PLANT RESPONSES TO INTERNAL AND EXTERNAL SIGNALS
Explain how plants communicate between different parts of its body.
List 6 classes of plant hormones, describe their major functions, and note where they are produced in the plant.
Explain how a hormone may cause its effect on plant growth and development.
Describe the evidence that suggests other factors that may control apical dominance.
Give examples and explain how plants respond to different stimuli in their environment.
Plants Use Chemicals to Communicate
Plants use chemicals not just to communicate to the living world but also for communicating between different
parts of the its body, thereby optimizing the response of the whole organism. Chemical communication within
plants is an exploding field of research. It has also been discovered that plants have an attribute that animals do
not: The cytoplasmic bridges between plant cells (plasmodesmata), unlike the analogous structures in animals (gap
junctions), can dilate enough that macromolecules such as proteins can move from cell to cell. This recent research
builds on a long history of classic experiments that provided the first clues that mobile signaling molecules called
hormones are internal regulators of plant growth.
General Characteristics of Plant Hormones
A hormone is a signaling molecule that is produced in low concentrations by one part of an organism
and transported to other parts, where it binds to a specific receptor and triggers responses in target cells and
tissues. In animals, hormones are usually transported through the circulatory system, a criterion often included in
definitions of the term. However, plants don
t have circulating blood to transport hormone-like signaling molecules.
Thus, many plant biologists prefer the broader term plant growth regulator to describe organic compounds, either
natural or synthetic, that modify or control one or more specific physiological processes within a plant. The terms
plant hormone and plant growth regulator are used about equally, but for historical continuity this text will use the
and adhere to the criterion that plant hormones are
active at very low concentrations
Although plant hormones are produced in very low concentrations, a tiny amount of hormone can have a
profound effect on plant growth and development. Virtually every aspect of plant growth and development is under
hormonal control to some degree. Each hormone has multiple effects, depending on its site of action, its
concentration, and the developmental stage of the plant. Conversely, multiple hormones can influence a single
process. Plant hormone responses commonly depend on both the amounts of the hormones involved and their
relative concentrations. It is often the interactions between different hormones, rather than hormones acting in
isolation, that control growth and development. These interactions will become apparent in the following survey of