Feeling the heat
By Zoe Dunford from the John Innes Centre and The Sainsbury Laboratory
16 April 2012
Plant scientists from Norwich Research Park are gathering evidence on the precise ways in which plants are affected by rising temperatures and falling rainfall levels. Their findings could show us if anything can be done to help crops cope with the increased risks from drought and disease. Research to help plants cope with a changing climate.
Some summers can be too hot to handle. In 2003, 35,000 people died in Europe as a result of a record heat wave. Too much sun, too much heat and not enough water makes us sick.
The same goes for plants, including those that feed us and our livestock. Less water means lower productivity, leaves grow more slowly and cereals produce smaller, lighter grains because the grain-filling period is reduced.
Higher temperatures can also make plants more vulnerable to attack. Pests and diseases can become more virulent or are able to survive in new locations. Others become less virulent but the crops become more susceptible to their effects.
In 2003 French wheat yields were down by a fifth and Italian maize yields were down by over a third. Nothing like it had been seen since 1540, the year that Henry VIII got hitched to wives four and five, when a heatwave beginning in April lasted seven months. Such extreme temperatures are likely to become more frequent according to IPCC climate model projections.
It is also predicted that every 1ºC temperature rise will cause direct losses in crop yields of 2.5-16%, with further losses expected from rising sea levels, increased salinity and decreased soil moisture.
Researchers at the John Innes Centre (JIC), which receives strategic funding from BBSRC, and The Sainsbury Laboratory are starting to unravel how plants respond to temperature and how their interactions with pests and diseases are likely to change. This knowledge may show us if it is possible to breed crop varieties that can to adapt to a changing climate.
Plants that glow
In 2010, Scientists studying luminescent laboratory plants at the JIC discovered that plants have a built-in genetic thermometer, which they use to control their development.
By studying plants that emit a gently glow when the temperature is increased, Dr Vinod Kumar and Dr Phil Wigge, were able to screen for mutant plants that could no longer sense the proper temperature. One plant behaved as though it was hot all the time.
"It was amazing to see the plants," said Kumar, who discovered the mutant plant. "They grew like plants at high temperature even when we turned the temperature right down."
Together with Dr Wigge, Kumar found that this mutant has a single defect in its DNA packaging system, which means that the plant switches on all of its genes as if it is at a high temperature even when it is cold. Normal plants can change the way their DNA is packaged according to temperature. As it becomes warmer, plant DNA becomes less tightly wrapped. This allows genes to be switched on in response to temperature.
What they are trying to find out next is whether this is a direct effect of temperature on DNA or is more indirect.
If they can develop a deep enough understanding of the process they hope to be able to breed plants that respond to temperature differently and are less sensitive to temperature shocks.
Waking up from winter
Buds were bursting early across the UK this year following yet another warm March only to be threatened by a returning cold snap. Warm weather can trigger flowering, even out of season, but it is important for plants to blossom at the right time of year.
Scientists at JIC have recently unpicked why temperature has such a powerful effect. Their research, which was published in Nature, has led to the identification of the switch that accelerates flowering time in response to temperature.
Flowering is triggered by a special molecule, called Florigen. Florigen itself is activated by many signals, including increasing day length during spring. Some plants rely more on temperature, others more on day length to control key stages in their life cycle. While the pathway that activates Florigen in response to daylength has been known for many years, how temperature exerts its control has been a mystery until now.
Led by Dr Phil Wigge, the JIC team have found a gene called PIF4 that triggers Florigen production at warmer temperatures but is unable to act when it is cold. At lower temperatures, plants do still flower eventually but via other pathways. Any acceleration triggered by PIF4 is lost as it does not bind to and switch on the Florigen gene.
"What is striking is that temperature alone is able to exert such specific and precise control on the activity of PIF4," said Wigge.
"Our findings explain at the molecular level what we observe in our gardens as the warmer temperatures of spring arrive. It also explains why plants are flowering earlier as a result of climate change."
