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Mode of Action of Phytohormones

Phytohormones seem to act through modification of gene expression. The stimulating effect of auxins on stem growth has been shown to be strongly inhibited by an antibiotic, actinomycin D. Actinomycin D exerts its inhibitory influence in a precise way, that is, it inhibits the transcription process in protein synthesis (the synthesis of RNA). The inhibition of auxin action by actinomycin D suggests that at least one effect of auxin is dependent on the transcription process. This has been further verified by the discovery that when isolated plant nuclei are treated with IAA, a marked increase in the synthesis of RNA follows. However, when actiomycin D is added, this increase in RNA is prevented. The effects of gibberellins act by activating genes in cells. As in the case of auxins and gibberellins, evidences indicate that at least some of the effects of cytokinins are also brought about by selective gene activation. However, most of the actions of phytohormones are due to interaction among various growth regulators and often it becomes difficult to associate a specific response with a particular phytohormone.

Regulation of Plant Development

An adult plant develops from the zygote by a series of steps such as the formation of the embryo, embryonic development which starts with the germination of the seed and gradually the seedling developes into a mature plant ready for reproduction, once again. Developmental changes do not stop at that level. Gradually the plant undergoes what we call, ageing and becomes old and senescent leading ultimately to natural death.

In most plants three definite phases can be recognised in their whole life cycle. These are (a) seed germination and vegetative growth, (b) mature reproductive phase and (c) senescence and death. All these phases in the life cycle of a plant are controlled and regulated by both internal and external factors and a particular developmental process is often regulated by more than one growth regulator. In many species the process of development is under tight control from the time the seed sprouts until the plant dies.

Seed Germination

In flowering plants, fertilisation of the egg nucleus occurs within the embryo sac (refer to the earlier chapter for details). The embryo sac and the surrounding tissue give rise to the seed and during these developmental changes the embryo arises from the zygote. Once the embryo is formed it stops further development and remains in a state of suspended animation. This situation is called dormancy. The dormant seed loses water and all its metabolic activities come to a stand still. At this stage the seed is separated from the parent plant and dispersed. The seed will show further development only when conditions favorable for germination are provided.
  1. Conditions of Germination
    Most seeds germinate in the presence of adequate supply of water, a suitable temperature and an adequate supply of oxygen. In some species seed germination occurs even in darkness. However, in certain seeds the dormancy is broken only with additional signals from the environment.

    The range of suitable temperatures for germination also varies from species to species. For example, wheat grains germinate between 1 and 35°C and maize grains between 5 and 45°C. The optimum temperature for germination is about 17°C for wheat and 22°C for maize.

    Seeds of some plants require higher oxygen content and those of others can germinate even in the absence of oxygen. The seeds of some hydrophytes germinate even if they are completely submerged in water. In peas, as the seed coats are impermeable to oxygen, respiration during germination is generally anaerobic until the seed coats are ruptured.
  2. Imbibition of Water
    The intake of water by the dehydrated seed is the first step in germination. This is called imbibition. This is followed by hydration of cell walls and protoplasts. Imbibition generally takes 1 to 3 hours, after which hydration of the embryo and endosperm occurs. This leads to an increase in respiration and other metabolic processes.
  3. Mobilisation of Food Resources
    Food reserves in the seed are in the form of complex polymers such as proteins, nucleic acids and polysaccharides like starch. These complex substances cannot be utilised by the embryo from the surrounding storage tissue (endosperm) unless they are digested to their monomeric units, such as amino acids, nucleotides and glucose. As the embryo becomes active during germination imbibing water, it secretes gibberellins. Gibberellins trigger a crucial series of events resulting in the breakdown of proteins into amino acids. These, in turn, are used in the formation of digestive enzymes such as amylases, proteases and ribonucleases for the breakdown of starch, proteins and nucleic acids respectively. As a result of the actions of these enzymes as well as the others, amino acids, sugar and nucleotides are released, which the embryo utilises for its metabolism during development.
  4. Process of Germination
    The first external evidence of seed germination is the emergence of radicle in dicots and the emergence of plumule in monocots. The plumule in dicots emerges slightly later in some species and much later in some others. In most dicots and in some monocots the hypocotyl elongates and raises the cotyledons above ground which is called epigeal germination. However, in most monocots and some dicots such as peas and oaks, the hypocotyls do not elongate and the cotyledons remain underground (hypogeal germination).

Patterns of Early Shoot Development: (a) Corn (b) Bean (c) Pea

  1. Phytochromes
    Of all the factors that help in breaking seed dormancy and initiate the process of germination, light seems to be the major influence, at least in some plants. Light is known to prevent or retard the germination of seeds of tomato, onion and some lilies. In lettuce seeds, exposure to red light promotes germination whereas if far red light exposure is given immediately after red light treatment, there is almost complete inhibition. However, if the seeds are again treated with red light, germination is again set in. These studies may be represented as follows:
    R (red light) ...........................................germination
    R + FR (far red) .......................................no germination
    R + FR R................................................germination
    R + FR + R + FR.......................................no germination

    In other words after repeated red, far-red light treatments it is the last treatment which determines the seed germination response. If red, they germinate; if far red, they remain dormant. Such red-far-red reversibility is observed in many other aspects of plant development.

