Mechanisms Underlying Inhibition Of Stomatal Immunity By Environmental Factors

Plants have evolutionarily established a complicated innate immune system that can counter pathogenic infection. Being pores, stomata constitute a natural entry site for potentially harmful microbes. To prevent microbial invasion from the leaf, stomata close promptly upon perception of the pathogen-associated molecular patterns (PAMPs) by cognate receptors on the guard cell surface, and this represents an important layer of plant innate immunity at the pre-invasive level. There is a growing body of evidence suggesting that plant defense against pathogenic microorganisms can be altered under certain environmental conditions; e.g. severe outbreaks of bacterial diseases in the field are often associated with periods of heavy rain or high relative air humidity. However, the underlying mechanisms still remain unclear. We hypothesize that a subset of environmental factors can alter plant hormonal dynamics, and on this basis inhibit or abolish stomatal immunity.To test it, we used Arabidopsis-pseudomonas syringae and Vicia faba L.-E.coli as model systems, respectively. We exploited stomatal bioassay, laser confocal scanning microscopy, enumeration of bacterial population and artificial environmental manipulation to screen for environmental factors that could inhibit stomatal immunity and then investigate the mechanisms underlying these phenomena. The results obtained in these experiments are as follows:1. Bacteria-induced stomatal closure could be abolished under low [CO2] (50 ppm) or high relative air humidity (RH≥90%) conditions, which may result in impairment of plant disease resistance and lead to outbreaks of bacterial diseases.2. Stomatal immunity requires ABA; ABA receptor protein (PYR/PYL), protein phosphatase type 2C (PP2C), and the calcium-dependent protein kinase CPK6 are involved in the biotic stress signal transduction mediated by ABA, but the OST1 protein kinase is not a critical regulatory element in this response.3. Foliar application of ABA could rescue stomatal immunity in wild-type Arabidopsis (Col-O) plants under high relative air humidity; and in the ABA 8’-hydroxylase deficient mutant cyp707ala3, stomatal immunity was not impaired under high air humidity.In summary, our study revealed:(1) stomata might prioritize their responses to multiple biotic and abiotic signal inputs;(2) ABA is required for stomatal immunity, and its signal transduction pathway in guard cells is different but overlap between abiotic and biotic stress responses; (3) inhibition of stomatal immunity by high relative air humidity might be due to the increase in ABA degradation metabolism, catalyzed by ABA 8’-hydroxylase. This work reinforces our understanding of molecular mechanisms underpinning stomatal immunity and provides a new avenue for the control of air-borne bacterial diseases in plants under some adverse environmental conditions.