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Essay on Plant Breeding–Objective and Modes of Reproduction

Objectives of Plant Breeding:

i. Higher yields

ii. Improved Quality

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iii. Disease and insect resistance

iv. Photo insensitivity

v. Synchronous Maturity

vi. Non shattering characteristics

vii. Varieties for new seasons

viii. Moisture stress and Salt tolerance

Mode of Reproduction

(a) Sexual Reproduction:

It means fusion of male and female gametes to form a zygote, which develops into an embryo.

In crop plants meiotic division of specific cells stamen and pistil yields microspores and megaspores respectively. This is followed by mitotic division of the spore nuclei to produce gametes like microspores and megaspores respectively.

Sporogenesis:

Production of microspores and megaspores is known as sporogenesis.

Microspores are produced in anthers, while megaspores are produced in ovules.

1. Microsporogenesis:

Each anther has four pollen sacs, which contain pollen mother cells. Each pollen mother cells undergoes meiosis to produce four haploid cells. This process is known as microsporogenesis.

2. Megasporogenesis:

A single cell in each ovule differentiates into a megaspore mother cell. The megaspore mother cell undergoes meiosis to produce four haploid megaspores. Only one is functional.

Gametogenesis:

The production of male and female gametes in the microspores and the megaspores respectively is known as gametogenesis.

1. Microgametogenesis:

During the maturation of pollen, the microspore nucleus divides mitotically to produce a generative and a tube nucleus. When the pollen lands onto the stigma of a flower pollination occurs shortly after pollination, the pollen germinates. The pollen tube enters the stigma and grows through the style. The generative nucleus undergoes a mitotic division to produce two male gametes. The pollen tube enters the ovule through a small pore, micropyle and discharges the two sperms into the embryosac.

2. Megagametogenesis:

The nucleus of a functional megaspore divides mitotically to produce four nuclei. Now megaspore nucleus undergoes three mitotic divisions to produce eight nuclei. Three of these nuclei move to one pole and produce a egg cell and two synergid cells. Another three nuclei migrate to the opposite pole to give rise to antipodal cells. The two nuclei remaining in the centre is called polar nuclei. The megaspore thus develop into a mature embryo sac. The development of embryosac from a megaspore is known as megagametogenesis.

(b) Asexual reproduction:

Asexual reproduction – Does not involve fusion of male and female gametes.

(a) Vegetative reproduction:

1. Underground Stems

i. Tubers

ii. Bulb

iii. Rhizomes

2. Subaerial Stems

i. Stolon

ii. Suckers

iii. Runners.

3. Bulbils – garlic

4. Artificial vegetative reproduction

i. Grafting, layering

ii. Tissue culture.

(b) Apomixis – Seeds are formed but the embryos develop without fertilization.

1. Adventive embryony – embryo by nucellus, integeument and chalaza:

i. There is no formation of embryosac. e.g. Mango, Citrus.

2. Apospory – non reduced embryo by vegetative cells other than embryosac.

i. Here embryosac is formed. e.g. Heiraceum, Ranunculus.

3. Diplospory – embryo formed by embryosac cells.

Mode of Pollination:

Self Pollination:

Pollen fall on to the stigma of the same flower mechanisms promoting self pollination.

Mechanism promoting self pollination.

i. Cleistogamy – Flowers do not open e.g. cereals.

ii. Chasmogamy – Flowers open, but only after pollination has taken place e.g. in cereals.

iii. Position of anthers in relation to stigmas ensure self pollination e.g. tomato and brinjal.

iv. Flowers open but the stamens and the stigma are hidden by other floral organs e.g. pea and soybean.

Cross Pollination:

Mechanism promoting cross pollination:

1. Dicliny:

The flowers are either staminate (male) or pistillate (female).

(a) Monoecy:

Staminate and pistillate flowers occur in the same plant, either in the same inflorescence e.g. castor, mango.

