Plant embryology

In short

Plant embryology (reproductive morphology of angiosperms) covers everything from the formation of spores inside the anther and ovule, through pollination and double fertilisation, to the development of endosperm, embryo and seed. It explains how ploidy changes across generations, why endosperm is triploid, how hybrid barriers arise and how breeders bypass them. It is a scoring, diagram-heavy block in B.Sc and M.Sc papers and a standing section in CSIR-UGC NET Life Sciences, GATE XL (Botany), ICAR-NET/ARS, IARI and university entrance tests. Applied embryology is the daily working knowledge of India's seed and planting-material industry — hybrid seed production, male sterility, embryo rescue, anther-culture haploids and polyembryonic mango and citrus rootstocks. The Indian school of the subject is largely P. Maheshwari's and B.M. Johri's (Delhi), whose terminology the syllabi still follow.

Explained

Anther, ovule and the ground plan

• Life cycle: diplohaplontic alternation — sporophyte (2n) → meiosis → spores (n) → gametophytes (n) → gametes → zygote (2n). Angiosperms are heterosporous and endosporic: both gametophytes develop inside the spore wall and inside the parent plant.
• Anther: typically tetrasporangiate (four microsporangia, two lobes). Wall layers from outside in — epidermis, endothecium (fibrous, aids dehiscence), 1–3 middle layers, tapetum.
• Tapetum: nutritive and decisive layer. Two types — secretory (glandular), cells stay in place; amoeboid (periplasmodial), protoplasts invade the locule. It supplies callase (releases microspores from the tetrad), sporopollenin and Ubisch bodies for exine, pollenkitt, and the S-gene proteins of sporophytic self-incompatibility. Tapetal failure = male sterility.
• Dehiscence occurs at the stomium by differential drying of the endothecium.
• Ovule (megasporangium): funicle, chalaza, nucellus, one or two integuments, micropyle. Appendages — obturator (guides the pollen tube), hypostase, epistase, aril, caruncle (Ricinus).
• Ovule types by orientation: orthotropous (Piper, Polygonum), anatropous (commonest), hemianatropous, campylotropous (Brassicaceae), amphitropous, circinotropous (Opuntia).
• By integuments: unitegmic (sympetalous families), bitegmic (most polypetalous), ategmic (some parasites). By nucellus: tenuinucellate (thin, sac close to epidermis) and crassinucellate (massive nucellus).

Microsporogenesis and the male gametophyte

1. Hypodermal archesporial cells → primary parietal cells (wall) and sporogenous tissue → microspore mother cells (PMC, 2n).
2. Meiosis in each PMC → a microspore tetrad (n); cytokinesis successive in most monocots, simultaneous in most dicots. Tetrad arrangement may be tetrahedral, isobilateral, decussate, T-shaped or linear; grains may stay united as massulae or a pollinium (Calotropis, orchids).
3. Pollen wall: exine of sporopollenin (highly resistant — basis of palynology and fossil pollen) with intine of pectocellulose; apertures described by the NPC system (number, position, character), e.g. monocolpate (monocots), tricolporate (many dicots).
4. Male gametophyte: microspore nucleus divides unequally → large vegetative (tube) cell + small generative cell. The generative cell divides to give two male gametes. Pollen is shed 2-celled in about 70% of families and 3-celled in Poaceae, Brassicaceae, Asteraceae.
5. Germination (siphonogamy): on a compatible stigma the tube emerges through an aperture, grows through the style by tip growth with a tip-focused Ca²⁺ gradient, and is guided by the synergid's filiform apparatus and LURE-type peptides. Entry into the ovule is porogamous (through the micropyle, usual), chalazogamous (Casuarina, Betula) or mesogamous (Cucurbita).

