Abstract
Heat shock factors (Hsfs) play a central regulatory role in acquired thermotolerance. To understand the role of the major molecular players in wheat adaptation to heat stress, the Hsf family was investigated in Triticum aestivum. Bioinformatic and phylogenetic analyses identified 56 TaHsf members, which are classified into A, B, and C classes. Many TaHsfs were constitutively expressed. Subclass A6 members were predominantly expressed in the endosperm under non-stress conditions. Upon heat stress, the transcript levels of A2 and A6 members became the dominant Hsfs, suggesting an important regulatory role during heat stress. Many TaHsfA members as well as B1, C1, and C2 members were also up-regulated during drought and salt stresses. The heat-induced expression profiles of many heat shock protein (Hsp) genes were paralleled by those of A2 and A6 members. Transactivation analysis revealed that in addition to TaHsfA members (A2b and A4e), overexpression of TaHsfC2a activated expression of TaHsp promoter-driven reporter genes under non-stress conditions, while TaHsfB1b and TaHsfC1b did not. Functional heat shock elements (HSEs) interacting with TaHsfA2b were identified in four TaHsp promoters. Promoter mutagenesis analysis demonstrated that an atypical HSE (GAACATTTTGGAA) in the TaHsp17 promoter is functional for heat-inducible expression and transactivation by Hsf proteins. The transactivation of Hsp promoter-driven reporter genes by TaHsfC2a also relied on the presence of HSE. An activation motif in the C-terminal domain of TaHsfC2a was identified by amino residue substitution analysis. These data demonstrate the role of HsfA and HsfC2 in regulation of Hsp genes in wheat.
Highlights
Temperate cereal crops such as wheat often encounter heat stress during the reproductive stage in warm-climate wheat production regions (Wardlaw and Wrigley, 1994)
It was demonstrated that a HsfC member was a transcriptional activator of heat shock protein (Hsp) genes; this has not been shown in any other plant species. These analyses provide some fundamental insights into the role of heat shock factor (Hsf) in mediating wheat adaptation to heat stress through regulation of Hsp genes as well as their potential roles in adaptation to other major abiotic stresses
Fifty-six TaHsf members were identified through a combination of sequence analysis of wheat expressed sequence tags (ESTs) and nucleotide collection databases in NCBI, followed by confirmation and sequence extension analyses using wheat genome sequences in the CerealDB, and isolation of 15 TaHsf cDNAs in this study (Table 1)
Summary
Temperate cereal crops such as wheat often encounter heat stress during the reproductive stage in warm-climate wheat production regions (Wardlaw and Wrigley, 1994). The acquisition of thermotolerance in plants relies on acclimatization to permissive high temperatures, during which time a large number of heat protection genes such as those encoding heat shock proteins (Hsps) and ROS scavengers are induced (Kotak et al, 2007; Mittler et al, 2012). At least three nGAAn repeats are required for effective Hsf binding in Drosophila (Xiao et al, 1991) and in Arabidopsis (Kumar et al, 2009)
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