(a) Enumerate the types of fruiting bodies of Ascomycota and Basidiomycota. Write the various steps of ascus formation in…

(a) Fruiting Bodies of Ascomycota and Basidiomycota; Ascus Formation

Ascomycota produce sexual fruiting bodies known as ascocarps, which are categorized into three main types based on their morphology and spore release mechanism. The Apothecium is a cup-shaped, open structure where asci are exposed on the hymenium, facilitating passive spore dispersal; examples include Peziza and Ascobolus. The Perithecium is a flask-shaped, closed structure with a narrow opening called an ostiole, protecting the asci; Neurospora and Claviceps exhibit this form. The Cleistothecium is a completely closed, spherical sac that ruptures to release spores, as seen in Erysiphe (powdery mildews) and Penicillium (sexual stage). A fourth type, the pseudothecium (ascostroma), is a cavity in a stroma in which asci develop without a true wall (Venturia, Pleospora).

Basidiomycota produce basidiocarps, primarily classified by their shape and spore-bearing surface. The Agaric (mushroom) features a pileus and stipe with gills, e.g., Agaricus bisporus. Polypores (bracket fungi) are woody, shelf-like structures with pores on the underside, e.g., Polyporus. Puffballs (Lycoperdon) are spherical and closed, releasing spores like dust. Clavarioid forms are club-shaped, e.g., Clavaria. Phalloid forms, such as stinkhorns (Phallus), have a distinctive stalk and cap, often with foul odors to attract vectors.

Ascus Formation Ascus development in Ascomycetes is a highly regulated process. After plasmogamy between the ascogonium and antheridium, ascogenous hyphae grow out carrying pairs of compatible nuclei (n + n). The tip of an ascogenous hypha bends back to form a hook or crozier, and its two nuclei divide simultaneously (conjugate division). Two septa then cut off a uninucleate tip cell and a uninucleate stalk (basal) cell, leaving a binucleate penultimate cell at the bend; the tip cell later fuses with the stalk cell, which can form a new crozier. The penultimate cell (ascus mother cell) undergoes karyogamy (fusion of two haploid nuclei) to form a diploid nucleus. This is followed by meiosis I, producing two haploid nuclei, and meiosis II, resulting in four haploid nuclei. A subsequent mitotic division yields eight haploid nuclei. Each nucleus, with some surrounding cytoplasm, is then enclosed by a wall (free-cell formation) to form eight ascospores within the ascus. Asci may be unitunicate or bitunicate, and in many species the ascospores are forcibly discharged through an apical pore or operculum.

(b) Modes of Infection and Dissemination of Plant Viruses

Plant viruses infect hosts through specific entry points and disseminate via various vectors. Mechanical inoculation occurs when plant sap containing virions is transferred to wounds caused by pruning, harvesting, or natural abrasions. Grafting transmits viruses through vascular connections, particularly in perennial crops like citrus and grapes. Seed transmission is rare but significant in legumes (e.g., Bean Common Mosaic Virus) and cereals. Pollen transmission allows viruses to move from male to female flowers during pollination. Dodder (Cuscuta) acts as a parasitic vector, connecting to host plants and transmitting viruses through its haustoria.

Dissemination is heavily reliant on insect vectors. Aphids transmit viruses in non-persistent (short duration, e.g., Cucumber Mosaic Virus), semi-persistent (e.g., Beet Yellows Virus), or persistent (long duration, e.g., Potato Leafroll Virus) styles. Leafhoppers and whiteflies are key vectors for many rice and vegetable viruses. Nematodes (e.g., Xiphinema) transmit viruses like Grapevine Fanleaf Virus through soil. Fungal vectors such as Olpidium and Polymyxa transmit viruses such as Tobacco Necrosis Virus and Beet Necrotic Yellow Vein Virus. Within the plant, a virus multiplies in the first cells, moves from cell to cell through plasmodesmata with the help of virus-encoded movement proteins (for example, the 30 kDa protein of TMV), and then spreads systemically through the phloem with the flow of sugars, producing local lesions or systemic symptoms. Human agency also contributes to dissemination through the movement of infected planting material, seeds, and tools across regions. In India, the Tungro virus of rice is disseminated by the green rice leafhopper (Nephotettix spp.), causing significant yield losses in paddy fields.

(c) Male and Female Strobili of Gnetum; Why It Is a Gymnosperm

Male Strobilus The male strobilus of Gnetum is a compact, strobiloid structure. It consists of a central axis bearing pairs of decussate bracts that fuse to form cup-like collars (cupules) at the nodes. Above each collar lie the male flowers (microsporangiophores), each enclosed in a small sheath (perianth). They are arranged in 3–6 rings per collar, topped by a ring of abortive ovules. Each microsporangiophore bears two (rarely one) unilocular microsporangia at its tip. The microsporangia produce microspores that develop into male gametophytes with prothallial cells, which eventually produce two sperm cells.

Female Strobilus The female strobilus has a similar axis with collars, but each collar bears a single ring of 4–10 ovules above it. Each ovule has a nucellus surrounded by three envelopes: an outer envelope (often interpreted as a perianth), a middle envelope, and an inner integument that projects as a long micropylar tube which secretes a pollination drop. The ovules are naked, not enclosed in an ovary.

Gymnosperm Justification Gnetum is classified as a gymnosperm primarily because its ovules are naked, not enclosed within an ovary. Although it exhibits double fertilization (the second male gamete fuses with another free nucleus of the female gametophyte), it does not form triploid endosperm via triple fusion; instead, it produces a free-nuclear endosperm. The female gametophyte is free-nuclear and lacks archegonia (a specialised gymnosperm feature), and the haploid female gametophyte serving as nutritive tissue and the retention of tracheids with bordered pits alongside vessels in its xylem further support its gymnosperm status, distinguishing it from angiosperms which have triploid endosperm and true flowers and fruits.