2008 Annual Report
Objective 2: Characterize genetic differences in disease resistance of solanaceous hosts in response to Phytophthora and Streptomyces scabies. Sub-objectives: a. Characterize differences in gene expression in both hosts and pathogens to identify pathogenicity determinants, applying RT-PCR, microarray analysis, and functional genomics. b. Map genes for resistance to common scab onto potato chromosomes, and develop molecular markers for common scab resistance.
Objective 3: Identify plant defense responses in tomato induced by the biocontrol agent F. oxysporum strain CS-20 and determine their contribution to observed biocontrol of Fusarium wilt. Other biocontrol-host-pathogen systems and biocontrol mechanisms will be included as time and resources permit, and as indicated by results. Objective 4: Identify and characterize genetic traits and gene expression related to biocontrol ability in the biocontrol fungus F. oxysporum strain CS-20. Objective 5: Characterize interactions between the biocontrol fungus F. oxysporum strain CS-20 and other components of the tomato rhizosphere community.
Genetics of common scab resistance in commercial potato cultivars. In the initial stage of identifying molecular markers for common scab resistance, reciprocal genetic crosses were set up between resistant and susceptible potato varieties by a scientist. Resistant X resistant and resistant X susceptible combinations were chosen, resulting in eight families to test for segregation of resistance. During this year, the first sets of true potato seed progeny were obtained and grown to produce tubers for common scab susceptibility testing. The tubers from two families are currently being tested for common scab resistance.
Discovery of rare mating type. In 2007, we reported the isolation and characterization of a rare mating type of Phytophthora infestans from tomato hosts, growing in heavily blighted fields in two locations in Taiwan. Characterization of pathogens with biologically significant markers with information on mating type (thus likely frequency of sexual reproduction) was the fundamental aim of this study. In several countries, pathogen populations were found to be more genetically diverse on tomato and potato hosts when two mating types are present. The progeny derived from the resultant mating should be more genetically diverse than the parental strains. This may exert an influence on population variability by allowing recombination of agriculturally undesirable characteristics, for example, fungicide resistance increased ability to infect tubers. Not only were both mating types isolated on tomato hosts, but the recovered strains were unusually damaging to potato hosts in these locations. The increase in the frequency of the mating types detected in Taiwan is consistent with the increase in frequency that has taken place in some European countries. This information would possibly allow us to determine the mechanisms and tempo of genetic change within late blight populations. This information will benefit other scientists, plant breeders and various agricultural agents working with crop protection.
Sequencing of the Phytophthora infestans genome. An international effort was made to provide a completed genome sequence for public use. Work was performed at the Broad Institute (MIT) with meetings to complete gene annotation. At three times the size of other Phytophthora genomes, this represented a major accomplishment. Almost two-thirds of the genome was found to be repetitive sequences that may have cryptic genes within the region. This sequence will be a fundamental tool in deciphering the mechanisms by which P. infestans generates variable populations, and how it infects plants.
2. Discovery of unique mechanism for pathogenicity factor variability in Phytophthora.
The potato late blight agent Phytophthora infestans, belongs to the most destructive pathogen genus recorded. With a continual proliferation of new isolates, P. infestans has proven intractable to control by standard plant breeding methods. Part of this difficulty lies in the lack of understanding of the genetic mechanisms driving P. infestans variability. We provided the first report of alternate intron processing for Phytophthora, wherein a single gene can produce multiple variants of a pathogenicity factor. This was also demonstrated for P. sojae (Phytophthora root rot of soybean) and P. ramorum (sudden oak death). This information can be used to track variations in new isolates and lead to design of more effective pathogen inhibitor proteins. This addresses the National Program 303 Component 1. Disease Diagnosis: Detection, Identification and Characterization of Plant Pathogens. Problem Statement 1B: Detection, Identification, Characterization, and Classification of Pathogens.
3. Identification of P. infestans A2 mating type in Taiwan. Variability in the plant pathogen Phytophthora can be achieved asexually or sexually. Sexual reproduction allows for formation of oospores, which can serve as long term survival structures in soil. Outbreaks of tomato late blight in Taiwan were investigated through collection of numerous isolates island-wide. While most isolates were A1, a limited number of A2 isolates were recorded, indicating the ability of sexual reproduction of P. infestans in Taiwan. This information is useful in limiting plantings of tomato in oospore infected soils. This addresses the National Program 303 Component 1. Disease Diagnosis: Detection, Identification and Characterization of Plant Pathogens. Problem Statement 1B: Detection, Identification, Characterization, and Classification of Pathogens.
Costanzo, S., Ospina-Giraldo, M., Deahl, K.L., Baker, C.J., Jones, R.W. 2007. Alternate Intron Processing of Family 5 Endoglucanase Transcripts from Genus Phytophthora. Eukaryotic Cell. 52:115-123.
Wanner, L.A. 2007. Population studies of Streptomyces in dedicated common scab fields. Canadian Journal of Microbiology. 53(9):1062-1075.
Qu, X., Wanner, L.A., Christ, B.J. 2008. Using the TxtAB Operon to Quantify Pathogenic Streptomyces in Potato Tubers and Soil. Phytopathology. 98:405-412.
Deahl, K.L., Jones, R.W., Black, L.L., Wang, T.C., Cooke, L.R. 2008. First report of the occurrence of the A2 mating type of Phytophthora infestans on tomato crops in Taiwan, Republic of China. Plant Disease. 92:978.