In this article, we will discuss about the development process of plants in detail.
Contents
Development of Plants:
If a callus tissue is grown in artificial culture media, it may continue to grow indefinitely without any tissue differentiation. This is growth without any development. The normal plant, however, shows both growth and development. The fertilised egg cell develops into an embryo, the seed develops into a seedling, the seedling produces vegetative organs for some time and then switches on to its reproductive phase, i.e., flowering, and ultimately the fruit is developed. When the reproductive phase is completed it may be followed in annuals by senescence and death.
The physiological factors, controlling the development of the plant, have only in recent times been critically analysed. The development of a plant from the vegetative to the reproductive stage is one of the commonest of all natural observations. In case of annuals, it is observed that only when the vegetative growth is completed, reproductive phase begins. It is apparent that vegetative growth and reproduction are largely antagonistic to each other.
In case of perennials, the plants produce vegetative parts during some period of their life history and reproductive organs in the other. The most important thing is that the two phases—vegetative growth and reproduction—do not usually take place simultaneously. It is only when one stops, the other begins.
Photoperiodism:
In all parts of the world except tropics and subtropics marked seasonal variations occur in the length of the daylight period.
The phenomenon of photoperiodism is the development of plants in relation to the length of day and night. It is one of the most notable of all reactions of plants to their environment.
The foundations of our present knowledge on photoperiodism were laid when Garner and Allard showed in 1920 that flowering of many plants could be induced or prevented simply by controlling the length of the daylight period.
In trying to carry out breeding experiments with a giant variety to tobacco, they found that the plants would not flower when growth is in the open in summer, although they would do so and profusely in the greenhouse in autumn or winter. After many unsuccessful attempts Garner and Allard were able to induce these tobacco varieties to flower profusely in the open during summer months by reducing the day-length to correspond with that which the plants get when they flowered in greenhouses in winter. The day-length period was artificially reduced by simply moving the potted plants into a dark room after exposure to the desired number of hours of daylight.
The realisation dawned on them suddenly that growth of the plant and formation of reproductive organs are strongly influenced by the length of the day, most plants having a particular optimum requirement in this respect—the photoperiod (duration of light). Some plants flowered most rapidly when the day-length was about 12 hours or less, others when the day-length was about 12 hours or more. Some plants belonging to the second group were found to flower even in continuous illumination.
As instances multiplied, it was found that plants could be classified into three broad groups:
Short-day plants (SDP):
They are really long-night plants (LNP); flowering best when subjected to night periods longer than a critical duration as in many tropical plants, cereals, tobacco, Crysanthemum sp., Xanthium sp., etc.; also most of the spring and autumn flowering species of temperate regions. It is a varietal character. All Aman varieties of rice are short day plants.
Long-day plants (LDP):
Flowering best if subjected to light periods longer than a critical duration. Most of the agricultural plants of the temperate regions are long-day plants e.g., wheat, potato; and also many summer-flowering plants of the temperate regions. Species growing in high latitude (60° and further north) are mostly long-day types. Potato is a long-day plant for flower formation but it is a short-day one for tuberisation. Economic cropping of potato can only be obtained if potato is grown in short-days.
Indifferent or day-neutral plants:
The flowering of plants of this group is independent of day-length. This group includes tomato, maize and a large number of tropical plants which have no known flowering rhythm, but which produce flowers all the year round. Flowering in tomatoes seems to be controlled by temperature effect—an alternation between night temperature of 15°G and a day temperature of 25°C, appears to be optimal for flowering in tomatoes. This phenomenon has rightly been termed thermoperiodism or thermo-periodicity. Other plants also frequently show similar responses. Aush varieties of rice belong to this group, flowering only after a fixed number (‘period’) of days.
The photoperiodic effect has been found in all types of plants, herbaceous, woody, annual, biennial, perennial and even in animals. Though it is true that the most important effect of day-length is upon the flowering of plants, the day-length or the night-length also affects many other phases of the physiology of the plant. Many trees do not enter their rest period in the autumn if long days are maintained instead of short days.
