Making space for the pore: Structural characterization of stomatal movements when guard cells and epidermal cells interact

Abstract

Stomatal pores on vascular plants modulate plant gas exchange to increase CO2 uptake and to prevent excessive dehydration and loss of xylem function due to cavitation. Stomatal mechanisms control plant capacity for productivity and ecosystem carbon water fluxes. The stomatal mechanism present among many angiosperms succeeds in increasing gas exchange capacity by moving both guard cells and adjacent epidermal cells and forming pores wider than would be possible with only guard cell movements. This dissertation seeks to further understand guard-epidermal cell (GC/E) interactions among angiosperms and characterize the effect of these interactions on stomatal response to water loss. A key question regarding stomatal mechanics is the extent to which pore formation results from changes in guard cell shape (oval to round) versus expansion into space previously occupied by adjacent epidermal cells. There are few direct observations of GC/E interactions in intact leaves. In chapter 1, I use cryo-SEM and 3D confocal microscopy to describe guard cell shape differences between closed and open stomata of Vicia faba and Tradescantia virginiana. I then characterize the three-dimensional interactions between guard cells and epidermal cells as stomata move from a closed to an open state. My results demonstrate that stomatal opening in these two angiosperms is entirely the result of guard cells moving into and deforming adjacent epidermal cells. Angiosperms are credited with the use of GC/E interactions since many of their eudicot members have this stomatal mechanism and the presence of GC/E interactions explains the high photosynthetic productivity seen among them. Yet angiosperm species that remain in understory environments and have low photosynthetic and gas exchange capacities have not been tested for the presence of GC/E interactions. In chapter 2, I determine that GC/E interactions are present in several ANITA grade species, suggesting that this stomatal mechanism might be widespread among angiosperms. I also show that the interaction between guard cells and epidermal cell introduces a substantial degree of variability in stomatal openings within and between species. The high variation in stomatal opening is possible because of GC/E interactions. Stomatal models and experimental studies have shown that the GC/E interactions also causes transient stomatal openings in response to hydropassive dehydration of the leaf. This stomatal opening in the ‘wrong’ direction of eventual stomatal closure response to water loss is considered a tradeoff for having a GC/E mechanism. In chapter 3, I explore the risk of wrong way stomatal openings (WWR) to leaf dehydration, especially in droughted plants with already decreased plant water status. I find that WWR magnitudes in V. faba, Vigna unguiculata and Vitis vinifera, decreases with drought. The results of this thesis clarify that every step of stomatal opening in angiosperms is a result of guard-epidermal interactions. Advancing our understanding of GC/E interactions will be important if we are to integrate tissue level responses of stomata into an understanding of plant-environmental interactions at all scales.