Role of plasmodesmata and plasmodesmata localizing proteins in systemic immunity.

Systemic acquired resistance (SAR) is a highly desirable form of resistance that protects against a broad-spectrum of pathogens. SAR involves the generation of a mobile signal at the site of primary infection, which arms distal portions of a plant against subsequent secondary infections. A number of diverse chemical signals contributing to SAR have been isolated and characterized. Among these, salicylic acid (SA) functions in parallel to azelaic acid (AzA) and glycerol-3-phosphate (G3P), and both AzA and G3P function downstream of the free radicals nitric oxide and reactive oxygen species. We now show that phloem loading of AzA and G3P occurs via the symplast, whereas that of SA occurs via the apoplast. The symplastic transport of AzA and G3P is regulated by plasmodesmata localizing protein (PDLP) 5, which together with PDLP1 also plays a signaling role in SAR. Together, these results reveal the transport routes of SAR associated chemical signals, and the regulatory role of PDLPs in SAR.