An interview with Dr. Ravnskov of the Aarhus University, Denmark.
Dr. Sabine Ravnskov is Associate professor at the Department of Agroecology, Aarhus University in Denmark. Her research covers microbial interactions in soil and roots, with special emphasis on Arbuscular Mycorrhiza (AM) and pathogen interactions in relation to plant health. This research involves both fundamental and applied approaches. Her fundamental research approach is to reveal mechanisms behind the antagonistic potential of AM fungi against plant pathogens. The more applied perspective is to use this knowledge in the development of IPM strategies. We decided to interview Dr. Ravnskov for a larger audience because her presentation at the European Root Health Forum 2012 on the effect of AM fungi on roots – and thereby on plant health – gained a lot of interest from the participants. Also on the Root Health Network on LinkedIn, questions about the role of AM in root health have been raised in various discussions.
What makes AM fungi such a special partner for plants?
AM fungi are symbiotic fungi associated with almost all plant species (80-90%) among the major crops, with the exception of plants of the genus Brassicaceae (e.g. canola). They can be found in all soils. AM fungi have co-evolved with plants for 450 million years and therefore developed a very specific symbiotic interaction with plants. Once they have established a symbiotic relationship with a plant, these fungi help the plant to take up nutrients (especially P), increase the plant’s stress tolerance, and impact the microbial community of the rhizosphere. I call this function of AM for the plant an “ecosystem service.”
What can AM add to root health in a modern farming system?
If root health is defined as the absence of a disease, AM can increase plant tolerance against diseases and thereby root health. Arbuscular Mycorrhiza also has an impact on the composition of the microbial community in the root system. With respect to nutrient uptake, AM enhances, amongst many others, the uptake of phosphorus (P) – yet the P uptake aspect is best known and most studied. It was thought for a long time that the high P content in modern farming systems with high P input and availability has a negative effect on the function of AM in terms of P transport to the plant. It has now been found that – although mycorrhization often is reduced under these conditions – the symbiosis is still active and supports plant P uptake.
How does a mycorrhization impact the plants morphology?
The mycorrhization of plants is not always correlated to growth enhancement and can even result in reduced plant growth. It was thought that this growth reduction effect was due to the fact that, during the establishment of the symbiotic relationship, AM fungi were consuming plant carbon without providing phosphorus to the plant. However, in more recent studies, plants did show significant uptake of P from the AM fungus pathway, although plant growth was reduced during this vegetative stage. These results underline that the AM symbiosis is an integrated part of plant P uptake irrespective of the growth of the plant. (Most studies in the past were conducted on plants at the vegetative growth stages) More studies focusing on functional aspects of AM up to harvest are needed to more clearly understand the role of AM in plant production. At Aarhus University, AM influence on yield and quality of yield of different crops grown under abiotic and biotic stress conditions are studied in on-going experiments. Another morphological change induced by AM is that mycorrhizal roots are less branched and thicker because the hyphae of the fungus are replacing the finer roots for nutrient uptake and soil exploration. We have shown in greenhouse experiments that the amount of P required by a mycorrhized plant to obtain similar growth and yield as compared to a plant without mycorrhization could be reduced from 40 ppm to 20 ppm. Reduction of P fertilization, which I think is currently overused in many cases, will become of increasing importance for agriculture in the future since P is a limited commodity.
Is the amount of AM in the soil sufficient or can you envisage artificial inoculations of the soil with AM? Where would a step like this make sense?
When plants are directly drilled into field soil, an artificial inoculation is not needed for the predominant agricultural soils of Europe and North America. Inoculation with AM fungi can be beneficial when polluted soils are recultivated (e.g. mining areas) or for high value crops pre-grown in the greenhouse in artificial growth media before being transplanted to the field. This pre-inoculation of transplants in the greenhouse ensures that a pre-grown plant has sufficient mycorrhizal functioning in relation to plant nutrient uptake and increased tolerance against abiotic and biotic stress. For some crops, pre-inoculation with AM fungi can stimulate plant growth and thereby shorten the time to harvest of this crop.
