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Title: Symbiotic interactions between the parasitic weed Striga hermonthica and the arbuscular mycorrhizal fungus Rhizophagus irregularis in rice
Author: McNally, A.
ISNI:       0000 0004 6060 4738
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2016
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The parasitic weed Striga hermonthica and the arbuscular mycorrhizal (AM) fungus Rhizophagus irregularis are both obligate root symbionts that appear to have opposing functions in planta. S. hermonthica is a parasite that presents a serious threat to subsistence agriculture in sub-Saharan Africa (SSA), whereas R. irregularis can enhance host fitness by providing nutrients and inducing plant defence against a range of pathogens and parasites. Previous research into the interaction between AM fungi and Striga has found a suppressive effect of the fungus on parasite infection. This has been attributed to a reduction in parasite-seed germinating strigolactones (SLs) released from the host roots as a result of AM-facilitated nutrient enhancement. Given their requirement for association with host roots, Striga and AM fungi enter an inherent competition to acquire resources, and alter host physiology in order to achieve their optimum fitness. It is therefore likely that the interaction will involve more than just the pre-attachment SL signalling mechanism. The aim of this thesis was to develop our understanding of this interaction by exploring symbiont success and the effect of the interaction on host growth and physiology. In chapter 2, the host plant, rice, was grown in pots with both S. hermonthica and a commercially available R. irregularis inoculum. R. irregularis acted as a highly mutualistic symbiont. However, in combination with S. hermonthica, there was an increase in S. hermonthica infection. Root metabolome analysis indicated opposing effects of each symbiont on host defence chemistry at the time of harvest. I hypothesised that the down-regulation of host defences known to occur during the early stages of AM colonisation were responsible for the increase in S. hermonthica infection, and that high nutrient supply coupled with nutrient provision by the fungus allowed the host plant to support the increase in parasite demand for host nutrients. In chapter 3, I manipulated the nutrient supply to the host to see how this alters the ability of the host to support S. hermonthica. Hairy root cultures of R. irregularis isolate 09 were used instead of the commercial inoculum used in chapter 2. Large differences in root growth between host plants supplied with high and low nutrient treatments made comparisons of symbiont success problematic. Interestingly though, the isolate of R. irregularis used in this chapter had a commensal relationship with the host in contrast to the mutualistic isolate used in chapter 2. Furthermore, there was little alteration in S. hermonthica infection in the presence of the fungus. These observations showed the importance of AM fungal identity for host plants and on subsequent competition with S. hermonthica. In chapter 4, I used rhizotrons to investigate how the order of colonisation/infection alters symbiont success. R. irregularis isolate 09 was used to colonise plants in rhizotrons before, after and at the same time as S. hermonthica. Early arrival afforded a priority effect, which suppressed the invading symbiont, and each symbiont appeared to exclude the other from specific roots. Overall, this thesis has expanded our knowledge of the AM-Striga-host interaction by exploring mechanisms beyond SL signalling. I have shown that the impact of an AM fungus on Striga is context-dependent and may not always be beneficial by reducing parasite infection. Furthermore, the order of symbiont arrival and its effects on host physiology may determine symbiont success.
Supervisor: Scholes, Julie ; Cameron, Duncan Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available