Unlike animals, plants can manufacture all the chemical substances they need for survival, given only light, water, carbon dioxide and trace elements from the soil. They manufacture all the amino acids, proteins, carbohydrates, nucleic acids and other compounds , and among these are hormones (from the Greek horman, meaning "to stimulate").
By definition (originally from animals), a hormone
In plants, some hormones operate in the same tissues in which they are manufactured, and others are transported for use to different locations.
Plants don't have glands to produce hormones: various tissues throughout the plant body produce hormones.
Recall the generalized model for a hormone-triggered signal transduction pathway:
A given plant hormone may:
The sensitivity of a plant tissue to a given hormone may be altered either by
In addition to the five hormone classes above, other chemical substances exhibiting hormone-like activity have more recently been discovered.
The compound on the far right is the active ingredient in commercial preparations such as "RootTone." It is used to stimulate the growth of adventitious roots and to reduce fruit drop in commercial crops. This synthetic auxin, too, can break down into toxic, carcinogenic compounds.
Site of Production - primarily leaf primordia, meristems, and maturing seeds, though all plant tissues can produce small amounts of this hormone.
Transport - IAA is the only plant hormone known to be transported in a polar (i.e., unidirectional) fashion. Transport takes place via the vascular parenchyma cells, and may be
IAA also may be transported in a non-polar fashion via the phloem conducting cells.
Auxin's Mode of Action Revealed
Until very recently, the exact mechanism of action of auxin was not fully understood. But recent research is helping to solve this long-standing mystery.
1. Short-lived (5-10 minute lifespan), rare, plant proteins known as Aux/IAA proteins are believed to be DNA-binding transcription factors involved in the regulation of secondary (as opposed to primary or early) auxin-response genes.
2. Aux/IAA proteins are believed to be repressors.
3. Auxin binds to a specific plasma membrane receptor, T1R1, which is a type of f box protein. Thus bound, it promotes (ubiquitin-dependent) degradation of Aux/IAA repressor proteins.
4. With repressors removed, auxin-response genes are transcribed and translated, and response to auxin is initiated.
5. F-box receptors in addition to T1R1 have recently been identified, all of them with an affinity for auxin. (Mutant plants lacking any of these receptors are insensitive to auxin.)
Parthenocarpy and Stenospermocarpy
Ever wonder how your banana, pineapple, watermelons, oranges or grapes can develop without seeds? Wonder no more.
Parthenocarpy - in some species, fruits mature without ovule fertilization. In some species, pollination is required to stimulate fruit production, in others, pollination produces larger, tastier parthenocarpic fruit, and in still others, pollination is not required. (Most seedless citrus must be pollinated; bananas and pineapples need not be.)
Parthenocarpic fruits do occur in nature, and some species produce both parthenocarpic and seeded fruit in the same crop. Why waste the energy? It could be an important defense against fruigivorous insects.
Stenospermocarpy - This process requires both pollination and fertilization, because it results from the abortion of the developing embryo in the seed, at which point the seed stops developing. Seedless grapes are a result of stenospermocarpy, and you can see teh remnant of the aborted seed in most seedless grapes.
Seedless grapes are usually smaller than seeded grapes because the developing seeds produce another hormone, gibberellin, that promotes the fruits increase in size and sugar content. By spraying seedless grapes with gibberellins artificially, the farmer can produce large, marketable grapes.
After about another decade of fruitless (har!) research, Folke Skoog was able to purify, but not isolate the substance. Carlos Miller later analyzed a DNA breakdown product that had similar activity to the mystery coconut substance, and this led to the discovery that the substance he found (kinetin) was related to the plant hormones (though probably not found naturally in plants) that were later named cytokinins for their role in promoting cell division. Miller isolated a natural cytokinin from corn and named it zeatin, after the corn genus. It is still the most active and powerful of the known plant cytokinins.
Where found? - Growing tissues and wounded areas.
Transport - Acropetal, from root to shoot, via the vascular tissues. Effects
In the former case, the cell tends to remain meristematic. In the latter, it begins the genetic cascade towards differentiation and maturation into a specific cell type.
In tissue culture, a plant cell given
Many ripening fleshy fruits will undergo a notable increase in the rate of cellular respiration, and are known as climateric fruits (e.g., Solanaceous fruit such as tomatoes, pome fruits (apples; pears), avocados). Others undergo a gradual reduction in respiration as they ripen, and are known as non-climateric fruits (e.g., citrus, grapes, strawberries).
In climateric fruits, ethylene synthesis increases as ripening commences, and ethylene promotes many of the ripening features.
Commercially, fruits that are picked green (e.g., tomatoes, bananas, etc.) can be gassed with ethylene in transport so that they ripen as they travel, and are thus less easily bruised during transport.
Transport - ABA is produced in leaves, root cap and stems. It is transported via xylem, phloem to the target tissues.
As mentioned at the start of this lecture, most hormones operate at the molecular level by acting as transcription factors or by affecting transcription factors, and hence, DNA transcription and translation.
As you'll learn in Genetics (if you haven't already), it's All About RNA. Stay tuned.