The relationship between strigolactones and Striga hermonthica infection in cereals

Cereal production in Africa is under increasing constraint due to the obligate, out-crossing, hemiparasitic weed Striga hermonthica (Del.) Benth, a member of the Scrophulariaceae family. Striga parasitizes roots of cereals like sorghum, pearl millet, maize and upland rice. It has infested about 40% of the African agricultural land, resulting in severe yield losses or even complete crop failure worth US billion per annum. The subsistence farmers or approximately 300 million African people lose about 20-80% of their crop because of this weed. This considerable damage by Striga is due to the fact that existing control measures are often ineffective. Since much of the damage occurs underground during the early stages of parasitism, there is a need to develop control strategies that target the weed prior to attachment and emergence. A crucial step in the lifecycle of Striga is the induction of germination by strigolactones, signalling molecules secreted by the roots of its host. These strigolactones could be an important target to control this weed at the pre attachment phase. Control methods targeted at the germination and attachment phase, based on low strigolactones, might prove to be more effective and result in reduced infestation of this weed in cereal crops. In present thesis we studied the relationship between strigolactones and Striga infection in cereals and explored opportunities for lowering Striga damage at the germination or attachment phase. To this end different aspects like strigolactone biosynthetic inhibitors, genetic variation for strigolactone production, and the effect of fertilizers on strigolactone production were investigated in laboratory studies and – when possible - in the field in Kenya and Mali. The first investigation was on the use of carotenoid inhibitors to see the possibilities of strigolactone reduction in the roots of plants by blocking carotenoid biosynthesis. We postulated in this study that the (mild) inhibition of carotenoid biosynthesis by carotenoid inhibitors, could lead to a reduced production of strigolactones and decreased Striga germination and infection. Very low concentrations of four different carotenoid inhibitors (fluridone, norflurazon, clomazone and amitrole) were applied to rice either through irrigation or through foliar spray. Irrigation application of all carotenoid inhibitors and spray application of amitrole significantly decreased strigolactone production. Application of carotenoid inhibitors caused 61-75% reduction in Striga germination and 65-94% reduction in Striga attachment. The study shows thatthe reducing effect of carotenoid inhibitors (which, in much higher concentrations are widely used as herbicides) on strigolactone secretion and subsequent Striga germination and attachment may be developed into an attractive Striga control technology. Another experiment was aimed at assessing the pre-attachment Striga resistance based on low strigolactone production. We hypothesized that low strigolactones producing crop cultivars might possess pre-attachment Striga resistance due to less germination. For this purpose a set of 18 upland cultivars of NERICA and their parents were screened for strigolactones production and Striga infection parameters like germination, attachment, emergence and Striga dry biomass. NERICA 1 and CG14 produced significantly less strigolactones and showed less Striga infection while NERICAs 7, 8, 11 and 14 produced the highest amounts of strigolactones and showed the most severe Striga infection. This study shows that genetic variation for pre-attachment Striga resistance exists in NERICA rice due to variation in strigolactones. This could be highly relevant for breeding programs aimed at the development of Striga resistant cultivars. In another similar study we hypothesized that variation in strigolactone production in rice might be interconnected with the tillering phenotype and that this link could affect Striga infection. In this study the genetic variation was tested in a series of rice varieties collected from all over the world for strigolactone production, tillering phenotype and Striga infection. Rice cultivars like IAC 165, IAC 1246, Gangweondo and Kinko produced high amounts of the strigolactones, displayed low amounts of tillers and induced high Striga germination, attachment, emergence as well as Striga biomass. In contrast to this, rice cultivars such as Super Basmati, TN 1, Anakila and Agee showed low production of strigolactones and also low Striga germination and infection but high tillering. These results show that genetic variation in strigolactone production results in variation in tillering and also in Striga infection. The tillering phenotype could possibly be used as an easy indicator of the strigolactone production in a breeding programme for Striga resistance. Some experiments were also designed with the aim to quantify the relationship between strigolactones and Striga germination and attachment and to explore the mechanism responsible for the reported reduction in Striga parasitism in the field after fertilizer application. Different levels of nitrogen and phosphorous were applied under greenhouse conditions using rice, maize and sorghum. For maize and sorghum, a parallel study was carried out under field conditions in Kenya and Mali to study the translation of greenhouse results to the field. Application of N and P effectively suppressed Striga infection in the greenhouse in all three crop species and the reduction strongly correlated with reduced secretion of strigolactones into the rhizosphere and the Striga germination induced by these exudates. Although the field results with maize in Kenya were less consistent than in the greenhouse, especially with respect to P effect, still there was a trend that fertilizer application reduced Striga infection. These results show that the positive effect of fertilizer against Striga is at least partly due to a reduction in strigolactone production and as a consequence of that lower Striga germination and subsequent attachment. Overall it can be concluded that there is a good correlation between strigolactones andStriga germination, attachment and biomass. We found this using strigolactone biosynthesis inhibitors, genetic variation and using fertilizer application. These technologies can hence be exploited as an important tool to target Striga at a very early phase of its life cycle. The practical field application of these strategies requires further research but could lead to effective Striga control components that can be used in Integrated Striga Management.