2.2.3. Plant Architecture

Plant architecture is a complex trait attributed to stem and leaf morphologies, including plant height, leaf size, leaf shape, leaf angle, petiole length and lateral shoot growth. It affects the planting density, yield and quality of broccoli [50]. Several QTLs for plant architecture-relevant stem and leaf traits have been reported [23,27,32,51,52]. Before the release of reference genomes, researchers mapped QTLs associated with leaf lamina width on linkage groups C01 and C07 [51], stem width on LG5 [23], leaf apex on linkage groups C06 and C07 [43], leaf shape on linkage group C3 [43], leaf length on linkage group C7 [43], wing petiole length on linkage group C7 [43] and lobe number, wing number, leaf shape and lamina petiole length on linkage group C3 [43]. In recent years, in addition to focusing on heading traits, Stansell et al. mapped several QTLs for leaf morphology and lateral shoots. Four QTLs for leaf apex, two QTLs for leaf margin and leaf-associated hotspot genomic regions, *Lea3* on C03 0.7–1.7 Mb and *Lea7* on C07 37.0–39.5 Mb were identified. A *GRF1-INTERACTING FACTOR 1* (*GIF1*) homolog (*Bo7g093130*) within major QTL *LA\_C07@36.6* may be responsible for the narrow leaf phenotype, and a *LATE MERISTEM IDENTITY1* ortholog (*BoLMI1, Bo3g002560*) near the major *LM\_C03@0.7*, explaining over 40% phenotype variation, may be responsible for leaf margin phenotype [33]. Three lateral shoot growth-associated QTLs, *LT\_C03@5.9*, *LT\_C04@15.0* and *LT\_C09@9.0*, are located on C03, C04 and C09, although no likely candidates were predicted [33]. Huang et al. constructed a genetic linkage map using a broccoli DH population and identified QTLs for plant height (PH), maximum outer petiole length (PL) and leaf width (LW), including

major QTLs *phc1* for PH on chromosome 1, *plc6*−*2* for PL on chromosome 6 and *lwc3*−*1* for LW on chromosome 3 [52].

## 2.2.4. Stem Development

Broccoli hollow stem is an undesirable phenotypic disorder showing symptoms of cracks in the internal stem tissue [53,54]. It reduces the quality of broccoli products because hollow stems can result in (1) yield reduction, as harvested broccoli comprise partially edible stalks; (2) secondary pathogen infection and rotting of stems and florets [38,39]. The incidence of hollow stems increases when plants grow rapidly, triggered by, for example, high levels of nitrogenous and warm weather but also varies in different broccoli accessions, indicating that this trait is largely genetically determined and can be controlled by breeding resistant varieties [53,54]. However, relevant studies on this trait are very limited. Yu et al. constructed a genetic map using specific locus-amplified fragment (SLAF) sequencing in a double-haploid segregation population of broccoli and defined nine QTLs on C02, C03, C05, C06 and C09 for hollow stems, among which *QHS. C09-2* could explain 14.1% of the phenotypic variation [55].
