Beating the heat is one of the goals most vigorously pursued by plant geneticists. A solution is not yet in sight, but after so many years of research, it is clear that there are several avenues worth exploring. The three most important things are testing, testing, testing.
The first consideration is that plants can adopt different strategies to survive when water is scarce. You can distinguish between drought resistance and water use efficiency, or go subtle by talking about drought avoidance, drought escape, and drought tolerance.
Another basic premise is that drought can vary in intensity and duration, so that a plant capable of tolerating moderate stress may still succumb under more extreme conditions. Further complicating matters is the fact that, to be adopted by farmers, future crops will have to prove not only more resilient but also as productive as the varieties they are intended to replace. Two strategies are being pursued at the University of Milan with the help of CRISPR.
The first falls into the avoidance typology and aims to reduce transpiration by half-closing stomata, the pores responsible for gas exchange in the leaves. The second aims to anticipate flowering, to harvest before the drier months arrive, and is considered a form of escape. Lucio Conti is involved in both, and I asked him to give an overview of the state of the art.
“The history of our research on stomata regulation started many years ago from studies performed on the model plant Arabidopsis thaliana and is now continuing on tomato as part of the Biotech Project funded by the Ministry of Agriculture,” Conti says. The researchers led by Chiara Tonelli had identified a transcription factor expressed in the pair of guard cells coontrolling the of the stomatal opening but not in other parts of the plant. So they had good reason to hope that repressing it would not interfere with other physiological processes.
“The gene is called MYB60 and it is an interesting target because when it is inactivated with a mutation, the plant saves water and can better withstand drought,” Conti explains. The pores do not close completely, which is good, because a drastic reduction in gas exchange would compromise the plant’s performance. We are lucky that the gene has also been found in other dicotyledonous species, for example in grapevine, tobacco and tomato, and that it has since become easier to edit genomes, thanks to CRISPR.
“This allowed Massimo Galbiati and me to develop tomato plants with different deletions. Having lines with different mutations will allow us to choose the best ones,” Conti adds. Selection is underway and the first greenhouse observations look promising, but we will have to wait until spring to collect reliable data. “It would be important to do field trials as well, so we hope that authorizations for open field tests with genetically modified plants will be unblocked as soon as possible,” comments Chiara Tonelli, who has been appointed president of the Italian Life Sciences Federation.
Plant species have different water needs, and it is unfortunate that the equivalent of MYB60 has not been found in cereals. Tonelli gives us the numbers: “To get a tomato you need 13 liters of water, for an apple 70 liters, for a kilo of corn, wheat and rice you need nine hundred, one thousand and three thousand liters respectively.” Agriculture consumes 70 percent of fresh water, in comparison domestic uses and industry use 8 and 22 percent, she says. “Drought is the first cause of loss of production, it causes migration and conflict. In short, it is important to develop crops that are able to grow well in conditions of water scarcity but also in normal conditions, because it is not possible to know whether a year will be more or less dry,” Tonelli sums up.
One way is to make plants more responsive to early signs of water stress by acting on certain plant hormones. “In drought, abscisic acid or ABA is a key chemical signal, and we would like to act on this signal cascade. By deactivating the genes that restrain it, we could get the plants sensitized and therefore more ready to activate defensive strategies,” Conti explains. There are many genes involved, so it will be necessary to study them one by one and in different combinations to find the right balance between responsiveness and growth.
The ABA pathway affects many processes besides stomatal regulation, including the timing of flowering in which Conti is particularly interested. “It is one of the crucial traits, because if flowering is scheduled within a certain time window, the plant can make seeds before the drought comes.” Basically, it’s a matter of compressing the reproductive phase into the rainy season, playing ahead of the dry months. It is not certain, however, that what works for Arabidopsis will be successful in crops, which have lost some of their plasticity due to the selection done so far.
“A study by François Tardieu compared maize varieties grown over more than 50 years. It shows that breeders have acted on constitutive traits such as flowering and leaf architecture, but not on stomata regulation, which could have been disadvantageous for productivity in a hitherto relatively favorable scenario,” Conti says. Now, however, the looming climate crisis is changing the cost-benefit ratio for this and other traits, such as the dramatic increase in root mass triggered by the DRO1 gene (another avoidance strategy). “Several groups are working on this because having more developed roots allows you to reach water deeper, although investing in roots is a cost to the plant and the benefit is lost if the soil is shallow,” Conti says.
In short, drought tolerance is an extremely complex character, or rather set of characters, and many paths will need to be explored before effective tricks can be found. In the past, researchers have tried classical breeding and even genetic engineering, but water stress-resistant varieties on the market can be counted on the fingers of one hand. Hopefully, increased awareness of the challenge posed by climate change and the advent of a versatile tool such as genome editing will help accelerate research and bring tangible results.