Comparative Transcriptomic Profiles of Striga Resistance Mechanisms within Hosts and Non Hosts Plant Interactions

Abstract

Majority of the staple cereals cultivated in the sub-Saharan Africa are heavily infested by Striga spp., causing tremendous economic losses annually. A durable and broad spectrum approach to Striga control is via the deployment of resistant varieties. However, there is breakage of this resistance due to the introduction of novel variants of Striga necessitating the need to explore non-hosts that are closely related to the parasitic host range. More so, a dual approach to deciphering the virulence strategies employed by Striga during host colonization is essential. This study capitalizes on molecular signatures in; (i) post attachment resistance mechanisms in both hosts (rice and pearl millet) and non-host (Brachiaria mulato II) to underscore new Striga-resistance mechanisms (ii) comparative transcriptome profile, to unravel genetic factors and effectors during Striga infection of rice(IAC_165 and Nipponbarre), pearl millet(29AW and SOSAT) and Brachiaria mulato II genotypes. Mechanisms of Striga resistance of rice, pearl millet and brachiaria were determined via rhizotron assays in a completely randomized design with 3 biological replicates. A comparative transcriptome study of host and non-hosts Striga interactions using dual stance RNA sequencing was conducted. In the study, rice and pearl millet are the host plants while brachiaria is the non-host. In rice, IAC_165 is the susceptible genotype, while Nipponbarre is the resistant rice genotype. For pearl millet, 29AW is resistant, while SOSAT is susceptible. In order, to understand the enriched processes, gene ontology (GO) enrichment analysis was done on the differentially expressed genes upon Striga infection. Prediction of the pathways enriched upon Striga infection was performed using Kyoto Encyclopedia of Genes and Genomes (KEGG). Nipponbare and Brachiaria mulato II showed intact seed coats and a mechanical barrier through the sections. A comparison of the transcriptome profiles of the hosts and non-host pointed to the enhanced expression of defense, secondary metabolite and cell wall reinforcement genes. Notably, less gene expression was observed in the brachiaria as the non-host interaction. An in-depth analysis using Self Organizing Maps (SOMs) on the virulence factors during rice (IAC_165 and Nipponbarre), pearl millet (29AW and SOSAT) and non-host (Brachiaria) interaction exhibited a temporal clustering of genes during the early and late phase of infection as opposed to compatibility status. There was an observation of the induction of genes involved in enzymatic and non-enzymatic cell wall loosening. Weighted gene co-expression network analysis (WGCNA) homed in on cell wall modification processes resulting from interactions with a resistant (Nipponbare) compared to a susceptible (IAC 165) rice cultivar. This section of the study showed strong fortification at the Striga-rice cell walls involving Striga on the offense and rice as the host on the defense; as a result depicting the variance between a successful or failed parasite invasion. Striga activates enzymes that degrade the host cell wall to enable entry; expansins allow cell elongation and pectin methyl esterase inhibitors facilitate rigidity during infection. Susceptibility pointed to an inactivated immune response processes. Host resistance activates immune responses controlled by plant hormones to strengthen the host cell wall via polysaccharides and lignin accumulation. These findings have far-reaching implications to create durable and broad-spectrum resistant genotypes for Striga management program in sub-Saharan Africa.