Location: Sugarbeet and Bean Research2009 Annual Report
1a. Objectives (from AD-416)
1) Enhance and release sugarbeet germplasm that represents substantially improved populations enriched for novel genetic combinations for the unique Eastern U.S. growing regions; 2) Develop and exploit sugarbeet and other species nucleotide sequence information for marker-assisted gene discovery, including development and release of simple sequence repeats (SSR) and single nucleotide polymorphism (SNP) markers; and 3) Develop simplified phenotyping assays for priority biotic and abiotic stress resistance and early season development suitable for molecular analyses.
1b. Approach (from AD-416)
Traditional sugarbeet population improvement approaches will be deployed for open pollinated, self-incompatible germplasm for release to industry. Production of improved populations will follow from mother root selection under field, greenhouse, or laboratory conditions of one or more germplasm sources, followed by random inter-mating, and harvest of seed from either individual plants, genetically related individuals, or as an entire population. Selfed families will be created from self-fertile materials generated to dissect the genetic control of high priority disease resistances. A program of phenotypic selection is followed by selecting mother roots from field nurseries, selfing these hybrids in the greenhouse, and applying molecular markers. Molecular markers will be developed from sugar beet nucleotide sequences, located to one of the nine beet chromosomes, and compared with segregation of disease and agronomic traits to identify genetic control. Mapped molecular markers will also be used to integrate sugar beet Bacterial Artificial Clones on the genetic linkage map for eventual isolation of specific genes that control agronomic and disease traits. Transcript profiling will be employed for gene discovery, however these tools are new for germplasm enhancement and their use has not been well explored. Examining transcript of profiles during sugar beet development, and during abiotic and biotic stress will allow deduction of important physiological and biochemical clues to the plant responses to stress and development that can be used towards more rigorous application in germplasm enhancement. The prevalence of different sugar beet pathogens in the Michigan agro-ecosystem will be ascertained, and used to develop high priority targets for transcript profiling. Differential disease reaction to Fusarium oxysporum, for instance, as an example of a newly discovered pathogen for Michigan, will form the basis for transcript profiling experiments that will better characterize the disease infection process and assist in identifying targets of opportunity for breeding intervention. Populations and their progeny showing good agronomic and disease performance will be folded into the general agronomic and disease nursery evaluations, and released to industry as enhanced germplasm.
3. Progress Report
Germplasm enhancement activities continue with individual field trials and greenhouse seed increases in Michigan encompassing agronomic evaluation, selection for resistance to Cercospora leaf spot, Fusarium yellows, and Rhizoctonia seedling and root diseases, selection for sugar beet cyst nematode resistance and salt tolerance during germination, and assessment of stand establishment potential. Over 1,100 distinct entries, not including those submitted for the official leaf spot nursery, are being evaluated. From these trials, individuals with superior characteristics will be selected after evaluation of their performance during the current growing season. From FY08, over 3,000 roots were selected, vernalized, and selfed in the greenhouse for inbred seed production, and an equal number selected for greenhouse and field seed production for open-pollinated population enhancement. Multiple wild and unadapted germplasms have been incorporated into these population improvement schemes. Marker discovery and genetic analyses activities were productive with demonstration of marker utility in extending the genetic map and predicting the occurrence and location of genes involved in sucrose yield and water content. Additional nucleotide sequence acquisition for marker development using single nucleotide polymorphism technologies was initiated using next generation sequencing. A coalition of ARS researchers, and university and industry scientists is being formed to obtain a full-length sugar beet genome sequence. Simplifying phenotypic selection through development of new methods and molecular dissection yielded very positive results. Synergistic interactions between root pathogens including Fusarium, Rhizoctonia, and Rhizopus were demonstrated. The developmental profile of germinating sugar beet seeds before radicle eruption was completed and molecular investigations of stress responses during germination suggest the first 24 hours after imbibition is critical for expression of seedling vigor. The second year of a comprehensive survey of sugarbeet disease-causing fungal pathogens present in the Great Lakes growing areas is continuing - the first survey in over 25 years. Results confirm presence of known pathogens and suggest involvement of others whose precise roles are being ascertained, and unlike the previous year, Aphanomyces seedling disease is a recurring issue for Michigan growers.
1. Release of SR98 germplasm with seedling resistance to Rhizoctonia. Stand establishment and persistence of sugarbeet is critical for grower profitability, and only 60% of seed planted consistently survives to harvest. Both biotic and abiotic interactions affect establishment, and seedling diseases are the major persistence problem. Of the myriad fungi, Rhizoctonia damping-off appears to be a key target for breeding genetic resistance, and no resistance has been yet described until recently in poor agronomic germplasm. SR98 was bred from this source for higher sucrose and smooth root, which reduces adhered soil by 50% and halves factory soil disposal costs. In addition to SR98 being the first agronomically acceptable germplasm with Rhizoctonia damping-off resistance, SR98 is resistant to the crown and root rot phase of the disease, unlike all smooth-root germplasms release to date, and is resistant to at least one form of Fusarium, which is a root rot of increasing importance in the Great Lakes growing region. SR98 is expected to be used worldwide as a source of resistance for the production of hybrids adapted to local growing areas with similar disease pressures.
2. Transition from juvenile to adult plant growth in sugar beet. The most vulnerable phase of sugar beet growth is the period from germination to roughly eight weeks post emergence, and growers are understandably anxious during this period since economic returns are dependent on sufficient numbers of harvested beets since only 60% of beets are harvested from seed planted, on average. One of the hallmarks of stand persistence is acquisition of adult disease resistance that marginally affects stands, versus acute seedling death prior to this. Molecular investigations were carried out to follow root development during this critical growth period, with the result that as many as 25% of the genes expressed earlier than five weeks of age were not expressed later, and vice versa. Thus, there appears to be a transition from juvenile to adult plant growth that coincides with acquisition of stand persistence. The discovery of this transition will allow sugar beet physiologists and pathologists to account for this phenomenon in their experimental designs, as well as afford breeders a target for selecting germplasm that can transition to adult plant growth more quickly.
3. Anatomical barrier to Rhizoctonia fungal growth in expression of seedling damping-off resistance. Seedling damping-off is characterized by rapid infection and death of infected seedlings. The resistance reaction of EL51 sugarbeet was investigated using microscopy and differential staining of fungus and plant tissues. Results showed that in a susceptible host using a virulent pathogen, the pathogen was able to breach the endodermis barrier. In the resistant host sugarbeet, the pathogen’s growth was arrested at or near the endodermis. The observation of the fungus’ exclusion from water conducting tissues suggests one physiological mechanism for resistance. This observation will allow breeders an easy screening target for developing resistant hybrids as well as provide plant pathologists and molecular biologists a target for understanding the molecular basis of this unique host-pathogen interaction.
Hanson, L.E., Duckert, T.M., McGrath, J.M. 2009. Beta PIs from the USDA-ARS NPGS evaluated for resistance to Cercospora beticola, 2008. Plant Disease Management Reports. The American Phytopathological Society. 3:V017. DOI: 10.1094/PDMR01. Available: http://www.plantmanagementnetwork.org/pub/trial/PDMR/reports/2009/V017.pdf.