The Function and Regulation of Chloroplast Targeting of AZI1, a Key Factor in Plant Systemic Immune Signaling

Abstract

Recognition of microbes and microbe‐associated molecular patterns by plant cell receptors induces signaling cascades that prime broad‐spectrum defense responses to suppress subsequent infections. Primed plants can activate stronger defense responses more quickly than unprimed plants when challenged by pathogens and other stresses. Systemic priming requires the long‐distance movement of signals produced during initial exposure events. Distinct responses are induced depending on whether roots are colonized by specific commensal microbes or if leaves are infected by certain pathogens. The protein AZI1, a member of the lipid transfer protein superfamily, is essential for both root‐stimulated and leaf‐stimulated defense priming. AZI1 is also needed for uptake and mobilization of azelaic acid (AZA). A defense signal derived from chloroplast lipid oxidation, the oxylipin AZA stimulates priming when applied to leaves or roots.Functional AZI1‐GFP traffics among chloroplast outer envelope membranes (OEMs), ER, PM, and intercellular channels called plasmodesmata. The targeting of AZI1 to chloroplast OEMs requires a newly described bipartite protein targeting signal featuring an N‐terminal hydrophobic domain and an internal region called the proline‐rich region. AZI1’s association with chloroplasts and plasmodesmata is suggestive of a pathway by which AZI1 may act in the intercellular mobilization of a chloroplast‐derived lipid signal(s) such as AZA. Consistent with this hypothesis, AZI1 increasingly targets to chloroplasts under infection conditions. This infection‐induced chloroplast accumulation requires the kinase MPK3, which, like AZI1, is essential for priming defenses and is reported to phosphorylate AZI1 in vitro.To better understand the molecular basis of plant immunity, it is essential to assess the location and movement of chloroplast‐derived priming signals; here, I report on the function of AZI1’s localization to chloroplasts and the regulation of this chloroplast targeting using the model plants Arabidopsis thaliana and Nicotiana benthamiana. A more thorough understanding of priming will have profound agricultural benefits via genetic engineering or breeding of crops. Enhanced priming will boost immunity only during infection and would reduce fitness costs associated with constitutively active defenses.Support or Funding InformationThis work is supported by the NSF, the MCB training grant (T32 GM007183), the Ford Predoctoral Fellowship, and the HHMI Gilliam Fellowship for Advanced Study.