PLS221                                              Instructor: Carlos F. Quiros

Apiaceae: Daucus carota var. sativum, carrot

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(slides)


Carrot

Origin and Distribution and Domestication

Related Species

Cytogenetics

Floral Biology

Male sterility

Most important traits:

Root color

Quality

Disease Resistance

Markers and Mapping

Carrot as a model research organism for tissue culture

Biotechnological applications



Carrot, as an underground vegetable ranks second after potatoes in the Mediterranean Region. It is an excellent source of sugars, vitamins A and C and fiber in the diet. It is also an important source of nutrition, especially for infants.

Carrots belongs to the genus Daucus, including species ranging from x=7 to 11.

The cultivated species is D. carota 2n=2x=18

It belongs to the section Carota, which includes 11 other species, all found in the Mediterranean Region.

D. carota consists of 13 subspecies divided into two groups:

A) Carota group.

The cultivated species belongs to var. sativum. Wild forms of this species are widespread through out the world. These belong to var. carota.

Other ssp in this group are: maritimus, major, azoricus, parviflorus.

B) Gingidium group: It includes seven subspecies

All subspecies subspecies of D. carota can cross freely with each other, and hybridization occurs in nature in many cases.

The related wild species D. capillifolius can be crossed to D. carota. This species together with the carota subspecies forms part of the Carota Complex.

The genus Daucus has more than 80 species.

These are centered North and South of the Mediterranean Sea, spreading to North Africa, SW. Asia and Ethiopia.

There are two main types of cultivated carrots:

1) Eastern/Asiatic carrots: These are often called anthocyanin carrots because or their purple roots, although some have yellow roots. They have pubescent leaves giving them a gray-green color, and bolt easily. The greatest diversity of these carrots is found in Afghanistan, Russia, Iran and India. These are possible centers of domestication, which took place around the 10th century.

Anthocyanin carrots are still under cultivation in Asia, but are being rapidly replaced by orange rooted Western carrots.

2) Western or Carotene carrots: These have orange, red or white roots. Most likely these carrots derived from the first group by selection among hybrid progenies of yellow Eastern carrots, white carrots and wild subspecies grown in the Mediterranean.  The first two originated by mutation. These carrots may have originated in Turkey.

Carotene carrots are relatively recent, from the 16, 17th century. Orange carrots were first cultivated in the Netherlands. Our present cultivars seem to originate from long orange varieties developed there. Adaptation to northern latitudes has been accompanied by change in photoperiod response.
 
 

Eastern carrots

Western carrots

Roots branched, yellow, reddish-purple to purple black

Roots unbranched, yellow, orange or red, occasionally white

Slightly dissected leaves

Strongly dissected leaves

Greyish-green pubescent foliage 

Bright green, sparsely hairy foliage

Annual

Biennial

Carrots were first introduced to the US in the 17th century by European settlers.

The main carrot germplasm centers are found in Wellesbourne, UK. In the US, USDA maintains a germplasm collection in Iowa. In addition to these two, other collections exist around the world.

Floral Biology:

Carrot is a biennial crop but it is grown as an annual. Protandry in the flowers promotes outcrossing by insects. Inbreeding results in severe depression, although it is still possible to produce inbred lines for F1 hybrid production due to the increased demand of plant uniformity for precise planting equipment and value-added products.

Like celery the carrot flowers are organized in compound inflorescences called umbels, comprising 50 or more umbellets, each with 50 flowers. There are approximately 1000/umbel. Flowers are perfect, or functionally male and female. The main umbel, often called king umbel, produces the highest seed yield and quality.

Seed germination affects uniformity of crop since less developed embryos take longer to germinate.

Emasculation is rarely performed for variety production, since it is inefficient, the flowers are small and each produces only two seeds. Hybrid production is based on cytoplasmic male sterility.

