Research progress of heat stress transcription factors (Hsfs) in horticultural plants

Abstract: Under natural conditions, plants are constantly subjected to various stresses. In particular, abiotic stress is one of the primary causes for crop loss worldwide, which greatly reduces crop productivity. In order to improve crop yield, it is urgent to elucidate the potential molecular mechanism of plant stress to abiotic stress. With the global warming, high temperature stress has become an important abiotic stress affecting plant growth and crop yield. When plants are subjected to heat stress, a series of responses will be triggered, thus leading to the rapid accumulation of heat shock protein. They play critical important roles in the process of plant resistance. Genes regulated the expression of heat shock proteins are called plant heat shock transcription factor (Hsf). As one of crucial regulators in regulation network, plant heat shock transcription factor, could bind to HSE cis-acting elements in promoters of stressinducible genes and respond to multiple biotic and abiotic stresses. Hsfs are classified into three classes in plants, including HsfA, HsfB, and HsfC, respectively. Similar to all non-plant Hsfs, the HR-A/B region of class B is compact, however, members of class A and C have an extended HR-A/B region due to an insertion of 21 (HsfAs) and 7 (HsfCs) amino acid residues between the HR-A and HR-B parts, respectively. HsfAs has attracted a lot of attention, but few on class B (HsfBs) and class C (HsfCs). Theidentification of the Hsfs family were previously reported only in a few model plant species such as Arabidopsis, tomato, and rice. In recent years, the availability of the ever-increasing number of complete plant genomes and EST sequences, lead to the identification of a large numbers of Hsfs families from more than 20 plant species at the genome-wide scale. For instance, there are 21 Hsf encoding genes in Arabidopsis, 26 in tomato, 25 in pepper, 25 in apple, and 19 in grapevine. Similar to many other species, plant Hsf proteins share a well-conserved modular structure, and consist of five parts, including Nterminal DNA-binding domain (DBD), oligomerization domain (OD or HR-A/B region), nuclear localization signal (NLS), nuclear export signal (NES) and acidic C-terminal domain (CTD), respectively. Plant Hsfs are core regulators in regulation of heat- stress (HS), low temperature, salt, and drought. Therefore, it is necessary to clarify the function of Hsfs that participate in various abiotic stresses. Among the major abiotic stresses, HS has an independent mode of action on the physiology and metabolism of plant cells, and has a negative effect on plant growth and development, which may lead to catastrophic loss of crop productivity. HsfAs play critical roles in regulating the expression of Hsp genes in heat stress. At present, the functional investigation of Hsfs under HS condition was mainly concentrated in HsfsA1, HsfsA2 and HsfsB. HsfsA1 is a main regulator of heat shock response and cannot be replaced by other Hsfs. The function and structure of HsfA2 are similar to those of HsfA1, playing a key component of plant cell heat shock signal transduction. In contrast to HsfAs, HsfBs have no transcriptional activity due to lack of the activator domain. These indicate striking species-specific deviation in the functional diversification of some members of the Hsfs family. In this review, genome-wide expression profiling of plant Hsfs genes under heat stresses has been investigated extensively in various horticultural plants. The expression of Hsfs in apple, citrus, eggplant and strawberry were either regulated or downregulated by HS. Furthermore, Hsfs play an important role under different abiotic stress conditions. Talking about the study on Hsfs, high temperature stress is still an important factor, and the response mechanism of Hsfs to other stresses is relatively less, especially to low temperature stress and biological stress. In previous studies, some heat shock proteins involved in low temperature stress response were identified by our research group. The data showed that a large number of Hsps are involved in cold response, including 6 down-regulated genes and 4 up-regulated genes. This paper summarized the structural characteristics, classification and the function of plant heat shock transcription factors in response to various stress conditions, including high temperature stress, drought stress, high salt stress and other stresses. In addition, the development and application prospects of plant heat shock transcription factors in future research hotpots were discussed, aiming to provide the basis and reference for elucidating the function and molecular mechanism of Hsfs in plants. A combination of advanced high throughput technologies will provide the critical information to elucidate the whole complexity of Hsfs in abiotic stress responses and different signaling pathways.