Microbial associations with plants

The roots of almost all plants form mutualistic associations with fungi, known as mycorrhizae, which serve to enhance the uptake of water and mineral nutrients, especially phosphate, by the plants. The beneficial effect of a mycorrhizal association is particularly noticeable in soils with a poor phosphorus content. In return, the plant supplies reduced carbon in the form of carbohydrates to the fungi. Unlike other plant-microorganism interactions that occur in the rhizosphere, mycor-rhizal associations involve the formation of a distinct,

Table 15.4 Microorganism-microorganism associations

Microorganism

Microorganism

Type of relationship

Fungi

Alga/blue green

Mutualism (Lichen)

Amoeba,

Methanogenic

Mutualism

flagellates

archaea

A mycorrhiza is a mutually beneficial association between plant roots and a species of fungus.

The rhizosphere is the region around the surface of a plant'sroot system.

Figure 15.1 Symbiosis in Riftia, the giant tube worm. Found in deep-sea hydrothermal vents, Riftia acts as host to sulphur-oxidising bacteria. Energy and reducing power derived from sulphide oxidation are used to fix CO2 via the Calvin cycle and provide the worm with organic carbon. From Prescott, LM, Harley, JP & Klein, DA: Microbiology 5th edn, McGraw Hill, 2002. Reproduced by permission of the publishers

Figure 15.1 Symbiosis in Riftia, the giant tube worm. Found in deep-sea hydrothermal vents, Riftia acts as host to sulphur-oxidising bacteria. Energy and reducing power derived from sulphide oxidation are used to fix CO2 via the Calvin cycle and provide the worm with organic carbon. From Prescott, LM, Harley, JP & Klein, DA: Microbiology 5th edn, McGraw Hill, 2002. Reproduced by permission of the publishers

Box 15.1 Once bitten, twice shy

The fungus Acremonium derives reduced carbon compounds and shelter by living within the tissues of the grass Stipa robusta, and in return deters animals from grazing on it. It does this by producing various alkaloids which, ingested in sufficient amounts, are powerful enough to send a horse to sleep for several days! The horse clearly does not relish the experience, as it avoids the grass thereafter. The Acremonium passes to future generations through the seeds, so the relationship between plant and fungus is perpetuated. The nickname of 'sleepy grass' is self-explanatory!

Figure 15.2 Endomycorrhizae. Section through a plant root colonised by an endomycor-rhizal fungus. Note the spreading, 'tree-like' arbuscles

integrated structure comprising root cells and fungal hyphae. In ectomycorrhizae the plant partner is always a tree; the fungus surrounds the root tip, and hyphae spread between (but do not enter) root cells. In the case of the more common endomycorrhizae, the fungal hyphae actually penetrate the cells by releasing cellulolytic enzymes. Arbuscular mycorrhizae are found in practically all plant types, including 'lower' plants (mosses, ferns). They form highly branched arbuscules within the root cells that gradually lyse, releasing nutrients into the plant cells (Figure 15.2). In contrast to pathogenic fungi, mycorrhizal fungi are often rather non-specific in their choice of 'partner' plant.

An interesting example of mutualistic association concerns the endophytic (='inside plant') fungus Acremonium (Box 15.1).

The ability of crop plants to thrive is frequently limited by the supply of available nitrogen; although there is a lot of it in the atmosphere, plants are unable to utilise it, and instead must rely on an inorganic supply (both naturally-occurring and in the form of fertilisers). As we saw in Chapter 7, however, certain bacterial species are able to 'fix' atmospheric nitrogen into a usable form. Some of these, notably Rhizobium spp. form a mutualistic relationship with leguminous plants such as peas, beans and clover, converting nitrogen to ammonia, which the legume can incorporate into amino acids. In return, the bacteria receive a supply of organic carbon, which they can use as an energy source for the fixation of nitrogen.

The free-living Rhizobium enters the plant via its root hairs, forming an infection thread and infecting more and more cells (Figure 15.3). Normally rod-shaped, they proliferate as irregularly-shaped bacteroids, densely packing the cells and causing them to swell, forming root nodules.

