Phenotypic, physico-chemical and genetic responses of maize associated with resistance to Fusarium verticillioides

Links, Stefan (2019-04)

Thesis (MScAgric)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: Maize (Zea mays L.) is integral to the Southern African diet and is a major component of animal feed products. Fusarium verticillioides is an ubiquitous fungal pathogen that contaminates maize grain and is the primary causal organism of Fusarium ear rot (FER) of maize. Grain contamination by F. verticillioides results in two key issues being kernel rot and mycotoxin, specifically fumonisins, contamination. Thus, grain yield and grain quality can be significantly impacted. Ingestion of fumonisin-contaminated grain has been associated with certain health implications in humans and animals such as oesophageal cancer and neural tube defects in humans as well as brain, heart and kidney defects in animals. Cultural and biological management strategies have been explored but despite some advantages for production, these remain inefficient for the management of FER and fumonisin accumulation in maize. Furthermore, there are no registered fungicides available for FER/fumonisin management. Plant resistance offers the most promising approach to managing FER/fumonisins that is both sustainable and environmentally sound. Resistance to F. verticillioides is, however, complex and requires a comprehensive understanding of the mechanisms that contribute to it. Several factors mediate resistance at different levels of infection yet their relative importance and how they correlate to resistance is not well characterised. This study aimed to determine the importance of structural, physico-chemical and genetic traits for resistance. The correlation of these traits with specific infection indicators including FER severity, F. verticillioides growth and fumonisin accumulation in maize grain was also established. Maize inbred lines and commercial cultivars were inoculated with F. verticillioides and different characteristics were analysed. Structural characteristics included silk length, husk coverage, pericarp thickness and kernel hardness. The physico-chemical properties analysed in this study comprised pH, moisture content, total nitrogen and carbon as well as phenolic acids. Finally, reverse transcription quantitative PCR was used to evaluate the genetic response of maize grain to F. verticillioides using pathogenesis-related genes (PR1 and PR5) and as well as peroxidase gene expression. There were significant linear correlations between F. verticillioides growth and fumonisin accumulation in both trials. Significant fungal infection only occurred in one of the two localities (Vaalharts) evaluated. Multifactor analysis revealed no significant relationships between the levels of resistance (structural, physico-chemical and genetic) and infection indicators. However, partial least squares analysis revealed strong associations between individual characteristics and infection indicators. Significant relationships between the genetic response of PR5 and peroxidase genes and infection indicators were determined. Furthermore, significant inverse associations between pH and carbon to nitrogen ratio and infection indicators were found. This study highlighted the importance of physico-chemical and genetic response, in relation to well-established infection indicators commonly used to select resistant genotypes. Furthermore, this study has provided evidence that pH, C/N ratio and the expression of defence-related genes can be, used by breeders, as additional indicators of resistance to FER/fumonisins.

AFRIKAANSE OPSOMMING: Mielies (Zea mays L.) maak ‘n integrale deel uit van die Suid-Afrikaanse dieet en is ‘n belangrike komponent van voedsel produkte vir diere. Die alomteenwoordige patogeen, Fusarium verticillioides, kontamineer mielie graan en is die primêre veroorsakende patogeen van Fusarium kopvrot (FKV) van mielies. Graan kontaminasie deur F. verticillioides veroorsaak twee belangrike kwessies naamlik kern vrot en mikotoksien (spesifiek fumonisiene) kontaminasie. Sodoende word beide die graan opbrengs en -kwaliteit beïnvloed. Die inname van fumonisien gekontamineerde graan was al geassosieer met sekere gesondheid implikasies onderskeidelik in mense, soos esofageale kanker en neurale buisdefekte, en brein-, hart- en nier defekte in diere. Verbouïngspraktyke en biologiese beheer strategieë was al ondersoek, maar ten spyte van sekere voordele vir produksie, bly hierdie metodes onvoldoende vir die beheer van FKV en die opbouing van fumonisiene in mielies. Daarbenewens, is daar geen geregistreerde swamdoders beskikbaar vir FKV/fumonisien beheer nie. Plant weerstand bied die mees belowende benadering om FKV/fumonisien te beheer, wat beide volhoubaar en omgewingsvriendelik is. Nietemin is die weerstand van F. verticillioides kompleks en benodig ‘n omvattende begrip van die meganismes wat bydra tot dit. Verskeie faktore bemiddel weerstand by verskillende vlakke van infeksie, maar hul relatiewe belangrikheid en hul verwantskap tot weerstand is nog nie goed beskryf nie. Hierdie studie se doel was om die belangrikheid van die strukturele, fisio-chemiese en genetiese kenmerke van weerstand te bepaal. Die verwantskap van hierdie kenmerke met spesifieke siekte aanwysers, insluitend die erns van FKV, F. verticillioides groei en fumonisien akkumilasie in mielie graan was ook bevestig. Mielie teellyne en kommersiële kultivars was geïnokuleer met F. verticillioides en gevolglik was verskillende faktore geanaliseer. Strukturele eienskappe soos lengte, skaal dekking, perikarp dikte en kern hardheid was ingesluit. Die fisio-chemiese eienskappe sluit die pH, voginhoud, totale stikstof en koolstof sowel as fenoliese sure in. Verder was omgekeerde transkriptoom kwantitatiewe PKR gebruik om die genetiese reaksie van mielie graan tot F. verticillioides te bepaal, met die gebruik van patogenese-verwante gene (PR1 en PR5) asook as peroksidase geen uitdrukking. Daar was beduidende lineêre verwantskappe tussen F. verticillioides groei en fumonisien ophoping in beide van die proewe. Beduidende siekte ontwikkeling het net plaasgevind in een (Vaalharts) van die twee lokaliteite wat geevalueer was. Multifaktor analise het gewys dat geen beduidende verwantskappe tussen die groepe van weerstand indikators (struktureel, fisio-chemies en geneties) en infeksie aanwysers was nie. Nietemin, het “partial least squares’’ analise ʼn sterk assosiasie getoon tussen individuele karaktereienskappe en infeksie aanwysers. Beduidende verwantskappe tussen die genetiese reaksie van PR5 en peroksidase gene en infeksie parameters was bepaal. Ten einde laaste was daar beduidende negatiewe assosiasies tussen pH en koolstof tot stikstof infeksie aanwysers gevind. Hierdie studie het lig gewerp op die belangrikheid van fisio-chemiese en genetiese reaksie, in verwantskap tot bestaande infeksie aanwysers wat algemeen gebruik word vir die seleksie van weerstandbiedende genotipes. Die studie verskaf ook bewyse dat pH, C/N verhouding en die geen uidrukking van vededigingsverwante gene kan deur plantetelers gebruik word, as addisionele aanwysers van weerstand tot FKV/fumonisiene.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/105637
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