**1. Introduction**

Broccoli (*Brassica oleracea* L. var. *italica*) is a member of the Brassicaceae family and is widely cultivated as an important vegetable crop worldwide [1,2]. It produces edible hypertrophic reproductive organs (floral head and stalk), with rich health benefits and nutritious properties, such as vitamin A, vitamin K, calcium, magnesium and anticancer bioactive compounds, including glucosinolates, sulforaphane, selenium and flavonoids [3–5].

The *italica* group arises from the cultivation and domestication of *Brassica oleracea* (CC genome; 2n = 18) in the Mediterranean region. Accurate knowledge about the cultivation of *B. oleracea* mustard plants can be traced to the Hellenic culture, starting in approximately the 6th century BC [6]. By distinguishing the *B. oleracea* cultivars, 'Broccoli' is probably a colloquial Latin word for any projecting shoots of the cabbage family [6]. Broccoli-like varieties were developed from selections of desirable *B. oleracea* types during the past 2000 years and formed various broccoli landraces mainly in Italy [6–9]. The broccoli variety 'Vrocculi o Sparaceddi' is considered the first domesticated form of wild brassica from which broccoli originated [10]. During the past 300 years, the heading broccoli has greatly improved, largely attributed to selection by Danish and English horticulturists [6]. For a long time, the consumption of broccoli as a vegetable was confined to the Italian peninsula and it was grown mainly as sprouting broccoli cultivars [10]. With the breeding and improvement of calabrese broccoli varieties, a particular type producing large and compact heads more similar to cauliflower, broccoli spread and gained popularity worldwide [10,11]. Various broccoli landraces were introduced to the United Kingdom in the 1700s and to the United States in the 1800s and became popular after World War II [11]. Broccoli was initially introduced into several southern provinces of China in the 1980s and has been a popular vegetable widely grown in China. In recent years, China, with a cultivation area

**Citation:** Han, F.; Liu, Y.; Fang, Z.; Yang, L.; Zhuang, M.; Zhang, Y.; Lv, H.; Wang, Y.; Ji, J.; Li, Z. Advances in Genetics and Molecular Breeding of Broccoli. *Horticulturae* **2021**, *7*, 280. https://doi.org/10.3390/ horticulturae7090280

Academic Editor: Yuyang Zhang

Received: 6 August 2021 Accepted: 1 September 2021 Published: 3 September 2021

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of over 80,000 ha, has become the largest producer of broccoli in the world [12]. Driven by scientific evidence that broccoli is beneficial to human health, the market demand is still increasing in the main broccoli-producing countries, including China, the US and India [12–14].

With the development of molecular biology technology and functional genomics, a large number of studies on broccoli have been performed. Marker-assisted selection (MAS) and genetic transformation were combined with conventional breeding to improve broccoli for high yield, quality, resistance to biotic and abiotic stresses, etc. We review the recent progress on the genetics and molecular breeding of broccoli, focusing on desirable agronomic traits, male sterility, abiotic stress resistance, disease resistance, secondary metabolites and genetic transformation. Postharvest yellowing (or prolonging shelf life) of broccoli is also a research hotspot that has been reviewed recently and thus is not included in this review [2]. Broccoli improvement by genetic engineering was reviewed in 2016 [1], so relevant advances in recent years from 2016 to 2021 are included in this review.
