Biochemical and physiological responses of common bean infected by Sclerotinia sclerotiorum mediated by oxalic acid and phosphites and diagrammatic scale for assessment of white mold severity

• Ano de defesa: 2018
• Autor(a) principal: Nacarath, Inaia Rhavene Freire Fagundes
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: eng
• Instituição de defesa: Universidade Federal de Viçosa
• Palavras-chave em Português: Phaseolus vulgaris; Feijão; Sclerotinia sclerotiorum; Fotossíntese; Mofo branco; Fitopatologia
• Link de acesso: https://locus.ufv.br//handle/123456789/27459

White mold, caused by Sclerotinia sclerotiorum, is one of the most devastating diseases of common bean worldwide. The wide range of hosts (>400 species) and the fungus’ ability to survive for many years on the soil as sclerotia make disease management very difficult. Despite the importance of white mold, many aspects of the host-fungus interaction remain elusive. Here, a series of experiments was conducted to assess the biochemical and physiological changes mediated by oxalic acid (OA, the main pathogenic material secreted by S. sclerotiorum), phosphites in the common bean-S. sclerotiorum interaction as well as a diagrammatic scale for assessment of white mold severity (WMS). Firstly, we performed biochemical and physiological analyses to investigate the role of oxalic acid (OA) during S. sclerotiorum infection. To this end, common bean plants were sprayed with water or OA (referred to as –OA and +OA plants, respectively) and either challenged or not with a wild type (WT) and an OA-defective mutant (A4) of S. sclerotiorum. Irrespective of OA spray, WT isolate was more aggressive than A4 isolate, and spraying OA increased OA concentration in the leaflets and the aggressiveness of both isolates. Biochemical limitations were found to be behind S. sclerotiorum- induced photosynthetic impairments, notably for the +OA plants inoculated with WT isolate. Inoculated plants were not able to fully capture and exploit the collected energy as a result photochemical dysfunctions, which were potentiated by OA. With exception of catalase activity (CAT), there were increases in the activities of antioxidant enzymes, remarkably for plants that were inoculated with WT isolate. OA attenuated most of such increases, thereby increasing generation of superoxide and hydrogen peroxide and the concurrent damage to the host cell membranes, as evidenced by the increases in malondialdehyde (MDA) concentration. In the second experiment, we investigated physiological changes in common bean plants that were sprayed with water, zinc (Zn) or copper (Cu) phosphites and challenged or not with S. sclerotiorum. In vitro assays revealed that Zn and Cu phosphites inhibited fungal growth in a dose-dependent manner, but the latter was more fungitoxic. Lesion area and white mold severity were reduced by Zn and Cu phosphites, but the former was more effective. Sclerotinia sclerotiorum infection dramatically impaired plant photosynthesis, but such impairments were greatly abrogated in the Zn and Cu phosphite-treated plants. Concentrations of chlorophyll a + b and carotenoids were decreased in inoculated plants, but lower decreases were recorded in the presence of Zn and Cu phosphites. In the last study, we investigated whether Zn and Cu phosphites potentiate biochemical defenses of common bean against white mold. Histopathological analysis showed fewer fungal hyphae and less collapse of the mesophyll cells in the Zn and Cu phosphite-treated plants relative to the control. The S. sclerotiorum-triggered accumulation of reactive oxygen species, OA and MDA were constrained as a result of Zn and Cu phosphites sprays. In general, the activities of the antioxidant and defense enzymes at early stages of pathogen infection were higher in Zn- and Cu-sprayed plants than in their non-sprayed counterparts. Concentrations of total soluble phenols and lignin-thioglycolic acid derivatives in the inoculated plants were not affected by the Cu phosphite treatment, but they were higher and lower, respectively, in the Zn phosphite treatment at 60 hai than in the control. In conclusion, our results showed that OA enhance biochemical limitations to photosynthesis, photochemical dysfunctions and oxidative stress in common bean leaflets during the infection process of S. sclerotiorum. Additionally, we provide novel evidences of the potential of Zn and Cu phosphites in attenuate the physiological impairments and shed light on the biochemical defense mechanisms involved in the Zn and Cu phosphites-mediated suppression of white mold in common bean. In the fourth experiment, we develop SADs consisting of eight color images of diseased leaflets with severity values that ranged from 0.4 to 53.7%. Twenty raters [10 experienced (ER) and 10 inexperienced (IR)] validated the SADs by assessing the same set of 50 images twice, the first without SADs and the second using it as an aid. The SADs significantly improved both accuracy and precision for IR, whereas for ER only precision (r) was improved by SADs. The SWM estimates were also more reliable because inter-rater reliability (coefficient of determination, R 2 ) was significantly increased for both ER and IR by using SADs. Therefore, the SADs presented is thought to be a valuable tool to provide accurate, precise and reliable estimates of the SWM on common bean.