Ethylene antagonizes <scp>ABA</scp> responses in stomatal movement and seed germination by upregulating <scp>ABI1</scp> and <scp>ABI2</scp> at both the transcript and protein levels

The intricate interplay among plant hormones is crucial for the fine-tuning of stomatal movements, which are vital for plant growth and stress responses. While it is well documented that ethylene inhibits abscisic acid (ABA)-induced stomatal closure, the detailed interactions between their core signaling pathways remain elusive. In this study, we discovered that ethylene is dependent on the canonical EIN2-EIN3 signaling pathway to suppress ABA-induced stomatal closure by enhancing the activity of protein phosphatase 2C (PP2C), the key negative regulator of ABA signaling, and consequently reducing the phosphorylation levels of SnRK2.6/OST1, a core positive regulator of ABA signaling. Our findings in loss-of-function mutants for ABA-INSENSITIVE1 (ABI1) and ABI2 demonstrate that these proteins are the principal PP2Cs mediating the interaction between ethylene and ABA-induced stomatal movement. Moreover, we demonstrated that ethylene significantly upregulates the gene expression and protein accumulation of ABI1 and ABI2 in the presence of ABA. Using genetic and molecular assays, we further uncovered that ethylene directly regulates ABI1 and ABI2 transcription through ethylene-responsive factors. Additionally, we found that ethylene enhances the stability of ABI1 and ABI2 proteins by inhibiting their ABA-induced degradation. Collectively, our research elucidates how ethylene, through the EIN2-EIN3 signaling pathway, upregulates PP2Cs to inhibit ABA-induced stomatal closure, with effects contingent on ABA presence. These findings reveal a novel negative feedback regulatory mechanism in ABA guard cell signaling. Additionally, during seed germination, ethylene also upregulates ABI1 and ABI2 to counteract ABA's effects, aligning with the mechanism observed in stomatal closure. These results collectively suggest a general model for ethylene's antagonism of ABA responses, thereby advancing our understanding of how plants precisely regulate multiple processes through hormonal crosstalk.