**1. Introduction**

Plants live in complex environments where multiple abiotic stresses, such as salt, drought and extreme temperature, may seriously restrict their growth and development [1]. As sessile organisms, plants cannot move to avoid these stresses and, thus, they have developed mechanisms, such as enhanced expression of tolerance-related genes, in response to heat stress [2,3]. To survive and acclimatize under adverse environment conditions, plants have established self-defense mechanisms during the course of long-term evolution. Previous studies have shown that under heat stress (HS), plant cells respond rapidly to high temperatures by inducing the expression of genes encoding heat shock proteins (Hsps), which are involved in preventing heat-related damage and confer plant thermotolerance in strawberry, walnut, barley and grapevines [4–7]. Many Hsps function as molecular chaperones in preventing protein misfolding and aggregation, consequently maintaining protein homeostasis in cells and inducing acquired thermotolerance in plants [8]. The expression of Hsps is controlled and regulated by specific types of transcription factors called heat shock factors (Hsfs), which normally exist as inactive proteins [9].

Currently, many plant *Hsf* and *Hsp* genes from various species have been isolated and comprehensively studied. Based on their approximate molecular weights and sequence homologies, Hsps are classified into five families, namely, the small *Hsp* (*sHsp*), *Hsp60s*,

**Citation:** Gong, C.; Pang, Q.; Li, Z.; Li, Z.; Chen, R.; Sun, G.; Sun, B. Genome-Wide Identification and Characterization of *Hsf* and *Hsp* Gene Families and Gene Expression Analysis under Heat Stress in Eggplant (*Solanum melongema* L.). *Horticulturae* **2021**, *7*, 149. https://doi.org/10.3390/ horticulturae7060149

Academic Editor: Yuyang Zhang

Received: 16 May 2021 Accepted: 3 June 2021 Published: 10 June 2021

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*Hsp70s*, *Hsp90s* and *Hsp100s* [10]. The expression of *sHsp* is positively correlated with thermostability [11]. As chaperones, *Hsp60* proteins participate in the folding and aggregation of many proteins transported to organelles, such as chloroplasts and mitochondria [12]. *Hsp70* chaperones, together with their co-chaperones, make up a set of prominent cellular machines that assist with a wide range of protein folding processes in almost all cellular compartments [13]. In Arabidopsis TU8 mutants, the downregulation of *Hsp90* expression leads to mutants that are more sensitive to heat. In *Arabidopsis thaliana* seedlings, fungi producing *Hsp90* inhibitors increase the expression of the *Hsp101* and *Hsp70* genes, resulting in the enhancement of plant heat resistance [14]. Arabidopsis has at least 21 *Hsf* s [15]. *HsfA1a*, *HsfA1b* and *HsfA1d* act as the main positive regulators of the heat shock response [16] and HsfA2 can enhance the thermotolerance of plants [17]. Above all, *Hsfs* and *Hsps* play crucial roles in plant thermotolerance. The *Hsf* and *Hsp* gene families have been extensively studied in the model plant *Arabidopsis thaliana* and in non-model plants, such as rice (*Oryza sativa*) [18], poplar (*Populus trichocarpa*) [19], maize (*Zea mays*) [20] and Chinese cabbage (*Brassica rapa* L. ssp. *pekinensis*) [21].

Eggplant (*Solanum melongema* L.) is an important economic solanaceous crop, ranking third, after potato and tomato. Eggplant is primarily cultivated in East Asia, South Asia, the Middle East and northern Africa. The optimal temperature for eggplant growth and development ranges from 22 ◦C to 30 ◦C. With global warming, the temperature in subtropical and tropical regions is often above 35 ◦C, resulting in serious heat injury in eggplant, including limited plant growth, reduced productivity and damaged quality [2]. Thermotolerance is an important agronomic trait for eggplants, but the molecular mechanisms of heat tolerance remain elusive. Hsfs and Hsps play core roles in the signal transduction pathways involved in plant response to heat stress. Due to the vital regulatory functions of *Hsf* and *Hsp* genes in plant responses to heat stress, *Hsf* and *Hsp* genes in eggplant under heat stress were studied. The eggplant genome was sequenced and assembled [22], enabling the characterization of the eggplant *Hsf* and *Hsp* families and their responses to heat stresses at the molecular level. Therefore, genome-wide identification of *Hsf* and *Hsp* genes in eggplant was conducted to infer their expansion and evolutionary history. RNA-seq data and quantitative real-time RT-PCR analyses were used to explore their expression difference in the thermotolerant line 05-4 and the thermosensitive line 05-1 as elicited by naturally increased temperature. The results provide a relatively complete profile of the *Hsf* and *Hsp* gene families in eggplant and elucidate their relationship with thermotolerance, which provides a foundation for further functional research on these genes in eggplant. Furthermore, these findings could potentially be useful for understanding the mechanism of thermotolerance mediated by Hsfs and Hsps in eggplant.
