Be careful what you breathe in

Unlike animals, which breathe through airways lined with pathogen-trapping defenses, plants get air through tiny pores in their leaves that all but invite bacteria to sneak in. So how do plants keep pathogens out? They slam their pores, or stomata, shut.

Stomata are flanked by guard cells that swell when triggered by bacteria, thus closing the pores. Besides being fascinating in its own right, this defense response has implications for human health – stomata have recently been shown to block some, but not all, types of the fecal bacteria that can contaminate leafy greens and other fresh produce, causing food poisoning outbreaks.

A recent paper in PLOS Biology shows that guard cells contain an enzyme called lipoxygenase that makes stomata close in response to pathogens, overturning a previous theory that this process is regulated by the plant hormone abscisic acid.

An Abscisic Acid-Independent Oxylipin Pathway Controls Stomatal Closure and Immune Defense in Arabidopsis. (2013) PLoS Biol 11(3): e1001513. doi:10.1371/journal.pbio.1001513

Stomata are microscopic pores that are present in the epidermis of the aerial parts of higher plants, such as the leaves. These pores, which are flanked by a pair of cells called guard cells, regulate transpiration and the exchange of gas between leaves and the atmosphere. It is well documented that the phytohormone abscisic acid (ABA) is a key regulator that controls the osmotic pressure in guard cells, allowing pore size to be adjusted in response to environmental conditions. Recently, stomata have also been shown to play an important role in the innate immune response. Indeed, upon contact with microbes, plants actively close stomata to prevent the entry of microbes and the consequent colonization of host tissue. This response is known as the stomatal defense response. However, the molecular mechanisms that regulate this defense response are not well understood. Using a variety of approaches, we show in this study that LOX1, a gene that encodes lipoxygenase (LOX) in guard cells, plays a major role in stomatal defense in the model plant Arabidopsis thaliana. Mutations in LOX1 impair stomatal closure and make plants more susceptible to the bacterium Pseudomonas syringae pv. tomato. We also show that several LOX-derived metabolites, the oxylipins, are potent inducers of stomatal closure. Finally, we provide evidence to show that ABA plays only a minor role in stomatal defense response, specifically by modulating this response.