Localization of DIR1 at the tissue, cellular and subcellular levels during Systemic Acquired Resistance in Arabidopsis using DIR1:GUS and DIR1:EGFP reporters.

BACKGROUND: Systemic Acquired Resistance (SAR) is an induced resistance response to pathogens, characterized by the translocation of a long-distance signal from induced leaves to distant tissues to prime them for increased resistance to future infection. DEFECTIVE in INDUCED RESISTANCE 1 (DIR1) has been hypothesized to chaperone a small signaling molecule to distant tissues during SAR in Arabidopsis. RESULTS: DIR1 promoter:DIR1-GUS/dir1-1 lines were constructed to examine DIR1 expression. DIR1 is expressed in seedlings, flowers and ubiquitously in untreated or mock-inoculated mature leaf cells, including phloem sieve elements and companion cells. Inoculation of leaves with SAR-inducing avirulent or virulent Pseudomonas syringae pv tomato (Pst) resulted in Type III Secretion System-dependent suppression of DIR1 expression in leaf cells. Transient expression of fluorescent fusion proteins in tobacco and intercellular washing fluid experiments indicated that DIR1's ER signal sequence targets it for secretion to the cell wall. However, DIR1 expressed without a signal sequence rescued the dir1-1 SAR defect, suggesting that a cytosolic pool of DIR1 is important for the SAR response. CONCLUSIONS: Although expression of DIR1 decreases during SAR induction, the protein localizes to all living cell types of the vasculature, including companion cells and sieve elements, and therefore DIR1 is well situated to participate in long-distance signaling during SAR.

Forward and reverse genetics to identify genes involved in the age-related resistance response in Arabidopsis thaliana.

SUMMARY Age-related resistance (ARR) occurs in numerous plant species, often resulting in increased disease resistance as plants mature. ARR in Arabidopsis to Pseudomonas syringae pv. tomato is associated with intercellular salicylic acid (SA) accumulation and the transition to flowering. Forward and reverse genetic screens were performed to identify genes required for ARR and to investigate the mechanism of the ARR response. Infiltration of SA into the intercellular space of the ARR-defective mutant iap1-1 (important for the ARR pathway) partially restored ARR function. Inter- and intracellular SA accumulation was reduced in the mutant iap1-1 compared with the wild-type, and the SA regulatory gene EDS1 was also required for ARR. Combining microarray analysis with reverse genetics using T-DNA insertion lines, four additional ARR genes were identified as contributing to ARR: two plant-specific transcription factors of the NAC family [ANAC055 (At3g15500) and ANAC092 (At5g39610)], a UDP-glucose glucosyltransferase [UGT85A1 (At1g22400)] and a cytidine deaminase [CDA1 (At2g19570)]. These four genes and IAP1 are also required for ARR to Hyaloperonospora parasitica. IAP1 encodes a key component of ARR that acts upstream of SA accumulation and possibly downstream of UGT85A1, CDA1 and the two NAC transcription factors (ANAC055, ANAC092).