The cool colour
Everything in nature is a result of evolution. If it does not have a purpose, nature will not spend energy on doing it. Flowers are amazing, and I am still amazed by how nature is involved in making such perfect interactions. Colours in flowers have lots of purposes; some are to attract specific pollinators, and others are to tell how pollinators should land! The compounds that give flowers their colour sometimes protect against environmental and non-environmental factors. In this article, I wrote about the role of blue, focusing on flowers and anthocyanins (compounds that give the colour blue in flowers and vegetables).
Also, I need to thank my friend Dylan for asking why the colour blue is more prevalent in cold environments. My curiosity took advantage of me, so this article is for you.
Photo by Nuta Teodora
In Autumn
In autumn, days become shorter, and exposure to daylight decreases drastically. When this happens, leaves reduce photosynthesis, and chlorophyll breaks down at a higher rate than synthesised. This leads to the leaves turning yellow, and other compounds such as yellow flavonols, orange carotenoids, and red to purple anthocyanins become more prevalent in the leaves. This can all be delayed with cytokinins, a plant hormone class that promotes cytokinesis (cell division and thus regeneration) in plants and roots. The down expression of these hormones is linked to leaf senescing. This class of hormones is directly involved in evergreen plants not losing their leaves in winter!
The red and purple colours on the leaves are due to anthocyanins; they start being produced by the leaves once the leaf loses ~50% of its chlorophyll. Anthocyanins are present in the vacuoles of the cells, and when the vacuole’s pH is basic, anthocyanins result in blue colourations and acidic pH results in red colours.
Anthocyanidins
Anthocyanins are a class of flavonoids whose colour depends on the intravacuolar environment. There are over 600 different anthocyanidins in nature. Because of their antioxidant properties due to their positively charged ion, they protect plants against biotic (environment) or abiotic (non-environmental) stresses, which might be an advantage against climate change. Anthocyanins also play a role in plant reproduction by attracting more pollinators with bright colours and protecting young vegetative tissue from photoinhibition and photobleaching under light stress without compromising photosynthesis! Fruit without anthocyanins was shown to be more sensitive to photoinhibition during development.
Also, an experiment by Malone et al. showed that anthocyanidins could change feeding behaviour; they genetically modified tobacco plants to express MYB transcription factors on their leaves. This TF is involved in anthocyanidins expression. Plants with the expression of this TF were not eaten by Helicoverpa armigera. The mortality of this pathogen increased substantially after being fed the transgenic leaves. Also, the pathogens did not feed from the leaves naturally. The transgenic leaves showed high concentrations of several anthocyanins and other polyphenolics, including chlorogenic, caffeic acids, and rutin.
Transgenic tomatoes with higher anthocyanidin levels have also been shown to be more tolerant to heat! Fruits synthesise anthocyanidins, which protect them during development. Thanks to their antioxidant properties, these compounds protect the photosynthetic apparatus from excess UV radiation and scavenge free radicals.
For those interested in the biochemistry of this compound’s synthesis, the diagram below is a schematic representation of anthocyanidins biosynthesis.
But do plants have what it takes?
Anthocyanin compounds have been studied for more than 50 years; in fact, the CHS gene from parsley was first cloned in 1983. To synthesise anthocyanin, plants need structural genes that encode the enzymes directly involved in anthocyanin synthesis (like CHS and CHI) and regulatory genes that control the transcription of structural genes. Biology is this complicated, yes.
Regulatory genes usually have a lot on their plate; they control the biosynthesis and expression of many different structural genes simultaneously. The R gene family, for example, determines the spread, timing and amount of anthocyanin pigmentation in maize. An2 and An4 control the expression of anthocyanin genes: An2 in the flower limb and An4 in anthers. Other genes involved are Sn, Lc, B, C1 and PI. All these genes showed high similarity, suggesting they evolved from the same ancestor gene.
But why is blue the cool colour
Environmental factors influence anthocyanin synthesis: high irradiance, UV/blue light, and low temperatures promote anthocyanin biosynthesis. At the same time, hot temperatures lead to its degradation. The tomato fruit exposed to the sun had more anthocyanin concentration than the shaded counterparts; Guo and Wang (2010) showed that higher UV irradiation increased anthocyanin content in tomatoes compared to white light. This was also conducted with blue light and darkness. UV-B photons cause cellular damage (they promote the generation of photoproducts in DNA) and directly damage proteins. When this radiation starts acting on plant leaves, the plant senses this and transcribes proteins and enzymes involved in flavonoid synthesis, including anthocyanin. These compounds absorb the 280-315 nm region, acting as UV filters, preventing DNA damage. They are natural sunscreen! Specifically, radiation leads to CHS, DFR, and F3H transcription of the flavonoid pathway in lettuce leaves. Studies with Arabidopsis showed increased transcription of R2R3-MYB activator genes when exposed to blue and red light. Additionally, high temperatures decrease the expression of the genes required for the early and late steps of the anthocyanin biosynthesis pathway in Arabidopsis seedlings.
The process is not fully understood when it comes to temperature, but it is known that low temperature induces anthocyanin accumulation. There is a theory that the mechanism is the same as light since anthocyanin biosynthesis at low temperatures needs light. Some transcription factors are known to be involved: SlAN2, SlAN1, and SlJAF13. At high temperatures, there is an inhibition of the expression of anthocyanin activators and enhancing of repressors. For example, peroxidase activity increases in berries and petunias at high temperatures. Overexpression of VviPrx31, a gene for a grapevine class III peroxidase, caused anthocyanin degradation in petals at heat stress. Also, high temperatures activate E3 ubiquitin ligase and repress positive regulators for anthocyanin synthesis HY5.
Qin et al. planted two grape berries in various climate conditions during five natural stages. Climate has been shown to significantly impact anthocyanin synthesis and degradation and the antioxidant properties of grape berries. The berries were redder at lower temperatures, light, and high humidity, which confirmed high anthocyanin content and better antioxidant properties than those in hot environments.
Also, anthocyanins are susceptible to pH, and this can be observed in hydrangeas, in which anthocyanins turn into one of their isoforms. Hydrangeas react with levels of aluminium ions in the soil; more ions mean more acidic soil, making the flower blue. Lower aluminium in the soil makes the flower turn pink. Some gardeners water their hydrangeas with aluminium sulphate drenches so they keep having blue petals. This needs to be done carefully since aluminium is toxic to plants; it is, in fact, one of the major factors that inhibit plant growth and development in acidic soils by affecting nutrient and water uptake. Interestingly, metal ions, like iron and magnesium, increase anthocyanin stability by forming complexes with anthocyanins.
When I started researching this article, I had no idea how cool these compounds were. They protect plants from pathogens and radiation. Blue is not very common in plants, but this colour has been shown to attract more bees! Flowers with blue tones tend to produce more nectar, and even plants that do not produce enough blue for their flowers produce a blue ‘halo’ to attract more bees! These compounds, like most things in nature, are tightly regulated; and might give an advantage against climate change.
