Plant hormone
[10] Some phytohormones also occur in microorganisms, such as unicellular fungi and bacteria, however in these cases they do not play a hormonal role and can better be regarded as secondary metabolites.
Plant hormones are not nutrients, but chemicals that in small amounts promote and influence the growth,[13] development, and differentiation of cells and tissues.
Plants lack glands to produce and store hormones, because, unlike animals—which have two circulatory systems (lymphatic and cardiovascular) —plants use more passive means to move chemicals around their bodies.
Initial research into plant hormones identified five major classes: abscisic acid, auxins, brassinosteroids, cytokinins and ethylene.
[16] This list was later expanded, and brassinosteroids, jasmonates, salicylic acid, and strigolactones are now also considered major plant hormones.
Since it was found in freshly abscissed leaves, it was initially thought to play a role in the processes of natural leaf drop, but further research has disproven this.
Since ABA dissipates slowly from the tissues and its effects take time to be offset by other plant hormones, there is a delay in physiological pathways that provides some protection from premature growth.
Stress from water or predation affects ABA production and catabolism rates, mediating another cascade of effects that trigger specific responses from targeted cells.
The roots then release ABA, which is translocated to the foliage through the vascular system[19] and modulates potassium and sodium uptake within the guard cells, which then lose turgidity, closing the stomata.
[29] These released transcription factors then bind to DNA that leads to growth and developmental processes[29] and allows plants to respond to abiotic stressors.
They also help delay senescence of tissues, are responsible for mediating auxin transport throughout the plant, and affect internodal length and leaf growth.
Cytokinins counter the apical dominance induced by auxins; in conjunction with ethylene, they promote abscission of leaves, flower parts, and fruits.
[31] Among the plant hormones, the three that are known to help with immunological interactions are ethylene (ET), salicylates (SA), and jasmonates (JA), however more research has gone into identifying the role that cytokinins play in this.
[32] For example, pathogen resistance involving cytokinins was tested using the Arabidopsis species by treating them with naturally occurring CK (trans-zeatin) to see their response to the bacteria Pseudomonas syringa.
Cytokinin defense effects can include the establishment and growth of microbes (delay leaf senescence), reconfiguration of secondary metabolism or even induce the production of new organs such as galls or nodules.
Unlike the other major plant hormones, ethylene is a gas and a very simple organic compound, consisting of just six atoms.
As the new shoot is exposed to light, reactions mediated by phytochrome in the plant's cells produce a signal for ethylene production to decrease, allowing leaf expansion.
In numerous aquatic and semi-aquatic species (e.g. Callitriche platycarpus, rice, and Rumex palustris), the accumulated ethylene strongly stimulates upward elongation.
This response is an important mechanism for the adaptive escape from submergence that avoids asphyxiation by returning the shoot and leaves to contact with the air whilst allowing the release of entrapped ethylene.
[40] It was later discovered that GAs are also produced by the plants themselves and control multiple aspects of development across the life cycle.
It helps in the growth of the stem[citation needed] Jasmonates (JAs) are lipid-based hormones that were originally isolated from jasmine oil.
[47] Jasmonyl-isoleucine (JA-Ile) accumulates in response to herbivory, which causes an upregulation in defense gene expression by freeing up transcription factors.
It was originally isolated from an extract of white willow bark (Salix alba) and is of great interest to human medicine, as it is the precursor of the painkiller aspirin.
Like MeJA, methyl salicylate is volatile and can act as a long-distance signal to neighboring plants to warn of pathogen attack.
[50] Salicylic acid (SA) serves as a key hormone in plant innate immunity, including resistance in both local and systemic tissue upon biotic attacks, hypersensitive responses, and cell death.
Some of the SA influences on plants include seed germination, cell growth, respiration, stomatal closure, senescence-associated gene expression, responses to abiotic and biotic stresses, basal thermo tolerance and fruit yield.
[51] The result was that injecting SA stimulated pathogenesis related (PR) protein accumulation and enhanced resistance to tobacco mosaic virus (TMV) infection.
Therefore with increased internal concentration of SA, plants were able to build resistant barriers for pathogens and other adverse environmental conditions[53] Strigolactones (SLs) were originally discovered through studies of the germination of the parasitic weed Striga lutea.
It was found that the germination of Striga species was stimulated by the presence of a compound exuded by the roots of its host plant.
[69] Another derivative of SA, sodium salicylate has been found to suppress proliferation of lymphoblastic leukemia, prostate, breast, and melanoma human cancer cells.
