Dark Side of the Stomata: New mutations controlling Guard Cell Aperture

One of the most fundamental environmental stimuli plants experience is the daily light-dark cycle – failure to coordinate biological processes with the external light-dark cycle results in significant decreases in plant productivity (see Dodd et al. 2004 for a really nice paper exploring this). One of the most important processes to regulate in response to the daily cycle is transpiration; in C3 plants* guard cells must open during the day to allow CO2 influx for photosynthesis, but also must close at night to reduce excess water loss. Much work has been done on understanding the opening of stomata in response to light, but little has been done on the prolonged closure of stomata at night. However, a new paper in Plant Physiology identifies new mutants which may give us insight into the control of stomata at night.

The authors of the study screened a population of Arabidopsis thaliana plants which had been chemically mutagenised with EMS (note this is an example of forward genetics). The researchers were looking for plants which were cooler than expected during the night – as transpiration will result in evaporative cooling, plants with open stomata will be cooler than those where the stomata are closed. The lines generated were named open all night long (opal) mutants, with 5 separate loci being identified through the screen.

Thermal imaging of the opal mutants. Arabidopsis rosettes shown above, and false-colour thermal images shown below. Image reproduced from Plant Physiology.
Thermal imaging of the opal mutants. Arabidopsis thaliana rosettes shown above, and false-colour thermal images shown below. Image reproduced from Plant Physiology.

Analysis of gas exchange and stomatal aperture in the opal mutants confirmed that the lines identified were defective in their ability to regulate their guard cells at night. The opal mutants were also tested for their responsiveness to the stomatal closing stimuli CO2; and abscisic acid (ABA), however there were no significant differences between the wildtype controls and the opal mutants when treated with these stimuli. The authors therefore propose that OPAL induced closure and CO2/ABA induced closure are independent from each other.

The study doesn’t map the opal mutations, so for the moment we don’t know what the molecular basis of the stomatal phenotype is. However these mutants do now give us a tool for understanding dark-regulation of guard cells, so I await the cloning of the OPAL genes with interest.

*Of course things are different in CAM plants, which we will meet later in the IA PoO lectures.

Reference: OPEN ALL NIGHT LONG: The Dark Side of Stomatal Control. Costa et al. (2015) Plant Physiology 167:289-94 http:/​/​dx.​doi.​org/​10.​1104/​pp.​114.​253369

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