Series 3: Plant Hormones

Hello everyone! Welcome to one of my favorite areas ever in plant science - PLANT HORMONES! This is going to a fun one so thank you for reading! Anoushka, a member of Plantastic Science and regular reader has offered her amazing skills in this month’s article and I know that her insights have really helped shape it into a thrilling read. If you are interested in writing articles or getting involved, please do not be afraid to get in touch! Let’s get rolling….

Plant hormones, often known as phytohormones, are chemical signals. They are produced in a wide range of organelles within the plant, and are transported by extracellular fluids (tissue fluids) to other parts of the organism. Their role is to signal to specific parts of the plant to perform a certain function to aid its survival. This includes a wide range of functions, such as defending itself against pathogens (these are microorganisms that cause disease), developing plant organs or responding to environmental stresses. The roles of plant hormones are so diverse, so let’s delve a little deeper!

A Common Misconception

A common misconception is that plant hormones are nutrients. However, since nutrients are not generated by the organism and must be taken in from the environment, hormones are not nutrients but rather chemical signals. That being said, in the same way plants need a variety of different nutrients e.g. nitrates and phosphates to survive, they also need a wide variety of hormones too! So, let’s explore more.

Auxins

  Auxins are created when a protein called tryptophan undergoes a two-step oxygenating pathway ending with indole-3-acetic acid. What this means is that the protein has gained an oxygen or lost electrons. This process occurs in both the stem tips and in the roots. These auxins are then transported via the apoplast. This involves pH-based diffusion and has several facilitators in the cell membrane, some of which are nitrogen based. Auxin enters the cell nucleus and has affects the DNA. Some molecules called auxin response transcriptors (or ARFs for short), are able to turn certain genes on or off, which change the way a cell behaves. By being transported by the phloem, auxins can reach many parts of the plant.

   Auxins are the chief regulators of growth. This growth is dictated by the changes in the environment of the plant, known as external stimuli. For example, auxins help cause the downward growth of roots (called geotropism), which maximizes the water uptake by the roots. Phototropism is also heavily influenced by the presence of auxins. This is where the plant grows towards the light to maximize the process of photosynthesis. When light hits a stem, more auxins are transported to the dark side causing it to grow at a faster rate than the side exposed to the light. This means that the plant grows at an angle to expose a greater proportion of the plant to the light.

Apical dominance, the process by which a central, main root grows at a greater rate than lateral roots is also dictated by auxins!

Gibberellins

There are over 136 gibberellins (GA’s) with only 4 being bioactive. When they bind with calcium, they enter the cell DNA to start cell growth. They can be found wherever plant cell growth is occurring. There are differences in gibberellins between fungi, lower plants and more developed plants. Gibberellin concentration is reduced when the plant is under stress and temperature. The relationship between gibberellin and other plant hormones is also pretty amazing. Auxin levels and gibberellin levels are directly proportional whereas gibberellin and ABA have an inversely proportional relationship. The GAs work by inhibiting a repressor mechanism, DELLA, via a multistep pathway. They are synthesized in plant stem cells, called meristems, and are transported by the vascular system. Gibberellins are responsible for a whole host of functions: the breakdown of starch to sugar in a seed, stem elongation, flowering and fruit formation!

Cytokinins

  There are 2 types of cytokines, adenine based and phenylurea based. In plants, only adenine based cytokines can be found though! They are produced in the roots and are responsible for plant cell division. Why do plants need cell division? Well, to put it simply to grow and repair its tissues and create gametes which can be used in reproduction! They influence this cell division and differentiation by interacting with receptors that activate signaling pathways involving histidine kinases and response regulators.

 Abscisic Acid (ABA)

  ABA is produced in the roots, stems, leaves and seeds. It is produced in the chloroplasts and contains 3 binding proteins:

1. ABA Receptors (PYR/PYL/RCAR proteins): These proteins are part of a larger family known as the PYR/PYL/RCAR (Pyrabactin Resistance 1/PYR1-like/Regulatory Component of ABA Receptors) family. They bind ABA and undergo conformational changes that enable them to interact with and inhibit protein phosphatases 2C (PP2Cs), which are negative regulators of ABA signaling.

