Validating cassava brown streak disease resistance in selected transgenic cassava clones

Abstract

Cassava production is being challenged by prevalence of severe cassava brown streak disease (CBSD). Although conventional breeding has been used to introduce genes of resistance into cassava, only tolerant varieties that degenerate overtime have been generated. Currently, no cultivars with high levels of resistance to CBSD are available to farmers. RNAi technology was used to transform farmer-preferred cultivar TME 204, two events of transgenic TME 204 were conventionally crossed with two farmer-preferred cultivars, TME 14 and NASE 14. Eight superior F1 transgenic clones identified after evaluation in the field, these clones were graft challenged with the virulent CBSV in the screen house. The aim of the study was to contribute towards development of cassava genotypes with durable resistance to CBSD. The experiment was set up in a Completely Randomized Design with six replicates at the National Crop Resources Research Institute (NaCRRI). The bug grafting technique was used to inoculate superior transgenic cassava lines and TME 204 and NAROCASS 1 control cultivars by inserting a bud from symptomatic TME cultivar carrying CBSV. Data was collected on foliar incidence and severity, root incidence and severity, and Reverse transcription polymerase chain reaction (RT-PCR) analysis for detection of Cassava brown streak virus (CBSV) in the leaves of superior transgenic cassava lines and TME 204 and NAROCASS 1 control cultivars. Results indicated that foliar incidence and severity varied significantly among genotypes (p<0.001). The control TME 204 cultivar developed foliar symptoms with CBSD incidences of 100% by 42 days post inoculation (Pi). In contrast, all the eight transgenic lines and NAROCASS 1 control showed no foliar symptoms. Average CBSD foliar severity ranged from 1 to 2.76. At 21 weeks Pi, the plants were harvested, roots slices to assess CBSD incidence and severity. There was 100% incidence of root necrosis in the cultivar TME 204, all the transgenic cassava lines and NAROCASS 1 control remained free of root necrosis. Average CBSD root severity was 1 to 3.54 on a scale of 1-5. Absence of detectable CBSV in all the transgenic plants and NAROCASS 1 was confirmed by RT-PCR. Data presented shows resistance of eight transgenic clones to CBSV under screen house conditions