CMS in carrots:

Similar to onions and Brassica, this is due to interaction of mitochondrial and nuclear genes, thus indicating that mitochondria participation is necessary for normal floral development. Mitochondrial dysfunction due to DNA rearrangements resulting in chimeric genes often results in reduced respiration, function that sometimes can be restored by certain nuclear genes (Linke et al 2003).Usually the last steps in pollen formation are affected, without changing female structure or floral architecture. In rare cases homeotic-like changes occur changing floral structure, and this is seen in petaloid and carpeloid cms in carrot.

There are two main sources of cms in carrots:

1) Petaloid and carpeloid: stamens modified to petals, found by Munger (1953) or carpels (Linke et al, 2003).

This cms system was found in wild carrot.

2) Brown anthers: Anthers dried and deformed due to degeneration, found by Welch and Grimball (1947) in cv. Tendersweet. Welch was a lettuce breeder at the Dept. of Veg Crops at UC Davis.

Newer sources for both types of CMS have been found in the recent years.

In the US petaloid cms is mostly used for hybrid seed production because it is more stable than brown anthers, however in Europe and Asia the latter is preferred because it is more stable there than petaloid cms.

The inheritance of cms is not simple and has not been completely solved:

Both types of cms are dominant and may be determined by triplicated nuclear genes, one dominant (M), the two recessive (t and l), which interact with sterile cytoplasm

[S]MMlltt = male sterile

restorer: [S]MmLltt or [S]MmllTt

Maintainer: [N] MMlltt

As mapping progresses in this crop we will have a better understanding on the inheritance of restoration.

Male sterile lines produce less seed than fertile ones after pollination, perhaps because the flowers are less attractive to pollinators and to presence of reduced nectaries.

Carpeloid cms seems to result from disrupted interaction between the gene products of nuclear and mitochondrial genes leading to reduced expression of some of the MADS box genes, which are know to be involved in floral development (Linke et al, 2003).There seem to be a mitochondrial effect on MAD-box factors specifying organ development (Carlsson et al 07).

For seed production, care must be taken to isolate breeding plots from wild carrots in the area, since cross pollination occurs. In most cases, these hybrids will display deformed or forked roots.

Cytogenetics:

The nine carrot chromosomes are small and uniform in length. The DNA content in this species is 0.98 pg DNA per 1C nucleus (473 Mbp). 40% of the DNA is highly repeated. Three chromosomes have satellites.

Aneuploids have been reported only in tissue culture cell lines. Tetraploid and hexaploid cell lines are also quite common.

Two natural polyploid species have been reported:

D. glochidiatus tetraploid

D. montanus hexaploid

Haploids: Only 1 obtained in 600 seedlings spontaneously. An haploid cell line was maintained for basic studies. Also induced by pollen irradiation, but not used extensively.

Interspecific and intergeneric hybridization

There is enough genetic variation among the cultivated classes of carrots to derive any root type or regional adaptation. However, crosses of domesticated x wild carrot are sometimes necessary, mostly for disease and pest resistance. These crosses derive satisfactory domesticated types, but larger populations, more time, and more backcrosses to domesticated stocks are necessary.

Several x=9 species are valuable sources of germplasm for nematode and carrot fly resistance:

D. capillifolius

D. syrticus

D. carota ssp. hispanicus

D. carota x D. capillifolius hybrid is fully fertile

Interspecific differences in chloroplast and mitochondrial DNA have been detected.

cpDNA is more conserved, making possible to detect differences by restriction analysis among some subspecies and species. It has a size of 155 kb.

On the other hand, mtDNA much less conserved, making possible to detect varietal differences. It has a size of 386 to 468 kb.

Somatic hybrids by protoplast fusion:

In somatic hybrids, rearrangements of mt genomes can be detected. Carrot protoplast fusion with parsley (n=11) has been successful but the regenerants were not analyzed. Other more distant hybrids include:

Intergeneric hybrids: carrot and Petunia

Interkingdom: HeLA (human ) and carrot

Inheritance of most important traits:

Root color is one of the most important traits of carrots. Orange carrots contain alpha and beta carotenes and are an important source of pro vit A in the US.Yellow carrots contain xanthophylls (mostly lutein) and red carrots contain lycopene. White carrots are devoid of carotenoids.

