CDS (I) 2021 General Knowledge · Question 93 of 120

Correct — C, Cytokinin. Cytokinins are the class of plant hormones that promote cell division — the name is built on cytokinesis, the division of the cell itself. The first one found, kinetin, was isolated in 1954 from autoclaved herring sperm DNA and named for exactly that power; the first natural one, zeatin, came out of maize endosperm about a decade later and takes its name from Zea, the maize genus. Cytokinins are made chiefly in the roots and move up to the shoot, and they are richest where division is fastest — root tips, developing fruits and seeds. In tissue culture a high cytokinin-to-auxin ratio is what pushes a callus into forming shoots. A supervisor asked for the hormone that raises the rate of cell division has one honest answer on this list.

• (a)Abscisic acid — The plant's brake rather than its accelerator. It closes stomata under water stress, enforces seed and bud dormancy and hastens leaf fall — it slows growth instead of adding divisions.
• (b)Gibberellins — Gibberellins lengthen stems, break seed dormancy and cause bolting in rosette plants. The famous effect is a taller plant from longer cells and internodes, which is not the same as more cells.
• (d)Auxin — Auxin's signature effect is cell elongation — it drives phototropism and apical dominance and is used in rooting powders. It works with cytokinin in culture, but the division-promoting half of that partnership is cytokinin.

Plant growth is regulated by a small set of hormones with distinct jobs. Auxins drive cell elongation and directional growth; gibberellins drive stem elongation and germination; cytokinins drive cell division and delay leaf ageing; abscisic acid inhibits growth and closes stomata; ethylene ripens fruit. Growth in a plant organ is the product of two separate processes, how many cells are made and how much each one stretches, and the hormones divide along that line.

Read the stem for the exact process named. It says 'increasing the cell division', not 'increasing the height', and that single word decides between cytokinin and gibberellin. A student who has only memorised 'gibberellin makes plants tall' will pick (b), because taller does look like more growth. The check that separates them is what changes inside the stem: gibberellin-treated plants have longer cells, cytokinin-treated tissue has more of them. The one detail worth carrying is the culture ratio — cytokinin high relative to auxin gives shoots, auxin high relative to cytokinin gives roots.

• Cytokinins promote cell division, or cytokinesis, in plant roots and shoots.
• Kinetin, isolated in 1954 from autoclaved herring sperm DNA, was the first cytokinin named.
• Zeatin, obtained from maize endosperm, was the first naturally occurring cytokinin identified, and is named after the genus Zea.
• Most adenine-type cytokinins are synthesised in the roots and travel upward.
• Abscisic acid is the growth inhibitor of the set — stomatal closure, dormancy and leaf fall.

• Equating 'growth' with 'cell division' — gibberellin makes a plant taller by stretching cells, not by adding them.
• Placing cytokinin among the inhibitors because it delays leaf yellowing; delaying senescence is a growth-favouring effect.
• Assuming every plant hormone is made in the shoot — cytokinins come mainly from the roots.

As a hormone-to-effect match, or as an applied scenario like this one where the stem names the process and the candidate has to name the hormone.

• practice — not a real PYQ

  Which one of the following plant hormones is chiefly responsible for closing the stomata when a plant is short of water?

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

  Answer(c) Abscisic acid — the stress hormone of the set, which closes stomata, enforces dormancy and is often called the plant's growth inhibitor.

• practice — not a real PYQ

  In plant tissue culture, a callus is most likely to be induced to form shoots when the medium contains

  • (a)a high auxin to cytokinin ratio
  • (b)a high cytokinin to auxin ratio
  • (c)abscisic acid alone
  • (d)no hormones at all

  Answer(b) a high cytokinin to auxin ratio — cytokinin dominance pushes the callus towards shoot formation, while auxin dominance pushes it towards roots.