Lecture 9 Notes
Plant Hormones
| I. What is a hormone?
A. A hormone is a chemical produced by the plant that elicits specific reactions in certain cells, usually after exposure to only very small concentrations. Sometimes a pathogen, such as a fungus or bacteria, can also produce the chemicals. Transport from one cell to another is not a requirement in plants as it usually is in animals. B. The main hormones are: 1. auxins 2. gibberellins 3. cytokinins 4. abscisic acid 5. ethylene 6. others (jasmonic acid for one) C. Let's go through what each hormone does, and how it does those things. 1. Auxins (I use plural because there are several forms depending on the plant species). Real name is indole acetic acid (has a COOH group, which makes it an acid). a. In the 1880's, Charles and Francis Darwin performed some experiments on how plants perceive light. Coleoptiles (sheath covering newly germinated grass plants) tend to bend toward the light. They wondered - how and where does the coleoptile perceive the direction of light. b. They put little covers over the tips of some, over the base of others. In yet others, they cut off the tip. Only those with intact tips, and without a cover over the tip bent. Whenever the tip was cut off, or covered, no reaction to the light. c. This suggested to them that the tip perceived the direction of the incoming light, and that perhaps something was transported from the tip to the base where the bending occurred. d. Then, Boysen-Jensen, a Dutch researcher, severed the tip, put it back on the coleoptile, but with a piece of agar between the base and tip (agar is gelatin and can absorb chemicals and desorb them later) or a piece of mica (preventing anything from diffusing from the tip to coleoptile base). The coleoptiles with the mica did not bend. Those with the agar did, again suggesting transport of some compound from tip to base. e. Later, in the 1920's, Fritz Went used the cut tips as bioassays to determine what the compound was. He allowed cut tips to exude the mysterious compound into the agar, then put the agar back on a decapitated coleoptile. If the coleoptile was then exposed to light from one side, it bent, even though it didn't have the tip anymore, just a slab of agar. f. This showed that some chemical was responsible for the bending, not just the tip. g. If he put the plants in conditions where the light came from above, or in the dark, but put the agar on only one side of the coleoptile, the coleoptile bent. h. This showed that most likely, the chemical was moving from the light side to the dark side, and that it stimulated cell growth on the dark side, which would cause the coleoptile to bend toward the light. i. Went named this unknown compound auxin, meaning in Greek, "to grow". j. Later the chemical structure was determined from human urine (humans concentrate this chemical becuase they eat plants which contain it). k. What does auxin do? 1. apical dominance - the pyramidal shape of pine trees, and the inhibition of axillary buds is due to auxins from the apical meristem. High concentrations prevent axillary buds from forming, and keeps branch growth in check. If you decapitate a plant, the axillary buds are released and you get bushier plants - horticulturists do this to make nicer looking foliage plants. 2. cell division and elongation - auxins stimulate cells to enlarge. Auxin causes cell walls to get loose so that cellulose microfibrils can slip, allowing the cell to expand due to turgor pressure. 3. rooting - cut stems will root more readily if dabbed with auxin powders. 4. phototropism - is responsible for plants growing toward the light. Turns out that light forces auxins to the dark side, stimulating growth on that side, which forces the plant to grow toward the light. 5. gravitropism - when plants are laid on their side, the shoot grows up, and the roots grow down. Why? i. when laid on their side, auxins accumulate on the lower sides of the stem. ii. shoot cell enlargement is stimulated by the accumulation of auxin on lower side, forcing the stem to turn upwards. iii. roots though, are more sensitive, and the auxin on the lower side inhibits growth. Thus, cells on the upper side grow more, and the root turns down. 2. Gibberellins - or gibberellic acid (it is an acid, has COOH group on it). a. discovered just prior to WWII in Japan by rice researchers. Plants were infected with a fungus Gibberella, causing it to grow tall and then fall over. Called foolish seedling disease. b. later, the compound was isolated, and found to occur naturally in plants, albeit in low (billionths of a molar) concentrations. c. causes the following: 1. stem elongation - dwarf plants have a genetic defect wherein they can not metabolize or produce the appropriate gibberellin. If supplied from outside artificially, plant will send up tall shoots and not be dwarf anymore. Also, plants with rosettes will produce a flowering shoot in their second year, due to production of gibberellins. 2. flowering - can cause some plants to flower out of season if applied. 