Functional characterisation of the EDS1–MYC2 regulatory node in Arabidopsis

Plants rely on a multi-layered, cell autonomous immune system to combat a plethora of pathogens. While a first, broad defence response is sufficient to restrict growth of most invaders, some pathogens have evolved means to overcome this. In an evolutionary arms race plants have evolved intracellular receptors that recognise host-adapted pathogens and initiate a sustained and potent immune response call ETI (effector-triggered immunity). EDS1 (ENHANCED DISEASE SUSCEPTIBILITY 1) and its signalling partners PAD4 (PHYTOALEXIN DEFICIENT 4) or SAG101 (SENESCENCE-ASSOCIATED GENE 101) form a central convergence point for those intracellular receptors and act as a decision- making node for SA (salicylic acid) dependent and SA independent transcriptional reprogramming. Despite our increasing knowledge about plant immunity the molecular function of the EDS1/PAD4 complex and how these proteins are regulated remains unclear. Recent work established an antagonistic regulation between the JA (jasmonic acid) key TF (transcription factor) MYC2 and EDS1. While MYC2 enhances bacterial virulence by repressing the EDS1 promoter, ETI activated EDS1 represses MYC2 signalling and dampens pathogen growth. This cross-regulation represents an intersection of ETI and JA signalling and allows fine-tuning of the plant’s immune response. How EDS1 controls MYC2 accumulation and activity is not known. Here, I show regulation of MYC2 abundance and MYC2 transactivation activity by EDS1 family proteins. Further, I present evidence for the underlying molecular mechanisms of this regulation and identify new components in this pathway. Specifically, PAD4 and SAG101 but not EDS1, stabilise MYC2 protein while EDS1 but not PAD4 or SAG101, promote MYC2 transactivation activity. Thus, protein accumulation is not indicative of MYC2 transcriptional output. MYC2 activity promotion is lost in a JAZ repressor uncoupled MYC2 variant (MYC2s) or when co-expressed with the bacterial virulence protein avrRPS4, indicating i) regulation of JAZ proteins by EDS1 and ii) immunity context dependent regulation of MYC2. Functional characterisation of this regulation shows that besides JAZ repression MYC2 is phosphorylated in an EDS1 dependent manner. In eds1-2 plants MYC2S123 is phosphorylated, suggesting that EDS1 either represses a protein kinase or activates a protein phosphatase. I show interaction of the protein kinase EDR1 (ENHANCED DISEASE RESISTANCE 1), a negative regulator of plant immunity, with PAD4 and with MYC2 in Arabidopsis (Arabidopsis thaliana) protoplasts. Whether EDS1 regulates MYC2 via EDR1 or another, so far unknown, component remains to be tested.Results presented in this work provide insights into the regulatory relationship of EDS1 and MYC2. EDS1 promotes MYC2 transactivation activity as shown by enhanced MYC2 target gene expression. Molecularly, EDS1 regulates MYC2 via JAZ proteins and likely via MYC2 phosphorylation. More detailed analysis will be necessary to address this entirely. Ultimately, the impact of the presented data depends on the functional relevance of this regulation. For this, in planta experiments in pathogen challenged and unchallenged conditions will be key.