Modes and sites of action of lipid transfer proteins important for long distance defense priming in Arabidopsis.

CECCHINI NM; SCHLÄPPI MR; GIFFORD AN; GREENBERG JT

Congreso; XVI International Congress on Molecular Plant-Microbe Interactions, IS-MPMI.; 2014

Priming of defense responses is a major mechanism behind the immunological ?memory? observed during plant systemic acquired resistance (SAR). A lipidic signal, azelaic acid (AZA), was identified as a chloroplast-derived mobile priming factor. The products of two essential SAR-components, AZI1 and DIR1, are related to lipid transfer proteins (LTPs), suggesting that AZA could be translocated by these LTPs. To discern the possible roles of AZI1/DIR1 during SAR/AZA-priming, we analyzed their subcellular locations, possible interacting proteins, and if they affected AZA systemic movement. AZI1 localizes exclusively to the membrane fraction and AZI1:GFP resides in vesicles-like structures actively trafficking between ER and plasma membrane, whereas DIR1:GFP localizes mainly to the ER and apoplast. Remarkably, AZI1 also is targeted to the outer membrane of chloroplasts, enriched in contact sites between ER/chloroplasts. These sites are proposed as main places were lipid exchange occurs. Moreover, after infection, the fraction of native AZI1 present in Arabidopsis chloroplasts increases, suggesting a key role for the chloroplast pool of this protein. Tissues co-expressing AZI1:GFP/DIR1:RFP showed co-localization to the ER. In agreement, co-immunoprecipitation studies revealed interactions between AZI1 and DIR1 as well with other members of these LTPs-related protein families. Using 14C-AZA radiotracer assays, we found that AZI1, but not DIR1, is required for maximal movement of AZA to distal tissues. These results strongly suggest the existence of a LTPs complex(s) acting in different steps/places during SAR establishment, probably moving a lipidic signal(s), such as AZA, intracellularly as well as over long distance to the systemic tissues.This work was supported by a grant to JTG from the NSF and U. S. Department of Energy, Office of Biological and Environmental Research