How do plants protect themselves against sunburn?

A pair of proteins trigger protective systems when a plant's blasted with UV radiation. Belinda Smith reports.

Arabidopsis leaf surface cells before (top) and after sun exposure. The green fluorescence shows how a type of receptor called UVR8 has accumulated in the nucleus of those cells.
Sylvain Loubéry / UNIGE

Sunburn – for some, it's an unavoidable part of summer. But while we have the luxury of slapping on sunscreen, putting on extra clothes or simply going inside, plants in full sun usually have little choice but to cop it.

Being in full sun has its advantages, of course – more sunlight means more photosynthesis and growth. But with the good light comes the bad, burning ultraviolet-B (UV-B) radiation.

So how do plants avoid sunburn?

Biologists at the University of Geneva in Switzerland, led by Roman Ulm, found a plant's response to a blast of UV-B is regulated by a pair of proteins – one a receptor, the other an enzyme.

Their work was published in the Proceedings of the National Academy of Sciences.

UV light, in small doses, is good for plants – it influences growth and development. (A little can be beneficial for people too because we synthesise vitamin D from UV-B on our skin.)

But – for plants and animals – too much is a terrible thing.

Crops over-exposed to UV-B are stunted, less productive and grain quality drops. They're also more susceptible to disease.

Plants are usually quite good at churning out a host of self-defense mechanisms against UV-B, such as enzymes to repair sun damage, and antioxidants, which neutralise damaging free radicals produced by UV-B rays.

How these protection systems arise has been largely a mystery until recent years.

Ulm's lab, in 2011, discovered a type of receptor found in cells of thale cress (Arabidopsis thaliana) which absorbs UV-B rays and initiates the plant's defence response. They called it UV-B resistance 8, or UVR8.

But the steps between receptor activation and protection are largely unknown. So in their most recent work, Ulm and his team looked at what happened to these UVR8 receptors immediately post-activation.

UVR8 receptors usually hang around in the cell's cytosol – the liquid in which cellular bits and pieces, such as chloroplasts (which photosynthesise), are suspended.

But when irradiated by UV-B, a UVR8 receptor splits into pieces and moves into the cell nucleus.

The nucleus, which houses DNA, is the control centre of the cell. And once enough UVR8 accumulates within, genetic signals kick into gear to produce UV-B protection.

The process takes only a few hours.

Crucial to UVR8's translocation to the nucleus is an enzyme called COP1. Ulm and colleagues found plants genetically engineered to lack COP1 couldn't shift UVR8 from the cytosol to nucleus when blasted with UV-B.

COP1, they realised, is a kind of shuttle – dragging across large quantities of UVR8 into the nucleus relatively quickly and kicking off sunburn defence systems before too much damage is done.

Next, the researchers wish to determine exactly how UVR8 and COP1 in the nucleus regulate genetic expression of protective mechanisms.

Explore #plants #botany #UV
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