Phyllosphere
The phyllosphere is in the ecology of the area, formed by the surfaces of leaves and leaf sheaths as a habitat for other organisms. The phyllosphere represents the largest biological surface of the earth and is especially, populated by a number of microorganisms, primarily bacteria, yeasts and filamentous fungi. In adaptation to a nutrient-poor habitat with special surface ( water-repellent cuticle of the leaves ) and to rapidly changing conditions - for example with regard to humidity and radiation - developed the Phyllosphärenbewohner a wide range of survival strategies.
To elucidate the so far only partially known biodiversity and the complex interactions that extend to the role of Phyllosphärengesellschaften in global biogeochemical cycles, the relatively young field of molecular biology methods Phyllosphärenforschung also served. Lag initially the main focus on the study of pathogens in plants, in the meantime, the importance of the numerous neutral or even promoting acting on their base Phyllosphärenbewohner was detected, which can be used for example in biological plant protection.
- 4.1 Special
- 4.2 colonization strategies
- 5.1 bacteria, yeasts and fungi
- 5.2 Other groups of organisms
- 6.1 Within the phyllosphere
- 6.2 With man and atmosphere
Terms
The term " phyllosphere " In the mid 1950s, in analogy to the rhizosphere was coined ( the area around the plant roots ) for the boundary layer between the leaf and atmosphere. It is derived from the ancient Greek names φύλλον phyllon, leaf ' and σφαῖρα sphaira, sphere ', ' ball ' fro. The spatial delineation of Phyllosphärenbegriffes ( in English sometimes referred to as leaf area) is handled in the literature inconsistent and partly not limited to leaves and leaf sheaths, but extended to the surfaces of all aboveground plant organs, including buds, flowers, fruits and stems. Sometimes, instead of or in addition, the term phylloplane used partly synonymous with the phyllosphere, partly in strict restriction on the actual leaf area, while the term " phyllosphere " depending on the author also deeper areas may include how the substomatären cavity below the stomata or the apoplast with.
The phyllosphere is inhabited usually by numerous microorganisms. These organisms are often associated with the concept " epiphyte ". However, since this generally " epiphytes " means ( and also includes vascular plants, while the Phyllosphärenforschung largely focused on the micro-organisms of direct leaf surfaces ), many authors use the more specific term for Phyllosphärenbesiedler " epiphyllous ", which is also used in the present article; the existing especially in the tropical mountain rain forest epiphyllous vascular plants are not considered generally to phyllosphere, as they themselves are covering ground and leave the zone of immediate leaf surface. Strictly speaking, however epiphyllous are only those organisms that colonize the leaf tops, residents of leaf undersides, however, " Hypophylle ". Epi - and Hypophylle are in turn combined under the generic term " foliicol ", a term that has become especially visible with the naked eye organisms such as mosses or lichens at least partially enforced in the literature.
Importance
The habitat (Habitat) phyllosphere is the largest biological surface of the earth. On the basis of satellite data, the terrestrial total leaf area is estimated at about 640 million square kilometers to 1 billion square kilometers, which is about 125 % to 200 % of the earth's surface. The phyllosphere thus offers an expansive, also richly structured habitat. As settlers prevail microorganisms whose observation is commonly the key feature of Phyllosphärenforschung. These are especially bacteria (whose number of individuals dominated by far), followed by yeasts, filamentous ( thread-like ) fungi and possibly other groups of organisms: plant viruses, archaea, Myxomycetes, green algae, mosses, lichens, ferns, protozoan and invertebrates.
Conservative estimates put the total number of bacteria in the phyllosphere of 1026. A plant leaf bears per square centimeter typically one million to 10 million bacteria. Some groups of plants, such as citrus species or conifers are, however, significantly weaker populated with sometimes less than 1,000 cells per cm ².
