Abstract
Maize lethal necrosis (MLN) is a viral disease with a devastating effect on maize production. Developing and deploying improved varieties with resistance to the disease is important to effectively control MLN; however, little is known about the causal genes and molecular mechanism(s) underlying MLN resistance. Screening thousands of maize inbred lines revealed KS23-5 and KS23-6 as two of the most promising donors of MLN resistance alleles. KS23-5 and KS23-6 lines were earlier developed at the University of Hawaii, United States, on the basis of a source population constituted using germplasm from Kasetsart University, Thailand. Both linkage mapping and association mapping approaches were used to discover and validate genomic regions associated with MLN resistance. Selective genotyping of resistant and susceptible individuals within large F2 populations coupled with genome-wide association study identified a major-effect QTL (qMLN06_157) on chromosome 6 for MLN disease severity score and area under the disease progress curve values in all three F2 populations involving one of the KS23 lines as a parent. The major-effect QTL (qMLN06_157) is recessively inherited and explained 55%–70% of the phenotypic variation with an approximately 6 Mb confidence interval. Linkage mapping in three F3 populations and three F2 populations involving KS23-5 or KS23-6 as one of the parents confirmed the presence of this major-effect QTL on chromosome 6, demonstrating the efficacy of the KS23 allele at qMLN06.157 in varying populations. This QTL could not be identified in population that was not derived using KS23 lines. Validation of this QTL in six F2 populations with 20 SNPs closely linked with qMLN06.157 was further confirmed its consistent expression across populations and its recessive nature of inheritance. On the basis of the consistent and effective resistance afforded by the KS23 allele at qMLN06.157, the QTL can be used in both marker-assisted forward breeding and marker-assisted backcrossing schemes to improve MLN resistance of breeding populations and key lines for eastern Africa.
Highlights
Increasing trade and travel coupled with weak phytosanitary systems are accelerating the global spread of devastating crop pests and diseases (McDonald and Stukenbrock, 2016; De Groote et al, 2021)
The present study identified a novel QTL for Maize lethal necrosis (MLN) resistance in the genetic background of two sister lines KS23-5 and KS23-6, on the basis of a source population constituted at the University of Hawaii, United States, using germplasm from Kasetsart University, Thailand
These two lines showed excellent resistance to MLN when tested under artificial inoculation at the MLN Screening Facility at Naivasha, Kenya, and serve as trait donors for improving MLN resistance in maize breeding pipelines in Africa
Summary
Increasing trade and travel coupled with weak phytosanitary systems are accelerating the global spread of devastating crop pests and diseases (McDonald and Stukenbrock, 2016; De Groote et al, 2021). The maize lethal necrosis (MLN) is one such transboundary maize disease that emerged in eastern Africa in 2011 (Mahuku et al, 2015; Prasanna et al, 2020). MLN results from the synergistic interaction of Maize chlorotic mottle virus (MCMV) with any of the cereal viruses of the Potyviridae family including Sugarcane mosaic virus (SCMV), Wheat streak mosaic virus (WSMV), and Maize dwarf mosaic virus (MDMV) (Mahuku et al, 2015; Prasanna et al, 2020). MLN in eastern Africa was found to be due to synergistic interaction between MCMV and SCMV (Wangai et al, 2012). The disease was exacerbated because of practices such as monocropping, besides lack of resistant maize varieties, and complicated nature of MLN spread and development (Beyene et al, 2017). A review by Marenya et al (2018) showed that, improving agronomic practices through maize-legume rotation would be a useful approach for MLN control, application of such a method may not be feasible over large geographic areas in eastern Africa. Marenya et al (2018) highlighted the importance of using MLN tolerant maize varieties and estimated the benefits to the tune of US$245–756 million and US$195–675 million in Ethiopia and Kenya, respectively
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