Plant hormones-Morphactins and others
Morphactins are a group of substances which act on morphogenesis and modulate the expression of plants. Chemically, they are the derivates of fluorene compounds. Fluorene by itself is inactive, but the addition of COOH group in the 9th position makes it active. Majority of morphactins, as synthetic compounds, have very diverse effects on plant growth and development. Ex. Chloroflurenol, Flurenol, Methyl benzilate, Methyl chloroflurenol, Methyl dichloroflurenol.
Most of these components have transitory effect and they are degraded rapidly in plants. The half life varies from compound to compound and plant to plant. Moreover, cellular and environmental factors have greater influence on the stability of them. Sometimes, the applied morphactins may be modified by glycosylation or demethylation. They are translocated in plants basipetally as well as acropetally through both sieve tubes and xylem elements.
The effect of morphactins is very interesting. Particularly in the presence of other natural hormones, they exhibit both synergistic and antagonistic effects, which is however depends upon the relative concentrations. Generally, morphactins have adverse effects on plant morphogenesis. They inhibit seed germination, but sprouting, growth of seedling, internode elongation etc. In many cases, they depolarize cell division which probably leads to distorted morphogenesis. Morphactins are very effective in inducing lateral bud development so tillering will be profuse. Strangely, some morphactins stimulate flowering in certain short day plants.
In many respects, morphactins resemble ABA in inducing seed dormancy, bud dormancy, suppressing stem elongation, etc. Most of their effects can be reversed by GA3 treatment. The overall effect of morphactins appears to be polyvalent anti-regulators.
Other identified plant growth regulators include:
· Brassinosteroids - are a class of polyhydroxysteroids, a group of plant growth regulators. Brassinosteroids have been recognized as a sixth class of plant hormones, which stimulate cell elongation and division, gravitropism, resistance to stress, and xylem differentiation. They inhibit root growth and leaf abscission. Brassinolide was the first identified brassinosteroid and was isolated from extracts of rapeseed (Brassica napus) pollen in 1979.
Yi Zhang and John Turner at the University of East Anglia found that when leaves of the model plant Arabidopsis are wounded, cell division in the apical meristem is reduced, growth of the plant is arrested within days, and the new leaves grow to only one-half of their normal size although the size of leaf cells is unaffected.
Unexpectedly, the suppression of cell division in the apical meristem occurs through a signal pathway initiated by the wound hormone, jasmonate, which is synthesized in the damaged mature leaves. Mutant Arabidopsis lines unable to synthesize or to respond to jasmonate are not only larger than normal plants, but their growth is not reduced by the wound stress.
The Jasmonates are a group of plant stress hormones that naturally occur in plants following exposure to certain types of stresses, including pathogen and herbivore attacks. (+/-)-Methyl jasmonate is a mixture of trans (3R, 7R and 3S,7S) isomers. Methyl jasmonate induces the synthesis of proteinase inhibitors in plant leaves.1 In cancer cells, it suppresses proliferation and induces apoptosis.2 More specifically, methyl jasmonate inhibits hexokinase that is bound to mitochondria.3 As hexokinase is overexpressed in cancer cells and contributes to cancer cell growth and survival, methyl Jasmonates disruption of mitochondrial hexokinase activity selectively targets, and kills, cancer cells. Methyl jasmonate derivatives also have potential as anti-inflammatory agents.
There are approximately 60 steroidal compounds known as brassinosteroids named after the first one identified, brassinolide, which was found in mustard pollen. Effects include:
1. Stimulation of stem elongation.
2. Inhibition of root growth and development.
3. Promotion of ethylene biosynthesis and epinasty.
Salicylates have been known to be present in willow bark for quite some time. Salicylic acid is synthesized from the amino acid phenylalanine. Effects include
