Molecule of the Month: Auxin

As the bitter, cold winter comes to a slow end in the Northern Hemisphere we are reminded of the warmth and light that spring gladly offers. The icy patches are replaced by the buds and shoots of daffodils, tulips and crocuses that may have planted in the dead of winter.

This re-awakening of Mother Nature is an exuberant affair with plants sensing many factors such as temperature, wind and light allowing these shoots and eventual flowers, to come to establishment. The orchestrated and complicated affair from seed, shoot, to flower is dictated by many signalling molecules that talk and interact with one another.

Auxin plays a large role in the growth and the primary establishment of a plant and even allows the plant to respond to light, an essential source of energy. Within agriculture, synthetic auxin has also been developed as a way of controlling the growth of weeds. However, auxin does not work alone, and other signalling molecules can dictate its production.

Power to Plants

Auxin (Figure 1)1 is a plant hormone that is described to influence nearly every aspect of plant growth and development2. Many biochemical and experimental studies have presented that the amino acid, tryptophan is used by plants to make this fundamental hormone3.

Figure 1:

Most notably, auxin is able grow and elongate cells allowing certain structures of the plant, such as roots and shoots to respond to light4. An effective response to light, known as phototropism, provides multiple developmental and growth cues suggesting evolutionary selection5.

Moving Parts: Roots & Shoots

The power of auxin in guiding the stem can be seen when light is shone onto the plant at an angle. When the light source is at an angle (Figure 2)6 auxin is able to accumulate on the shaded sides of the stem, leaving the lit sides of the stem with little to no auxin. The gradient of auxin leads to the elongation of the cells within the shaded part of the plant, compared to the lit parts. This causes the stem to bends its way towards the light, which is called positive phototropism5.

Figure 2: Auxin is able to guid the stems towards a light source, presenting positive phototropism.

Although, auxin is shown to elongate cells within the shoot, when auxin is present within the root it actually inhibits this process allowing the root to bend away from light7. This is an example of negative phototropism presenting the dual yet opposing effects of auxin within the shoot and root. (Figure 3)8

Figure 3: Shows the dual, opposite effects auxin has on the shoots are roots of the plant.

However, negative phototropism has come into wider questioning as particular studies on garden cress have presented that 50% of the roots show a positive phototropic response, growing towards, instead of away from light9. This may suggest that this response is different between species and that other factors, such as gravity, may play a larger role in the direction of root growth.

More than light?

Aside from auxin guiding the stem or the root of the plant either away or towards light, this hormone is heavily implicated in root development. Root systems are an integral part of the plant as it enables it to invade land, giving it anchorage and also facilitating the uptake of water to survive dry conditions10.

Processes including the initial production of the and normal development of the root are all mediated by auxin11. For example, a study found that a gradient of auxin within cress is crucial for maintaining root stem cell identity presenting its integral role in the establishment of the plant12.  Especially within the root, maintaining stem cell identity is important as plants need to grow roots at a moment’s notice if it becomes damaged.

Stem cells, present in humans and plants alike, are the main source of “immature cells” that are able to form a wide range of specialised cells. However, stem cells in the root require a specialised tissue, the quiescent centre, to maintain them13. Studies on corn and cress have shown that auxin plays a substantial role in the formation, establishment, and maintenance of these quiescent centres within root systems14,15.   

Agricultural auxin

As plant hormones have been found to influence nearly every stage of development, they can be utilised and manipulated towards the use of agriculture to increase yield by destroying unwanted plants.

Compounds that mimic auxin can be utilised as an herbicide that go onto control the growth of unwanted plants, usually killing them. Although, auxin is known to stimulate a variety of growth and development processes, at high concentrations, man-made auxin it is able to disrupt growth and fatally damage the plant16

However, since the widespread and continual use of herbicides worldwide, scientists are worried about various species of weeds that are showing resistance towards synthetic auxin. The first species of weeds resistant to auxin was documented in 1957 but this has since expanded to include a total of 36 species as of 201817. The presence of these resistant weeds remains at relatively low level, when compared to herbicides acting in a different way to auxin. However, they encourage scientists to further understand the biochemical processes that are taking place that lead to resistance.

