Guard cell

Photosynthesis depends on the diffusion of carbon dioxide (CO2) from the air through the stomata into the mesophyll tissues.

Guard cells contain phototropin proteins which are serine and threonine kinases with blue-light photoreceptor activity.

[5] The phototropins trigger many responses such as phototropism, chloroplast movement and leaf expansion as well as stomatal opening.

[6] This was done by adding phosphopeptides such as P-950, which inhibits the binding of 14-3-3 protein, to phosphorylated H+-ATPase and observing the amino acid sequence.

[citation needed] Opening and closure of the stomatal pore is mediated by changes in the turgor pressure of the two guard cells.

Guard cells have cell walls of varying thickness(its inner region, adjacent to the stomatal pore is thicker and highly cutinized[7]) and differently oriented cellulose microfibers, causing them to bend outward when they are turgid, which in turn, causes stomata to open.

These two things are crucial in causing the stomatal opening to close preventing water loss for the plant.

Potassium channels and pumps have been identified and shown to function in the uptake of ions and opening of stomatal apertures.

(b) Anion channels are activated by signals that cause stomatal closing, for example by intracellular calcium and ABA.

This transport channel was found to cause either an influx or efflux of malate depending on the concentrations of calcium.

[40] When the transporter is knocked out from guard cell vacuoles there is a significant reduction in malate flow current.

[40] Guard cells perceive and process environmental and endogenous stimuli such as light, humidity, CO2 concentration, temperature, drought, and plant hormones to trigger cellular responses resulting in stomatal opening or closure.

These signal transduction pathways determine for example how quickly a plant will lose water during a drought period.

Using Arabidopsis thaliana, the investigation of signal processing in single guard cells has become open to the power of genetics.

A challenge for future research is to assign the functions of some of the identified proteins to these diverse cell biological processes.

[43][44] The density of the stomatal pores in leaves is regulated by environmental signals, including increasing atmospheric CO2 concentration, which reduces the density of stomatal pores in the surface of leaves in many plant species by presently unknown mechanisms.

The genetics of stomatal development can be directly studied by imaging of the leaf epidermis using a microscope.

Several major control proteins that function in a pathway mediating the development of guard cells and the stomatal pores have been identified.