Pepper asparagine synthetase 1 (CaAS1) is required for plant nitrogen assimilation and defense responses to microbial pathogens

Asparagine synthetase is a key enzyme in the production of the nitrogen-rich amino acid asparagine, which is crucial to primary nitrogen metabolism. Despite its importance physiologically, the roles that asparagine synthetase plays during plant defense responses remain unknown. Here, we determined that pepper (Capsicum annuum) asparagine synthetase 1 (CaAS1) is essential for plant defense to microbial pathogens. Infection with Xanthomonas campestris pv. vesicatoria (Xcv) induced early and strong CaAS1 expression in pepper leaves and silencing of this gene resulted in enhanced susceptibility to Xcv infection. Transgenic Arabidopsis (Arabidopsis thaliana) plants that overexpressed CaAS1 exhibited enhanced resistance to Pseudomonas syringae pv. tomato DC3000 and Hyaloperonospora arabidopsidis. Increased CaAS1 expression influenced early defense responses in diseased leaves, including increased electrolyte leakage, reactive oxygen species and nitric oxide bursts. In plants, increased conversion of aspartate to asparagine appears to be associated with enhanced resistance to bacterial and oomycete pathogens. In CaAS1-silenced pepper and/or CaAS1-overexpressing Arabidopsis, CaAS1-dependent changes in asparagine levels correlated with increased susceptibility or defense responses to microbial pathogens, respectively. Linking transcriptional and targeted metabolite studies, our results suggest that CaAS1 is required for asparagine synthesis and disease resistance in plants.