Reproduction in Plants: 3 Modes | Plant Breeding

The mode of reproduction in crop plants may be broadly grouped into three categories: Vegetative, Apomictic and Sexual.

Mode # 1. Vegetative or Asexual:

In this type of reproduction the vegetative parts of the plants act as propagule in place of seed. This mode is mostly found in all those plants where there is no seed set, long reproduction cycle and heterozygosity exists.

The following organs can act as propagule:

1. Modified Stem:

Underground modified stems like rhizome (ginger, turmeric, banana), tuber (potato), bulb (onion, garlic), corn (colocasia, yam), stolon (strawberry), sucker (chrysanthemum, menthe) are used for propagation.

2. Stem Cutting:

In many of the fruit crops the artificially produced clones or stem cuttings are used as propagative material. In sugarcane the nodal portions of stems, in fruit crops like mango, litchi, lemon, grapes – the different methods like layering, grafting, budding are applied to get the stem clone.

3. Normal or Modified Root:

Normal roots of wood-apple, citrus and many such trees are used as units for propagation. Modified roots such as tuberous root (sweet potato), fasciculated root (dahlia, asparagus) are used as propagule.

4. Bulbils:

In some plants the flower bud modified into globose bulb which are called bulbils can be used as multiplication unit.

Significance:

Asexual or vegetative reproduction leads to perpetuation of the same genotype with great conservation. It is very much advantageous because large number of genetically identical individuals can be obtained irrespective of the degree of heterozygosity of the genotype. At any stage of breeding programme if a breeder gets any desirable clone it can be maintained through vegetative means.

Mutation breeding, i.e., the search for desirable mutants both through natural and artificially induced mutation is very much helpful in case of vegetatively reproducing plants as the sexual reproduction can be avoided. The method of polyploid breeding is also useful as the induced bud can be used as propagule and polyploidy can be maintained without the intervention of meiotic segregational disturbances.

Mode # 2. Apomixis:

Apomixis is the phenomenon where there is no normal fertilisation of the egg cell, hence no normal development of embryo from the egg cell. However, embryo may develop from an un-fertilised egg cell or from a cell other than the egg cell within the embryo sac or from the cell outside the embryo sac (Fig. 2.1).

The plants produced by apomixis are called apomictic which are of two types: obligate, producing only apomictic embryos, and facultative, producing both apomictic and normal embryos. This phenomenon where the substitution of sexual process occurs by asexual methods is known as apomixis (apo = away from, mixis = act of mixing), the plants are called apomictic.

Types of Apomixis:

Non-Recurrent Apomixis (Haploid Seed/Plant Formation):

Here the seed/plant develops from the un-fertilised egg cell or any other un-fertilised haploid cell of embryo sac. In this type of apomixis the first event of normal sexual cycle, meiosis takes place, the embryo sac is formed normally, but due to various reasons the fertilisation does not take place.

The reasons may be:

(i) Absence of pollen tube;

(ii) Inability of the tube to discharge its contents;

(iii) No attraction between male and female nuclei;

(iv) Early degeneration of sperms;

(v) Maturation of egg and sperm is not synchronized. ,

(a) Haploid Parthenogenesis:

Here a haploid embryo develops from an un-fertilised egg cell of the embryo sac, e.g., in Orchis maculata, Platenthera chlorantha, Cephalanthera damasonium – the pollen tube enters but fails to fertilize.

(b) Haploid Apogamy:

It is the development of haploid embryo from any haploid cell of the embryo sac other than the egg cell. In Lilium martagon, Erythraea centaurium – two pro-embryos are formed; one is from fertilized egg (zygotic cell) and another from the synergid cell (haploid cell).

Recurrent Apomixis:

Here the meiotic event does not take place either in any somatic cell or the meiocyte cell fails to undergo meiosis, thereby produces the embryo sac with diploid cells only. These diploid cells directly give rise to diploid embryos, called apospory (Fig. 2.2).

(a) Generative Apospory:

The diploid egg cell develops into embryo without fertilisation, one particular variety of Parthenium argentatum exhibits.

(b) Somatic Apospory:

The diploid cell of nucellus or integument develops directly into diploid embryo sac and the diploid embryo is developed from the diploid un-fertilised egg cell. This phenomenon is found in Allium, Meliss, Crepis etc.

Agamospermy:

This term is used to categories the plants which has the seed habit for propagation but not through normal sexual cycle, either meiosis or syngamy or both are eliminated. This includes either the embryos which may develop from cell of the unreduced female gametophyte or directly from the diploid sporophytic cell of the ovule, such as nucellus or integuments. Diplospory, apospory and adventive embryony, all are included within this category.

Adventive Embryony:

In this case the gametophytic generation is completely eliminated. It is very much close to vegetative propagation, but the plants here retained the seed habit, i.e., the diploid embryo is developed from any diploid cell outside the embryo sac but matures into embryo within the embryo sac, zygotic embryo degenerates or competes with apomictic embryo. This kind of embryos is found in Citrus, mango, etc.

Diplospory:

Here the MMC differentiates into sexual ovules, but it does not enter into meiosis, it produces the embryo sac directly by mitotic division, all the cells within the embryo sac are diploid, and the embryos are formed.

Significance:

Apomictic plants tend to conserve the genetic structure and are also capable of maintaining heterozygote advantages. Due to prohibition of fertilisation process, apomixis is the way for exploitation of maternal influence or perpetuation of maternal individuals or maternal properties.

