The role of Plant Growth Regulators in the interaction between the parasitic weed Striga Hermonthica and rice

Parasitic plants of the genus Striga are obligate, hemiparasitic weeds that infect the roots of their host plants in order to access nutrients. Striga hermonthica infects staple cereal crops such as maize, sorghum, millet and rice and is considered to be the major biotic factor limiting cereal production in sub-Saharan Africa (SSA). Infection leads to a rapid onset of severe morphological alterations in the host, such as a stunting and thinning of the main stem, a suppression of secondary branching, stomatal closure and a reduction in the rate of photosynthesis. These changes, in combination with a loss of carbon, nitrogen and inorganic nutrients, from the host to the parasite lead to reductions in yield of between 40 - 100%. Whilst multiple hypotheses have been proposed to explain the early onset of morphological changes in the host including, exchange of mRNA between host and parasite, movement of a phytotoxin from parasite to host and Striga-induced alterations in the amounts of plant growth regulators (PGRs), none have been conclusively proven. The aim of this thesis was to test the hypothesis that alterations in above ground morphology, particularly the suppression of stem growth and tillering in rice are due to changes in amounts of PGRs.
In order to test this hypothesis a detailed analysis of alterations in the morphology of the rice cultivar IAC 165 was carried out over the time course of infection by S. hermonthica. This was combined, for the first time, with measurements of the amounts of auxins, gibberellins, cytokinins (CKs) and abscisic acid (ABA) in the roots, stems and leaves of uninfected and S. hermonthica-infected plants. These analyses revealed that S. hermonthica-infected rice plants had greatly altered profiles of PGRs and importantly there were reductions in the amounts of bioactive gibberellins and CKs within the stem/stem base, two PGRs that are crucial in promoting stem growth and tillering in rice respectively.
A genetic approach, utilising the strigolactone (SL) biosynthetic and signalling dwarf mutants of rice, was taken to further test the hypothesis that alterations in PGRs lead to the S. hermonthica-induced stunting of the main stem and suppression of tillering in rice. As many PGRs regulate stem growth I hypothesised that these mutants would still show a S. hermonthica-induced stunting of the main stem. Data were consistent with this hypothesis. As SLs suppress tiller bud outgrowth I also hypothesised that these mutants would not show a S. hermonthica-induced suppression of tillering. This analysis revealed that S. hermonthica still caused a suppression of tillering in all the SL mutants suggesting that alterations in SL biosynthesis or signalling are not the main, or only, factor suppressing tillering in S. hermonthica-infected rice. An analysis of the expression of genes known to be involved in tiller bud outgrowth in wildtype cultivars IAC-165 and Shiokari suggested that a suppression in the biosynthesis of cytokinins in the stem bases repressed tiller bud outgrowth.
The analysis of the SL mutants unexpectedly showed that they exhibited a clear increase in susceptibility to S. hermonthica compared to the wildtype. The hypothesis that alterations in PGRs in the roots of these mutants altered the structure of the cell wall and/or the expression of genes involved in cell wall remodelling allowed the parasite to penetrate into the root more easily than wildtype roots was tested by (a) measuring the rate of progress of the parasite endophyte through the root cortex (b) determining whether the cell walls exhibited differential amounts of pectins and hemicelluloses using fluorescent labelled antibodies, (c) profiling the expression of genes involved in cell wall remodelling and (d) by growing rice in the presence of exogenously applied IAA and GR24. These studies showed that there was a faster rate of infection by S. hermonthica through the root cortex of the dwarf mutants compared to the wildtype. Profiling of the expression of cell wall remodelling genes revealed that their expression was higher in the dwarf mutants compared to the wildtype (in the absence of Striga) consistent with the hypothesis that loosening of the cells walls increased susceptibility to the parasite. In addition plants grown with an exogenous supply of IAA or GR24 also led to an increase in susceptibility to S. hermonthica. Infection of the rice cultivar IAC 165, led to an alteration in the composition of the host primary cell wall.
In conclusion this study has shown for the first time that PGRs are involved in regulating the S. hermonthica-induced changes in above ground morphology and that they can modulate the susceptibility of rice roots to S. hermonthica. Further investigation into these mechanisms will provide an avenue for the production of new rice varieties, which show lower yield losses upon infection with S. hermonthica. This will significantly impact the lives of people within SSA.