After a seed is fertilized, the embryo will begin undergoing mitosis to produce millions of new cells, all genetically identical. How do these genetic clones become specialized cells such as xylem and phloem? In many ways, the mechanism of cell specialization are the same as in animals. Plant hormones, chemicals produced by one cell and affecting the growth and development of other cells, induce many cellular changes. A hormone influences cells equipped with appropriate cellular receptors (located on the surface of the cell membrane).

What are the different types of plant hormones? Below, five major hormones (and their functions) are listed:

A. Gibberellins – Decades ago, farmers in Japan noticed that rice plants would ‘bolt’ when a certain fungus grew on their stems. It turns out that the fungus was releasing gibberllins, hormones that cause the stem to elongate. Functions: Stem Elongation and Breaking Dormancy.

B. Auxins – Affect cell elongation, especially during phototropism and gravitropism.

C. Cytokinins – The name sounds like cytokinesis (the stage following mitosis, in which the cell is actually cleaved into 2 individual cells), which reflects its activity in promoting cell division and leaf expansion.

D. Abscisic Acid – In plants, abscission is the dropping of leaves or fruits. Abscisic acid in named for its role in this process. The release of AA causes plants to enter dormancy, in which they remain live but not actively adding new growth for a period of time.

E. Ethylene – An interesting gas that speeds up fruit ripening (can also lead to senescence, or process leading to death, of seeds, leaves and fruits – for this reason, don’t store seeds in your fridge because the ethlylene from fruits may lead to seed death). Fruits are commonly shipped before they are ripe and exposed to ethylene to promote ripening.

Tropism is growth in response to a stimulus.

Gravitropism: Growth in response to gravity. Roots display positive gravitropism, meaning they grow towards the source of gravity. The tip of the root distributes auxins (along with other growth inhibitors) slow the elongation of cells. Gravity causes the auxins to build up on one side of the root, resulting in the slowing of the growth of root cells.

Phototropism: Growth in response to light. In the case of light, the cells on the side of the stem away from light have an increase in auxins, causing them to grow faster than the side exposed to light.

Thigmotropism: Growth in response to touch/pressure.

Circadian Rhythms: Repeated, daily cycles displayed by living organisms.

Photoperiod: Response to amount of light/dark in the period of a day. A pigment, phytochrome, changes shape based on amount of red-light it absorbs. This molecule appears to allow plants to respond to changing lengths of night.

This above situation is due to phytochrome. The mechanism of flowering is not completely understood at this time, likely involving a hypothetical hormone known as floragen. Flowering is a hot topic of research, with significant implications for life on earth!

Many plants have mechanisms to survive unfavorable conditions. The major method involves a dormant period, during which plants don’t actively grow.

Abscission: The loss of leaves, fruit, etc. due to activity of abscic acid.

Senescence: Death of all or part of a plant.