Isolation and characterization of tomato Hsa32 encoding a novel heat-shock protein

Heat-shock (HS) response is universal and many HS protein (Hsp) genes are highly conserved in all organisms. Plants, being sessile, may require unique genes to tolerate frequent temperature fluctuation in nature environment. However, this aspect has not been well explored. Here, we report the study on a novel Hsp gene, Hsa32, which encodes a HS-associated 32 kDa protein. Hsa32 cDNA was isolated from a subtractive library prepared from tomato HS-induced versus control mRNA. Unlike the multigene families of many well-characterized Hsps, only one copy of Hsa32 exists in the tomato genome as shown by Southern blot. In addition to HS, the gene was also induced by salt, osmotic and oxidative stresses but to a much lower extent than by heat. Under non-HS conditions, LeHsa32 transcripts were more abundant in fruits, flowers and cultured cells. Multiple alternatively spliced forms of Hsa32 transcripts were observed in the HS-treated tomato flower but not in the leaf. Heterologous expression of a fusion protein of Arabidopsis HsfA1b and GUS in tomato results in the up-regulation of Hsa32 without heat treatment. So far, homologs of tomato Hsa32 could be found mainly in land plants and some microbes. Phylogenic analysis suggests that Hsa32 shares a common ancestor with bacterial phosphosulfolactate synthase. Identification of Hsa32 supports the idea that plants possess special features in HS response.