​The defect in a major semi-dwarfing gene of rice, sd-1, leads to cultivars with short, thick culms and improved lodging resistance. The gene is related to which one of the following phytohormones?

Correct option is A

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The sd-1 (semi-dwarfing 1) gene is linked to gibberellins (GA), which are plant hormones that regulate growth and development. The mutation in the sd-1 gene leads to a reduction in gibberellin biosynthesis, causing the rice plant to have shorter stems (dwarf phenotype). This mutation has been widely used in breeding semi-dwarf rice varieties, significantly contributing to increased yield and lodging resistance during the Green Revolution. Lodging resistance is crucial because it prevents the plant from falling over under adverse conditions, thereby enhancing productivity.

Information Booster:

1. Gibberellins (GA): Plant hormones involved in stem elongation, seed germination, and flowering. Mutations in GA biosynthesis or signaling pathways often lead to dwarf phenotypes.
2. The sd-1 gene encodes an enzyme in the GA biosynthetic pathway. Mutations here reduce gibberellin levels.
3. Semi-dwarf rice varieties have short, sturdy stems that resist lodging, making them suitable for high-yield farming.
4. The Green Revolution relied heavily on such genetic modifications, improving crop yield worldwide.
5. Mutation in sd-1 specifically affects internode elongation, reducing stem height while maintaining overall plant health.

Additional Knowledge:

(a) Gibberellins:
This option is correct. Gibberellins regulate plant height by promoting cell elongation. In the case of the sd-1 gene, a mutation leads to a reduction in gibberellin biosynthesis, resulting in the dwarf phenotype that prevents lodging and improves yield potential.

(b) Abscisic acid:
This option is incorrect. Abscisic acid (ABA) is primarily associated with stress responses, such as drought tolerance, by regulating stomatal closure and seed dormancy. It does not play a significant role in determining plant height or lodging resistance in rice.

(c) Jasmonic acid:
This option is incorrect. Jasmonic acid is involved in defense mechanisms against herbivores and pathogens, as well as wound responses. It is unrelated to the semi-dwarfing gene or plant height regulation.

(d) Salicylic acid:
This option is incorrect. Salicylic acid is primarily associated with systemic acquired resistance (SAR) in plants against pathogens. It has no direct role in controlling stem elongation or lodging resistance.