sd-1 Gene and Rice Semi-Dwarfism

The question asks about the phytohormone related to the defect in the major semi-dwarfing gene of rice, known as sd-1. This defect results in a cultivar with short, thick culms and improved lodging resistance.

Understanding the Role of Phytohormones

Phytohormones, or plant hormones, are chemical messengers that regulate various aspects of plant growth and development. Different phytohormones have distinct roles:

• Gibberellins (GAs): Primarily known for promoting stem elongation, seed germination, and flowering.
• Abscisic acid (ABA): Involved in dormancy, stress responses (like drought), and inhibiting growth.
• Jasmonic Acid: Plays a role in plant defense responses against herbivores and pathogens, and also in development processes.
• Salicylic acid: Involved in plant defense mechanisms, particularly against biotrophic pathogens, and systemic acquired resistance.

The sd-1 Gene and Gibberellins

The sd-1 gene is a well-studied gene responsible for semi-dwarfism in many high-yielding rice varieties, including the 'miracle rice' developed during the Green Revolution. Research has shown that the sd-1 gene encodes for an enzyme involved in the biosynthesis of Gibberellins, specifically GA20 oxidase-2.

A defect or mutation in the sd-1 gene leads to reduced production of active Gibberellins. Since Gibberellins are crucial for promoting stem elongation, a deficiency results in shorter stems (dwarfism). The reduced height and thicker culms contribute to improved lodging resistance, meaning the plants are less likely to fall over in strong winds or heavy rain.

Therefore, the defect in the sd-1 gene is directly linked to the metabolism of Gibberellins. Reducing the active levels of this growth-promoting hormone causes the semi-dwarf phenotype observed in rice cultivars with the sd-1 defect.

Conclusion

Based on the function of phytohormones and the known mechanism of the sd-1 gene, the defect in this gene is related to Gibberellins.