MAMP pre-treatment protectsFv/Fm suppression by P. syringae DC3000 infection.
Previous work showed that leaves pre-treated with flg22 24 hpi with virulent P. syringae DC3000 restricted effector induced suppression of maximum dark-adapted quantum efficiency (Fv/Fm ) levels (de Torres Zabalaet al. , 2015). To determine if this observation was true for other peptide elicitors, wild type, Col-0, A. thaliana leaves were pre-treated with the bacterial peptides flg22 (1 µM), elf18 (1 µM) and the fungal peptide chitin, (chi; 100 µg/ml) then 16 hpi with DC3000. All pre-treatments protected challenged leaves from DC3000 induced suppression of Fv/Fm over a 24 h period (Figure 1A). Figure 1B illustratesFv/Fm images at 18 hpi showing pre-treated leaves (flg, elf, chi) have healthyFv/Fm responses (red/orange leaves) whereas reduced Fv/Fm due to DC3000 infection following H2O (mock) pre-treatment is characterised by their distinctive green/blue colour.
Flg22, elf18 and chitin are recognised by the plant cell surface pattern recognition receptors (PRR) FLS2, EFR and Cerk1-2, respectively. Elf18 pre-treatment of fls2 leaves primed the plant and this cross-protection resulted in no change toFv/Fm during DC3000 infection (de Torres Zabala et al. , 2015). These data indicate that activation of different MTI receptors can abrogate effector mediatedFv/Fm suppression. Consistent with this hypothesis, flg22 pre-treatment on efr1 (Figure 1C, D) or cerk1-2 leaves (Figure 1E, F) results in protection against DC3000 mediated Fv/Fm suppression over a 24 h period. The level of protection offered by flg22 toefr1 and cerk1-2 mutants is comparable to the Col-0 control (Figure 1C, E). By contrast, pre-treatment of elf18 onefr1 plants (Figure 1C, D) and chitin on cerk1-2 plants (Figure 1E, F) failed to prevent suppression ofFv/Fm following DC3000 infection. The Fv/Fm images at 18 hpi illustrate healthy (red/orange) flg22 pre-treated leaves on Col-0,efr1 and cerk1-2 plants compared to suppression ofFv/Fm (green/blue) induced by DC3000 infection (Figure 1D, F) following H2O treatment. In addition, elf18 pre-treatment protects cerk1-2 plants from reduced Fv/Fm during DC3000 infection (Figure 2A, B). Notably, cerk1-2 challenged leaves showed greater suppression of Fv/Fm compared to Col-0 (Figure 1D, 2A) indicating uncoupling chitin signalling may also weaken chloroplast immune responses. Surprisingly, while pre-treatment of efr1 plants with chitin protected from DC3000 induced suppression of Fv/Fm levels chitin pre-treatment failed to protect fls2 plants during DC3000 infection (Figure 2C-D), where DC3000 suppression ofFv/Fm was indistinguishable from H2O pre-treatment (Figure 2E-F).