Evolution of Guard-Cell Theories: The Story of Sugars. (June 2019)

• Record Type: Journal Article
• Authors: Granot, David; Kelly, Gilor
• Is Part Of: Trends in plant science. Volume 24:Number 6(2019)
• Page Start: 507
• Page End: 518
• Publication Date: 2019-06
• Subjects: stomata -- guard cell -- sugar -- sucrose -- hexokinase; Botany -- Periodicals; Botanique -- Périodiques; Botany; Periodicals; 580.5
• DOI: 10.1016/j.tplants.2019.02.009
• Languages: English
• ISSNs: 1360-1385

Abstract

Abstract : Stomata are dynamic pores in the impermeable cuticle that coats the aerial parts of vascular plants, allowing the entry of CO2 for photosynthesis and controlling water loss. They are composed of two guard cells that can swell or shrink due to an increase or decrease in their osmotic pressure, respectively. Swelling opens the stomata and shrinking closes the stomata. For more than a century, scientists have been working to uncover the nature of the osmolytes that modulate osmotic pressure in guard cells. Recent discoveries have undermined long-standing theories in this area, reversing the understood roles of sugars and demonstrating the evolution of scientific theories. Here, we describe the evolution of guard-cell osmoregulation theories with an emphasis on the role of sugars. Highlights: For many years, it was believed that sugars obtained from guard-cell starch degradation act as osmolytes that open stomata. Numerous studies have tried to validate the starch-sugar hypothesis by correlating guard-cell starch content, sugar content, and stomatal aperture. The discovery in the 1940s that ions are the primary osmolytes that open stomata led to a temporary decline in the popularity of the sugar theory. However, in the 1990s, a modified sugar theory based on correlations between sugar content and stomatal aperture became popular. Since 2013, functional studies have revealed that sugars do not act as guard-cell osmolytes but rather are sensed by hexokinase (a sugar-sensing enzyme) within guard cells to close stomata. Sugar production involves significant loss of water through the stomata and the closure of stomata by sugars coordinates sugar production with water loss.