NO contributes to cadmium toxicity in Arabidopsis thaliana by mediating an iron deprivation response

Abstract

Several studies have revealed that nitric oxide (NO), an endogenous mediator in diverse physiological processes, is produced in plants exposed to the toxic metal cadmium (Cd). It was first shown that exogenously applied NO protects plant tissues against the oxidative damages triggered by Cd, suggesting a putative role for NO in counteracting the deleterious effects of Cd. More recently, our team as well as other laboratories challenged this view and demonstrated that endogenously produced NO promotes the metal-induced reduction of root growth. We investigated more thoroughly the role of NO in mediating Cd effects in roots. We have shown that in Arabidopsis thaliana, the Cd-mediated NO production is sensitive to mammalian NO synthase inhibitors and occurs downstream of IRT1, a major iron transporter also involved in the uptake of Cd. Our data support a model in which this production might be related to the iron deprivation caused by Cd. Accordingly, we found that NO upregulates the expression of genes encoding proteins related to iron acquisition, including IRT1. This process might explain the ability of NO to amplify Cd uptake and, consequently, the toxic effects of the metal.

Keywords: Arabidopsis thaliana roots; IRT1; cadmium; iron acquisition; nitric oxide.

Figure 1

NO production and function in A. thaliana roots exposed to Cd. (A) NO production. Roots of 3-week-old A. thaliana irt1 and wild type (Wt) seedlings grown on plates were loaded with the NO-sensitive fluorophore diaminofluorescein diacetate (DAF-2DA; 20 µM) in 50 mM Tris-HCl pH 7.5 for 2 h. Then, roots were washed three times with distilled water to wash off excessive fluorophore and incubated for 7 h in the dark with CdSO₄ (200 µM). Digital images were captured with a cool charged-coupled device camera controlled with AxioVision software (Zeiss). (B) A hypothetical model depicting the involvement of NO in the plant response to Cd. Cd enters root cells through IRT1 (and most probably through other transporters), a major ferrous iron transporter in A. thaliana. The competition between Cd and iron leads to a reduction of the intracellular iron content and, consequently, favours NO synthesis. NO production might also result from other non-characterized mechanisms. Once produced, NO initiates an iron-starvation pathway promoting the upregulation of the expression of the iron uptake genes IRT1, FRO2 and FIT. In turn, the corresponding proteins synthesised de novo amplify Cd uptake, thus favouring the deleterious effects of Cd including the inhibition of root growth. pm: plasma membrane.