Inheritance of root color is complex and at least 5of interacting genes are involved resulting in colors that range from white to purple. . Most of this work was initiated by Imam and Gabelman (1968) and the most recent work on the subject can be found in the papers of Just et al (2007) and (2009). This includes the isolation and mapping of 24 four candidate genes in the carrot carotene pathway in 8 of the 9 linkage groups.Two are associated to a QTL for carotene accumulation.

A summary of the major genes involved in carrot color as well as genotypes and phenotypes based on the work of Just and Simon is listed below. (P. Simon pers. com. 2010)

YY Y2Y2 __=white (Y inhibits carotene accumulation)

yy Y2Y2 __ = white or yellow

yy Y2y2 __ = orange with yellow core in some backgrounds, yellow throughout in others

yy y2y2 y3ay3a = pale orange

yy y2y2 y3ay3b = orange

yy y2y2 y3by3b = dark orange

 

Y, and Y2 control most of the variation of carotene accumulation in carrots for alpha and beta carotene, respectively and are associated to the zeaxanthin epoxidase, carotene hydroxylase and carotenoid dioxygenase genes. Y3 locus is tentative (P. Simon pers. com. 2010).

Additionally, there are other genes, A and L for red carrots (Umiel and Gabelman (1972) andmodifier genes acting in the xylem, such asIO=intense orange xylem,O=orange xylem, (Kust 1971)yellow xylem is undesirable. rp=reduced pigmentationresulting from lower rate carotene deposition.

All these genes control different concentration of alpha and beta carotene, anthocyanin and lycopene.

Purple or anthocyanin (cyanidin) carrots, determined by another set of genes

P1P1 p2p2 = purple root, green foliage, P1 is dominant in some backgrounds but incompletely dominant in others

P1P1P2_ = purple root and foliage

p1p1 P2P2= purple foliage but not leaves in some backgrounds, non-purple roots and foliage in others

(P. Simon pers. com. 2010)

There is a wide range of variation in carotene content, from zero in white carrots to 450 ppm in Hi Carotene carrots with the potential to reach 950 ppm . Breeders have been constantly developing varieties with higher carotene content. (Simon andPollak 2009)

 

Breeding:

Most carrot hybrids are usually 3-way crosses, (AxB)xC. This is because the hybrid vigor in a single-cross F1 female seed parent produces much more seed than an inbred male-sterile parent. Single-cross hybrids, AxB, which are more uniform, are used when seed productivity is adequate, depending on the inbreds used.

For selection the root to seed method is used. Roots are harvested, and selections of the best are made, discarding off types. These are then re planted for intercrossing in isolated plots or cages.

Carrot cultivars can be classified in two major groups:

Temperate and sub-tropical. Most breeding efforts are on temperate types because of their higher value and larger market share.

Cultivars of one class do not perform well in regions outside their range of adaptation.

Temperate types include Nantes, Imperator, Danvers, and Chantenay.

In North America, Imperator types predominate for fresh market, including "cello" (whole carrots) and "cut and peel" or "baby" (lightly processed carrot pieces), and for processing as carrot slices. For processing as diced pieces cvs Chantenay, Danvers, and to a lesser extent Nantes types are often used.

In Europe, Nantes types are grown mostly for fresh market. Only whole carrots are sold fresh in Europe. Two other types, Berlicum and Amsterdam, are also grown for fresh market. Carrots for processing are these same as for North America plus another type, Flakkee.

Sub-tropical types include Kuroda, Brasilia, and Tropical Nantes

Sub-tropical carrot classes as the Temperate ones also vary by root shape. Kuroda resembles Danvers or Chantenay, while Brasilia, and Tropical Nantes resemble Nantes. All of these types are used for fresh market and for processing.

Breeding for Specific Characteristics

General traits:

Selection for stable maintenance of male sterility and for bolting resistance.

These must be evaluated in climates similar to these intended for production. Bolting resistance is evaluated in more cool winter production areas.

