Fusarium
head blight (FHB) is an economically important disease of small grains globally
and is primarily caused by Fusarium graminearum in North America.
Recently, microbial biocontrols have risen in importance as sustainable agents
of disease control. However, the path to implementation of microbial biocontrols
in agriculture will require an understanding of how microbiota impact both
plant performance overall and vary with inherent host disease resistance. Using
a full-factorial, controlled greenhouse experiment, we tested how bacterial
endophyte treatments impacted both plant physiology prior to disease onset and
FHB disease progression in Triticum aestivum (wheat). Seven bacterial
endophytes were tested as seed soaks on two wheat varieties varying in FHB
resistance. Leaf-level physiology and morphology measurements were made prior
to inoculation with F. graminearum, while F. graminearum toxin accumulation
(deoxynivalenol or DON) was quantified post-harvest. Bacterial endophyte
treatments strongly impacted the light dependent reactions of photosynthesis,
with changes in plant traits regulating light energy allocation and the
build-up of electrochemical energy storage across the thylakoid membrane.
Physiological responses were contingent on host variety. The direct effects of
bacterial endophytes on wheat response to infection were weak and dependent on
the inherent disease resistance of the host variety. However, disease outcomes
were indirectly mediated by bacterial impacts on plant traits, with proton
motive force traits emerging as common predictors of disease response across
both host varieties and other traits indicating potential trade-offs in host
response to bacterial inoculants and F. graminearum infection. Our results
provide an alternate mechanism for microbial biocontrol efficacy other than
direct antagonism with the pathogen inside the host. Furthermore, the
chlorophyll-fluorescence and absorbance-based markers assessed here may have
translational potential as a phenotyping tool for FHB susceptibility in wheat
and other small grains.