(b) Dioecy:

The male and female flowers are present on different plants e.g. papaya, datepalm.

2. Dichogamy:

Stamens and pistils of hermaphrodite flowers mature at different times.

(a) Protogyny:

Pistils mature before stamens e.g. bajra.

(b) Protandry:

Stamens mature before pistil e.g. maize.

3. Stigmas are covered with a waxy film e.g. alfalfa.

4. Self-Incompatibility.

5. Male sterility.

Male Sterility:

Male sterility is a reproductive deficiency of some plants where male organs in hermaphrodite flowers are rendered defunctioning. It relates particularly to non-viable pollen grains which are formed through a chain of vital processes during microsporogenesis.

Characteristics Features:

1. Better quality of hybrid seeds since human errors committed during detasselling are completely eliminated.

2. Larger quantity of hybrid seeds, since the production area no more remains a limiting factor, as also the manpower.

3. The possibilities of losses due to weather hazards at the time of detasselling are completely dissipated.

4. There is considerable wastage of plant energy in producing pollen grains in normal plants.

5. Male sterility can be utilized for developing mass reservoirs, particularly in outbreeders for the conservation of variability.

6. Male sterile lines can serve as tester genotypes for assessing the combining ability of a large number of stocks.

7. Male sterile plants produce seed only on cross pollination with functional pollen of other plants.

8. Presence of male sterility leads to heterozygocity in a species as it promotes out breeding and reduces homozygosity due to elimination of inbreeding.

9. It results from the action of nuclear genes or cytoplasmic genes or both Male sterility is caused due to pollen or anther abortion.

10. It occurs in nature from spontaneous mutations as well as can be induced artificially by chemical or physical mutagens.

12. It can be observed in all diploid species in crop plants both wild and cultivated if properly investigated.

Types of male sterility:

It is of five types —

(i) Genetic male sterility

(ii) Cytoplasmic male sterility

(iii) Cytoplasmic genetic male sterility

(iv) Chemical induced male sterility

(v) Transgenic male sterility.

Genetic Male Sterility (GMS):

The pollen sterility is caused by nuclear genes is termed as genic or genetic male sterility.

Reported in barley, wheat, maize, cotton, sorghum, lucerne, cucurbits, tomato and sugar beet.

The characteristics of GMS are:

1. Male sterility genes are usually recessive.

2. In majority of cases, sterility is caused by single gene. In few cases two or more genes control male sterility.

Merits:

i. GMS usually recessive and monogenic. Hence fertility restoration in the hybrid and crossing plants are relatively easy.

ii. Can be used for the production of hybrid seed both in seed as well as vegetatively Propagated crops:

iii. Generally doesn’t have undesirable agronomic characters.

iv. Requires less area and labour, because the breeder has to maintain only A and  lines.

Demerits:

i. Less stable

ii. 50% plants are fertile, which have to be removed every year. This increases the cost of hybrid seed.

Cytoplasmic Male Sterility:

Features —

1. Plants carrying particular type of cytoplasm are male sterile, but will produce seed if pollinators are present.

The Fi seeds (cross seeds) produce only male sterile plants, because their cytoplasm is derived entirely from the female gamete.

2. It consists of only A line and  line. A is male sterile and  is male fertile. For all other characters A and  lines are similar or isogenic.

3. CMS can’t be utilised for hybrid seed production without the use of restorer line because F1 seeds produce only male sterile Fi plants.

4. CMS can be used for development of hybrids in vegetatively propagated crops and ornamental crops, were grain is not the economic part.

5. CMS is not influenced by environmental factors such as low or high temperature.

Merits:

i. Highly stable and is not influenced by environmental conditions such as temperature and day length.

ii. CMS requires less area because breeder has to maintain only A and  lines.

Demerits:

i. Can’t be used for development of hybrids in those crops where seed is the economic product.

ii. Can be used only in asexually propagated crops viz., sugar cane, potato and forage crops.

iii. CMS is solely governed by plasma genes Therefore, it is impossible to restore fertility in the hybrid.

iv. CMS line may have inferior agronomic performance.

