Institute of Plant Protection BOKU Vienna

Mycorrhiza

Research groups

Phytopathology

Regulatory Mechanisms of Mycorrhization

Arbuscular mycorrhizal fungi (AMF) are symbiotic soil fungi which colonize roots of about 80% of vascular plants. The mycorrhizal symbiosis enhances the growth and survival of numerous plant species. The complex cellular relationship between the host roots and AMF requires a continuous exchange of signals, which in return affects the regulation of genes whose products participate in metabolic and structural changes that lead to the symbiosis.

The perception of the AMF by the plant before appressoria formation

More and more data about the presence of AM fungal signals perceived by the root before penetration or even appressoria formation are accumulated. Non-host plants are an interesting system to study the presence of AM fungal signals. Roots of plants of the AM non-host famililies such as the Brassicacea are not penetrated by AMF and in general no AM fungal structures are attached to the root surface, thus any changes in the root in presence of AMF should be due to diffusible AM derived signals. In roots of Brassicacea plants the activities of the two hydrolases ß-1,3-glucanase and chitinase and glucosinolate levels were altered when plants were inoculated with AMF and inoculation of the AM non-host plant Salsola kali with an AMF initiated resistance events in the roots.
To study the release of elicitor factors by AM fungal tissue roots have been challenged with spores and hyphal fragments and changes in the roots were studied. Interestingly in presence of a spore exstract of G. intraradices in Ri T-DNA-transformed tomato roots new polypeptides appeared. Application of spores and hyphal fragments of G. intraradices to roots of alfalfa resulted in alterations of the accumulation pattern of several flavonoids.
The biological function of these changes remains unclear, however, they point towards the presence of AM fungal-derived signals sensed by plants before appressoria formation. The non-specific character of these factors is suggested as even AM nonhost plants seem to sense the presence of an AMF. A so-called "Myc-factor" (analog to the Nod factor known from rhizobia) recently has been proposed.
Due to the inability to culture AMF axenically and the lack of a clear biological indicator in the plant, studies on the presence of a possible "Myc factor" are difficult, however, "the Myc factor" is more and more a matter of intensive studies and recent studies give promising new insight in understanding early signaling events leading to mycorrhization.

The root colonization by AMF
The regulation of AMF root penetration by the host plant

Depending on the P-availibility the AMF host status of a plant can vary. In general, in plants with a high P-status root colonization is reduced or absent, whereas low P-levels enhance root colonization. The exact mechanisms involved are still unknown, however, in several works a decrease of the cell membrane permeability in plants grown at high P-conditions resulting in a lower leakage of amino acids and sugars has been linked with a lower root colonization.
There seems a clear difference in the biological activity of root exudates from high P and low P plants. Whereas root exudates of low P plants stimulate hyphal branching, hyphal growth and root colonization, root exudates of high P plants show a reduced stimulation of hyphal growth and no stimulation of root colonization.
To summarize, depending on the P-status plants possibly have developped different strategies to regulate AM root colonization. A P-status dependent accumulation of secondary plant compounds in roots might translate in an altered root exudation. Plants with a high P-status, when challenged with AMF derived signals, can accumulate secondary plant compounds inhibitory to AMF, thus avoiding the costs of the symbiosis without P-nutritional profit, whereas in P-deficient plants, in absence and in presence of AMF derived signals, secondary plant compounds can be accumulated which stimulate root colonization, thus, improving the P-status of the plant via the P provided by the fungus.

The AMF gets established and the mature AM association

In host-pathogen interactions a range of defense mechanisms is activated in response to microbial attack. There is some information about defense responses during penetration of AMF in the root, the internal growth of the AMF in the root and the development of arbuscules, however, few data are available about the mature mycorrhizal association.
In roots of mature AM associations autofluorescing spots have been observed which have been identified as collapsed arbuscules. It has been hypothesized that the autofluorecence can be attributed to a different perception of the collapsing arbuscules by the plant. Whereas the active arbuscule possibly masks its presence in the root cell or blocks a reaction of the plant and thus in terms of activation of plant defense mechanisms remains unperceived by the plant, fungal cell-wall components released by the collapsing, disintegrating arbuscule could elicit plant defense responses e.g. the accumulation of phenolics. Phenolic compounds accumulating in plant cells as a plant defense response against microorganisms are known to fluoresce and thus might be responsible for the autofluorescence of collapsing arbuscules.
Flavonoids play a significant role in how plants interact with organisms in the environment and have been proposed as regulatory compounds in the AM symbiosis. Recently it has been reported that flavonoid levels in mycorrhizal plants are modulated by the developmental stage of the AM symbiosis. Whereas during root penetration and the establishment of the AMF in the root intermediate levels of a number of flavonoids were detected, at a late stage of root colonization characterized by viable arbuscules and abundant collapsed arbuscules, high levels of flavonoids such as the phytoalexin medicarpin could be detected in the root.
To summarize, although there are abundant data about the arbuscule as the "nutrition" organ in the AM symbiosis further studies are needed on the possible role of this organ apart from the nutritional aspect, such as the involvement in the regulation of mycorrhization in the mature association.