The phyllosphere is due to their large surface area is an important refuge and also a major resource for microorganisms dar. As absorb this organic matter and later release them again in a modified form, is - in addition to over the sheet already held mass exchange with the environment - including the phyllosphere in biogeochemical cycles involved. The number of microorganisms inhabiting the phyllosphere is sufficiently large to affect the global carbon and nitrogen cycles can.
While research into the rhizosphere can already look back on a long tradition, the properties of biodiversity and the interactions of Phyllosphärenbewohner with the animate and inanimate environment for only a few decades increased attention to be paid. Initially, the main interest of Phyllosphären Microbiology taught for economic reasons, to the study of disease (pathogens ) in plants in order to understand the mechanisms of its establishment, spread and adverse effects and to take appropriate countermeasures. Meanwhile, the important role of many neutral or even promoting acting on their base Phyllosphärenorganismen was detected. The importance of Phyllosphärenforschung is supported by regular professional events bill, including an international Phyllosphärensymposium, which takes place in a 5 - year intervals since the first event of its kind in 1970 in Newcastle upon Tyne. The 9th International Symposium on the Microbiology of Aerial Plant Surfaces was organized by the Oregon State University in August 2010.
Methods of investigation
The basis for the exploration of the often invisible to the naked eye communities of the phyllosphere the development of appropriate research methods. Deeper insights into complete kinds spectra are now made possible by modern techniques of molecular biology.
Classical methods
The classical research methods for microorganisms of the phyllosphere include the so-called "Leaf Printing". Here a leaf is gently onto a nutrient medium, such as an agar plate, pressed and pulled off again. After incubation, the now growing bacterial or fungal colonies are analyzed. A similar method is the technique of " leaf washing": These sheets are in a vessel with a liquid ( for example, saline or phosphate buffer), rinsed and washed off the cuticle microorganisms then cultured, for example, by plating on a nutrient medium. In both methods, then come to light, fluorescence or electron microscopy in combination with microbiological and biochemical methods are used to analyze the grown organisms more closely. Thus, the culture media may contain certain nutrients or inhibitory substances with which we can distinguish different groups of organisms.
Note, however, that only part of the epiphyllous microorganisms can be cultivated under laboratory conditions or forms colonies. Therefore, can be detected with the methods mentioned only a part of the actual existing species spectrum. And they only form the microbial population at a certain time from; to be able to capture the often significant during the course of leaf development changes, it requires numerous leaf analyzes over the entire development period of time.
Molecular biological methods
In addition to morphological or physiological research methods, methods are now available with which washed off of leaves samples on the entire genome ( genome, in this case, the metagenome ) of the organisms obtained can be assayed for. Often in this case the polymerase chain reaction ( PCR) is used, is reproduced with the genetic material to then unravel by gel electrophoresis and to examine specific markers. Such markers are the particular coding for parts of ribosome genes ( 16S rRNA in prokaryotes such as bacteria, 18S rRNA in eukaryotes such as fungi ). Comparable methods have recently been used with regard to the totality of the proteins ( metaproteome ) using mass spectrometric methods.
Other methods of investigation of the habitat phyllosphere based on the inoculation of leaf surfaces in the laboratory or field with suspensions, the defined bacterial or fungal spore ( single species or strains or mixtures ). Research object is then particularly their successful growth under different environmental conditions and interactions with existing colonizers. This often involves genetically modified bacteria are used as so-called bioreporters or biosensors for use. These have special reporter genes, often a fluorescent protein (eg the light originating from the jellyfish Aequorea victoria GFP gene ), whose activity can be detected readily. With the help of such bioreporter can be for example the presence and distribution of certain substances on leaves, seen as nutrients such as sugars, trace elements, or even water.