Cytoplasmic genetic male sterility:

When pollen sterility is controlled by both cytoplasmic and nuclear genes it is known as cytoplasmic genetic male sterility.

Features —

i. Male sterility is controlled by the cytoplasm and nuclear gene.

ii. System includes A, Â and R lines. A is male sterile line, Â is similar to A in all features but its male fertile and R restores fertility in F1 hybrid, hence its called restorer line (R). ‘B’ line is used to maintain the fertility and so its called maintainer line ‘B’ line.

Development of new male sterile and restorer lines:

New male sterile lines are developed by following the same procedure as that CMS. But the nuclear genotype of the pollinator strain must be rr in this transfer, otherwise fertility would be restored.

Restorer lines are developed by following the procedure:

Utilization:

i. Commercially used to produce hybrid seed in maize, bajra, sunflower, cotton, jo war etc.

ii. Widely used for hybrid seed production in both seed as well as vegetatively propagated species.

iii. CGMS is highly stable and reliable as its not affected by environmental factors.

Limitations:

1. Undesirable effects of cytoplasm:

Male strile cytoplasms have undesirable effects.

2. Unsatisfactory fertility restoration:

Restoration of fertility is not satisfactory and so these sources can’t be used in the hybrid seed production.

3. Unsatisfactory pollination:

Natural pollination is not satisfactory except in wind. Pollinated species (maize) which increase the production cost of hybrid seed by.reducing the production of the same.

4. Modifying genes:

Reduce the effectiveness of CGMS. During backcrosses while transforming male sterile cytoplasms, the nuclear genetic background may also be disturbed, which may lead to some pollen production by the male sterile lines.

5. Contribution of cytoplasm by sperm:

Sometimes, cytoplasm may be contributed by the sperm, which may lead to breakdown of male sterlity during long run.

6. Non-availability of a suitable restorer line:

In crops like wheat, polyploid nature of the crop and undesirable linkages with the restorer genes make it very difficult to develop a suitable restorer (R) line.

Chemically Induced Male Sterility:

This type of male sterility can be induced by chemicals called male gametocides. The major difference between mutagen induced and gametocide induced male sterility is that the former is heritable and the latter is non-heritable.

(Eg) for male gametocides: (Chemical hybridizing agents (CHA))

Ethanol – Rice, sugarbeet, wheat.

GA3 – Maize, onion, rice.

MH – Cucurbits, onion, tomato, wheat.

NAA – Cucurbits.

Sodium Methyl Arsenate – Rice.

Advantages:

1. It can be used with any variety as per need and there is no need of seed for isolation or transfer of cytoplasmic or genetic male sterility.

2. Maintainer and restorer lines need not be developed.

3. Saves time, labour and money.

4. This method is being used in China to develop rice hybrids.

Disadvantages:

1. Pollen abortion is ‘”complete and variable.

2. Treatments are effective only at specific stage of crop growth and effect is short lived.

3. Female fertility may also be reduced.

4. Production of undesirable side effects.

5. High cost of CHAs and their repeated application increases the cost of hybrid seed.

Transgenic Male Sterility:

The male sterility which is induced by the technique of genetic engineering which is induced by the technique of genetic engineering is called TMS so the foreign gene (transgene) is used for the induction of male sterility. The gene responsible for male sterility is isolated from microorganisms. It comes under genetic male sterility is heritable.

The TMS has been induced in tobacco by transferring a gene from B. amylolique faciens. Two types of transgenes are used, one gene (Bamase) causes male sterility, hence this gene is integrated in ‘A’ line. Another gene (Barstar) suppresses the male sterility gene (bamase) and leads to the restoration of fertility. This gene is transferred to restorer line.

