Which one among the following is not a plant hormone?

Correct — D, Thyroxine. Thyroxine is an animal hormone, secreted by the thyroid gland of vertebrates. It is built around iodine — four iodine atoms in each molecule, which is why it is written T4 — and its job is to set the basal metabolic rate, governing how fast the body burns fuel, and to drive growth and development, which is why a severe shortage in childhood stunts both body and brain. Plants have no glands and no thyroid, so nothing in a plant makes or responds to it. The other three are three of the five classical plant growth regulators. Auxins, of which indole-3-acetic acid is the natural example, drive cell elongation, are behind the bending of a shoot towards light and maintain apical dominance, the suppression of side buds by the growing tip. Gibberellins promote stem elongation, break seed and bud dormancy and trigger bolting; they were discovered through the bakanae or foolish-seedling disease of rice, in which a fungus makes infected plants shoot up abnormally tall. Abscisic acid is the odd one among the plant hormones in a different sense — it inhibits rather than promotes, enforcing seed dormancy and closing the stomata when a plant is short of water, which has earned it the name of the stress hormone. The remaining two classical regulators, not offered here, are the cytokinins, which drive cell division, and ethylene, a gas that ripens fruit.

• (a)Gibberellins — A plant hormone. They promote stem elongation, seed germination and bolting, and were discovered through the bakanae disease of rice, in which infected seedlings grow abnormally tall.
• (b)Abscisic acid — A plant hormone, and the main growth inhibitor among them. It enforces seed dormancy and closes stomata under water stress, which is why it is called the stress hormone.
• (c)Auxins — The best-known plant hormone group. They cause cell elongation, the bending of shoots towards light and apical dominance, and are used commercially in rooting powders and weedkillers.

Plant hormones, more properly plant growth regulators, are small molecules made in one part of a plant that act in tiny amounts to change growth elsewhere. Five groups are classical: auxins, gibberellins and cytokinins, which promote growth in different ways, and abscisic acid and ethylene, which inhibit or age tissue. Unlike animals, plants have no glands to make them and no bloodstream to carry them, so the substances are produced in ordinary tissue and move by diffusion or through the vascular system. Animal hormones such as thyroxine, insulin and adrenaline come from discrete endocrine glands and travel in blood to distant target organs. The word hormone is shared; the machinery behind it is not.

This is an odd-one-out item with an easy surface and a lesson underneath. The quick route is the category test: three names are familiar from plant physiology and the fourth from human biology, and 'thyroid' is audible inside 'thyroxine'. The lesson underneath is the difference the item is quietly built on. Plant growth regulators come from ordinary cells, act at extremely low concentrations, often do several unrelated things at once, and frequently work as a balance between two of them rather than singly — root and shoot formation in tissue culture, for instance, is set by the ratio of auxin to cytokinin rather than by either alone. Animal hormones are glandular, targeted and usually specific. A useful check when an examiner mixes the lists: if the substance has a named gland behind it, it is not a plant hormone. Thyroxine also carries an Indian public-health story worth knowing, since it cannot be made without iodine and India's universal salt iodisation programme exists to prevent the goitre and developmental damage that iodine deficiency causes.

• The five classical plant growth regulators are auxins, gibberellins, cytokinins, abscisic acid and ethylene.
• Thyroxine, or T4, is secreted by the thyroid gland of vertebrates, contains four iodine atoms and regulates the basal metabolic rate.
• Auxins such as indole-3-acetic acid cause cell elongation, phototropic bending and apical dominance.
• Gibberellins promote stem elongation, seed germination and bolting, and were discovered through the bakanae disease of rice caused by a fungus.
• Abscisic acid is a growth inhibitor that enforces seed dormancy and closes stomata under water stress.
• Cytokinins promote cell division, and ethylene is a gaseous regulator that ripens fruit.
• Plants have no endocrine glands; growth regulators are made in ordinary tissue and move by diffusion or through the vascular system.

• Assuming any substance called a hormone must belong to the same system; the word covers two quite different mechanisms in plants and animals.
• Confusing abscisic acid with an ordinary growth promoter. It is the inhibitor of the group.
• Forgetting ethylene because it is a gas rather than a liquid or solid; it is a full member of the classical five.

As an odd-one-out item like this one, as a hormone-to-effect match, or through an applied stem — which hormone would you use to induce cell division, break dormancy or ripen fruit.

• practice — not a real PYQ

  Which one of the following plant growth regulators is a gas at room temperature?

  • (a)Auxin
  • (b)Gibberellin
  • (c)Ethylene
  • (d)Abscisic acid

  Answer(c) Ethylene — a gaseous regulator that ripens fruit, which is why one ripening fruit hastens the ripening of others stored beside it.

• practice — not a real PYQ

  Thyroxine cannot be synthesised by the human body in the absence of which one of the following?

  • (a)Iron
  • (b)Iodine
  • (c)Calcium
  • (d)Zinc

  Answer(b) Iodine — each thyroxine molecule carries four iodine atoms, which is why iodine deficiency causes goitre and why edible salt is iodised.