Impact of Arbuscular Mycorrhizal Symbiosis on Plant Response to Biotic Stress: The Role of Plant Defence Mechanisms
Arbuscular mycorrhizal associations imply a remarkable reprogramming of functions in both plant and fungal symbionts. The consequent alterations on plant physiology have a clear impact on the plant response to biotic stresses. In this chapter we discuss the effects of the mycorrhizal symbiosis on plant susceptibility/resistance to potential deleterious organisms, including root and shoot pathogens, root parasitic plants and phytophagous insects, highlighting the mechanisms that may be operating in each particular case. Special attention is given to the modulation of plant defence responses in mycorrhizal systems, as it may affect all interactions. Finally we focus on the priming of jasmonate regulated plant defence mechanisms that seem to mediate the induction of resistance by arbuscular mycorrhizas. KeywordsBiotic stress-Bioprotection-Induced resistance-Priming-Plant defence-Biocontrol-Defence signalling-Jasmonates-Pathogens-Insects
H. Koltai and Y. Kapulnik (eds.), Arbuscular Mycorrhizas: Physiology and Function,
DOI 10.1007/978-90-481-9489-6_9, © Springer Science+Business Media B.V. 2010
Abstract Arbuscular mycorrhizal associations imply a remarkable reprogramming
of functions in both plant and fungal symbionts. The consequent alterations on plant
physiology have a clear impact on the plant response to biotic stresses. In this chap-
ter we discuss the effects of the mycorrhizal symbiosis on plant susceptibility/resis-
tance to potential deleterious organisms, including root and shoot pathogens, root
parasitic plants and phytophagous insects, highlighting the mechanisms that may
be operating in each particular case. Special attention is given to the modulation of
plant defence responses in mycorrhizal systems, as it may affect all interactions.
Finally we focus on the priming of jasmonate regulated plant defence mechanisms
that seem to mediate the induction of resistance by arbuscular mycorrhizas.
Keywords Biotic stress
Induced resistance • Priming
AM Arbuscular mycorrhiza
AMF Arbuscular mycorrhizal fungi
MIR mycorrhiza-induced resistance
SA Salicylic acid
Nm Non-mycorrhizal plants
Gm Glomus mosseae colonized plants
Hpi hours post inoculation
M.J. Pozo (*), S.C. Jung, J.A. López-Ráez, and C. Azcón-Aguilar
Department of Soil Microbiology and Symbiotic Systems, Estación Experimental
del Zaidín, CSIC, Prof. Albareda 1, 18008 Granada, Spain
Impact of Arbuscular Mycorrhizal Symbiosis
on Plant Response to Biotic Stress: The Role
of Plant Defence Mechanisms
María J. Pozo, Sabine C. Jung, Juan A. López-Ráez,
and Concepción Azcón-Aguilar
194 M.J. Pozo et al.
The establishment of the arbuscular mycorrhizal (AM) symbiosis implies remarkable
changes in the physiology of the host plant. The changes span from alterations in the
hormonal balance and transcriptional profile to altered primary and secondary metab-
olism (Hause et al. 2007; Liu et al. 2007; Schliemann et al. 2008; López-Ráez et al.
2010). This global reprogramming of plant functions has an impact on the plant inter-
action with the environment, modifying its responses to biotic and abiotic stresses. As
a result, mycorrhizal plants are generally more tolerant to environmental stresses. The
consequences go beyond the individual level as they may influence plant diversity
and productivity in terrestrial ecosystems (van der Heijden et al. 2008).
It should be noted that the impact of the symbiosis in terms of resistance/toler-
ance to biotic stresses differs among AM fungal isolates for a given plant-pathogen
interaction. Moreover, such impact can be modulated by environmental conditions.
Despite of this variability, general trends emerge from the multiple studies dealing
with mycorrhiza in diverse pathosystems. Generally, enhanced resistance to soil-
borne pathogens has been reported in AM plants. Furthermore, the symbiosis can
also impact plant interactions with above-ground attackers. In this case, the out-
come ranges from enhanced resistance to increased susceptibility, largely depend-
ing on the attacker life-style (Pozo and Azcón-Aguilar 2007).
Early works on mycorrhizas and biotic stresses were mostly descriptive (for
reviews see Schonbeck and Dehne 1989; Sharma et al. 1992; Linderman 2000).
Generally, reports have focussed on beneficial effects of the symbiosis, aiming at
using AM as potential biocontrol agents in integrated management programmes for
disease control (Sharma and Adholeya 2000; Harrier and Watson 2004; Whipps
2004; Mukerji and Ciancio 2007).