When a cross is made between A and  line the F1 is fertile. But this is an expensive method of obtaining male sterility. Maintenance of male sterility is a problem.

Self-Incompatibility (SI):

SI was first reported by Koelreuter. Here, pollen grains fail to germinate on the stigma of the flower that produced them. If some pollen grains do germinate, pollen tubes fail to enter the stigma. In many species, the pollen tubes fail to enter the stigma. In many species, the pollen tubes enters the style, but they grow too slow to effect fertilization before the flower drops. Sometimes fertilization is affected, but the embryos degenerate at a very early stage. All these conditions combinely referred as self-incompatibility.

Types:

SI can be classified on the basis of

(A) Flower morphology:

I. Heteromorphic System:

i. Distyly

ii. Tristyly

II. Homomorphic System:

i. Diallelic

ii. Poly allelic

(B) Genes involved:

(a) mono-allelic

(b) Diallelic

(c) Poly allelic.

(C) Site of expression:

(a)Stigmatic

(b) Stylar

(c) Ovarian.

(D) Pollen cytology:

(a) Binucleate

(b) Trinucleate

I. Heteromorphic system:

Here, flowers of different incompatibility groups are different in morphology.

(SI) in primula two types of flowers Pin and Thrum is called distyly.

i. Pin and thrum flowers produced on different plants. The only compatible mating is between pin and thrum flowers, which is governed by a single locus S; Ss produces thrum, while ss produces pin flowers.

ii. Here the incompatiblity reaction is governed by the genotype of the plant producing them.

iii. Allele ‘S’ is dominant over‘s’ and incompatibility system is hetero-morphic- sporophytic.

iv. Results of four types of crosses in primula.

Pin x Pin ss x ss Incompatible

Thrum x Thrum Ss x Ss Incompatible

Thrum x pin Ss x ss 1 Thrum: 1 Pin

Pin x Thrum sS x Ss 1 Thrum: 1 Pin

Tristyly:

In tristyly anthers and style have three positions, short, medium and long. (Eg) members of lythraceae.

Three positions of style are genetically controlled by two genes (Ss and Mm). The ‘S’ gives rise to short style, sM to medium style and sm to long style. The compatible matings in primula are long x medium, long x short and medium x short.

Homomorphic System:

The incompatibility reaction of pollen may be controlled by the genotype of the plant in which its produced (sporophytic) by its own genotype (gametophytic control). This system is very much important in crop plants. Its can operate in many ways like

1. The pollen grains do not germinate on the stigma of same flower. If they germinate the pollen tube fails to penetrate the stigma as in a rye, cabbage and radish.

2. Pollen grains may germinate, but there is retardation of pollen tube growth.

3. There is slow rate of pollen tube growth and it rarely reaches the ovule.

4. Pollen tube growth may be normal, but it does not release the male gamete.

Gametophytic System (GS):

Described by East and Mangelsdorf

i. Genotype of plant (Sporophyte) S1 S2 S3 S4

Genotype of pollen and gametes S1 S2 S3 S4

Incompatibility reaction of pollen grains S1 S2 S3 S4

Incompatibility reaction of style S1 S2 S3 S4

ii. The incompatibility reaction of pollen is determined by own genotype and not by the genotype of the plant on which it is produced. This is because the biochemical substance involved in the S| reaction of the pollen is produced after meiosis.

Sporophytic system (ss):

Discovered by Hugher and Babcock (1950).

In sporophytic system SI reaction is governed by single gene ‘S’ with multiple alleles. In general, the number of ‘S’ alleles is considerably larger in the gametophytic system than in the sponophytic system.

The incompatibility reaction of pollen is governed by the genotype of the plant on which the pollen is produced and not by the genotype of the pollen.

In sporophytic system, the ‘S’ alleles may exhibit dominance individual action (codominance) or competition.

i. Polygenes (modifying genes) are known to increase as well as decrease the activities of‘S’ alleles both in the GS and SS systems.