Megasporogenesis and the female gametophyte

• A hypodermal archesporial cell forms the megaspore mother cell (MMC, 2n), which undergoes meiosis. Behaviour of the four nuclei defines three classes:
• Monosporic (Polygonum type) — three megaspores degenerate; the chalazal one undergoes three mitoses → 8 nuclei, 7 cells: egg + 2 synergids (egg apparatus), 2 polar nuclei in the central cell, 3 antipodals. Found in over 80% of angiosperms; this is the type to draw unless asked otherwise.
• Bisporic (Allium type) — meiosis I completes, meiosis II in only one dyad cell; two nuclei, two mitoses → 8-nucleate sac.
• Tetrasporic — no wall after meiosis; all four nuclei participate. Adoxa type (one mitosis each → 8 nuclei, Polygonum-like organisation), Fritillaria/Lilium type (three chalazal nuclei fuse to 3n, so the sac has a 1n and a 3n polar nucleus), Peperomia type (16-nucleate, 8 polar nuclei), Plumbago type (synergids absent), Penaea and Drusa types.
• Egg apparatus: egg cell polar and highly vacuolate; synergids secrete attractants, absorb nutrients and receive the pollen tube (one becomes the persistent/degenerating synergid). Antipodals are nutritive, may proliferate (up to 300 in some grasses) or be absent.

Pollination, pollen–pistil interaction and double fertilisation

• Modes: autogamy, geitonogamy (same plant, different flower), xenogamy. Cleistogamy assures seed (Commelina benghalensis, Oxalis); chasmogamy allows outcrossing. Agents: anemophily, entomophily, ornithophily, chiropterophily, hydrophily (Vallisneria, Zostera).
• Pollen–pistil interaction is a recognition event: compatible pollen hydrates, germinates and the tube is accepted; incompatible pollen is arrested on the stigma or in the style.
• Self-incompatibility (SI): gametophytic — pollen's own haploid S-allele acts; in the S-RNase system the style's S-RNase degrades tube RNA (Solanaceae, Rosaceae, Plantaginaceae), while Papaveraceae (PrsS–PrpS, Ca²⁺-mediated) and Poaceae use non-RNase mechanisms; sporophytic — pollen coat protein (SP11/SCR) binds stigmatic receptor kinase SRK, acts on the tetrad's diploid parent genotype (Brassicaceae, Asteraceae); heteromorphic — distyly/tristyly (Primula, Linum, Oxalis). Overcome by bud pollination, mentor pollen, high CO₂, NaCl spray, or ovule/ovary culture.
• Double fertilisation (Nawaschin, 1898, in Lilium and Fritillaria; confirmed by Guignard, 1899) — unique to angiosperms. The tube discharges two gametes into a synergid:
• Syngamy: one gamete + egg → zygote (2n).
• Triple fusion: second gamete + two polar nuclei → primary endosperm nucleus (3n).
• Fusion of polar nuclei may precede gamete entry (premitotic) or follow it (postmitotic). Embryo-sac type, polar-nuclei number and ploidy, and apomixis all change endosperm ploidy (chalazogamy and siphonogamy are only routes of tube entry and growth, and do not) — compute it, never memorise "3n".

Endosperm

• Functions: nourishes embryo and seedling, a signalling and imprinting tissue, and the economic product in cereals (rice, wheat, maize endosperm = India's staple calories) and coconut.
• Types: nuclear (free nuclear divisions first, walls later or never — commonest; liquid coconut endosperm with solid wall layer), cellular (wall after every division — Petunia, Adoxa, Areca), helobial (first division gives unequal micropylar and chalazal chambers, further behaviour differing — Helobiae monocots).
• Specialisations: ruminate endosperm (Areca catechu, nutmeg), endosperm haustoria that invade nucellus or even beyond the ovule (Santalum, Acanthaceae), aleurone layer (proteinaceous, source of α-amylase in germinating barley/rice), mosaic endosperm in maize.
• Perisperm is persistent nucellus, not endosperm (black pepper, beet, Coffea).
• Ploidy conflict: interploidy crosses often fail because the 2:1 maternal:paternal endosperm balance is upset — the basis of triploid block and of many interspecific barriers.
• Endosperm culture yields triploid plants (useful for seedlessness) and coconut endosperm liquid ("coconut milk") was the classic cytokinin-rich tissue-culture supplement.

