Marker-assisted selection in Gossypium spp. for Meloidogyne incognita resistance and histopathological characterization of a near immune line

The root-knot nematode, Meloidogyne incognita, is one of the most important parasites that cause economic losses in the cotton crop. Plant genetic resistance is the most desirable strategy to control this pathogen. Sources of resistance in cotton have been known for several years but only a few resistant commercial varieties have been released. Cotton breeding lines were developed using marker-assisted selection in early generation plants to introgress root-knot resistance genes from two different sources: M-315 or CIR1348. Phenotyping was carried out in greenhouse conditions to validate the molecular markers associated with the resistance genes in the breeding lines and confirmed by genotyping. The markers targeting QTLs from M-315 resistance source were highly efficient in the selection of plants resistant to M. incognita, with all plants expressing a reproduction factor inferior to 0.08. CIR1348 resistance source markers were also very efficient in selecting resistance; however, some segregation events revealed the need for fine mapping of the resistance QTLs. To clarify the resistance mechanisms, present in the germplasm derived from the M-315 resistance source, the resistant line CNPA 17-26 B2RF (triple cross [BRS 368RF × M-315] × [BRS 430B2RF]) was chosen for histopathological characterization of plant-nematode interaction and compared with the susceptible FiberMax 966 (FM 966). The second-stage juveniles (J2) penetrated equally in both genotypes. In the histopathological study, a strong blue fluorescence was visualized in the tissues around the nematode (hypersensitivity reaction, HR), mainly at the beginning (from 2 to 6 DAI) in the cortex and central cylinder of the resistant plant, indicating accumulation of phenolic compounds in the roots. At 9 DAI, giant cells in the early stage of subdivision next to nematodes were observed in the central cylinder of the resistant plant, and phenolic compounds were also shown around the nematode. At 12–40 DAI these initial cells were completely degraded with the presence of phenolics involving the nematodes and initial giant cells. No fully developed giant cells or mature females were observed, only fourth-stage juveniles (J4s), and males were frequently visualized at 34 DAI. This resistance mechanism characterizes near-immunity, and so no enlarged females and no egg production were observed. In susceptible control, it was possible to visualize feeding sites well developed from 6 to 30 DAI. Females reached maturity at 26 DAI, and eggs were observed at 30 DAI. Our results suggested that the resistance (near-immunity) of the line CNPA 17-26 B2RF was related to early (2–12 DAI) defense responses that totally prevented nematode reproduction.