Abstract
The rapid increase in heat shock proteins upon exposure to damaging stresses and during plant development related to desiccation events reveal their dual importance in plant development and stress tolerance. Genome-wide sequence survey identified 20 non-redundant small heat shock proteins (sHsp) and 22 heat shock factor (Hsf) genes in barley. While all three major classes (A, B, C) of Hsfs are localized in nucleus, the 20 sHsp gene family members are localized in different cell organelles like cytoplasm, mitochondria, plastid and peroxisomes. Hsf and sHsp members are differentially regulated during drought and at different seed developmental stages suggesting the importance of chaperone role under drought as well as seed development. In silico cis-regulatory motif analysis of Hsf promoters showed an enrichment with abscisic acid responsive cis-elements (ABRE), implying regulatory role of ABA in mediating transcriptional response of HvsHsf genes. Gene regulatory network analysis identified HvHsfB2c as potential central regulator of the seed-specific expression of several HvsHsps including 17.5CI sHsp. These results indicate that HvHsfB2c is co-expressed in the central hub of small Hsps and therefore it may be regulating the expression of several HvsHsp subclasses HvHsp16.88-CI, HvHsp17.5-CI and HvHsp17.7-CI. The in vivo relevance of binding specificity of HvHsfB2C transcription factor to HSE-element present in the promoter of HvSHP17.5-CI under heat stress exposure is confirmed by gel shift and LUC-reporter assays. Further, we isolated 477 bp cDNA from barley encoding a 17.5 sHsp polypeptide, which was predominantly upregulated under drought stress treatments and also preferentially expressed in developing seeds. Recombinant HvsHsp17.5-CI protein was expressed in E. coli and purified to homogeneity, which displayed in vitro chaperone activity. The predicted structural model of HvsHsp-17.5-CI protein suggests that the α-crystallin domain is evolutionarily highly conserved.
Highlights
When plants are challenged by drought and temperature stresses, a wide array of interconnected cellular stress response systems is triggered
Details of all the genes encoding for barley small heat shock proteins (sHsp) and heat shock factor (Hsf) are represented in the Tables 1 and 2 respectively
The cis-motif ABRE [42] required for ABA response is present in the promoters of all the Hsf genes except in HsfA2c, suggesting that these Hsf genes are involved in ABA mediated signal transduction
Summary
When plants are challenged by drought and temperature stresses, a wide array of interconnected cellular stress response systems is triggered. Heat shock responses (HSR) are temperaturerelated defense activities and include the induction of evolutionarily conserved chaperone proteins known as heat shock proteins (Hsps) Based on their molecular size, they are classified into different classes i.e Hsp100, Hsp, Hsp70/DnaK, Hsp60/GroE and small heat shock proteins (sHsps) [1]. SHsps are highly expressed in developmental stages like zygotic embryonic tissues, pollen maturation, embryogenesis and during seed maturation [4,5,6,7] How these large gene family members of sHsps are finely regulated by a defined set of potential heat shock transcription factors (Hsfs) to control many vital processes important during plant development and stress response is largely obscure
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