Plants coexist with diverse microorganisms, and their interactions have resulted in coevolution. Higher plants have immune systems with two different levels to resist pathogen invasion and multiplication. The gene-for-gene concept characterizes genetic control by plant resistance gene products and pathogen avirulence proteins, which are pathogen-secreted effectors that manipulate host processes in favour of the pathogen (Goehre & Robatzek, 2008; Gouveia et al., 2017; Wang et al., 2019); their direct or indirect molecular interactions initiate effector-triggered immunity (ETI), which is essential to conventional race-specific resistance breeding programmes (Peng et al., 2018). Moreover, microbial-originated invariant structures, known as microbial-associated molecular patterns (MAMPs), are able to activate pattern-triggered immunity (PTI) in various cultivars of numerous species, resulting in the elicitation of plant stomatal closure and disease resistance (Boller & He, 2009; Chisholm et al., 2006). Chitin is a polymer of N-acetyl-d-glucosamine that can be obtained from natural sources, such as arthropod exoskeletons or fungal cell walls. It is a PTI inducer that elicits stomatal closure and disease resistance, and has been applied to control crop disease (Liu et al.,

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 SHAMsensitive 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.

Stomatal movement participates in plant immunity by directly affecting the invasion of bacteria, but the genes that regulate stomatal immunity have not been well identified. Here, we characterised the function of the bZIP59 transcription factor from Arabidopsis thaliana, which is constitutively expressed in guard cells. The bzip59 mutant is partially impaired in stomatal closure induced by Pseudomonas syringae pv. tomato strain (Pst) DC3000 and is more susceptible to Pst DC3000 infection. By contrast, the line overexpressing bZIP59 enhances resistance to Pst DC3000 infection. Furthermore, the bzip59 mutant is also partially impaired in stomatal closure induced by flagellin flg22 derived from Pst DC3000, and epistasis analysis revealed that bZIP59 acts upstream of reactive oxygen species (ROS) and nitric oxide (NO) and downstream of salicylic acid signalling in flg22‐induced stomatal closure. In addition, the bzip59 mutant showed resistance and sensitivity to Sclerotinia sclerotiorum and Tobacco mosaic virus that do not invade through stomata, respectively. Collectively, our results demonstrate that bZIP59 plays an important role in the stomatal immunity and reveal that the same transcription factor can positively and negatively regulate disease resistance against different pathogens.

The back cover cover image is based on the Original Article The cell‐type specific role of Arabidopsis bZIP59 transcription factor in plant immunity by Zhiqiang Song et al., https://doi.org/10.1111/pce.14299