My students & I spent our last couple of tutorials talking about how the mammalian immune system functions: innate immunity, acquired immunity, the whole lot. Our immune system is a wonderful & complex thing. But, just as we tend to overlook the fact that plants show a variety of behavioural responses to their environment, I suspect most people wouldm't recognise that plants also have an immune system of sorts.
Like animals, plants must find ways of preventing invasion by various bacteria & viruses. In animals, the first level of defence involves the skin, which offers a physical barrier to microbes (aided by secretions from the skin & mucous membranes). Of course, plants also have an outer skin, the epidermis, but this is pierced by openings - lenticels in the stem & of course stomata in the leaves - which would appear to offer easy entry points for invaders. Pathogens can also enter plant tissues through wounds caused by feeding insects
This is particularly true of bacteria: fungal hyphae can grow directly between or through plant cells, so for fungi the stomata are almost irrelevant. One such bacterium is called Pseudomonas sygringae, strains of which cause a number of disease symptoms in a diverse range of plant species. However, until fairly recently scientists tended to study the progression of infection once organisms like P.syringae had actually entered plant tissues, rather than the process of infection itself (assuming, I suppose, that the microbes had unimpeded entry via the stomata).
Once a pathogen has entered a mammal's body, the various cellular & non-cellular defences kick in. Phagocytic white blood cells engulf the invaders, antibodies bind to & tag them for destruction, chemical signals cause inflammation... But plants don't have the same mobile cellular defences, nor do they have the ability to generate antibodies, relying instead on 'the innate immunity of each cell and on systemic signals emanating from infection sites' ( Jones & Dangi 2006). So this makes it all the more important to keep as many pathogens as possible out of the plant's tissues, & highlights the fact that stomata would appear to offer a clear & open pathway in. In 2006 Melotte et all used the plant Arabidopsis to see if this was really the case.
The team knew that, under controlled growing conditions, their plants would have around 75% of their stomata open after being exposed to at least 3 hours of light. They then inoculated the leaves with bacteria, & found that within 2 hours the percentage of stomata that were open dropped to around 30%. (There was no change in control leaves inoculated with water.) In addition, they incubated stomatal peels (where epidermis is stripped from a leaf & mounted on a microscope slide) with plant-pathogen bacteria & observed them. The bacteria moved selectively towards open stomata - & in less than an hour after the bacteria were added to the slide, stomata had closed. However, they'd opened again 2 hours later. Interestingly, the human pathogen E.coli O157:H7 also induced stomatal closing, but in this case the stomata did not subsequently reopen. This suggests that the plant pathogen has evolved some way of overcoming the plant's initial response.
When Melotte & his colleagues (2006) looked further into what was going on, they found was that in Arabidopsis the guard cells that surround each stoma are able to recognise bacterial antigens. This means that there's a receptor protein on the cell membrane that detects the antigens, & that this in turn triggers a sequence of events within the cell that causes the stoma to close, shutting out the bacteria. This turns out to be similar to the pathway controlling stomatal closure when planted are stressed, althouth the latter is mediated by abscisic acid. (It seems that in the case of P.syringae, & other plant pathogens, the bacterium has responded by evolving virulence factors that, among other things, stimulate stomatal re-opening - rather like an arms race between the microbe and the plant!)
Plants - they really are more complex that our earlier imaginings :-)
J.D.G.Jones & J.L.Dangi (2006) The plant immune system. Nature 444: 323-329. doi.10.1038/nature05286
M.Melotte, W.Underwood, J.Koczan, K.Nomura & S.Y.He (2006) Plant stomata function in innate immunity against bacterial invasion. Cell 126: 969-980. doi 10.1016.j.cell.2006.06.054