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Recent Advances in Sexual Propagation and Breeding of Garlic

Einat Shemesh-Mayer and Rina Kamenetsky Goldstein

Institute of Plant Sciences, Agricultural Research Organization, The Volcani Center, Beit Dagan, Israel

ABSTRACT

The restoration of flowering ability, sexual hybridization, and seed production in garlic (Allium sativum L.) has resulted in an increase in genetic variability available to agriculture and has opened new avenues for the breeding of this important crop. In this review, the current status of flower development, fertility, hybridization, sexual propagation, and seed production in garlic is discussed. We summarize the main stages in the life cycle of garlic from true seeds to flowering and bulb formation, and recent advances in our understanding of floro- and gametogenesis. Flowering and fertility of garlic are tightly regulated by environmental conditions, and therefore the seed production cycles in various climatic zones are complex and challenging. Recent establishment of modern molecular tools and the creation of large transcriptome catalogs provide a better understanding of the molecular and genetic mechanisms of flowering and fertility processes, and accelerate the breeding process by using molecular markers for desirable traits.

KEYWORDS: Allium sativum, environmental regulation, fertility, genetic regulation, hybridization, male sterility, seed production

1. I. INTRODUCTION
2. II. HORTICULTURAL DIVERSITY AND GENETIC RESOURCES
3. III. LIFE CYCLE AND THE FLOWERING PROCESS
   1. A. Seed and Seedling Development
   2. B. Annual Life Cycle and Florogenesis
   3. C. Environmental and Genetic Control of Flowering
4. IV. FERTILITY BARRIERS
   1. A. Morphology and Anatomy of the Individual Flower
      • 1. The Male Gametophyte
      • 2. The Female Gametophyte
   2. B. Environmental and Genetic Control of Male Sterility
5. V. UNLOCKING VARIABILITY BY SEXUAL REPRODUCTION
   1. A. Morphological Variability in Seedling Populations
   2. B.Environmental Regulation of Seedling Development
   3. C. Molecular Markers in Variable Garlic Populations
6. VI. CONCLUDING REMARKS
7. LITERATURE CITED

I. INTRODUCTION

Garlic (Allium sativum L.) is one of the most popular vegetable crops, being cultivated in different continents for flavor, nutrition, and medicinal purposes. The wild ancestor of cultivated garlic probably originated in Central Asia, and was gathered by seminomadic tribes about 10?000?years ago. Later, traders introduced plants to the Mediterranean Basin, India, and China, and from there garlic spread across various regions of the world (Engeland 1991; Etoh and Simon 2002). Widespread geographical distribution of cultivated garlic resulted in its adaptation to different climatic conditions and in the development of many local types and varieties with specific morphological and physiological traits.

Taxonomically, A. sativum belongs to the section Allium of the genus Allium. Among 114 species in this section, about 25 are closely related to the cultivated plant, such as A. tuncelianum (Kollman) Özhatay, Mathew, Siraneci from Turkey and A. moschatum L. from the Caucasus (Mathew 1996). A. sativum is not found in native populations, but most garlic relatives grow wild in regions characterized by relatively cold winters and hot and dry summers, have garlic-like taste and smell, and are used by local populations as food and nutraceuticals.

Similar to many wild Allium species, the ancestors of garlic from Central Asia probably produced flowers, seeds, and relatively small bulbs. However, since the development and growth of flowering scapes consume energy at the expense of storage organs, it is likely that human selection for early maturation of large garlic bulbs deprived the developing scapes of nutritional supplies. Cultivated garlic has lost its flowering potential and fertility, and today commercial production is based exclusively on vegetative propagation (Etoh 1985; Etoh and Simon 2002). Consequently, garlic breeding has been limited to selection from established genetic variation, and breeding was attempted only via mutation and in vitro techniques (Takagi 1990). In recent years, flowering ability was restored in several garlic genotypes, and an increase in garlic variability was achieved via sexual hybridization and seed production (Etoh 1983b; Etoh et al. 1988; Pooler and Simon 1994; Inaba et al. 1995; Jenderek 1998, 2004; Jenderek and Hannan 2000; Jenderek and Zewdie 2005; Kamenetsky et al. 2005; Kamenetsky 2007).

