● The `color{Brown}"plant growth regulators (PGRs)"` are small, simple molecules of `color{violet}"diverse chemical composition"`.

● They could be `color{violet}"indole compounds"` (indole-3-acetic acid, IAA); `color{violet}"adenine derivatives"` (`N_6`-furfurylamino purine, kinetin), derivatives of `color{violet}"carotenoids"` (abscisic acid, ABA); `color{violet}"terpenes"` (gibberellic acid, GA3) or `color{violet}"gases"` (ethylene, C2H4).

● Plant growth regulators are variously described as `color{violet}"plant growth substances"`, `color{violet}"plant hormones"` or `color{violet}"phytohormones"` in literature.

● The PGRs can be broadly divided into `color{violet}"two groups"` based on their functions in a `color{violet}"living plant body"`.

● One group of PGRs are involved in `color{violet}"growth promoting activities"`, such as cell division, cell enlargement, pattern
formation, tropic growth, flowering, fruiting and seed formation.

● These are also called `color{violet}"plant growth promoters"`, e.g., auxins, gibberellins and cytokinins.

● The PGRs of the other group play an important role in `color{violet}"plant responses to wounds"` and `color{violet}"stresses of biotic and abiotic"` origin.

● They are also involved in various `color{violet}"growth inhibiting activities"` such as dormancy and abscission.

● The PGR `color{violet}"abscisic acid"` belongs to this group.

● The `color{violet}"gaseous PGR, ethylene"`, could fit either of the groups, but it is largely `color{violet}"an inhibitor"` of growth activities.

● Interestingly, the discovery of each of the `color{violet}"five major groups of PGRs"` have been accidental.

● All this started with the observation of `color{Brown}"Charles Darwin"` and his son `color{Brown}"Francis Darwin"` when they observed that the coleoptiles of canary grass responded to `color{violet}"unilateral illumination"` by growing towards the light source (phototropism).

● After a series of experiments, it was concluded that the `color{violet}"tip of coleoptile"` was the site of transmittable
influence that caused the `color{violet}"bending of the entire coleoptile"`.

● Auxin was isolated by `color{Brown}"F.W. Went"` from tips of coleoptiles of `color{violet}"oat seedlings"`.

● The ‘bakane’ (`color{violet}"foolish seedling"`) a disease of rice seedlings, was caused by a fungal pathogen `color{violet}"𝘎𝘪𝘣𝘣𝘦𝘳𝘢𝘭𝘭𝘢 𝘧𝘶𝘫𝘪𝘬𝘶𝘳𝘰𝘪"`.

● `color{Brown}"E. Kurosawa"` reported the appearance of symptoms of the disease in `color{violet}"uninfected rice seedlings"` when they were treated with `color{violet}"sterile filtrates"` of the fungus.

● The `color{violet}"active substances"` were later identified as `color{violet}"gibberellic acid"`.

● `color{Brown}"F. Skoog"` and his co-workers observed that from the `color{violet}"intermodal segments"` of tobacco stems `color{violet}"the callus"` (a mass of undifferentiated cells) proliferated only if, in `color{violet}"addition to auxins"` the nutrients medium was supplemented with one of the following: extracts of `color{violet}"vascular tissues"`, `color{violet}"yeast extract"`, `color{violet}"coconut milk"` or DNA.

● `color{Brown}"Skoog and Miller"`, later identified and `color{violet}"crystallised the cytokinesis"` promoting active substance that they termed
kinetin.

● During mid-1960s, `color{violet}"three independent researches"` reported the `color{violet}"purification and chemical characterisation"` of three different kinds of inhibitors: `color{violet}"inhibitor-B, abscission II"` and `color{violet}"dormin."`

● Later all the three were proved to be `color{violet}"chemically identical"`.

● It was named `color{violet}"abscisic acid (ABA)"`.

● Cousins confirmed the `color{violet}"release of a volatile substance"` from ripened oranges that `color{violet}"hastened the ripening"` of stored unripened bananas.

● Later this volatile substance was identified as `color{violet}"ethylene, a gaseous PGR"`