*2.2. Desirable Agronomic Traits*

## 2.2.1. Heading

Broccoli produces edible reproductive organs characterized by proliferation and developmental arrest of floral buds [17]. Floral head quality is the most important agronomic trait selected by breeders. With forward and reversed genetic approaches, some genes/loci related to head formation have been identified, but the genetic basis remains elusive [34].

Some works tried to identify homologs of the Arabidopsis floral meristem identity genes *LEAFY* (*LFY*), *APETALA1* (*AP1*) and *CAULIFLOWER* (*CAL*) and implied that *BoCAL* and *BoAP1* are involved in curding in cauliflower, a subspecies similar to broccoli but different in the developmental stage of the reproductive meristem at harvest [35–37]. Subsequent studies suggest that heading is quite complex in both cauliflower and broccoli, which seems not to be controlled solely by these floral genes [34].

In the 1990s, researchers started to construct genetic maps by crossing broccoli cultivars/inbred lines with various materials, including broccoli cultivar/landrace, cabbage, cauliflower, kale and Chinese kale, to detect loci of important traits, such as disease resistance, head morphology, nutritional quality and flowering/maturation time [38–42]. Several quantitative trait loci influencing head traits, including head weight, head height/width and floret height/width, have been identified, but the early constructed genetic maps are hard to unify [23,42,43] due to the differences in plant germplasm, marker types and linkage group nomenclature and the lack of *B. oleracea* reference genomes before 2014. Using a double-haploid BolTBDH mapping population derived from Early Big (broccoli DH line) and TO1000DH3 (nonhead Chinese kale), Stansell et al. identified heading-quality QTLs, including *BU\_C04@51.5*, *BR\_C09@49.5*, *HC\_C09@48.8*, *HU\_C09@48.8*, *HE\_C09@47.7* and *OQ\_C09@49.5* (Table S1), and found genomic regions of approximately 49 Mb on C09 harboring *FLOWERING LOCUS C* (*FLC*) homologs *Bo9g173400* and *Bo9g173370*, as hotspots contributing largely to over 40% phenotypic variance of the heading phenotype [33]. In another study, three head quality QTLs, *qCQ-2*, *qCQ-3* and *qCQ-6*, associated with subtropical adaptation were identified [21]; and specific haplotype combinations of candidates *BoFLC3* in the interval containing *qCQ-3* and *PERIANTHIA* (*PAN*, a bZIP-transcription factor required for *AGAMOUS* activation) in the interval containing *qCQ-6*, were supposed to adapt broccoli to high ambient temperature and short daylength. Along with these key head-related traits, QTL mapping for bud morphology was also reported by Stansell et al. and Lin et al. [21,33]. These studies provide genetic information and breeding materials for improving broccoli varieties.