3. seed germination - embryo, in response to wetting and higher temperatures, begins producing gibberellic acid (GA), which migrates to a special layer in the seed called the aleurone layer. Aleurone layer is stimulated to produce amylase, a digestive enzyme. This breaks the starch in the endosperm down into sugars that are assimilated by the embryo. Beer industry uses barley seed as a source of sugar for the yeast, and often puts in GA to stimulate the conversion of starch to sugar. 3. Cytokinins - a nucleic acid a. people doing tissue culture work were having a problem getting their cells divide. Began looking for a cell division hormone. b. discovered accidentally in Folke Skoog's lab by a postdoc, Carlos Miller, degraded herring sperm DNA. Suggested compound was a nucleic acid. c. eventually found to be a kinetin. Zeatin is one from corn. Coconut milk is liquid endosperm, and contains cytokinins. It was known to cause cells to divide. d. what do cytokinins do? 1. stimulate cell division - without cytokinins, cells might duplicate their DNA, but the cell does not divide, resulting in cells with multiple copies of their DNA. 2. delay cell senescence - cells sprayed with cytokinins remain viable for a longer period of time. Some insects that build galls on or in leaves synthesize cytokinin analogs that keep the plant tissue around them alive viable for a long time. Pretty neat co-evolutionary adaptations. 4. Abscisic acid (ABA) - another acid. a. a research group in England found that sap from birch trees, if fed to lettuce seed, was non-inhibitor in the spring. But sap from trees in the fall prevented the seed from germinating. Suggested a germination inhibitor was being produced in the fall. b. this group thought that inhibitor might be a dormancy hormone, so they named it dormin. c. another group in the US was studying leaf abscision. They isolated a compound they called abscisin. d. each group found their compounds within a few days of each other. When they compared notes, they found they had isolated the same compound. Eventually, they decided on the name abscisic acid. e. what does ABA do? 1. closes stomata - under water stress, leaves produce ABA, which causes stomata to remain closed. Does this by not allowing K+ to build up in the leaf - the leaf remains leaky to them, and hence, no osmosis, no turgor. Prevents excess water loss when there is drought. 2. maintains seed dormancy - as seeds dry out (think of it as controlled water stress), the seed produces ABA. As long as the ABA is there, no germination. But when wetted, the ABA is metabolized away, and the seed can germinate. Helps the plant make sure it's seeds germinate at the right time of year. 5. Ethylene - only gaseous hormone (H2C=CH2) a. produced from any cell after the plant is disturbed (wounding, shaking, etc.). b. requires oxygen and ATP to make c. causes: 1. leaf abscision - although it was thought originally that ABA caused leaves fall off, it now turns out that the responsible agent is ethylene. Works at very low concentrations (1-5 parts per billion!). 2. induces flowering in Bromeliads - pineapples are bromeliads. When smoke nearby sugar cane field in Hawai'i drifted over the fields, it caused all the plants to flower synchronously. This saved a lot of labor costs in the harvesting of pineapples. Now, they are sprayed with a compound that the plants metabolize into ethylene. 3. fruit ripening - as fruits ripen, they go through a stage called the climacteric. Here, respiration rates go way up, and digestive enzymes break down the unripe fruit, convert starches to sugars, and make the fruit soft and sweet. This whole process is initiated by exposure to ethylene. By controlling this in large warehouses, we are able to have ripe apples in the winter. To prevent ripening, warehouses are depleted of oxygen, and the temperature is lowered. To begin ripening, workers raise the oxygen levels and temperatures. D. Flowering 1. Plants tend to flower at certain times of the year. a. long day plants (LDP) - flower when the days are long, nights short. b. short day plants (SDP) - flower when the days are short, nights long. c. day neutral plants - flower anytime. 2. But studies in the 1930's and 1940's showed it was not the day that was important, but rather, the night! Long day plants should really be called short night plants, and short day plants should be called long night plants. Here is a summary of some experiments and whether they induce flowering: D = 1 hour of day; N = 1 hr of night; V = vegetative; F = flowers. The "O" represents a short burst of light in the night or a short burst of dark in the day.
Photoperiods
LDP SDP
Summary of results: for LDP plants, any photoperiod that has a short night, whether naturally, or if interrupted by a burst of light (thus creating two short nights),will flower. What happens during the day does not matter. For SDP plants, any photoperiod with a long night will result in flowering. Again, what happens during the day is irrelevant. 3. How does
the plant sense daylength?
660 nm light------>>>> P660----------------->>>>>>
P730
b. at night, P730 reverts back to P660, but slowly
over several hours.
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