Bean Metal-Responsive Element-Binding Transcription Factor Confers Cadmium Resistance in Tobacco

Abstract

Cadmium (Cd) is highly toxic to plants. Modulation of Cd-responsive transcription is an important way for Cd detoxification in plants. Metal-responsive element (MRE) is originally described in animal metallothionein genes. Although functional MREs also exist in Cd-regulated plant genes, specific transcription factors that bind MRE to regulate Cd tolerance have not been identified. Previously, we showed that Cd-inducible bean (Phaseolus vulgaris) stress-related gene2 (PvSR2) produces a short (S) PvSR2 transcript (S-PvSR2) driven by an intronic promoter. Here, we demonstrate that S-PvSR2 encodes a bean MRE-binding transcription factor1 (PvMTF-1) that confers Cd tolerance in tobacco (Nicotiana tabacum). PvMTF-1 expression was up-regulated by Cd at the levels of RNA and protein. Importantly, expression of PvMTF-1 in tobacco enhanced Cd tolerance, indicating its role in regulating Cd resistance in planta. This was achieved through direct regulation of a feedback-insensitive Anthranilate Synthase a-2 chain gene (ASA2), which catalyzes the first step for tryptophan biosynthesis. In vitro and in vivo DNA-protein interaction studies further revealed that PvMTF-1 directly binds to the MRE in the ASA2 promoter, and this binding depends on the zinc finger-like motif of PvMTF-1. Through modulating ASA2 up-regulation by Cd, PvMTF-1 increased free tryptophan level and subsequently reduced Cd accumulation, thereby enhancing Cd tolerance of transgenic tobacco plants. Consistent with this observation, tobacco transiently overexpressing ASA2 also exhibited increased tolerance to Cd. We conclude that PvMTF-1 is a zinc finger-like transcription factor that links MRE to Cd resistance in transgenic tobacco through activation of tryptophan biosynthesis.

Transcription Factors PvERF15 and PvMTF-1 Form a Cadmium Stress Transcriptional Pathway

“…We previously revealed a Cd-responsive transcription factor (such as PvMTF-1)/Cd response element (such as the metal response element) working model (Sun et al, 2015). However, unlike the metal response element, ACE is a positive element and interacts with a Cd-responsive PvERF15 transcription factor to regulate Cd-induced transcription.…”

“…They trigger the expression of Cd detoxification genes and, thus, converge Cd stress signals Chmielowska-Bąk et al, 2014). Although the downstream targets of these transcription factors have been identified extensively (Shim et al, 2009;Farinati et al, 2010;Wu et al, 2012;Sun et al, 2015;Chen et al, 2016;He et al, 2016), the upstream transcriptional regulatory pathways that modulate their responses to Cd remain unclear.…”

“…We previously identified bean (Phaseolus vulgaris) METAL RESPONSE ELEMENT-BINDING TRANSCRIPTION FACTOR1 (PvMTF-1) as a Cd-responsive transcription factor (Sun et al, 2015). PvMTF-1 contains a zinc finger-like domain and confers Cd tolerance of tobacco (Nicotiana tabacum) plants through activating Trp biosynthesis (Sun et al, 2015).…”

“…PvMTF-1 contains a zinc finger-like domain and confers Cd tolerance of tobacco (Nicotiana tabacum) plants through activating Trp biosynthesis (Sun et al, 2015). The expression of PvMTF-1 is driven by an intronic promoter located in the bean STRESS-RELATED GENE2 (PvSR2) locus (Qi et al, 2007;Sun et al, 2015;Yang et al, 2015).…”

“…Interestingly, the expression of PvMTF-1 is induced by Cd stress (Sun et al, 2015), suggesting that it should be activated by putative upstream transcription factors in the Cd response pathways. In this study, we identified and functionally characterized a bean ethylene response factor, PvERF15, involved in Cd-induced PvMTF-1 activation and Cd tolerance.…”