| dc.description.abstract | Cassava brown streak disease (CBSD) is one of the seven major constraints threatening food security worldwide. The challenge in managing the disease is lack of an effective and sustainable method of mitigating CBSD with use of resistant cultivars being the most effective method. Despite decades of in depth studies on molecular characterization and diversity of cassava brown streak viruses (CBSV), there is limited information on physiological and biochemical changes in the CBSV- cassava pathosystem. The objective of this study was therefore to screen farmer preferred cassava cultivars for resistance to CBSV and further investigate the role of antioxidant enzymes in resistance of cassava to the virus. Thirteen cultivars of cassava were graft inoculated with CBSV infected cassava stems and screened for resistance to CBSD in the glass house. The disease symptoms were scored on a scale of 1 - 5 at 8 and 12 weeks post inoculation (wpi). The physiological parameters and antioxidant enzyme responses of the cassava cultivars following infection with CBSV were quantified at 8 and 12 wpi. From the study, cultivars Karibuni, Tajirika and Karembo were tolerant to CBSD, whereas cultivars TME 204, TME 7, TME 14, Kibandameno, KME-1, Ex-Mariakani, Ex-ndolo, MM96/7151, Ebwanatereka 1 and Ebwanatereka 2 were susceptible to CBSV. The amount of hydrogen peroxide (H2O2) was significantly higher (P ≤ 0.05) in the inoculated susceptible (128.6-407.1nmol/g FW) than in the inoculated tolerant plants (231.7-275 nmol/g FW) at 8 wpi when compared to their respective controls. At 12 wpi, there was no significant difference (P ≥ 0.05) in the amount of hydrogen peroxide in the inoculated plants of all the cultivars with respect to their controls. There was no significant difference (P ≥ 0.05) in the amount of malondialdehyde in the inoculated (1.90-7.24 nmol/g FW) and non-inoculated plants (1.71-5.27 nmol/g FW) of all cultivars at 8 wpi. However, at 12 wpi, the amount of malondialdehyde was significantly higher (P ≤ 0.05) in the inoculated susceptible plants (2.23-25.63 nmol/g FW) than in inoculated tolerant plants (3.36-5.71 nmol/g FW) when compared to their controls. The amount of chlorophyll was significantly lower (P ≤ 0.05) in the inoculated (21.87-8.55 nmol/g FW) compared to the non-inoculated plants (24.06-12.28 nmol/g FW) of susceptible cultivars at 8 wpi and12 wpi. There was no significant difference (P ≥ 0.05) in the amount of chlorophyll in the inoculated (10.29-10.84 nmol/g FW) and the non-inoculated (11.11-12.26 nmol/g FW) plants of tolerant cultivars. There was no significant difference (P ≥ 0.05) in the amount of catalase in the inoculated (0.88-2.86 μmol/g FW) and non-inoculated (0.2-1.28 μmol/g FW) plants of all cultivars at 8 wpi. However, at 12
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wpi the amount of catalase was significantly higher (P ≤ 0.05) in the inoculated (1.61-3.44 μmol/g FW) compared to the non-inoculated (0.39-1.38 μmol/g FW) plants of all cultivars. The amount of peroxidase and was significantly (P ≤ 0.05) higher (2.42-2.67 μmol/g FW) and (2.17-2.42 μmol/g FW) in inoculated plants of tolerant cultivars at 8 and 12 wpi respectively. The amount of ascorbate peroxidase was significantly (P ≤ 0.05) higher (4.86-6.85 μmol/g FW) and (6.75-7.64 μmol/g FW) in inoculated tolerant plants at 8 and 12 wpi respectively compared to the non-inoculated tolerant plants. There was no significant difference (P ≤ 0.05) in the amount of peroxidase and ascorbate peroxidase (1.34-2.74 μmol/g FW) and (2.18-5.26 μmol/g FW) respectively at 8 wpi and (1.17-2.52 μmol/g FW) and (2.68-6.85 μmol/g FW) respectively at 12 wpi in inoculated compared to the non-inoculated plants of susceptible cultivars. Karibuni, Tajirika and Karembo were identified as tolerant cultivars and had an increase in the amount of peroxidase and ascorbate peroxidase enzymes upon inoculation with CBSV. Cultivars tolerant to CBSD showed lower lipid peroxidation compared to the susceptible ones. The accumulation of hydrogen peroxide was lower in the infected tolerant plants compared to the susceptible plants. The amount of chlorophyll in susceptible plants reduced dramatically upon CBSV infection, whereas there was little or no reduction in chlorophyll in their tolerant counterparts. Therefore, ascorbate peroxidase and peroxidase enzymes could play a vital role in tolerance of cassava plants to CBSV infection. The two enzymes therefore could be useful tools in breeding strategies by plant breeders to induce resistance to CBSD in cassava cultivars susceptible to the disease. The two enzymes are also useful in agriculture for selection of cassava cultivars resistant to CBSD | en_US |