Embryogeny, seed development and nutrition

• Dicot (Crucifer/Onagrad type, Capsella bursa-pastoris): zygote divides transversely → terminal cell ca and basal cell cb. ca → quadrant → octant → dermatogen, periblem, plerome; cb → suspensor and hypophysis (root cap and part of radicle). Stages: proembryo → globular → heart-shaped → torpedo → mature embryo with plumule, two cotyledons, hypocotyl, radicle.
• Johansen's types of embryogeny (by the plane of the first zygotic division and the fate of ca and cb): Crucifer (Onagrad), Asterad, Solanad, Caryophyllad, Chenopodiad — all with a transverse first division; Piperad with a longitudinal first division.
• Monocot (Graminad/Sagittaria type): terminal cell forms most of the embryo; mature grain embryo has a single scutellum (cotyledon), coleoptile, coleorhiza, plumule, radicle and often an epiblast.
• Nutrition of the embryo comes in sequence from the degenerating synergid, endosperm, nucellus/perisperm, and haustoria (suspensor, endosperm, antipodal, synergid haustoria).
• Seed: integuments → testa and tegmen; nucellus → perisperm if retained; funicle scar = hilum; ovule → seed, ovary → fruit. Seeds are endospermic (albuminous) — castor, cereals — or non-endospermic — pea, gram, groundnut, where cotyledons store food.
• Dormancy and storage: orthodox seeds survive desiccation and are banked at low moisture and −20 °C (NBPGR, New Delhi); recalcitrant seeds (jackfruit, mango, cocoa, many RET forest species) cannot be dried and need in-vitro or cryo conservation of embryonic axes.

Apomixis, polyembryony and applied embryology

• Apomixis = asexual seed formation. Diplospory (MMC-derived unreduced sac — Taraxacum), apospory (somatic-cell sac — Hieracium), adventive/nucellar embryony (Citrus, Mangifera, Opuntia), parthenogenesis (egg without fertilisation) and apogamy (synergid/antipodal embryo). Pseudogamy needs pollination for endosperm but not for the embryo (Poa).
• Why breeders want it: apomictic seed is a clone, so hybrid vigour would be fixed permanently and farmers could resow F₁ seed. Indian work includes pearl millet and forage grasses; polyembryonic mango rootstocks (Olour, Vellaikolamban) and Rangpur lime/rough lemon citrus rootstocks are used commercially because nucellar seedlings are true-to-type and uniform.
• Polyembryony: simple vs multiple; cleavage polyembryony (common in conifers), nucellar, integumentary, synergid or antipodal in origin.
• Techniques: embryo rescue (culturing hybrid embryos before they abort — used in Brassica, cotton, Vigna, grapes), ovule and ovary culture, in vitro (test-tube) fertilisation (Nicotiana, Zea), anther/microspore culture for haploids and doubled-haploid lines (rice and wheat DH breeding at ICAR institutes), endosperm culture for triploids, somatic embryogenesis for mass clonal propagation.
• Male sterility (genic, cytoplasmic, CMS, and chemically induced) removes the need for hand emasculation in hybrid seed production of pearl millet, rice, sunflower, cotton and vegetables; its anther-level basis is tapetal and pollen abortion.
• Indian legal and standards context (verify current editions before quoting): variety protection and DUS testing under the Protection of Plant Varieties and Farmers' Rights Act, 2001 (PPV&FR Authority) use floral, pollen and seed descriptors; seed quality and labelling under the Seeds Act, 1966 and Seeds (Control) Order, 1983, with germination and purity limits in the Indian Minimum Seed Certification Standards; isolation distances for hybrid and foundation seed plots are crop- and class-specific (a few hundred metres for cross-pollinated crops such as maize and sunflower — read the current standard, these are revised); access to germplasm under the Biological Diversity Act, 2002 (as amended 2023); transgenic material under the Rules, 1989 of the Environment (Protection) Act, 1986 through GEAC/IBSC.

