Pseudomonas syringae
S.R. Sparrow Smith

Pseudomonas syringae is a Gram-negative, aerobic, rod-shaped bacteria.  One distinguishing characteristic is its polar flagella.  It is a Gamma Proteobacteria of the order Pseudomonadales in the family Pseudomonadaceae.  It has been assigned over fifty pathovars, or subspecies, based on the type of plant they infect, many of which were once considered individual species until DNA hybridization techniques showed them all to be P. syringae.  Examples include P. syringae pv. atrofaciens which attacks wheat, P. syringae pv. japonica which attacks barley, and P. syringae pv. pisi which attacks peas.  The name P. syringae itself came from the first species on which it was isolated, the lilac tree, or Syringa vulgaris.

While it is normally considered a plant pathogen, each strain of P. syringae is specific to the plant species it infects; colonies frequently develop on non-host plants to which they are not pathonogenic.  This multiplicity of pathovars is indicative of the organism’s great diversity of metabolism, however for the most part they are chemoheterotrophs that rely on their specific host plant as a nutrient source, typically living on plant leaves.  However, P. syringae can also survive as saprophytes, living off of dead or organic matter.  Besides differences in metabolism, gene sequencing has identified other anomalies of the various P. syringae pathovars, including their plasmids.  While some strains have no plasmids, other strains have one or even two.  These plasmids are unique to each particular strain, and are implicated in a variety of phenotypic expressions—of particular interest is the property of ice nucleation.

Wild-type P. syringae produce ice-nucleating surface proteins (called Ina proteins for Ice Nucleation-Active), which are the primary agent for frost damage in plants.  Because the Ina proteins cause water to freeze, the epithelium of the plant leaves is damaged; however, this has adaptive value for P. syringae since it renders the nutrients in underlying tissues available to the bacteria.  For crop farmers, the solution to the P. syringae frost damage problem has been the capacious use of the “ice minus” mutant form of the bacteria, which do not produce Ina proteins.  The mechanism for their antagonism with the “ice plus” strain is simple competition for resources—where “ice minus” bacteria predominate, frost damage is significantly reduced.

Another agricultural use of P. syringae, also thought to rely on competitive antagonism, has been its employment as a biocontrol agent fungi that cause decay on harvested fruits.  For this purpose the saprophytic strain is utilized, and has been marketed under the name BioSave (other strains have also been marketed by other companies for the same purpose).  P. syringae can be used to control blue molds caused by Penicillium, gray mold, Mucor rot on apples and pears, green mold on citrus fruits, crown molds on bananas, dry rot on potatoes, and in prevention of the foodborne pathogen E. coli in apple wounds.  Clearly, its benefits to the agricultural world are profound; however, its greater potential may be yet untapped.

The reason I chose this organism is because of the peculiarity of its ice nucleation properties.  Because of its ability to form ice crystals, one of the more novel commercial uses of P. syringae has been in the production of artificial snow for ski resorts.  What is most curious about this bacteria is that recent evidence seems to indicate that P. syringae is also responsible for the formation of most natural snow as well, not to mention much of the rainfall we receive.  While the concept of bioprecipitation (as it is called) was proposed over 25 years ago, recent research by LSU scientist Brent Christner indicates that the atmospheric distribution of these bacteria is much greater than we once thought.  So, while dust and soot are also capable of forming the nucleus of ice crystals, it is likely that P. syringae is actually responsible for the majority of it; furthermore, the bacteria catalyze freezing at much lower temperatures, and may even be responsible for processes that trigger precipitation.

Clearly, the implications of this are far-reaching.  It is suspected that the propagation pattern of these bacteria follows that of the water cycle; bacteria infect a plant via rainfall, then reproduce and are aerosolized into the atmosphere again.  This would seem to indicate that the spurious introduction of P. syringae populations could result in the ability to manipulate localized precipitation levels—a distinct advantage for geographic areas suffering drought.  Obviously, if this is the case, the same technology in the wrong hands could be used as a type of “agricultural warfare”—inducing undesirable periods of droughts or flooding (a possibility that has not yet been alluded to by current research).  On a less catastrophic level, perhaps a method of simply quantifying atmospheric P. syringae could lead to a more practical, widespread usage—namely, improved accuracy of weather forecasting.


*Disclaimer - This report was written by a student participaring in a microbiology course at the Missouri University of Science and Technology. The accuracy of the contents of this report is not guaranteed and it is recommended that you seek additional sources of information to verify the contents.


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