Doctoral Degrees (Plant Sciences)
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Browsing Doctoral Degrees (Plant Sciences) by Subject "Active oxygen species"
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Item Open Access Biochemical events associated with rust resistance in sunflower(University of the Free State, 2003-11) Mohase, Lintle; Van der Westhuizen, A. J.; Pretorius, Z. A.English: The biochemical resistance response of sunflower (Helianthus annuus L.) to rust (Puccinia helianthi Schw., pathotype UVPhe 2) was investigated in a comparative study using susceptible (S37-388) and resistant (PhRR3) sunflower cultivars and related to rust development. In addition, the potential of a plant activator, benzothiadiazole (BTH), to induce rust resistance in susceptible sunflower was investigated. Rust infection induced higher enzyme activities of the apoplastic pathogenesis-related (PR) proteins, β-1,3-glucanase, chitinase and peroxidase in the resistant than susceptible plants. The accumulation of β-1,3-glucanase isoenzymes was confirmed by Western blots. According to the labelling data of the immunogold localisation studies, β-1,3-glucanases were concentrated in the mesophyll cell walls during the rust resistance response. As the enzyme activities of the PR proteins were induced, the development of infection structures in the resistant plants was retarded, i.e. the number of vesicles, infection hyphae and haustorium mother cells. An early transient accumulation of H2O2 in the infected resistant plants is proof of the involvement of active oxygen species (AOS) in the resistance response. Active oxygen species’ burst correlated with an early induction (after 3 h of infection) of the activities of AOS-generating enzymes, NADPH oxidase, superoxide dismutase (SOD) and the NADH-dependent peroxidase. The phenolic, salicylic acid (SA), has been implicated in the resistance response as a potential signal molecule. The increase in SA content occurred with a concomitant increase in the activity of phenylalanine ammonia-lyase (PAL), a key enzyme in the synthesis of phenolic compounds. Induction of the activity of lipoxygenase, a key enzyme in lipid biosynthesis, in infected resistant plants, suggests that lipid-derived signals may play a role in the rust resistance response. As rust infection progressed, the number of aborted substomatal vesicles increased in resistant than susceptible plants. The number of infection hyphae, haustorium mother cells and rust colonies, which all favoured disease expression, were lower in the resistant than susceptible plants. Furthermore, in the resistant cultivar, haustorium mother cells were more often associated with host cell necrosis. The hypersensitivity index exceeded one, 96 hours after infection, indicating that the resistant plants were exhibiting the hypersensitive reaction (HR). Since it was found that SA is a possible signal for rust resistance reactions, the effectiveness of BTH, a functional analogue of SA, to induce the resistance response was investigated. This was measured in terms of activities of defence-related enzymes, β-1,3-glucanase and peroxidase. Benzothiadiazole induced an increase in the activities of these enzymes and further reduced disease symptoms by about 30%. In this study some of the biochemical components of the rust resistance response in sunflower have been identified and related to the development of rust infection structures. Furthermore, the potential of BTH as a plant activator in the control of rust diseases in sunflower has been demonstrated. The information gathered in this study contributes to a better understanding of the mechanisms in sunflower involved in rust resistance, which eventually could help to manipulate sunflower for improved resistance.