Plant Hormones and Auxin

Plant Hormones Overview

• Plant hormones are organic substances that, in minute amounts, modify a plant's physiological processes.
• Plant hormones are an assorted set of chemical substances influencing plant growth, development, and other physiological processes.
• Plant hormones are produced by a plant and active elsewhere than at the site of production.

Auxin

• Derived from the Greek word "auxein," meaning "to grow".
• Auxin is synthesized in apical meristems, developing seeds, and young leaves.

Auxin Experiment

• Light is responsible for triggering phototropism that is sensed at the coleoptile tip.

Polar (Basipetal) Auxin Transport

• Polar (basipetal) auxin transport involves a directional and active flow of auxin molecules through plant tissues.
• The protonation state of auxin is dictated by the pH of the environment because auxin acts as a weak acid.
• Polarity is established in the cell, with efflux carriers positioned asymmetrically on the plasma membrane, usually towards the base of plant cells, directing auxin flow towards the roots.

Nonpolar Auxin Transport

• Nonpolar auxin transport passes through the phloem in the assimilate, moving from synthesis locations, the source, to utilization points, the sink.

Auxin Actions: Tropisms

• Phototropism and gravitropism are auxin actions.
• A higher auxin concentration on one side of the stem typically causes elongation on that side.
• In roots, a higher auxin concentration inhibits elongation.

Auxin Actions: Embryo Development

• Gradients of auxin guide the patterning of the embryo into organs such as the shoot apex, primary leaves, cotyledons, stem, and root from the zygote's first mitotic division.

Auxin Actions: Vascular Tissue Differentiation

• Auxin controls cell differentiation of vascular tissue.

Auxin Actions: Leaf Development and Arrangement

• The formation of new leaves in the apical meristem is initiated by the accumulation of auxin.
• Already-developing leaves deplete the surrounding cells of auxin to ensure new leaves don't form in close proximity.
• Auxin controls the precise patterning of the epidermal cells of the developing leaf.

Auxin Actions: Root Initiation and Development

• Localized auxin accumulation in root epidermal cells initiates lateral and secondary root formation.
• Auxin stimulates the development of adventitious roots in numerous plant species.
• Regarding auxin concentration gradient, the production of new cells at the meristem is increased as you go up or at the tip.
• In the area of differentiation, as you go down, or at the region of differentiation, promotion of the elongation and differentiation of root cells

Auxin Actions: Shade Avoidance

• Parts of plants that can access sunlight experience cell elongation in a shade-avoidance response.

Auxin Actions: Interactions with Other Growth-Regulating Hormones

• Auxin is essential for other growth-regulating hormones such as cytokinin.
• Cytokinin promotes cell division, but only in the presence of auxin.

Auxin Actions: Apical Dominance

• The inhibition of axillary bud or lateral bud, formation results from the downward flow of auxins produced in the apical meristem.

Auxin Actions: Flowering and Fruit Development

• Auxin encourages flowering and fruit setting, followed by ripening.
• As seeds reach maturation, they liberate auxin into the surrounding flower tissues, which subsequently grow into the fruit enveloping the seeds.

Auxin Actions: Prevention of Abscission

• Abscission, regulated by interactions between auxin and ethylene.
• During the growing season, young leaves and fruits produce abundant auxin, suppressing ethylene activity to keep them attached to stems.

Auxin Actions: Interactions with Ethylene

• As seasons shift, auxin levels decrease, enabling ethylene to commence senescence, or aging.

Cytokinin

• Cytokinins are adenine derivatives promoting cytokinesis or cell division.
• Naturally occurring Zeatin is a cytokinin examples, kinetin is an example of a synthetic cytokinin
• Cytokinins are produced in root tips and other actively dividing young structures such as embryos and fruits, as well as in wounded tissue
• Cytokinins are transported through the xylem.

Cytokinin Actions: Seed Germination

• The endosperm of monocot seeds stores cytokinin zeatin
• Zeatin moves from the endosperm to the root tip, stimulating mitosis when the corn kernel germinates.

Cytokinin Actions: Plant Development

• Cytokinins are involved in leaf formation and delay leaf tissue senescence and affect chloroplast development.
• Impacts the effects of auxin when regulating shoot and root development, inhibit apical dominance by stimulating axillary bud development, and inhibit the formation of lateral roots while auxin initiates lateral roots

Cytokinin Actions: Plant Development and Callus Growth

• Shoots form when cytokinin is applied to a mass of cells, or callus and roots develop if the applied hormone is auxin.

Cytokinin Actions: Gravitropism

• The function of gravitropism is similar to auxin.
• Cytokinins build up at a root's lower side when turned on its side, impeding elongation
• As the upper surface elongates, the root bends downwards.

