2.1.1. Heat Stress

Broccoli production faces challenges of demand to extend plant areas and maintain production security under extreme weather brought by climate change [13,15]. Broccoli is suitable for growth in cool weather with optimal temperatures ranging from 15 to 23 ◦C during the early stages of floral development [16]. High temperatures above 25 severely reduce broccoli quality because (1) most broccoli germplasms require vernalization at temperatures below 23 ◦C and superoptimal temperatures would even result in no head formation; (2) some broccoli germplasms do not require vernalization, but floral development under high temperatures (e.g., above 30 ◦C) results in undesirable traits, such as bracting, uneven head surface and sizes of buds, discoloration or even brown bead, making the broccoli products unmarketable; and (3) high temperatures during the head maturity stage decrease broccoli yield [15,17,18]. In recent years, substantial progress has been made in creating heat-tolerant breeding lines and genetically controlling heat tolerance in broccoli. In the USA, researchers have made efforts to achieve sustainable broccoli production under heat tolerance in the main production area on the east coast, supported by projects (National Institute of Food and Agriculture (NIFA) Project No. 2010-51181-21062 and the USDA Vegetable Brassica Research Project (CRIS No. 6080-21000- 019-00D)) [16,19,20]. In Asia, researchers are trying to introduce broccoli to subtropical and tropical regions, such as in Taiwan, China and Indonesia [21,22].

The ability to produce high-quality heads by several broccoli germplasms under heat stress is considered a quantitative trait controlling multiple positive loci [13,15]. Lin et al. identified 31 QTLs for head size and weight phenotypes of broccoli grown in hightemperature seasons (average 36.4 ◦C day/25.9 ◦C) [23]. Branham et al. constructed a high-density genetic map by genotyping-by-sequencing of a DH broccoli segregating population for heat tolerance and identified five QTLs and one positive epistatic interaction between *QHT\_C03* and *QHT\_C05*, explaining 62.1% of phenotypic variation [15]. Using a new DH population of broccoli, Branham et al. performed whole-genome resequencing of bulked segregants and identified two novel heat tolerance QTLs, of which *QHT\_C09.2* may explain the negative correlation between maturity and heat tolerance [13].

Using reversed genetic approaches, a heat-stress-related broccoli catalase gene was cloned, and ectopic expression of this gene in *Arabidopsis* can enhance heat tolerance, but whether it plays a role in maintaining a high-quality head under high temperatures is still unknown [24,25]. In addition, benefiting from improved sequencing techniques and the release of reference genomes, some researchers performed omics-related studies and identified differentially expressed microRNAs/genes and potential pathways involved in heat tolerance [26,27].
