Victoria Munkager:
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Challenges such as climate change and a growing world population has increased the need for
sustainable solutions that can increase crop production. Microbial inoculants have been proposed to
bring forth the next green revolution to agriculture as alternatives to chemical fertilizers and
pesticides. Alleviation of abiotic stress, pathogen attack, and nutrient deficiency are some of the many
plant benefits attributed to microorganisms. Yet, microbial inoculants have had limited success as
commercial, effective products, in part due to our insufficient understanding of the many underlying
mechanisms controlling how plants, rhizosphere microorganisms and soil interact. We need to further
our fundamental understanding of this complex system, not only to attain more effective microbial
inoculants for agriculture, but also to ensure that they truly are sustainable alternatives to chemical
fertilizer and pesticides without any unintended side-effects. In this thesis, I present work illustrating
that abiotic conditions can alter the plant-microbiota interaction. When soil is nitrogen-limited, the
soil microbiota induces nitrogen deficiency in plants. I also show that shoot biomass was reduced by
12% regardless of nitrogen availability, when plants were inoculated with soil microbiota. This
specific plant-microbiota interaction was deleterious to plant growth even in conditions free of abiotic
stress and pathogen infection. It appears that interacting with microorganisms, is not necessarily costfree
for plants. Furthermore, I show that artificial selection of microbiotas associated with low shoot
greenness also leads to some plants to exhibiting symptoms of iron deficiency. This finding illustrates
that although microbiota engineering can successfully be applied to alter a plant trait, we may also in
the process inadvertently alter one or several other plant traits. This highlights why it is essential that
we further our understanding of how microorganisms affect plants and under which conditions. In
the thesis, I also present a new experimental design for studying plant-microbiota-soil interactions
under highly controlled conditions. Due to the complexity of the plant-microbiota-soil interaction,
we need such systems wherein microbial contamination, from seeds and the surrounding
environment, is minimized to decrease stochastic variation, so that we gain a mechanistic
understanding of how microorganisms affect plants. My work and results are a small step towards
closing the many knowledge gaps that prevent us from predicting the net effect specific
microorganisms will have on a specific plant under specific abiotic conditions. It is important not
only to focus on the benefits that microorganisms can provide to plants but also to understanding the
deleterious interactions. When we fully understand how and when soil microorganisms impede plant
growth and health, we may also be able improve plant growth via microorganisms by minimizing the
deleterious interactions.