Introduction to Phytohormones

A brief introduction to phytohormones.

Table of Contents

Click Here for the Related IntroNotes Article: Propagating Pothos & Why We Prune

Why do we prune? It helps promote more proximal growth and development of the plant, but how? The easy answer that “the plant can spend more energy on growing wide as opposed to tall”, whilst charmingly personifying doesn’t actually help to explain the increased rate of lateral growth. Furthermore it doesn’t really provide any mechanistic justification for not just simply letting your plant grow un-pruned until it has achieved the desired lateral development.

 And yet we see pruning the most distal parts seems to promote lateral growth. Why is that? To answer this, let’s have a quick refresher on plant endocrinology.

Plant Hormone Basics

There are five major groups of phytohormones (plant hormones) discussed in modern botany:

  • Abscisic Acid
  • Auxins
  • Cytokinins
  • Ethylene
  • Gibberellins

Below is a (very basic) description of what they do.

Abscisic Acid (ABA)

Abscisic Acid (ABA) is the chief water stress signal. When roots detect that water is at a critical level they release ABA into the plant vasculature. Similar to a human neurotransmitters and neural action potential functioning, ABA triggers an influx of Ca2+ and membrane depolarisation of stomatal guard cells (the cells which open/close stomata, the microscopic windows in a leaf which allow water and air to be breathed in and out of the plant). This results in efflux of Ca2+, K+, and associated anions. The loss of these ions, draws water out of the guard cells, mediating stomatal closure, reducing potential water losses by transpiration.

 At this point it is worth mentioning the topic of Meristems, locations of embryonic plant tissue capable of dividing and growing branches/roots. Meristematic tissue can be located apically (end of stems/vines/roots) or lateral (branch points).

Auxins

Auxins care about length over width. This is to say they promote apical dominance, longer stems and roots with less lateral/axillary budding. Produced chiefly in the apical meristems, Auxins signal for lateral buds (dormant embryonic meristems) to remain dormant, causing growth to be predominantly apical rather than lateral. Thus in the case of Epipremnum aureum pruning, we are removing apical meristems, promoting development of lateral buds, and overall fullness of the plant, rather than length.

 In a related manner, Auxin is the chief of phototropism, the curving growth of plants towards a light source. Produced in the stem tip, Auxin localises to the darker side of the plant. Thus, when the auxin promotes more pronounced growth on this side, the overall effect is to arch the stem towards the light. The same processes facilitates gravitropism (aka geotropism), the growth of root in the direction of gravity by gravitationally-dictated auxin distribution in the roots. This is the process which is being exploited by exogenous Auxin when industrial ‘Rooting Hormone’ is added to plants or cuttings.

Cytokinins

Cytokinins (not to be mistaken for human kinins of the Kinin-Kallikrein system) promote cell division and inhibit aging of greenery. Their function is far too complex to attempt even a basic discussion here, but in short, Cytokinins work in a system of balances and ratios with Auxin to drive embryonic patterning and differentiation, callus formation.

Ethylene

Ethylene (aka Ethene in organic chemistry, H2C=CH2)  is the cause of the proverbial ‘rotten apple’ spoiling the bunch, and ripe bananas ‘infecting’ green bananas with their ripe-ness. The only known gaseous plant hormone, Ethylene signals for the opening of flowers, the ripening of fruit, the shedding (abscission) of leaves, and ultimately rotting of proverbial apples. The culmination of a series of biosynthetic operations and the Methionine/Yang cycle (which gives me Citric Acid/Krebb’s undergrad war flashbacks), Ethylene is formed by the oxidation of ACC. Thus the agricultural world has found that keep produce in high-CO2, low-O2 environments reduces ripening and rotting.

Gibberellins

Gibberellins (GAs) main concern is internode elongation, with lesser roles in germination, differentiation, and embryonic patterning, and synthesised in areas of active plant growth. Plants with low/defective/inhibited GAs present with brachytic dwarfism (internodal shortening). This causes short stems and close clustering of nodes (and thus leaves/branches/flowers etc). Conversely GA excess results in long stems, sparse branching, and elongated plants. The understanding of this hormone plays an important role in forming high-yielding varieties of agricultural goods (dwarf cereals, elongated sugarcane).

Sowhenever we hear someone discussing plant-pruning in the name of lateral growth, we can thank our little friends Auxins and Gibberellins.


Banner image edited from photo by Mokkie 

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