A key factor determining the effect of the symbiosis on interactions with other
organisms seems to be the extension of root colonization by the AM fungi (AMF).
With some exceptions (Caron et al. 1986; García-Garrido and Ocampo 1988;
St-Arnaud et al. 1997; Kapoor 2008), reports on mycorrhizal protection against
pathogens show the requirement of a well established symbiosis prior to the chal-
lenge with the attacker (Rosendahl 1985; Cordier et al. 1998; Slezack et al. 2000;
Khaosaad et al. 2007). The first mechanism proposed to be involved in mycorrhiza-
induced protection was the improvement of plant nutrition and the consequent
compensation of the damages caused by the pathogen. However, studies including
nutrient supplemented controls showed that AM effects cannot be regarded as a
mere consequence of improved phosphorus nutrition (Trotta et al. 1996; Fritz 2006;
Liu et al. 2007). As we advance in our knowledge on the physiology and regulation
of the AM symbiosis, we start to understand the diversity of mechanisms underly-
ing the impact of the symbiosis on plant interactions with other organisms. In addi-
tion to the nutritional aspects, changes in the plant architecture, root exudation and
in the microbial populations in the rhizosphere, and the activation of plant defence
mechanisms may all be relevant. Their individual contribution to the final
outcome will depend on the organisms involved and the timing of the interactions
1959 Impact of Arbuscular Mycorrhizal Symbiosis on Plant Response to Biotic Stress
( Azcón-Aguilar and Barea 1996; Whipps 2004). In this chapter we will discuss the
possible mechanisms affecting the different types of plant-attacker interactions,
with special emphasis in those involving plant defence responses.
2 Impact of the AM Symbioses on Soil-Borne Pathogens
It is widely accepted that AM symbioses reduce the damage caused by soil-borne
pathogens. Many studies revealed a reduction of the incidence and/or severity of
diseases as root rot or wilting caused by diverse fungi such as Fusarium, Rhizoctonia,
Macrophomina or Verticillium, bacteria as Erwinia carotovora, and oomycetes as
Phytophthora, Pythium and Aphanomyces. A comprehensive review of those studies
was compiled by Whipps (2004). Similarly, a reduction of the deleterious effects by
parasitic nematodes such as Pratylenchus and Meloidogyne has been reported in
mycorrhizal plants (Pinochet et al. 1996; de la Peña et al. 2006; Li et al. 2006).
Additional reports showed protection to other soil pathogens as Armillaria melea in
grapevine (Nogales et al. 2009), broadening the range of pathosystems in which AM
symbioses may have a protective effect. The effectiveness against such diverse range
of attackers confirms the broad spectrum character of the induced resistance associ-
ated to the AM symbiosis.
Studies comparing different fungal species or isolates highlighted that the degree of
protection is highly dependent on the AMF involved (Kobra et al. 2009). Interestingly,
many studies point to a higher protector effect of Glomus mosseae in comparison to
other AMF (Pozo et al. 2002; Utkhede 2006; Ozgonen and Erkilic 2007).
Several mechanisms may operate simultaneously in the enhanced resistance of
mycorrhizal plants to soil pathogens. In addition to a possible competition for pho-
tosynthates between the AMF and the pathogen, competition for colonization sites
has been demonstrated. For example, in tomato roots, full exclusion of Phytophthora
from arbusculated cells was evidenced (Cordier et al. 1998). Mycorrhizal coloniza-
tion is also known to induce changes in the root system architecture and morphol-
ogy (Schellenbaum et al. 1991; Norman et al. 1996). These changes may alter the
dynamics of infection by the pathogen, although direct evidences of such correla-
tion are lacking. An altered pattern of root exudation may also impact the develop-
ment of the pathogen. Mycorrhizal colonization leaded to modifications in root
exudates composition that significantly reduced the sporulation of Phytophthora
fragariae (Norman and Hooker 2000) and altered the chemotactic response of the
zoospores of Phytophthora nicotianae (Lioussanne et al. 2008). Since root exudates
are key factors in shaping soil microbial communities (Badri and Vivanco 2009),
the changes in exudation into the mycorrhizosphere may result in alteration of the
microbial communities including possible antagonistic organisms. This may be the
reason underlying the biocontrol of pathogens in non-AM species by co-culture
with mycorrhizal plants (St-Arnaud et al. 1997).
