Diverse chromosomal locations of quantitative trait loci for tolerance to maize chlorotic mottle in five maize populations

Authors: Jones, Mark; Penning, Bryan; JAMANN, T - University Of Illinois; GLAUBITZ, J - Cornell University; ROMAY, C - Cornell University; Buckler, Edward - Ed; Redinbaugh, Margaret

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/28/2017
Publication Date: 12/29/2017
Citation: Jones, M.W., Penning, B., Jamann, T.M., Glaubitz, J.C., Romay, C., Buckler IV, E.S., Redinbaugh, M.G. 2017. Diverse chromosomal locations of quantitative trait loci for tolerance to maize chlorotic mottle in five maize populations. Phytopathology. 108(6):748-758. https://doi.org/10.1094/PHYTO-09-17-0321-R.
DOI: https://doi.org/10.1094/PHYTO-09-17-0321-R

Interpretive Summary: In sub-Saharan Africa, maize or corn is a staple crop that is used as food and a cash crop by small farmers, and pest and disease outbreaks are key constraints to maize productivity and food security. In September 2011, a serious disease outbreak, later diagnosed as maize lethal necrosis (MLN), was reported on maize in Kenya. The disease has since been identified in Ethiopia, South Sudan, Kenya, Uganda, Tanzania, Rwanda, Burundi and the Democratic Republic of the Congo, where farmers can lose up to 90% of their crop to the disease. MLN is caused by infection of maize with two different viruses named maize chlorotic mottle virus (MCMV) and sugarcane mosaic virus (SCMV). Ultimately, control of this important disease will likely require multiple approaches, including controlling populations of the insect vectors that transmit the viruses, clean seed, clean soil and host resistance. Of these, host resistance is the most economically viable and environmentally sustainable approaches to MLN control. However, few reliable sources of MLN resistance were known and well over 90% of the maize hybrids and lines tested were highly susceptible to the disease. Because we know a lot about resistance to SCMV in maize and little about MCMV resistance, we focused our research on identifying and characterizing MCMV resistance in maize. We identified five maize lines that developed fewer symptoms than susceptible lines after inoculation with MCMV, three of these lines were also resistant to SCMV and four lines developed fewer symptoms after inoculation with both viruses (MLN). We tested the maize lines to see if the lack of observed symptoms was associated with a lack of MCMV in the plants. Although a reduction in virus presence in maize lines developing fewer symptoms has been found for other maize-infecting viruses, in this case the 'resistant' maize lines had high levels of MCMV in leaves. Therefore, we call these maize lines 'tolerant' of MCMV rather than 'resistant' to the virus. To identify genes or quantitative trait loci (QTL) for MCMV resistance, we developed populations for each of the five lines by crossing them with a highly susceptible maize line. We found one to four QTL in each population that were associated with virus tolerance. The QTL we identified were different in each populations, occuring on different chromosomes and having different genetic qualities (i.e., some looked like recessive genes and others looked like additive genes). These results are in contrast to previous studies of virus resistance in maize, in which QTL for virus resistance occurs in the same genetic regions regardless of the maize population, and suggest that different maize line-specific mechanisms are controlling virus tolerance. Our results will benefit maize breeders and hybrid seed companies by providing plant materials and information required to use marker-assisted selection for the development of MCMV and MLN tolerant maize hybrids for disease control. In the future, it will be important to determine whether combining the QTL from the different maize lines into one new line will provide better symptom reduction and prevent the accumulation of MCMV in plants.

Technical Abstract: The recent rapid emergence of maize lethal necrosis (MLN), caused by coinfection of maize with maize chlorotic mottle virus (MCMV) and a second virus usually from the family Potyviridae, is causing extensive losses for farmers in East Africa, Southeast Asia and South America. Although the genetic basis of resistance to potyviruses is well understood in maize, little was known about resistance to MCMV. The responses of five maize inbred lines (KS23-5, KS23-6, N211, DR and Oh1VI) to inoculation with MCMV, sugarcane mosaic virus and MLN were characterized. All five lines developed fewer symptoms than susceptible controls after inoculation with MCMV; however, the virus was detected in systemic leaf tissue from each of the lines similarly to susceptible controls, indicating the lines were tolerant of MCMV rather than resistant to it. Except for KS23-5, the inbred lines also developed fewer symptoms after inoculation with MLN than susceptible controls. To identify genetic loci associated with MCMV tolerance, large F2 or recombinant inbred populations were evaluated for their phenotypic responses to MCMV and the most resistant and susceptible plants were genotyped by sequencing. One to four quantitative trait loci (QTL) were identified in each tolerant population using recombination frequency and positionally based mapping strategies. In contrast to previous studies of virus resistance in maize, the chromosomal positions and genetic character of the QTL were unique to each population. The results suggest different, genotype specific, mechanisms are associated with MCMV tolerance in maize. These results will allow for the development of markers for marker-assisted selection of MCMV and MLN tolerant maize hybrids for disease control.