Stomata, the minute pores on plant leaves, are crucial for regulating gas exchange and water transpiration. Plants control stomatal aperture through the opening and closing of guard cells, which is essential for various physiological functions. Moreover, stomata serve as the first line of defense against pathogen invasion. Plants can sense pathogen-associated molecular patterns (PAMPs) via stomata and trigger a series of defense responses, including stomatal closure, to impede pathogen entry. In this process, reactive oxygen species (ROS) and hydrogen sulfide (H₂S) act as important signaling molecules, coordinating stomatal behavior and activating plant immune mechanisms. Mitochondrial β-cyanoalanine synthase (CAS-C1) plays a pivotal role in catalyzing the synthesis reaction to generate H₂S, potentially influencing plant immune responses. Although the role of H₂S in plant physiology is widely recognized, the specific role of CAS-C1 in mitochondria and how it regulates stomatal immune responses still require further exploration.
Recently, a research team published a study titled “Mitochondrial β-Cyanoalanine Synthase Participates in flg22-Induced Stomatal Immunity” in Plant, Cell & Environment. The aim was to disclose the role of mitochondrial β-cyanoalanine synthase (CAS-C1) and its product hydrogen sulfide (H₂S) in regulating plant stomatal immune responses induced by the bacterial PAMP flg22, providing a new molecular mechanism for enhancing crop disease resistance, promoting research in the field of plant immune signaling, and potentially offering insights into the role of signaling molecules in other biological systems.
Firstly, the research team analyzed the stomatal closure ability of cas-c1 mutants and control plants after flg22 treatment by measuring changes in stomatal aperture. The study found that the stomatal closure ability of cas-c1 mutants was impaired in response to flg22, indicating that CAS-C1 plays a crucial role in the mitochondrial H₂S-mediated stomatal closure signal. Additionally, using chemiluminescence and fluorescent protein sensor technologies, the production of ROS in the apoplast and cytoplasm of cas-c1 mutants and control plants under flg22 stimulation was detected. Under flg22 treatment, the production of apoplastic ROS in cas-c1 mutants was significantly reduced, suggesting that endogenous H₂S is essential for ROS production.
By treating with mitochondrial electron transport chain inhibitors (such as rotenone, 2-Thenoyltrifluoroacetone, and antimycin A), the research team discovered that the flg22-induced stomatal closure response was inhibited, indicating that mitochondrial function is vital for stomatal closure. Through spray inoculation with Pseudomonas syringae pv. tomato DC3000, the researchers found that the bacterial growth in cas-c1 mutants was significantly higher than that in the wild type one day after inoculation, but the difference between the wild type and mutants decreased after three days, suggesting that CAS-C1 plays an early defense role in mitochondrial-mediated stomatal immunity.
Finally, the study also found that the mitochondrial H₂S donor AP39 could induce stomatal closure, and this effect was dependent on RBOHD, implying that RBOHD plays a crucial role in mediating mitochondrial H₂S signals. Further research revealed that the depletion of endogenous H₂S impaired RBOHD-mediated apoplastic ROS production, indicating that H₂S is crucial in regulating RBOHD activity and stomatal closure signals.
In conclusion, this study directly confirmed the significant role of the mitochondrial H₂S source CAS-C1 in plant stomatal immune responses and uncovered its key role in regulating stomatal closure and ROS production. This provides a new perspective and profound insights into how plants defend against pathogens by modulating mitochondrial function and H₂S signals.
For reference of: https://doi.org/10.1111/pce.15155