 2006; Mohammed et al, 2011; Åkesson et al, 2014). In China, for example, approximately 7% of the soil is cadmium (Cd) contaminated, 0.5% of which is severely polluted (Zhang et al, 2015). NRT1.1 Regulates Cadmium Tolerance soil by excessive use of chemical fertilizers and pesticides, utilization of industrial wastewater and sludge, and atmospheric deposition (Woodis et al, 1977; He and Singh, 1994; Wong et al, 2003; Ottosen et al, 2007; Roberts, 2014). Susceptibility to Cd2+ stress is species-specific in plants (Chen, 1996), which provides opportunities to select and breed Cd2+-tolerant species/varieties. This requires a complete understanding of the underlying mechanisms of Cd2+ tolerance in plants
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