Impact of Stomatal Density and Morphology on Water-Use Efficiency in a Changing World

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• The development of crop varieties able to sustain or improve yields with less water input is, therefore, a priority for crop research. Almost all water used for plant growth is lost to the atmosphere by transpiration through stomatal pores on the leaf epidermis. By altering stomatal pore apertures, plants are able to optimize their CO2 uptake for photosynthesis while minimizing water loss.

• Changes in climate are already negatively affecting the yields of staple crops in agricultural areas around the world

• Agriculture will need to adapt quickly to ensure that water is used more efficiently, while maintaining food security in a world where human population is rapidly growing

• Moreover, the development of crop varieties that have improved water-use efficiency (WUE) under predicted future climates will also be critica

• WUE is the amount of CO2 fixed in photosynthesis (A) relative to the amount of water vapor lost to the atmosphere

• Stomata are microscopic structures consisting of a pair of specialized guard cells that surround a central pore.

• By actively adjusting guard cell turgor pressure, plants can alter stomatal pore aperture, thereby moderating gas exchange rates between the leaf interior and the atmosphere

• stomatal density (SD, number of stomata per unit of area)

• The impact of water availability on stomatal development is less understood,

• how targeted modifications of stomatal traits affect physiological responses in crop plants, especially in field experiments.

• Here, we discuss our current understanding of how alterations in SS, SD, and stomatal morphology contribute to altered WUE and drought tolerance with particular emphasis on the latest advances in crop species.

• Dynamic adjustments to the opening degree of stomatal pores are responsible for regulating gs in the short term, allowing plants to quickly reduce water loss according to external cues (Farquhar and Sharkey, 1982).

• Several approaches to improve drought tolerance and water-use efficiency through the modification of stomatal traits have been tested in the model plant Arabidopsis thaliana

• For example, when water becomes limited, signals such as reduced hydraulic conductivity and increased abscisic acid (ABA) arise, causing guard cell turgor pressure decreases, which result in reduced stomatal aperture and gs

• Over evolutionary time, various stomatal traits have altered, potentially aiding in adapting plant species to new environments (Taylor et al., 2012; Drake et al., 2013; Haworth et al., 2018).

• changing stomatal traits may improve WUE without such undesirable yield penalties (Yu et al., 2013; Hughes et al., 2017; Caine et al., 2019).

• In Arabidopsis, the overexpression of AtEPF2 results in plants with particularly low SD

• This has allowed researchers to begin to test the implications of targeted manipulations in stomatal density in grasses, a family of plants that comprises many important food crops.

• Taken together, these data indicate that it is possible to improve WUE by altering gsmax and gs using genetic engineering tools

• Although results are yet to be demonstrated in the field, in overexpressing orthologs of Arabidopsis SDD1 in maize and tomato, respectively, Liu et al. (2015) and Morales-Navarro et al. (2018) have been able to reduce leaf SD, leading to reduced water consumption and improved drought tolerance in both crops, as well as improved WUE in maize