Unit 7: Anther culture (Androgenesis) - Subjective Questions

Androgenesis is the process of in vitro development of haploid plants from the male gametophyte (microspores or immature pollen grains) present within the anther.

Significance:

• Produces haploid and eventually homozygous diploid (doubled haploid) plants.
• Rapidly develops pure homozygous lines, saving several generations of conventional inbreeding.
• Useful in plant breeding for developing new varieties.
• Enables mutation studies since recessive mutations are directly expressed in haploids.
• Facilitates genetic and genomic mapping.
• Helps in the study of gametoclonal variation.

Conclusion: Microspore culture is preferred when pure haploids are required, whereas anther culture is easier and widely used routinely.

Procedure for anther culture:

1. Selection of flower buds: Choose healthy plants and select flower buds at the correct stage (microspores at uninucleate to early binucleate stage).

2. Surface sterilization: Sterilize the flower buds using 70% ethanol for 30 sec followed by 0.1% mercuric chloride or sodium hypochlorite (1-2%) for a few minutes, then rinse with sterile distilled water 3-4 times.

3. Dissection of anthers: Under aseptic conditions in a laminar air flow chamber, carefully remove the anthers using sterile forceps and needle. Avoid damaging the anther and remove the filament.

4. Checking microspore stage: A test anther is squashed and stained (e.g., with acetocarmine) to confirm the correct developmental stage.

5. Inoculation: Place the intact anthers horizontally on the surface of the culture medium (e.g., MS or Nitsch medium) supplemented with suitable growth regulators.

6. Incubation: Incubate cultures at 25 ± 2°C, often with an initial cold pretreatment (4°C) to enhance response.

7. Regeneration: Microspores develop into embryos or callus, which regenerate into haploid plantlets.

8. Chromosome doubling: Treat haploids with colchicine to obtain doubled haploids (homozygous diploids).

The developmental stage of the microspore is the most critical factor determining the success of androgenesis.

Key points:

• The most responsive stage is generally the uninucleate to early binucleate stage of microspore development.
• At this stage, the microspore can be diverted from its normal gametophytic (pollen) pathway to a sporophytic (embryogenic) pathway.
• Too early (tetrad stage): microspores are immature and rarely respond.
• Too late (mature pollen): cells are committed to gametophytic development and fail to induce embryos.

Determining the stage:

• Correlated with bud size, petal-to-sepal ratio, or anther colour.
• Confirmed by acetocarmine squash and microscopic examination.

Selecting anthers at the correct stage maximizes the frequency of embryoid/callus induction.

Colchicine is an alkaloid extracted from Colchicum autumnale that acts as a spindle inhibitor (anti-mitotic agent).

Mechanism:

• It binds to tubulin and prevents spindle fibre formation during mitosis.
• As a result, duplicated chromosomes fail to separate, causing chromosome doubling in the cell.

Necessity in haploid production:

• Haploid plants (n) are sterile because they cannot undergo normal meiosis and produce viable gametes.
• Colchicine treatment doubles the chromosome number, converting haploid (n) into homozygous diploid (2n), i.e., doubled haploid.
• The doubled haploids are completely homozygous and fertile, valuable for breeding programmes.

Application: Applied to haploid seedlings, callus, or plantlets typically at concentrations of 0.1–0.5% for a specific duration.

In anther culture, microspores can develop into plantlets through two pathways:

1. Direct Androgenesis (Embryogenesis):

• Microspores directly develop into embryoids without an intervening callus phase.
• Embryoids pass through globular, heart, torpedo, and cotyledonary stages.
• These germinate directly into haploid plantlets.
• Preferred because there is less genetic variation and no somaclonal aberration.

2. Indirect Androgenesis (Callus mediated / Organogenesis):

• Microspores first form a callus.
• The callus is transferred to a regeneration medium with suitable growth regulators.
• Plantlets are formed via organogenesis or somatic embryogenesis.
• More prone to somaclonal variation and chromosomal abnormalities.

Note: The pathway depends on the genotype, medium composition, and hormonal balance.

The success of anther culture depends on several factors:

1. Genotype: Some species and cultivars respond more readily; genotype strongly influences the frequency of androgenesis.

2. Stage of microspore: The uninucleate to early binucleate stage is optimum.

3. Physiological state of donor plant: Healthy, young donor plants grown under optimal conditions give better responses.

4. Pretreatment:

• Cold pretreatment (4°C) of buds enhances embryogenesis in many species.
• Heat shock treatment is effective in some species (e.g., Brassica).

5. Culture medium: Composition (e.g., MS, Nitsch, N6), sugars, and growth regulators (auxins and cytokinins) affect the response.

