2011 Annual Report
1a.Objectives (from AD-416)
To determine the susceptibility of commercial geranium (Pelargonium) cultivars to bacterial wilt caused by Ralsonia solanacearum, and whether certain plants may act as asymptomatic hosts; to determine whether susceptibility is correlated with genetic relatedness of the host cultivar; to determine the susceptibility of other greenhouse-grown ornamental crops to Ralstonia solanacearum; to compare isolates of the bacterium from different outbreaks to correlate with geographic origin; to determine freeze survival of the bacterium in soil; to investigate chemical control measures; and to identify genes associated with the survival and pathogenicity at different temperatures, evaluate pathogen populations in relation to their ability to cause pathogenicity to agricultural crops at various temperatures and determine potential environmental fate and movement in the United States.
1b.Approach (from AD-416)
An empirical approach will be used to examine the genetic and physiological capability of exotic strains of bacterial wilt to access their ability to become established and spread within U.S. agriculture. Once introduced Ralstonia solanacearum is know historically to be spread via river systems. The project will further address the survival of distinct genetic populations in various water types and assess the ability to infect and survive under different temperature regimes.
Ralstonia solanacearum is distributed in tropical and subtropical areas of the world. R. solanacearum race 3 biovar 2 (R3B2) strains are found in the coldest areas of their range and are thought to be pathogenic at low temperatures. Growth and twitching motility characterization of selected strains of Ralstonia were completed in order to identify potential phenotypes associated with pathogenicity at low temperature. In addition to profiles of cellular proteins, proteomics experiments were conducted to identify secreted proteins differentially expressed at warm and cool temperatures.
Real time RNA expression studies (qRT-PCR) of R. solanacearum strains in contact with plant roots demonstrated that twitching motility is down-regulated at 18°C when compared with expression at 30°C in strains that are not pathogenic at low temperature, while it is not in strains that are pathogenic at the same temperature. This result suggests that twitching motility is involved directly or indirectly with efficient colonization of the plant host rhizosphere and/or subsequent invasion of roots by the pathogen. Bioinformatics analysis of the set of proteins differentially expressed in our proteomics study revealed a high percentage of hypothetical or non-annotated proteins which could represent novel functions involved in virulence at low temperatures. The analysis also identified proteins with functions associated with oxidative-stress responses and known virulence factors. Preliminary RNA expression experiments for 6 protein candidates seem to confirm a correlation between virulence and expression at low temperature. Currently, qRT-PCR tests are being repeated to confirm our preliminary results, and deletion mutants of two candidate genes are in progress to test their involvement in virulence at low temperature.
Because of its ability to survive and kill solanaceous crops such as potato under cool climate conditions, R3B2 is designated as a “Select Agent” under the Agricultural Bioterrorism Protection Act of 2002. In this work, bacterial wilt strains currently present in the United States that have the potential to survive and infect tomato and potato plants at low temperatures have been identified. It is important to determine the potential environmental fate and movement of these populations within the United States and to identify their virulence determinants at low temperature. If genes involved in establishment under specific temperate conditions are identified for R3B2 and other races of Ralstonia, new methods of control can be developed to safeguard U.S. tomato and potato production.
Research activities under this agreement were monitored by phone calls, e-mails, and reports.