Date Published: July 6, 2017
Publisher: Public Library of Science
Author(s): Feng Wen, Xiaozhu Wu, Tongjian Li, Mingliang Jia, Xinshen Liu, Peng Li, Xiaojian Zhou, Xinxin Ji, Xiaomin Yue, Keqiang Wu.
The heat shock protein 70s (Hsp70s) and heat shock factors (Hsfs) play key roles in protecting plant cells or tissues from various abiotic stresses. Brachypodium distachyon, recently developed an excellent model organism for functional genomics research, is related to the major cereal grain species. Although B. distachyon genome has been fully sequenced, the information of Hsf and Hsp70 genes and especially the regulatory network between Hsfs and Hsp70s remains incomplete. Here, a total of 24 BdHsfs and 29 BdHsp70s were identified in the genome by bioinformatics analysis and the regulatory network between Hsfs and Hsp70s were performed in this study. Based on highly conserved domain and motif analysis, BdHsfs were grouped into three classes, and BdHsp70s divided into six groups, respectively. Most of Hsf proteins contain five conserved domains: DBD, HR-A/B region, NLS and NES motifs and AHA domain, while Hsp70 proteins have three conserved domains: N-terminal nucleotide binding domain, peptide binding domain and a variable C-terminal lid region. Expression data revealed a large number of BdHsfs and BdHsp70s were induced by HS challenge, and a previous heat acclimation could induce the acquired thermotolerance to help seedling suffer the severe HS challenge, suggesting that the BdHsfs and BdHsp70s played a role in alleviating the damage by HS. The comparison revealed that, most BdHsfs and BdHsp70s genes responded to multiple abiotic stresses in an overlapping relationship, while some of them were stress specific response genes. Moreover, co-expression relationships and predicted protein-protein interaction network implied that class A and B Hsfs played as activator and repressors, respectively, suggesting that BdHsp70s might be regulated by both the activation and the repression mechanisms under stress condition. Our genomics analysis of BdHsfs and BdHsp70s provides important evolutionary and functional characterization for further investigation of the accurate regulatory mechanisms among Hsfs and Hsp70s in herbaceous plants.
Abiotic stresses, such as heat, cold, drought, and salinity, are the most harmful factors concerning the growth and productivity of crops, which represent seriously threat to agriculture and cause the huge loss of crop yield worldwide by more than 50% annually . For instance, temperature is a major environmental factor that affects plant growth and productivity. If plant expose to high temperature beyond a threshold level for a period of time, it will cause significant adverse impact on almost all aspects of plant development and growth, including leaf damage, accelerated leaf senescence, ROS burst, and reduced photosynthesis capacity, and may drastically reduce plant biomass production and economic yield [2,3]. Since plants are especially dependent on environmental factors because of a sessile lifestyle, they have to evolve a spectrum of molecular programs to help them adapt to changing environmental conditions. In plants, the heat stress (HS) response is highly conserved and refers to multiple pathways and regulatory networks . The plant cells respond to HS by inducing the transcription of genes encoding heat shock proteins (Hsp), which are involved in preventing or repairing the damage caused by elevated temperature and thus confer increased thermotolerance . For instance, Hsp70 gene family is a type of Hsps, plays key roles in protecting plant cells or tissues from heat stress as well as other environmental stresses, by function in degradation of misfolded and truncated proteins as molecular chaperones . One of the regulatory networks that control the expression of Hsps is the heat shock factors (Hsfs) network. Hsfs are transcription factors (TFs), which can regulate the expression of Hsps by recognizing heat shock elements (HSEs) within the promoters of Hsps.
Identification and characterization of Hsf and Hsp70 genes in a grass model-species would help to better understand the evolutionary processes and functions of these gene families. In this study, 53 members of B. distachyon Hsf and Hsp70 gene families were identified. Genetic characterizations analyses (phytogenesis, chromosomal localization, gene duplication, protein structure) and abiotic stresses induced expression profile have been systematically investigated. Phylogenetic tree revealed that BdHsfs and BdHsp70s can be divided into three and six subfamilies, respectively. Most of subfamilies contained members from rice, Arabidopsis and B. distachyon, suggesting that the functions of most of Hsfs and Hsp70s were conserved during evolution. In addition, gene duplication analysis implied that Hsfs and Hsp70s might be the result of genomic rearrangements and expansions during the process of evolution, for instance, five of cHsp70s, which were located at the end of chromosome I in tandem duplication region, might be originating from BdcHsp70-1 and/or BdcHsp70-6. A heat-induced expression profile showed that HS-induced BdHsf expression can induce the acquired-thermotolerance to prevent plant from severe HS challenge. Expression heatmap and correlation analysis of BdHsfs and BdHsp70s showed that the response of Hsfs and Hsp70s to multiple abiotic stresses exhibited extensively overlapped and distinct expression pattern, suggesting that some genes were important in responding to multiple environmental stresses, and others were stress specific response genes. Moreover, the co-expression network implied that there was a complex transcriptional regulatory network between B. distachyon Hsfs and Hsp70s, and BdHsp70s might be regulated by both the activation and the repression mechanisms. Our study provided genetic characterizations and expression analysis of Hsfs and Hsp70s genes in B. distachyon under multiple stresses conditions which could improved our understanding for further investigating the accurate regulatory mechanisms among Hsfs and Hsps in herbaceous plants.