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The [phyllosphere] refers to the above-ground surfaces of a plant as a habitat for microorganisms, with a heavy emphasis on leaf surfaces. All plants are home to a wide variety of microorganism communities including bacteria, fungi, and yeasts. Some microorganisms benefit the plant, others are plant pathogens and can potentially damage or kill the host plant (4). The majority of bacterial colonists on any given plant have no noticeable effect on plant growth or function.

Estimates suggest that the roughly 1 billion square kilometers of worldwide leaf surfaces host more than 10^26 bacteria, which are the most abundant colonizers of this habitat. Overall, microbiota in this ecosystem are large enough to have an impact on both global carbon and nitrogen cycles. Further more the phyllosphere mircroorganisms influence their hosts at the level of the individual plants.

With repeated and rapid constant change of environmental conditions occurring on leaf surfaces, the phyllosphere is recognized as a hostile environment to bacteria. Leaf surfaces are often dry and temperatures can reach 40–55°C under intense sunlight. During the night, however, leaves are frequently wet with dew and at cool temperatures (5–10°C) (2).

Microbes that live in the Phyllosphere are called [Epiphytes]

Physical environment

The leaf surface is exposed to rapidly changing temperature and relative humidity. Also the repeated alternation between presence and absence of moisture due to rain and dew (3). The leaf also provides limited nutrient resources to bacterial colonists. The rapid and severe changes in the physical conditions of the phyllosphere creates a hostile microbe environment.

Several factors may influence the microhabitat experienced by bacteria on leaves. First, the leaf itself is surrounded by a very thin laminar layer in which moisture emitted through stomata may be sequestered, alleviating the water that stress epiphytes (4).

Second, some cells in a leaf bacterial population, particularly in [plant-pathogenic] populations locally invade the interior of the leaf, avoiding the stresses on the exterior of the leaf by residing in substomatal chambers or other interior locations. While some phytopathogens have the option of avoiding stresses, most epiphytes must tolerate them in some way (Lindow, S).

"Under the microscope, aerial plant leaves resemble eerie landscapes, with deep gorges, tall peaks and gaping pits that riddle the waxy surface." -Leveau, J. (2009)

Epidermal cells produce hills and valleys that will determine the shape and size of low areas on the surface, which will influence the shape and spread of water droplets on the plant. The first contact between immigrating bacteria and a leaf normally occurs at the plant cuticle. The waxy layer, which has different three-dimensional crystalline structures on different plant species and can change as leaves age. These modifications limit passive diffusion of nutrients and water vapor from the plant's interior onto the surface and defines the hydrophobicity of the leaf. Thick waxy cuticles have thus been thought to interfere with bacterial colonization of plants by limiting diffusion of nutrients and inhibiting the wetting of the leaf surface (Lindow, S).

Because the Phyllosphere is a hostile environment for the residing microorganisms physical parameters contribute to stressful conditions, such as UV radiation, temperature shifts, and the presence of reactive oxygen species. Adaptation to stressful conditions was reflected by the detection of various proteins, assigned to diverse bacterial genera and detected in all analyzed samples. Among these proteins were superoxide dismutase, catalase, DNA protection proteins, chaperones, and proteins involved in the formation of the osmoprotectant trehalose (1).

Biological interactions

There is evidence that bacteria form large and heterogeneous aggregates on plant surfaces. Microscopic examinations of colonized leaves show that plant surfaces have many epiphytes occuring on them in large mixed-bacterial-species aggregates that also harbor fungi. While large numbers of solitary bacterial cells occur on plants, a few large masses of apparently mixed bacterial species can be found.

Such aggregates can constitute between 30 and 80% of the total bacterial population on certain plant species. These assemblages, with an extent and structure similar to those of biofilms that develop in aquatic habitats, are probably found only on long-lived leaves in moist climates such as the tropics or wet temperate regions such as the Pacific Northwest.

The conglomerates on most other plants, while still sizable, are best known as aggregates. The formation of aggregates by bacteria on plants has major implications for the ability of these microbes to colonize and survive the harsh environment of the phyllosphere (1); it may provide them with a means to modify their immediate environment in the habitat . The production of extracellular polysaccharide (EPS), which is considered to form a major part of the bacterial aggregate matrix, may benefit epiphytes in the phyllosphere to see a photo of phyllospheric bacteria click the link microbial interactions

Water availability is one of the most highly fluctuating factors on leaf surfaces. The heavy EPS slime within aggregates can shield the bacteria from desiccation stress by buffering the matric and osmotic potentials of their surroundings. Furthermore, EPS helps to protect plant-associated bacteria from reactive oxygen species, which are often encountered on plants. It has been demonstrated that aggregated bacteria resist oxidative stress better than planktonic bacteria (1).

