Molecule of the Month: Pollen

Pollen is the source of all plant life on Earth. Coming in a different range of consistencies, colours, and types pollen is integral to not only the life cycle of the plant, but also serves agricultural purposes.

Plants can undergo different types of reproduction, depending on whether their flowers contain both male and female sex organs or only either one. Therefore, self-pollination within plants is a very common process and there are emerging theories that it causes the overall decline in the plant’s health and livelihood. However, there is emerging evidence that this may not be as true as first thought.  

Pollen is an incredibly useful tool which plants use but it can be a source of irritation when it concerns humans. Sneezing, runny nose and watery eyes are caused by the various lipids and other compounds that make pollen. The immune system responds to these compounds causing hayfever.

Over the coming years however, climate change has drastically altered the environment causing the pollen producing season to be lengthened within some areas. Even certain invasive plants have been reported to produce a greater amount of pollen compounding the severity and prevalence of hayfever within parts of the world.

Pollen: The giver of life

Flowering plants are a great evolutionary feature of plants as they contain reproductive organs allowing for eventual fertilisation and creation of seeds for the next generation1. Unlike mammals, many flowers can contain both male and female reproductive organs and are termed bisexual flowers. Conversely, flowers have only male or female organs, are named unisexual flowers.

Plants can contain male and female sex organs (Figure 1)2, or either one depending on if they produce unisexual flowers. Pollen grains come from the male reproductive organ, called the stamen, present on plants and trees. However, the pollen granules are produced on the anther, at the end of the filament.

Figure 1: Shows the location of the reproductive organs present on flowers (left) and the structure of the male (middle) and female (right) reproductive organs.

The female sex organ, carpels, contains the stigma, style and the ovaries. The stigma receives the pollen from the same or different plant, moves down the style, where the ovaries lie for fertilisation to happen.

The whole process of a grain of pollen fertilising a plant, is known as pollination and can produce the next generation of offspring3. However, this process also has agricultural importance as mature ovaries, containing the seeds of the next generation, produces fruits and grains that the human diet and agricultural industries has depended on for generations.

Busy bees

Plants can pollinate in a wide variety of ways however, up to 75% of crops species benefit from animal pollination from species such as bees, flies, butterflies and bats4. Although, bees are thought to be one of the major contributors in global crop pollination, other pollinators (Figure 2)5 are able to provide a similar benefit to the pollination of crops6.

Figure 2: Even though Bees are considered to be the main pollinator species there are many different species, pictured above, which contribute to pollination on the whole.

The importance of pollinators, such as bees, have been highlighted in many different studies. A paper looking at cotton and sesame crops to be sold within Burkina Faso found that pollination by bees significantly increased yield quantity and quality by up to 62%7. However, when these crops were not pollinated in this way, there was a sizeable gap in crop quality and quantity in both cotton and sesame varieties presenting on a small scale, that pollinators have a large economic value.

Many academics have tried to quantify the economic value that pollinators have with various methods that have either under-estimated or over-estimated such figures8.  Costs can additionally fluctuate depending on their crop type and their country of origin.

For example, crops which were dependent on pollinators in Brazil had an agricultural income of $45 billion per year. The contribution of these specific pollinators within this study was found to be at least $12 billion, presenting a sizeable contribution9. On a more worldwide scale, pollinator species are estimated to make up 9.5% of the world’s food production value10.

However, it is important to note that the importance of pollinator species does not stop at their economic benefit to humans, but instead, also provides a large ecological benefit too. Conserving the habitat of pollinator species such as bees, bats, and moths enhances the overall biodiversity and can even protect soil and water quality by protecting soil erosion and other delicate natural processes11.

Self-love?

Pollination of a flower can occur in a wide variety of ways, with plants even being able to pollinate themselves (Figure 3)12 if they contain male and female reproductive organs on the same plant. Self-pollination is thought to be a process used by plants to respond effectively to rapid environmental change, such as the decrease in pollinator species, therefore still ensuring the creation of the next generation13.

Figure 3: Shows the process by which self-pollination happens on an unisexual lilly. Any of the offspring are genetically identical to one another

However, there is no genetic diversity involved in self-fertilisation, which is thought to eventually lead to inbreeding depression where there is an overall decline in the plant’s fitness14. Although, the process of self-fertilisation within plants might be advantageous within the short term, these organisms have the possibility of having higher extinction rates much later. Even though this theory is well accepted, as it has some empirical evidence to back it up, the underlying genetic and ecological causes are not well understood15.

