Submitted to: Journal of Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/10/2008
Publication Date: 4/1/2009
Citation: Hirano, S.S., Kinscherf, T.G., Upper, C.D., Willis, D.K. 2009. Population Dynamics of Pseudomonas Syringae pv. Tomato Strains on Tomato Cultivars Rio Grande and Rio Grande-Pto Under Field Conditions. Journal of Phytopathology. 157(4):219-227. Interpretive Summary: We are studying the mechanism by which bacteria are able to cause plant disease. Towards this end, we are analyzing the Pseudomonas syringae pv. tomato (the causative agent of bacterial speck disease of tomato - an important field disease in the USA). We hope to gain an understanding of the molecular biology of the genes and gene products that are required for the disease process by the bacteria and resistance to disease by the tomato host. Ultimately, this understanding will provide information of use to breeders and plant molecular biologists that will enable the development of plant lines resistant to disease organisms. This manuscript describes the field analysis of the growth of disease-causing bacteria (P. syringae pv. tomato) on resistant and susceptible tomato cultivars. A surprising finding was that resistant tomato plants can inhibit the growth of the invading bacteria on the leaf surface. This result is not predicted by current models of disease resistance. It is likely that our discovery of the inhibition of bacterial growth on plant surfaces in the field by a plant disease resistance gene explains the longevity of this particular resistance (Pto) through years of planting. These results illustrate an additional trait of plant resistance that should be examined to increase the long-term effectiveness of disease resistant cultivars.
Technical Abstract: Previous studies revealed complex interactivity between bacterial avirulence factors and plant resistance genes in growth chamber experiments. We examined the effects of the Pto resistance locus on the field population dynamics of P. syringae pv. tomato (Pst) SM78-1 strains on the nearly isogenic tomato lines Rio Grande (RG, race 0 susceptible) and Rio Grande-Pto (RG-Pto, race 0 resistant). In notable contrast to the paradigm strain DC3000, SM78-1Smr grew well under field conditions and caused ample bacterial speck disease on susceptible RG. Mean population sizes of SM78-1Smr were roughly 4 to 5 orders of magnitude larger on RG than RG-Pto plants. While most leaflets from RG-Pto field plots had limited numbers of SM78-1Smr, some leaflets bore 105 to 106 CFU/leaflet. Bacteria isolated from these leaflets caused typical speck lesions on RG-Pto plants in growth chamber assays and had phenotypes appropriate to Race 1. Additional race shift mutants were isolated from RG-Pto plants inoculated with SM78-1 in growth chamber experiments. Mutants isolated from both environments were shown by hybridization to be deleted for avrPto but not for avrPtoB. One such mutant, M00T56, caused typical bacterial speck lesions in growth chamber pathogenicity assays on both RG-Pto and RG plants, but had smaller population sizes on RG-Pto plants in the field than its progenitor had on susceptible RG. These results reinforce the idea that racial competence in the growth chamber is not equivalent to field fitness, while also hinting at why the Pto locus has been so durable in nature.