1. Thermogenesis in Arum flowers.
2. Plant pathogen resistance-stimulates plant pathogenesis protein production.
3. Reported to enhance longevity of flower (practice of adding a bit to cut flowers).
4. Reported to inhibit ethylene biosynthesis.
5. Reported to inhibit seed germination.
6. Blocks the wound response.
7. Reverses the effects of ABA.
Jasmonates are represented by Jasmonate and its methyl ester. They were first isolated from the jasmine plant in which the methyl ester is an important product in the perfume industry. Jasmonic acid is synthesized from linolenic acid which is an important fatty acid. Jasmonates have a number of effects such as:
1. Inhibition of many processes such as growth and germination.
2. Promotion of senescence, abscission, tuber formation, fruit ripening, pigment formation, and tendril coiling.
3. They appear to have important roles in plant defense by inducing proteinase synthesis- a defense mechanism against fungi
Practical applications of Plant Hormones:
Agriculturists all over the world have developed certain unusual methods by which they successfully cultivate the crop plants. It is only in recent years plant physiologists discovered how plant hormones can be effectives used in agriculture, horticultures, pomiculture and other related fields. As described earlier, plant hormones have a wide variety of effects and most of these responses are concentration dependent. Fortunately phyto chemists have also identified many synthetic hormones, some of which are more potent than natural hormones. Experimentation and experience have shown that the judicial use of hormones or combination of hormones can e employed in agriculture and related industries to get the maximum benefit. Quantity and quality of agricultural products are very important factors in the agricultural economics. How best the phytohormones can be utilized in this direction requires imagination and training.
There are many areas in agriculture, horticulture, pomiculture, moriculture, etc., where phytohormones can be used in successful cultivation to obtain greater yield. The high percentage of germination of sown seeds in the field has a bearing on the output. Pretreatment of seeds with IAA, NAA, GA, etc. has been found to be very effective not only in increased the percentage of germination but also in the total yield of the crop plants. Suitable concentration and combination have to be determined for each and every crop plants.
The overall growth of plants, number of tillers and branches that produce from every plant in the field contribute to the total yield. Use of GA or IAA greatly enhances the growth of plants and total area of leaf surfaces. Some morphactins can also be used to produce more tillers. In the case of sugar cane, use of GA has been found to increase the length of the internodes and also the sugar content.
Plants can be multiplied by vegetative propagation. Many horticultural plants are propagated by this way. The success of this method depends upon rooting. Hormones like NAA and IBA are very effective in inducing roots in stem cuttings. These hormones can also be used fro in grafting propagation. This way most of the plants can be propagated in large numbers and is quick time.
Use of hormones like IAA, NAA, IBA and Gibberellins ensures fruit setting and many of the fruits which develop from such hormone treatments are seedless, larger in size and sweeter in content. In many cases the total yield will be very high. Quantitatively and qualitatively such products yield more income to the farmer. Grapes, apples, oranges, mangoes and other fruit yielding crop plants can be treated with some of these phytohormones for the better yield. Furthermore, these hormones prevent premature falling of fruits, otherwise nearly 50-70% of the set fruits fail to mature and most of them fall off because of the formation of abscission layer in their stalks.
Preservation of agricultural products before marketing is another area in which hormones can be used effectively. Tubers, rhizomes, bulbs and such products sprout while they are stored. This will affect the farmer in terms of financial gain. Some of the synthetic hormones like NAA, maleic hydrazide can be used to preserve the said products for quite a period of time, thus one can improve the keeping quality of agricultural products.
Another judicial use which can be commercially exploited is the use of GA and other hormones in inducing flowering in unseasonal periods. But one should known which hormone is effective on which plant; otherwise phytohormones fail to produce the desired results.
Today farming is labour oriented and economically it is becoming impossible. Particularly removing weeds in the field is a nuisance, costly and time consuming. However specific synthetic hormones are available in the market to destroy the weeds selectively. 2, 4-D, 2, 4, 5-T can be used in paddy fields to destroy weeds. Similarly, monocot grasses can be destroyed by using specific weedicide hormones. Sometimes, water hyacinth and such water plants grow and multiply so fast they spread and establish their population in large tracts of tanks and streams. This causes considerable damage to water storing capacity of tanks. For example, 2 methyl 4 chloro 5 isopropyl phenoxyacetic acids can be effectively used to destroy water hyacinth and save the tanks. Thus one can use different hormones for different purposes, it can be for good or for bad, and it is like a knife with two cutting edges. It is left to the man who uses it.