Survival of Ice Nucleation-Active and Genetically Engineered Non-Ice-Nucleating Pseudomonas syringae Strains after Freezing

History of Discovery and Environmental Role of Ice Nucleating Bacteria

The phenomenon of biological ice nucleation that is exhibited by a variety of bacteria is a fascinating phenotype, which has been shown to incite frost damage to frost-sensitive plants and has been proposed to contribute to atmospheric processes that affect the water cycle and earth's radiation balance. This review explores the several possible drivers for the evolutionary origin of the ice nucleation phenotype. These bacteria and the gene required for this phenotype have also been exploited in processes as diverse as reporter gene assays to assess environmentally responsive gene expression in various plant pathogenic and environmental bacteria and in the detection of foodborne human pathogens when coupled with host-specific bacteriophage, whereas ice nucleating bacteria themselves have been exploited in the production of artificial snow for recreation and oil exploration and in the process of freezing of various food products. This review also examines the historical development of our understanding of ice nucleating bacteria, details of the genetic determinants of ice nucleation, and features of the aggregates of membrane-bound ice nucleation protein necessary for catalyzing ice. Lastly, this review also explores the role of these bacteria in limiting the supercooling ability of plants and the strategies and limitations of avoiding plant frost damage by managing these bacterial populations by bactericides, antagonistic bacteria, or cultural control strategies.

Characterization of Pseudomonas syringae pv. syringae, Causal Agent of Citrus Blast of Mandarin in Montenegro

Citrus blast caused by bacterium Pseudomonas syringae is a very important disease of citrus occuring in many areas of the world, but with few data about genetic structure of the pathogen involved. Considering the above fact, this study reports genetic characterization of 43 P. syringae isolates obtained from plant tissue displaying citrus blast symptoms on mandarin (Citrus reticulata) in Montenegro, using multilocus sequence analysis of gyrB, rpoD, and gap1 gene sequences. Gene sequences from a collection of 54 reference pathotype strains of P. syringae from the Plant Associated and Environmental Microbes Database (PAMDB) was used to establish a genetic relationship with our isolates obtained from mandarin. Phylogenetic analyses of gyrB, rpoD, and gap1 gene sequences showed that P. syringae pv. syringae causes citrus blast in mandarin in Montenegro, and belongs to genomospecies 1. Genetic homogeneity of isolates suggested that the Montenegrian population might be clonal which indicates a possible common source of infection. These findings may assist in further epidemiological studies of this pathogen and for determining mandarin breeding strategies for P. syringae control.

Inferring the evolutionary history of the plant pathogen Pseudomonas syringae from its biogeography in headwaters of rivers in North America, Europe, and New Zealand

Nonhost environmental reservoirs of pathogens play key roles in their evolutionary ecology and in particular in the evolution of pathogenicity. In light of recent reports of the plant pathogen Pseudomonas syringae in pristine waters outside agricultural regions and its dissemination via the water cycle, we have examined the genetic and phenotypic diversity, population structure, and biogeography of P. syringae from headwaters of rivers on three continents and their phylogenetic relationship to strains from crops. A collection of 236 strains from 11 sites in the United States, in France, and in New Zealand was characterized for genetic diversity based on housekeeping gene sequences and for phenotypic diversity based on measures of pathogenicity and ice nucleation activity. Phylogenetic analyses revealed several new genetic clades from water. The genetic structure of P. syringae populations was not influenced by geographic location or water chemistry, whereas the phenotypic structure was affected by these parameters. Comparison with strains from crops revealed that the metapopulation of P. syringae is structured into three genetic ecotypes: a crop-specific type, a water-specific type, and an abundant ecotype found in both habitats. Aggressiveness of strains was significantly and positively correlated with ice nucleation activity. Furthermore, the ubiquitous genotypes were the most aggressive, on average. The abundance and diversity in water relative to crops suggest that adaptation to the freshwater habitat has played a nonnegligible role in the evolutionary history of P. syringae. We discuss how adaptation to the water cycle is linked to the epidemiological success of this plant pathogen.

