Today's entry, organized by Connor Fitzpatrick, is the fourth in a BPotD series for UBC Research Week. The photographs and write up come courtesy of Dr. Fred Sack, Professor and Head, Department of Botany.

Each leaf contains thousands of pores, stomata, which allow gas exchange between the atmosphere and the shoot. Stomata are cellular valves central to plant survival because they allow carbon dioxide to enter leaves where it is used to make sugars in photosynthesis. Stomata are also adaptive because they close down when water loss becomes too great. Efficient gas exchange seems to require that valves be spaced apart from each other since it is rare in nature to find two stomata in direct contact.

My lab pioneered the discovery of genes required for stomatal formation and spacing. We first determined how stomata develop and are distributed in the model eudicot Arabidopsis. As in all plants, stomatal formation requires an initial division that is unequal in size and fate, generating a smaller cell and a larger cell. After the smaller cell becomes oval in profile, it divides equally thus producing the two young guard cells that develop into the stoma. Meanwhile the larger cell produced by the unequal division can in turn divide asymmetrically. Normally this “piggyback” (iterative) division is oriented so that the new small precursor cell does not contact the previously formed one, a placement that generates the minimal one-celled separation between stomata. This placement probably requires intercellular communication, a conclusion reinforced when we identified the TOO MANY MOUTHS gene which encodes a probable receptor. Defects in TMM induce spacing violations, suggesting that it normally receives spatial cues used to correctly orient “piggyback” divisions. TOO MANY MOUTHS acts exclusively in the cells that form stomata as shown by the distribution of green fluorescent protein in the accompanying picture (red shows the cell walls; note that stomata are still forming in this picture; reproduced from Nadeau and Sack, Science). Thus this gene, which is conserved in monocots as well, controls the division behavior of islands of stem cells distributed throughout the epidermis of the developing shoot.

We also found that a different gene, FOUR LIPS, is required to ensure that there is only one equal division of the GMC (the guard mother cell is a precursor to guard cells). Mutations in FLP induce extra, abnormal, equal divisions resulting in four guard cells (lips) in a row (“stoma” comes from the Greek for “mouth”). We found that FLP is a transcription factor that regulates genes involved in cell cycling. Additional genes in this pathway are being identified in collaboration with Erich Grotewold at Ohio State University. It is likely that restricting GMC divisions to one (failsafe) would be strongly selected for in evolution since the control of water loss and the efficiency of carbon dioxide uptake are critical for plant survival.

The first photograph was taken using cryoscanning electron microscopy. The second photograph was taken using confocal laser scanning microscopy. The red channel shows the cell outlines (cell walls labeled with propidium iodide), and the green channel shows where the gene TMM is expressed.