Question

Which hormone is anti-auxin?

Answer 1

Auxin is a plant growth regulator hormone that is produced in the apices of the plant and helps to control growth. The word ‘anti-auxin’ in very simple words means the chemicals/compounds/hormones that act against auxin or inhibit the physiological functions of auxin.

Complete answer:
Auxins are actually IAA, IBA, NAA or 2,4-D or such natural chemicals that may have similar structures and functions. They promote cell division, have an effect called apical dominance, promote flowering, promote abscission of older leaves and flowers. They are also used in parthenocarpy to obtain seedless fruits.
Now, the anti-auxins are certain chemicals/compounds that mask the effect of auxins or inhibit their action.
The common anti auxin hormone known widely is PCIB or p-para chloro phenoxy isobutyric acid. This hormone actually competes with auxin to its binding sites (shows competitive inhibition). Thus, auxin cannot bind with its dedicated receptors and so the definite physiological function is not exhibited.
Other examples include NPA (1-N-Naphthylphthalamic acid) and TIBA (2,3,5-triiodobenzoic acid) which block auxin transport. By blocking the transport of auxin, they prevent its actions.

Note:
Anti-auxins are chemicals that prevent the action of auxin. Auxin is a natural PGR (Plant growth regulator) that contributes to the growth of the plant and performs some other physiological functions too. The anti-auxin prevents the action of this hormone by showing competitive inhibition with its binding sites or receptors like in the case of PCIB, or by preventing the transport of auxin as in the case of NPA and TIBA.