4.3 VAM - the beneficial fungi that feed plants

by Sally Smith, University of Adelaide

Most plants do not just have roots, they have VAMs - vesicular arbuscular mycorrhiza.  VAMs are fungi which live in a harmonious relationship with plant roots.  This is a symbiosis in which the fungi provide the plant with extra nutrients from the soil, especially phosphorus and zinc, in exchange for sugars provided by the plants.

About 80% of all plants, including most field crops and many trees, harbour the fungi as an integral and normal component of their root systems. 

As with all fungi VAMs also help hold soil particles together.

How VAMS live

VAM fungi grow inside plant roots. Hyphae absorbs nutrients beyond the reach of roots.	Summer: plants absent. Fungi survives in the dead root fragments or as large spores. Fungi are dormant when soil is dry.	After rain VAM germinates from dead root fragments and spores-colonising new roots.

There are about 150 species of the fungi, which may have small preferences for different soil types and environments, but in general they are all capable of colonising roots of all susceptible plants (see Table 1 for susceptible/host crops), which is an important factor in their management.

Some plants, especially trees like Eucalypts, orchids and some heathland shrubs, have a different type of symbiotic fungus which works in a similar way to VAM.  If you are interested in revegetation you also need to consider the range of different beneficial fungi that may be important.

Can I see VAM in soil or roots?

VAMs cannot be seen unless the root is stained and viewed under a microscope.  The VAM fungi do not cause any disease, so there is no discolouration or root distortion.  This makes it difficult to determine whether they are present in the roots or the soil.  However, the chances are they will be there and working to improve the nutrient uptake of your crops and the stability of your soil. 

Lack of VAMs will reduce plant growth, but this again may be hard to determine in a paddock situation.

Figure :
Mycorrhizal (VAMs) and phophate responses in low phosphorus soil.

How does the fungus-plant relationship work?

VAMs extend the plant root system and the whole mycorrhiza (fungus plus plant) can exploit the soil nutrients much more effectively than the plant alone.

Some plant nutrients, such as phosphorus (P) and zinc (Zn), move very slowly in the soil solution.  Therefore, when a plant removes these nutrients from the soil near the root, there can be a delay before they are replaced at the root surface.  A zone of nutrient depletion may occur near the root slowing down plant nutrient uptake.

The fungi grow out into the soil, sometimes several centimetres from the root and pick up nutrients at a distance where they are still readily available.  The fungal strands (hyphae) then transport the nutrients quickly back to the plant – a kind of rapid transit system - overcoming the slow movement in the soil.  Tolerance to drought can be increased as the rapid transit system overcomes slow movement of nutrients in dry soil.  There is no good evidence that the fungi actually transport water.

Additionally, the hyphae are very, very narrow (only about 10 millionths of a metre across, or less).  This means that they have a huge surface area for nutrient absorption and they can squeeze into soil pores that are not accessible to roots which will be 10 times, or more, the width of a VAM fungal hypha.

VAM hyphae growing out of the roots bind soil particles together, like a ‘sticky string bag’.  This improves soil stability and can help to prevent erosion.

The benefits do not come absolutely free, because the fungus needs sugars provided by the plant.  Under most conditions, the plant produces sugars to spare, so the ‘cost’ of supporting the fungi is well invested.  This results in enhanced nutrient uptake and more effective use of fertilisers.

Do all plants host VAM fungi?

About 80% of plant species, including many important crops, do form VAM.  Table 1 lists common crops and whether they are host or non-hosts.

Some important non-hosts which never form VAM are canola and other members of the cabbage family, lupins and beets (see Table 1). 

Other families of crop plants do host the fungi, but the degree to which they respond to the symbiosis is variable and often relates to the speed of root growth and development of root hairs by the plant and to soil conditions, particularly nutrient levels.

A knowledge of which crops are non-hosts and which are highly responsive could help improve crop productivity, especially in soils with low nutrient availability.  Ideally, highly responsive crops should not follow non-hosts.

