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).