Key points

• Microsporogenesis - meiosis in a pollen mother cell giving a tetrad of four haploid microspores.
• Tapetum - innermost anther wall layer; supplies sporopollenin, callase and SI proteins; its failure causes male sterility.
• Male gametophyte - 3-celled at most: one vegetative (tube) cell and two male gametes; shed 2-celled or 3-celled.
• Polygonum type - monosporic, 8-nucleate, 7-celled sac; the standard angiosperm embryo sac (>80% of species).
• Egg apparatus - egg + two synergids; the filiform apparatus of the synergid attracts and receives the pollen tube.
• Double fertilisation - syngamy (2n zygote) plus triple fusion (3n primary endosperm nucleus); exclusive to angiosperms.
• Endosperm types - nuclear (commonest), cellular, helobial; perisperm is nucellar, not endosperm.
• Fritillaria type - tetrasporic with a 3n polar nucleus, so its endosperm is pentaploid, not triploid.
• Embryogeny types - Crucifer, Asterad, Solanad, Caryophyllad, Chenopodiad (transverse first division) and Piperad (longitudinal).
• Suspensor and hypophysis - basal-cell derivatives; the hypophysis contributes the root cap/radicle region.
• Monocot embryo - scutellum, coleoptile, coleorhiza, epiblast; dicot embryo has two cotyledons and a hypocotyl.
• Self-incompatibility - gametophytic (pollen's own S-allele, S-RNase) vs sporophytic (parental genotype, SRK/SCR).
• Apomixis - seed without fertilisation; diplospory, apospory, nucellar embryony, parthenogenesis, pseudogamy.
• Nucellar embryony - why polyembryonic mango and citrus seedlings are true-to-type rootstocks.
• Embryo rescue - culture of hybrid embryos that would abort from endosperm failure in wide crosses.
• Anther culture - route to haploids and doubled-haploid lines, shortening varietal breeding by several generations.
• Orthodox vs recalcitrant seeds - desiccation-tolerant and bankable vs desiccation-sensitive (mango, jackfruit).
• Ploidy rule - count the contributing nuclei every time; endosperm ploidy depends on the embryo-sac type.

Formulas and examples

Example 1 — ploidy map of a Polygonum-type sac, 2n = 24 (so n = 12).

• Megaspore mother cell = 24; megaspore = 12; egg, each synergid, each antipodal, each polar nucleus = 12 each.
• Zygote = 12 + 12 = 24 (2n).
• Primary endosperm nucleus = 12 + 12 + 12 = 36 (3n).
• Nucellus and integuments (sporophytic, maternal) = 24; testa = 24; embryo = 24; endosperm = 36.

Example 2 — the same plant with a Fritillaria-type sac.

• Micropylar polar nucleus = 12 (n); chalazal polar nucleus = 3 × 12 = 36 (3n); male gamete = 12.
• PEN = 12 + 36 + 12 = 60 = 5n (pentaploid endosperm). Zygote stays 24.

Example 3 — pollen output and breeding system.

• A flower has 5 anthers, each with 4 microsporangia containing 250 pollen mother cells → 5 × 4 × 250 = 5,000 PMC.
• Pollen grains = 5,000 × 4 = 20,000. Ovules per flower = 40.
• P/O = 20,000 / 40 = 500 → far above the autogamous range (tens) and consistent with facultative xenogamy; obligate outcrossers typically run into the thousands.

Example 4 — pollen viability, seed set and self-incompatibility.

• 500 grains scored; 420 stained → fertility = 420/500 × 100 = 84%.
• Cross-pollination: 126 seeds from 180 ovules → 126/180 × 100 = 70%.
• Self-pollination: 12 seeds from 180 ovules → 12/180 × 100 = 6.67%.
• ISI = 6.67 / 70 = 0.095 (<0.2) → a strongly self-incompatible plant; pollen is good, so the failure is recognition, not sterility.

Example 5 — divisions needed to build the gametophytes.

• Female (Polygonum): 1 meiosis (2 divisions) + 3 free mitoses. Mitotic nuclear divisions = 1 + 2 + 4 = 7, giving 8 nuclei from one functional megaspore.
• Male: 1 meiosis per PMC → 4 microspores; then pollen mitosis I (vegetative + generative cell) and pollen mitosis II (two gametes) = 2 mitoses per grain; a 2-celled grain completes the second division in the stigma or tube.

Where it is used

• Hybrid seed industry (Nuziveedu, Mahyco, Rasi, Kaveri, ICAR institutes): CMS/male-sterile lines, controlled pollination, isolation distances and seed certification under the Seeds Act, 1966 — roles such as seed production officer, seed certification officer at State Seed Certification Agencies.
• Plant breeding and biotechnology labs: embryo rescue in wide crosses, anther-culture doubled haploids, ovule culture, somatic embryogenesis for clonal planting material (banana, sugarcane, oil palm).
• Horticulture and nurseries: polyembryonic mango and citrus rootstocks, seedless triploid watermelon and banana, grafting and true-to-type propagation (ICAR-IIHR Bengaluru, ICAR-CISH Lucknow).
• Seed testing and germplasm conservation: germination, purity and viability testing to ISTA-aligned Indian standards; NBPGR gene bank for orthodox seeds and in-vitro/cryo banks for recalcitrant species.
• Variety protection and regulation: DUS testing under the PPV&FR Act, 2001 uses floral, pollen and seed descriptors; GEAC/IBSC dossiers for transgenics require pollen-flow and outcrossing data.
• Palynology and allied fields: forensic and archaeological pollen analysis, aerobiology and pollen-allergy calendars, honey authentication (melissopalynology), petroleum and Quaternary stratigraphy (BSIP Lucknow).

