(a) Describe the five distinct stages in the life cycle of Puccinia graminis with suitable diagrams. 20 (b) With suitable…

**(a) Life cycle of Puccinia graminis tritici (black stem rust of wheat)**

P. graminis is an obligate parasite. It is macrocyclic, producing all five spore stages, and heteroecious, completing its cycle on two unrelated hosts: wheat (primary host) and barberry (Berberis vulgaris, alternate host).

Stage 0: pycnial (spermogonial) stage, on barberry, n.

• Basidiospores infect the upper leaf surface and form flask-shaped pycnia sunk in the tissue, each opening by an ostiole fringed with periphyses.
• Pycnia produce tiny uninucleate pycniospores (spermatia) in nectar, and flexuous receptive hyphae protrude from the ostiole.
• Each pycnium is of one mating type (+ or −). Insects carry spermatia to receptive hyphae of the opposite type, and spermatisation (plasmogamy without karyogamy) restores the dikaryon (n+n).

Stage I: aecial stage, on barberry, n+n.

• Dikaryotic mycelium forms cup-shaped aecia ("cluster cups") on the lower leaf surface, bounded by a peridium.
• The aecia release chains of orange, polyhedral, binucleate aeciospores.
• Aeciospores cannot reinfect barberry. They are wind-borne to wheat.

Stage II: uredinial stage, on wheat, n+n.

• Brick-red pustules (uredinia) on stems, leaf sheaths and leaves bear uredospores.
• Each uredospore is one-celled, oval, stalked, binucleate, finely spiny (echinulate) and has four equatorial germ pores.
• This is the repeating stage: new crops of uredospores every 10–14 days cause epidemics. Uredospores enter through stomata.

Stage III: telial stage, on wheat, n+n then 2n.

• As the crop matures, black telia appear on stems.
• Teliospores are two-celled, spindle-shaped, thick-walled, dark and long-stalked.
• Karyogamy in each cell gives a diploid nucleus, and the spore overwinters as a resting spore.

Stage IV: basidial stage, on stubble or soil, n.

• In spring each teliospore cell germinates into a promycelium (basidium).
• Meiosis takes place, the basidium becomes four-celled, and each cell bears a basidiospore on a sterigma: two of + type and two of − type.
• Wind carries basidiospores to barberry.

Indian context and management

• Survival between seasons: barberry plays little role in the Indian plains, where summers are too hot for the fungus to survive. K. C. Mehta showed that uredospores survive the summer on off-season wheat and grasses in the Nilgiri and Palni hills and the Himalaya, and are wind-borne to the plains each season (the "Puccinia pathway").
• Virulent races: new races such as Ug99 (TTKSK), first found in Uganda in 1999, overcome widely used resistance genes such as Sr31.
• Control: resistant varieties carrying Sr genes, gene deployment by zone, rust surveillance, barberry eradication (practised in the USA and Europe), and fungicides (propiconazole, tebuconazole).

(b) Industrial uses of microorganisms; advantages of immobilised enzymes and cells

(i) Industrial uses

• Alcohol and beverages: Saccharomyces cerevisiae is used for beer, wine and fuel ethanol from molasses. Zymomonas mobilis also produces ethanol.
• Organic acids: citric acid from Aspergillus niger; lactic acid from Lactobacillus delbrueckii; acetic acid (vinegar) from Acetobacter aceti; gluconic acid from A. niger.
• Antibiotics: penicillin from Penicillium chrysogenum, streptomycin from Streptomyces griseus, tetracyclines from Streptomyces aureofaciens, and cephalosporins from Acremonium chrysogenum.

Enzymes:

• α-amylase from Bacillus amyloliquefaciens and B. licheniformis, used in starch liquefaction.
• Proteases from Bacillus for detergents and leather.
• Glucose isomerase from Streptomyces for high-fructose syrup.
• Pectinases from Aspergillus for fruit-juice clarification.
• Recombinant chymosin for cheese.
• Amino acids and vitamins: glutamic acid (MSG) from Corynebacterium glutamicum, lysine, vitamin B₁₂ from Propionibacterium and Pseudomonas denitrificans, and riboflavin from Ashbya gossypii.
• Recombinant pharmaceuticals: human insulin made in E. coli or yeast, hepatitis B vaccine made in yeast, and interferons.
• Steroid transformation: Rhizopus hydroxylates progesterone, a step in producing cortisone.
• Food: yoghurt (Lactobacillus bulgaricus, Streptococcus thermophilus), cheese (Penicillium roqueforti, P. camemberti), bread (yeast), and single-cell protein (Spirulina, Methylophilus methylotrophus, Fusarium venenatum mycoprotein).
• Agriculture and environment: biofertilisers (Rhizobium, Azotobacter, Azospirillum, blue-green algae, mycorrhizae), biopesticides (Bacillus thuringiensis, Trichoderma), biogas from methanogens, bioremediation (oil-degrading Pseudomonas), and bioleaching of copper and uranium by Acidithiobacillus ferrooxidans.

(ii) Advantages of immobilised enzymes and cells

Immobilisation confines enzymes or whole cells to a support, keeping them active. Methods include adsorption on carriers, covalent binding, cross-linking with glutaraldehyde, entrapment in calcium alginate or polyacrylamide, and microencapsulation.

• Reuse: the catalyst is used again and again, lowering cost.
• Easy separation: the product is free of enzyme or cells, which simplifies downstream processing.
• Continuous processing: packed-bed or fluidised-bed reactors can run continuously without cells washing out.
• Stability: better tolerance of temperature, pH and solvents, and a longer working life.
• Higher efficiency: high catalyst density gives better productivity, and whole cells can carry out multi-step reactions and regenerate cofactors.

Industrial examples: immobilised glucose isomerase for high-fructose corn syrup; penicillin acylase for making 6-APA, the base of semi-synthetic penicillins; aminoacylase for resolving L-amino acids (Tanabe, 1969, the first industrial use); and immobilised yeast for continuous ethanol and beer production.

**(c) Ginkgo biloba as a "living fossil": the evidence of its reproductive structures**

A "living fossil" is a living species that closely resembles fossils millions of years old and is the lone survivor of an otherwise extinct group. Ginkgo biloba is the only surviving species of the Ginkgoales, a group well represented from the Permian–Triassic onwards. Leaves and reproductive organs almost identical to the living tree are known from the Jurassic (Ginkgo yimaensis, about 170 million years ago) and the Cretaceous–Paleogene (G. adiantoides). The tree survived the Cretaceous–Paleogene mass extinction and the Ice Ages, persisting as relict populations in China, and was saved largely by cultivation around temples.

Reproductive features that are ancient

• Dioecy and dwarf shoots: trees are male or female, and reproductive organs arise in the axils of leaves on short (spur) shoots.
• Male strobilus: a loose, catkin-like axis bearing many stalked microsporophylls. Each carries two pendant microsporangia, and wind-dispersed pollen develops into a male gametophyte.
• Female structure: there is no cone. A long stalk (peduncle) forks and bears two erect, naked (orthotropous) ovules, each seated on a collar. Usually only one matures.

Ovule:

• A single integument that becomes three-layered in the seed: an outer fleshy, foul-smelling sarcotesta, a stony sclerotesta and an inner papery endotesta. The "fruit" is therefore a seed, not an ovary.
• A nucellus with a pollen chamber.
• A large, green, cellular female gametophyte with two or three archegonia under an archegonial chamber.

Pollination and fertilisation:

• Pollen is caught in a pollination drop and drawn into the pollen chamber.
• The pollen tube is haustorial: it absorbs food from the nucellus instead of carrying gametes.
• Months later it releases two large, top-shaped sperms with a spiral band of many flagella. They swim through fluid in the archegonial chamber to the egg.
• This zooidogamy was discovered by Sakugoro Hirase in 1896. Among living seed plants it is shared only with cycads, recalling the swimming sperm of pteridophytes and early seed plants.
• Other primitive traits: fertilisation may take place late, around the time the ovules fall, and the embryo develops without dormancy. The fan-shaped leaves have open dichotomous venation.

Swimming sperm, the haustorial pollen tube, naked ovules on a simple stalk and a large, green, cellular female gametophyte are all features of Palaeozoic–Mesozoic seed plants. Together with a fossil record almost unchanged since the Jurassic, they justify calling Ginkgo a living fossil and a link between pteridophyte-like and advanced seed plants.