Microbial processes

Certain bacteria in the Phyllosphere can increase the saturation of leaves via production of compounds with surfactant properties. This ability occurred in 50% of the Pseudomonas strains tested. Because of the hydrophobic nature of the cuticle, it is likely that increased saturation of these habitats allows diffusion of substrates, making them more readily available to epiphytic bacteria (Lindow, S). Biosurfactants facilitate the moving of bacteria on the phylloplane, this phenomena was suggested for tolaasin, a toxin produced by Pseudomonas tolaasi. The water film created by the surfactant could spread the bacteria across the leaf surface to areas where nutrients are more abundant. The production of biosurfactants may be one trait which bacteria can alter their habitat to exploit it more efficiently (3).

"Schematic diagram representing various hypothetical bacterial-habitat modifications in the phyllosphere, such as the release of nutrients from plant cells and bacterial cell dispersal effected by the production of syringomycin, which may act both as a phytotoxin and as a surfactant (A); the release of saccharides from the plant cell wall, caused by bacterial secretion of auxin (B); and protection from environmental stresses via production of EPS in bacterial aggregates(C)" Communities in the phyllosphere are thought to be limited by carbon availability, and it may be expected that access to carbon compounds on leaves is a major determinant of epiphytic colonization. There is evidence that small amounts of nutrients, such as simple sugars (glucose, fructose, and sucrose) leach from the interior of the plant.

Communities in the phyllosphere are thought to be limited by carbon availability, and it may be expected that access to carbon compounds on leaves is a major determinant of epiphytic colonization. There is evidence that small amounts of nutrients, such as simple sugars (glucose, fructose, and sucrose) leach from the interior of the plant. (1).

Key Microorganisms

There are several microorganisms that have great effects on the phyllosphere. The Two main categories that will be focused on will be pathogens and Nitrogen fixers. Pathogens are biological agents that cause harm to their hosts usually through disease and infection. There is a constant battle between pathogens and good bacteria to maintain control of the host. In humans pathogens are fought through the immune system. Pathogens can vary from irritating to deadly.

Nitrogen fixation is one of the most important biological interactions on the face of the earth. The Nitrogen cycle is a complex system which microorganisms preform such tasks as fixation, mineralization and nitrification. About 80% of earth's atmosphere is nitrogen but it is unavailable for biological use, making nitrogen fixing bacteria extremely important for sustaining life.


There are several microbes that live on the Phyllosphere which are of great importance in regards to humans. These microbes account for a large amount of the food borne illnesses in the United States: Campylobacter,Escherichia coli O157:H7, Salmonella, [Cryptosporidium], [Toxoplasma] and [Cyclospora]. A large amount of the food crop regulatory commissioning focus's on eliminating these bacteria and protozoa.

Two independent studies have shown that [Salmonella enterica] and [Escherichia coli] have the ability to colonize corn, bean, and cilantro plants under humid conditions, albeit to lower population levels than those of common bacterial epiphytes. Unlike P. syringae, they failed to grow on leaves under dry conditions. However, S. enterica survived dry conditions on cilantro leaves and recovered to gain major population sizes under wet conditions (1). The fitness of some human enteric pathogens in the phyllosphere, as well as the distribution on plants of Enterococcus spp. and of common opportunistic pathogens such as Pseudomonas aeruginosa and Burkholderia cepacia, encourage technological innovation in the field of crop harvesting and food safety with regards to virus's.

The study of bacterial colonizers of leaves has been restricted mostly to aerobic bacteria and has also been driven by the importance of investigating the ecology of plant-pathogenic bacteria because of their negative effect on plant productivity. The microbial ecology of the phyllosphere has been viewed mainly through the biology of gram-negative bacteria such as Pseudomonas syringae and Erwinia (Pantoea) spp. In recent years, the association of multiple outbreaks of food-borne illness with fresh fruits and vegetables has raised concern about the possible preharvest contamination of plants with human pathogens. Surveys of the occurrence of enteric pathogens on produce showed that Salmonella spp. and Shigella spp. were detected in up to 4% of the samples (2).

Helpful Epiphytes

Phyllosphere bacteria can promote plant growth and both suppress and stimulate the colonisation and infection of tissues by plant pathogens. Fungal endophytes of leaves may deter herbivores, protect against pathogens and increase drought tolerance. Interactions in the phyllosphere zone determine the extent to which human pathogens are able to colonise and survive on plant tissues, an area of great importance with the rise in cases of human disease associated with consumption of fresh salad, fruit and vegetable produce.

