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Arabidopsis thaliana

Arabidopsis thaliana
Arabidopsis thaliana

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.

11 Comments

All I can say is WOW!

P.S. Such beauty even in microscopics.

I'll add that Arabidopsis thaliana, or thale cress, was the first plant to have its entire genome sequenced.

The Arabidopsis Information Resource, or TAIR, is a central resource for Arabidopsis information. It includes guidelines for nomenclature, e.g., the genes TMM and FLP.

So plants have microscopic "mouths" as shown in the first photo? Whoa...

Coprosma repens, mirror plant, has such large stomata that they can be seen with the naked eye, a handy identifying marker since it's so unusual. Keep up the great work.

Wonderful to see such important and impressive parts of plants, along with our usual perspective. Thanks!

thank you all
this is quite an event for me
to have so many people be so shareing
and lead me on pathways unknown to me

the camera work is so good
conner happy to have you around







Thanks for these fantastic pictures and information.

A whole book about thale cress is Seed To Seed by Nicholas Harberd, 2006 : he describes both what can be seen with the naked eye and the hidden molecular mechanisms...the story of the last 10 years of discovery in his own lab (UK), as the team works to understand the genetic control of the growth of thale cress...etc.

amazing! first glimpse looked like compound heart valves, vascular for sure, but maybe human but compound! thanks for the view in insights!

As a retired physician, I enjoy reading articles such as this and comparing them to similar molecular advances in medicine. However, I can only wish that we had such names for genes as TOO MANY MOUTHS or FOUR LIPS instead of the gibberish of numbers and letters assigned by medical researchers. It also makes them vastly easier to remember! Thanks for the fascinating information.

bev

Ihave some wildtype arabidopsis under lights in my office, this is a very interesting piece of info for me and my co-workers. I use mine to infect them with powdery mildew :(
Susanne

I'm looking for a beautiful micro photograph of either corn leaves or sugar cane leaves showing the stomata. It's for an article about plant production for biofuel in American Scientist magazine. I'm having difficulty finding images for these specific plants. Any help would be greatly appreciated!

Barbara Aulicino, Art Director
American Scientist

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