UC San Diego

Plants defend against pathogens through metabolic reprogramming and de novo synthesis of various proteins and defense metabolites, which requires mobilization of nitrogen through nitrogen-releasing enzymes. Plant-type L-asparaginase (ASPG) is an essential nitrogen-releasing enzyme, which catalyzes the conversion of asparagine to aspartic acid. However, little is known about the activation mechanism of this enzyme in relation to plant immune pathways. Here, we defined the variable-loop serine site phosphorylation of the enzyme asparaginase after AtPep1- induced immune responses. Furthermore, we set out to examine the role of this serine site phosphorylation in enzyme cleavage, and the subsequent effect of nitrogen availability on Arabidopsis thaliana immune responses against necrotrophic fungus Botrytis cinerea. It was observed that double mutant Arabidopsis plants (aspga+b) lacking both potassium-dependent (ASPGB1) and potassium-independent (ASPGA1) enzymes are susceptible to fungal attack. Additionally, our analysis further revealed that the phosphoabolishing transgenic Arabidopsis line aspga+b: ASPGA1.1 S169A -YFP is similarly susceptible to B. cinerea. Taken together, this data shows the variable-loop serine site phosphorylation plays a role in asparaginase enzymatic activity and implicates ASPGA1 as an essential influencer of defense output in Arabidopsis.