Habitat
Special
The habitat phyllosphere represents the geno - and phenotypic flexibility of potential colonizers special challenges, because the leaf surface is a highly non-uniform habitat. Structurally and functionally distinct regions of the sheet as stomata ( stomata ), leaf veins, hairs ( trichomes ) and epidermal cells not only provide a different " topography ", but also vary considerably in terms of water retention, thickness of the cuticle or permeability to be herbal substances. The cuticle of the leaves is usually the substrate to which the organisms of the phyllosphere first come into contact or which is populated by them. It is an extracellular lipophile biopolymer consists mainly of cutin, mono-or superposed in the waxes of different composition. Its structure shows an often highly complex three-dimensional and crystalloid structure which can be subject to substantial changes in the course of leaf development and eroded in older leaves often.
In addition to the structural features of the base of the phyllosphere habitat is characterized by more specific physico-chemical conditions. It is characterized by frequent and rapid changes in moisture available. This is particularly supplied by rain, fog or dew and depends to a large extent also of leaf surface structures and their wettability. More often rapid fluctuations of embossed factors radiation ( especially UV light ), temperature conditions, wind, and nutrient availability. This reflects the phyllosphere differ from the rhizosphere, for the largely constant or slowly changing conditions are typical. Total reigns in the phyllosphere before a low water and nutrient supply; only a small portion of the transported sheet in the nutrients and the water passes to the outside. Available for microorganisms, for example, the guttation fluid of some plant species, which also contains nutrients. The resources available for settlers nutrients the phyllosphere however mostly caused only a small part of the leaf itself, since the cuticle is a barrier for polar molecules such as sugars or amino acids, but mainly from the atmosphere ( nitrogen-containing compounds, pollen ) or as honeydew from insects. Wachstumsbegrenzend for microorganisms is often primarily the availability of carbon compounds, in the second place, the availability of nitrogen.
Colonization strategies
The colonization of the phyllosphere happens especially by rain, Wind transport of snow (possibly hundreds of kilometers away ) or insects. The distribution of microorganisms on leaves is very inhomogeneous. Leaf veins, leaf undersides or shaded, near the base areas of glandular trichomes or stomata usually have a denser microbial colonization than other leaf zones. It is advantageous to the mobility of some microorganisms, which allows them to take positive steps to move to appropriate locations. For mounting on the hydrophobic leaf surface in addition to a passive adhesion are also active mechanisms to bear. Thus, an attachment, for example, extracellular water-insoluble glycoproteins or polysaccharides. Some bacteria have this special training, such as thin, filamentous protein appendages or tubes ( pili ) or microfibrils of cellulose. Often microorganisms form on sheets larger colonies or aggregates. A number of species is able to develop the so-called biofilm. Biofilms are thin slimy layers that represent a complex matrix of extracellular polysaccharides and other biopolymers, and are known from a variety of habitats. They offer it often embedded in large numbers organisms - often a broad spectrum of bacteria, yeasts and filamentous fungi - favorable environment, for example with respect to pH or ionic strength, and provide total protection against dehydration and environmental influences dar.
Some species are able to deliver surfactants that reduce the surface tension as so-called biosurfactants. Thus, the wettability of hydrophobic leaf surfaces is improved with water and thus increases its availability for microorganisms. An effective biosurfactant for example, is produced by Pseudomonas syringae syringomycin.
Depending on the rapidly changing environmental conditions, the population density of phyllosphere strong temporal fluctuations. So rainfall not only lead to a settlement, but also a washout existing Blattbesiedler. Heavy precipitation can therefore draw significant changes in population density and structure by itself, but favor by at least a temporary humidification turn a new settlement. Also, the arrival time is essential for potential colonizers: colonizer usually find more nutrients than before later comers and take on less competition. Survival benefits in the phyllosphere have organisms that have special adaptation mechanisms to survive even rapid environment changes using a tolerance or avoidance strategies. In addition to creating their own environments through biofilms (see above), many microorganisms of the phyllosphere in a position to shield pigments ( usually pink, orange or yellow in color ) to produce, giving them protection from UV radiation. Since radiation carries an increased mutation risk, efficient DNA repair systems can be beneficial additionally or alternatively. For example, such a Pseudomonas syringae responsive to UV -B radiation are known mechanisms.
