A recent report in Cell has revealed that rather than being passive ports of entry, stomata have an active role in the innate immune response of plants to bacterial challenge, and that bacteria have evolved specific virulence factors to counteract this defence reaction.

Unlike fungal pathogens, bacterial pathogens cannot penetrate plant tissues directly. Instead, they rely on entry though wounds or natural openings in the plant surface. Stomata are microscopic pores in the plant epidermis that allow gaseous exchange and transpiration. Each stomatal pore is surrounded by two guard cells that control stomatal opening and closing in response to changing environmental conditions, such as light and humidity.

In this work, Maeli Melotto and colleagues studied the response of Arabidopsis thaliana stomata to challenge with the plant pathogen Pseudomonas syringae pv. tomato DC300 (Pst DC300). After 1 hour of incubation with Pst DC300, the average width of the stomatal aperture decreased dramatically, and after 2 hours the number of open stomata was reduced by ∼70%.

Is stomatal closure stimulated by a specific bacterial component, such as a PAMP (pathogen-associated molecular pattern)? Melotto et al. found that two well-known PAMPs — a flagellin-derived peptide and lipopolysaccharide — stimulated stomatal closure. In A. thaliana, the innate immune response can be either salicylic acid (SA)-dependent or SA–independent and the PAMP response was found to be part of the SA-dependent pathway. Stomatal closure is regulated by an abscisic acid (ABA) signalling pathway in guard cells, and Melotto et al. found that the PAMP-induced closure also involved ABA signalling.

The induction of stomatal closure was not restricted to Pst DC300 and was also observed with the human pathogen Escherichia coli O157:H7. Interestingly, the length of the induced closure differed between these two pathogens — with E. coli O157:H7, closure persisted for the 8-hour duration of the experiment, whereas Pst DC300-induced closure was reversed after ∼3 hours. This suggested that Pst DC300 might have evolved a mechanism to reopen the stomata. Pst DC300 has two main virulence factors, a type III secretion system (T3SS) and the phytotoxin coronatine. Analysis of the response to Pst DC300 mutants deficient in coronatine or with a defective T3SS demonstrated that coronatine is the virulence factor involved in suppressing stomatal closure, and it was shown to function downstream of ABA.

So, in addition to their key role in gaseous exchange and transpiration, plant stomata also function as 'innate immune gates'. Rather than being able to freely enter plant tissues through the stomata, Pst DC300 triggers initial stomatal closure through the detection of PAMPs and the ABA signalling pathway. The bacteria then counteract this defence response by secreting coronatine, which causes the stomata to reopen. Given that stomata are present in all vascular plants, the authors speculate that PAMP-induced stomatal closure could be a widespread phenomenon and that the inhibition of this defence response might have been a key adaptation in the evolution of plant pathogens.