ions were introduced, while in the AtALMT6-GFP mutant a huge inward rectifying current was observed. When the transporter is knocked out from guard cell vacuoles there is a significant reduction in malate flow current. The current goes from a huge inward current to not much different than the WT, and Meyer et al hypothesized that this is due to residual malate concentrations in the vacuole. There is also a similar response in the knockout mutants to drought as in the WT. There was no phenotypic difference observed between the knockout mutants, the wild type, or the AtALMT6-GFP mutants, and the exact cause for this is not fully known. Signal transduction 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. Guard cells have become a model for single cell signaling. Using Arabidopsis thaliana, the investigation of signal processing in single guard cells has become open to the power of genetics. Cytosolic and nuclear proteins and chemical messengers that function in stomatal movements have been identified that mediate the transduction of environmental signals thus controlling CO2 intake into plants and plant water loss. Research on guard cell signal transduction mechanisms is producing an understanding of how plants can improve their response to drought stress by reducing plant water loss. Guard cells also provide an excellent model for basic studies on how a cell integrates numerous kinds of input signals to produce a response (stomatal opening or closing). These responses require coordination of numerous cell biological processes in guard cells, including signal reception, ion channel and pump regulation, membrane trafficking, transcription, cytoskeletal rearrangements and more. A challenge for future research is to assign the functions of some of the identified proteins to these diverse cell biological processes. Development During the development of plant leaves, the specialized guard cells differentiate from “guard mother cells”. 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