1997 2ND PLACE

Sweetness and Light

By Matthew Barrett

With the exception of Triffids, plants don't have a habit of getting up and walking around. Nor do they hop, scuffle or fly. Being quite literally rooted to the spot presents a big problem for plants: running away from a hungry herbivore or looking for a sunny spot when it gets a little shady are not options. When the going gets tough, the tough simply cannot get going. It's a case of Adapt or Die.

Despite their apparently placid nature, plants are constantly undergoing rapid adjustments in response to their environments. Within seconds of brushing past a plant in your garden, an internal communication system has warned of potential attack and the plant starts to shore-up defences. If you stick a plant in darkness, photosynthesis (the process by which plants make food from light, water and carbon dioxide) shuts down. The plant will start to nibble away at its food reserves. This happens every night. Keeping the plant like that for a day or two will get it seriously worried about starving and it will bring in emergency survival plans.

Photosynthesis produces sugar, so in the light, plants accumulate sugar. Sugar is then broken down to provide energy and the building blocks for growth. Getting the balance right between sugar manufacture and usage is critical: too much unnecessary manufacturing relative to usage for growth, could mean losing the edge against a faster growing competitor. On the other hand, failure to accumulate sufficient sugar for storage and periods of famine could prove fatal.

What's more, different parts of plants have different needs at different times. A growing potato tuber starts off hungry for sugar, which it stores as starch, but then becomes the lifeline source of sugar for the shoots which give rise to new potato plants. That sprouting spud at the back of your kitchen cupboard is a lifepod desperately sending out scouts in search of a new existence.

It has been known for some time that altering access to sugar can directly trigger feasting or fasting responses in plants. Feeding sugar to illuminated plant cells switches off genes (the units of information which control all living processes) involved in photosynthesis. After all, why waste effort manufacturing sugar when it's being provided free of charge? Conversely, removing sugar from the same cells and plunging them in darkness switches on 'starvation genes' involved in finding alternatives for energy and growth. Being able to mimic changes in the plants' internal sugar balance would provide a massive potential benefit to agriculture. Trick a potato plant, say, into devoting more sugar to tuber growth compared to less edible tissues, and the result could be bigger, more numerous tubers. Alternatively, conning crops into prioritising early growth more than usual, thereby out-competing weeds, could reduce the need for herbicides.

The problem is, in order to mimic changes in the sugar balance, you have to understand how sugar switches genes on and off. At the moment, 'sugar sensing', the process by which plants regulate sugar manufacture and allocation in response to changes in the sugar balance, is a black box. Recently, a picture has emerged which suggests that hexokinase, the enzyme (a minuscule protein-based machine) -which carries out the very first step in sugar breakdown, has two jobs. Its most obvious job is to label sugar for the conveyor belt of energy production and growth. The second, more subtle, function of hexokinase is to signal to relevant cellular processes what the sugar status is at any time. It is akin to the security guard at the factory gate not just checking in the sugar delivery, but actually letting all the factory floors know what they should be doing, as soon as it comes through the gate.

In our lab at the University of Edinburgh, Dr. Steve Smith co-ordinates a European project devoted to identifying all the components of the sugar sensing chain. How do we go about finding all the links in the chain? One approach is to take a gene you know is switched on or off by sugar and use it as 'bait' to capture the gene one step higher in the chain of command. Continue long enough with this molecular fishing trip and hexokinase might, one day, end up in the net.

A plant's versatility in responding rapidly and correctly to the sugar balance is a critical element in its adaptation to a constantly changing environment. The plant kingdom is arguably the most successful on earth. Plants boast the greatest mass of living stuff, the biggest organism (the giant sequoia - no competition at up to 680 tons) the oldest living thing on the planet (5000 year old bristle-cone pines) and they occupy every corner of the globe, from desert to rainforest. Perhaps it pays to have a sweet tooth.