What are VEGETABLE HORMONES and how are they classified?

The multicellular organisms with specialized tissues they need to have some kind of messenger that allows cells of different organs and tissues to communicate with each other in order to coordinate their actions. These messengers are chemical compounds They are called hormones. Hormones are present in higher organisms, both animals and plants. In this lesson from a PROFESSOR we tell you what are plant hormones and how are they classified.

Index

1. What are plant hormones? Easy definition
2. Classification of plant hormones
3. Cytokines: Germination
4. Auxins: Vegetative growth
5. Gibberellins: Vegetative growth and flower initiation
6. Ethylene: Reproduction and maturation
7. Abscisic Acid: Maturation and aging

Plant hormones o phytohormones They are chemical messengers that plant cells use to control their development and adapt it to environmental conditions. These plant chemical messengers are defined as hormones since they meet the two requirements that define this type of compound:

1. The center of synthesis is different from the place of action of hormones. That is, hormones are synthesized by a tissue or organ and cause a certain effect in a different tissue or organ.
2. The effect caused by a hormone is proportional to concentration to which you are.

Thus, the synthesis of hormones by a certain type of cell, the variation of its concentrations or their degradation provide information to other cells that have active receptors for such information.

Hormones acquire special importance in the plant organisms since it is about sessile organisms (which are fixed in the middle and cannot be moved). This condition makes them especially sensitive to adverse conditions since they cannot escape from them.

Thanks to the existence of a great diversity of hormones, vegetables have an elaborate response system to environmental stress, be it caused by other living organisms, such as predators (herbivores) and pathogens (fungi, bacteria, viruses or parasitic insects); or by adverse environmental conditions that negatively affect its development (drought, salinity of the water or soil, temperature fluctuations).

Image: Garden Adeniums

Plant hormones they act sequentially throughout the life cycle of the plant, so that in each of the stages one group predominates of hormones involved in the control of different processes of growth and tissue differentiation.

Inside of classification of plant hormones Two large groups of phytohormones can be distinguished:

Growth hormones

• Cytokines
• Auxins
• Giberalines

Stress hormones

• Ethylene
• Abscisic Acid

To better understand the action of the different groups of plant hormones, in the following sections We will discuss the different groups in the order in which they exercise their function throughout the biological cycle of the plant.

They are the predominant plant hormones in the first stage of the plant cycle, in which it is produced germination and rooting of the vegetable. They are very abundant in meristematic tissues. Meristematic tissues are embryonic tissues capable of growing and differentiating into specialized tissues. In particular they are found root hair level (root meristematic tissue).

Once the development of the roots has begun, the cytokines move towards the upper parts of the plant where they produce the stem and leaf growth. These new tissues produce a new type of hormone, which will be predominant in the new stage of the plant's cycle: Auxins.

Cytokines are also important anti-senescence agents, because high concentrations of this type of phytohormones prevent the levels of abscisic acid from growing, which is responsible for the aging processes of vegetables.

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They are plant hormones specialized in cell differentiation processes. Auxins stimulate cell division, growth and tissue specialization processes.

The synthesis of Auxins by newly formed tissues gives rise to the beginning of the second stage of the biological cycle of the plant: the stage of vegetative growth. Especially high concentrations of auxins are found in those areas of plants where growth is important, such as the apical parts of the stems and roots.

Auxins are produced by the meristematic tissue of the new leaves and descend towards the lower parts of the plant where they combine their action with that of cytokines. Depending on the ratio between auxins and cytokines, one type of growth or another is produced. If the auxin ratio is higher cytokine predominates root growth. In particular, auxins play an important role in the development of secondary roots.

If the ratio reverses and there is higher proportion of cytokines which of auxins then predominates foliage growth. In addition to causing vegetative growth, auxins play a fundamental role in the distribution of food to all areas of the plant. On the one hand, they intervene in the processes of vascular differentiation (formation of the conducting vessels of the phloem and xylem) and also direct the mobilization of nutrients from the roots to the aerial part of the plant.

Regulation of tropisms (Gravitropism) It occurs when the roots of the plants lose their verticality, they adopt new positions in relation to the force of gravity. Although auxins predominate in the vegetative growth stage, they take part in the regulation of all stages of the life cycle of the plants. We must remember that plants do not have a limited growth period, as happens with animals, but that they grow throughout their entire lives.

Gibberellins intervene in the vegetative growth stage along with auxins, which have started the process. The movement of auxins from the leaves to the roots causes the initiation of gibberellin synthesis.

The main effect of gibberellins is to enhance growth in height. When gibberellins become the major phytohormones in the plant organism, they start theflowering and reproduction stage.

Ethylene regulates the reproduction process and the maturation of the fruits of the vegetables. Vegetables produce ethylene in two different circumstances:

Physiological ethylene

is the one produced by the plant under normal conditions during the flowering and fruit formation stage. Ethylene causes the displacement of different auxins, which control the distribution of food between the different parts of the plant, to ensure the arrival of nutrients to the developing fruits.

Ethylene is produced by the fruit itself in formation and triggers the ripening process of this one. As the plant ages, the amount of Ethylene increases and causes the synthesis of Abscisic Acid that will give rise to the processes of the final stage of the biological cycle of plants.

Ethylene by stress

Ethylene synthesis can also occur under unfavorable conditions for the plant. It has been observed that plants growing on hard and compact soils the roots produce Ethylene that accumulates in and around the rootThis causes the roots to stop growing in length and do so in thickness.

In the final stage of life cycle of vegetables, the predominant plant hormone is Abscisic Acid. This hormone is produced mainly by the roots and regulates ripening processes (along with Ethylene) and senescence or aging of the plant. In the same way that happened with Ethylene, we can distinguish between abscisic acid that acts under normal physiological conditions and abscisic acid produced under stress conditions.

• Physiological Abscisic Acid: The senescence process has two different aspects, depending on the parts of the plant involved in the process. Senescence can affect certain tissues and organs as they mature (leaves, flowers and fruits) or the plant as a whole. These are programmed cell death processes mediated by the presence of Ethylene and causes the leaf fall (absition) and the seed dormancy (state of inactivity in which the seeds are dormant seeds, a period in which their germination is inhibited).
• Abscisic acid by stress: Under stress conditions, Abscisic Acid migrates rapidly from the roots, where it is synthesized, to the leaves, causing a series of Effects aimed at minimizing the possible damage caused by adverse conditions: the stomata of the leaves are closed to prevent moisture loss, reduces the level of Auxins, stops the growth of leaves but not roots and causes dormancy seeds.

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