Article: "Perception of solar UV radiation by plants: photoreceptors and mechanisms"

[aphalo]

We have recently published in the journal Plant Physiology an Update titled Perception of solar UV radiation by plants: photoreceptors and mechanisms. The article is available under open access at https://doi.org/10.1093/plphys/kiab162. (This link should work for everybody, but if it does not for you, please let me know and I will send you a PDF reprint.)

 

Rai N, Morales L O, Aphalo P J. 2021. Perception of solar UV radiation by plants: photoreceptors and mechanisms. Plant Physiology 186: 1382–1396.

 

Abstract:

About 95% of the ultraviolet (UV) photons reaching the Earth’s surface are UV-A (315–400 nm) photons. Plant responses to UV-A radiation have been less frequently studied than those to UV-B (280–315 nm) radiation. Most previous studies on UV-A radiation have used an unrealistic balance between UV-A, UV-B, and photosynthetically active radiation (PAR). Consequently, results from these studies are difficult to interpret from an ecological perspective, leaving an important gap in our understanding of the perception of solar UV radiation by plants. Previously, it was assumed UV-A/blue photoreceptors, cryptochromes and phototropins mediated photomorphogenic responses to UV-A radiation and “UV-B photoreceptor” UV RESISTANCE LOCUS 8 (UVR8) to UV-B radiation. However, our understanding of how UV-A radiation is perceived by plants has recently improved. Experiments using a realistic balance between UV-B, UV-A, and PAR have demonstrated that UVR8 can play a major role in the perception of both UV-B and short-wavelength UV-A (UV-A_sw, 315 to ∼350 nm) radiation. These experiments also showed that UVR8 and cryptochromes jointly regulate gene expression through interactions that alter the relative sensitivity to UV-B, UV-A, and blue wavelengths. Negative feedback loops on the action of these photoreceptors can arise from gene expression, signaling crosstalk, and absorption of UV photons by phenolic metabolites. These interactions explain why exposure to blue light modulates photomorphogenic responses to UV-B and UV-A_sw radiation. Future studies will need to distinguish between short and long wavelengths of UV-A radiation and to consider UVR8’s role as a UV-B/UV-A_sw photoreceptor in sunlight.

 

[Guest]

I understand maybe one percent of this document, but would an applied outcome be to make better indoor grow lights or possibly predict suitable outdoor locale such as region or elevation?

[aphalo]

I think the main take home message is that plants have mechanisms to integrate the "signals" sensed through multiple photoreceptors (the pigments that once activated by light/UV radiation trigger a response, initially at cellular level) and this allows plants to extract the information they use to adjust to the environment they are growing in. It still remains to be convincingly demonstrated but what we see is that plants seem to distinguish among wavelength with better resolution than what has been earlier assumed based on the properties of the individual photoreceptors. If we keep in mind that plants have about a dozen different photoreceptors in some five or six "families" and colour vision in humans allows us to distinguish many thousands of colours with only three types of photoreceptors it is quite likely that plants also "distinguish" quite many "colours" and respond to them.

 

Another rather surprising fact that had bothered me is that the peak of absorption of the UVR8 "UV-B photoreceptor" is very near 280 nm, in other words a wavelength not present in sunlight at ground level. So one possible explanation is that evolution has selected for this "oddity" because "using" the long-wavelength tail of the absorption spectrum counterbalances the increase in photon irradiance (flux of photons) with increasing wavelength in the solar spectrum (the very steep slope of the solar spectrum in the UV region). My idea is that a hypotetical pigment with maximum absortion at say 300 nm would "see" far too many UV-A photons to allow "sensing" of UV-B. UVR8 and other plant photoreceptors are large molecules that have broad peaks of absorption so a "narrow bandpass" pigment was probably an option unavailable during the course of evolution.

 

Yes, this is relevant to grow lights. The assumption usually seen in advertising that red + blue is best because of the absorption spectrum of chlorophyll is a fallacy. Not only because the optical properties of leaves make green light useful for photosynthesis, but because wavelengths from shorter than 280 nm to longer than 750 nm are sensed by plants and this sensing regulates many aspects of function, morphology and of the timing of development events (such as seed germination, flowering, etc.).

 

A scientifically accurate but accesible book on plants' sensory abilities is:

Chamovitz D. 2017. What a Plant Knows: A Field Guide to the Senses. (Updated and Expanded Edition.) New York: Scientifc American. 

