PLS221                                              Instructor: Carlos F. Quiros

Liliaceae: Allium sativum, Garlic

List of references (see Onion)


Origin and distribution

Horticultural types

Sterility in garlic

True garlic seed, a recent breakthrough

Markers and biotechnology


 Garlic: A. sativum, is a diploid species of 2n=2x=16. Use extensively fresh for cooking and dehydrated. Its pungency and typical flavor is due to the presence of methyl allyl disulfide. In garlic, alliinase catalyses the formation of allicin which gives fresh garlic its characteristic smell. The therapeutic value of this crop is widely recognized for various ailments.

The main centers of variation for garlic are found in the mountain regions of the former Soviet Union, in Central Asia, and pre-forest regions of the Caucasus. The Mediterranean, China and India are secondary centers of variation.

A. longicuspis, is supposed to be a diploid species from Central Asia closely related to garlic, and is postulated as its probable ancestor. However, morphologically this “species” is undistinguishable from garlic. Furthermore, isozymes and DNA-based markers fail to separate it from some forms of cultivated garlic.

The main germplasm resources for garlic are in Wageningen, Netherlands, Wellesboune, UK, Tsukuba Japan and USA at the USDA germplasm centers.

Most garlic cultivars are sterile, so this crop is reproduced vegetatively by cloves.

Often garlic cultivars are classified as botanical varieties based on their ability to bolt and flower:

Var sativum: non-bolting or rarely bolting types. If it bolts, the floral stem has a soft neck and many cloves in the base. (cross section)

Var. ophioscorodon: bolting with snake like twisting or coiled flower stems. Some with topsets and flowers, with few cloves in the base. (cross section). Some are fertile.

No polyploid forms are found in garlic, although some varieties might be triploid.

Induced polyploidy have been accomplished by colchine treatment of garlic cloves. Also meristem culture results in aneuploid and polyploid plants. The resulting polyploids display delayed growth, being unsuitable as new varieties. Therefore, polyploidy has not been successfully applied to improve this crop.

It is difficult to explain the extensive variability in size, color and shape found in garlic cultivars, unless extensive somatic mutations occur in this species.  Most likely these have been accumulated after years of clonal propagation.

The wild, putative ancestor is also seedless, being reproduced by topsets or bulbils.

Because of the obligate apomictic nature of garlic, no classical breeding has been possible in this crop, although this is changing with the reports of fertile cultivars. Induced mutagenesis has been attempted with limited success.

Probable causes for widespread sterility in garlic:

There are many possible causes for sterility, some of which are quite speculative:

1) Natural hybridity between two unknown species resulting in sterility. This is unlikely, since there are some varieties that are fertile.

2) Physiological, or bacterial diseases. Sometimes pollen obtained by antibiotic treatments.

3) Genic male sterility due to tapetum degeneration after microspore formation. Gene now identified (Mf).

4) Female sterility.

5) Cytoplasmic male sterility

6) Accumulation of chromosomal aberrations due to its prolonged vegetative propagation.

7) Flower buds unable to compete with fast developing bulbils or top sets.

Evidence for some of these has been reported, but none have been very convincing.

Chromosomal aberrations are common in garlic, due to multiple translocations sometimes involving 8 or even 10 chromosomes. However, some varieties have a normal karyotype, yet they are sterile.

True garlic seed

In the past 25 years, researchers in Germany, Japan and US have been able to obtain seeds from a few garlic clones, and to grow them successfully into seedlings. Furthermore, a private company, Basic Vegetables in Vacaville, CA have now a garlic breeding program based on these recent findings.

As a first step to obtain seed, garlic accessions with normal meiosis and with ability to produce some pollen were selected by researchers after extensive germplasm screening. Those producing true botanical seed belong to the ophioscorodon variety. Pooler and Simon (1994) report this work.

