Understanding resistance in inter-specific rice cultivars to the parasitic witchweed Striga

• Main Author: Cissoko, Mamadou
• Other Authors: Scholes, Julie
• Published: University of Sheffield 2012
• Subjects: 632
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.577372

The root hemi-parasitic witchweeds Striga hermonthica and S. asiatica are considered the most important biotic constraint to cereal crop production in sub-Saharan Africa (SSA). These parasites infect the staple cereal crops (rice, maize, sorghum and millet) resulting in considerable yield losses. Control of these parasites is very difficult as the Striga seed bank is widespread and damage to the crop occurs long before the parasite emerges above ground. Resistant cultivars are considered to be an effective and affordable component of an integrated Striga management strategy but very few are available to farmers as sources of resistance to Striga are relatively scarce and little is known about the molecular genetic basis of resistance to this parasite. Rice is an economically important cereal crop in SSA that is mostly cultivated by resource-poor farmers. Both cultivated rice species, Oryza sativa (L.) and Oryza glaberrima (Steud.), are grown in Africa. To take advantage of superior traits from each species, AfricaRice Center and partners developed inter-specific rice cultivars called NERICA (NEw RICe for Africa) for rain-fed upland ecosystems. Because of their high yields, even on low nutrient soils where Striga spp. are prevalent, the NERICA cultivars have been widely adopted by farmers. Despite this, very little is known about their resistance to different species and ecotypes of Striga. The aims of this study are to determine how resistant and/or tolerant the upland NERICA cultivars are to different species and ecotypes of Striga under controlled environment and Striga-infested field conditions, to identify whether resistance is broad spectrum or specific to particular ecotypes of Striga and to characterize the phenotype of the resistance at a histological level. Finally using a Chromosome Segment Substitution Line (CSSL) population derived from a cross between an O. glaberrima cultivar MG12 (donor parent) and an O. sativa cultivar Caiapo (recurrent parent), the genetic basis of post-attachment resistance to Striga is investigated. The NERICA rice cultivars showed different susceptibilities to both S. hermonthica and S. asiatica species under controlled environment conditions. Some cultivars showed good broad-spectrum resistance against several Striga ecotypes and species whilst others showed intermediate resistance or were very susceptible. In addition, some cultivars showed resistance to a particular ecotype of Striga but were susceptible to others. The phenotype of a resistant interaction was often characterized by necrosis at the host parasite interface and an inability of the parasite to penetrate the host root endodermis. In general, the most resistant NERICA cultivars grew better than the very susceptible cultivars although even a small number of parasites caused a reduction in above ground host biomass. There was however, genetic variation for tolerance to Striga (the ability to grow and yield well in the presence of Striga) amongst the NERICA cultivars. The NERICA cultivars were also grown in field trials at Kyela in Tanzania (under S. asiatica infestation) and at Mbita Point in Kenya (under S. hermonthica infestation) in 2010 and 2011 to determine the impact of environment on the expression of resistance. The resistance of the NERICA cultivars against S. hermonthica and S. asiatica, in the field, was broadly similar to that observed in the laboratory although there were some exceptions. These results allow us to recommend particular cultivars for Striga-infested regions but they also illustrate the necessity of understanding the genetic basis of resistance to different ecotypes of Striga for breeding of durable resistance (and pyramiding of appropriate resistance genes) in host cultivars adapted to different rice agro-ecosystems in sub-Saharan Africa. Sixty four lines of an inter-specific CSSL population and the parent cultivars MG12 and Caiapo were phenotyped for resistance to S. hermonthica. MG12 showed good resistance to S. hermonthica whilst Caiapo was very susceptible. The CSSLs showed a range of susceptibility to the parasite, however, only two CSSLs showed the same strong resistance phenotype as MG12. Graphical genotyping and a Quantitative Trait Loci (QTL) analysis revealed a large QTL on chromosome 12 (designated STR12.1) which explained at least 80 % of the variation for resistance in the population and suggests that resistance to S. hermonthica (in MG12) is due to one (or a few genes) of major effect. This finding opens the way for the identification of candidate Striga resistance genes (through fine mapping approaches) and their transfer to farmer-preferred cultivars via marker assisted breeding.