Cross-talk

Auxin can be seen as they key hormonal regulator of the processes concerning both the root and the stem, but other plant hormones such cytokinin and ethylene can participate in these processes, even crosstalk with one another.

For example, ethylene is an important signal in inhibiting root growth but its eventual effects on the individual cells are mediated by auxin synthesis and transport. Within cress, ethylene increases auxin synthesis that goes to stop the cells of the root from elongating or getting larger18.

Nevertheless, auxin is an integral signalling hormone that influences a wide range of processes from stem cell maintenance to stem and root growth in the response to light. Although, some studies suggest that negative phototropism within roots may have other influences involved, synthetic auxin is widely used within agriculture to destroy weeds.

As the sunshine of spring and summer arrive, the shoots and possibility even the roots of many plants will be influenced under the guide of auxin.

References:

1.         Indole-3-acetic acid. Wikipedia (2021). [Cross-checked with academic material]

2.         Weijers, D., Nemhauser, J. & Yang, Z. Auxin: small molecule, big impact. J. Exp. Bot. 69, 133–136 (2018).

3.         Zhao, Y. Auxin Biosynthesis. Arab. Book Am. Soc. Plant Biol. 12, (2014).

4.         Ma, L. & Li, G. Auxin-Dependent Cell Elongation During the Shade Avoidance Response. Front. Plant Sci. 10, (2019).

5.         Liscum, E. et al. Phototropism: Growing towards an Understanding of Plant Movement[OPEN]. Plant Cell 26, 38–55 (2014).

6.         IGCSE Biology 2017: 2.85: Understand the Role of Auxin in the Phototropic Response of Stems. IGCSE Biology 2017 http://igcse-biology-2017.blogspot.com/2017/06/285-understand-role-of-auxin-in.html (2017).

7.         Eliasson, L., Bertell, G. & Bolander, E. Inhibitory Action of Auxin on Root Elongation Not Mediated by Ethylene. Plant Physiol. 91, 310–314 (1989).

8.         Auxin is a hormone that controls growth in plants cells. https://mammothmemory.net/biology/plants/sexual-reproduction-in-plants/auxin.html.

9.         Kutschera, U. & Briggs, W. R. Root phototropism: from dogma to the mechanism of blue light perception. Planta 235, 443–452 (2012).

10.       Kenrick, P. & Strullu-Derrien, C. The Origin and Early Evolution of Roots1. Plant Physiol. 166, 570–580 (2014).

11.       Overvoorde, P., Fukaki, H. & Beeckman, T. Auxin Control of Root Development. Cold Spring Harb. Perspect. Biol. 2, (2010).

12.       Tian, H., Niu, T., Yu, Q., Quan, T. & Ding, Z. Auxin gradient is crucial for the maintenance of root distal stem cell identity in Arabidopsis. Plant Signal. Behav. 8, (2013).

13.       Scheres, B. Stem Cells: A Plant Biology Perspective. Cell 122, 499–504 (2005).

14.       Kerk, N. & Feldman, L. The quiescent center in roots of maize: initiation, maintenance and role in organization of the root apical meristem. Protoplasma 183, 100–106 (1994).

15.       Della Rovere, F. et al. Auxin and cytokinin control formation of the quiescent centre in the adventitious root apex of arabidopsis. Ann. Bot. 112, 1395–1407 (2013).

16.       Grossmann, K. Auxin Herbicide Action. Plant Signal. Behav. 2, 421–423 (2007).

17.       Busi, R. et al. Weed resistance to synthetic auxin herbicides. Pest Manag. Sci. 74, 2265–2276 (2018).

18.       Xu, P. et al. Integration of Jasmonic Acid and Ethylene Into Auxin Signaling in Root Development. Front. Plant Sci. 11, (2020).

Picture references

Pots on shelf image by Min An from Pexels

Person with plant image by Daria Shevtsova from Pexels

Shoots image by Markus Spiske from Pexels

Roots image by Karolina Grabowska from Pexels

Agriculture image by  freestocks.org from Pexels

Leaf image by  Markus Spiske from Pexels

Split image by Freepik from www.flaticon.com

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