The seed producing varieties will form a combination of flowers and bulbils in their inflorescences. For successful seed production, the bulbils must be removed early on from the inflorescences to allow normal flower development. Floral stems are cut from the base and inserted in containers with water. This strategy seems to delay flower senescence. From pollination to seed takes 2-3 months. Pollen after germination grows down the style normally and can be detected by fluorescent dyes. After 24 hours the pollen tubes reach the ovary.

Seeds have low vigor, so these must be scarified and germinated in agar medium. Large variability among seedlings was observed. Variants not observed in the whole collection were recovered. Crosses between different plants are highly heterotic.

Recovered plants were normal diploids, with normal meiosis.

Success of true botanical seed is clone-specific and environment-dependent. Topset removal is essential, otherwise flowers will dry before anthesis.

Of 200 varieties tested, 10 produced seed. Only 1 variety produced enough pollen to be used as male parent to pollinate other five varieties used as female parents. Pollen fertility varied from 0 to 19%.

There seems to be an association between anther color and pollen fertility. Purple anthers tend to have fertile pollen, whereas yellow anthers have sterile pollen. Gene for male fertility (Mf) identified.

Seedling tested by isozymes indicate that they derived by cross pollination, although Shemesh (2008) confirmed that the main breeding system of garlic is allogamy. Botanical seed propagation releases great variability that  will be useful  to breeders.   

True garlic cracks open field of garlic breeding if it is indeed practical. Seed might even be stored for 10 years.

Hybrids between A. longicuspis and A. sativus have been reported.

Pooler and Simon (1993) report the characterization of the US garlic collection with isozymes and lately part of the collection by AFLPs.

Phylogenetic studies indicate that A. longicuspis should belong to A. sativum, since both species share the same isozyme and molecular markers. There is no justification to separate them as different species.

17 different electrophoretic phenotypes were disclosed. These seem to associate to floral traits. The following clusters were obtained in the basis of isozymes:

1) Early senescing, non-fertile flowering clones.

2) Non- pollen producing ophioscorodon types

3) Most pollen producing ophioscorodon types.

4) Most of the non-bolting sativum types.

RAPD, AFLP and other DNA-based markers are also available in garlic and have been used for variety identification. Some of these have been found associated to fertility. The accessions in the USDA germplasm can be classified in six major clusters on the basis of molecular markers.

Mapping: Botanical seed propagated garlic has opened up inheritance studies, including gene mapping. Ipek et al (2005) and Zewdie (2005) have produced the first linkage maps. Ipek et al produced two maps with over 100 AFLP markers, including mapping of genes for alliinase, chitinase, sucrose metabolism and chalcone synthase. Zewdie produced a 37 marker map in 9 linkage groups. Most of the markers were derived from onion sequences. A male fertility locus was mapped. Genetic distortion was common.

Biotechnology: The use of these techniques is quite limited in garlic. Basically only meristem culture have been used for virus cleaning of the clones. Regeneration from protoplasts has also been reported.

A Garlic EST database with over 21,000 sequences produced in Korea. It is based on cDNA library.

Transformation:
 Successful transformation of garlic based on Agrobacterium tumefaciens has been reported by Kondo et al. 2000. For this purpose calli from primordial meristerm was used. A more efficient protocol reported by Kenel et al 2009.

Garlic as a functional food: Garlic is used in traditional medicine for the treatment of many ailments. In the past few years extensive research is taking place trying to demonstrate that indeed garlic is useful to prevent a number of diseases and disorders. These studies include its role as an anti-inflammatory, preventing atherosclerosis. This is probably due to the antioxidant activity of S-allyl cysteine. Also saponins in garlic have been reported to lower the risk of cardiovascular diseases.

Other studies indicate that garlic contains di-allyl sulfide, gamma-glutamyl-Se-methylselenocysteine and lectins, which are naturally occurring anticancer agents.

 

Garlic also has been reported to boost the immune system and to lower the cholesterol level.

 

Lanzotti et al (2006) reviews the main compounds reported in onions and garlic as well as assay methodology.

 

 



 Return to syllabus

Last modified, May 6, 2010
© Carlos F Quiros, 1998