Halving the water requirements of crops

Water is increasingly making headlines in many countries where, year on year, we’re seeing declines in rainfall resulting in shortages. Cape Town, in South Africa, was on the verge recently of having to turn off the taps completely. And in parts of Australia, farmers are weathering some of the worst drought conditions ever documented. And even where the rainfall is reliable, rising population - and therefore water consumption - mean that there may still be shortages. One of the biggest consumers, globally, is agriculture. So a breakthrough by Mike Blatt, at the University of Glasgow, that’s enabled him to halve plant water requirements, could be a game changer. He’s done it by adding a light-sensing gene to the guard cells that open and close pores on the leaves - these are called stomata - that the plant uses to absorb CO2 so it can grow. This modification means the plants can respond more rapidly to changes in light intensity, so they become much more water efficient. Chris Smith spoke to Mike, to find out more...

Mike - One of the biggest resource demands for plant growth is water. Every attempt people have made in the past to improve photosynthesis, carbon gain and biomass production of a plant, results in increased demand of water simply because the plants rely on small pores in the leaf surface, so-called stomata, for both CO2 entry, CO2 being used by photosynthesis to make sugars, these pores also are, unfortunately a pathway for water loss -  a bit like you and me breathing, we breathe in oxygen to carry out respiration but at the same time we have to breathe out, and when we breathe out we lose water.

Chris - So by virtue of the fact that they have to get the CO2 into the leaf and the inside of the leaf has a lot of water, by allowing the CO2 in, it’s an inescapable consequence that water is going to leak out and be lost and therefore the plant has to maintain a constant supply of water that it's basically throwing away to the atmosphere?

Mike - In a sense yes. Water loss in itself actually drives an engine of circulation within the plants, so it's not necessarily a bad thing but it doesn't necessarily have to be as extravagant as it is in some plants and that's particularly true of crops.

Chris - So what have you done here that you think can improve on this process?

Mike - What we've done is to show that it is possible to increase the water efficiency to the plant and also to increase the efficiency of carbon capture by introducing a new way for the guard cells that surround the stomatal pore to gain and lose solutes which is what drives their opening and closing. And by doing so we've managed to accelerate the rate at which they move, and that acceleration better matches the variation in photosynthesis that the plants see over the course of the day as clouds move overhead or will pass by and that means that the plants are more efficient in their ability to prevent water loss when they don't need to carry out gas exchange.

Chris - Now when you say these guard cells, when we look at these pores on the underside of the leaves down a microscope, you can see these cells that are literally like the gatekeepers which change their shape to allow the pore to open or close and therefore allow water out and carbon dioxide in, when they're open. You're saying that you can manipulate how those cells do their job to make them do it much faster, so that there literally tethering whether they're open or closed much more rapidly to the ambient conditions?

Mike - Precisely, exactly.

Chris - But how have you done that? What have you actually done to the cells?

Mike - We've introduced a synthetic ion channel. In this case we've introduced a channel which we've created to connect the activity of the channel to light, so the channel becomes active when the light is on, it shuts down shortly after the light is switched off so we basically providing a new pathway for solute uptake and loss from the guard cells, and it's that movement of solute in and out of the guard cells that drives the accelerated stomatal movements.

Chris - And how much of a difference does this make to the a) consumption of water and b) the rate at which the plant actually grows?

Mike - In terms of the increase in biomass that we see in the plants, is on the order of a factor of two, and likewise the water use efficiency improved more or less on the same order, a bit less than a factor of two.

Chris - Now could you easily confer what you've done on your laboratory test plants on important plants we grow to keep humans fed, things like soy, things like maize, things like rice, wheat, other cereals and so on?

Mike - In progress. We'll revisit this chapter in another couple of years. We're now looking to get the selection of very similar light dependent controls into a number of crop species including brassica and barley as a starting point. It will be very interesting to get into maize which is obviously tremendously important agriculturally, and also in rice.

Chris - And assuming this does work in those other species, what could be the implications of this?

Mike - If it actually works out it means that in crop species it means that we may well have a number of crops available to farmers in the course of the next decade or so that are substantially more efficient in their use of water, which means that the amount of water that agriculture demands in the field goes significantly downwards. At the moment, agriculture consumes 70% of all freshwater resources on the planet. If we can reduce that by even 20 or 30% that would be a very significant impact on agriculture and on our water use globally.