Gibberellin (GA)

• Isolated by Japanese researchers in the 1930s, gibberellin (GA) is a growth-promoting substance found in fungi cultures that parasitize rice crops.
• Gibberellins are synthesized in young leaves, seed embryos, root and stem apical meristems
• Gibberellins are transported through the vascular tissue.

Gibberellin Actions: Shoot Elongation

• Shoot elongation results from both cell division and cell elongation.
• When a little amount is applied to a bush or Dwarf bean , the stem starts to grow quickly.

Gibberellin Actions: Seed Germination

• Seed dormancy, a state of inhibited growth, is broken.
• Cold exposure or light is required to germinate.

Gibberellin Actions: Bolting

• Bolting correlates to increased daylength and temperature.

Abscisic Acid (ABA)

• Abscisic acid responds to stressful environmental conditions such as dehydration, cold temperatures, shortened day lengths, and exposure to salt water or salinated soils.
• Abscisic acid in root and leaves as they mature.
• Abscisic acid is transported through the vascular tissue.

Abscisic Acid Actions: Seed Maturation and Inhibition of Germination

• Abscisic acid is essential for seed maturation and enforced dormancy by preventing germination and supporting the synthesis of storage proteins.
• It is important that seeds do not sprout too early under conditions that do not favour the coming Winter or start of the dry season.

Abscisic Acid Actions: Bud Dormancy

• Promotes the development of winter buds; it encourages the apical meristem to become a dormant bud.
• New leaves that sprout above the meristem get modified to form stiff bud scales that will keep the meristem safe from mechanical harm and prevent it from becoming dry through winter.

Abscisic Acid Actions: Cellular Protection from Dehydration

• Abscisic acid responds to dehydration and regulates guard cells, closure and opening

Abscisic Acid Actions: Cellular Protection from Dehydration Gene Encoding

• Abscisic acid activates gene expression that encodes proteins that defend cells, in seeds and vegetative tissues, from the effects of dehydration.

Ethylene

• Ethylene is a volatile gas made up of a simple molecule.
• Ethylene is synthesized in aging tissues, such as those that are wilting or ripening, and nodes of stems
• Ethylene is transported via air as gas.

Ethylene Actions: Fruit Ripening

• During ripening, ethylene promotes the conversion of starch and acids into sugars.
• Unripe fruits, such as avocados, accelerates ripening when placed in a sealed bag, since the gas discharged speeds up the ripening of the remaining fruit.

Ethylene Actions: Abscission

• When auxin levels drop, ethylene starts senescence, causing cell death at the point where the stem connects to the leaf stem, a special layer of cells, the abscission layer, is what structures from the foot of the petiole or fruit stalk.
• The more distal separation layer and more proximal protective layer are only some of the portions, of the two parts of the abscission layer.
• Nutrients are transported into the stem to prevent wastage prior to abscission As the separation layer deteriorates, the leaf detaches and falls to the earth lightly, and the protective layer contains suberin to act as a seal.

Other Signalling Molecules

• Brassinosteroids are synthesized in young tissues and do not travel far from their synthesis location. They are important in a number of physiological and development processes.
• They have a positive effect on apical dominance, seed germination, gravitropism, lateral root formation, cell differentiation within the vascular tissue, and resistance to freezing.
• Inhibits fruit dropping and root growth.
• Systemin distributes itself effectively in the plant when fabricated, it is a polypeptide that triggers the plant's response to wound from pests that feed on the plant, stimulating the production of jasmonic acid.

Systemic Acquired Response

• Salicylic acid causes the systemic (whole body) response or (SAR) by infection or parasites.
• If a cell gets infected by a parasite or pathogen, a localized reaction occurs.
• This is referred to as hypersensitive response or (HR).

Oligosaccharins

• They are short chains of simple sugars that aid in plant defence when bacterial and fungal infections occur.
• Act locally in the injury site and can be delivered to other tissues.
• Enzymes, like glucanase and chitinase, can split the wall in which they release the oligosaccharins to stimulate the production of plant defence compounds and are found inside Cytoplasm

Strigolactones

• Promote seed germination in certain plan species and inhibits lateral apical development with assistance from auxins.
• Supports the creation mycorrhizae, which are symbiotic relationships between plant roots and fungi

florigen

• Florigen initiates flowering and is included within the organizing storage and structure of the plant.
• A systemic marker from leaves and goes up to the shot's meristem to encourage flowering. The system also responds to the time of year, or the cues regarding daylength.
• Florigen is a protein created from FLOWERING LOCUS T (FT) which is highly conserved within the plant.
• Regulation of flowering is important so that it allows for easier cultivation on ranches or farmlands and the flowering time will occur at a certain period.
• Extreme temperatures and unfavourable timing may diminish yield of the crops where photosynthetic growth or poor fertility is caused, such as heat stress, and poor replication.