Os01g0749300

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Annotated Information

Function

Fig.01 Identification of Amino Acid Residues in the DBD of Ta HsfA4a That Are Critical for Cd Tolerance

Fig.02 Enhanced Cd Resistance in Ta HsfA4a–Overexpressing Rice Plants

Fig.03 Plant Proteins with Homology to Ta HsfA4a

Os HsfA4a is a rice ortholog of Ta HsfA4a which plays a role as Cd tolerance gene of Wheat. Ta HsfA4a is the most similar to the class A4 Hsfs that confer Cd tolerance to a Cd hypersensitive yeast strain identified Heat shock transcription factor A4a(HsfA4a). Cd tolerance is enhanced in rice plants expressing Ta HsfA4a and decreased in rice plant with knocked-down expression of Os HsfA4a. 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. Os HsfA4a confers Cd tolerance in the root of rice by upregulating metallothionein(MT) gene expression in planta. HsfA4a is unique not only in the high level of tolerance that it confers, but also in its Cd specificity. Os HsfA4a greatly enhances Cd tolerance in rice, but does not significantly enhance their tolerance to the other heavy metals, namely, lead, zinc, cobalt, manganese, silver, mercury, and iron.

Expression

The DBD binding domain of Os HsfA4a is critical for Cd tolerance. The region for the direct binding to DNA is conserved. However, a few animo acids outside the direct DNA binding region are different in Os HsfA4a when compared with other Hsfs. Ala-31 and Leu-42 in the DBD domain of Ta HsfA4a are critical for its ability to confer Cd tolerance. As a transcription factor, HsfA4a is expected to function by regulating the expression levels of other genes. CUP1 contributes to HsfA4a-induced Cd tolerance by acting as a downstream target of HsfA4a. Overexpression of HsfA4a enhances Cd tolerance in rice. The small, Cys-rich metallothionein proteins are the main chelators of Cd and other heavy metals in animals. HsfA4a confers Cd resistence in rice via upregulation of metallothionein, at least in part. MT genes are important targets of HsfA4a and MT expression is necessary for Cd tolerance in plants. HsfA4a is upregulated by Cd treatment and it in turn upregulates the expression of target genes, including MTs, and thereby confers Cd tolerance in planta. The site where HsfA4a confers Cd tolerance is the root and not the shoot. HsfA4a most likely activates multiple Cd tolerance mechanisms, similarly to the mechanisms described for MTF-1 in animal cells. The Cd-responsive induction of the Os HsfA4a genes is rapid and strong in rice root, and their expression induced a well-known Cd tolerance gene, metallothionein, leading to Cd tolerance in the plants.

Evolution

Plant genomes contain far more Hsf genes than yeast or animal ges and have evolved functionally specialized Hsfs to respond to a variety of stresses. For example, some class A4 members might have evolved to confer Cd tolerance via a few amino acid changes in their DBDs (e.g., Ala-31 and Leu-42 in Ta HsfA4a; Gly-31 and Ala-42 in Os HsfA4a). The yeast Hsf (HSF1) provides an example of the acquisition of a new function via a single amino acid change in the DBD. Normally, HSF1 does not activate the CUP1 gene; however, a mutation of Tyr to Phe in the DBD of HSF1 allowed it to activate the CUP1 gene and thereby confer resistance to Cd and Cu. The novel function of class A4 Hsfs as Cd tolerance factors evolved after the evolutionary split between monocot and dicot plants because there are no other dicot genes that are highly homologous to Ta HsfA4a in their DBDs.

Labs working on this gene

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

References

Donghwan Shim,Jae-Ung Hwang,Joohyun Lee,Sichul Lee,et al.(2009).Orthologs of the Class A4 Heat Shock Transcription Factor HsfA4a Confer Cadmium Tolerance in Wheat and Rice.J.The Plant Cell.21:4031-4043. Chuang WANG,Qian ZHANG,Hui-xia SHOU.(2009).Identification and expression analysis of OsHsfs in rice.J.Journal of Zhejiang University.4:291-300. Kotak, S., Vierling, E., Baumlein, H., and von Koskull-Doring, P.(2007). A novel transcriptional cascade regulating expression of heat stress proteins during seed development of Arabidopsis. Plant Cell.19: 182–195.