Disease and pest resistance:

Screening is done in seedlings:

Alternaria leaf blight resistance and powdery mildew.

Black crown (A. radicina).

Northern and Southern root-knot nematode being tested for feasibility.

Inoculation of root slices are being tested for resistance to cavity spot and Erwinia soft rot resistance.

Field testing is also used to breed for resistance to carrot fly, powdery mildew, cavity spot, Cercospora leaf blight, Xanthomonas blight, and two mycoplasmal diseases, aster yellows and virus complex motley dwarf.

Simply-inherited resistance is known for powdery mildew, Cercospora leaf blight, and Northern root knot nematode, but environment affect in many cases the level of resistance.

Quality:

Flavor and eating quality are important traits in the development of new cultivars. Selection for low levels of volatile terpenoids, which result in turpentiney flavor, and high levels of free sugars, which are responsible for sweetness in raw carrots, can be performed chromatographically in the laboratory. Nevertheless organoleptic taster panels can also perform this function as well as to evaluate for texture. Sugar levels can also be estimated by assessing soluble solids levels. These three traits constitute the most important sensory attributes of unprocessed or lightly processed carrots. Sugar levels are also important for processed carrots.

The allele Rs is responsible for high reducing sugar content in carrots.

Color is another important trait, due to carotene content. Visual selection for dark orange internal root color has been an important parameter for increasing carotene content. Selection above 150 to 250 ppm total carotene requires spectrophometric assessment. This is especially important for the development of carrots to be used as carotene sources. A high carotene mass population was developed by Peterson at the U. of Wisconsin. This trait may be controlled by complementary gene action. Some selections were as high as 700ppm (fresh wt).

Markers and Mapping:

As in other crops various types of markers systems are available to the carrot breeder and genetics. The goal is to perform in the future marker-assisted selection and QTL applications.

Approximately 1000 RFLP, 15 RAPD, 250 AFLP and 30 other molecular or biochemical markers such as isozymes have been mapped in several carrot populations to date, with genetic maps from 800 to 1300 cM resulting (Schultz et al. 1994). A more recent map including RFLPs, AFLPs and microsatellites in a population segregating for root color and sugar content has been reported (Vivek and Simon, 1999). Santos and Simon (2004) has expanded this map based on a dark x white carrot F2 population. 24 caroteniod biosyntesis genes mapped (Just et al 2007).

Several linkage have been reported:

One marker to nematode resistance (Meloidogyne javanica)

Two markers for seed spines.

Two markers for northern root-knot nematode, (Meloidogyne hapla) resistance

One marker for purple root color.

A number of important quantitatively-inherited traits in carrot have also been studied including greening, flavor terpenoids, and soluble solids content of the storage root.

Polymorphic insertion sites of transposons DcMaster (PIF/Harbinger-like)have been mapped by Grzebelus et al (2006). Seems to be specific of the Apiacea family. They have been found useful for testing hybrid seed purity.

Biotechnological Applications

Complete plastid genome done in carrot (Ruhlman et al 2006) 115,911 bo, 115 unique genes, 21 duplicated genes. It maight open up the possibility of using plastid transformation in the future.

Carrot tissue culture systems are very well-developed since carrot was an early model system for regeneration and demonstration of totipotency by F.C. Steward in the 1960ís. Therefore carrot is readily manipulated in tissue culture and genetically transformed, although transformed plants have not been yet tested in the field.

Micropropagation is simple to do but it is not commercially applicable since it results in plants with malformed roots.

Transformation have been reported by Agrobacterium tumefaciens (Tukuji and Fukuda, 1999). The main goals have been to develop herbicide resistance and insect resistance carrots. Other applications include generation of new male sterility types and to improve root quality for color or flavor.

Resistance to Sclerotinia and Botrytis have been reported by transformation of carrots with thaumatin-like protein from rice, a pathogenesis related protein (Chen and Punja, 2002).



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    Last modified: May 11, 2010
    © Carlos F Quiros, 1998