Fertility restoration and seed production have opened a new stage of genetic research into garlic. Similar to many other perennial monocots, Allium species possess a large genome size (7-32?Gb) (Ricroch et al. 2005). Despite its domestication, garlic has maintained its ploidy level (2n?=?2x?=?16), and the diploid garlic nuclear genome is estimated at 15.9?Gbp, 32 times larger than the genome of rice (Arumuganathan and Earle 1991; Fritsch and Friesen 2002; Kik 2002). Therefore, full sequencing of the garlic genome is still a challenging task, but transcriptome assembly using next-generation sequencing (NGS) might be efficiently employed for the generation of functional genomic data. At the same time, an enormous amount of genetic and molecular data, collected in model plants over recent decades, can be translated to commercial crops by using various experimental tools such as candidate genes, library screening, expressed sequenced tags (ESTs), and genomic, transcriptomic, proteomic, and metabolomic databases (Leeggangers et al. 2013).

New genetic variability obtained by sexual hybridization, in combination with research results, has provided solid ground for a new phase in garlic breeding (Pooler and Simon, 1994; Jenderek and Hannan 2004; Kamenetsky et al. 2004a, 2015; Jenderek and Zewdie 2005; Shemesh et al. 2008; Shemesh-Mayer et al. 2015a). Generation of garlic S1 families provided the first source of variability for genetic studies for breeding purposes (Hong and Etoh 1996; Jenderek 2004; Jenderek and Zewdie 2005). In Israel, a breeding program was established 10?years ago and is currently focused on sexual hybridization and selection of superior garlic plants, the introduction of new useful traits that are uncommon in commercial clones, and the development of new cultivars for different climatic zones. In this review, the current status of sexual propagation, hybridization, and seed production in garlic is discussed.

II. HORTICULTURAL DIVERSITY AND GENETIC RESOURCES

During a long cultivation history, garlic plants were grown in diverse climatic and biogeographic regions. They exhibit wide variations in bulb size, shape and color, number and size of cloves, peeling ability, maturity date, flavor and pungency, bolting capacity, and numbers and sizes of topsets and flowers in the inflorescence (Figure 1.1) (McCollum 1976; Astley et al. 1982; Astley 1990; Hong and Etoh 1996; Lallemand et al. 1997; IPGRI, ECP/GR, AVRDC 2001; Kamenetsky et al. 2005; Meredith 2008). A strong interaction between genotype and environment has led to a variety of phenotypic expressions (Lallemand et al. 1997; Portela 2001; Kamenetsky et al. 2004b; Meredith 2008).

Figure 1.1 Morphological variation in garlic cultivars, propagated vegetatively in various climatic areas

(Source: C. Aaron and R. Adams, poster, 2017, with permission.)

Depending on the ability to develop a flower stem, garlic producers distinguish between softneck and hardneck varieties (Engeland 1991; Meredith 2008). However, from a physiological point of view, the terminology bolters and nonbolters is more accurate. Depending on the traits of scape elongation and inflorescence development, garlic varieties were classified by Takagi (1990) as: (i) nonbolters, which normally do not form a flower stalk or produce cloves inside an incomplete scape; (ii) incomplete bolters, which produce a thin, short flower stalk, bear only a few large topsets, and usually form no flowers; and (iii) complete bolters, which produce a long, thick flower stalk, with many topsets and flowers. It was observed that these traits might be altered by different environmental conditions, but the mechanisms of their regulation are still unknown.

Based on morphological and physiological phenotype, worldwide garlic cultivars were classified into several horticultural groups, reflecting the broad diversity of the crop. The group named Purple Stripe, which includes bolting hardneck cultivars, is considered to be genetically closest to the origin of garlic. The other groups include the Artichoke, Asiatic, Creole, Glazed Purple Stripe, Marbled Purple Stripe, Middle Eastern, Porcelain, Rocambole, Silverskin, and Turban types (Meredith 2008). These groups vary in bolting ability and bulb structure. Moreover, plant performance is affected by environment, and therefore phenotypes of the same variety...