As a result, they fail to develop frost resistance and are readily killed by winter cold. Shedding of leaves in many plants (e.g., yellow poplar, Liriodendron) can be stopped by artificially illuminating the plants, i.e., giving long days in autumn or in early winter; the plants remain evergreen.
Short-day plants will flower even when exposed to long-day conditions, provided they have already been previously exposed to a sufficient number of short days. Long- day plants will similarly flower in short days if they have been previously exposed to a sufficient period of long days. This carry-over of the effect is called photoperiodic induction (or after-effect).
As has been said before long-day plants will flower even in continuous illumination and do not really require any dark intervals whatsoever for flower initiation.
In short-day plants, however, conditions are entirely different—a dark period is obligatory; however it needs a light period for photosynthetic purpose. The flowering of short-day plants in short days thus happens because the dark periods are long and certainly not because the light periods are short.
In conjunction with experiments by Hamner and Bonner and also by Cajlachjan, (1936) who called the hypothetical flower forming hormone, florigen, the general assumption was that, in all plants a specific substance is manufactured in the leaves, from whence it is translocated to the growing points, in which it induces the formation of floral initials. The rate of movement of this flowering stimulus from the leaves of certain plants has also bean measured.
In 1961, Lincoln, Mayfield and Cunningham claimed success (?) when they prepared an alcohol-extract at very low temperature from flowering Xanthium plants
44 faros] which induced flowering in similar plants under non-induced photoperiods. The degree of flowering, however, was only small, although a significant fraction, of the full potential. On this basis, Lincoln et al claimed the first preparation of a natural extract capable of imparting a reproducible flowering stimulus by external application. It was later claimed to have been substantiated by Carr (1964).
The active fraction isolated was acidic in nature and Lincoln and others (1964) felt that the acidic nature of the active fractions warranted assigning the name florigenic acid to this compound, which even now remains, by and large, clearly hypothetical.
For the flowering response light stimulus is apparently perceived by a phycocyanin- like pigment (distinct from chlorophyll). Recent findings also indicate that this pigment named phytochrome is chemically related to phycobilins of-algae. The active spectrum of flower formation is strikingly similar to the absorption spectrum of allophycocyanin. It seems certain that two forms of the same pigment are involved and the induction of photoperiodic response depends upon a reversible photoreaction of the photomorpho- genic pigment.
Phytochrome is a chromoprotein (biliprotein) which exists in two interchangeable forms—one form, R phytochrome, absorbs short-red light in the vicinity of 660 nm and is thereby converted into the other form, FR phytochrome, which absorbs in the far-red region—730 nm—and brings about the reverse reaction.
This may enable one response induced by red light or white light containing red be promptly cancelled by an exposure to far-red light or vice versa. However, in one direction, this reversion of FR phytochrome to R phytochrome can also proceed in the dark—by a slow, temperature-controlled, metabolic pathway.
At the end of the daylight period it is apparent that all the phytochrome is in the far-red absorbing form and the slow night reversion starts.
For a short-day plant, e.g., chrysanthemum, even a single flash of light (or exposure to red light) will prevent flowering response, for such an exposure to light, however brief, re-converts all the R phytochrome (formed at night prior to exposure) into the FR form thereby altering the effective concentration promoting flowering in short-day plants. The same light-flash treatment, as will readily be understood, induces flowering in long-day plants.
The same perceptive system has been found to be present in certain seeds, e.g., lettuce, tobacco, etc., which require light, particularly short-red light, for germination. Such seeds can be caused to germinate or prevented from germinating by alternate exposure to red and far-red light—the last exposure determining the response—if red, the seeds germinate; if far-red, germination does not take place.
It is imperative in all cases that alternate light exposures follow each other fairly promptly for cancellations to occur. If delayed, i.e., if the ‘trigger’ or the chain of reactions for a particular response has already been set in motion—it cannot be reversed.
