And drought tolerance is just the tip of the Iceberg for these hard working gate keepers called Guard Cells!
Tomato Leaf Stomate
Like the picture of the tomato leaf above, all plant's have these oval pores called stomata (singular stoma). I suppose microscopically they would be a sort of peppered like polka dots. Now surrounding these Stomata pores are guard cells that control the rate of moisture escape and carbon dioxide intake. Under dry conditions, these guard cells simply relax and the pores close. When moisture is more abundant, they stiffen and the pore slowly opens. But how do they accomplish this ? How do they know when it is dry and what causes these pores to open their doors ?
Information has come along in the past explaining the amazing communications networks within plants and this subject is no exception. But another paper has recently come out in the journal, Current Biology: which found something interesting about those guard cells knowing how to make Abscisic acid (ABA) from scratch. It's this abscisic acid which is used to close the stoma.
"Stomata are pores on the leaf surface, bounded by two guard cells, which control the uptake of CO(2) for photosynthesis and the concomitant loss of water vapor. In 1898, Francis Darwin showed that stomata, close in response to reduced atmospheric relative humidity (rh); however, our understanding of the signaling pathway responsible for coupling changes in relative humidity to alterations in stomata aperture is fragmentary. The results presented here highlight the primacy of abscisic acid (ABA) in the stomatal response to drying air. We show that guard cells possess the entire biosynthesis pathway and that it appears unregulated by positive feedback by ABA. When wild-type Arabudopsis and the ABA-deficient mutant aba3-1 were exposed to reductions in relative humidity, the aba3-1 mutant wilted, whereas the wild type did not. However, when aba3-1 plants, in which ABA synthesis had been specifically rescued in guard cells, were challenged with dry air, they did not wilt. These data indicate that guard cell-autonomous ABA synthesis is required for and is sufficient for stomatal closure in response to low relative humidity. Guard cell-autonomous ABA synthesis allows the plant to tailor leaf gas exchange exquisitely to suit the prevailing environmental conditions."
|credit: Science Direct|
Figure 4. Complementation of the aba3-1 Wilty Phenotype by Guard Cell-Specific ABA SynthesisExcised leaves ofArabidopsis WT plants supplied with water were able to adjust their stomatal aperture when challenged for 3 hr with dry air and survived under these conditions (upper panel) as described previously . aba3-1 plants, lacking stress-induced ABA, were unable to close their stomata and wilted in consequence of excessive water loss (middle panel). ABA3 expression under the control of the guard cell-specific MYB60-promoter led to ABA synthesis in the aba3-1 mutant background in a guard cell-specific manner and thus complemented the wilty phenotype of aba3-1 mutants (lower panel).
There are actually more and more amazing this going on here. The whole mechanism and it's various components are not completely well understood as the above abstract and the sentence I've highlighted do indicate. But there are far more signaling mechanisms triggered by differing sensors going on here than a mere guarding of the gates by opening and closing pores. These openings and closing and other functions are triggered by other environmental factors aside from humidity and dry air. Apparently other signals can effect this area. Here's a great link I found that goes into incredible detail
See, Julian Schroeder's Lab page: "The Clickable Guard Cell: Electronically linked Model of Guard Cell Signal Transduction Pathways"
Light-induced stomatal opening
Guards cells respond to a multitude of signals including temperature, partial CO2 pressure, light, humidity, and hormonal stimuli. For the majority of signals the molecular identity of the censors is not known, with the notable exception of blue light.
ABA-induced stomatal closing
One of the best understood plant signaling networks is the one triggered by ABA in guard cells, causing stomatal closure. As shown in Figure 3 , many genes encoding for positive as well as negative regulators of guard cell ABA signaling have been identified in Arabidopsis . The gene(s) encoding the ABA receptor(s), however, remained elusive. Most of the ABA signal transduction components were found by classic "forward" increased or reduced sensitivity to ABA.
|Credit: UC San Diego - Schroeder's Lab|
|Credit: Ross E. Koning 1994|
Very informative and fascinating read.
When were the major classes of plant hormones identified and who is associated with their identification?
"Absicisic Acid - Philip F. Wareing discovered large amounts of a growth inhibitor in the dormant buds of Ash Trees and Potatoes that he called Dormin. Several years later in the 1960s, Frederick T. Addicott (1913-2008) reported the discovery in leaves and fruits of a substance capable of accerating absicission that he called abscisin. It was soon discovered that Dormin and Abscisin were identical chemically."
Another important factor in the guard cells opening and closing of these gates based on environmental censor response is natural weather modification and creation through the release of VOCs we know as aerosols which are necessary for the nuclei by which water vapor is able to form droplets which leads to cloud formation.
"Aerosols such as Isoprenes, especially over forests where they are released by trees and other vegetation can actually create & cause cloud formation. These isoprenes are dispensed through those same gates by those guard cells as referenced above. Under certain atmospheric conditions such aerosol releases are triggered which sets in motion climate creation events. Removal of forests and other vegetation is bringing about this climate change.~@
However, isoprenes also can mix with human pollutants and create worse pollution over populated areas. Unfortunately, the idiots pimping biofuels who are being blamed for this increase in pollution have an answer. This was reported January 6, 2013. Here's a genius quote from the article:
Poplar, willow or eucalyptus trees, all used as fast-growing sources of renewable wood fuel, emit high levels of the chemical isoprene as they grow, the study said. Isoprene forms toxic ozone when mixed with other air pollutants in sunlight.
Siting new biofuel plantations far away from polluted population centres would help limit ozone formation, the study suggested. Genetic engineering might be used to reduce isoprene emissions, it said.
There appears no end to the stupidity and retardation when it comes to profits obsessed science. What would happened if they had their way and stopped these trees from emitting isoprene and any other volatile organic compounds which aid in cloud formation, and/or haze over cities when mix with other human caused pollutants ? Destroy a trees ability to produce these and climate further suffers. Especially if these polluted genetics ever get out into the wild and infect other trees in nature. Think you have Global Warming now, wait till GMO industries have their way in Industrial Forestry.