Regulation of Stomatal Defense by Air Relative Humidity

It has long been observed that environmental conditions play crucial roles in modulating immunity and disease in plants and animals. For instance, many bacterial plant disease outbreaks occur after periods of high humidity and rain. A critical step in bacterial infection is entry into the plant interior through wounds and natural openings, such as stomata, which are adjustable microscopic pores in the epidermal tissue. Several studies have shown that stomatal closure is an integral part of the plant immune response to reduce pathogen invasion. In this study, we found that high humidity can effectively compromise Pseudomonas syringae-triggered stomatal closure in both Phaseolus vulgaris and Arabidopsis (Arabidopsis thaliana), which is accompanied by early up-regulation of the jasmonic acid (JA) pathway and simultaneous down-regulation of salicylic acid (SA) pathway in guard cells. Furthermore, SA-dependent response, but not JA-dependent response, is faster in guard cells than in whole leaves, suggesting that the SA signaling in guard cells may be independent from other cell types. Thus, we conclude that high humidity, a well-known disease-promoting environmental condition, acts in part by suppressing stomatal defense and is linked to hormone signaling in guard cells.The phyllosphere is one of the most diverse niches for microbe inhabitation. Numerous bacteria can survive and proliferate on the surface of the plant without causing any harm (Lindow and Brandl, 2003). However, for a bacterial pathogen to cause disease, it must penetrate through the plant epidermis and be able to survive and proliferate inside the plant. The mode and mechanism of penetration into the plant tissue is a critical step for infection, especially for bacterial pathogens that rely on natural openings and accidental wounds on the plant surface to colonize internal tissues (Misas-Villamil et al., 2013). Stomata are an example of such openings, providing one of the main routes through which the foliar pathogen Pseudomonas syringae transitions from avirulent epiphytic to virulent endophytic lifestyles (Melotto et al., 2008). This abundant opening in the epidermal tissue is not a passive port that allows unrestricted entry of microbes. It has been shown that plants are able to respond to human and plant bacterial pathogens by actively closing the stomatal pore (McDonald and Cahill, 1999; Melotto et al., 2006; Gudesblat et al., 2009; Zhang et al., 2010; Roy et al., 2013; Arnaud and Hwang, 2015), a phenomenon described as stomatal immunity (Sawinski et al., 2013). Several lines of evidence point to the complexity of this response and show that stomatal closure is an integral basal plant defense mechanism to restrict the invasion of pathogenic bacteria into plant tissues (Ali et al., 2007; Melotto et al., 2008; Zhang et al., 2008; Gudesblat et al., 2009). However, certain bacterial pathogens, such as Xanthomonas campestris pv campestris (Gudesblat et al., 2009), P. syringae pv syringae (Pss) B728a (Schellenberg et al., 2010), and P. syringae pvs tabaci, tomato, and maculicola (Melotto et al., 2006), can successfully cause disease by producing toxins that overcome stomatal immunity. Specifically, P. syringae pv tomato (Pst) DC3000 uses coronatine (COR) as such a toxin.In this study, we focused on elucidating environmental regulation of stomatal-based defense against bacterial invasion. Changes in environmental conditions, such as air relative humidity (RH), light, and carbon dioxide concentration regulate guard cell turgidity that consequently alters stomatal aperture size and the basic functions of stomata in plants, i.e. exchange of photosynthetic gases and regulation of water loss by transpiration (Zelitch, 1969; Schroeder et al., 2001; Fan et al., 2004). In natural conditions, plants are exposed to both biotic and abiotic stresses, and guard cells need to prioritize their response to the simultaneous occurrence of these stresses. For instance, it is a common observation that severe outbreaks of bacterial disease in the field are often associated with periods of heavy rain or high air humidity (Goode and Sasser, 1980). Mechanical wounding of plant tissues by rain might be one way that allows pathogens to bypass the stomatal route and gain unprecedented access to the plant interior. Additionally, the formation of large bacterial aggregates under high humidity on the leaf surface (Monier and Lindow, 2004) and splashing of bacteria during rain may also contribute to the spreading of disease at a higher rate. Interestingly, to ensure infection in the laboratory, researchers commonly expose plants to very high humidity for an extended period after surface inoculation. Here, we demonstrate that high RH compromises stomatal defense in Arabidopsis (Arabidopsis thaliana) and common bean (Phaseolus vulgaris) against P. syringae, allowing more bacteria to enter the leaf tissue and contributing to severe infections. Compromised bacterial-triggered stomatal closure due to high RH is accompanied by changes in plant hormone signaling in Arabidopsis. Specifically, high RH leads to activation of the jasmonic acid (JA) signaling pathway and down-regulation of the salicylic acid (SA) signaling in guard cells. These results connect plant physiology with epidemiology and advance the current understanding of foliar bacterial infection in plants.