The ability to colonize deeper areas of the leaf and other plant organs is mainly limited to pathogens ( pathogens ). Many fungi are capable of penetrating the cuticle by hyphae, which allows them not only a mechanical attachment, but also often a direct tapping of nutrients of the leaf. They form the transition to Endophytismus, but this is often not sharply separable from Epiphytismus and purely epiphyller lifestyle and in some species may be dependent on environmental conditions or developmental stage because of their lifestyle. Usually a methodical differentiation takes place in that a surface - sterilization is overcome or not. Pathogenicity is not necessarily linked to endophytic lifestyle.
Species composition and succession
In addition to the population density and species composition of Phyllosphärengesellschaften is not static, but characterized by high dynamics, whereby in addition to environmental factors such as weather or radiation, for example, air pollution may play a role. Because of the high heterogeneity of the phyllosphere, there are attempts to transfer biogeographic models of island biogeography on the micro scale of the phyllosphere and thus to explain the between adjacent sheets partially widely divergent microbial colonization structure.
Bacteria, yeasts and fungi
In temperate latitudes in the course of the growing season often observed several groups of microorganisms epiphyller a temporal sequence (succession ): In young, unfolding leaves dominate in general - at the beginning rather low nutrient supply - bacteria. Among the readily culturable under laboratory conditions types are frequently found representatives oxygen breathing ( aerobic ) groups of the genera Pseudomonas, including Pseudomonas syringae (one of the most studied organisms in the phyllosphere ), Corynebacterium, Erwinia, Bacillus and Xanthomonas. Also common, but more difficult to prove because of their slower growth and more specific claims in the laboratory are facultative methylotrophic (ie the use of methanol qualified as a carbon and energy source) bacteria of the genus Methylobacterium. Furthermore, cyanobacteria live on leaves, among other things from the genera Anabaena, Nostoc, and Scytonema Aulosira. If the metagenomic analyzes can be found in many cases still unknown species in the samples, which prove that the knowledge of the bacterial epiphyllous still has significant gaps. So 78 out of 37 known bacterial species and 12 genera hitherto nameless alone were detected in sugar beet leaves.
If, in the course of leaf development increased nutrients available (for example in the form of pollen, dust, or excrement blattbewohnender arthropods, especially sugary honeydew ), yeasts come to the fore. Frequently represented genera Cryptococcus, Sporobolomyces, Rhodotorula, Torulopsis, and Aureobasidium. Species of the genus Sporobolomyces yeasts are among the most successful colonizers of the phyllosphere, as they can spread by means of the so-called firing Ballistosporen efficiently from one sheet to another.
With the progress of leaf senescence ( senescence ) in the fall of the cohabitation of the phyllosphere is increasingly dominated by filamentous fungi. They originate from the groups of ascomycetes ( Ascomycota ), Mushroom Fungi ( Basidiomycota ) and of the artificial group of Imperfect Fungi, lists the types of uncertain systematic position. Demonstrated, for example, the genera Epicoccum, Alternaria and Stemphylium, including leaf -damaging species. Existing honeydew also provides food for mainly saprophytic, ie living on dead organic material types, such as the so-called Rußtaupilze. In more progressive leaf senescence almost only saprophytes, such as representatives from the genera Ascochytula, Leptosphaeria, Pleospora and Phoma appear. Allochthonous (ie originating from other habitats, especially the floor) include the genera Cryptococcus (see above), Myrothecium and Pilobolus represented amongst several other well in the phyllosphere. An inventory on the leaves of Mediterranean plants yielded 1029 strains of filamentous fungi and yeasts strains 540, 36 and 46 could be assigned to different types. Not all species of bacteria as in the known under the fungi and yeasts so far at a distance.
