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Orthologs of the Class A4 Heat Shock Transcription Factor HsfA4a Confer Cadmium Tolerance in Wheat and Rice^[C],[W]

^a POSTECH-UZH Global Research Laboratory, Division of Integrative Biology and Biotechnology, Pohang University of Science and Technology, Pohang, 790-784, Korea
^b Division of Molecular and Life Science, Biotechnology Research Center, Pohang University of Science and Technology, Pohang 790-784, Korea
^c Institute of Plant Biology, University of Zurich, 8008 Zurich, Switzerland

² Address correspondence to ylee{at}postech.ac.kr.

Cadmium (Cd) is a widespread soil pollutant; thus, the underlying molecular controls of plant Cd tolerance are of substantial interest. A screen for wheat (Triticum aestivum) genes that confer Cd tolerance to a Cd hypersensitive yeast strain identified Heat shock transcription factor A4a (HsfA4a). Ta HsfA4a is most similar to the class A4 Hsfs from monocots. The most closely related rice (Oryza sativa) homolog, Os HsfA4a, conferred Cd tolerance in yeast, as did Ta HsfA4a, but the second most closely related rice homolog, Os HsfA4d, did not. Cd tolerance was enhanced in rice plants expressing Ta HsfA4a and decreased in rice plants with knocked-down expression of Os HsfA4a. An analysis of the functional domain using chimeric proteins constructed from Ta HsfA4a and Os HsfA4d revealed that the DNA binding domain (DBD) of HsfA4a is critical for Cd tolerance, and within the DBD, Ala-31 and Leu-42 are important for Cd tolerance. Moreover, Ta HsfA4a–mediated Cd resistance in yeast requires metallothionein (MT). In the roots of wheat and rice, Cd stress caused increases in HsfA4a expression, together the MT genes. Our findings thus suggest that HsfA4a of wheat and rice confers Cd tolerance by upregulating MT gene expression in planta.