Many factors can cause and affect cell growth in the plant such as external (environmental) and internal factors; one of the most important internal factors is plant growth hormones. Many hormones required for cell growth, such as auxins, gibberellins, brassinosteroids, ethylene, jasmonates, salicylic acid, strigolactones and cytokinins which able to accelerate or promote growth, but, some hormone-like abscisic acid has an adverse effect on growth which increases seed dormancy by inhibiting cell growth. Also, plant hormones are able to breakdowns dormancy for many plants and can alleviate abiotic stress (salinity, extreme temperatures and, drought,…) which led to enhance germination and improve growth for many plants, whether naturally occurring in the plant or by adding it to the plant in its artificially formed or in the form of bio- or nano-fertilization in order to increase the productivity and improve its efficiency under extreme conditions. Therefore, this chapter will highlight and will provide data for the positive or/and negative effect of these hormones on many plants to achieve a rapid germination method. It will also shed light on the relationship of these hormones to some enzymes to accelerate growth.
- plant hormones
- seed germination
- cell growth
Plant hormones (phytohormones) are not nutrients, but chemicals and not all plant cells respond to hormones, but those cells that do are programmed to respond at specific points in their growth cycle. The greatest effects occur at specific stages during the cell’s life, with diminished effects occurring before or after this period .
Plants need hormones at very specific times during plant growth and at specific locations. They also need to disengage the effects that hormones have when they are no longer needed. The production of hormones occurs very often at sites of active growth within the meristems, before the cells have fully differentiated. After production, they have sometimes moved to other parts of the plant, where they cause an immediate effect; or they can be stored in cells to be released later. Plants can also break down hormones chemically, effectively destroying them. Plant hormones frequently regulate the concentrations of other plant hormones [2, 3].
2. Importance of plant hormones
The small amounts of plant hormones promote, control, influence and develop the growth from embryo to reproductive development, also, stress tolerance and pathogen defense. According to the importance of plant hormones in this chapter will be divided into two main points: first: the effect of plant hormones on germination and growth of plants under internal or external suitable conditions, second: the effect of plant hormones on the germination and growth of plants under internal or external unsuitable environmental conditions.
2.1 The effect of plant hormones on germination and growth of plants under internal or external suitable conditions
Seed germination is attracted to the effective growth of the embryo when appropriate environmental conditions are present, leading to seed rupture and the appearance of a small plant. There are five basic steps to germination: water imbibition, enzyme activation, initiation of embryo growth, rupture of the seed coat and emergence of seedling, seedling establishment [4, 5].
In the second step stage of germination (enzyme activation), after the absorption of water through the natural openings in the casing of the seed and spread through the tissues of the seed, gibberellins which activate the formation of the hydrolytic enzymes, mainly α- amylase in the aleurone cells, which are responsible for hydrolysis of storage macro-molecules such as starch and proteins and convert them into available forms to the embryo, usage to increase in size, and raise the osmotic content of the seed, to increase water potential [6, 7].
In addition, plant hormones have an important role in plant growth not only germination such as cytokinins (CKs) which influence cell division, the formation of shoot and helping in delay tissues senescence [8, 9]. Also, the ratios of Cytokinins and auxins affect most major growth periods during a plant’s lifetime . Also, Peptide hormones, control of cell division, expansion, and play crucial roles in plant growth and development . Furthermore, gibberellins (Gas) strongly promote cell elongation in seedlings [12, 13]. It can also affect cell cycling in plant . Meanwhile, the responses of
For enhances and increases plant hormones production in the plants, many studies have proved the need to add plant hormones either directly (GA3, kinetin and cytokinins) [17, 18] or indirectly (humic substances, manures, magnetite, natural zeolites,
2.2 The effect of plant hormones on the germination and growth of plants under internal or external unsuitable environmental conditions
Sometimes even under favorable germination conditions (an adequate water supply, a suitable temperature and the normal composition of the atmosphere) seeds do not germinate. In this case, seeds are considered dormant. Seed dormancy is defined as an inactive phase in which the growth and development are deferred and the respiration is greatly reduced [28, 29]. Seed coat dormancy involves the mechanical restriction of the seed coat. GA releases this dormancy by increasing the embryo growth potential, and/or weakening the seed coat so the radical of the seedling can break through the seed coat. ABA affects the testa or seed coat growth characteristics, including thickness, and affects the GA-mediated embryo growth potential .