For farmers, knowing the best time to harvest a crop is crucial for ensuring they get the best quality produce. Many vegetables are harvested just before flowering or, in the case of brassicas such as broccoli, with young or developing flower shoots.
The timing of plant development can also be strongly influenced by the length and severity of cold experienced over the winter. In the case of crops like winter wheat, a long period of cold accelerates flowering.JIC scientists are uncovering the genes that control this process.
In one project Dr Judith Irwin and team are focusing on flower development in brassicas. Brassicas are grown across the length and breadth of the UK, in a vegetable market worth £330 million, from Jersey, where the Gulf Stream ensures it rarely falls below freezing, to Fife in an area of Scotland, where the UK's record lowest temperature was recorded.
Dr Irwin's research aims to identify whether it will be possible to develop new varieties adapted to varying conditions and year-round demand, for example by breeding for a more predictable flowering time. It will also help breeders plan for warmer winters.
Plants that save water
Plant pores, called stomata, are essential for life. They are tiny conduits for the Earth's carbon and water cycles, enabling plants to regulate water loss and take up the gases they need.
In Jordan over half of the cultivated land is used to grow wheat, yet periodic droughts make harvests vulnerable. If scientists were able to develop plants that close their stomata when they are starved of water, the plants would be better able to cope. Research at JIC is underway to make stomata more responsive to drought conditions. In collaboration with Jordanian scientists, Dr Wendy Harwood is studying guard cells that control the opening and closing of stomata in response to stress. The aim is to transfer the findings to Jordanian wheat varieties.
Keep off our crops
Rising temperatures and increasing drought may be the most obvious threats to agriculture posed by climate change, but the greatest threat to yields could actually be from pests and diseases. For example, some insect vectors, such as leafhoppers that spread small bacteria called phytoplasmas, are sensitive to cold and climate change could provide them with more opportunities to spread to new areas.
The significance of this problem has only recently gained recognition and is an emerging area of research. JIC scientists are investigating how disease resistance in crops could be undermined and which diseases and insect pests might spread or become more virulent as temperatures rise.
Plants become more susceptible to disease under warmer growing conditions, even if the pathogens themselves become less virulent. At the moment, the only way to deal with this is by 'priming' seeds using pesticides. Dr Vinod Kumar has been carrying out fundamental research, supported by a BBSRC Fellowship, to better understand the mechanisms involved in the effect of temperature on crops and crop diseases in the hope that they will come up with an alternative solution that can be bred into new varieties.
A wild grass growing on coastal plains in Israel and South Lebanon could hold the key to stopping one of the world's most threatening cereal diseases: the Ug99 strain of stem rust which is destroying wheat across Africa and moving into the Middle East and Asia.
"The immediate fear in terms of food security is that Ug99 will reach the Northern Punjab in Pakistan and India where nearly a fifth of the world's wheat is grown," said Dr Brande Wulff from the Sainsbury Laboratory (TSL) on Norwich Research Park.
The wild grass is immune to Ug99 and work is underway at TSL to clone several resistance genes and introduce them as a package into wheat.
"We hope to create a formidable obstacle to the pathogen," said Wulff.
However, many populations of the grass are on the edge of extinction as the land they grow on is prime real estate.
Seed banks around the world are at the front line in preserving biodiversity. As well as powerful resistance to crop diseases, wild plants can harbour traits for improved human nutrition, higher yields and more efficient use of nitrogen, sulphur and water.
The seed collections at JIC hold more than 9,500 wild and cultivated wheat varieties, 10,500 of barley, 3,000 types of oats and 3,300 peas from around the world. Some varieties are over 300 years old. They would not have experienced the Mediterranean climate that persisted for much of 1540 but they may be crucial for helping crops cope with the heat waves of the future.
- Historical Warnings of Future Food Insecurity with Unprecedented Seasonal Heat. Science DOI: 10.1126/science.1164363