Location:2011 Annual Report
1a. Objectives (from AD-416)
Develop control strategies that will minimize yield and fiber quality losses caused by nematodes and microbial pathogens that are emerging as significant impediments to sustained profitability by cotton producers. Of particular concern is the spread of the reniform nematode in the southern U.S. and south Texas, the emergence of a new disease called South Carolina seed rot in the southeast, the introduction of a particularly virulent isolate of Fusarium oxysporum f. sp. vasinfectum that was inadvertently imported from Australia on cottonseed and that could adversely affect 50% of U.S. cotton production, and the identification of race 4 of F.o.v. for the first time in the U.S. To address these emerging diseases we will: 1) Complete the introgression of reniform nematode (Rotylenchulus reniformis) resistance from Gossypium longicalyx and G. barbadense into Upland cotton (G. hirsutum) and develop markers for the trait suitable for marker-assisted selection; 2) Describe mechanisms of pathogenesis and identify virulence genes in the bacterial pathogen(s) causing the disease known as South Carolina seed and boll rot; and develop detection methods for the pathogen(s); and 3) Determine whether fusaric acid contributes to differences in virulence among races and biotypes of Fusarium oxysporum f. sp. vasinfectum to provide a basis for developing strategies for controlling Fusarium wilt of cotton.
1b. Approach (from AD-416)
1) Introgression of Reniform Nematode Resistance: Standard procedures for cotton flower emasculation and pollen transfer will be followed for backcross breeding. After each cross, progeny will be bioassayed for resistance to the reniform nematode, and the most highly resistant progeny will be retained for subsequent crossing, self seed generation, and DNA extraction for marker development. Plants will be grown under field conditions and evaluated for agronomic performance. Standard techniques and commercially available technology will be used for cotton DNA extraction. PCR amplification, electrophoresis, and fragment size detection will be utilized to identify molecular markers. 2) South Carolina Seed and Boll Rot: A mutagenesis system will be used to identify genes involved in production, regulation, and/or secretion of factors that cause boll rot. Based on these results, a set of predicted gene sequences associated with pathogenicity will be used to develop a PCR based method for detecting seed and boll rotting bacteria in field samples. Bolls from greenhouse grown plants will be used in initial testing to determine the efficacy of the developed amplification system. 3) Relation of Fusaric Acid to Virulence of Fusarium oxysporum f. sp. vasinfectum (F.o.v.): Biotypes of F.o.v. will be monitored for their ability to produce high levels of phytotoxins, and virulence of biotypes will be determined. The biosynthesis of those phytotoxins that correlate with virulence will be determined by feeding labeled substrates to the pathogens. Genes involved in the biosynthesis of these phytotoxins will be identified. Knock-out mutants will be generated to assess the role of specific phytotoxins in virulence and pathogenicity.
3. Progress Report
In FY 2011, in project work to develop new cotton germplasm that is resistant or immune to the reniform nematode, more than 8,000 F2 plants from F1 parents were selected for the Renlon (reniform nematode-resistant) gene and its molecular marker (known as BNL 3279_115). These F2 plants were screened for recombination of the reniform resistance marker and resistance to stunt in reniform nematode-infested soils. On the basis of the data obtained, 60 of these plants were chosen for additional evaluation for resistance to the reniform nematode. In South Carolina boll and seed rot research, stink bugs infected with a boll rotting pathogen were allowed to feed on greenhouse-reared bolls. These bolls did not show visible exterior boll damage and usually showed an interior boll-wall blister that is characteristic of stink bug feeding. However, some bolls showed neither of these symptoms of stink bug feeding, yet their fiber/seed were damaged by the pathogen. These later bolls, when examined microscopically, did show evidence of stink bug feeding; this feeding apparently introduced the pathogen that caused the seed/fiber damage. In our Fusarium oxysporum f. sp. vasinfectum work, we established that fusaric acid is an important virulence factor by cloning the associated biosynthetic genes, generating the gene knock-out mutants, and testing the resulting mutants in a pathogenicity assay. This is the first direct proof that fusaric acid is a virulence factor in fungal plant pathogens and provides a basis for developing new control strategies for Fusarium and other fungal wilt diseases.
1. New cottons resistant to both reniform and root knot nematodes. The reniform and root knot nematodes have historically been, and remain, major obstacles to efficient and profitable cotton production in many areas of the U.S. Cotton Belt. Cottons resistant to the root knot nematode have been available to producers for some time, but only recently has cotton germplasm resistant to the reniform nematode been developed and released to the cotton breeding and research communities. ARS scientists at College Station, TX, have now developed new cotton germplasm that contains genetic elements that convey resistance to both root knot and reniform nematodes. The reniform-resistant (Renbar) gene is from germplasm generated recently by the ARS research team at College Station, TX, and at Mississippi State University and ARS Stoneville, MS, and the root knot resistance gene is from the breeding line M315. The new dual resistant cotton germplasm has been increased sufficiently to permit release of seed to breeders and other interested parties. This work will greatly facilitate development of new commercial cottons that have inherent resistance to both of these nematode pests.
Pshenichonov, E., Khashimova, N., Akhunov, A., Golunbenko, Z., Stipanovic, R.D. 2011. Participation of chitin-binding peroxidase isoforms in the wilt pathogenesis of cotton. American Journal of Plant Science. 2:43-49.