Chitosan signaling in guard cells requires endogenous salicylic acid

Integrative regulatory mechanisms of stomatal movements under changing climate

Global climate change-caused drought stress, high temperatures and other extreme weather profoundly impact plant growth and development, restricting sustainable crop production. To cope with various environmental stimuli, plants can optimize the opening and closing of stomata to balance CO2 uptake for photosynthesis and water loss from leaves. Guard cells perceive and integrate various signals to adjust stomatal pores through turgor pressure regulation. Molecular mechanisms and signaling networks underlying the stomatal movements in response to environmental stresses have been extensively studied and elucidated. This review focuses on the molecular mechanisms of stomatal movements mediated by abscisic acid, light, CO2 , reactive oxygen species, pathogens, temperature, and other phytohormones. We discussed the significance of elucidating the integrative mechanisms that regulate stomatal movements in helping design smart crops with enhanced water use efficiency and resilience in a climate-changing world.

Xylooligosaccharides induce stomatal closure via salicylic acid signaling-regulated reactive oxygen species and nitric oxide production in Arabidopsis

Xylooligosaccharides (XOS) are the major coproducts of biofuel production and the most representative functional sugar enhancing animal physiology. However, little is known regarding the biological relevance of XOS to plants. Here, we found XOS triggered stomatal closure in Arabidopsis in a dose-dependent manner. Pamarcological data showed that XOS-induced stomatal closure was markedly inhibited by catalase (CAT, a reactive oxygen species [ROS] scavenger), salicylhydroxamic acid (SHAM, a peroxidase inhibitor), and 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO, a nitric oxide [NO] scavenger). Moreover, XOS induced the production of ROS and NO in guard cells of Arabidopsis. ROS production was strongly restricted by CAT and SHAM, but was unaffected by treatment with diphenyleneiodonium chloride (DPI, an NADPH oxidase inhibitor) or cPTIO. NO production was suppressed by CAT, SHAM, and cPTIO, but not by DPI. The elevation of ROS level mediated by SHAM-sensitive peroxidases occurred upstream of NO. Additionally, XOS-triggered stomatal closure and ROS and NO accumulation were significantly impaired in npr1 (salicylic acid signaling) mutant plants, but were not in jar1 (jasmonic acid signaling) or ein2 (ethylene signaling) mutant plants. Furthermore, XOS-induced stomatal closure was unaffected in both ost1 and atrbohD atrbohF (abscisic acid [ABA] signaling) mutant plants. Therefore, these results indicated that the biotic sugar, XOS, can elicit stomatal closure via salicylic acid signaling-mediated production of ROS and NO, in a manner independent of ABA signaling.

Acibenzolar-S-Methyl Activates Stomatal-Based Defense Systemically in Japanese Radish

Acibenzolar-S-methyl (ASM) is a well-known plant activator, which is a synthetic analog of salicylic acid (SA). Recently, copper fungicides and antibiotics are major strategies for controlling bacterial diseases. However, resistant strains have already been found. Therefore, there is an increasing demand for sustainable new disease control strategies. We investigated the ASM disease control effect against Pseudomonas cannabina pv. alisalensis (Pcal), which causes bacterial blight on Japanese radish. In this study, we demonstrated that ASM effectively suppressed Pcal disease symptom development associated with reduced bacterial populations on Japanese radish leaves. Interestingly, we also demonstrated that ASM activated systemic acquired resistance (SAR), including stomatal-based defense on ASM-untreated upper and lower leaves. Reactive oxygen species (ROS) are essential second messengers in stomatal-based defense. We found that ASM induced stomatal closure by inducing ROS production through peroxidase. These results indicate that stomatal closure induced by ASM treatment is effective for preventing Pcal pathogen invasion into plants, and in turn reduction of disease development.

Salicylic acid receptor NPR1 is involved in guard cell chitosan signaling

Chitosan (CHT) induces stomatal closure and thus plays a crucial role in plants to adapt to the adverse environments. Our previous results of a SA-deficient mutant nahG suggest that endogenous salicylic acid (SA) is involved in the CHT signaling in guard cells. Here in order to make the involvement definite, we examined stomatal responses to CHT of another SA-deficient mutant, sid2, and an SA receptor mutant, npr1-3. The sid2 mutation impaired CHT-induced stomatal closure and reactive oxygen species production and both impairments were complemented with exogenous SA application. Moreover, the CHT-induced stomatal closure is disrupted in the npr1-3 mutant. These results suggest that endogenous SA is involved in the CHT-induced stomatal closure via the SA receptor, NPR1.Abbreviations: SA: salicylic acid; ABA: abscisic acid; ROS: reactive oxygen species; NPR1: nonexpresser of pathogenesis-related genes1; CHT: chitosan; DAB: 3,3'-diaminobenzidine.

Effects of Light and Daytime on the Regulation of Chitosan-Induced Stomatal Responses and Defence in Tomato Plants

Closure of stomata upon pathogenesis is among the earliest plant immune responses. However, our knowledge is very limited about the dependency of plant defence responses to chitosan (CHT) on external factors (e.g., time of the day, presence, or absence of light) in intact plants. CHT induced stomatal closure before dark/light transition in leaves treated at 17:00 hrs and stomata were closed at 09:00 hrs in plants treated at dawn and in the morning. CHT was able to induce generation of reactive oxygen species (ROS) in guard cells in the first part of the light phase, but significant nitric oxide production was observable only at 15:00 hrs. The actual quantum yield of PSII electron transport (ΦPSII) decreased upon CHT treatments at 09:00 hrs in guard cells but it declined only at dawn in mesophyll cells after the treatment at 17:00 hrs. Expression of Pathogenesis-related 1 (PR1) and Ethylene Response Factor 1 were already increased at dawn in the CHT-treated leaves but PR1 expression was inhibited in the dark. CHT-induced systemic response was also observed in the distal leaves of CHT-treated ones. Our results suggest a delayed and daytime-dependent defence response of tomato plants after CHT treatment at night and under darkness.