The phytochrome system is also the sensing mechanism that brings about changes in the growth of a developing seedling—in the dark, a great elongation and no opening of leaf-buds, and in light, short stem internodes and expansion and unfurling of leaves. The same response follows on exposure to red light which can again be nullified by a prompt exposure to far-red.
Phytochrome was finally isolated from etiolated plant materials, e.g., pea, corn seedlings, spinach leaves, etc., in the spring of 1959. It is very different from chlorophyll and is largely proteinaceous in nature, most probably, as we already know, a biliprotein like allophycocyanin.
It is a pigment having a molecular weight of about 120,000 and is believed to be associated with membranous structures of all green plants from algae to angiosperms. There is good evidence that it controls permeability of metabolites across membranes. According to Borthwick and Hendricks, the signal of alteration in the level of metabolites is ultimately transmitted to the genome in an unknown manner which then responds by an alteration of its function, i.e., an alteration in the products of transcription and translation. According to Galston phytochrome is associated with the nuclear membrane.
Changes in the patterns of RNA synthesised in red and far red light have been observed. Antimetabolites which initiate RNA and protein synthesis also inhibit flowering.
As in the case of auxins, phytochrome exhibits both fast and delayed actions. The fast action is believed to be mediated through the conversion of phytochrome from one form to another due to acceptance or removal of H+; a membrane potential is developed resulting in the movement of protons. The fast reaction can be demonstrated within a few seconds. Several enzymes are inducible by phytochromes and many are activated.
The manifestation of physiological changes like emergence of radicle or plumule, biochemical effects like formation of the red pigment in tomato cuticle takes much longer time. The extreme example of the delayed effect is exhibited by the photoperiodic effect or flowering. The link between the fast and the delayed process is not understood at present.
How Does A Plant Flower?
As in the case of growth itself, there is strong evidence of a hormonal control of flowering. The response certainly occurs in the growing points, yet the growing points themselves (regions of response) do not have to be exposed to any particular day-length. The region of perception of stimulus for flower formation, the leaf, even a single one, if subjected to the necessary photoperiod (day-length) may induce growing points to flower.
It has been possible to transfer this floral stimulus from one plant to another by grafting. Certainly then, if the stimulus, is a chemical substance, it is synthesised in the leaf and translocated to the regions where flower formation takes place at the growing points. Some believe that the chemical substance in question is the same one that controls growth—hormones. Indoleacetic acid inhibits flowering in concentrations which promotes vegetative growth in general.
Gibberellins promote the flowering of usually long-day plants. In several plants cytokinins have also hastened flowering. Consequently it may appear that the naturally occurring hormones themselves or through their interaction with “flowering hormone” —whatever be its nature—are responsible for flowering.
Chailakhyan has modified his original florigen concept now to imply a balance between anthesins (flower forming substance) and gibberellins. More recently he has shown, along with Anton Lang and others that graft transmissible inhibitors, which ate produced under non-inductive conditions play an important role in the regulation of flowering.
A balance of flower forming and inhibiting substances proposed many years ago by Wellensiek and others has also been visualised. The inhibitor(s) has not yet been isolated. It is interesting to mention that level of abscisic acid is affected remarkably by photoperiods and has been shown to possess marked effects on flowering of plants. Other inhibitors may also be involved. There is a net appearance of florigen in the leaf which can be transported to the region of flower formation through the living tissues such as phloem.
And once the flowering hormone or florigen has reached the growing bud, differentiation begins almost immediately, transforming the vegetative bud into a floral one. Although the exact nature of these changes is not clear, we do know that an active bud or its equivalent is necessary. Evidences discussed previously and above thus show the existence of several processes in the photoperiodic response to flowering.
The sequence of stages may be summarised as follows:
There is no absolute requirement for the second high-intensity light process since plants will flower even in continuous darkness after the first photoperiodic induction. The suggestion is that florigen appears unstable until it is fixed in some manner by the second high-intensity light process following the dark period.
The existence of a universal, graft-transmissible flower-hormone (florigen) has been questioned in recent years.
It has been realised for some time that there are actually multiple paths to floral induction in many plants and this, perhaps, cannot be readily explained by supposing the universality of florigen concept (Evans, 1971).
