Mechanisms underlying changes in the morphology of rice plants infected with the parasitic weed Striga hermonthica

Parasitic plants of the genus Striga (also known as witchweeds) are root hemiparasites that cause devastating losses in crop production in sub Saharan Africa (SSA). Two of the most important species, S. hermonthica and S. asiatica, infect the staple cereal crops of SSA including maize, sorghum, millet and rice. These parasitic weeds represent the major biotic constraint to cereal crop production. They cause losses in yield of 40-100% and negatively impact the quality of life of millions of the poorest subsistence farmers. Infection of crop plants by Striga causes severe stunting of the host plant, thinning of stems and in plants such as rice, suppression of tillering. In rice the number of tillers produced is very important as grain yield is often correlated with tiller number. At present the mechanisms underlying changes in the morphology and architecture of Striga-infected plants are not fully understood and the aim of this thesis was to investigate the potential role of plant growth regulators in the suppression of tillering and the stunting rice plants infected with S. hermonthica. In Chapter 2 a detailed analysis of the changes in growth and architecture of the susceptible rice cultivar IAC165 following infection by S. hermonthica was carried out. This revealed that Striga-infected rice plants showed severe stunting, decreased biomass, increased root to shoot ratio, thinner stems and suppression of tillering. In addition, this study revealed, for the first time that the suppression of tillering in Striga-infected rice plants to an inhibition in the formation and outgrowth of lateral buds. In Chapter 3 the hypothesis that strigolactones are involved in the suppression of tillering in Striga-infected rice plants was investigated by analysing the effect of the parasite on the suppression of tillering in rice plants with altered concentrations of strigolactones (plant growth regulators involved in the regulation of branching in plants). Strigolactone concentration was modified by (a) the use of strigolactone mutants (d10-1, a strigolactone biosynthestic mutant and d3-1 a strigolactone signalling mutant) and (b) the application of a carotenoid biosynthetic inhibitor (fluridone) or a synthetic analogue of strigolactone (GR24). These studies revealed a partial role for strigolactones in the inhibition of tillering. In addition, there was an increase in the expression of CCD8 (encoding a strigolactone biosynthetic enzyme) in the leaf axils of S. hermonthica - infected plants, consistent with a role for strigolactones in the suppression of tillering. The effect of S. hermonthica on the biosynthesis of strigolactones in infected roots was also investigated using CCD8:GUS transgenic rice plants. These studies revealed increased expression of CCD8 in cells of the vascular bundle as S. hermonthica penetrated into the root and initiated fusion of xylem vessels with its host, suggesting a role for strigolactones during the infection process In Chapter 4 the potential roles of multiple plant growth regulator pathways in the changes in plant morphology of rice plants infected with S. hermonthica were investigated by profiling changes in gene expression in the roots, stem and leaves of control and Striga-infected plants using Affymetrix microarrays. This is the first study to analyse whole genome expression in the whole plant simultaneously following infection by S. hermonthica. This study showed the profound impact of S. hermonthica on the regulation of auxin metabolic, signalling and transport pathways of infected plants, revealing the importance of this plant growth regulator in plant parasitism.