Interview questions

• Q: Why is endosperm usually triploid? A: Because the primary endosperm nucleus forms by triple fusion — one haploid male gamete plus the two haploid polar nuclei of the central cell.
• Q: Give a case where endosperm is not triploid. A: The Fritillaria/Lilium type has a 3n chalazal polar nucleus, so its endosperm is pentaploid; Peperomia type, with eight polar nuclei, gives 9n endosperm.
• Q: What is double fertilisation and why does it matter? A: Simultaneous syngamy and triple fusion in the same embryo sac, forming a 2n zygote and a 3n endosperm nucleus; it is unique to angiosperms and ensures nutrition is made only when an embryo is actually formed.
• Q: Which is the commonest embryo sac type and how many cells does it have? A: The monosporic Polygonum type — 8 nuclei in 7 cells: egg, two synergids, three antipodals and a binucleate central cell.
• Q: What does the tapetum do? A: Nourishes the developing microspores and provides callase, sporopollenin and Ubisch bodies for the exine, pollenkitt, and the S-proteins of sporophytic incompatibility; its degeneration causes male sterility.
• Q: Difference between gametophytic and sporophytic self-incompatibility? A: Gametophytic SI is determined by the pollen's own haploid S-allele and usually acts in the style via S-RNase; sporophytic SI is determined by the diploid genotype of the pollen parent and acts on the stigma surface through SRK–SCR recognition.
• Q: What is nucellar embryony and why is it commercially valuable? A: Adventive embryos arising from nucellar cells, so they are genetically identical to the mother; it gives uniform, true-to-type mango and citrus rootstocks.
• Q: Why would a breeder use embryo rescue? A: In wide or interspecific crosses the endosperm fails and the hybrid embryo aborts; excising and culturing it on a nutrient medium saves the hybrid.
• Q: How does anther culture shorten a breeding programme? A: Microspores regenerate haploid plants that, after chromosome doubling, are instantly homozygous doubled haploids — removing several generations of selfing.
• Q: What is perisperm and name a seed with it? A: Persistent nucellar tissue in the mature seed, as in black pepper, beet and coffee — it is maternal (2n) and not endosperm.
• Q: Distinguish orthodox and recalcitrant seeds with Indian examples. A: Orthodox seeds tolerate drying and freezing and can be gene-banked (wheat, chickpea); recalcitrant seeds are desiccation-sensitive and short-lived (mango, jackfruit, cocoa) and need in-vitro or cryo conservation.
• Q: If apomixis could be engineered into rice, what would change for the farmer? A: Hybrid vigour would be fixed in the seed, so saved seed from an F₁ crop would stay true, cutting dependence on annual hybrid seed purchase.

Common mistakes

• Writing "endosperm is 3n" for every case, without checking the embryo-sac type or counting the contributing nuclei.
• Calling the embryo sac "8-celled"; it is 8-nucleate but 7-celled, the central cell holding two polar nuclei.
• Confusing megasporogenesis (meiosis forming megaspores) with megagametogenesis (mitoses forming the sac), and the same error on the male side.
• Mixing up monosporic, bisporic and tetrasporic by counting nuclei instead of counting how many meiotic products enter the sac.
• Treating perisperm as endosperm, or the nucellus as gametophytic tissue — both are maternal sporophyte.
• Saying the pollen tube is attracted by the egg; it is received and guided by the synergid and its filiform apparatus.
• In labs: calling unstained pollen "dead" without a germination test — stainability overestimates viability; and reporting seed set without stating the ovule denominator.

Related topics

Plant anatomy · Plant reproduction and pollination biology · Plant tissue culture and micropropagation · Genetics and plant breeding · Seed technology and certification · Palynology