Growth and differentiation into various cell types and tissues and organs &endash; How are these stimulated in plants?

Are there "hormones" in plants?

("Phytohormones" was coined)

Chemical substances can be isolated that affect many aspects of plant development

(Also "synthetic" hormones are manufactured)

Hormones: Naturally occurring chemical messengers (organic) that "exert a profound influence on physiological processes."

In animals, hormones are produced in an organ (or tissue) and then travel to another part of the body via a concentration gradient.

In Plants: Hormone production is not as localized and is not necessarily sent from one tissue or organ (or even cell) to a far-removed target tissue in another part of the plant.

e.g. Auxin is synthesized in shoot and root apexes and influences elongation in the cells very close to its originating cells.

e.g. However, cytokinin is synthesized in root cells and transported to leaves, where action occurs.

There are many examples of both modes of action,

So, promoting action at a distance is NOT an ESSENTIAL plant hormone property (as it is in animal hormones)

Do plant hormones act or function in a concentration-dependent mode?

Controversy on hormone action in plants continues! (even into the 1990's)

• Do plant cells respond to hormone concentration? -- or &endash;
• Do the target cells become more or less sensitive to the hormone?

Plant hormones seem to be much less specific in their effects (i.e. one plant hormone can affect many different physiological processes.

(e.g. many different developmental events are influenced by auxin or cytokinin)

And also in plants:

1. Hormones do not act alone; they act in concert in plant growth and plant development

2. Hormones do not control all the physiological processes:

a. Genetics

b. Biochemicals(e.g. many different developmental events are influenced by auxin or cytokinin)

And also in plants:

1. Hormones do not act alone; they act in concert in plant growth and plant development

2. Hormones do not control all the physiological processes:

a. Genetics

b. Biochemicals

c. Environmental parameters(e.g. many different developmental events are influenced by auxin or cytokinin)

And also in plants:

1. Hormones do not act alone; they act in concert in plant growth and plant development

2. Hormones do not control all the physiological processes:

a. Genetics b. Biochemicals c. Environmental parameters

Factors that also affect development

Therefore, each plant hormone

• has a multiplicity of action
• is difficult to identify the effects with certainty

Is hormone a suitable term for plant regulatory chemicals?

Better terms?

• Plant growth substance &endash; or&emdash;
• Plant growth regulator

"Hormones are what physiologists call hormones" (A.C. Leopold, 1987)

Important for plant physiologists to not be overly influenced by animal studies.

Five Accepted Groups of Plant Hormones (overlap in their functions of regulating plant development events):

• Auxins
• Gibberellins
• Cytokinins
• Abscisic Acid
• Ethylene

Biologically active &endash;

• Brassinosteroids (lipids from pollen, stems, leaves, flowers), occurs in Brassica, Napus. Causes elongation of 2nd bean internode
• Polyamines (from arginine and lysine) influence synthesis and activity of cells (not well known!)

• Synthesized in apex of stem and of root
• Stimulate cell elongation
• Implicated in root initiation
• Play a role in differentiation of xylem and phloem cells
• Phototropism and other tropisms are auxin induced
• Controls auxiliary bud development
• Controls flower and fruit development

IAA (indole-3-acetic acid) &endash; principal plant auxin, there are other natural auxins and many synthetic auxins, including herbicides based on auxin chemistry

All have an acetic acid side chain on an aromatic ring.

See more of Chapter 15