How long (in days) does it take a seedling to develop a symbiosis with AM after planting?
The interaction between AM fungi and a seedling is established early in the germinating roots. A functional successful mycorrhization is already established a few weeks after planting. The time until successful mycorrhization depends to some extend on the plant species. It also depends on the proximity of AM inoculum to the seedling. We have conducted some greenhouse experiments in which we placed the AM fungi directly next to the seed, as well as with one (1) and two (2) cm distances to the seed. The closer the AM fungal inoculum was to the seed, the more the growth response of the plant. The response was delayed when the inoculum was placed in one (1) cm distance. No growth stimulation was seen when inoculum had been placed with two (2) cm distance to the seed.
How can AM defend the plant against diseases or reduce their impact?
AM can definitely increase plant tolerance against several root diseases. Yet although I have been working on this topic for 10-15 years, we still don’t know exactly how it works. There are different theories on how AM fungi can impact disease development. One is that the AM-stimulated changes in the plant’s morphology and growth pattern stimulated by AM have an impact on the ecological niche for the pathogen, and thereby for the incidence of disease. A second theory is that AM induce an increase of secondary metabolites in the plants and thereby defend the plant against root pathogens. Another possible mechanism is direct competition for carbon hydrates between AM fungi and the pathogen. The fact that AM fungi change the root exudation pattern can also have an impact on the pathogen. Finally, a theory based on the fact that mycorrhizal roots and hyphae are associated with a special bacteria community is that these bacteria play a role in the increased tolerance against diseases in AM plants. At Aarhus University, we have a collection of bacteria isolated from roots with mycorrhizal structures. More
of these bacteria have shown potential to control root diseases, indicating that these bacteria could play a key role in the antagonistic potential of AM against pathogens.
Which mechanism do you think is the most important one for disease reduction?
In my opinion, the mechanism underlying the increased tolerance of AM plants against pathogens could be the combination of several of these mechanisms. This pattern of mechanisms is adjusted depending on the abiotic and biotic environmental conditions in which the plants are grown. The theory of AM-induced plant resistance has been dominating in the literature; however in a study with clover, Pythium and different AM fungal species, we found that plant production of secondary metabolites depended on the combination of plant variety and AM fungal species. This result was not correlated with disease control. Furthermore, most of the known defense compounds in clover were down regulated and not up regulated by AM fungi. It seems difficult to draw general conclusions on this. Many pathogens are adapted to a certain plant species, whereas AM fungi are generalists and not plant species specific. Although the AM symbiosis can be established in most combinations of AM plant species and AM fungi, the function of the symbiosis depends on the combination of species in the symbiosis. Consequently, this is a rather academic issue, as in the field AM plants will always be colonized with several AM fungi. In a study with pea grown in different field soils, we found that the pea plants naturally were colonized with 26 AM fungal taxa at the same time.
How can this knowledge be transferred to the field?
As we are interested in how a beneficial AM symbiosis can be established in high value crops pre-grown in the greenhouse, we are currently running some field trials in which we investigate the influence of AM fungal inoculation of transplants on yield and quality of yield for onion and lettuce grown with different agrochemical spraying programs (conventional and reduced). In these crops, especially airborne diseases in early August cause yield losses. The fact that crops already are mycorrhizal when transplanted to the field may shorten time to harvest and help to avoid the disease season for these crops by an earlier harvest. Parameters that will be further looked at in these studies include disease severity, yield quantity and quality, nutrient and vitamin content of crops and AM fungal colonization.
What other root health aspects can AM impact?
Besides influencing plant nutrient uptake and plant growth and tolerance against pathogens, mycorrhiza is also known to enhance stress resistance of plants against abiotic stress – for example towards drought stress. We are currently looking at this aspect in wheat, studying the effect of mycorrhization on wheat varieties grown under drought stress conditions.
Dr. Sabine Ravnskov was interviewed by Dr. Melanie Goll, Technical Innovation Manager, Syngenta. Melanie holds a Ph.D. in plant pathology and focuses on seed treatment development, especially fungicides and nematicides.