High light enhances Fv/Fmreduction during P. syringae DC3000 infection.
High light can result in photoinhibition and ROS accumulation around
photosystem II. Excess light can be absorbed by the light-harvesting
complexes and dissipated as heat via thermal energy dissipation (qE),
linked to non-photochemical quenching mechanisms (NPQ; (Holt, Fleming
and Niyogi, 2004)). High light also promotes perinuclear clustering of
chloroplasts with genetic encoded H2O2biosensors providing compelling evidence for cROS transport from the
chloroplast stroma to the nucleus following high light exposure
(Exposito-Rodriguez et al. , 2017). These data suggest that high
light may pre-activate host immunity to DC3000 infection, therefore we
examined Fv/Fm dynamics during
DC3000 infection under standard (normal light) or high light conditions.
A “normal” light (120 µmol m-2s-1)
cycle of 1 h (as above), comprised 40 min of light before dark adaption,
for 24 cycles. To ensure the duration of high light exposure would
encompass early pathogen infection events, including expression of
effector genes and assembly of the Type-III Secretion System (T3SS), we
used a regime of 2.5 h high light (650 µmol
m-2s-1) prior to dark adaption,
enabling Fv/Fm measurements to be
captured 8 times over a 26 h period. In comparison to DC3000 or
DC3000hrpA challenge under normal light conditions (Figure 6A,
B), high light resulted in a dramatic initial decrease ofFv/Fm within the first 6.5 hpi
for both DC3000 and DC3000hrpA challenges.Fv/Fm in DC3000hrpAtreated leaves partially recovered did not regain levels observed under
normal light condition (Figure 6C, D). By contrast, leaves infected with
DC3000 showed strong decreases inFv/Fm over the entire 26 h. These
were consistently significantly lower than that observed in infected
leaves under normal light (Figure 6A-D). Interestingly, under high light
conditions flg22 and elf18 pre-treatment failed to prevent the majority
of the suppression of Fv/Fm and
in fact show infection dynamics very similar to that observed in DC3000
challenged Col-0 leaves. These data imply high light was the dominant
driver of Fv/Fm (Figure 6E, F).
Consistent with this, high light treatment at the onset of infection
even further suppressed the Fv/Fm infection dynamics observed for the bkk1-1/bak1-5 plants
(Figure 6G, H).
To ascertain the impact of high light inducedFv/Fm suppression we enumerated
bacterial growth under high light and normal light conditions. As the
strong Fv/Fm suppression under
high light exhibited by DC3000 challenge is reminiscent of ETI responses
(Littlejohn et al. , 2021) it was surprising that high light
enhanced susceptibility (Figure 7,
Supp Fig 3b). Interestingly, already hypersusceptiblebkk1-1/bak1-5 plants were even more susceptible to DC3000
infection under high light (450 µmol
m-2s-1) suggesting that high light
uncouples immunity through pathways independently of those guarded by
classical MTI signalling (Figure 7). Interestingly, there was no
significant difference in bacterial growth observed for fls2 in
comparison to Col-0 plants under high light (Figure 7). This apparent
insensitivity of fls2 plants to high light warrants further
investigation. Notably, plants pre-adapted to high light were no more or
less susceptible than plants exposed to high light immediately after
DC3000 challenge (Figure 7 and Supp Figure 3). Despite showing the
accumulation of anthocyanin compared to the cognate control plants under
120 µmol m-2s-1 (Supp Fig 3a),
plants that had been acclimatised to high light treatment for 5 days
showed similar enhanced susceptibility (Supp Fig 3b). This is despite
significant accumulation of anthocyanin which are associated with
accumulation of defensive metabolites (Gould, 2004; Schaefer and
Rolshausen, 2006; Lev-Yadun and Gould, 2008). Thus, HL pre-adaptation is
not required to elicit enhanced susceptibility, it is only required
co-incident with pathogen infection to significantly enhance bacterial
growth, and this is additional to that achieved by uncoupling classical
MTI defences.