However, considering that nearly 14% of the worlds’ plants undergo self-fertilisation as their only mode of reproduction, there is some clear advantage to this process16. Self-fertilisation can additionally purge the disadvantageous characteristics from the plant lineage. This alongside, the creation of modern evolutionary models, has casted doubt into whether plants who undergo self-reproduction fail to thrive over a longer period of time17.

Runny nose, sneezing and watery eyes

The way in which pollen contributes to our agricultural and ecological world is vast, however, to humans at least, this golden substance can be the source of much discomfort. Hayfever, characterised by sneezing, a runny nose and watery eyes, is due to the immune system responding to particles of pollen within the air18.

Pollen is made up of distinct biochemical compounds such as the enzyme NADPH oxidase, and other pollen proteins and lipids. Interestingly, each one of these compounds is able to stimulate a non-specific immune response19

The enzyme NADPH oxidase can transfer a negative charge to oxygen thereby creating a group of molecules called reactive oxygen species (ROS)20. These ROS are important especially within plant development as they induce pollen tube growth during the fertilisation process21.

However, when NADPH oxidase within pollen make their way into the lungs, they can also produce ROS within the lining of the tissue. The presence of ROS, attracts specialised immune cells called neutrophils (Figure 4)22 which release chemicals that causes wide scale inflammation23. This pathway of inflammation caused by pollen is backed up by empirical studies done in both mice and humans24.

Figure 4: Neutrophils are a specialised type of immune cell which causes inflammation of the airway when activated by the presence of pollen.

Just like with any dysregulated immune response, this is only a small part of a wider process going on in the background. Antibodies produced in response to pollen, such as IgE, plays a huge role in the allergic response seen within hayfever25. Furthermore, lipids within pollen grains are thought to shift the immune response towards producing cells that can release these antibodies26.

A changing climate

Pollen and pollinator species are integral to both human and environmental health however, when our climate changes to its extremes it can have a wide scale impact on agriculture, pollinator species and even the severity of allergies.

A 2019 study looking at how pollen load and temperature has changed, over a 20-year period, in 17 different locations, found that 12 of them exhibited significant increases in seasonal and annual pollen load. Additionally, these were significantly related to increases in both maximum and minimum temperature extremes27. The study also found that in most locations the duration of the pollen season was increasing on an average of 0.9 days a per year.

Given the longer and more intense pollen seasons, alongside the introduction of invasive species, such as ragweed, it is unsurprising to hear hayfever prevalence is rising within Europe28.

Food for thought?

It is important to consider the ways in which climate change impacts hayfever however, other environmental factors such as diet and cleanliness can influence how the immune system responds to pollen.

Older papers have shown that a high intake of unsaturated fats is positively associated with hayfever29. This study additionally presented that consumption of specific fats, such as fish oils, were found to decrease the incidence of hayfever among the participants. This provides some evidence that diet can influence how the immune system responds to pollen, but more detailed and modern studies are needed to bolster this argument.

Pollen is a wonderous molecule, that shapes our ecosystems and provides a strong foundation off which human agriculture has thrived. The different types of reproduction involved alongside the shifting of pollen season brings to attention how integral this molecule is in maintaining ecological and human health. 

References:

1.         Pereira, A. M. & Coimbra, S. Advances in plant reproduction: from gametes to seeds. J. Exp. Bot. 70, 2933–2936 (2019).

2.         saba4mi. Sexual Reproduction in Plants : Science | TET Success Key. https://www.tetsuccesskey.com/2014/01/sexual-reproduction-in-plants.html.

3.         Gilbert, S. F. Pollination. Dev. Biol. 6th Ed. (2000).

4.         Bartomeus, I. et al. Contribution of insect pollinators to crop yield and quality varies with agricultural intensification. PeerJ 2, (2014).

5.         Plant a Pollinator-Friendly Garden | ReThink Red Deer. https://rethinkreddeer.ca/node/39.

6.         Rader, R. et al. Non-bee insects are important contributors to global crop pollination. Proc. Natl. Acad. Sci. 113, 146–151 (2016).

7.         Stein, K. et al. Bee pollination increases yield quantity and quality of cash crops in Burkina Faso, West Africa. Sci. Rep. 7, (2017).

8.         Allsopp, M. H., de Lange, W. J. & Veldtman, R. Valuing Insect Pollination Services with Cost of Replacement. PLoS ONE 3, (2008).