Lack of response does not mean that the beneficial fungi are absent.  VAMs will continue to multiply in all host crops regardless of the crop’s responsiveness.  This can have positive benefits for a responsive crop later in the rotation.

Table 1 provides a rough guide to crop responsiveness, but it is important to note that there will be variations with cultivar and soil conditions.

How are VAM affected by soil conditions?

To grow and reproduce, VAM fungi need living plants which are hosts.  However, they are adapted to survive as ‘resting stages’ in most soil types and conditions around the world, including hot and dry, wet and frozen soils.

They are present in soils of all textures, from sandy soils to those with a high clay content and are also present at a wide range of soil pH.

A mixture of species is usually present, adapted to the local conditions.

The spores and infective root fragments can survive very well in hot conditions as long as the soil is dry, which is important for cropping in Mediterranean climates, like South Australia.  Spores will become active in moist conditions, but if host plants are absent, they will die.  False breaks may reduce, but certainly not eliminate, colonisation when the crop finally gets going.

VAM do not use soil organic matter as a food source.  Different species can associate with all host plant species (but not the non-hosts, of course).  Host plants will provide sugars for the fungi and so help to maintain populations.

Rotations

Rotations that include either long, bare fallow (especially when the soil is wet) or non-hosts will reduce VAM populations.

The effect of bare fallow has been shown by research in Queensland, where ‘Long Fallow Disorder’ has been found to be caused by low populations of VAM.

This is because in warm moist soils without plants, the VAM spores germinate, but as they cannot find a plant, they die.  If fallow persists for 12 months or longer, the VAM spores can effectively be wiped out.

In South Australia, long fallows are not used and often the soil is dry in the summer, so germination does not occur and problems are much less likely.

Non-host crops, like canola, also reduce VAM populations and the amount of VAM in the roots of the following crops.  At present is seems that one year of canola will not create a major problem, but if several years of canola or mustards are grown for soil fumigation, then the VAM will be reduced, together with the disease organisms.

Tillage

Conventional tillage and other soil disturbance has a negative impact on VAM function.  It breaks up fungal threads in soil and destroys their connections with the plant so that they cannot work to increase uptake of nutrients.

Soil compaction

This not only reduces root growth, but reduces the benefits of VAM.  Research is in progress to find out how the fungal threads grow through compacted soil and whether some fungi are able to do this better than others.

Fertiliser

High fertiliser applications, especially phosphorus, reduce the plant’s need for VAM and can reduce the fungal populations too.  The effect varies with the responsiveness of the crop.  Wheat essentially loses its VAM partner when fertilisers are high, but peas, beans and many pasture legumes may still have the VAM and benefit from them, but to a lesser degree.

Pesticides and soil fumigants

Some fungicides, if they get into the soil, will reduce VAM populations.

Most herbicides do not seem to have a direct chemical effect on VAM, but they do kill the plants and, therefore, reduce the living food source of the VAM fungi.

Soil fumigants eliminate all soil biota, including VAM. This can be a problem in horticulture, especially if the crop is particularly responsive to VAM.

Stubble management

Retaining stubble will return nutrients to the soil and the VAM will help to take these directly to the plants.  Stubble burning kills VAM, especially hot burns.  Some research has shown that burning stubble from a peanut crop reduced the percentage of the root length of the next crop from 72% to 16%.  Taking into account differences in the crop growth, this translated to a reduction of VAM-colonised roots from 12 metres per plant, to 1.5 metres per plant.

Organic management has been shown to increase VAM populations in the roots of crops.

Do VAM interact with other soil organisms?

VAMs compete with other members of the soil biota for soil nutrients and, hence, increase the competitive ability of their host plants.

They increase nodulation and nitrogen fixation in legumes by supplying the phosphate that is essential for effective nodulation.

VAM can increase the tolerance of plants to some diseases and pests by compensating for root damage and may even have direct negative effects on the disease-causing organisms themselves.

Some soil animals graze on VAM hyphae and spores, but unless the populations are very high and out of balance the grazing may actually help to keep the fungi young and vigorous and release nutrients from the dead hyphae.

4.3 VAM - the beneficial fungi that feed plants

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