Root-Knot Nematode Resistance Genes
Genetic and Physical Locations of Root-Knot Nematode Resistance Genes in Cotton
Crop & Soil Sciences
Our mapping results validated the effects of the qMi-C14 resistance locus, delimiting the QTL to a smaller region, and identified tightly linked SSR markers to improve the efficiency of marker-assisted selection. The candidate genes identified warrant functional studies that will help in identifying and characterizing the actual qMi-C14 defense gene(s) against root-knot nematodes.
Plant-parasitic nematodes are important pests in agriculture and are found across a broad range of climatic conditions. The genus Meloidogyne includes more than 70 species; four species – M. incognita, M. arenaria, M. javanica and M. hapla – account for approximately 95% of the total crop area infested by this genus. The Southern root-knot nematode (Meloidogyne incognita, RKN) is the most important parasitic nematode of Upland cotton inflicting economic losses through direct damage to the plant root system and indirectly through increasing severity of other root diseases such as Fusarium Wilt caused by Fusarium oxysporum f. sp. vasinfectum. Yield loss due to RKN has dramatically increased in the U.S. from 1% in 1987 to 5.5% in 2006, resulting in losses of more than 300 million pounds (4.4%) of cotton valued over $235 million. Recommended management practices to control RKN include crop rotation and nematicide application. The broad host-range of RKN leaves cotton growers with few profitable options to adopt crop rotation as a means of nematode management. Nematicides applied at planting fail to provide season long protection; moreover, the future availability of most widely used cotton nematicide, aldicarb, is uncertain.
Host plant resistance can be an efficient RKN management tool in cotton production. Resistant varieties can offer good to excellent pest control efficacy and are also environmentally sound alternatives. Cotton germplasm with RKN resistance has been extant since the development of Auburn 623 germplasm line in 1974 and the resistance in this line has been transferred to lines with acceptable agronomic qualities. Yet, until recently none of the currently grown commercial cotton cultivars expresses a high level of resistant to root-knot nematodes. The slow breeding progress for developing RKN resistant cultivars is partly due to the current screening process that is tedious, time consuming and destructive for identifying resistance genotypes. Molecular marker offers an alternative screening process for identifying resistance genotypes in breeding programs. The development of diagnostic markers for genes conditioning RKN resistance will accelerate the transfer of these genes to genotypes or germplasm for new cultivar development.
QTL mapping identified major genes conferring resistance to RKN; the qMi-C11 locus predominately affects root gall suppression whereas the qMi-C14 locus largely reduces egg production but has little effect on galling. While the main effects of each QTL appeared to serve as the major genetic basis in conferring resistance for both galling and egg production phenotypes, our analysis also indicated that additive x additive epistatic interaction was important in suppressing nematode egg production, resulting in a near-immunity to infection when both QTLs are present. Our recent mapping results validated the effects of the qMi-C14 resistance locus, delimiting the QTL to a smaller region, and identified tightly linked SSR markers to improve the efficiency of marker-assisted selection. By using the reference genome sequence of G. raimondii, we identified 20 candidate genes encoding disease resistance protein homologs in the newly defined 2.3 Mb region flanked by two SSR markers. Resequencing of an RKN resistant and susceptible G. hirsutum germplasm revealed non-synonymous mutations in only four of the coding regions of candidate genes, and these four genes are consequently of high interest. The candidate genes identified warrant functional studies that will help in identifying and characterizing the actual qMi-C14 defense gene(s) against root-knot nematodes.