9.         Giannini, T. C., Cordeiro, G. D., Freitas, B. M., Saraiva, A. M. & Imperatriz-Fonseca, V. L. The Dependence of Crops for Pollinators and the Economic Value of Pollination in Brazil. J. Econ. Entomol. 108, 849–857 (2015).

10.       Vanbergen, A., Heard, M., Breeze, T., Potts, S. & Hanley, N. Status and value of pollinators and pollination services. (2014).

11.       Wratten, S. D., Gillespie, M., Decourtye, A., Mader, E. & Desneux, N. Pollinator habitat enhancement: Benefits to other ecosystem services. Agric. Ecosyst. Environ. 159, 112–122 (2012).

12.       Self-pollination | botany. Encyclopedia Britannica https://www.britannica.com/science/self-pollination.

13.       Cheptou, P.-O. Does the evolution of self-fertilization rescue populations or increase the risk of extinction? Ann. Bot. 123, 337–345 (2019).

14.       Drake, J. M. Population Viability Analysis. in Encyclopedia of Ecology (eds. Jørgensen, S. E. & Fath, B. D.) 2901–2907 (Academic Press, 2008). doi:10.1016/B978-008045405-4.00654-6.

15.       Wright, S. I., Kalisz, S. & Slotte, T. Evolutionary consequences of self-fertilization in plants. Proc. R. Soc. B Biol. Sci. 280, (2013).

16.       Goodwillie, C., Kalisz, S. & Eckert, C. G. The Evolutionary Enigma of Mixed Mating Systems in Plants: Occurrence, Theoretical Explanations, and Empirical Evidence. Annu. Rev. Ecol. Evol. Syst. 36, 47–79 (2005).

17.       van Ginkel, M. & Flipphi, R. C. H. Why Self-fertilizing Plants Still Exist in Wild Populations: Diversity Assurance through Stress-Induced Male Sterility May Promote Selective Outcrossing and Recombination. Agronomy 10, 349 (2020).

18.       Ross, A. & Fleming, D. Hayfever — practical management issues. Br. J. Gen. Pract. 54, 412–414 (2004).

19.       Hosoki, K., Boldogh, I. & Sur, S. Innate responses to pollen allergens. Curr. Opin. Allergy Clin. Immunol. 15, 79–88 (2015).

20.       Tarafdar, A. & Pula, G. The Role of NADPH Oxidases and Oxidative Stress in Neurodegenerative Disorders. Int. J. Mol. Sci. 19, (2018).

21.       Dharajiya, N., Boldogh, I., Cardenas, V. & Sur, S. Role of pollen NAD(P)H oxidase in allergic inflammation. Curr. Opin. Allergy Clin. Immunol. 8, 57–62 (2008).

22.       Matsumura, D. What are neutrophils and what do they do? | Berkeley Institute International. https://berkeley-institute.com/what-are-neutrophils-and-what-do-they-do/ (2020).

23.       Boldogh, I. et al. ROS generated by pollen NADPH oxidase provide a signal that augments antigen-induced allergic airway inflammation. J. Clin. Invest. 115, 2169–2179 (2005).

24.       Hosoki, K., Boldogh, I. & Sur, S. Neutrophil recruitment by allergens contribute to allergic sensitization and allergic inflammation. Curr. Opin. Allergy Clin. Immunol. 16, 45–50 (2016).

25.       Galli, S. J., Tsai, M. & Piliponsky, A. M. The development of allergic inflammation. Nature 454, 445–454 (2008).

26.       Gilles, S. et al. Pollen allergens do not come alone: pollen associated lipid mediators (PALMS) shift the human immue systems towards a TH2-dominated response. Allergy Asthma Clin. Immunol. 5, 3 (2009).

27.       Ziska, L. H. et al. Temperature-related changes in airborne allergenic pollen abundance and seasonality across the northern hemisphere: a retrospective data analysis. Lancet Planet. Health 3, e124–e131 (2019).

28.       Schmidt, C. W. Pollen Overload: Seasonal Allergies in a Changing Climate. Environ. Health Perspect. 124, A70–A75 (2016).

29.       Trak-Fellermeier, M. A., Brasche, S., Winkler, G., Koletzko, B. & Heinrich, J. Food and fatty acid intake and atopic disease in adults. Eur. Respir. J. 23, 575–582 (2004).

Image references:

Field photo by Pixabay from Pexels

Purple flower photo by Pixabay from Pexels

Lilly flower photo by  Trina Snow from Pexels

Sneezing person photo by Gustavo Fring from Pexels

One world poster photo by Markus Spiske from Pexels

Cheese photo by Karolina Grabowska from Pexels

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