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The Mycorrhizal Fungus - A Lifelong Partner Of The Grapevine
André Meyer
Key words: mycorrhizae, grapevines, fungus, root, colonisation, soil, hyphae.
Introduction
Arbuscular mycorrhizal (AM) fungi are an exceptional group of beneficial fungi. They are present in most soils and are capable of forming associations with the root systems of the great majority of vascular plant species, where they act as bioregulators and protectors, and facilitate the uptake of mineral nutrients (Lovato et al., 1996). This article concerns the relationship between AM fungi and the grapevine, a topic that has received little attention in the popular viticultural press to date.
Fungi have different forms and behaviour patterns. Some digest decaying wood, or organic material in the soil and produce conspicuous fruiting structures. Others, including the arbuscular mycorrhizal (AM) fungi are microscopic and can only be examined properly through a microscope after being stained with special dyes. All AM fungi are soil-borne and are most abundant in that region of the soil which immediately surrounds the roots. This is referred to as the rhizosphere or, sometimes, as the mycorrhizosphere. The AM fungi form networks in the rhizosphere and also extend into the roots. Since AM fungi are unable to grow and reproduce in regions of the soil where host plant roots are absent, they are considered to be obligate biotrophs. Arbuscular mycorrhizal fungi are nevertheless capable of surviving in the bulk soil as dormant spores, germinating if a root moves into their vicinity.
In mycorrhizal relationships, the fungus acts as an extension of the host’s root system, greatly facilitating the uptake of slowly diffusing soil nutrients, such as phosphorus (P), as well as water. This uptake is made possible by the very extensive nature of the network of nutrient detecting thread-like filaments of which the fungus is composed. In return, the plant provides the fungus with a congenial environment rich in food materials, notably carbon compounds produced during photosynthesis, which the fungus cannot produce for itself. This is commonly referred to as a bi-directional transfer of nutrients (Smith et al., 1994). The essence of the symbiotic relationship is that both host and fungus benefit.
Grapevines have much coarser root systems than annual crops and so are more likely to benefit from a mutualistic relationship with AM fungi (Baylis, 1970; Crush, 1973; Linderman, 1988). It is therefore not surprising that AM are commonly encountered when field-grown vine roots are studied under the microscope. In fact, vine roots, in common with roots in general, exude chemical compounds that increase host susceptibility to colonisation by AM fungi. Production of such exudates varies according to the time of the season and to the host’s growth stage. Consequently, grapevines vary in terms of their ability to attract specific AM fungal species with season and growth stage. Single plants may host a number of AM fungal species at the same time.
Process of establishment
Since AM fungi live partially within the roots of grapevines, they must first establish themselves within the root tissue before they can render any benefit to the host. There are two main phases in the establishment of a functional symbiosis between AM fungi and grapevines, namely, an external phase (in the soil) and an internal phase (inside the grapevine’s root tissue) (Fig. 2).
External phase: In the external phase, the fungus produces thread-like filaments, called hyphae. Hyphae have various morphologies and functions. Some are fertile, others absorptive, and some are infective. Collectively, the vast network of hyphae in the soil around the root is known as the mycelium. Absorptive hyphae are extremely thin, in some cases only one fifth of the diameter of a root hair. This characteristic enables absorptive hyphae to penetrate soil pores that are inaccessible to even the finest roots. The overall volume of soil that is exploited for nutrients and water by a mycorrhizal root system is therefore considerably greater than that which is accessible to a non-mycorrhizal root system. Fertile hyphae bear spores which become distributed through the body of the soil. The spores, which are usually spherical, thick-walled, resting structures, germinate under suitable conditions to produce the thick infective hyphae which are responsible for root colonisation.
A sequence of recognition events leads up to the morphological and physiological integration of the partners in the symbiosis (Nagahashi & Douds, 1997). Infective hyphae are apparently induced to grow towards young, actively growing grapevine roots by a kind of sensing mechanism. These infective hyphae may originate from older roots that are already infected, from spores that are attached to infected roots, or from isolated, detached spores in the soil. As a rule, hyphae only colonise fine roots, not those that have become woody.
