Advantages of Pollen Culture over Anther Culture (With Diagram)

The below mentioned article provides the advantages of pollen culture over anther culture.

Anther cul­ture is an efficient way for the production of hap­loids from the microspores present within the in­tact anther.

In this process, there is always the possibility that somatic cells of the anther that are diploid, will also respond to the culture con­dition and so produce unwanted diploid calluses or plantlets.

Sometimes, the development of mi­crospore inside the anther may be interrupted due to growth inhibiting substances leaking out of the anther wall in contact with nutrient medi­um. In attempts to avoid these problems, the culture of free pollen has been investigated.

The culture of pollen offers the following additional advantages:

(i) Overcrowding of pollen grain in anther is eliminated and isolated pollen grains are equally exposed to nutrient medium.

(ii) Unwanted growth of the anther wall and other associated tissue are eliminated.

(iii) The steps of androgenizes can be observed starting from single cell.

(iv) Various factor governing androgenizes can be better regulated.

(v) Pollen is ideal for uptake, transformation and mutagenic studies as pollens can be uniformly exposed to chemicals and phys­ical mutagens.

(vi) Pollen may be directly transformed into an embryoid. So it is very suitable for un­derstanding biochemistry and physiology of androgenizes.

(vii) Higher yields of haploid plants per anther could be expected in pollen culture than the anther culture.

Homozygous Plants:

Haploid plants derived from either anther culture or pollen cultures are sterile. These plants contain only one set of chromosomes. By dou­bling their chromosome number, the plants can be made fertile and the resultant plants will be homozygous diploid or isogenic diploid (Fig 11.4). These homozygous diploid plants show the normal meiotic segregation. The fer­tile homozygous diploid plants are more impor­tant than the sterile haploid plants. Homozy­gous diploid plants can be used as pure lines in breeding programme.

Haploids can be diplodized by a number of methods:

Colchicine Treatment:

Colchicine has been utilized widely as a spindle inhibitor to induce chromosome dupli­cation and to produce polyploid plants. This method has been employed for obtaining homo­zygous diploid plants from haploid culture. The young, plantlets while still enclosed within an­ther, are treated with 0.5% colchicine solution for 24-48 hrs. Treated plantlets are replanted in the medium after thorough washing. In case of mature haploid plantlets, 4% colchicine-lanoline paste may be applied to the axil of the leaves.

Endomitosis:

It is known that haploid cells are unstable in culture and have a tendency to undergo endomitosis, i.e., chromosome duplication without nu­clear division. This property can be used for ob­taining homozygous diploid plants.

In this pro­cess, a small explant of stem from a haploid plant is cultured on auxin-cytokinin added medium where the segment forms the callus tissue. Dur­ing callus growth, diploid homozygous cells are produced by endomitosis. Now large number of isogenic diploid plants can be obtained by organogenesis.

Fusion of Pollen Nuclei:

Homozygous diploid callus or embryoids may form by the spontaneous fusion of two sim­ilar nuclei of the cultured pollen after first divi­sion. In Brassica, the frequency of spontaneous nuclear fusion in microspore is high in culture.

Significance of Haploid Plants:

In a diploid cell the chromosomes exist in homologous pairs. The genes for specific charac­ters are also formed in pairs which are known as allelic gene pairs. For an example, T gene (for tallness) is an allele of t gene (for dwarfness) and vice versa in heterozygous condition. Each allele is located on one of the pair of homologous chro­mosome at a particular gene locus.

Although each allele controls the same genetic trait (height of the plant), yet, they may control a contrasting phenotypic expression (tall/dwarf) of that trait. In heterozygous condition, the activity of only one of the alleles is expressed phenotypically, the allele is said to be dominant (suppose T gene). On the other hand, the activity of the other al­lele which is not expressed phenotypically until, it is separated from dominant allele, is said to be recessive (suppose t gene).

A chromosome contains a number of domi­nant and recessive genes whose allelic forms are present on the homologous partner chromosome at the same gene loci. In heterozygous diploid, only dominant alleles are expressed phenotypically. Homologous chromosome separates dur­ing meiosis. Pollen grains receive only one set of homologous chromosomes.

As a result, in pollen-derived haploid plants, all the recessive genes, along with dominant genes, will be ex­pressed phenotypically as there is no masking of recessive gene by dominant genes. Since the hap­loid plants are sterile, diploid fertile plant can be made by doubling the same existing chromo­some.

As a result, the dominant as well as re­cessive genes will be doubled at their respective loci. So, even in diploid condition, all the reces­sive genes will be expressed phenotypically. Such diploid plant is also called homozygous plant or isogenic diploid plant. Therefore, comparing the heterozygous diploid plant with homozygous diploid plant, one can easily identify the reces­sive characters which are not possible to identify in heterozygous condition.

Crossing over is an essential feature in the meiotic cycle by which random exchange of ge­netic material (genetic recombination) between two homologous chromatids takes place. The exchange is of great importance because it pro­duces a new gene combination. As a result, four haploid nuclei, produced from a single diploid nucleus, differ from one another. Therefore, hap­loid plants, derived from four haploid pollens of a pollen tetrad, are significant because the plants will differ genotypically.

Why the Haploid Plants are Sterile?

In haploid plants, each chromosome is rep­resented only once and this is the reason there is no zygotene pairing in first meiotic division. Thus, all the chromosomes appear as univalent. During anaphase I, each chromosome moves free­ly and form generally more than two groups. Ga­metes with less than the haploid number are gen­erally not viable; therefore, haploid plants are highly sterile.