On the other hand, the analysis of photochrome action in flowering, suggests that at least two separate photoperiodic processes, one favoured by high and the other by low FR-phytochrome level, are required for flowering in both LDP and SDP plants. The two processes may proceed simultaneously, however, in light sources, giving intermediate proportions of FR-form. Translocation studies (Evans, 1966, King and Evans, 1968) also suggest that there may be at least two primary photoperiodic stimuli for floral induction.
It seems, according to Evans (1971) that we may have to discard our present attractive concept of a universally-active single florigen. And the case for doing so is already pretty strong, according to Evans (1971).
Vernalisation:
Some winter varieties of cereals, e.g., wheat or rye must be sown in temperate regions in the early autumn in order to make them flower in the following summer. If they are sown in the spring they continue to grow vegetatively with abundant tillering but fail to come to ear or flower much later than those sown in autumn of the year preceding. They thus contrast with spring varieties which planted in spring, flower in the same year.
If, however, seeds of winter varieties are moistened with enough water to increase their weight to about 20-60%, allowing germination to begin but not such as to encourage rapid growth (when tips are just emerging) and kept at a temperature close to freezing, e.g., 0-5°C for about 3 or 4 weeks, they can be sown in the spring to flower at about the same time as the previously autumn-sown seeds.
This treatment is known as vernalisation. Plants requiring vernalisation are, of course, confined to temperate and polar regions. Thus vernalisation is an agricultural practice of treatment of seeds with low temperature in initial stages of seedling development, accelerating its development. This treatment induces a rapid development towards a physiological older condition and results in shortening of the interval between sowing and flowering.
The flowering factor, may be the hypothetical vernalin, can actually be translocated from a vernalised plant to a non-vernalised one and cause it to flower. An unvernalised plant of Hyoscyamus can also be brought to flower by grafting on to it a scion of the annual variety which does not require vernalisation. By grafting it has been possible to show that ‘vemalin’ is distinct from photoperiodic florigen. By vernalisation treatment, winter varieties of cereals can be made to behave as spring varieties which planted in the spring, flower and come to ear the same year.
Gregory and Purvis first put out the concept that growing tips perceive the vernalisation stimulus of low temperature. Wellensiek (1961-1970) from his studies with leaves and isolated roots, arrived more or less to the same conclusion; that dividing cells are necessary for the action of low temperature, no matter where they occur in the plant; so it seemed that dividing cells were prerequisites for vernalisation.
However, Chakravorti (1970) from his work with Linum, concluded that the cells of the fully formed epidermis are capable of perceiving the vernalisation stimulus. This has long been claimed by Efeikin (1958), working with sugar-beet, who showed that the perception of low- temperature stimulus, necessary for flowering, is not restricted to the local changes in meristems.
Lysenko stressed the distinction between growth and development; that they are not identical phenomena and advocated what was once known as the theory of phasic development. According to this theory the development processes of an annual plant consists of a series of distinct phases which occur in strict predetermined sequence. The onset of any one phase will only take place when the preceding phase is completed.
In winter varieties of wheat, low temperature is necessary for a certain phase of development, the thermophase, which must be passed before the next phase, the light-controlled photophase can begin (day-length being the controlling factor). Recent researches, however, indicate that the different developmental processes cannot be as clearly separated as suggested by Lysenko. Nevertheless it is true that flowering is undoubtedly influenced both by temperature and day-length to which the plants are exposed and vernalisation may accelerate a part of the course of development.
Vernalisation has been developed in Europe, particularly in Russia where the significance of this treatment has been demonstrated and found to be economically very profitable. In Russia, the severe cold during the winter months sometimes kills the autumn-sown winter cereals. Vernalisation method is now used extensively to avoid this severe loss due to killing of seeds.
The winter cereal seeds are now vernalised and sown like ordinary spring varieties. Other possibilities arising from the vernalisation treatment, e.g., introduction of a crop plant into a new area with a growing season shorter than that under which it normally grows, are being explored. Very satisfactory results have also been obtained in this line.
