Abstract
DIR1 is a lipid transfer protein (LTP) postulated to complex with and/or chaperone a signal(s) to distant leaves during Systemic Acquired Resistance (SAR) in Arabidopsis. DIR1 was detected in phloem sap-enriched petiole exudates collected from wild-type leaves induced for SAR, suggesting that DIR1 gains access to the phloem for movement from the induced leaf. Occasionally the defective in induced resistance1 (dir1-1) mutant displayed a partially SAR-competent phenotype and a DIR1-sized band in protein gel blots was detected in dir1-1 exudates suggesting that a highly similar protein, DIR1-like (At5g48490), may contribute to SAR. Recombinant protein studies demonstrated that DIR1 polyclonal antibodies recognize DIR1 and DIR1-like. Homology modeling of DIR1-like using the DIR1-phospholipid crystal structure as template, provides clues as to why the dir1-1 mutant is rarely SAR-competent. The contribution of DIR1 and DIR1-like during SAR was examined using an Agrobacterium-mediated transient expression-SAR assay and an estrogen-inducible DIR1-EGFP/dir1-1 line. We provide evidence that upon SAR induction, DIR1 moves down the leaf petiole to distant leaves. Our data also suggests that DIR1-like displays a reduced capacity to move to distant leaves during SAR and this may explain why dir1-1 is occasionally SAR-competent.
Highlights
Plants respond to pathogen infection locally at the individual cell level and can acquire resistance in tissues distant from the initial site of infection
Exudates collected from mock-inoculated leaves consistently contained less protein per exudate (∼3 μg ml−1) compared to exudates collected from leaves induced for Systemic Acquired Resistance (SAR) (∼30 μg ml−1), suggesting that additional proteins enter the phloem during SAR induction (Figure S1)
The absence of DIR1 antibody signals in exudates collected from mock-inoculated leaves is consistent with experiments done with untreated Arabidopsis petiole exudates performed by Guelette et al (2012)
Summary
Plants respond to pathogen infection locally at the individual cell level and can acquire resistance in tissues distant from the initial site of infection. SAR is defined as a defense response induced by certain local infections resulting in broad-spectrum resistance in distant tissues to normally virulent pathogens (Kuc, 1982). Cucumber and Arabidopsis demonstrated that the SAR response occurs in stages that include induction, movement of a long distance signal(s), perception of the signal(s) which primes the plant for the manifestation stage in which the plant responds to normally virulent pathogens in a resistant manner [reviewed in Champigny and Cameron (2009)]. Induction of SAR is initiated when a necrotizing pathogen infects a leaf and results in either the formation of a localized hypersensitive response (HR) and local resistance, or in disease-induced necrosis (Kuc, 1982). Recent studies in tobacco (Liu et al, 2010a) and Arabidopsis (Mishina and Zeier, 2007) suggest that cell death is not required to induce SAR
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