Other groups of organisms
The conditions in subtropical and tropical regions, especially the tropical rain forests with year round high temperatures and corresponding humidity range (as well as the fact that many plants bear perennial foliage in the tropics ) allow in addition to microorganisms other groups of species to colonize there leaf areas. These include epiphyllous ( foliicole ) lichens, of which over 800 species are known, but 616 for the Neotropics, including for example, representatives of the genera Arthonia, Bacidia, Byssoloma, Mazosia, Porina, Strigula and Tricharia. Among the also very species-rich foliicolen mosses dominate foliose liverworts Lejeuneaceae the family; in 1996 were around 1,000 liverworts with epiphyller of life known, including species of the genera Cololejeunea, Ceratolejeunea, Drepanolejeunea and Colura. Also from oceanic influenced climate zones of higher latitudes are isolated observations of liverworts on long lasting leaves before, so when Ivy ( Hedera helix) in southern England. If with increasing leaf age the cuticle degraded and higher wettability with water, see also green algae for appropriate conditions. This is true in temperate climates, especially for the multi-year needles of conifers, which are often populated from the second year of members of the genus Chlorococcus.
Only since the end of the 1990s it is known that the phyllosphere in tropical regions - also represents a habitat for organisms group of slime molds ( Myxomycetes ) - in temperate climates only exceptionally. While mosses and lichens usually settle on leaf tops, prefer the slime molds with their various stages of development rather less rain or sun-exposed areas, ie leaf undersides or secondary, formed by epiphyllous liverworts microhabitats.
Protozoa that are known to feed in the soil and water of bacteria, could be frequently found on leaves. One example is the Heutierchen ( Colpoda cucullus ) having the ability to form cysts in dehydration to rapidly when wetted to be active again. Some common invertebrate inhabitants of the phyllosphere include nematodes. At least large parts of their lives to keep many kinds of other animal groups in the phyllosphere, including tardigrades, annelids, snails, mites, double tails, springtails, dust lice, aphids, butterflies and Hymenoptera, especially ants.
Interactions
Within the phyllosphere
Interactions between the organisms of the phyllosphere play on the one hand between the colonizers among themselves, on the other hand between the colonizers and their host base, so the sheet. The phyllosphere favored by the close aggregation of microorganisms colonizing them the gene exchange. Such high rates were detected by horizontal gene transfer also across species by transfer of bacterial plasmids, which makes the phyllosphere likely to become an important source or a hotspot of microbial biodiversity. Also, interactions between different epiphyllous groups exist that go beyond mere competition, such as leaf -damaging fungi can turn colonized by bacteria, ie these are parasitized, such as Neurospora crassa by Pseudomonas syringae.
The organisms in the phyllosphere can exercise as commensals neutral, as pathogens ( pathogens ) negative or positive as symbionts influences on their surface. There are various ways by which epiphyllous can affect a leaf to their advantage and thereby damage it more or less. The influence may be about to express plant hormones ( phytohormones), toxins (toxins ) or substances exist that increase the permeability (permeability) of cell membranes, so as to increase the availability of nutrients from the leaf. Positive effects ( mutualism ) for the host plant can arise, for example, if their leaves are colonized by nitrogen fixers, their growth is enhanced by the phytohormones epiphyllous or whose presence leads to a suppression of other organisms with pathogenic effect. In many cases, no clear assignment of organisms is possible because environmental factors and the development cycle a say in particular bacterial species, whether they act neutral or phytopathogenic.
The visible with the naked eye Blattbesiedler - such as mosses, lichens or green algae - are not directly harmful usually populated for leaves, but can decrease with increasing coverage of their photosynthetic performance. Parasitism is less common in these groups, but does happen. Examples are green algae of the genus Cephaleuros and lichens of the genus Strigula where Cephaleuros species are symbiotic partners.