The actual temperature requirement varies considerably from species to species. The mechanism of this temperature effect is possibly controlled by a hormone and the name vernalin has been suggested for this unknown hypothetical substance. It is thought that vernal-in is either a precursor of florigen or a catalyser essential for florigen formation. Alternatively vernalisation reaction may simply consist in the removal of restriction to the production of the largely hypothetical flower hormone, florigen.
Some plants can be vernalised after an appreciable amount of vegetative growth has taken place. Biennial plants such as beet generally grow vegetatively during the first season and flower only after they have spent the winter in the soil. The low temperature stimulus is perceived by the plant in winter, persists within the plant and subsequently causes flowering in the following summer. The perception of the low- temperature simulus in the biennial is restricted to the apical bud and is independent of the leaves and can be transmitted to another plant across a graft.
The low-temperature requirement for vernalisation of buds or seeds may be determined by a single gene.
Vernalisation effect is, however, reversible. Completely vernalised rye or wheat may entirely revert to the normal condition on being dried for several weeks or simply by storing them under anaerobic conditions (devernalisation).
Polarity, Restitution, Correlation and Differentiation:
Polarity:
In the development of a plant (and animals) growth does not proceed at random producing a formless mass of living cells but is an orderly process that gives rise to specific three dimensional forms of organ or body. Growth in one region or dimension is related to growth in the others and thus the plant becomes an integrated individual. A notable feature of these bodily forms is the presence in them of an axis which establishes a longitudinal dimension for an organ or the plant.
Along the axis and symmetrically to it, the lateral structures develop. The two ends of poles or the axis are usually different both as to structure and physiological activity.
Thus a typical vascular plant has a major axis with the root at one end and shoot at the other, with lateral appendages, e.g., leaves, branches and lateral roots arranged symmetrically around it. Growth is usually more rapid parallel to the axis than at right angles, resulting usually, but by no means invariably, in an elongate form. These patterns appear very early in the development of an organism as the result of differences in growth or in planes of cell division. This characteristic orientation of an organism, which is typically bipolar and axial, is termed polarity.
Polarity may manifest itself in a variety of ways. The structures at the two ends of the axis are very much unlike as in the cases of root and shoot, stem end and flowering end and petiole and leaf blade. The movement of certain substances may take place in one direction along the axis but not in the other, thus showing polarity in physiological activity. Individual Cells show polar behaviour in the plane, of division and in the different characters of the two daughter cells formed.
By axiate polarity is then understood as the property of the plant or an organ which determines the contrast between apex and base. In higher plants, this polarity is already determined in the fertilised egg cell and once established it seems to be strictly followed. When a piece of stem is cut off the plant this will usually reform roots and often shoots. The roots are always produced at the basal end of the axis whereas shoots develop near the apical end.
This polarity seems to be due to polar transport of auxin in cells and tissues where it always moves from apex to base. This causes auxin accumulation at the base which favours root formation and a deficiency of auxin at the apex which favours shoot initiation. Thus it has never been possible to transform the shoot end into root end and conversely by simply inverting a plant or a piece of stem (except by treatment with morphactins).
Polarity in rare cases may be determined by external factors, in Fucus zygotes, rhizoids always form on the side opposite to the direction of the source of light and the first wall is laid down at right angles to this direction. If the zygotes are, however, exposed to polarised light, the rhizoids tend to develop in the plane of polarisation.
It has also been found that when Fucus zygotes are subjected to centrifugal force, rhizoids seem to grow out from the centrifugal pole. In Fucus eggs, even the shape of the egg-cell sometimes modifies egg polarity!
Restitution:
By restitution is commonly understood the formation of new organs which as a rule follows the mutilation of a plant and can take place in situations where no active growth would have occurred in an uninjured plant. Two types of restitution are commonly observed: (i) new development occurring at the wounded surface and (ii) at some distance from the wounded surface;
(i) The production of a lost organ from the injured surface though not uncommon in lower plants such as algae and fungi is certainly of very restricted occurrence in higher plants. Only tissues that are still actively dividing—meristematic or embryonic—are capable of this.