Nitrogen Fixers

N2 fixation is thought to occur mostly on the leaf surfaces (not in the leaf interior) and cyanobacteria associated with epiphytes are likely to represent the key N2-fixing bacteria in this environment. Also bacteria such as diazotrophic γ-proteobacteria could be connected in N2 fixation processes. In tropical regions, N2-fixing cyanobacteria belonging to the genera Scytonema, Oscillatoria, Microcoleus and Stigonema have been reported to colonize the phyllosphere of forest plants (Abell, G).

Examples of organisms within the group

There are many organisms which live in the Phyllosphere, they vary from microscopic to several inches in length. The most Common organism in the Phyllosphere is bacteria, mostly bacilli and are not detrimental to the plant, although there are around 100 species that are classified as pathogens and are harmful to the plant or primary consumers.

Fungi, pathogens, cyanobacteria, Oomycetes, phytoplasma, nematodes, protozoa and even small parasitic plants have been found in the Phyllosphere. Also algae, bryophytes (mosses, liverworts) and lichens have been found to live in the phyllosphere. Because the phyllosphere is such a varying ecosystem many microorganisms have grown unique niches to their particular habitat, even to their particular plant. With different areas having different amounts of sun, wind, moisture, macro/micro nutrients, leaf surface area and microbial populations it is reasonable to assume that no two phyllospheres are completely the same.

Current Research

Research into the characteristics of microbial life in the phyllosphere is of great commercial importance to the agricultural industry for two reasons. First, understanding the survival of plant disease-causing bacteria and fungi is vital for developing new ways to control their spread. Second, there has been a recent rise in the number of food poisoning cases associated with fruit and vegetables contaminated with bacteria such as Salmonella and E. coli O157:H7. This is particularly true of fresh fruits and salads which are not cooked prior to consumption. Preventing these outbreaks by developing better decontamination strategies is important to protect public health.

Immediate-term information is needed to guide growers, Cooperative Extension, the diagnostic service industry, shippers, and processors in the development of on-the-farm management practices to prevent these microbial pathogens from being introduced during productionand at harvest.

One form of research being done that many people might not think about is crop dusting. Planes fly over crops and dump certain chemicals onto the plants that have certain effects on specific micro/macro fauna. This is a continuous research process to determine which pesticide/bacterial agent works the best in specific conditions over long periods of time with no long term detrimental effects.

Another step being taken in terms of research is screening. The process of screening is critical in developing of biocontrol agents. The success of subsequent stages depends on the sucess rate of a screening procedure to identify an appropriate candidate. Many useful bacterial biocontrol agents have been found by researching zones of inhibition in Petri plates. This method does not identify biocontrol agents with different action modes such as parasitism, induced plant resistance or forms of competition (7).

Screening methods for parasitism include burying and retrieving propagules of the pathogen to isolate parasites. In regards to competition, methods include searching for microbes that colonize sterile soil rapidly and can exclude other organisms that attempt to invade the space.


(1) Lindow, S. Brandl, M. 2003. "Microbiology of the Phyllosphere." Applied and Environmental Microbiology. Vol. 69, No.4. 0099-2240

(2) Delmonte, N., Knief, C., Chaffron, S., 2009. "Community proteogenomics reveals insights into the physiology of phyllosphere." National Academy of Sciences.

(3) Machowicz-Stefaniak, Z., Krol, E. 2006. "Biotic effect of caraway phyllosphere fungi on the pathogenic fungus." Department of Plant Pathology University of Life Science. 2-8

(4)Suslow, T. 2005. "Microbial Food Safety IS Your Responsibility." Vegetable Research Information Center. 1-6.

(5) Howplantswork. 2009. "Life in the Phyllosphere: What Microbes Commonly Dwell on the Surface of Leaves?."

(6) Whipps, J.M. Hand, P. Pink, D. 2008. "Phyllosphere microbiology with special reference to diversity and plant genotype." 2-34

(7) Gardener, B., Frafel, D. 2002. "Biological Control of Plant Pathogens: Research, Commercialization, and Application in the USA." Plant Health Progress. 1-18

(8) Abell, G., Richter, A. 2008. "Nitrogen fixation by phyllosphere bacteria associated with higher plants and their colonizing epiphytes of a tropical lowland rainforest of Costa Rica." The IMSE journal. 2, 561–570

Edited by student of Angela Kent at the University of Illinois at Urbana-Champaign.