Many pathogens have life cycles that involve survival and multiplication in nonhost environmental habitats. For human pathogens, numerous studies have revealed how adaptation to environmental habitats is linked to the evolution of their pathogenicity and emergence of pathogens. For plant pathogens, the link between adaptation to nonhost habitats and pathogenicity has not been explored. Here we have examined the genetic and phenotypic diversity of the plant pathogen Pseudomonas syringae in headwaters of rivers on three continents and compared it to that of strains from crops. This model pathogen was chosen because it is widely abundant in habitats associated with the water cycle and in particular in pristine waters outside agricultural regions. This work reveals that there is considerable exchange of populations between freshwater and agricultural habitats and that those in the former contribute considerably to the diversification of P. syringae.

Physiological and ecological significance of biological ice nucleators

When a pure water sample is cooled it can remain in the liquid state at temperatures well below its melting point (0 degrees C). The initiation of the transition from the liquid state to ice is called nucleation. Substances that facilitate this transition so that it takes place at a relatively high sub-zero temperature are called ice nucleators. Many living organisms produce ice nucleators. In some cases, plausible reasons for their production have been suggested. In bacteria, they could induce frost damage to their hosts, giving the bacteria access to nutrients. In freeze-tolerant animals, it has been suggested that ice nucleators help to control the ice formation so that it is tolerable to the animal. Such ice nucleators can be called adaptive ice nucleators. There are, however, also examples of ice nucleators in living organisms where the adaptive value is difficult to understand. These ice nucleators might be structures with functions other than facilitating ice formation. These structures might be called incidental ice nucleators.

Genetically engineered microorganisms to rescue plants from frost injury

Ice nucleation active bacteria belonging to genera Pseudomonas, Xanthomonas and Erwinia contribute to frost damage to plants by initiating the formation of ice in plants that would otherwise supercool and avoid the damaging ice formation. The biological control of frost injury can be achieved by the application of non-ice nucleation active bacteria to the plant surfaces before they become colonized by Ice+ species. ice genes have been cloned from Pseudomonas and isogenic Ice- derivatives constructed via genetic manipulations. These genetically engineered microorganisms (GEMs) have been released into the environment to control the frost damage. The incidence of frost injury to the plants has, thereby, been reduced by 50-85% during natural frosts. These GEMs do not survive in soil and show no aerial dispersal in the environment.

Efficacy of burning, tillage, and biocides in controlling bacteria released at field sites and effects on indigenous bacteria and fungi

Decontamination treatments of burning and biocide application, alone and in combination with tillage, were evaluated for their ability to reduce populations of bacteria applied to the leaves of plants in field plots. In addition, the effects of these control methods on indigenous leaf and soil bacteria and fungi were assessed. Field plots of bush beans (Phaseolus vulgaris), sprayed with the bacterium Pseudomonas syringae, Pseudomonas fluorescens, or Erwinia herbicola, received the following treatments: (i) control, (ii) tillage, (iii) burning, (iv) burning plus tillage (burn-tillage), (v) Kocide (cupric hydroxide), (vi) Kocide plus tillage, (vii) Agri-Strep (streptomycin sulfate), and (viii) Agri-Strep plus tillage. Leaves and soil from the plots were sampled at 1 day before and at 1, 3, 7, 10, 14, 21, and 30 days after application of the decontamination treatments. The burn and burn-tillage treatments produced the most significant reductions in bacterial populations. The Agri-Strep treatment was more effective than the Kocide treatment in eliminating applied bacteria, but neither biocide produced consistent or persistent control. In contrast, the tillage treatment, alone or in combination with the Agri-Strep or Kocide treatments, had a short-term stimulatory effect and increased populations of applied bacteria and also levels of indigenous fungi and bacteria. Agri-Strep and Kocide treatments caused significant reductions in indigenous bacterial populations up to 14 days after application and in indigenous fungal populations on day 7 after application. Our results suggest that conventional plant disease control methods may not provide satisfactory control of genetically engineered microorganisms and indicate a need for further development of effective and selective methods to control release microorganisms at field sites.

Evaluation of Four Aerobiological Sampling Methods for the Retrieval of Aerosolized Pseudomonas syringae

The Andersen six-stage impactor, the SAS (Surface Air System) impactor, the AGI-30 impinger, and gravity plates were evaluated for the retrieval of aerosol-released Pseudomonas syringae . The upper limits of the impactor samplers were exceeded at a spray concentration of 10 7 CFU/ml, indicating that these samplers are not appropriate for monitoring high airborne concentrations. Decreased cell concentrations were retrieved with increased sampling time for the Andersen and AGI samplers, indicating that a minimum sampling time is preferable for monitoring aerosolized vegetative cells.