Autoregulation of mycorrhization

Legumes can form a symbiosis not only with AMF, but also with nodule-forming, N-fixing rhizobial bacteria. From the plant's perspective, the development of the rhizobial association is a beneficial, but also costly process. Thus legumes developed regulatory mechanisms to control the number of nodule. Already existing nodules systemically inhibit subsequent nodulation in other parts of the root system. This feedback control is termed autoregulation.
To limit energy expense of the host a similar mechanism seems plausible in the AM symbiosis. In split-root systems of barley it has been observed that once AMF have colonized roots on one half of a split root system, further root colonization by an AMF on the other half of the split root system is reduced. The mechanism is unspecific, that means, pre-colonization of one half of a split-root system with G. mosseae, G. intraradices or Gi. rosea suppressed subsequent colonization on the other half of the split-root system by G. mosseae.
In another study it has been shown that in a split-root system pre-colonized on one side with an AMF, the suppressional effect on AMF root colonization is increasing with the degree of root colonization. The degree of root colonization was linked with the period plants were inoculated. Barley plants which were inoculated earlier showed a higher degree of root colonization than plants which were inoculated inoculated later.
The appearance of autofluorescing collapsed arbuscules in the mature AM association has been discussed above (see chapter: The AMF gets established and the mature AM association). As an autoregulatory effect in mycorrhizal plants is only observed at high colonization levels that means at a later stage of the symbiosis, further root colonization of a mycorrhizal plant might not be suppressed depending on high levels of root colonization, but when arbuscules start to collapse and thus activate defense responses of the plant thus reducing further root colonization by AMF.

Autoregulation of mycorrhization and nodulation

Growing evidence is brought forward that a number of processes in the rhizobial and mycorrhizal association are similarly regulated. Autoregulation of mycorrhization and nodulation also seems to share some common features. Autoregulation of nodulation occurs only after a critical number of nodules is formed on the root system. Similarly in split-root systems of barley a clear suppressional effect on further root colonization in mycorrhizal barley plants was only observed when one side of the split-root system was heavily mycorrhizal, but not at lower colonizati on levels.
Recent results show other interesting parallels in the autoregulation of both symbioses. It has been reported that in alfalfa plants an established mycorrhizal association systemically suppresses not only further mycorrhization but also nodulation. Vice versa, nodulation on one side of a split-root system systemically suppresses later mycorrhization on the other side of the split-root system. These results point towards a similar regulatory mechanism in the rhizobial and the arbuscular mycorrhizal association suppressing either nodulation or mycorrhization.

The root exudation of mycorrhizal plants

There is abundant information on the stimulatory effect of root exudates from non-mycorrhizal plants on different stages during the development of the AM symbiosis. Recently root exudates of tomato mutants resistant to AMF colonization (Myc- phenotyp) showed an inhibitory effect on spore germination demonstrating that root exudates can be important factors in the control of colonization of host plants by AMF.
The mechanisms regulating the mycorrhizal autoregulation are still unclear, however, recently it has been suggested that an altered root exudation might be at least partially be responsible for the altered susceptibility of mycorrhizal plants to further root colonization by AMF.
Root exudates of mycorrhizal plants have been reported to exhibit a different effect on AMF than root exudates from non-mycorrhizal roots. Root exudates of non-mycorrhizal plants stimulate hyphal growth, however, no stimulatory effect on hyphal growth could be detected when root exudates of mycorrhizal plants where tested.
Alteration of the exudation pattern seem not to be limited to mycorrhizal roots, but, through a plant-mediated mechanism, occur also in non-mycorrhizal roots of a mycorrhizal plant. When the effect of root exudates of a split-root sytem of cucumber, with one side mycorrhizal and the other side non-mycorrhizal, on root colonization was tested, the exudates from both sides exhibited a similar inhibitory effect showing that changes of the exudation pattern in mycorrhizal plants are systemic, that means not limited to mycorrhizal root (Vierheilig et al. 2003).
All these data indicate alterations of the root exudation pattern in mycorrhizal plants, which have an effect on the microbial population including AMF around mycorrhizal roots. An analysis of root exudates from mycorrhizal plants and non-mycorrhizal plants to identify the compounds involved in the differing effect of these exudates seems most interesting.

Perspectives

While the existence of a "Myc factor" is an exciting finding which needs further studies to elucidate the character of the involved signal and its significance for the establishment of the symbiosis, the mechanism regulating further root colonization by AMF in already mycorrhizal plants could be interesting in another context.
There are abundant data about a bioprotective effect of mycorrhizal plants on soilborne fungal pathogens. The mechanisms involved are still a matter of debate, however, it is generally accepted that a well established AM symbiosis, characterized by the formation of arbuscules, is a pre-requisite for a mycorrhizal-induced increase in resistance. This reminds of the autoregulatory mechanism of mycorrhization discussed above, also depending on a well established AM symbiosis.
Moreover, looking at the alterations in the root exudation of a mycorrhizal plant it is tempting to speculate that not only the susceptibility of the plant to AMF but also to other soilborne fungi is affected. These data are in line with the hypothesis suggesting that plants colonized by AMF while trying to limit their costs of the AM symbiose also aquired bioprotection against pathogenic fungi. It seems plausible that an already mycorrhizal plant develops only one mechanism to repulse colonization by fungi, not discriminating between AM fungi and soil borne pathogenic fungi. Although the presented data do point towards "one mechanism, two symptoms" for autoregulation and bioprotection in mycorrhizal plants, further studies are needed to test the validity of the presented hypothesis.