Guest Post. Barring the gates: How plants defend against invading bacteria
This week I have swapped blogs with S.E. Gould (@labratting) from Scientific American. Visit her blog to see my post on communication between legume plants and bacteria during symbiosis.
Barring the gates: How plants defend against invading bacteria
Bacteria will exploit any opportunity to invade a new living space, in particular taking advantage of any easily-colonisable entrances into other living organisms. In plants one of these entrances is a doorway between the interior of the leaf and the outside air in order to exchange gases. Plants require carbon dioxide in order to carry out photosynthesis, and this has to be brought into the interior of the leaves. Similarly the excess oxygen produced must leave the cells before it builds up to toxic levels. In order to achieve this, plants have small holes in the leaves formed by two curved cells, known as guard cells, with a gap between them. These gaps are called stomata (singular stoma).
The stomata aren’t kept open all the time, but close and open at different times of the day depending on how much the plant needs them. This is controlled by the plant hormone abscisic acid which regulates the amount of water found in the guard cells. When the guard cells fill with water they expand and arch outwards – ballooning out to form two large curves with the pore between them. When they lose water they shrink back down and close the pore. Abscisic acid can regulate the water content of the guard cells by controlling ion channels in the guard cells, changing the osmotic pressure compared to surrounding cells and causing water to diffuse in or out.
As these stomata form a passageway between the outside world and the interior of the plant, it makes sense to shut them in the presence of bacteria. Researchers exploring stomata control found that stomata tend to shut when the plant is exposed to bacteria. However when they used a strain of plants with the gene for the enzyme LOX1 removed the stomata remained open. LOX1 is involved in the production of fatty acids and has nothing to do with abscisic acid, or the abscisic acid regulation of guard cell water content.
The graph shows four strains of plant; the wild type, two mutants without the gene for LOX1 and a mutant which has had the LOX1 gene removed and then added back in on a little loop of DNA. The size of the stomata is shown under four conditions with the control, normal light, on the left. In the light the guard cells are curved open to keep the stomata nice and wide. The two sets of bars in the middle show the plants challenged with two bacterial cultures and the one on the far right shows plants challenged with flagella proteins – small bits of the bacteria. What this shows is that the LOX1 enzyme is crucial for closing the stomata in response to bacteria, and that only certain specific bacterial proteins are required to stimulate this response.
While the two pathways for stomata closure are triggered by different events (abscisic acid by environmental factors and LOX1 by bacteria) it is likely that they both work on similar downstream proteins, in particular the ion channel proteins that control the water content of the guard cells. The ion channel SLAC1 was found to be required for both bacteria-mediated and environmental stomatal closure. In this way plants have two separate mechanisms for closing stomata, one to keep them in the optimal position for the appropriate surrounding environment and the other to slam them shut in the presence of potentially invasive bacteria.
Don’t forget to read my post “The manipulative friend: bacterial hijacking of plant symbiosis signalling” on S.E. Gould’s blog.
1. Montillet J-L, Leonhardt N, Mondy S, Tranchimand S, Rumeau D, et al. (2013) An Abscisic Acid-Independent Oxylipin Pathway Controls Stomatal Closure and Immune Defense in Arabidopsis. PLoS Biol 11(3): e1001513. doi:10.1371/journal.pbio.1001513
2. Meadows R (2013) How Plants Shut out Bacteria. PLoS Biol 11(3): e1001514. doi:10.1371/journal.pbio.1001514