AUXIN

CYTOKININS

ETHYLENE

click on the to learn more about each hormone

INTRODUCTION

One very active area of research today is on the mechanisms by which cells interpret signals inside and out. Perhaps the best system to study external signal perception and transduction is plants since plant cells are extremely well equipped to sense the external environment. Plant cells are able to perceive and respond to signals from the external environment by recognizing internal and external signals through receptor protiens. External signals like light, temperature, or gravity often cause internal signals such as changes in homone concentration, ion concentration, or cytoskeletal arangement. The plant system is ideal for studying signal systems because there are many ways in which researchers can measure isolated stimuli and responses in plants. Changes in growth paterns or direction can be measured to varify that these sessile organisms are able to receive and respond to stimuli. Determining the mechanisms with which these organisms perceive signals is not as easy. Only recently with the advent of various molecular techniques have researchers been able to get at the mechanisms of the signaling systems in plants. This document will review recent research on the perception of the above hormone signals during the growth and development of plants.

These are a few hormones that are believed to be involved in an enormous variety of responses. This document is in no way able to gather all research available on the signaling pathways of these hormones. It is my goal to present the research regarding receptor protiens involved with these homones. Much intresting research is available on how these receptor protiens amplify the hormonal signal by intervening in regular cellular processes. Plant hormones are present in micromolar or submicromolar concentrations therefore it is essential in the hormone signal pathway that the hormone signal be amplified for a response to occur. This signal amplification may occur by a number of mechanisms, one of which is the controll of gene activity.

POSSIBLE SITES OF SIGNAL INTERVENTION

Figure 1. Possible sites of homonal control of gene activity. We can see that activation of genes represents a method of amplification that can lead to many copies of an important cellular product. Various control points in the flow of genetic information from DNA to a molecular product can be seen in figure 1. These processing steps are controlled by enzymes whose actions might be regulated by hormones.

The low concentrations of hormones in plants require more than just the amplification of the signal. There are three main parts of a response system:


1**THE HORMONE MUST BE PRESENT IN SUFFICIENT QUANTITY IN THE PROPER CELLS.

2**THE HOMONE MUST BE RECONGNIZED AND BOUND TIGHTLY BY EACH OF THE GROUPS OF CELLS THAT RESPOND TO THE HORMONE (TARGET CELLS)

3**THE RECEPTOR PROTIEN MUST CAUSE SOME METABOLIC CHANGE THAT LEADS TO AMPLIFICATION OF THE HORMONAL SIGNAL.


The mechanism through which the hormone is able to confer information to plant cells is not well understood. Animal researchers are familiar with a signaling pathway involving inositol tri-phosphate that may be present in plants. This pathway would explain many hormonal signals that appear be involved with phosphoralation and dephosphorylation activity.


THE INOSITOL TRI-PHOSPHATE SIGNALING PATHWAY
Figure 2. Model for initial hormone transduction at the plasma membrane. Binding of a hormone to its receptor causes activation (+) of nearby phospholipase c (PLC). PLC hydrolyzes a membrane lipid, phosphatidylinositol-4,5-bisphosphate (PIP₂) to release inositol-1,4,5-triphosphate (IP ₃)and a diacylglycerol (DAG). IP₃ moves to the tonoplast in plant cells, where it combines with a receptor that activates (+) a CA2+ pump or transporter that moves CA2+ from the vacuole to the cytosol. DAG, which remains membrane-bound, activates protein kinase c (PKC). PKC is also activated by Ca2+ released from the vacuole, so various enzymes become phosphorylated by PKC. Calcium also activates other protein kinases and other enzymes, when free or bound with calmodulin. IP₃ loses phosphates by hydrolysis to form IP₂ and IP₁ which is then converted back to phosphatidylinositol (PI) and other phosphoinositide lipids (PIP and PIP₂) in the plasma membrane. (Salisbury and Ross, 1992)

TO SEE MORE RESEARCHER NAMES AND TOPICS OF SIGNAL RESEARCH:

LINK TO PLANT SENSORY SYSTEMS: A NASA/NSF RESEARCH NETWORK

or

LIST OF REFERENCES