Professor University of Kentucky Lexington, Kentucky
Body of Abstract: Systemic acquired resistance (SAR), a form of broad-spectrum whole-plant immunity, involves the generation and systemic transport of mobile signals which prepare the plant against subsequent secondary infections. Molecular, biochemical, and genetic analysis of SAR signaling has identified numerous chemically diverse SAR activators, several of which are considered systemically mobile. Of these, glycerol-3-phosphate (G3P) also regulates a novel root-shoot-root signaling pathway, that enables strain-specific exclusion of poor nitrogen-fixing rhizobia in legumes. Notably, none of the chemical SAR inducers accumulate or move within the same time frame as the SAR signal. We describe the identifications of phased 21-nucleotide (nt) trans-acting small interfering RNA3a (tasi-RNA) as the early mobile signal of SAR. This tasi-RNAs are synthesized within 4 hours of pathogen infection and quickly transported via the plasmodesmata. Knockout mutations in the encoding precursor or RNA silencing components essential for tasi-RNA biogenesis compromise SAR without altering levels of other known SAR inducers. We find that tasi-RNA mediated cleavage of specific Auxin response factors (ARF) is essential for SAR activation.