“Give us this day our daily sunlight”, plants might pray, if they were Christian. Light is, after all, their main source of energy, captured by photosynthesis. But the machinery of photosynthesis isn’t always the best tool to detect light, and plants have an array of molecular sensors to detect small amounts of light in different colours. So how do you unpick the effects of different light receptors? In short, you break them and see what happens.
One important group of light receptors are the phytochromes, which are particularly good at picking up red light (along with far red, just beyond the end of the spectrum our eyes can pick up). In Arabidopsis (the standard plant scientists study), there are no fewer than five of them, inventively named phytochrome A to phytochrome E. Now a group of scientists in Argentina have succeeded in breaking all five of them together (creating a quintuple mutant, in technical terms). That’s no mean feat, because as you break bits, it becomes harder to get the plants to grow at all. But it’s interesting, because the five phytochromes can sometimes sub in for each other, hiding the effects of breaking each one separately.
Here’s a brief summary of what the mutant plants did (or didn’t, mainly):
- Unlike plants with any phytochromes working, they didn’t germinate (or only very rarely) in response to any kind of light. The researchers got round this using a plant hormone, gibberellin, which triggers germination.
- When a seedling reaches the surface, it should open out its seed-leaves and start making true leaves. The mutants didn’t do this in red light (which would be sensed by phytochromes), but did in blue light (which can be sensed by another group of light receptors, cryptochromes). This shows that cryptochromes can work without any phytochromes, which wasn’t previously clear.
- They did make some chlorophyll in response to light, so other light sensors must be involved there.
- They didn’t respond to changes in the amount of far red light, which is exactly what phytochromes would normally pick up. Plants use red and far red light to ‘see’ other plants, which absorb red light, but reflect and transmit more far red.
- When mutant plants growing in white light were moved to red light, their leaves started to die (senesce), and growing leaves stopped expanding.
- They could still produce a 24 hour rhythm—the circadian clock can work without phytochromes.
The only small question I have is about where phytochromes are involved in other systems: could the mutants leave partial or malformed phytochrome proteins, which are enough to interact with other proteins, although they don’t function as light sensors? The mutations involved are described in the paper, but I don’t know enough about molecular biology to say precisely what the results of those would be.
Strasser, B., Sanchez-Lamas, M., Yanovsky, M., Casal, J., & Cerdan, P. (2010). Arabidopsis thaliana life without phytochromes Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0910446107