As soon as contact is made with the root epidermal layer the hypha grows along the surface of the root for a short distance before enlarging to form an organ known as an appressorium between adjacent epidermal cells. Numerous such infection points usually form on the surface of a single root.
Internal phase: From the appressorium, the hypha penetrates the root surface and spreads through the root tissue. Penetration of the root tissue marks the beginning of the internal phase. How it does so depends on the anatomy of the root, and on the fungal species involved. In the case of Arum-type spreading the hypha enters the root between epidermal cells (the entry point) and spreads by growing along the air channels between the cells (the intercellular spaces). In contrast, in Paris-type spreading, the hypha directly penetrates the outer (epidermal) cell and spreads through the underlying cortex by passing through, rather than around, the cells. This intracellularly type of spreading pattern results in the characteristic looped arrangement of hyphae shown in Fig. 3).
As spreading progresses, various structures are formed by the AM fungus. These include vesicles, arbuscules, and, in exceptional cases, spores (Fig. 4). Vesicles are balloon-like structures and are mainly responsible for the storage of such plant metabolites as lipids. Vesicles may form within, or between, the host root cells. A major sub-group of AM fungi however, does not form vesicles inside root tissue. Instead, they form vesicle-like structures outside the root in the soil. These external vesicles form clustered swellings on external soil hyphae and are often ornamented with spines or knobs. They are believed to have a similar function to that of the normal vesicles, except that they form part of the external phase. Arbuscular mycorrhizal fungi also form tree-shaped or bush-like structures. These are specialised organs which play a highly active role in the exchange of materials between the fungus and the host cell. These exchange structures (arbuscules) develop inside root cortical cells and link with the external absorptive hyphae through which soil nutrients are transferred to the host.
Beneficial effects
Protection against disease: Young vines are highly susceptible to infection by pathogenic fungi. The AM fungus competes with pathogenic fungi for infection sites on the grapevine root surface. Since the AM fungi are highly competitive, pathogens are largely excluded by this process (Vigo et al., 2000). Although fungal pathogens compete with AM fungi for carbon and other nutrient sources (Graham, 2001), the AM fungi usually outperform the pathogen. By these means, and without any form of outside assistance, AM fungi are capable of being highly effective biological control agents (Vigo et al., 2000). In cases where AM fungi do not provide complete control over a specific disease, they nevertheless contribute to the defence of the host by engaging in a combined effort with other biological control agents (Nemec, 1997), sometimes including such other symbionts as root nodulating bacteria (Dar et al., 1997). Arbuscular mycorrhizal fungi may also produce chemical compounds with antimicrobial properties (toxins) that prevent other, unwanted, fungi from entering the roots, thereby ensuring their own establishment within the host.
Improving soil structure and aggregate stability: The vast network of hyphae produced by the AM fungus, contributes to the structure and stability of agricultural soils (Bearden & Peterson, 2000). Glomalin, an insoluble glycoprotein produced by AM fungal hyphae, is particularly important for the aggregation and stabilization of soils (Wright et al., 1999) since it has the ability to bind soil particles together. The integrity of the delicate soil hyphal network is, however, extremely sensitive to any form of disruption, of which the most violent is mechanical cultivation, such as ploughing. Disruption or detachment due to soil disturbances is particularly severe in the topsoil where cultivation-induced reductions in spore density may be expected (Kabir et al., 1998). Thus, if the abundance of AM fungi is to be maintained, damaging cultivation management practices must be avoided. Where possible, zero tillage should be encouraged, rather than conventional tillage practices.
Applying organic amendments to soil: By improving the soil organic content, the number of spores may be considerably increased (Cuenca et al., 1998). Application of organic amendments, such as compost and manure, has been shown to positively affect spore numbers in soil (Douds et al., 1997). Also, AM fungi may have direct access to organic P through the production of specific P-releasing enzymes, the so-called extracellular phosphatases, by the hyphae (Koide & Kabir, 2000).
Sowing cover crops in vineyards: Cover crops play host to a wide range of AM fungi. Once sown, they set in motion a vast underground process of hyphal growth and spore production (Fig. 5). Under these circumstances, pieces of infected root, spores and hyphae that have survived in the soil from the preceding season all act as propagules or sources of inoculum for the cover crop. In addition, because the root systems of the cover crop and grapevine become intertwined, fresh supplies of inoculum are passed on to grapevine.
