(a) Explain the mode of action of organophosphate and Bacillus thuringiensis which are used in pest management. (20 marks) (b)…

**(a) Mode of Action: Organophosphates and *Bacillus thuringiensis***

Organophosphates (OPs) function as broad-spectrum neurotoxins. Their primary mechanism involves the inhibition of the enzyme acetylcholinesterase (AChE) in the synaptic cleft of the nervous system: the OP phosphorylates the serine hydroxyl group at the enzyme's active site, and the phosphorylated enzyme recovers only very slowly (and not at all after 'ageing'). Many OPs are thions (P=S), such as malathion and chlorpyrifos, which are activated in the insect body to their oxon (P=O) forms, the actual inhibitors. Under normal conditions, AChE hydrolyzes the neurotransmitter acetylcholine (ACh) after it has triggered a nerve impulse. When OPs bind to the active site of AChE, ACh accumulates in the synapse, leading to continuous stimulation of post-synaptic receptors. This results in uncontrolled muscle contractions, paralysis, and death in the target organism. Because AChE is conserved across many animals, OPs are non-specific, posing significant risks to mammals, birds, and beneficial insects, which complicates their use in Integrated Pest Management (IPM).

In contrast, Bacillus thuringiensis (Bt) produces crystal (Cry) proteins that are toxic only to specific insect orders, primarily Lepidoptera, Diptera, and Coleoptera. The mode of action is highly specific: the crystal (protoxin) is ingested by the susceptible larva, dissolves in the alkaline midgut (pH above about 9.5 in caterpillars) and is cleaved by trypsin-like proteases to the active toxin. The toxin binds specific receptors on the brush border of midgut epithelial cells (cadherin, aminopeptidase-N, alkaline phosphatase), oligomerises and inserts into the membrane, forming pores. Feeding stops within hours. This disrupts the osmotic balance, causing cell lysis, gut paralysis and perforation, and death from starvation and septicaemia as spores and gut bacteria invade the body cavity. Because mammals have an acidic stomach and lack these receptors, Bt is safe for them. While Bt is generally considered safe for non-target organisms, it is not universally harmless; certain beneficial Lepidoptera, such as silkworms and some wild butterflies, possess receptors that make them susceptible to specific Cry toxins. This specificity allows Bt to be integrated into IPM strategies with minimal disruption to ecological balance compared to OPs.

(b) Phytochrome: Structure, Forms, and Function

Phytochrome is a photoreversible chromoprotein that acts as a light sensor in plants, regulating growth and development. It consists of a protein moiety and a chromophore, phytochromobilin, which absorbs light. Phytochrome exists in two interconvertible forms: Pr (red-absorbing) and Pfr (far-red-absorbing).

Pr is the biologically inactive form, absorbing light maximally at 660 nm (red light). Upon absorption of red light, Pr undergoes photoconversion to Pfr. Pfr is the biologically active form, absorbing light maximally at 730 nm (far-red light). Pfr is less stable than Pr and can revert to Pr through two mechanisms: photoconversion by far-red light and a slow, temperature-dependent thermal process known as dark reversion.

The physiological roles of these forms are distinct. The Pr-to-Pfr conversion is critical for seed germination in light-sensitive seeds, as Pfr promotes the breakdown of dormancy. In mature plants, the ratio of Pfr to Pr determines shade avoidance responses; a high Pfr/Pr ratio indicates full sunlight, while a low ratio (due to far-red enrichment from reflected light) triggers elongation to escape shade. Furthermore, phytochrome mediates photoperiodism, influencing flowering time by sensing day length. On conversion to Pfr, phytochrome moves from the cytoplasm into the nucleus, where it inactivates PIF (phytochrome-interacting factor) transcription factors, switching on light-regulated genes. Of the five phytochromes in Arabidopsis, phyA (light-labile) mediates very-low-fluence and far-red high-irradiance responses, while phyB (light-stable) mediates red/far-red reversible low-fluence responses and shade avoidance. The interplay between Pr and Pfr allows plants to adapt their growth strategies to environmental light conditions.

(c) The Cereal Production-Consumption Gap

India now produces more cereals than it directly consumes: foodgrain output exceeded 330 million tonnes in 2023–24, central stocks of rice and wheat often run above buffer norms, and India is the world's largest rice exporter. The reasons for this gap between production and consumption are:

• Falling direct cereal consumption: rising incomes and urbanisation have shifted diets towards pulses, milk, eggs, fruit, vegetables and processed foods, so per capita cereal consumption has declined over three decades (NSS and HCES consumption surveys).
• Production incentives skewed to rice and wheat: assured MSP and open-ended procurement in Punjab, Haryana and a few other states, input subsidies and irrigation favour these two cereals regardless of demand.
• Weak effective demand among the poor: low purchasing power and nutrition gaps coexist with surplus stocks.
• Losses and leakages: post-harvest losses of cereals are estimated at about 4–6% (ICAR-CIPHET, 2015), with storage losses in covered-and-plinth stocks and PDS leakages.
• Other uses: growing diversion to feed, starch and ethanol, and exports, change the balance from year to year.
• Regional imbalance: surplus states in the north-west and deficit states in the east and north-east depend on long-distance FCI movement.

To rectify this: diversify cropping from surplus rice and wheat towards pulses, oilseeds, millets and horticulture through price and procurement support for these crops (for example, state crop-diversification incentives and millet procurement); expand decentralised procurement in eastern states; reduce storage losses through steel silos, hermetic storage and the world's largest grain storage plan in the cooperative sector (2023); use surplus grain productively through PDS fortification, open market sales, exports and regulated ethanol use; promote value addition under the Pradhan Mantri Kisan Sampada Yojana; and raise the purchasing power and dietary diversity of the poor so that consumption and nutrition, not just output, improve.