Mechanism of SI:

1. Stigmatic inhibition (pollen stigma interaction):

Inhibition of pollen germination or pollen tube growth occurs on the stigma. Normally it is confined to sporophytic system. This type of inhibition is found in Radish, Cabbage, Cauliflower, Sunflower etc.

2. Stylar inhibition:

Inhibition of pollen tube growth in the stylar region is a common feature in SI plants with binucleate pollen. This inhibition is restricted to GS. Pollen tubes usually swell and burst of the apex in the upper region of the incompatible style, (eg) Beta, Helianths, Fagopyrum.

3. Ovarian inhibition (pollen tube style interaction):

Self incompatibility reaction occurs only when the pollen tubes reacts the ovary. Such inhibition occurs in the ovary in the species those have hollow styles. (Eg) Lilium, Anona etc.

Elimination of SI:

1. Doubling the chromosome number in case of single locus gametophytic system.

2. Isolation of self fertile (Sf) mutations.

3. Self-Incompatibility alleles may be transferred from related or same species through back cross.

Methods to Overcome SI:

Temporary self fertility can be achieved in many ways so then the SI is fully functional in the selfed progeny. Such fertility is called Pseudo fertility and is brought about by

1. Bud pollination.

2. End of season pollination.

3. Mentor pollen technique (recognition pollen).

4. High temperature exposure to pistils – Tomato, Brassica.

5. Irradiation by x-rays/y-rays – Solanaceae.

6. Grafting.

7. Double-pollination.

8. Other techniques such as.

(i) Injecting styles with immuno suppressant.

(ii) Treatment of pistil with phytohormones.

(iii) Steel brush pollination.

Table 1. Development of fruits:

OvaryFruits
Ovule

Integuments

Nucleus

Two polar nuclei + sperm nucleus

Egg nucleus + sperm nucleus

Seeds

Tests (seed coat)

Perisperm

Endosperm 3n

Zygote — embryo (diploid 2n)

Centres of Origin (Vavilov):

Name of the centreAreas coveredPlants
1. Chinese centreMountaineous regions of central western ChinaBuck wheat, Soybean, Onion, Bamboo, Cherry, Citrus, Sugarcane Tea etc.
2. Indian centre
(a) Main centreAssam and MyanmarRice, Sugarcane, Mango, Orange, Jute, Coconut Pepper etc.
(b) Indo MalayanIndo-China and Malayan ArchipelagoBanana, Sugarcane, Clove, Coconut, Manila hemp, Pepper.
3. Central AsiaticKashmir, N WFP of Punjab, Afghanistan, Uzbekistan, Tian-ShanWheat, Pea, Beans, Cotton, Carnet, Garlic, Apple, Almond, Apricot.
4. Near EasternInterior of Asia-minor, Transcaucasia, Iran, highlands of Turkmenistan.Cherry, Pomegranate, fig, Almond, Clove,
5. MeditrenaneanBorders of Mediterranean seaCabbage, Turnip, Clove, Mustards, Duram and emmer wheat, Pepper etc.
6. AbyssinianEthiopia, Somalia, EritreaWheat and Barley, Sesame, Castor, Coffee, Okra.
7. South Mexican and Central AmericanSouth Mexico, Guatamala, Honduras, CostaricaMaize, Bean, Sweet-potato, Papaya, Tobacco, Upland cotton.
8. South AmericanHigh mountainous areasMany potato spp, Tomato, Pumpkin,
(a) Peruvian- Bolivian

(b) Chile

Egyptian cotton, Tobacco.

Common potato

(c) Brazilian-ParaguianGroundnut, Cassava, Pineapple, Rubber and Cashew nut.

Primary gene centre: Native wild relatives, dominant genes, diversing due to ancient cultivation.

Secondary gene centre: Devoid of relatives recessive characters, ecological diversity, farming practice, human migration patterns and internal biological dynamics of hybridisation, segregation-selection.

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