**1. Introduction**

Global warming poses a serious threat to agriculture, as it threatens crop productivity and food safety worldwide [1,2]. Various strategies have been utilized to facilitate the breeding or engineering of thermotolerant crops, including regulating the expression of heat shock (HS) transcription factor (HSF) and heat shock response (HSR) genes, as well as the use of molecular markers [1,3,4].

In a model plant *Arabidopsis thaliana*, three types of tolerance responses to heat exposure have been identified: basal thermotolerance, acquired thermotolerance, and warming tolerance [5–7]. Plants with basal thermotolerance can survive when grown at 21–22 ◦C (normal growth condition), exposed to 42–45 ◦C for 0.5–1 h, and examined after 5–7 days. Plants with acquired thermotolerance can survive when grown under normal conditions, transferred to 36–38 ◦C (moderate heat stress) for 1.5 h (referred to as "priming"), recovered at 21–22 ◦C for 2 h, subjected to over 45 ◦C, and examined after 5–7 days. Plants with warming tolerance (as opposed to heat-stress tolerance) survive when grown under normal conditions, subjected to 12 ◦C for 2 days, and treated with warming conditions (27 ◦C) for 3 h. The long-term adaptation of plants to warmer growth conditions is thought to induce developmental reprograming. Identifying and applying warming-related genes in *Brassica* crop species poses a major challenge for crop breeding for improved tolerance to global warming.

Several marker genes for thermotolerance responses are currently available [8]. Genes encoding an exportin family protein (*XPO1A*, AT5G17020) and heat shock protein (HSP) 101 (*HSP101*, AT1G74310) are markers for basal thermotolerance. Acquired thermotolerance by priming is divided into two categories: short-term and long-term acquired thermotolerance. *HSP101* expression represents short-term acquired thermotolerance, while long-term acquired thermotolerance is characterized by the expression of several marker genes, including those encoding Rotamase FKBP1/FK506-binding protein 62 (*ROF1/FKBP62*, AT3G25230), ROF2/FKBP65 (AT5G48570), heat HSF factor A2 (*HSFA2*, AT2G26150), HSP101, and heat stress-associated 32 kD (*Hsa32*, AT4G21320). During the long-term acquired thermotolerance response, the expression levels of small HSP genes (*sHSP*s), *HSP70*s, *ROS* genes, and ascorbate peroxidase (*APX*) also increase [7].

Warming treatment does not trigger the expression of HSR genes, but other genes, such as *HSP70* (AT3g12580) [6] and *Phytochrome-Interacting Factor4* (*PIF4*, AT2G43010) [9–11], are core components in this process. PIF4 controls morphological acclimation to high temperatures (HT) via auxin [9,12]. Phytochrome B (PhyB) was recently shown to function upstream of PIF4 [13]. Changes in ambient temperatures induce alternative splicing of a large number of genes [14]. Due to increases in global temperatures, the mechanism used by plants to sense small variations in ambient temperatures is becoming an increasing focus of study. It is important to elucidate whether crops that have long been cultivated in regions with different climates, such as Chinese cabbage, have developed similar responses to warming to those found in *Arabidopsis*.

Two inbred Chinese cabbage lines, Chiifu and Kenshin, have different geographic origins: Chiifu originated in temperate regions, whereas Kenshin originated in subtropical and tropical regions. Kenshin has long been used as a breeding stock to develop heat-tolerant *Brassica* species [15,16]. In addition, these two inbred lines show different electrolyte leakage rates in response to HT exposure and different expression of many genes [17]. The long history and intensive breeding of these two Chinese cabbage lines make them promising targets for transcriptome analysis after warming treatment to identify warming-related genes in this crop. These genes could then be used to develop molecular markers and to generate climate-change-resilient *Brassica* crops under global warming conditions. In the current study, we used the Br135K microarray (Version 3) to identify differentially expressed genes (DEGs) upon warming treatment in Chiifu and Kenshin Chinese cabbage, confirmed their expression patterns by qRT-PCR, and further characterized the expression of patterns of several candidate warming-relating genes. The results of this study lay the foundation for breeding Chinese cabbage lines with improved tolerance to warming conditions.