2. Protein Phosphatase 2C (PP2C) proteins: These proteins are not ABA-binding proteins per se but are critical in the ABA signalling pathway. When ABA binds to its receptors, the PP2Cs are inhibited. This inhibition is crucial for the activation of downstream responses.

3. SnRK2 Kinases: These are a family of serine/threonine protein kinases that become activated following the inhibition of PP2Cs by ABA-bound receptors. The activation of SnRK2 kinases

 ABA can be converted by beta glucosidase to be stored as an inactive ester. Under stress, it gets converted back to ABA. Therefore, ABA is stored in multiple locations in the plant. When converted to its active form, it is transported by the phloem. It The level of ABA remains fairly constant under normal circumstances. However, extreme cold, extreme heat, drought, pests, and high metal exposure start the cascade of increased levels of ABA. This can result in stomata closing, delayed budding, seed germination stopping, and slowed cell division.

 Ethylene

Ethylene is a gaseous hormone synthesized in nearly all plant cells. It acts by binding to its receptors, which are part of a signaling cascade that affects gene expression and protein production. Ethylene biosynthesis a part of the Yang cycle, which is a 2-step enzymatic process associated with plant growth and stress response. It then enters the plant via gas receptors. It leaves the plant by diffusion.

Ethylene inhibits DNA synthesis, cell division and inhibits root, stem and leaf growth.   It accelerates the ripening process of fruits by regulating enzymes that break down cell wall components.  Increased ethylene levels are also present in response to mechanical stress and pathogen attack - modulating the plant’s defensive response.

If you have ever placed an unripe fruit next to a ripe one, in the hope your fruit will ripen faster, you are one smart cookie! Since ethylene is gaseous, it is able to exit from one fruit and signal to another to get cracking with its ripening!

Brassinosteroids

Brassinosteroids are synthesized in 3 different pathways from sterol precursors in all areas of the plant. They are produced within the cell’s endoplasmic reticulum, transported through the cytoplasm, then cross the cell wall to the outside. This is the start of brassinosteroid, or BR signaling pathway. There are 3 types of brassinosteroids, which depend upon the carbon number! They act through a receptor-mediated signaling pathway that involves phosphorylation. On a cellular effect level, they change the hydrogen pump, and cross cell walls via a kinase pump. They enter the nucleus and change transcription.

 Under regular conditions, they stimulate seed production, pollen production, fruit, root growth ,xylem growth, stomata development and leaf growth. It importantly regulates photomorphogenesis and modulates light’s effects. When challenged by temperature, drought, pests and soil contaminants like cadmium, brassinosteroids will dictate the plant’s response and resistance. Brassinosteroids interact with other hormones, especially giberrellins and auxins, in promoting growth.

 Hormonal Chemical Interactions

There are multiple hormonal interactions with interactions being additive, antagonistic and synergistic. Some common ones are:

-Auxin+Cytokinin: auxins and cytokinins are synergistic and determine whether a cell will divide or differentiate.

-Gibberellin+Auxin Interaction: Gibberellins and auxins work together to regulate stem elongation and fruit development.

-ABA+Giberrellin: They are antagonistic in germination.

-ABA + Ethylene: They have antagonistic effects on growth and stress response, balancing growth inhibition, and stress tolerance.

Plant hormones are pivotal in orchestrating plant growth, development, and adaptation to environmental changes. Understanding their mechanisms and interactions provides insight into fundamental biological processes and offers potential applications in agriculture and biotechnology, such as developing crops with improved growth characteristics or stress resilience. As research advances, the complexity of hormone signaling and regulation continues to unravel, revealing deeper layers of plant biology and potential for innovative agricultural practices.

September 2023