Plant Growth: phytohormones

https://www.youtube.com/watch?v=EZ5tU45Ti_g

Plant Growth: Auxins and Gibberellins | Biology for All | FuseSchool

Abscisic acid (also called ABA) is one of the most important plant growth inhibitor. It was discovered and researched under two different names before its chemical properties were fully known, it was called dormin and abscicin II. Without ABA, buds and seeds would start to grow during warm periods in winter and be killed when it froze again.

Auxins are compounds that positively influence cell enlargement, bud formation and root initiation. They also promote the production of other hormones and in conjunction with cytokinins, they control the growth of stems, roots, and fruits, and convert stems into flowers.

Brassinosteroids are a class of polyhydroxysteroids, the only example of steroid based hormones in plants. Brassinosteroids control cell elongation and division, gravitropism, resistance to stress, and xylem differentiation. 

Cytokinins or CKs are a group of chemicals that influence cell division and shoot formation. They were called kinins in the past when the first cytokinins were isolated from yeast cells. They also help delay senescence of tissues, are responsible for mediating auxin transport throughout the plant, and affect internodal length and leaf growth. 

Ethylene is a gas that forms through the breakdown of methionine, which is in all cells. Ethylene affects cell growth and cell shape; when a growing shoot hits an obstacle while underground, ethylene production greatly increases, preventing cell elongation and causing the stem to swell. 

 Ethylene affects cell growth and cell shape; when a growing shoot hits an obstacle while underground, ethylene production greatly increases, preventing cell elongation and causing the stem to swell. 
It was later discovered that GAs are also produced by the plants themselves and they control multiple aspects of development across the life cycle.