The attention of the research has been focused for a long time on the numerous phytopathogenic organisms of the phyllosphere. Nebenblatt destroyers as the many mildew - causing fungi or fire blight pathogen ( Erwinia ) can also favor frost damage by ice build some Erwinia or Pseudomonas species. Leaves contain many biological antifreeze substances that prevent clear even at temperatures below 0 ° C freezing. Bacteria containing the so-called ice- gene, as different strains ( pathovars ) of Pseudomonas syringae reduce the freezing tolerance of leaves, so that there will be early, cell -destroying ice crystal formation. To these harmful effects oppose it came to the first field trials with genetically modified bacteria at all, because it could be shown that bacterial strains can successfully compete without the ice gene with phytopathogenic strains. Appropriate bacterial strains are now sold commercially.
To combat the economically very important in orchards fire blight pathogen Erwinia amylovora non-chemical -based competition strategies have been developed in terms of a biological control also, based on the fact that early application of antagonistic microorganisms can suppress subsequent colonization by Erwinia successful. Generally find the positive in economic terms effects of some microorganisms in the form of so-called Biological Control Agents ( BCA) for the control or prevention of plant diseases growing interest. These are able to control pathogen on leaf surfaces or reduce. Here different mechanisms come into question as competition for nutrients, occupation of ecological niches or active inhibition of other species by releasing substances such as acids, cell- resolution active enzymes, or antibiotics or fungicidal substances.
Phytohormone of the auxin group are widespread among bacteria of the phyllosphere and can be effective in many areas of plant development. You can promote plant growth in a positive way, which is also true for the Phytohormongruppe of cytokinins produced by some Methylobacterium species. Disadvantages for the host arise when triggered by phytohormones hyperplasia or leaf deformation or plant galls. Number of Votes hormone substances can also stimulate the Phyllosphärenbewohner a nutrient release of the blade to the outside, for example through formation of ion channels in cell membranes, with the consequence of an increased efflux of metabolites. Indole - 3-acetic acid (IAA, auxin is the most important representative), for example, stimulates the release of saccharides from plant cell walls. In the extreme case, the effect of Votes substances by epiphyllous can up to the disintegration (lysis ) lead of leaf cells.
Of the leaves of many plants carrier in tropical regions is known that they are regularly colonized by nitrogen-fixing ( diazotrophic ) microorganisms. A part of the fixed nitrogen can be taken up and used by the plant host to leaf. Play a key role in particular diazotrophs cyanobacteria, which in turn often have a narrow, partly symbiotic bond to epiphyllous mosses. It is believed that such communities make a significant contribution to nitrogen deposition in tropical rain forests. Thus, nitrogen fixation rates for a premontane rain forest in Costa Rica indicated in the phyllosphere 2-5 kg per hectare per year (mainly by members of the genus Scytonema causes ). The actual capacity of the phyllosphere nitrogen fixation and its importance in the global nitrogen cycle is not yet known, however inadequately.
With humans and atmosphere
Direct to human nutrition are important on the one hand produced by fungi toxins (mycotoxins ) that are, however, usually produced by endophytes. Observed recently, case clusters of food intoxications prove the other hand, the risk that maturing fruit or leaf surfaces of vegetables can be with human pathogenic enterobacteria, such as Salmonella or Shigella, already inhabited before harvest ( for example, by irrigation with untreated water or fertilizer). Contrary to previous view such bacteria can not only survive but also proliferate on such surfaces, this especially in humid conditions. Overall, the biology of enteric bacteria on plant surfaces, however, is still poorly understood.
Since surfaces represent an important sink for reactive trace gases in the atmosphere in general, even the phyllosphere a not insignificant role to play in the regulation and removal of air pollutants such as ozone, sulfur dioxide, ammonia, and others in this regard. Basically provide the complex sheet structures with their wax pads and large surfaces diverse deposit options for fine particulate matter ( dust) or aerosols. How far apart from the only partially understood physico- chemical effects on leaf surfaces are also interactions with microorganisms of the phyllosphere of importance here, is poorly understood. It is known that Phyllosphärengesellschaften sensitive to air pollutants such as sulfur dioxide or nitrogen oxides. On the other hand, the occurrence serve heavy metal resistant bacteria in the phyllosphere as positive bioindicator for certain atmospheric pollutants.