Rhizobium requires oxygen as a terminal electron acceptor in oxidative phosphory-lation, but as you may recall from Chapter 7, the nitrogenase enzyme, which fixes the nitrogen, is sensitive to oxygen. The right microaerophilic conditions are maintained

Root nodules are tumour-like growths on the roots of legumes, where nitrogen fixation takes place.

Figure 15.3 Nitrogen-fixing bacteria form root nodules in legumes. (a) Rhizobium cells attach to root hairs and penetrate, forming an infection thread and spreading to other root cells. (b) Root nodule formation.

by means of a unique oxygen-binding pigment, leghaemoglobin. This is only synthe-sised by means of a collaboration between both partners. Nitrogen fixation requires a considerable input of energy in the form of ATP (16 molecules for every molecule of nitrogen), so when ammonia is in plentiful supply the synthesis of the nitrogenase enzyme is repressed.

Farmers have long recognised the value of incorporating a legume into a crop rotation system; the nodules left behind in the soil after harvesting the crop appreciably enhance the nitrogen content of the soil.

Legumes are not the only plants able to benefit from the nitrogen-fixing capabilities of bacteria. The water fern Azolla, which grows prolifically in the paddy fields of southeast Asia, has its nitrogen supplied by the blue-green bacterium Anabaena. When the fern dies, it acts as a natural fertiliser for the rice crop. Anabaena does not form root nodules, but takes up residence in small pores in the Azolla fronds. Nitrogen fixation takes place in hetero-cysts, specialised cells whose thick walls slow down the rate at which oxygen can diffuse into the cell, providing appropriate conditions for the oxygen-sensitive nitrogenase.

The alder tree (Alnus spp.) is able to grow in soils with poor nitrogen content due to its association in root nodules with the nitrogen-fixing actinomycete Frankia. The filamentous Frankia solves the problem of nitrogenase's sensitivity to oxygen by compartmentalising it in thick-walled vesicles at the tips of its hyphae, which serve the same function as the heterocysts of Anabaena.

Many microorganisms, particularly bacteria and yeasts, are to be found living as harmless commensals on the surface structures (leaves, stem, fruits) of plants.

Plant disease may be caused by viruses, bacteria, fungi or protozoans. These frequently have an impact on humans, especially if the plant affected is a commercially important crop. Occasionally the effect on a human

Unlike higher plants, ferns do not possess true roots, stems and leaves. The structure equivalent to a leaf is called a frond.

Organisms that grow on the surface of a plant are called epiphytes. They frequently live as commensals.

Table 15.5 Some microbial diseases of plants

Causative agent

Type of microorganism

Host

Disease

Heterobasidion

Fungus

Pine trees

Heart rot

Ceratocystis

Fungus

Elm trees

Dutch elm disease

Puccinia graminis

Fungus

Wheat

Wheat rust

Phytophthora infestans

Water mould

Potato

Potato blight

Erwinia amylovera

Bacterium

Apple, pear tree

Fire blight

Pseudomonas syringae

Bacterium

Various

Chlorosis

Agrobacterium

Bacterium

Various

Crown gall disease

Tobacco mosaic virus

Virus

Tobacco

Tobacco mosaic disease

population can be catastrophic, as with the Irish famine of the 1840s brought about by potato blight. A number of microbial diseases of plants are listed in Table 15.5.

We have already encountered the soil bacterium Agrobacterium tumefaciens in Chapter 12, where we saw how it has been exploited as a means of genetically modifying plants. A. tumefaciens is useful for introducing foreign DNA because it is a natural pathogen of plants, entering wounds and causing crown gall disease, a condition characterised by areas of uncontrolled growth, analogous to tumour formation in animals. This proliferation is caused by the expression within the plant cell of genes that encode the sequence for enzymes involved in the synthesis of certain plant hormones. The genes are carried on the T-DNA, part of an A. tumefaciens plasmid, which integrates into a host chromosome. Also on the T-DNA are genes that code for amino acids called opines. These are of no value to the plant, but are utilised by the A. tumefaciens as a food source.

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