The use of apomixis in plant breeding can be summarised:

1. Rapid multiplication of genetically, uniform individuals without any risk of segregation.

2. Heterosis or hybrid vigour can be utilised for recurring production of seeds of F₁ hybrids.

3. From generation to generation the maternal effect can be exploited.

Mode # 3. Sexual:

This process of reproduction involves the fusion of male and female gametes and formation of seed, so it is called the process of amphimixis. Depending on the nature of pollination the plants can be categorized as self-pollinated and cross pollinated.

Sexually reproducing crop plants produce a special structure, called flower which bears the essential whorls like androecium and gynoecium. Androecium consists of stamen and gynoecium consists of carpel. The male and female gametes are produced in microspores and megaspores respectively.

Sporogenesis:

Production of microspores and megaspores is known as sporogenesis. In another lobe there are pollen sacs which contain numerous PMC (Pollen mother cell) undergoing meiosis, produces microspores or pollen, the process is called micro-sporogenesis. Inside the ovary the ovules are present in which the MMC (Megaspore mother cell) undergoes meiosis and produces 4 megaspores out of which 3 degenerate and one survives, this process is called mega-sporogenesis.

Gametogenesis:

During maturation of pollen, the microspore nucleus divides mitotically to produce generative and vegetative nucleus. The generative nucleus then divides to form male gametes or sperms. The pollen along with pollen tube and sperms is called micro-gametophyte and the production of sperms is known as micro-gametogenesis.

The nucleus of functional megaspore divides mitotically three consecutive times to produce eight nuclei, which get arranged in the embryo sac. The embryo sac contains generally one egg cell, two synergids, two central nuclei and three antipodals. All these cells are haploid and this process is called as mega-gametogenesis.

Fertilisation:

Fusion of one of the two sperms with the egg cell, to produce a diploid zygote, is known as fertilisation, and the fusion of the remaining sperm with the secondary nucleus leading to the formation of primary endosperm nucleus known as double fertilisation or triple fusion.

The zygote divides mitotically to produce ‘diploid embryo. The primary endosperm nucleus produces endosperm by mitotic division; it may be absorbed completely in legumes or may form endospermous seed in cereals.

Significance in Generating and Fixing Genotypic Variation:

Among the sexually reproduced crop plants there are two categories, self-pollinated and cross pollinated. Both the categories differ in their genetic constituent, as self-pollination leads to homozygosity whereas cross pollination tends towards heterozygosity.

In self-pollination, the genotype AA or aa will remain homozygous, whereas the genotype Aa will segregate into homozygous and heterozygous in 1: 1 ratio, as a result in each generation the homozygosity will be increased by one half and heterozygosity will be reduced by one half.

Due to cross pollination in the self-pollinated crop the heterozygosity develops, or the heterozygosity may develop in a population through natural mutation. Cross pollination is the way for developing the variations in genotype and phenotype in a population.

Heterozygosity helps in developing vigor which is known as heterosis or hybrid vigor whereas self-pollination continuously will bring in the breeding depression due to attainment of homozygosity.

In plant breeding variation of characters among the population is most wanted. The breeder will select a particular character according to the need for that crop. So the characters which will be selected should be heritable through generations.

The total mechanism of heredity is dependent upon the behaviour of chromosomes, which carry the genes. Variations are originated through chromosome rearrangement, genetic recombination, mutation, structural and numerical changes of chromosome.

In heredity the chromosomes are important, as their distribution in germ cells determine the specific heritable character to the progeny. The heritable characters may be regulated by single gene or by multiple genes. The inheritance pattern of a particular character is studied by progeny testing which is a basic procedure in plant breeding.

In a breeding programme always the superior characters are selected which may or may not be controlled by dominant gene. Depending on that the homozygosity or heterozygosity in a population is wanted, i.e., if the heritable character is controlled by dominant gene then heterozygosity is attained but if the phenotype is controlled by recessive gene then homozygosity is desirable in a population.

The genes are situated on chromosomes, the recombination phenomenon during meiotic crossing over exchanges the chromosomal segments which controls the genetic distribution. The genes are the determinants for a particular character expression. Depending on their express-ability the dominant and recessive nature are determined, the genes occur always in the alternative forms called as alleles.

Due to mutation the changes in gene may occur, which also may be heritable or non-heritable. If the change is permanent and if it alters the phenotypic characters expression and if it is heritable through succeeding generations then mutation plays an important role in plant breeding.

In plant breeding when a breeder selects any particular mode of breeding programme then it affects the genetic criteria fixation in that population as he selects a particular desirable character.

Normally in a population all the characters may be evenly distributed or the dominant characters are prevailed, but whenever a breeder is choosing for a particular criterion then the genetic constituent of that population gets changed. A particular gene gets advantage to be fixed in that population.

Suppose in a breeding programme of barley one breeder is selective for the “white and smooth awn” character, both of them are recessive characters, so in a dihybrid cross programme only one out of 16 plants will bear this character and then selfing of that plant will lead to homozygous population, which will change the total genetic constituent of the population.

Back-crossing is also another phenomenon of changing and fixation of genetic constituent in a population. The parent holding more desirable characters is used as recurrent parent in a back-crossing programme which develops the homozygosity due to repeated back-crossing. Back-crossing programme is more opted when the character controlled by recessive genes are wanted in a population.

In dihybrid crossing programme the homozygous recessive is only one out of sixteen, but in a back-crossing programme it is one out of four. So the large number of population is not needed to get the homozygous line.