Origins and Evolution of Stomatal Development

Environmental sensing and morphological plasticity in plants

All creatures on earth are affected by their surrounding environment. Animals can move and escape unfavorable environmental changes, whereas plants must respond to environmental stimuli. Plants adapt to changes with cellular-level responses to short-term environmental changes, but may adapt to changes in the environment by regulating their development and growth. In this review, we considered changes in atmospheric CO₂ concentrations, dry/wet moisture conditions, flooding, and temperature as examples of environmental stimuli. We mainly focused on leaf morphology and stomatal density as examples of developmental and growth patterns of plants in response to the environment.

Diurnal Variation in Gas Exchange: The Balance between Carbon Fixation and Water Loss

Stomatal control of transpiration is critical for maintaining important processes, such as plant water status, leaf temperature, as well as permitting sufficient CO2 diffusion into the leaf to maintain photosynthetic rates (A). Stomatal conductance often closely correlates with A and is thought to control the balance between water loss and carbon gain. It has been suggested that a mesophyll-driven signal coordinates A and stomatal conductance responses to maintain this relationship; however, the signal has yet to be fully elucidated. Despite this correlation under stable environmental conditions, the responses of both parameters vary spatially and temporally and are dependent on species, environment, and plant water status. Most current models neglect these aspects of gas exchange, although it is clear that they play a vital role in the balance of carbon fixation and water loss. Future efforts should consider the dynamic nature of whole-plant gas exchange and how it represents much more than the sum of its individual leaf-level components, and they should take into consideration the long-term effect on gas exchange over time.

The Membrane Transport System of the Guard Cell and Its Integration for Stomatal Dynamics

Stomatal guard cells are widely recognized as the premier plant cell model for membrane transport, signaling, and homeostasis. This recognition is rooted in half a century of research into ion transport across the plasma and vacuolar membranes of guard cells that drive stomatal movements and the signaling mechanisms that regulate them. Stomatal guard cells surround pores in the epidermis of plant leaves, controlling the aperture of the pore to balance CO₂ entry into the leaf for photosynthesis with water loss via transpiration. The position of guard cells in the epidermis is ideally suited for cellular and subcellular research, and their sensitivity to endogenous signals and environmental stimuli makes them a primary target for physiological studies. Stomata underpin the challenges of water availability and crop production that are expected to unfold over the next 20 to 30 years. A quantitative understanding of how ion transport is integrated and controlled is key to meeting these challenges and to engineering guard cells for improved water use efficiency and agricultural yields.

Hornwort Stomata: Architecture and Fate Shared with 400-Million-Year-Old Fossil Plants without Leaves

As one of the earliest plant groups to evolve stomata, hornworts are key to understanding the origin and function of stomata. Hornwort stomata are large and scattered on sporangia that grow from their bases and release spores at their tips. We present data from development and immunocytochemistry that identify a role for hornwort stomata that is correlated with sporangial and spore maturation. We measured guard cells across the genera with stomata to assess developmental changes in size and to analyze any correlation with genome size. Stomata form at the base of the sporophyte in the green region, where they develop differential wall thickenings, form a pore, and die. Guard cells collapse inwardly, increase in surface area, and remain perched over a substomatal cavity and network of intercellular spaces that is initially fluid filled. Following pore formation, the sporophyte dries from the outside inwardly and continues to do so after guard cells die and collapse. Spore tetrads develop in spore mother cell walls within a mucilaginous matrix, both of which progressively dry before sporophyte dehiscence. A lack of correlation between guard cell size and DNA content, lack of arabinans in cell walls, and perpetually open pores are consistent with the inactivity of hornwort stomata. Stomata are expendable in hornworts, as they have been lost twice in derived taxa. Guard cells and epidermal cells of hornworts show striking similarities with the earliest plant fossils. Our findings identify an architecture and fate of stomata in hornworts that is ancient and common to plants without sporophytic leaves.

Wheat leaf epidermal pattern as a model for studying the influence of stress conditions on morphogenesis

The leaf epidermis of a monocotyledonous plant is a widely used  model system for studying the differentiation of plant cells, as it  contains readily observable specialized cells. The approach proposed  in this paper uses a growing cereal leaf to study stress-induced dynamic changes in morphogenesis. In the process of formation, the linear leaf of wheat remains in the stationary growth phase for long. This fact permits us to observe a  series of successive morphogenetic events recorded in the cellular  structure of the mature leaf. In studying the cellular architecture of  the wheat leaf epidermis, we obtained and processed confocal 3D images of wheat leaves stained with fluorescent dyes. This  procedure allows an accurate morphometric description and  determination of quantitative characteristics of the leaf epidermal  pattern. Low temperatures are among the factors limiting the  growing of crop plants in the temperate zone. In the present work,  we show significant aberrations of stomatal morphogenesis in the  epidermis of boot leaves of wheat varieties Saratovskaya 29 and  Yanetskis Probat in response to cold stress. We found that  nonfunctional stomata predominated in the zone of maximum  manifestation of stress, whereas in the zones formed before and  after the stress impact, the developmental anomalies come to the  disturbance in the morphogenesis of subsidiary cells. In  Saratovskaya 29, a significant amount of ectopic trichomes formed  in rows predetermined to stoma formation. The proposed approach  can provide standardized qualitative and quantitative data on  stressinduced morphogenesis aberrations in wheat leaf epidermis.  Subsequently, these data can be used for verification of computer  models of leaf morphogenesis. Further study of the mechanisms of  the effect of cold stress on morphogenesis will add to the search for  additional opportunities to increase wheat yields in areas of risky  agriculture.