In higher plants, formation of new organs at the injured surface is most frequently seen in the growing points of roots. If the growing tip of a root is removed by a transverse cut, not more than 0-5 mm from the tip, the tip may be formed again. A longitudinally split root tip, tends to complete itself in such a way so as to give rise to two distinct growing points. This type of regeneration is, however, rarely seen in the growing points of shoot or in leaf primordia.
Recent evidences strongly favour the idea that all these regenerations of organs due to injury is controlled by the specific hormone—the wound hormone, traumatin (or traumatic acid)—formed at the site of injury.
(ii) The formation of organ at some distance from the wounded surface is of widespread occurrence in plant kingdom. In algae and fungi it is commonly seen that an organ which has been lost is soon replaced either by the formation of a new one in the vicinity of the wound or by the outgrowth of one which was in rudimentary and dormant condition.
In higher plants if the growing point is destroyed, a new growing point may be developed from the meristem above the younger leaf primordia. Restitution here is restricted to meristematic cells and is certainly hormone controlled. In other cases, older and fully grown mature cells may return to meristematic conditions and thus tissues may also be regenerated from mature, parenchymatous cells.
When water- conducting xylem vessels are sometimes interrupted, new xylem vessels may be formed from the neighbouring thin-walled cells to re-establish connection.
The phenomenon of restitution has great economic importance in horticulture. Plants of horticultural importance may be rapidly multiplied from cuttings without the aid of seeds. In Begonia leaves, the origin or restitution of shoots are commonly observed.
Correlation:
If roots or shoots are decapitated (the apex is removed) they fail to show any tropic response. If, however-, they are exposed to the stimulus, e.g., light, gravity, etc., and then decapitated (before any positive or negative curvature can occur), the stumps show a definite response. Thus the tip is necessary for the perception of stimulus, (of light, gravity, etc.) but not for the response, which takes place further down the tip. This control of the response of one part of a plant by another part (sensitive tip) is known as correlation.
Differentiation:
Cells in different organs of a plant though apparently different, are identical in the sense that they are totipotent and each cell theoretically, having the same set of genes as an embryonic cell, is capable of giving rise to a complete plant. When a stem cell of a cutting develops into the root initials or when a leaf cell in Biyophyllum develops into a bud capable of developing into a new plant, this becomes evident.
The reason why morphology of the different organs of a plant composed of cells containing the same number of chromosomes, having the same nucleotide sequence in its DNA and hence the same genetic information is so different, is that the DNA is differently repressed in different organs.
When a young tissue section is transferred to a medium containing all nutrients essential for maintenance of life activity (White’s or Murashige and Skoog’s medium), an undifferentiated mass of cells (a callus) grows and will continue to grow indefinitely on such a medium if transferred at regular intervals. However, if the ratio of some nutrients in the medium, particularly hormones like auxin and cytokinin, are altered, differentiation may start and plantlets may be produced, which may later grow into normal plants capable of flowering and fruiting.
However, all plants do not respond similarly; thus, while some may respond to kinetin, others prefer zeatin and still others may need benzyladenine. In the cases of auxins also IAA, NAA and 2, 4-D are not always equally effective. There are also plants which do not respond to any such treatment. While these studies indicate that hormonal substances play very important roles in differentiation, we have to know a lot more about their interaction and the cell components concerned with differentiation.
In recent years single cell and protoplasts are used extensively in probing the secrets of differentiation and other life processes. Success has been claimed with respect to fusion of protoplasts belonging to same and even different species or genera—although regeneration of intact plants has been difficult. Unicellular algal cells, plasmids and isolated DNA molecules have been incorporated in protoplasts and plant cells.
It has been claimed that the nif gene (responsible for nitrogenase activity) has been introduced in eukaryotic cells, but such cells have not survived through several generations. The cause of this failure is not known. Obviously many hurdles have to be crossed before such efforts become successful. We can certainly be optimistic, since progress in this field is very rapid.