Q.4 A growing shoot of a germinating seedling encounters an underground obstacle.
Which one of the following hormones elicits ‘triple response’ to the underground
obstacle?
(A) Auxin
(B) Cytokinin
(C) Ethylene
(D) Gibberellins

Ethylene elicits the triple response in seedlings encountering underground obstacles.​

This classic response, observed in etiolated seedlings growing in soil, involves three key changes that help the shoot navigate barriers without damage. The correct answer is (C) Ethylene.​

Triple Response Explained

The triple response consists of hypocotyl swelling (radial thickening), inhibited hypocotyl elongation, and exaggerated apical hook formation. These modifications allow the seedling to exert more force, grow horizontally around obstacles, and protect the shoot meristem during soil penetration. Ethylene production increases under mechanical stress from soil or obstacles, activating EIN3/EIL1 signaling to coordinate this adaptive morphology.​

Option Analysis

• (A) Auxin: Promotes cell elongation, gravitropism, and root obstacle avoidance via PIN-mediated transport and TIR1/AFB signaling, but does not trigger the triple response.​

• (B) Cytokinin: Drives cell division, shoot meristem maintenance, and seed development; inhibits root growth but lacks role in obstacle-induced triple response.​

• (C) Ethylene: Correct; directly induces the triple response to soil pressure or obstacles, optimizing etiolated growth.​

• (D) Gibberellins: Stimulate stem elongation and seed germination via DELLA degradation, opposing ethylene’s inhibition; no involvement in triple response.​

The triple response to underground obstacles enables growing shoots of germinating seedlings to navigate soil barriers effectively. This adaptive mechanism, crucial for seedling emergence, relies on a specific hormone triggered by mechanical stress.​

What Triggers Triple Response?

When a seedling’s shoot encounters an underground obstacle, ethylene biosynthesis ramps up in response to soil pressure. This gaseous hormone activates a signaling cascade involving EIN3/EIL1 and ERF1, leading to the classic triple response: radial hypocotyl swelling, reduced elongation, and pronounced apical hook. These changes prevent meristem damage and facilitate horizontal growth around impediments.​

Role of Ethylene in Detail

Ethylene optimizes etiolated seedlings for subsurface conditions, with mutants like ein3eil1 failing to emerge from soil. It inhibits cell elongation via WDL5 upregulation and coordinates cotyledon-hypocotyl activities based on soil depth. Unlike light-grown responses, this is an innate soil adaptation, not just an ethylene assay artifact.​

Why Not Other Hormones?

Auxin guides tropic responses but promotes elongation, countering the inhibitory phase. Cytokinins focus on division and yield, irrelevant here. Gibberellins extend stems, opposing the response.​