    The basis of red, far-red effect on plants is the existence of a bluish protenacious pigment called phytochrome. Phytochrome exists in two interconvertible forms: Red light converts this pigment into one form and far-red converts that form into the other. The form of the pigment which absorbs red light is called Pr. By absorbing red light Pr is converted into Pfr, the far-red absorbing form of the pigment. Pr is stable in darkness whereas Pfr gradually disappears once it has been formed. Some of it is converted spontaneously in the dark to Pr; but another fraction is destroyed. Phytochromes have been found to be involved in a number of biological effects in plants. Apart from their influence on seed germination, they also have effects on the overall form of the plant, conversion of the plant to the reproductive state, movements of leaves and others. The mechanism by which phytochromes bring about their many effects is unknown. The current speculation is that they may regulate the movements of certain ions into and out of cells, specifically, sub-cellular organelles. Phytochromes may be functioning as 'channels' for movement of ion through membranes.

Spontaneous Conversion of Some Molecules in Dark
Behaviours of Phytochromes

Vegetative Growth Phase

Juvenility: With the emergence of the seedling above ground level, further development leads to the vegetative growth of the plant. This phase in the developmental history of plant life is also called juvenility which begins with the young seedling and lasts until the plant begins reproductive development. Juvenile plants have a high rate of anabolism and growth and differ from their mature counterparts in size and shape of leaves, growth patterns of stems and general absence of abcission.

The vegetative growth phase is characterised by a high metabolic rate. One of the other striking morphogenetic features of this phase of plant development is the difference in the leaf morphology when compared to that of the mature plant. In the garden bean plant, Phaseolus vulgaris, juvenile leaves are simple and opposite but the adult leaves are alternate and trifoliately compound. In certain plants such as cotton and morning glory, juvenile leaves change to adult stage through a large number of intermediate morphological forms at successive nodes. On a particular branch middle nodes will have adult leaves, whereas nodes at the base and those near the apex will posses juvenile leaves. Such a condition where morphologically different, juvenile and adult leaves are borne on the same plant is known as heterophyllous and the process heteromorphic development.

The juvenile leaves also differ with regard to their time of abscission. In the beach tree, for example, juvenile leaves fail to form abscission layers and remain intact on stems until growth is resumed next spring, but the leaves on mature branches abscise in the autumn. In general, as the juvenile branches continue to grow, they attain maturity in due course. In fruit trees also, juvenile and mature braches can be recognised at the same time. Besides leaves, other anatomical and morphological differences also exist in juvenile and mature plants.

Based on grafting experiments, it has been suggested that juvenile plants have some translocatable substance which can induce juvenility in a mature plant. Presence of strong geotropic response, apical dominance and capacity for formation of adventitious roots in juvenile plants suggests that they might have higher auxin content in them. Since the hormones are known to act through selective gene activation, the basis of the difference between the juvenile and mature stages of plants may be a shift in different kinds of mRNA and enzymes, regulated by the hormonal balance.
The onset of the maturation phase in plant development is characterised by reduction in vegetative growth and initiation of reproductive structures such as flowers. It has been found that the vegetative growth could be prolonged by the removal of flowers and floral buds. It is probable that certain substances produced in flowers or floral buds act as inhibitors of vegetative growth. As the plant enters the maturation phase, the overall auxin and gibberellic acid content in the plant body declines. On the contrary, the application of gibberellic acid and auxin at the mature phase of certain plants can convert them to the juvenile phase. These facts indicate that the onset of maturation is dependent on the levels of phytohormones in the plant body.

The duration of mature phase varies from species to species. In annuals and biennials, it lasts only a few weeks but in perennials, it is a matter of several years. In tree, for example, no sign of senescence is noticed for several hundred years. However, a few branches may senesce and abscise, and are replaced by new ones. In general, a change from maturity to the senescent phase is more gradual and less marked in woody plants. Nevertheless, in herbaceous plants, especially annuals and biennials the transition may be quite abrupt.


The transition from maturity to senescence generally involves few morphological changes and more of degenerative and deteriorative processes. Herbaceous perennials show what is called 'top senescence' wherein aerial shoots die annually but the roots and crown (the stem portion immediately above the roots) remain mature for several years. In woody perennials, the roots and aerial stems retain maturity for years, but the leaves and reproductive organs become senescent and die. Deciduous plants exhibit senescence and death of their leaves every autumn whereas the evergreen species retain their leaves for several years, after which they gradually undergo senescence and death.

Senescence in leaf, begins after it has attained full size. The first step is a decline in photosynthetic rate, then a decline in respiration followed by outward translocation of solutes, thus causing a decrease in dry weight. Hydrolysis of fats, proteins and starch exceeds their rate of synthesis. Chlorophyll breakdown exceeds chlorophyll synthesis, and leaves gradually turn yellow indicating thereby that carotenoids persist longer. Eventually, carotenoids are also broken down into volatile terpenoids.

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