Over-expression of Cyclic Nucleotide-Gated Ion Channel 2 (CNGC2) triggers hypersensitivity to virulent pathogens and elevated Ca ²⁺

The Arabidopsis Cyclic Nucleotide-Gated Ion Channel 2 (CNGC2), also known as Defense No Death 1 (DND1), is the most extensively studied plant CNGC and has been implicated in diverse physiological processes, including floral transition, responses to heat and humidity, and hormone signaling. Its role in immunity has received particular attention due to the autoimmunity phenotype observed in the cngc2/dnd1 knockout mutants. Interestingly, despite this hyperactivation of immunity, the mutant also exhibits impaired hypersensitive cell death—a hallmark of effector-triggered immunity (ETI)—as well as reduced reactive oxygen species (ROS) production and diminished Ca²⁺ influx in response to pathogen-associated molecular patterns (PAMPs) such as the bacterial flagellin peptide flg22. These contradictory phenotypes highlight the complex biological functions of CNGC2.

To date, most studies have focused on loss-of-function mutants. In this study, we performed a detailed characterization of CNGC2 overexpression lines to gain deeper insight into its role in immunity. Remarkably, overexpression of CNGC2 led to heightened susceptibility to two taxonomically distinct pathogens, despite the plants displaying wild-type morphology. Overexpression of CNGC2 rescued several cngc2 mutant phenotypes, including morphological defects and delayed flowering, yet these plants were also hypersensitive to elevated external Ca²⁺ levels. Furthermore, they exhibited attenuated responses to flg22, suggesting that CNGC2 does not act as a simple positive or negative regulator of immunity. Our findings reveal an essential role for CNGC2 where a balanced expression level is critical for maintaining Ca²⁺ homeostasis between the apoplast and cytosol, thereby influencing the generation of Ca²⁺ signals essential for immune responses.