Because of the root localization of both attacker and AMF, it is difficult to
discern the local or systemic character of the protection observed. However, the use
196 M.J. Pozo et al.
of split-root experimental systems allowing physical separation between AMF and
pathogens has confirmed a reduction of disease symptoms in the non-mycorrhizal
parts of the mycorrhizal root systems. Systemic protection at the root level has been
demonstrated against Phytophthora and Ralstonia in tomato (Cordier et al. 1998;
Pozo et al. 2002; Zhu and Yao 2004), against Gaeumannomyces in wheat (Khaosaad
et al. 2007), and against parasitic nematodes in banana plants (Elsen et al. 2008).
The systemic character of the induced resistance pointed to the involvement of
plant defences. Because of its relevance in all kinds of interactions, the implication
of plant defence mechanisms will be discussed in Section 7.
3 Effects of AM Symbioses on Root Parasitic Plants
Plants of the genera Striga and Orobanche parasitize different hosts around the
world, constituting one of the most damaging agricultural pests. These obligate
parasites attach to the roots of many plant species and acquire nutrients and water
from their host (Bouwmeester et al. 2003). Studies in African fields infested with
the hemiparasite Striga hermonthica evidenced that inoculation with AMF signifi-
cantly reduced the amount of parasitic plants in maize and sorghum cultures.
Accordingly, the use of mycorrhizas for integrated management of parasitic weeds
was proposed (Lendzemo et al. 2005; López-Ráez et al. 2009a).
Strigolactones are germinating stimulants for the seeds of root parasitic plants
(Bouwmeester et al. 2007). With the discovery of strigolactones as host detection
signals for AMF in the rhizosphere (Akiyama et al. 2005; see chapter by Bécard
et al.) a causal connection between AM and its impact on parasitic plants could be
established. Indeed, follow up experiments under controlled conditions confirmed
that AMF inoculation leads to a reduction of the impact of Striga, apparently related
to a reduction in strigolactones production (Lendzemo et al. 2007). Similarly, we
have observed that extracts from tomatoes colonized by G. mosseae induce less
germination of Orobanche ramosa seeds than those from non-mycorrhizal plants
(López-Ráez et al. 2009b). Moreover, a reduced production of strigolactones in a
tomato mutant correlated with less susceptibility to Orobanche (López-Ráez et al.
2008). All in all, it seems likely that a reduction in strigolactone production underlies
the decrease in the incidence of root parasitic plants on mycorrhizal plants.
4 Impact of AM Symbioses on Above-Ground Interactions
Studies dealing with AM effects on above-ground diseases are less abundant, and
apparently less conclusive. Early reports associated AM symbioses with enhanced
susceptibility to viruses (Whipps 2004), and it was generally accepted that AM
plants are more susceptible to shoot pathogens. However, recent studies dealing
with pathogens of different life styles have evidenced a more complex reality.
1979 Impact of Arbuscular Mycorrhizal Symbiosis on Plant Response to Biotic Stress
Biotrophic pathogens, such as powdery mildew and rust fungi (Blumeria, Oidium,
Uromyces) seem to perform better in mycorrhizal plants, although increased toler-
ance was often observed in terms of plant mass and yield (Gernns et al. 2001;
Whipps 2004). Concerning hemibiotrophs, the impact of the symbiosis varies from
no effect to reduction of the disease, for example, against Colletotrichum orbicu-
lare in cucumber (Lee et al. 2005; Chandanie et al. 2006). Finally, several studies
evidenced a positive effect on plant resistance to other shoot pathogens, including
the bacteria Xanthomonas campestris in Medicago (Liu et al. 2007), and the
necrotrophic fungus Alternaria solani in tomato (Fritz et al. 2006; De La Noval
et al. 2007). We have also confirmed that symbiosis with G. mosseae in tomato
induces systemic resistance to the necrotrophic fungus Botrytis cinerea (Jung et al.,
2009) (see Section 7). Recently, a positive effect of G. mosseae against Botrytis
cinerea has also been shown in roses, although dependent on environmental factors
(Moller et al. 2009).
Phytoplasma are specialized obligate parasites of phloem tissue transmitted by
insect vectors. AM establishment in tomato lead to a reduction of the disease symp-
toms caused by a phytoplasma of the Stolbur group (Lingua et al. 2002). Because they
inoculated through grafting with infected scions, potential effects on the insect vector
were ruled out. Thus, the protection is related to physiological changes in the mycor-
rhizal plant. Tolerance to phytoplasma disease was also reported in pear (García-
Chapa et al. 2004). Recently, a reduction in the titre of the Chrysanthemum yellows
phytoplasma has been shown in mycorrhizal chrysanthemum (D’Amelio et al. 2007),
confirming that mycorrhization can confer resistance to this type of pathogens.