6. Growth regulators: The ratio of auxin to cytokinin determines callus vs embryo formation.

7. Incubation conditions: Temperature, light/dark regime, and humidity.

8. Anther wall factors: Substances released from the anther wall can promote or inhibit microspore development.

Pretreatments are stress treatments given to flower buds/anthers before or during culture to switch microspores from the gametophytic to the sporophytic pathway.

1. Cold Pretreatment:

• Flower buds/anthers are held at 4°C for several days (1–14 days).
• Benefits:
  • Increases the number of embryogenic microspores.
  • Delays anther wall degeneration, protecting microspores from toxic substances.
  • Enhances survival and division of microspores.
• Effective in crops like rice, wheat, tobacco.

2. Heat Shock Treatment:

• Anthers/microspores are exposed to higher temperatures (30–35°C) for a short period.
• Effective in Brassica napus microspore culture.
• Induces reprogramming of microspores toward embryogenesis.

Mechanism: These stresses induce synchronous cell divisions and reprogram gene expression, diverting the microspore from pollen development to embryo formation.

Doubled haploids (DH) are completely homozygous diploid plants produced by doubling the chromosome number of a haploid, usually using colchicine.

Applications in plant breeding:

• Rapid production of homozygous lines: Achieves complete homozygosity in a single generation instead of 6–8 generations of selfing.
• Development of pure lines and varieties.
• Hybrid seed production: DH lines serve as parents in hybrid breeding.
• Genetic studies: Useful for mapping quantitative trait loci (QTL) and constructing genetic maps.
• Mutation breeding: Recessive mutations are readily expressed and selected.
• Reduced breeding time and cost.
• Gene fixation: Desirable gene combinations are instantly fixed.

Example: DH technology is widely used in barley, wheat, rice, rapeseed, and maize breeding programmes.

Surface sterilization removes microbial contaminants from the surface of explants while keeping internal tissue viable.

Procedure:

1. Collect healthy, unopened flower buds.
2. Wash under running tap water to remove dust.
3. Dip in 70% ethanol for 20–30 seconds.
4. Treat with a surface sterilant such as 0.1% mercuric chloride (HgCl₂) for 2–5 minutes or 1–2% sodium hypochlorite with a drop of Tween-20 (wetting agent).
5. Rinse 3–4 times with sterile distilled water to remove residual sterilant.
6. Perform all steps under a laminar air flow chamber using sterile instruments.

Importance:

• Prevents fungal and bacterial contamination that would otherwise overtake the culture.
• Since the anthers inside the bud are naturally protected, only the outer surface needs sterilization, keeping microspores intact and viable.
• Ensures healthy, contamination-free cultures essential for successful androgenesis.

The culture medium provides all nutrients required for microspore development into plantlets.

Common media: MS (Murashige & Skoog), Nitsch & Nitsch, and N6 medium are widely used.

Components and their roles:

• Macronutrients: Nitrogen (NO₃⁻, NH₄⁺), P, K, Ca, Mg, S — for growth and metabolism.
• Micronutrients: Fe, Mn, Zn, B, Cu, Mo — cofactors for enzymes.
• Carbon source: Sucrose (2–4%) provides energy; sometimes maltose gives better results in cereals.
• Vitamins: Thiamine, pyridoxine, nicotinic acid, myo-inositol.
• Growth regulators:
  • Auxins (2,4-D, NAA, IAA) for callus induction.
  • Cytokinins (BAP, kinetin) for shoot regeneration.
• Amino acids: Glutamine, serine may improve embryogenesis.
• Gelling agent: Agar (for solid medium); liquid medium is used in microspore culture.

Role: The balance of nutrients and hormones determines whether microspores form embryos or callus and subsequently regenerate into plantlets.

Summary: Haploids are the direct product of androgenesis, whereas doubled haploids are the fertile, homozygous product obtained after chromosome doubling.

Identifying the correct microspore stage is essential; two approaches are used:

1. Indirect method (Bud morphology):

• A correlation is established between the bud size / petal-to-sepal length ratio / flower colour and the microspore stage.
• Once established, buds of a particular size can be selected directly without dissection.
• This is quick and non-destructive for routine work.

2. Direct method (Cytological / Staining):

• A test anther is squashed on a slide.
• Stained with acetocarmine or DAPI (fluorescent stain for nuclei).
• Observed under a microscope to determine the nuclear stage:
  • Uninucleate stage: single central/peripheral nucleus (most responsive).
  • Binucleate stage: vegetative + generative nucleus.
• The uninucleate to early binucleate stage confirms readiness for culture.

Combining both methods ensures selection of the most androgenically competent microspores.

Advantages:

• Rapid production of haploids and homozygous doubled haploids.
• Saves considerable time compared to conventional inbreeding.
• Useful for genetic and mutation studies.
• Facilitates the recovery of recessive traits.
• Relatively simple and less labour intensive compared to microspore isolation.
• Aids in developing pure lines for hybrid breeding.

