12 April 2006
Reading: Chapters 19 and 22, Taiz and Zeiger's Plant Physiology
IV. Growth and Development
C. Plant hormones
a. Polar transport
c. Auxin receptors
Hormones are chemical signals sent and perceived by plant cells. The first step in perception of a hormone is its binding to a receptor, which is a protein that binds the hormone and sends theinformation that the hormone is present throughout the cell. Receptors that bind chemical signals, such as hormones, are often called "chemoreceptors".
In the case of auxins, there appear to be at least two classes of proteins that act as chemoreceptors. The first of these is ABP1, for "auxin binding protein 1". This auxin receptor was identified by chromatography. Auxin was attached to particles in a chromatography column and then cell extracts from plant cells responsive to auxin were passed through the column in the hope that auxin-binding proteins would stick to the auxin on the particles. The strategy worked and ABP1 was isolated. A variety of experiments have demonstrated that ABP1 participates in the increased H+ATPase activity that is part of auxin-stimulated acid growth (see page 441 in your text). A current model proposes that ABP1 associated with H+ATPases alters its conformation when it binds auxin and activates the ATPase. Immunofluorescence studies, in which a fluorescent antibody molecule specific for ABP1 was used to "stain" cell sections, showed that most of the ABP1 in plant cells is in the endoplasmic reticulum membrane. It is not clear if these ER-localized ABP1 molecules or only those found in the plasma membrane interact with H+ATPases. In another experiment, plant cell protoplasts were exposed to auxin with or without addition of antibodies to ABP1. The protoplasts exposed to antibodies showed no H+ATPase activation by auxin, suggesing that the antibodies interfered with binding of auxin to ABP1 and supporting the hypothesis that ABP1 is involved with H+ATPase activation. Another interesting result of this experiment was that, while expression of some auxin-stimulated genes were suppressed by the antibody treatment, others were not. This suggested that ABP1 was not the only auxin receptor.
Recently, a protein called TIR1 has been identified as one of the major auxin receptors in plant cells. TIR1 is present in the nuclei of plant cells that are receptive to auxin and it is known to bind auxins.
d. Auxin-regulated genes
The expression of many genes is stimulated by auxins. These auxin-responsive genes have been grouped into early and late responding genes.
i. Early genes
The early auxin-responsive genes are defined by the fact that translation inhibitors, such as cycloheximide, do not interfere with their expression in response to auxin treatment. This means that the mRNA for their transcription factors is already present in the cell at the time of auxin exposure, i.e. the transcription factors are expressed constitutively in plant cells responsive to auxins.
Early auxin-responsive genes are grouped into 5 classes (see page 455 in your text). Of these 5, the SAUR and GH3 families of genes are of unknown function. The AUX/IAA family of genes are repressors that appear to inhibit transcription of late auxin-responsive genes. The glutathione-S-transferase genes and ACC synthase are considered stress response genes, e.g. ACC synthase produces ethylene, which is often part of plant responses to physiological stress.
ii. Late genes
The late auxin-responsive genes are many and are thought to be under the control of the TIR1 auxin receptor. These genes may be thought of as late-responding but also as requiring more auxin than the early auxin-responsive genes.
The scheme that has been proposed for TIR1-mediated regulation of late auxin-responsive genes may be summarized as follows:
- Prior to auxin exposure, AUX/IAA proteins are bound to auxin-response factors (ARFs), which code for transcription factors of late auxin-resonsive genes.
-Auxin binds ti TIR1 proteins.
-The auxin-TIR1 complex binds to AUX/IAA proteins and causes them to be degraded.
-ARFs are expressed and transcription of late auxin-responsive genes begins.
Ethylene gas (C2H4) is a small, highly diffusible compound that acts as a plant hormone. It was discovered as a plant hormone 100 years ago when it was isolated as the active ingredient in gas lamp emissions that were observed to stunt growth of trees and other plants near gas street lamps.
Ethylene is produced in many plant tissues and diffuses readily between cells, tissues, and even to adjacent plants. A precursor of ethylene, 1-aminocyclopropane-1-carboxylic acid (ACC), can be transported long distance by the xylem.
Ethylene is made from the amino acid methionine in a cyclic pathway shown on p 521 of your text. The limiting step in ethylene synthesis is the production of ACC by the enzyme ACC synthase. ACC is then converted to ethylene by ACC oxidase in a reaction that requires oxygen.
b. Effects of ethylene
Ethylene has 3 types of effects on plant cells, inhibition of cell expansion, promotion of senescence and leaf abscission, and promotion of fruit ripening in some fruits.
i. Inhibition of cell expansion
Ethylene causes what is known as the "triple response" in plants: it causes plants to have short shoots, fat shoots, and increased lateral root growth. In seedlings, ethylene also stimulates root hair production by the radicle.
Ethylene inhibits cell expansion by altering the pattern of cortical microtubules in unexpanded cells. This causes alters the pattern in which cellulose microfibrils of the cell wall are produced. The altered pattern of microfibrils restricts cell expansion in all dimensions, making a short, fat cell.
Inhibition of cell expansion contributes to "thigmotropism" in plants, which is an inhibition of cell expansion by ethylene in response to physical contact. Thigmotropism is what causes some plants to twine around poles or other plants. It also allows seedlings to grow around rocks and other obstructions in their path to sunlight. Thigmotropism is also what makes plants from high wind environments shorter and stockier and better able to withstand the wind.
ii. Promotion of senescence and leaf abscission
Plant senescence is a process of controlled death that plant cells initiate in response to environmental signals. Ethylene promotes this process. Ethylene also stimulates leaf abscission, which is a late part of leaf senescence.
Ethylene stimulates senescence in many cut flowers. The small packet of salts that the florist gives you to put in the water for your cut flowers includes silver thiosulfate. Silver is a potent inhibitor of ethylene effects and prolongs the life of the cut flowers.
iii. Promotion of fruit ripening
Ethylene is involved in the ripening process of fruits known as "climacteric" fruits, which include apples, bananas, avocados, tomatoes, and others. These fruits produce a burst of ethylene as they ripen, which is followed by increased respiration and accelerated ripening. Exogenous ethylene stimulates ripening in these fruits. This is why 1 rotten apple spoils the whole barrel. One overripe apple produces ethylene, which diffuses to the others and accelerates their ripening. One way in which ethylene contributes to ripening is by increasing synthesis of cellulases and polygalacturonases, which soften cell walls in ripening fruit.
In agriculture, ethylene is used to control ripening. Ripening of fruits in the field can be synchronized with ethylene to reduce harvest costs. Also, some fruits (bananas, tomatoes) can be picked green and then ripened in a warehouse with ethylene.
Genetic engineering of the ethylene response has also been used to control ripening. Three approaches have been taken in tomatoes:
-Add an anti-sense gene for ACC synthase. Fruits of these plants show very delayed ripening but ripening can be easily induced by addition of exogenous ethylene. This strategy is being applied to bananas, melons, carnations, and petunias. Cut flowers from ACC synthase anti-sense petunias last weeks longer than cut flowers from un-engineered plants.
-Add a gene for ACC deaminase from bacteria. ACC deaminase competes with ACC oxidase for ACC and reduces the production of ethylene.
-Add an anti-sense gene for polygalacturonase. This was the approach taken to make the Flav'r Sav'r tomato, the first genetically-modified food brought to market.
All these approaches are effective in controlling ripening but none are in use today, in part because of consumer discomfort with the idea of genetically modified foods.