In above-ground interactions of mycorrhizal plants, two main mechanisms may
be operative. One would be the potential changes in nutrient levels of the host plant
and alterations of the source-sink relation within it, that may affect the suitability
of the plant for shoot attackers. The other would be the modulation of plant defence
mechanisms, as discussed in Section 7.
5 Effects of AM Symbioses on Phytophagous Insects
The mycorrhizal status of the host plant can also influence insect herbivore perfor-
mance, but the magnitude and direction of the effect depend upon the feeding mode
and life style of the insect (Hartley and Gange 2009; Koricheva et al. 2009).
Many different studies cover an ample range of mycorrhizal plant-insect interac-
tions under controlled or field conditions. Upon a comprehensive review of the
published data, Hartley and Gange (2009) concluded that, generally, mycorrhizas
have strong negative effects on rhizophagous insects, but effects on shoot-feeding
insects are weaker and more variable. Despite of this variability, some general pat-
terns emerge: generalist insects are usually adversely affected by mycorrhizas,
whereas specialist insects may often benefit. Furthermore, aphids usually perform
better on AM plants while leaf-chewing insects are usually negatively affected by
198 M.J. Pozo et al.
Such patterns may arise from the differential impact of nutritional and defence
aspects in the insect. While generalist insects are sensitive to plant defence mecha-
nisms, specialist herbivores are likely to be able to circumvent the defences of their
host and remain undetected. As a result, generalists may be affected by the
enhanced defence capacity of AM plants, while specialists will circumvent the
defences and may benefit from the improved nutritional status of the plant. The
negative effect on leaf-chewers is likely related to their susceptibility to jasmonate-
dependent defences (Peña-Cortés et al. 2004) potentiated in mycorrhizal plants (see
Section 8). In addition, AM can also have an impact on herbivores by affecting the
performance of their predators and parasitoids: in tomato, the volatile blends
released by AM plants can be more attractive to aphid parasitoids than those from
non-mycorrhizal ones (Guerrieri et al. 2004).
6 AM Symbiosis Modulate Host Defence Responses
As discussed earlier in this book and reviewed elsewhere (Gianinazzi-Pearson
1996; Harrison 2005; Parniske 2008) the establishment of a successful mutualistic
interaction requires a high degree of coordination between both partners. Plant and
fungus actively engage in the process of colonization, and a tight control of plant
defence mechanisms is necessary. Interestingly, the plant is able to restrict AMF
colonization once plants are already mycorrhizal, a phenomenon known as auto-
regulation (Vierheilig et al. 2008). The mechanisms operating in such autoregula-
tion may also impact plant interactions with pathogens.
The levels of several phytohormones (mainly salicylic acid (SA), jasmonates
(JAs), ethylene (ET) and abcisic acid (ABA)) fine-tune the defence responses in
plants through an intricate regulatory network (Pieterse et al. 2009). Remarkably,
the levels of these hormones seem to be altered in mycorrhizal plants (Hause et al.
2007; López-Ráez et al. 2010), probably affecting plant defence mechanisms.
There is evidence for the accumulation of defensive plant compounds in mycor-
rhizal roots, although to a much lower extent than in plant-pathogen interactions.
Activation of phenylpropanoid and oxylipin metabolism, accumulation of reactive
oxygen species and of specific isoforms of defence-related enzymes has been
reported in mycorrhizal roots (García-Garrido and Ocampo 2002; De Deyn et al.
2009; López-Ráez et al. 2010). These reactions, generally localized, may control the
development of the fungus inside the roots (Pozo et al. 2002; Dumas-Gaudot et al.
2000; García-Garrido and Ocampo 2002). Indeed, as obligate biotrophs, AMF share
similarities with biotrophic pathogens (Paszkowski 2006) and transcriptional profil-
ing of plant responses to AMF revealed some overlap with responses to biotrophic
pathogens (Güimil et al. 2005). Coherently, SA, a key regulator of plant defences
against biotrophs (Glazebrook 2005), seems to have a negative effect on AM colo-
nization (García-Garrido and Ocampo 2002; López-Ráez et al. 2010). Thus, it is
plausible that AMF repress SA-dependent responses in the host in order to achieve
a compatible interaction. Indeed, a delay in the accumulation of PR-1 proteins,
common markers of SA-dependent responses, has been observed in mycorrhizal
1999 Impact of Arbuscular Mycorrhizal Symbiosis on Plant Response to Biotic Stress
roots (Dumas-Gaudot et al. 2000). Even repression of defence responses triggered
upon pathogen attack has been reported when G. intraradices was co-inoculated
with Rhizoctonia solani (Guenoune et al. 2001). Although AMF are able to trigger
plant defence responses as evidenced in myc mutants, only weak and transient
defence responses are activated during compatible AM interactions (Liu et al.