Limitations:

• Genotype dependent — many important crops are recalcitrant.
• Plants may arise from the anther wall (somatic diploid tissue) rather than microspores, giving mixed ploidy.
• Albino plant regeneration is common, especially in cereals.
• Occurrence of somaclonal variation in callus-mediated regeneration.
• Variable and sometimes low frequency of response.
• Requires skilled handling and aseptic conditions.

Albinism refers to the regeneration of chlorophyll-deficient (white) plantlets that lack functional chloroplasts and cannot photosynthesize, thus dying eventually.

Occurrence:

• A major problem particularly in cereals such as rice, wheat, and barley.

Causes:

• Deletions or rearrangements in the plastid (chloroplast) genome.
• Impaired plastid differentiation during microspore development.
• Influenced by genotype, microspore stage, and culture conditions.

Management strategies:

• Selecting the optimal microspore stage for culture.
• Appropriate cold or heat pretreatment.
• Optimizing medium composition (e.g., using maltose instead of sucrose).
• Reducing the time microspores spend in the callus phase (favouring direct embryogenesis).
• Selecting responsive genotypes with low albino frequency.
• Controlling light and temperature during regeneration.

Growth regulators (plant hormones) control the direction and extent of microspore development in anther culture.

1. Auxins (e.g., 2,4-D, NAA, IAA):

• Promote cell division and callus induction.
• High auxin levels favour callus formation (indirect androgenesis).
• 2,4-D is a strong auxin commonly used for callus initiation.

2. Cytokinins (e.g., BAP, Kinetin):

• Promote cell division and shoot/organ regeneration.
• Stimulate bud and shoot formation from callus or embryoids.

Auxin : Cytokinin ratio — key determinant:

• High auxin : low cytokinin → root formation / callus.
• Low auxin : high cytokinin → shoot formation.
• Balanced ratio → embryogenesis or organogenesis.

Note: Some species undergo androgenesis on hormone-free medium (e.g., tobacco), while others require specific combinations. Proper optimization is essential for successful plantlet regeneration.

The physiological state of the donor plant greatly influences the androgenic response.

Important aspects:

• Age of the plant: Anthers from young, vigorously growing plants respond better than those from old plants.
• Growth conditions: Donor plants grown under controlled conditions (optimal light, temperature, nutrition, humidity) give higher response.
• Season / flowering time: Anthers collected at the beginning of the flowering season are often more responsive.
• Nutritional and health status: Nutrient deficiencies, disease, or stress reduce androgenic potential.
• Position of flower on the plant: Flowers from certain positions may respond better.

Reason: These factors affect the hormonal balance and metabolic status of microspores, determining whether they can be reprogrammed to the embryogenic pathway.

Conclusion: Maintaining donor plants under optimal conditions is a prerequisite for reproducible and high-frequency androgenesis.

Conclusion: Direct embryogenesis is generally preferred because it produces genetically uniform haploids with fewer abnormalities, while indirect organogenesis carries a greater risk of somaclonal variation but may be the only option in some species.

Chromosome doubling converts sterile haploids into fertile doubled haploids. Methods include:

1. Colchicine treatment (chemical):

• Most common method; inhibits spindle formation causing chromosome doubling.
• Applied to seedlings, shoot tips, or callus (0.1–0.5%).

2. Other antimitotic chemicals:

• Oryzalin, trifluralin, amiprophos-methyl (APM) — herbicidal spindle inhibitors, often less toxic than colchicine.

3. Spontaneous / Endomitotic doubling:

• Occurs naturally during callus culture due to endoreduplication or nuclear fusion.
• No chemical treatment needed but frequency is unpredictable.

4. Regeneration from callus:

• Prolonged callus culture may lead to spontaneous diploidization.

5. Nitrous oxide (N₂O) treatment:

• Applied under pressure to dividing cells to induce doubling.

Note: After doubling, ploidy is confirmed by chromosome counting, flow cytometry, or stomatal/pollen size measurement.

Since anther culture may yield plants of mixed ploidy (haploid, diploid, or higher), confirmation of ploidy is essential.

Methods of ploidy determination:

• Chromosome counting (cytology): Root tip or shoot tip cells are stained (e.g., acetocarmine) and chromosomes counted under a microscope — the direct and accurate method.
• Flow cytometry: Measures the DNA content of nuclei quickly and reliably for large numbers of plants.
• Stomatal guard cell size and chloroplast number: Haploids generally have smaller stomata with fewer chloroplasts than diploids.
• Pollen size and morphology.
• Plant morphology: Haploids are often smaller and sterile.

Importance of confirmation:

• To distinguish true haploids (from microspores) from diploids (from anther wall tissue).
• To select the correct plants for colchicine doubling.
• To ensure the resulting doubled haploids are homozygous diploids suitable for breeding.
• To detect any aneuploids or polyploids arising during culture.