Abstract
Excessive weed growth represents one of the major threats to crop production especially when reliance on herbicides is reduced. Biological weed control is an alternative, environmentally-sound method that, combined with other weed control practices, can contribute to an effective weed management in sustainable farming systems. Arbuscular mycorrhizal fungi (AMF) are
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widespread soil fungi that form symbiotic associations with the majority of land plants. Although AMF are mainly acknowledged for enhancing plant nutrient uptake and growth, these fungi are not always beneficial; interactions between plants and AMF can range from mutualistic to antagonistic, where plant growth is reduced. Previous studies have shown that AMF can have deleterious effects on weedy plant species but not much attention has been paid to antagonistic interactions between AMF and weeds, despite their applied potential. This PhD thesis aimed to gain a further insight on antagonistic effects of AMF on the growth of weeds in order to assess the potential role of AMF in weed management.
The effect of AMF on the growth of mycorrhizal and non-mycorrhizal (i.e. plants that do not establish a symbiosis with AMF) weed species was tested in several glasshouse experiments. The results presented in this thesis show that: (1) growth responses of mycorrhizal weed species to inoculation with AMF are variable but there seems to be predominance of negative responses, (2) negative effects of AMF on the growth of some weed species (e.g. Echinochloa crus-galli, Setaria viridis, Solanum nigrum) seem to be fairly consistent but (3) depend on AMF identity and abundance. In addition, (4) growth of non-mycorrhizal weeds (e.g. Stellaria media) can be strongly reduced in the presence of AMF mycelium supported by a coexisting mycorrhizal host but (5) this effect is most likely not due to the production of allelochemichals by AMF. Finally, (6) AMF can similarly reduce the growth and infect the roots of the model (non-mycorrhizal) species, Arabidopsis thaliana.
Although there seems to be predominance of antagonistic effects of AMF on weeds, the fact that not all the weed species are negatively affected by AMF and that not all AMF cause growth depressions on weeds indicates that AMF should not be considered as broad-spectrum weed biocontrol agents. Nonetheless, it indicates that AMF can drive weed community composition in agroecosystems. The role of AMF in weed management might therefore be more subtle; it is possible that manipulation of AMF via agronomic management may provide a means of keeping some weed populations below tolerance threshold levels and of directing weed communities dynamics as to reduce negative effects of weeds. In order to realize the potential role of AMF on weed management future research is needed, namely studies on the effects of AMF on the growth of weeds in field conditions. Furthermore, mechanisms explaining such effects should be tested. By showing for the first time mycorrhizal effects on the model plant Arabidopsis, my results suggest that Arabidopsis might serve as a valuable tool in unraveling the molecular basis of incompatible plant-AMF interactions, including the antagonistic effects on non-host weed species.
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