For example, in some moss species such as Physcomitrella patens and Funaria hygrometrica, guard mother cells undergo incomplete cytokinesis, which results in a single guard cell encasing a stomatal pore ( Sack and Paolillo, 1985 Chater et al., 2016). The myriad programs of epidermal growth and development adopted by different species result in a diversity of stomatal ontogeny ( Rudall et al., 2013). With an earlier appearance than vascular tissues and roots ( Peterson et al., 2010 Chen et al., 2017), stomata are thought to have evolved once ( Raven, 2002) and exist in almost all terrestrial plants except liverworts, although some liverwort species have a 16-cell barrel-shaped structure called the air pore complex that might serve a function similar to stomata ( Jones and Dolan, 2017). One of the most crucial adaptations for plants to colonize land is the innovation of stomata over 400 million years ago ( Edwards et al., 1992 Berry et al., 2010). We also raise open questions and provide a perspective on experimental approaches that could be used in the future to shed light on the composition and architecture of guard cell walls. In this review, we highlight and discuss the latest evidence for how wall polysaccharides are synthesized, deposited, reorganized, modified, and degraded in guard cells, and how these processes influence stomatal form and function. Although the walls of guard cells were long underexplored as compared to extensive studies of stomatal development and guard cell signaling, recent research has provided new genetic, cytological, and physiological data demonstrating that guard cell walls function centrally in stomatal development and dynamics.
In contrast to the walls of diffusely growing cells, guard cell walls have been hypothesized to be uniquely strong and elastic to meet the functional requirements of withstanding high turgor and allowing for reversible stomatal movements.
Guard cells, like other types of plant cells, are surrounded by a three-dimensional, extracellular network of polysaccharide-based wall polymers. Guard cells are pairs of epidermal cells that control gas diffusion by regulating the opening and closure of stomatal pores.
2Intercollege Graduate Degree Program in Plant Biology, The Pennsylvania State University, University Park, PA, United States.1Department of Biology, The Pennsylvania State University, University Park, PA, United States.Yue Rui 1,2†, Yintong Chen 1,3, Baris Kandemir 4, Hojae Yi 5, James Z.
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