Environmental conditions play crucial roles in modulating immunity and disease in plants. For instance, many
bacterial disease outbreaks occur after periods of high humidity and rain. A critical step in
bacterial infection is entry into the plant interior through
wounds or natural openings, such as stomata. Bacterium-triggered stomatal closure is an integral part of the plant immune response to reduce pathogen invasion. Recently, we found that high humidity compromises stomatal defense, which is accompanied by regulation of the
salicylic acid and
jasmonic acid pathways in guard cells. Periods of darkness, when most stomata are closed, are effective in decreasing pathogen penetration into leaves. However,
coronatine produced by Pseudomonas syringae pv. tomato (Pst) DC3000 cells can open dark-closed stomata facilitating
infection. Thus, a well-known disease-promoting environmental condition (high humidity) acts in part by suppressing stomatal defense, whereas an anti-stomatal defense factor such as
coronatine, may provide epidemiological advantages to ensure
bacterial infection when environmental conditions (darkness and insufficient humidity) favor stomatal defense.