Abstract
BackgroundCassava whitefly outbreaks were initially reported in East and Central Africa cassava (Manihot esculenta Crantz) growing regions in the 1990’s and have now spread to other geographical locations, becoming a global pest severely affecting farmers and smallholder income. Whiteflies impact plant yield via feeding and vectoring cassava mosaic and brown streak viruses, making roots unsuitable for food or trading. Deployment of virus resistant varieties has had little impact on whitefly populations and therefore development of whitefly resistant varieties is also necessary as part of integrated pest management strategies. Suitable sources of whitefly resistance exist in germplasm collections that require further characterization to facilitate and assist breeding programs.ResultsIn the present work, a hierarchical metabolomics approach has been employed to investigate the underlying biochemical mechanisms associated with whitefly resistance by comparing two naturally occurring accessions of cassava, one susceptible and one resistant to whitefly. Quantitative differences between genotypes detected at pre-infestation stages were consistently observed at each time point throughout the course of the whitefly infestation. This prevalent differential feature suggests that inherent genotypic differences override the response induced by the presence of whitefly and that they are directly linked with the phenotype observed. The most significant quantitative changes relating to whitefly susceptibility were linked to the phenylpropanoid super-pathway and its linked sub-pathways: monolignol, flavonoid and lignan biosynthesis. These findings suggest that the lignification process in the susceptible variety is less active, as the susceptible accession deposits less lignin and accumulates monolignol intermediates and derivatives thereof, differences that are maintained during the time-course of the infestation.ConclusionsResistance mechanism associated to the cassava whitefly-resistant accession ECU72 is an antixenosis strategy based on reinforcement of cell walls. Both resistant and susceptible accessions respond differently to whitefly attack at biochemical level, but the inherent metabolic differences are directly linked to the resistance phenotype rather than an induced response in the plant.
Highlights
Cassava whitefly outbreaks were initially reported in East and Central Africa cassava (Manihot esculenta Crantz) growing regions in the 1990’s and have spread to other geographical locations, becoming a global pest severely affecting farmers and smallholder income
The first two expanded leaves of five biological replicates were harvested representing time-point 0 (T0), and concurrently over the life cycle of the whitefly, leaves were collected at 12 h, 24 h, 7 days, 14 days and 22 days post-infestation for both COL2246 and ECU72 independently exposed to A. socialis colonies
Hierarchical metabolomics as a means of assessing complex natural variation The untargeted metabolomics approach used created a robust chemical fingerprinting of the cassava metabolome of Potential biochemical mechanisms conferring whitefly resistance Numerous examples exist of mutants/transgenic plants that are altered in the abundance of monolignol biosynthesis pathway intermediates [21,22,23,24,25], in a way reminiscent to the metabolic differences observed between COL2246 and ECU72
Summary
Cassava whitefly outbreaks were initially reported in East and Central Africa cassava (Manihot esculenta Crantz) growing regions in the 1990’s and have spread to other geographical locations, becoming a global pest severely affecting farmers and smallholder income. Cassava was first introduced into Africa during the 1500s, where it evolved into a staple food source, and got widely distributed across tropical regions during the 18th and nineteenth century [1]. Several key attributes have contributed to cassava as a food source in these regions; they include its ability to grow on marginal land with poor soil parameters and its high starch content providing dietary caloric value. Production in Africa and Central and South America is predominantly directed towards foodstuffs for human consumption, whilst the growing Asian markets dominate the export of cassava for industrial utilisation such as starch and biofuels
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