Plant hormones or ‘phytohormones’ are chemical signals that control plant physiology, including cell division and differentiation. Without phytohormones, plants would just be clumps of undifferentiated cells. Plant cells synthesize, store, and exchange these signalling hormones. There are several classes of phytohormones, defined by their chemical structure. Within each class there are various hormones with similar effects on plant physiology.

Hormones move within plant cells through diffusion and vascular tissue. Ethylene, a gaseous hormone involved with senescence and other developmental processes, can be transmitted through the air. The concentrations of hormones in plant cells fluctuates to provide a dynamic response to environmental conditions.

Auxins and Cytokinins

The main hormones responsible for plant morphological responses are auxins and cytokinins. Auxins are usually developed in shoots and transported to roots, where cell division and root development is the focus. This allows mediated distribution of auxins to various parts of the plant. Generally speaking, auxins promote cell elongation and inhibit lateral branching. Auxins also serve an important role in a plant’s ability to move towards light sources. Auxins gather in higher concentrations on unlit areas of the plant, resulting in elongation of shaded plant cells and bending toward the light. This process is continuous and responsive, allowing plants to track the path of light throughout the day.

Cytokinins play a fundamental role in plant organization and structure. Cytokinins generally promote shoot development while suppressing root development. Cytokinins develop in root cells and migrate to shoots through the xylem. They are also developed in shoots and migrate to roots through phloem tissues. Cytokinins also impact various root functions including nutrient signalling and uptake.

Put simply, auxins promote roots, while cytokinins promote shoots. When both hormones are present and balanced, both root and shoot development occurs. Other plant hormones, including gibberellins and ethylene, play ancillary roles in plant morphology. The relationships between these signalling hormones and systems is complex and not yet fully understood.

Auxins are involved in:

• Cell division and elongation
• Leaf formation, senescence and abscission
• Root formation and development
• Apical dominance; favouring vertical growth
• Fruit and flower development
• Phototropism and geotropism/gravitropism (plant response to light and gravity)

Cytokinins are involved in:

• Promoting cell division, cell enlargement and differentiation
• Promoting the development of adventitious roots and axillary buds, leaf formation and enlargement
• Breaking seed dormancy by increasing metabolic activity
• Reducing apical dominance
• Delaying senescence (leaves treated with cytokinin will stay greener for longer)
• Stimulating morphogenesis in tissue culture (shoot initiation/bud formation)
• Nutritional signalling

Gibberellins

Gibberellins are growth hormones that act as general plant growth stimulants. They promote germination by interrupting seed dormancy, they drive flower initiation, promote bud and trichome development, stem elongation, and root development. Gibberellins also help plants to respond to environmental stress. They may also promote male sex expression in plants.

Gibberellins are involved in:

• Cell elongation
• Germination
• Flower induction
• Trichome development
• Bud development
• Delaying senescence
• Root development
• Response to environmental stress
• Sex expression

Abscisic Acid

Abscisic acid works to inhibit growth, counteracting and balancing the effects of other phytohormones. It was named abscisic acid because it was originally thought to be responsible for abscission, however it was later discovered to play only a minor role in this process. It does, however, play a role in stress response and plant survival, preventing seeds from germinating in unfavourable conditions, and limiting growth during drought. Abscisic Acid promotes dormancy in seeds and buds and inhibits ripening in fruit. It also inhibits photosynthesis, delays cell division and reduces transpiration.

Abscisic Acid is involved in:

• Inhibiting shoot growth
• Promoting seed dormancy
• Stimulating the closing of stomata
• Halting transpiration
• Environmental stress response
• Inhibiting fruit-ripening
• Anthocyanin (colour) development
• Abscission of flowers and fruits
• Leaf senescence
• Promoting rooting

Ethylene

Ethylene is a gaseous phytohormone thats occurs naturally through the breakdown of methylene and is produced in all parts of the plant, particularly in cells undergoing senescence and in ripening fruit. Its action counteracts the effect of auxins, triggering the ageing process in plant cells. Its primary role is to promote senescence and the ripening of fruit. It also increases petiole length and internode distance. Ethylene plays a role in breaking seed dormancy and promoting germination.

Ethylene is fundamental to sexual expression in many plants, particularly those with female flowers that require more ethylene to develop than male flowers. Ethylene-inhibiting agents such as Sliver Tiosulphate can induce male flowers on female plants to create feminised seeds.

Ethylene is involved in:

• Stimulating and regulating fruit ripening
• Inducing seed germination
• Stimulating leaf senescence
• Inducing leaf abscission
• Enabling plants to survive in low oxygen environments (eg. flooding)
• Stress response, particularly to salinity
• Increases petiole and internode length
• Sex determination
• Female flower development

See A Guide to PGRs in The Grow Room to learn more.