Anther & Microspore Culture for Androgenic Haploid Production & Colchicine Diploidization
Androgenesis (Haploid Production): First discovered in *Datura innoxia* by Guha and Maheshwari in 1964, anther and isolated microspore culture enables immature male gametophytic cells (haploid microspores, $1n$) to switch developmental pathways—abandoning their normal gametophytic fate (mature pollen tube formation) and reprogramming into a sporophytic pathway to generate haploid embryos or callus.
Critical Development Window (The Uninucleate Stage):
The physiological state of the microspore is the single most critical factor determining androgenic responsiveness. Induction succeeds almost exclusively when anthers are excised at the mid- to late-uninucleate microspore stage (immediately preceding the first pollen mitosis). At this stage, microspores possess a single nucleus pushed toward the cell periphery by a developing central vacuole, remaining totipotent before asymmetric gametophytic transcription programs commit the vegetative and generative nuclei.
Stress-Induced Reprogramming:
Freshly excised floral buds must be subjected to an inductive physical stress shock—most commonly cold thermal shock (4°C for 48 to 72 hours). Cold pretreatment arrests gametophytic development, degrades gametophytic mRNAs, synchronizes microspore cell division, and synthesizes heat-shock/stress-response proteins that switch epigenetic chromatin marks toward embryogenesis.
Colchicine Diploidization & Doubled Haploid (DH) Breeding:
Haploid plants ($n$) are completely sterile because their single set of chromosomes cannot pair during prophase I of meiosis, resulting in irregular chromosome segregation and unviable pollen. To restore fertility, haploid meristems or seedlings are treated with colchicine (0.1 to 0.2% w/v), an alkaloid extracted from *Colchicum autumnale*. Colchicine binds to β-tubulin monomers, preventing microtubule polymerization and spindle assembly during mitosis. Chromatids replicate during S-phase but fail to segregate at anaphase, causing endoreduplication. Upon removal of the drug, cells divide with a doubled chromosome complement ($2n$). Because both chromosome sets are duplicated from a single gamete, the resulting Doubled Haploid (DH) is 100% homozygous at every genetic locus in a single generation, bypassing 6 to 8 generations of selfing in conventional plant breeding.
- Plant Material: Flower buds of *Nicotiana tabacum* cv. Petit Havana or *Datura innoxia* harvested when petals and sepals are of equal length.
- Cytological Stains: 1.0% Acetocarmine in 45% acetic acid; 2% Aceto-orcein.
- Culture Media:
– Induction Medium: Nitsch & Nitsch (N&N) or N6 basal medium + 20-30 g/L sucrose + 0.1 mg/L NAA + 1.0 mg/L BAP, pH 5.8, 0.8% agar.
– Regeneration/Plantlet Medium: Hormone-free 1/2 MS medium + 15 g/L sucrose + 0.8% agar. - Chromosome Doubling Solution: 0.2% (w/v) Colchicine solution in sterile water + 1% DMSO (permeation enhancer), filter-sterilized.
- Sterilizing Agents: 70% ethanol, 0.1% (w/v) Mercuric chloride ($HgCl_2$), sterile distilled water.
Evaluation Metrics:
1. Androgenic Anther Response (%):
$$text{Anther Response (%)} = frac{text{Number of anthers producing embryos}}{text{Total number of anthers plated}} times 100$$
2. Embryo Productivity:
$$text{Productivity} = frac{text{Total number of emerged embryos}}{text{Total number of responding anthers}}$$
- Injury to Anther Wall: When dissecting anthers, do not pinch or cut into the pollen sacs. Wounding the anther wall triggers diploid callus proliferation from somatic wall tissues (tapetum/endothecium), resulting in unwanted heterozygous diploid plants rather than gametophytic haploids.
- Colchicine Toxicity: Colchicine is a high-hazard spindle poison and mutagen. Never pipette by mouth. Collect all rinses in labeled toxic waste bottles. Colchicine exposure beyond 24 hours induces lethal chromosome polyploidy ($4n, 8n$) and shoot tip necrosis.
Ans: At this stage, microspores have just completed meiosis and have not yet undergone the asymmetric first pollen mitosis that irrevocably commits daughter cells to gametophytic differentiation (generative vs vegetative cells). The genome remains epigenetically plastic and totipotent, allowing stress to trigger an embryogenic developmental program.
Ans: Colchicine binds specifically to tubulin heterodimers, inhibiting the polymerisation of microtubules required for mitotic spindle apparatus assembly. Mitosis is arrested at metaphase; sister chromatids separate during anaphase without moving to poles (c-mitosis), resulting in a single restitution nucleus containing twice the chromosome number ($2n$).
Ans: Because the two chromosome sets in a doubled haploid arise by exact mitotic duplication of a single haploid gamete ($n$), every gene locus receives an identical replica of its single allele, creating 100% homozygosity ($AA$ or $aa$, never $Aa$) across all loci.
Ans: Cytogenetic chromosome counting confirms ploidy ($n = 12$ haploids arise solely from microspores; $2n$ from wall tissue). Furthermore, molecular DNA markers (SSR/RAPD) or isozyme analysis from heterozygous parents ($Aa$) show that microspore-derived doubled haploids segregate into uniform homozygous lines ($AA$ or $aa$), whereas wall-derived plants retain the heterozygous parental band pattern ($Aa$).
