Abstract
BackgroundHeat shock proteins (Hsps) constitute an important component in the heat shock response of all living systems. Among the various plant Hsps (i.e. Hsp100, Hsp90, Hsp70 and Hsp20), Hsp20 or small Hsps (sHsps) are expressed in maximal amounts under high temperature stress. The characteristic feature of the sHsps is the presence of α-crystallin domain (ACD) at the C-terminus. sHsps cooperate with Hsp100/Hsp70 and co-chaperones in ATP-dependent manner in preventing aggregation of cellular proteins and in their subsequent refolding. Database search was performed to investigate the sHsp gene family across rice genome sequence followed by comprehensive expression analysis of these genes.ResultsWe identified 40 α-crystallin domain containing genes in rice. Phylogenetic analysis showed that 23 out of these 40 genes constitute sHsps. The additional 17 genes containing ACD clustered with Acd proteins of Arabidopsis. Detailed scrutiny of 23 sHsp sequences enabled us to categorize these proteins in a revised scheme of classification constituting of 16 cytoplasmic/nuclear, 2 ER, 3 mitochondrial, 1 plastid and 1 peroxisomal genes. In the new classification proposed herein nucleo-cytoplasmic class of sHsps with 9 subfamilies is more complex in rice than in Arabidopsis. Strikingly, 17 of 23 rice sHsp genes were noted to be intronless. Expression analysis based on microarray and RT-PCR showed that 19 sHsp genes were upregulated by high temperature stress. Besides heat stress, expression of sHsp genes was up or downregulated by other abiotic and biotic stresses. In addition to stress regulation, various sHsp genes were differentially upregulated at different developmental stages of the rice plant. Majority of sHsp genes were expressed in seed.ConclusionWe identified twenty three sHsp genes and seventeen Acd genes in rice. Three nucleocytoplasmic sHsp genes were found only in monocots. Analysis of expression profiling of sHsp genes revealed that these genes are differentially expressed under stress and at different stages in the life cycle of rice plant.
Highlights
Heat shock proteins (Hsps) constitute an important component in the heat shock response of all living systems
We noticed two exceptions in this clustering pattern: (1) Hsp22.3-CVI along with recently categorized AtHsp21.7-CVI [8] was present on the clade with Acd genes and (2) Acd19.1 of rice was clustered with small heat shock protein (sHsp), showing their proximity to respective genes
Phylogenetic tree generated by alignment of amino acid sequence of rice sHsps and ESTs (Additional file 1) showed that these 5 sHsp genes grouped into distinct clusters on separate clades with sHsps from other plant species, reflecting that homologs of above mentioned genes were present in other plants (Figure 2)
Summary
Heat shock proteins (Hsps) constitute an important component in the heat shock response of all living systems. SHsps cooperate with Hsp100/Hsp and co-chaperones in ATP-dependent manner in preventing aggregation of cellular proteins and in their subsequent refolding. Heat shock response (HSR), defined as sum total of cellular high temperature-related defense (page number not for citation purposes). The characteristic feature of the sHsps is the presence of an evolutionarily-conserved sequence of 80–100 long amino acids called αcrystallin domain (ACD), located in the C-terminal region. Experiments have shown that sHsps form complexes with denatured proteins and prevent their aggregation. From these complexes, the target proteins are subsequently refolded by Hsp100/Hsp and co-chaperones in ATP-dependent manner during the recovery phase [3,4,5]. Expression of Hsps is controlled by heat shock transcription factors (HSFs) that bind to cisacting regulatory elements called heat shock element (HSEs) in the promoter region of the Hsp genes
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