Hormones also can mediate endosperm dormancy: Endosperm in most seeds is composed of living tissue that can actively respond to hormones generated by the embryo. The endosperm often acts as a barrier to seed germination, playing a part in seed coat dormancy or in the germination process. Living cells respond to and also affect the ABA:GA ratio, and mediate cellular sensitivity; GA thus increases the embryo growth potential and can promote endosperm weakening. GA also affects both ABA-independent and ABA-inhibiting process within the endosperm . In addition,  concluded that the prevented germination of some seeds of tomato , iris , and some varieties of cabbage was due to the present of inhibitors (ABA, parasorbic acid, and coumarin) which cases distributed in plants and to possess the property of inhibiting seed germination and other growth phenomena [5, 34].
Plant hormones affect seed germination and dormancy by acting on different parts of the seed such as  found that the inhibitors in seeds of peach were at least one of the factors controlling in germination by preventing or retarding cell division of the radical. In
The promotion of germination by gibberellin and cytokinins has been demonstrated in many seed species [38, 39]. Ref.  treated the seeds of
Ref.  found that fresh seed of
The effect of gibberellic acid and cytokinins were also recognized on the germination of other plant species seeds. Ref.  studied the effect of Kinetin at different concentrations on the seed germination of
Plant hormones can also alleviate abiotic stress such as drought, extreme temperatures, and salinity [52, 53]. The action of these hormones in response to situations of stress can be developed through synergistic or antagonistic activities . Also,  concluded that the plant growth regulators like ABA, JA, and ethylene are involved in the regulation of the plant response to abiotic stress. Cytokinins are also able to enhance seed germination by the alleviation of stresses such as salinity, drought, heavy metals and oxidative stress [56, 57, 58, 59]. Ref.  found that GA3 plays an important role in the growth and metabolism of microalgae
The decreased cytokinin and gibberellic acid (GA3) and increased abscisic acid contents are often observed responding in plants subjected to environmental stresses [63, 64]. Exogenous application of plant growth regulators [such as cytokinin or antioxidants (ascorbic acid) , Moringa (
Also, bio-fertilization has beneficial microorganisms that increasing plant hormones, which led to enhances yield, plant growth and nutrient uptake under various environmental conditions such as salinity [73, 74, 75, 76], drought and low fertility supply [77, 78, 79], especially that some endomycorrhizal fungi (Arbuscular mycorrhizal fungi) have been proven to improve drought stress; they colonize bio-trophically the root cortex and develop an extra-metrical mycelium that helps the plants to acquire mineral nutrients from the soil particularly those, which are immobile. They can under drought conditions stimulate growth-regulating substances, increase photosynthesis, improve osmotic adjustment, optimize hormonal balance and enhance water uptake .
Numerous studies have found also, that it can be alleviation of salt stress on peanut ; on pumpkin plants ; on
This chapter was indicated by many studies that the plant hormones, including IAA, cytokinins, ethylene, gibberellins, and brassinosteroids, can positively affect seed germination and seedling growth, for many plants as mentioned previously in the chapter, under favorable conditions. While ABA has an adverse on affect seed germination and the growth.
Also, this chapter sheds the light on the important role of soil bacteria in the production of plant hormones or as an alternative in the case of the low rate of plant hormones in the plant, which led to hence seed germination, growth, and hence crop production.
In addition, this chapter provided many studies that prove that the plant hormones very important to overcome dormancy or growth under stress condition. Also, shed the lights on the importance of the exogenous application of plant growth regulators (cytokinin or antioxidants, Moringa leaves extract, humic acid, or seaweed extract, bio- or nano fertilizers) for enhancing the productivity of plant hormones which led to increased cell growth.
Finally, it can be stated that the plant hormones are essential for cell growth, whether under normal conditions or under stress conditions.
Conflict of interest
The author declares that she does not have any conflict of interest.