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Abstract
In Arabidopsis thaliana, FRD3 (FERRIC CHELATE REDUCTASE DEFECTIVE 3) plays a central role in metal homeostasis. FRD3 is among a set of metal homeostasis genes that are constitutively highly expressed in roots and shoots of Arabidopsis halleri, a zinc hyperaccumulating and hypertolerant species. Here, we examined the regulation of FRD3 by zinc in both species to shed light on the evolutionary processes underlying the evolution of hyperaccumulation in A. halleri. We combined gene expression studies with the use of β-glucuronidase and green fluorescent protein reporter constructs to compare the expression profile and transcriptional and post-transcriptional regulation of FRD3 in both species. The AtFRD3 and AhFRD3 genes displayed a conserved expression profile. In A. thaliana, alternative transcription initiation sites from two promoters determined transcript variants that were differentially regulated by zinc supply in roots and shoots to favour the most highly translated variant under zinc-excess conditions. In A. halleri, a single transcript variant with higher transcript stability and enhanced translation has been maintained. The FRD3 gene thus undergoes complex transcriptional and post-transcriptional regulation in Arabidopsis relatives. Our study reveals that a diverse set of mechanisms underlie increased gene dosage in the A. halleri lineage and illustrates how an environmental challenge can alter gene regulation.
Highlights
Zinc is an essential micronutrient with numerous important functions in plants but becomes toxic when accumulated in excess (Broadley et al, 2007; Palmer and Guerinot, 2009; Nouet et al, 2011)
We further examined the transcriptional regulation of FRD3 by zinc in root and shoot tissues of A. thaliana
We examined the regulation of FRD3 expression in two closely related Brassicaceae, A. thaliana and A. halleri, with contrasting metal homeostasis (Verbruggen et al, 2009; Krämer, 2010; Hanikenne and Nouet, 2011)
Summary
Zinc is an essential micronutrient with numerous important functions in plants but becomes toxic when accumulated in excess (Broadley et al, 2007; Palmer and Guerinot, 2009; Nouet et al, 2011). -called metal hyperaccumulation, found in approximately 500 plant species, represents a rare and extreme adaptation of the metal homeostasis network. A. halleri diverged from Arabidopsis thaliana and Arabidopsis lyrata, two non-accumulator and non-tolerant species between 3 and 5.8 million years ago and 0.4 and 2 million years ago, respectively (Yogeeswaran et al, 2005; Clauss and Koch, 2006; Roux et al, 2011). These three species represent an ideal experimental model to examine the mechanisms of evolution of a naturally selected extreme trait
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