Strigolactones are a group of phytohormones that control developmental processes including shoot branching and various plant–environment interactions in plants. We previously showed that the strigolactone perception mutant more axillary branches 2 (max2) has increased susceptibility to plant pathogenic bacteria. Here we show that both strigolactone biosynthesis (max3 and max4) and perception mutants (max2 and dwarf14) are significantly more sensitive to Pseudomonas syringae DC3000. Moreover, in response to P. syringae infection, high levels of SA accumulated in max2 and this mutant was ozone sensitive. Further analysis of gene expression revealed no major role for strigolactone in regulation of defense gene expression. In contrast, guard cell function was clearly impaired in max2 and depending on the assay used, also in max3, max4, and d14 mutants. We analyzed stomatal responses to stimuli that cause stomatal closure. While the response to abscisic acid (ABA) was not impaired in any of the mutants, the response to darkness and high CO2 was impaired in max2 and d14‐1 mutants, and to CO2 also in strigolactone synthesis (max3, max4) mutants. To position the role of MAX2 in the guard cell signaling network, max2 was crossed with mutants defective in ABA biosynthesis or signaling. This revealed that MAX2 acts in a signaling pathway that functions in parallel to the guard cell ABA signaling pathway. We propose that the impaired defense responses of max2 are related to higher stomatal conductance that allows increased entry of bacteria or air pollutants like ozone. Furthermore, as MAX2 appears to act in a specific branch of guard cell signaling (related to CO2 signaling), this protein could be one of the components that allow guard cells to distinguish between different environmental conditions.

SUMMARY Jasmonic acid (JA) and salicylic acid (SA) regulate stomatal closure, preventing pathogen invasion into plants. However, to what extent abscisic acid (ABA), SA and JA interact, and what the roles of SA and JA are in stomatal responses to environmental cues, remains unclear. Here, by using intact plant gas‐exchange measurements in JA and SA single and double mutants, we show that stomatal responsiveness to CO2, light intensity, ABA, high vapor pressure deficit and ozone either did not or, for some stimuli only, very slightly depended upon JA and SA biosynthesis and signaling mutants, including dde2, sid2, coi1, jai1, myc2 and npr1 alleles. Although the stomata in the mutants studied clearly responded to ABA, CO2, light and ozone, ABA‐triggered stomatal closure in npr1‐1 was slightly accelerated compared with the wild type. Stomatal reopening after ozone pulses was quicker in the coi1‐16 mutant than in the wild type. In intact Arabidopsis plants, spraying with methyl‐JA led to only a modest reduction in stomatal conductance 80 min after treatment, whereas ABA and CO2 induced pronounced stomatal closure within minutes. We could not document a reduction of stomatal conductance after spraying with SA. Coronatine‐induced stomatal opening was initiated slowly after 1.5–2.0 h, and reached a maximum by 3 h after spraying intact plants. Our results suggest that ABA, CO2 and light are major regulators of rapid guard cell signaling, whereas JA and SA could play only minor roles in the whole‐plant stomatal response to environmental cues in Arabidopsis and Solanum lycopersicum (tomato).

Strigolactones are a group of phytohormones that control shoot branching in Arabidopsis thaliana. However, in recent years they have been shown to affect many other plant processes. We previously showed that the strigolactone perception mutant more axillary branches 2 (max2) has increased susceptibility to plant pathogenic bacteria as a result of more open stomata as well as alterations in hormonal signalling. Here we show that both, strigolactone biosynthesis-(max3 and max4), and perception mutants (max2 and dwarf14) are significantly more sensitive to Pseudomonas syringae DC3000. Moreover, in response to P. syringae infection, high levels of SA accumulated in max2 and this mutant was ozone sensitive. To search for the mechanisms that could explain pathogen-and ozone sensitivity we performed gene expression analysis and several different assays that explore the function of guard cells and regulation of guard cell signalling.Treatments with GR24 (a strigolactone analogue) resulted in very modest changes in defence-related gene expression. In contrast, guard cell function was clearly impaired in max2 and depending on the assay used, also in max3, max4 and d14 mutants. Moreover, stomatal responses to stimuli that cause stomatal closure in wild-type plants (darkness, high CO2 and ABA) were analysed in the strigolactone mutants. In darkness both strigolactone biosynthesis and perception mutants showed reduced stomatal closure, whereas the response to high CO2 was impaired only in max2 and d14. The response to ABA was not impaired in any of the mutants. To position the role of MAX2 in the guard cell signalling network, max2 was crossed with mutants defective in ABA biosynthesis (aba2), in guard cell ABA signalling (ost1) and a scaffold protein required for proper ion channel activity (ghr1). The stomatal conductance of double mutants was consistently higher than the corresponding single mutants, suggesting that MAX2 acts in a signalling pathway that functions in parallel to the well characterized guard cell ABA signalling pathway. We propose that the impaired defence responses of max2 is related to more open stomata that allows increased entry of bacteria or air pollutants like ozone. Furthermore, as MAX2 appears to act in a specific branch of guard cell signalling (related to CO2 signalling), this protein could be one of the elusive components that allow guard cells to distinguish between different environmental conditions.

