Investigating maize inbred line responses following infection by the Mycotoxigenic fungus Fusarium verticilliodes

[摘要] ENGLISH ABSTRACT: Fusarium verticillioides is an important fungal pathogen of maize and is associated with the crop wherever it is produced. It is the most common fungal contaminant of South African maize grain and causes Fusarium ear rot (FER). The presence of F. verticillioides can reduce grain yield and quality by the visible moulding and/or discolouration of the kernels which reduces its grading at silos. The fungus may also contaminate grain without visible symptoms and produce harmful secondary metabolites known as fumonisins. Fumonisins have been associated with a number of noxious effects on humans and animals. Their widespread occurrence in maize and maize-based products has led to numerous countries imposing maximum allowable limits of fumonisins in food and feed. Cultural practises, mainly focussed on optimal plant production as a disease management strategy, have been shown to reduce fumonisin contamination. However, host-plant resistance is considered the most feasible, economical and environmentally sound approach to manage F. verticillioides and its fumonisins.In this study the response of elite breeding lines to infection and fumonisin deposition by F. verticillioides was evaluated in a multi-environment trial over 2 years. These lines were evaluated with inbred lines previously characterised for their response to FER and fumonisin accumulation. The stability of the inbred line response across locations was also determined in order to identify lines with broad or specific adaptability for disease resistance. Inbred lines CML 390, RO 424W, US 2540W and VO 617y-2 consistently exhibited low FER severity (≤5%), fungal target DNA (≤0.1 ng μL-1) and fumonisin levels (≤5 mg kg-1) at most test locations. Line RO 424W was the most stable in its response to disease and fumonisin accumulation across environments. The elite inbred lines were highly susceptible to F. verticillioides and fumonisin accumulation with only CB-222 and CB-248 having intermediate resistance. These inbred lines may serve as sources of resistance in breeding programmes but can also be used in genomic and proteomic studies to better understand the genetic basis of resistance to F. verticillioides and its fumonisins.The response of Kenyan inbred lines, previously characterised for resistance to Aspergillus flavus and aflatoxins, to F. verticillioides and fumonisin contamination was evaluated in South Africa and Kenya. Common resistance mechanisms to these pathogens have been reported. The AER/aflatoxin-resistant lines were compared to South African lines previously characterised as resistant to F. verticillioides and fumonisin accumulation. Kenyan inbred lines CML 495, CML 264 and CKL05015 were most resistant to FER, F. verticillioides colonisation and fumonisin accumulation across localities. The inbred line CML 495 was also the most stable in its resistance response to F. verticillioides infection and fumonisin deposition, making it an attractive source of resistance for inclusion into a South African breeding programme. This study, therefore, provided further evidence that AER/aflatoxin-resistant lines appear to be a significant source of resistance to F. verticillioides and fumonisins.Genetic variability for resistance to F. verticillioides and fumonisin accumulation was generated by gamma irradiation of seven elite maize lines. Following mass irradiation, the lines were field planted and self-pollinated for four consecutive seasons (M0-M4 generations). The response of the M1 to M4 populations to F. verticillioides was evaluated visually for FER severity annually, and ears expressing less than 10% FER were advanced to the next breeding cycle. The M4 selections were also evaluated for F. verticillioides colonisation and fumonisin content. A number of selections derived from each inbred line were more resistant to FER, F. verticillioides colonisation and fumonisin accumulation when compared to their non-irradiated controls. However, no improved selections were obtained from inbred I-35.The transcriptional changes in maize induced upon infection by F. verticillioides were evaluated by next-generation RNA sequencing and monitored up to 7 days after inoculation (dai). Although an initial induction of defence-related transcripts associated with pathogen recognition, signalling molecules, pathogenesis-related genes, cell wall restructuring and secondary hormone-based signalling genes was observed 24 hours post inoculation (hpi), these were down-regulated 48 hpi. Plant responses did not prevent F. verticillioides from colonising maize kernels, as the target DNA of the pathogen continued to increase. At 72 hpi, genes involved in pathogenesis, G-coupled receptor signalling and response to oxidative stress were induced and may have resulted in the reduction of fungal contamination 7 dai. The transcriptional changes in maize suggest a delayed plant response to F. verticillioides infection and imply a pathogen-associated molecular pattern response characterised by a basal immunity. Several genes including PRRs, signalling molecules (protein kinases, calcium-dependant molecules, GTP-signalling and redox-associated molecules), PR protein-coding genes and those involved in secondary hormone signalling (auxins) that influence maize response to F. verticillioides warrant further investigation by genomic and proteomic approaches.

[发布机构] Stellenbosch University