Mitochondrial H₂S Production Regulates Stomatal Immunity

Guard cell signalling network encompasses a diverse array of molecular components, with hydrogen sulphide (H2S) emerging as a pivotal gasotransmitter in regulating stomatal responses (Papanatsiou et al. 2015; Shen et al. 2020; Chen et al. 2020; Zhou et al. 2021). In Arabidopsis, H2S production involves various enzymes distributed across distinct cellular compartments, including the cytoplasm, chloroplasts and mitochondria (Aroca et al. 2020; Liu et al. 2023). β-Cyanoalanine synthase (CAS-C1) is one of the enzymes that produce H2S, localized in mitochondria, where it catalyzes the reaction between hydrogen cyanide and cysteine to generate β-cyanoalanine and H2S (Yamaguchi et al. 2000; García et al. 2010). Variations in CAS-C1 expression in plants lead to altered root hair development and distinct responses to environmental stresses compared to wild-type (WT) plants (Watanabe et al. 2008; García et al. 2010; García et al. 2013; Arenas-Alfonseca et al. 2021). The cys-c1 mutants exhibit increased susceptibility to Botrytis cinerea but greater tolerance to Pseudomonas syringae and Beet curly top virus. This mutation also reduced leaf respiration, elevates reactive oxygen species (ROS) levels and induces the expression of AOX1a (alternative oxidase 1a) and PR1 (pathogenesis-related 1), suggested an altered immune response (García et al. 2013). Furthermore, plants overexpressing CYS-C1 showed enhanced cyanide detoxification, which helps reduce oxidative damage by maintaining higher antioxidant enzyme activity and improving salt stress resistance (Xu et al. 2023). Despite this, the specific role of CAS-C1 as a mitochondrial H2S synthase in the context of stomatal immunity remains inadequately explored and warrants further investigation. Recently, a study by Pantaleno et al. (2024) has elucidated the critical role of mitochondrial H2S and the enzyme CAS-C1 in mediating stomatal immunity against bacterial pathogen-associated molecular patterns (PAMPs), with a particular focus on flg22. This study underscores the intricate interactions between mitochondrial-derived H2S, ROS and stomatal responses, highlighting the critical involvement of mitochondrial function in plant defense mechanisms. These findings contribute a novel perspective to the understanding of plant immunity, revealing that mitochondria serve not only as energy producers but also as active participants in immune signalling pathways. The study by Pantaleno et al. (2024) demonstrated that CAS-C1 is crucial for flg22-induced stomatal closure. CAS-C1-deficient mutants exhibited compromised stomatal responses and increased susceptibility to P. syringae infection, thereby establishing mitochondrial-derived H2S as a key regulator of stomatal immunity. Additionally, flg22 treatment triggered a transient burst of apoplastic ROS, which was diminished in cas-c1 mutants, suggesting that mitochondrial H2S is integral to ROS modulation and linking mitochondrial function to the signalling pathways that regulate stomatal closure. Notably, Respiratory Burst Oxidase Homologue D (RBOHD) was identified as the principal mediator of H2S effects on ROS production, in contrast to RBOHF, highlighting the specific interaction between these signalling elements. The findings further indicated that normal mitochondrial function is essential for flg22-induced stomatal closure, as inhibition of mitochondrial complexes disrupted this response. This suggested that PAMP perception enhances mitochondrial activity, which is critical for executing stomatal closure. These results contributed a novel perspective on the integration of metabolic processes with immune responses in plants. Furthermore, the study drew parallels between mitochondrial H2S in stomatal responses to flg22 and its interactions with other signalling molecules, such as abscisic acid (ABA). Previous research has established that ABA induces mitochondrial H2O2 production (Postiglione and Muday 2023), and the current findings complement this by demonstrating that mitochondrial H2S also plays a significant role in stomatal closure. The study underscores the critical role of mitochondrial functions in plant immunity, emphasizing the connection between mitochondrial-derived H2S production and stomatal responses. This research highlights a vital component of the plant signalling network employed in pathogen defense. The findings suggest potential strategies for enhancing disease resistance in crops through targeted manipulation of mitochondrial signalling pathways. To fully elucidate the multifaceted role of mitochondrial H2S in plant immunity, future research should aim to clarify the precise mechanisms through which H2S influences ROS production and stomatal signalling. Additionally, it is important to investigate the contributions of other H2S-producing enzymes located in various cellular compartments. Such a comprehensive approach will deepen our understanding of H2S functionality within the broader framework of plant defense mechanisms. Furthermore, exploring post-translational modifications, such as the persulfidation of mitochondrial proteins, will be crucial for elucidating how these modifications regulate mitochondrial activity and impact stomatal responses. Collectively, these research avenues will provide a robust framework for deciphering the intricate signalling networks that underpin plant immunity. This work was supported by the National Natural Science Foundation of China (Grant 42370299). The authors have nothing to report.