Fig. 1.

Transmission electron microscopy (TEM) reveals large connections between the bulbous ends of dumbbell‐shaped guard cells (GCs). (a) Fluorescence and TEMs of sections of stomata of the grass species Brachypodium distachyon, Barley (Hordeum vulgare), Maize (Zea mays). Bars, 20 μm. Whilst each species has dumbbell‐shaped GCs flanked by a pair of lateral subsidiary cells (SC), the specific geometry of these differs between grass species, as can be seen when observing stomata in paradermal section. Planes of section for TEM images refer to those as described in (c). Transverse sections taken across the rod region (i) show the characteristic thick GC wall associated with grass stomata is present in each of the species. In comparison, the cell wall in the bulbous ends (ii) is relatively thinner. In the bulbous ends of each of the grass species, gaps in the cell wall connecting the protoplast of the adjoining GCs in the pair are present (highlighted using cyan arrowheads). Cell wall thickness in SCs is more uniform along the length of the stomatal complex, with no clear differentiation between the rod and bulbous end regions. SSC indicates the substomatal cavity, whilst epidermal pavement cells are labelled EPC. Below, images have been false‐coloured to highlight the GC wall (magenta) and protoplast (green). (b) Paradermal fluorescence and cross‐sectional TEMs of stomata in onion (nongrass monocot). In contrast to grass species, onion stomata are constituted of a pair of kidney‐shaped GCs and lack specialised SCs. Planes of section for TEM images as described in (d). In the mediolateral (iii) regions, GCs show variable wall thickness, having thicker ventral, anticlinal, and periclinal cell walls relative to the dorsal wall. In polar regions (iv), the GC wall is more uniform in thickness. The ventral wall in this region has no gaps and the GCs within a pair are isolated from one another. Below, images have been false‐coloured to highlight the GC wall (magenta) and protoplast (green). SSC indicates the substomatal cavity. Bars, 5 μm. (c) Grass stomata are composed of a pair of dumbbell‐shaped GCs (green), flanked by a pair of lateral SCs (white). Each GC can be split into distinct regions: a central rod region (transverse section shown by dashed line i) adjacent to the stomatal pore – where the cell wall (magenta) is thick – and two polar bulbous ends (transverse section shown by dashed line ii), which have a relatively thinner cell wall. In addition to a thinner cell wall, these regions additionally feature gaps in the ventral cell walls of the GCs, symplastically connecting the GC pair. GCs are symplastically isolated from their neighbouring SCs. (d) GCs (green) of onion and many other nongrass species are kidney‐shaped and encircle a central pore. Across the mediolateral axis of the complex (transverse section shown by dashed line iii), variable thickness of the cell wall is exhibited. The dorsal wall of the GC can be observed to be thinner than the ventral, periclinal, and anticlinal cell walls. Towards the polar regions of the GCs (transverse section shown by dashed line iv), cell wall thickness is more uniform, and the adjacent GCs are symplastically isolated from one another, having complete ventral cell walls in is a general feature of most nongrass species.