Plants require interaction between signaling pathways in order to differentiate and integrate stress responses and deploy appropriate defenses. The hormones ethylene, salicylic acid (SA), and jasmonic acid (JA) are important regulators of plant defenses. Numerous interactions between these signaling pathways are the cornerstone of robust plant immunity. Additionally, during the early response to pathogens, reactive oxygen species (ROS) act as signaling molecules. Here, we examined the extent of signal interaction in the early stages of Botrytis cinerea infection. To enable a comparison between B. cinerea infection with ROS signaling we subjected plants to ozone treatment, which stimulates an apoplastic ROS burst. We used a collection of single, double, and triple signaling mutants defective in hormone signaling and biosynthesis; and subjected them to B. cinerea infection and ozone treatment at different time points. We examined lesion size, cell death, and gene expression (both quantitatively and spatially). The two treatments shared many similarities, especially in JA insensitive mutants, which were sensitive to both treatments. Unexpectedly, a B. cinerea susceptible JA-insensitive mutant (coi1), became tolerant when both SA biosynthesis and signaling was impaired (coi1 npr1 sid2), demonstrating that JA responses may be under control from SA. Extensive marker gene analysis indicated JA as the main regulator of both B. cinerea and ozone defenses. In addition, we identified the transcription factor SR1 as a crucial regulator of PLANT DEFENSIN expression and cell death regulation, which contributes to resistance to B. cinerea. Overall, our work further defines the context of ROS in plant defense signaling.

Little is known about long-distance mesophyll-driven signals that regulate stomatal conductance. Soluble and/or vapor-phase molecules have been proposed. In this study, the involvement of the gaseous signal ethylene in the modulation of stomatal conductance in Arabidopsis thaliana by CO 2 /abscisic acid (ABA) was examined.We present a diffusion model which indicates that gaseous signaling molecule/s with a shorter/direct diffusion pathway to guard cells are more probable for rapid mesophylldependent stomatal conductance changes. We, therefore, analyzed different Arabidopsis ethylene-signaling and biosynthesis mutants for their ethylene production and kinetics of stomatal responses to ABA/[CO 2 ]-shifts.According to our research, higher [CO 2 ] causes Arabidopsis rosettes to produce more ethylene. An ACC-synthase octuple mutant with reduced ethylene biosynthesis exhibits dysfunctional CO 2 -induced stomatal movements. Ethylene-insensitive receptor (gain-of-function), etr1-1 and etr2-1, and signaling, ein2-5 and ein2-1, mutants showed intact stomatal responses to [CO 2 ]-shifts, whereas loss-of-function ethylene receptor mutants, including etr2-3;ein4-4;ers2-3, etr1-6;etr2-3 and etr1-6, showed markedly accelerated stomatal responses to [CO 2 ]-shifts. Further investigation revealed a significantly impaired stomatal closure to ABA in the ACC-synthase octuple mutant and accelerated stomatal responses in the etr1-6;etr2-3, and etr1-6, but not in the etr2-3;ein4-4;ers2-3 mutants.These findings suggest essential functions of ethylene biosynthesis and signaling components in tuning/accelerating stomatal conductance responses to CO 2 and ABA.