(a) What is micropropagation and how does it differ from traditional plant propagation methods ? Give an account of the…

(a) Micropropagation: Difference from Traditional Methods, Applications and Challenges

Micropropagation is the in vitro clonal propagation of plants from totipotent explants cultured on defined nutrient media under aseptic conditions. Its standard stages (Murashige) are: Stage 0, selection and preparation of the mother plant; Stage I, surface sterilisation and establishment of the explant in culture; Stage II, shoot multiplication (usually on MS medium with a cytokinin); Stage III, rooting (with an auxin); and Stage IV, hardening and transfer to the field.

Difference from traditional methods: traditional propagation relies on seeds, which segregate genetically, or on cuttings, layering and grafting, which are limited by seasons, low multiplication rates and the carry-over of systemic pathogens. Micropropagation works year-round, needs very little starting tissue and little space, and yields genetically uniform, pathogen-free propagules at an exponential rate.

Applications in crop improvement: it enables rapid clonal multiplication of elite genotypes of banana (e.g., Grand Naine), potato, sugarcane, strawberry, orchids and teak, and supplies disease-free seed tubers of Solanum tuberosum. Shoot-apical meristem culture eliminates systemic viruses, because high metabolic activity and rapid cell division outpace viral movement and the meristematic dome lacks vascular tissue. It also multiplies new hybrids, transgenic and male-sterile lines and doubled haploids quickly.

Conservation of endangered plants: it supports ex situ germplasm conservation, slow-growth storage and cryopreservation, and produces planting stock for reintroduction from very little material, e.g., Nepenthes khasiana, Pterocarpus santalinus (red sanders), Picrorhiza kurroa (kutki) and endemic orchids.

Challenges: somaclonal variation induced by prolonged callus phases; high capital and labour costs; physiological disorders such as hyperhydricity (vitrification); microbial contamination; recalcitrance of many woody species; and high mortality during ex vitro hardening and acclimatisation. A narrow clonal base also reduces genetic diversity.

(b) Polyembryony and Parthenocarpy; Classification and Significance of Parthenocarpy

Polyembryony is the formation of more than one embryo in a single seed, by cleavage of the proembryo or by adventive budding from maternal sporophytic tissue (nucellus or integuments), as in Citrus and Mangifera indica. It concerns the seed and embryo, and nucellar embryos are clones of the mother plant. Parthenocarpy, in contrast, is the development of fruit without fertilization, so the fruits are seedless; it is driven by endogenous auxin and gibberellin synthesis in the ovary tissues without syngamy. It concerns the fruit.

Classification of parthenocarpy:

Genetic (natural) parthenocarpy is of three kinds:

1. Vegetative: fruit develops without pollination, e.g., banana, pineapple, navel orange, some cucumber cultivars.
2. Stimulative: pollination or another stimulus is needed, but fertilization does not occur, e.g., Black Corinth grape, some orchids.
3. Stenospermocarpy: fertilization occurs but the embryo aborts, leaving only seed traces, e.g., Thompson Seedless grape. This is not true parthenocarpy, but the result is the same.

Environmental (facultative) parthenocarpy: low temperature, frost or high temperature impairs pollination and fertilization but still starts fruit growth, e.g., pear, olive, and tomato under cool nights.

Induced (chemical) parthenocarpy: auxins (IAA, NAA, 4-CPA, 2,4-D) in tomato and brinjal; gibberellic acid (GA₃) in grape and pear; cytokinins (CPPU) in kiwifruit and melons.

Significance: parthenocarpy gives seedless fruits with better eating quality, consumer preference and suitability for processing (juice, jam, raisins); it ensures reliable fruit set and yield when pollinators are scarce, in protected cultivation, or under unfavourable temperatures; and it allows sterile cultivars such as triploid bananas to be grown. Its drawbacks are that such crops must be propagated vegetatively and the fruits are sometimes smaller or misshapen.

(c) Cybrid Production, Genetic Difference from Hybrids and Applications

Process: protoplasts of both parents are isolated with cellulase and pectinase. To prevent biparental nuclear fusion, the donor protoplast is irradiated with X-rays or gamma rays to fragment its nucleus, or it is enucleated by centrifugation to give cytoplasts. The recipient protoplast is often metabolically inactivated with iodoacetate, so that only fusion products can divide. The protoplasts are then fused with polyethylene glycol (PEG) or by electrofusion. Heteroplasmic products are selected and cultured, and whole plants are regenerated by somatic embryogenesis or organogenesis. Organelle sorting-out during later divisions gives stable cybrids.

Difference in genetic composition: conventional sexual hybrids carry the nuclear genomes of both parents but usually only maternal organelles. Somatic hybrids combine the nuclear and organellar genomes of both parents. Cybrids carry the intact nuclear genome of only one parent (the recipient), together with the cytoplasmic organelles (chloroplast and mitochondrial DNA) of the donor parent, or a recombinant mixture of both parental cytoplasms.

Applications: cybrid technology transfers maternally inherited, organelle-encoded traits directly without disturbing an adapted nuclear genotype. Major examples:

1. Transfer of cytoplasmic male sterility (CMS), governed by mitochondrial genes, into elite lines of Brassica napus (e.g., Ogura CMS from radish, together with corrected chloroplasts) and Nicotiana tabacum for hybrid seed production.
2. Transfer of chloroplast-encoded triazine (atrazine) herbicide resistance from Brassica campestris into B. napus.
3. Creation of new nucleus–cytoplasm combinations in Citrus and potato.
4. Bypassing the sexual incompatibility barriers that prevent cytoplasm transfer by conventional backcrossing.