Researchers discovered that overexpressing a sugar-sensing enzyme called hexokinase in field-grown tobacco plants improved intrinsic water-use efficiency (iWUE) without reducing photosynthetic rates or biomass production.
According to a 2019 water report from the United Nations Food and Agriculture Organization (FAO), water deficit is currently one of the most significant limiting factors for global agricultural productivity, a factor exacerbated by global climate change. As a result, researchers around the world have been working to improve water-use efficiency in crops in order to better cope with water scarcity.
In a recent study published in the Journal of Experimental Botany, a team from the University of Illinois, the Volcani Center (Agricultural Research Organization, Israel), and the University of Cambridge found that by overexpressing a sugar-sensing enzyme, called hexokinase, in field-grown tobacco plants, they could improve intrinsic water-use efficiency (iWUE) without decreasing photosynthetic rates or biomass production.
Tobacco was used as a model crop because it is relatively easy to work with within the laboratory, greenhouse, and field. This crop produces results much faster than food crops, which are more difficult and time-consuming to modify and grow. As a result, tobacco was chosen as the first test crop to see if similar results could be demonstrated. After demonstrating success in the model crop, the researchers can confidently replicate the findings in food crops such as cassava, cowpea, rice, and soybean.
Our findings confirm that constitutive overexpression of AtHXK1 reduces productivity. We also demonstrated that guard-cell-targeted AtHXK1 overexpression could improve iWUE relative to wild-type without impairing CO2 assimilation. Nonetheless, this difference was strongly dependent on leaf age, and recent rainfall could have eliminated performance differences.
Liana Acevedo-Siaca
This study demonstrates the potential to generate plants with more conservative water-use throughout the growing season under field conditions and moderate water limitation, without significant yield penalty. For farmers, this could decrease soil water depletion throughout the growing season and reduce reliance on irrigation.
This research is part of the Realizing Increased Photosynthetic Efficiency (RIPE) project, which aims to increase global food production by developing food crops that convert the sun’s energy into food more efficiently, with funding from the Bill & Melinda Gates Foundation, the Foundation for Food & Agriculture Research, and the United Kingdom’s Foreign, Commonwealth, and Development Office.
To take in CO2, plants open tiny pores in their leaves called stomata during photosynthesis. When the pores are open, however, water can escape through transpiration. This forces plants to choose between losing too much water and absorbing too much CO2.
“Stomatal pores consist of a pair of guard cells that control the opening and closure of the pores,” said Liana Acevedo-Siaca, who led this study at Illinois during her time as a postdoctoral researcher. “Previous studies have shown that genetic manipulation of signal elements that trigger stomatal movement, such as overexpressing Arabidopsis Hexokinase 1 (AtHXK1) in the guard cells, can stimulate stomatal closure and adjust that trade-off for plants.” Acevedo-Siaca now works as an Associate Scientist in the Global Wheat Program at the International Maize and Wheat Improvement Center (CIMMYT) in Mexico.
It was previously shown that guard-cell-targeted expression of AtHXK1 can improve WUE in crops, as well as their tolerance to drought conditions and salinity stress because hexokinase signals to the pores that there is enough sugar, eliminating the need to fix more CO2. However, these previous studies were only evaluated in crops grown in controlled environments, such as greenhouses.
“To improve our understanding of the potential benefits of guard-cell-targeted AtHXK1, our study used two homozygous transgenic lines expressing AtHXK1 and a line that had guard-cell-targeted overexpression of AtHXK1 that were evaluated relative to wild-type field-grown tobacco to test WUE for traits related to photosynthesis and yield,” said Johannes Kromdijk, assistant professor at the University of Cambridge, who started this study in 2018.
“Our findings confirm that constitutive overexpression of AtHXK1 reduces productivity. We also demonstrated that guard-cell-targeted AtHXK1 overexpression could improve iWUE relative to wild-type without impairing CO2 assimilation. Nonetheless, this difference was strongly dependent on leaf age, and recent rainfall could have eliminated performance differences.”
The RIPE project and its sponsors are committed to ensuring Global Access and making the project’s technologies available to farmers in the most need.
