Abstract
Aim: Heavy metal pollution is serious in China, and abscisic acid (ABA) is an important stress hormone. How it regulates plant tolerance to cadmium remains unclear, so we aimed to explore the molecular mechanism responsible for enhanced cadmium resistance in Arabidopsis wild-type and mutant plants and Brassica napus seedlings.Methods: Arabidopsis/B. napus were cultured hydroponically for 28/15 days and then treated with 20/10 μM Cd/Cd+ABA (5 μM) for 3/4 days. Chlorophyll degradation rate, SPAD values, proline, MDA, ABA, , and Cd concentrations were measured in root vacuoles and protoplasts; root to shoot and Cd concentration ratios were determined and NRT1.5-, NRT1.8-, BnNRT1.5-, and BnNRT1.8-related gene expression was studied.Results: Cytoplasmic ABA levels in root cells of bglu10 and bglu18 Arabidopsis mutants were significantly lower than those in the wild-type, apparently making the latter more resistant to Cd. long-distance transporter NRT1.5 responded to ABA signaling by downregulating its own expression, while NRT1.8 did not respond. Concomitantly, proton pump activity in wild-type plants was higher than in the bglu10 and bglu18 mutants; thus, more and Cd accumulated in the vacuoles of wild-type root cells. ABA application inhibited Cd absorption by B. napus. BnNRT1.5 responded to exogenous ABA signal by downregulating its own expression, while the lack of response by BnNRT1.8 resulted in increased amount of accumulating in the roots to participate in the anti-cadmium reaction.Conclusion: NRT1.5 responds to the ABA signal to inhibit its own expression, whereas unresponsiveness of NRT1.8 causes accumulation of in the roots; thus, enhancing Cd resistance. In Arabidopsis, because of proton pump action, more and Cd accumulate in the vacuoles of Arabidopsis root cells, thereby reducing damage by Cd toxicity. However, in B. napus, the addition of exogenous ABA inhibited Cd absorption. Our data provide a sound basis to the theoretical molecular mechanism involved in hormone signaling during response of plants to heavy metal stress.
Highlights
Nitrogen (N) is an essential macronutrient that plays a key role in plant growth and development, and in crop yield (Hirel et al, 2007; Wang et al, 2012; Krapp et al, 2014; Ruffel et al, 2014; Vidal et al, 2014)
The expression of NRT1.8 was induced, there was almost no difference in fold change of NRT1.8 up-regulation between the wild-type and the ABA mutants (Figures 2C,D). These results indicated that NRT1.5, but not NRT1.8, responded to ABA signaling
We demonstrated that under Cd stress, NRT1.5 responded to the ABA signal and the expression level was downregulated, while NRT1.8 did not respond (Figure 2), which in turn caused more NO−3 to accumulate in the roots (Figure 4A), the Col0 anti-cadmium ability is improved
Summary
Nitrogen (N) is an essential macronutrient that plays a key role in plant growth and development, and in crop yield (Hirel et al, 2007; Wang et al, 2012; Krapp et al, 2014; Ruffel et al, 2014; Vidal et al, 2014). During growth and development plants inescapably experience various forms of unfavorable environmental conditions Under such circumstances, NO−3 plays a key role in the processes whereby plants try to prevent any potential damage. Studies showed that NRT1.8 was strongly upregulated by Cd stress in roots, while the nrt1.8-1 mutant showed a nitrate-dependent Cd2+-sensitive phenotype. This finding suggests that NRT1.8 regulated NO−3 distribution may play an important role in Cd2+ tolerance in plants (Li et al, 2010). NRT1.5 functions to mediate NO−3 reallocation to roots, stress-responsive gene expression and metabolism; salt, drought, and Cd2+ tolerance are affected by NRT1.5; further, the mRNA level of NRT1.5 is reportedly downregulated by salt, drought, and Cd treatments; lending support to the hypothesis that NO−3 reallocation to roots might be a common response to stress, coordinately regulated by the NRT1.8 and NRT1.5 (Chen et al, 2012)
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