Functional Characterization Of Striga Resistance Genes In Rice And In Vitro Culture Of The Parasite

ABSTRACT

Rice, the second most important cereal crop globally, is mainly grown for food and is a staple food for more than half of the world’s population. In sub-Saharan Africa biotic stresses and lack of resources are major production constraints to food production. An estimated yield loss of 16% attributed to weed infestation is recorded in the region with Striga spp. being the most prominent. Striga hermonthica and Striga asiatica constrain production of cereals while Striga gesnerioides parasitize cowpea in West Africa. Cultural control methods have for a long time been applied against Striga with only modest gains. Striga resistance in certain crops has been attributed to having particular genes or QTLs within the genome. Growth regulating factors (GRF) are plant specific transcription factors that play a role in parasitic interactions of Striga, nematodes and Arbuscular mycorrhiza. This study determined the fold change in gene expression of the 12 rice Growth Regulating Factors during Striga infection in a susceptible and a resistant rice genotype. All the 12 rice GRFs were expressed in rice roots. A relative fold change in gene expression of between 0.6 and 1.9 indicated that these genes could be playing a role in susceptibility or resistance to Striga infection. Rice GRF 1, 2, 10 and 12 were upregulated in the resistance rice, Nipponbare, at 4 days post infection while in the susceptible rice, Koshihikari, they were downregulated. To find out if heterologous expression of a resistance gene can induce resistance against Striga, rice was transformed with the resistance gene RSG3-301 and phenotyped for resistance to Striga hermonthica and Striga asiatica. Molecular analysis of the transgenic plants was done through PCR, RT-PCR and southern blot analysis. Arabidopsis thaliana transformed with RSG3-301 was also phenotyped for resistance to Striga gesnerioides. Heterologous expression of this gene did not induce resistance against any of the three Striga species. To develop a tool for functional validation of Striga genes, Striga hermonthica calli were transformed with GUS gene through Agrobacterium tumefaciens. Striga asiatica was found amenable to host free culture, a method that proved useful in production of clean explant for tissue culture. One-month-old whole seedlings and stem explants produced calli and there was no significant difference in callus induction frequency between the two. Identification of genes that can be used for development of Striga resistant crops and development of strategies for genetic transformation of Striga are critical in solving the menace in Africa.