Conclusions
Arbuscular mycorrhizal fungi are wholly natural components of the micro ecology of vineyard soils. They have the ability to improve nutrition and root resistance to attack by pathogens, and also contribute positively to soil structure and stability. The beneficial effects of AM fungi in vineyards are nevertheless largely unappreciated at present. Recognition of their importance as components of healthy soils, and the implementation of practices that favour their propagation, are important aspects of vineyard management.
For further information contact Andre Meyer at tel. +27 21 809 3100, fax +27 21 809 3400, E-mail: meyera@arc.agric.za
LITERATURE CITED
Baylis, G.T.S., 1970. Root hairs and phycomycetous mycorrhizas in Phosphorous deficient soils. Plant Soil 33, 713-716.
Bearden, B.N. & Peterson, L., 2000. Influence of arbuscular mycorrhizal fungi on soil structure and aggregate stability of a vertisol. Plant Soil 218, 173-183.
Brundrett, M., 2000 "Vesicular-Arbuscular Mycorrhizas" Section 3. Arbuscular Mycorrhizas. 21 June. http://www.ffp.csiro.au/research/mycorrhiza/vam.html, 28 November 2001.
Crush, J.R., 1973. The effect of Rhizophagus tenius mycorrhizas on ryegrass, cocksfoot and sweet vernal. New Pathol. 72, 965-973.
Cuenca, G., De Andrade, Z. & Escalante, G., 1998. Diversity of Glomalean spores from natural, disturbed and revegetated communities growing on nutrient-poor tropical soils. Soil Biol. Biochem. 30, 711-719.
Dar, G.H., Zargar, M.Y. & Beigh, G.M., 1997. Biocontrol of Fusarium Root Rot in the common bean (Phaseolus vulgaris L.) by using symbiotic Glomus mosseae and Rhizobium leguminosarum. Micro. Ecol. 34, 74-80.
Douds, D.D., Galvez, L., Franke-Snyder, M., Reider, C. & Drinkwater, L.E., 1997. Effect of compost addition and crop rotation point upon VAM fungi. Agric. Ecosyst. Environ. 65, 257-266.
Graham, J.H., 2001. What do pathogens see in mycorrhizas? New Phytol. 149, 357-359.
Kabir, Z., O’Halloran, I.P., Widden, P. & Hamel, C., 1998. Vertical distribution of arbuscular mycorrhizal fungi under corn (Zea mays L.) in no-tillage and conventional tillage systems. Mycorrhiza 8, 53-55.
Koide, R.T. & Kabir, Z., 2000. Extraradical hyphae of the mycorrhizal fungus Glomus intraradices can hydrolyse organic phosphate. New Phytol. 148, 511-517.
Linderman, R.G., 1988. Mycorrhizal interactions with rhizosphere microflora: The mycorrhizosphere effect. Phytopathol. 78, 366-371.
Lovato, P.E., Gianinazzi-pearson, V., Trouvelot, A. & Gianinazzi, S., 1996. The state of art of mycorrhizas and micropropagation. Advances in Horticultural Science 10: 46-52.
Nagahashi, G. & Douds Jr, D.D., 1997. Appressorium formation by AM fungi on isolated cell walls of carrot roots. New Phytol. 136, 299-304.
Nemec, S., 1997. Longevity of microbial biocontrol agents in a planting mix amended with Glomus intraradices. Biocontrol Sci. Technol. 7, 183-192.
Smith, S.E., Gianinazzi-Pearson, V., Koide, R. & Cairney, J.W.G., 1994. Nutrient transport in mycorrhizas: structure, physiology and consequences for efficiency of the symbiosis. Plant Soil 159, 103-113.
Vigo, C., Norman, J.R. & Hooker, J.E., 2000. Biocontrol of the pathogen Phytophthora parasitica by arbuscular mycorrhizal fungi is a consequence of effects on infection loci. Plant Pathol. 49, 509-514.
Wright, S.F., Starr, J.L. & Paltineanu, I.C., 1999. Changes in aggregate stability and concentration of glomalin during tillage management transition. Soil Sci. Soc. Am. J. 63, 1825-1829.
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