 

[Guest]

This is interesting. I will have to dive back into your paper and digest. There was that interesting and very insightful flow diagram. Your comment on the second paragraph reminds me of line noise error correction or even backwards support for deprecated coding. I have to ask as you speak of evolution. Was the UV at ground level the same two hundred or even two thousand years ago?

[dabateman]

Interesting that its about 280nm. 

Thats half 560nm which is about the solar peak I see commonly.

I wonder if the plant can amplify the local photons to induce that.

Like 2-photon microscopy.  If we want to excite a fluorochrome that is 400nm, you use two very closely timed 800nm photons. 

 

Also a major problem is the definition of UV. Its not clearly cut into sections. Yes there is UVB mostly cut off at about 315nm. But UVA will be reported as two regions. You use 315-350nm, that is close to what I have seen in regulator documents. So that good. The upper end can be indicated as 380nm or 400nm or even higher.

I have been seeing visible reported more recently as 380nm to 780nm.

So getting a good solid definition would be great.

[Bill De Jager]

Absolutely fascinating.  I had no idea that plants could sense different wavelengths like this, but it certainly makes sense since the balance of wavelengths will vary depending on environmental conditions that are relevant to plants' metabolic and growth decisions.

[Andrea B.]

Pedro, thank you so much for sharing your work with here on UVP. That is quite a fascinating abstract. I'm having flights of fancy about plants being able to "see".

 

Plants are utterly fascinating. To think that one little seed has the instructions for manufacturing all those compounds and chemicals which eventually give the plant (amongst a gazillion other structures) some photoreceptors!

[aphalo]

UVR8 is an unusual photoreceptor, with a few amino-acids(tryptophans) in the protein itself working as chromophores. As far as it is known there is no amplification allowing excitation by longer wavelength photons, but instead by multiple photons of wavelengths shorter than 340 nm or so. This mechanism was described last year by researchers from USA and China.

 

Li X, Ren H, Kundu M, Liu Z, Zhong F W, Wang L, Gao J, Zhong D. 2020. A leap in quantum efficiency through light harvesting in photoreceptor UVR8. Nature Communications 11. https://doi.org/10.1038/s41467-020-17838-6 (open access, but not an easy reading unless you have a background in chemistry or physics.)

 

Yes, although we have an ISO standard for UV wavebands, it is not always followed. The question is at least for plants how relevant those definitions are. The history of the definitions is also interesting.

 

Björn, LO 2015 Ultraviolet-A, B, and C, UV4Plants Bulletin. 2015(1), pp. 17–18. https://doi.org/10.19232/uv4pb.2015.1.12 (open access)

 

What follows is even less than a hypothesis at the moment, but I suspect that how plants sense UV-B with UVR8, a photoreceptor that also responds to solar UV-A, is to correct the "reading" by subtraction of an "estimate" of the out-of-band signal based on solar blue light sensed by cryptochrome photoreceptors. We know that a suitable signalling interaction downstream of these photoreceptors that would allow this exists, but we do not know if the interaction really implements a  correction that is quantitatively able to effectively do such correction.

 

Yes, photobology is indeed fascinating! I also find it fascinating how much struggling to measure UV-B radiation with array spectrometers made us look at plant's sensing of solar UV radiation from a perspective that helped us understand the biology.

 

About evolution, for as long as there has been a relatively high concentration of oxygen in the atmosphere there have not been UV-C present at ground level. Before CFC's and stratopheric ozone depletion UV-B radiation at ground level has not varied much over the last few centuries. It varies quite a lot with latitude, elevation above sea level, time of the year. During the day it more concentrated towards midday than longer wavelengths. Being more scattered (more diffuse) its proportion in shade tends to increase compared to in the open. There are a few "accidental" evolutionaary experiments "established" when crops have been transported from place to place. One interesting case are fava beans, a species that originated in the Mediterranean basin. It has been taken to Africa and South America where there are land races under cultivation in equatorial mountains, around 3000 m a.s.l., and also has a long history of cultivation in Northern Europe, including the Nordic Countries. We have done a couple of experiments with these, but they are far from conclusive as we had then only one genotype from Ecuador and a cultivar from Sweden.

 

Most photoreceptors are evolutionary old. For example cryptochromes are present also in humans and most animals. They play a double role as their sensitivity to light depends on the magnetic field. They are involved also in migratory birds' sensing of the Earth magnetic field. We know less about UVR8 because it was indentified as a photoreceptor rather recently, but it seems to be present at least in many/most plant species including mosses.

 

I hope that I haven't left too many of your questions and comments unanswered.