Answer: (D) Cytokine

Cytokines are signaling proteins primarily found in animals for immune responses, not plant hormones that regulate growth and development.

Option Analysis

• (A) Abscisic acid: This is a well-established plant hormone, often called the stress hormone. It promotes seed dormancy, stomatal closure during drought, and abscission of leaves or fruits.

• (B) Brassinosteroid: These are polyhydroxylated steroidal hormones in plants that control cell elongation, vascular differentiation, and stress tolerance. They function similarly to animal steroids but are specific to plants.

• (C) Ethylene: A gaseous plant hormone that triggers fruit ripening, leaf senescence, and responses to wounding or pathogen attack.

• (D) Cytokine: Not a plant hormone. Cytokines (note the correct spelling) are animal cell signaling molecules involved in immunity and inflammation; plants use cytokinins instead for cell division.

Which one of the following is not a plant hormone? If you’re prepping for GATE Life Sciences or biology exams, this question tests your grasp of phytohormones versus non-plant signals. Options include abscisic acid, brassinosteroid, ethylene, and cytokine—let’s break it down for clarity.

Core Plant Hormones Explained

Plants rely on hormones like auxins, gibberellins, cytokinins, abscisic acid, ethylene, and brassinosteroids for growth, stress response, and reproduction. These differ from animal hormones; for instance, abscisic acid induces dormancy and drought tolerance, while ethylene speeds fruit ripening.

Brassinosteroid enhances stem elongation and immunity, confirming its plant hormone status.

Why Cytokine Stands Out

Cytokine is not a plant hormone—it’s an animal immune regulator (e.g., interleukins). Plants have cytokinins (like zeatin) for cell division, but “cytokine” refers to animal biology, making it the correct exclusion.

Exam Relevance

In competitive exams like GATE, such questions distinguish classic hormones (abscisic acid, ethylene) from emerging ones (brassinosteroid) and distractors (cytokine). Focus on functions: stress (abscisic acid), ripening (ethylene), growth (brassinosteroid).