When you look at a leaf you may think about photosynthesis, seasons, plant diseases,…, but have you ever thought about what is living on and in the leaf? The term phyllosphere describes leaves as habitat for different microorganisms. Fungi, yeasts, bacteria and bacteriophages can be found in and on leaves. Their interplay with the plant can be beneficial or pathological, dependend on plant and bacterial genetics and physical aspects like the weather. Leaves as habitat are no difficult habitat if you compare the temperature maximum and minimum or radiation with other environments at which you find microorganisms (e.g. hot springs). The difficulties of this habitat are the (rapid) day-night changes of temperature, radiation, rain and wind, and the short existence of the habitat of several weeks in annual plants because of seasons. Nevertheless, many different bacteria can be found on leaves and the dominant species may change with plant age or with environmental conditions. One example of leaf-living bacteria is Pseudomonas syringae and it has a real paradoxial way of rewarding its own success. In 2000, Susan S. Hirano and Christen D. Upper published a review about P.syringae with the focus on its role in the leaf ecosystem. P.syringae is a known pathogen which creates lesions. Besides the lesions, P. syringae is known for its ability to nucleate supercooled water to form ice which damages the plant. The probabilty of both, the lesion formation and ice-nucleation, increases with increasing population size. That means that if the leaf offers good conditions for successful reproduction for P.syringae, it destroys its own habitat by lesion formation and ice-nucleation. That is paradoxial! Therefore, the paper claims that these overpopulations are just accidents. “The real function of P.syringae is to live on healthy leaves.” So maybe there is just a sensitive interplay between the host plant and the bacteria which normally restricts the population size. “Only when conditions become unusually favorable and population sizes of the bacteria become too large does the entire system crash, to the detriment of both host and bacteria.” The question is why this happens. Why the plant can't prohibit this process and creates a favorable habitat for its enemy. And why has P.syringae no feedback mechanism, which prevents the destruction of its own habitat? Maybe P.syringae just doesn’t care as it immigrates by the plant seeds and by the wind… Bacteria in the Leaf Ecosystem with Emphasis onPseudomonas syringae—a Pathogen, Ice Nucleus, and Epiphyte.
Susan S. Hirano, and Christen D. Upper Microbiology and molecular biology reviews 64.3 (2000): 624-653.
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