Temperature is an important environmental factor that affects plant growth and development, as well as resistance to pathogens. Low temperature promotes plant immune responses, and this process may involve the plant salicylic acid (SA) signaling pathway. However, the underlying mechanism by which low temperature signals coordinate SA signaling to regulate plant immune responses is not fully understood. 

In a study published in The Plant Cell, researchers from the Xishuangbanna Tropical Botanical Garden of the Chinese Academy of Sciences revealed the molecular mechanism of low temperature-enhanced plant immune responses. They demonstrated a critical function of the INDUCER OF CBF EXPRESSION 1 (ICE1) as an essential node bridging cold stimulus and plant immunity. 

To investigate the underlying relationship between low temperature and plant immunity, the researchers examined the effect of low temperature on plant immune responses against the hemibiotropic pathogenic bacteria, Pseudomonas syringae pv. tomato (Pst) DC3000.  

Mechanistic analyses revealed that the ICE1 protein physically interacts with the salicylic acid (SA) receptor NON-EXPRESSER OF PR GENES 1 (NPR1) and is involved in SA-mediated resistance against Pst DC3000. The ice1-2 mutant exhibited enhanced susceptibility to pathogen infection, including severe pathogen-induced water soaking symptoms and high pathogen proliferation, and decreased expression of PATHOGENESIS-RELATED GENE 1 (PR1). 

Most importantly, they found that ICE1 is a key regulator of low-temperature and salicylic acid signaling, as the coordinated function of cold- and SA- enhanced immunity was reduced in ice1-2 mutant plants. They also found that ICE1 interacts with TGACG-BINDING FACTORs (TGA) transcription factors to cooperatively stimulate PR1 expression under low temperature. 

"Therefore, the main underlying mechanism is that ICE1 activates the expression of PR1 and this activation is promoted by NPR1," said LI Shaoqin, first author of the study.  

"Our results not only provide evidence that ICE1 is an essential component of the SA signaling pathway at low temperature, but also enhance our understanding of the role of the NPR1-ICE1-TGA3 regulatory module in integrating the low temperature signal in activating plant immunity," said JIANG Yanjuan, a former researcher of XTBG.