2003). Thus, AM establishment seems to require inhibition of certain SA-regulated
defence responses. Remarkably, inhibition of SA responses is also necessary for the
Rhizobium–legume symbiosis (Stacey et al. 2006).
Modulation of plant defences during AM formation does not only occur in the
roots, but also in the shoots. Accumulation of insect anti-feedant compounds (Gange
2006; Pozo et al. 2009) and transcriptional up-regulation of defence-related genes (Liu
et al. 2007; Pozo et al. 2009) have been described in leaves of mycorrhizal plants. Also
a repression of certain defences may take place: a delay in the systemic accumulation
of PR1 upon treatment with SA or analogs has been reported in mycorrhizal tobacco
shoots (Shaul et al. 1999) and suppression of certain chemical defences has also been
reported (Bennett et al. 2009). This modulation may affect the interaction with shoot
attackers. To this regard, the reciprocal influence of below-ground and above-ground
interactions through their impact on plant defences is receiving increasing interest
(Bezemer and van Dam 2005; Erb et al. 2009). An additional level of complexity is
related to the altered volatile profile released by AM plants (Guerrieri et al. 2004;
Rapparini et al. 2008). Volatiles may play key roles in defence, for example, by attract-
ing natural enemies of potentially harmful insects, or by priming distal parts of the
plant for a more efficient activation of defences (Heil and Ton 2008).
We have recently shown that mycorrhizal colonization in tomato leads to
increases in the expression of defence related genes known to be regulated by JA
(Pozo et al. 2009). JA is a key regulator of plant defences against insects and
necrotrophic pathogens (Peña-Cortés et al. 2004; Pozo et al. 2005). Because of SA
and JA signalling pathways are interconnected, mostly in an antagonistic way
(Pieterse et al. 2009) their interplay may explain the pattern of enhanced resistance/
susceptibility of AM plants. If AM inhibits SA-regulated responses, the plant
would be more susceptible to pathogens resisted through these responses, i.e.
biotrophic pathogens. On the contrary, an induction of the JA signalling pathway
would make mycorrhizal plants more resistant to necrotrophic pathogens and
JA-sensitive insects (Pozo and Azcón-Aguilar 2007). Such pattern is more obvious
in shoot interactions, where modulation of plant defences seems to be the main mech-
anism. In roots, the relevance of this altered balance will be lower since other
mechanisms are operating simultaneously (see Section 3), and a reduction of the
disease is the most general outcome.
7 AM Symbiosis Primes JA-Dependent Responses
Upon detection of a potential attacker, a rapid and strong activation of the defence
mechanisms is crucial for resistance. Accordingly, pre-conditioning of plant tissues
for a quick and more effective activation of defences upon attack has important
200 M.J. Pozo et al.
ecological fitness benefits, and seems to be a common feature of the plant’s immune
system. This boost of basal defences is known as priming (Conrath et al. 2006;
Goellner and Conrath 2008). Priming seems to be the strategy followed by several
beneficial micro-organisms to enhance resistance in plants, avoiding a direct activa-
tion of defences which would be too expensive for the host in the absence of chal-
lenging attackers (Pozo et al. 2005; Van Wees et al. 2008).
Evidences for primed defence responses in mycorrhizas were first reported in
root tissues. Mycorrhizal transformed carrot roots displayed stronger defence reac-
tions at challenge sites by Fusarium (Benhamou et al. 1994). Similarly, mycor-
rhizal potatoes showed amplified accumulation of phytoalexins upon Rhizoctonia
infection (Yao et al. 2003). Priming for callose deposition seems to be responsible
for the protection achieved by G. intraradices against Colletotrichum in cucumber
(Lee et al. 2005). Recently, primed accumulation of phenolic compounds in AM
date palm trees has also been related to protection against F. oxysporum (Jaiti et al.
2008). Remarkably, priming is not restricted to AMF colonized areas of the roots,
but to the whole root system. This was first illustrated in tomato plants during
P. parasitica infection (Cordier et al. 1998; Pozo et al. 2002). Only AM plants, even
in non-mycorrhizal parts of the root system, formed papilla-like structures around
the sites of pathogen infection, preventing further spreading of the pathogen. They
also accumulated more PR-proteins than non-mycorrhizal plants upon challenge
(Cordier et al. 1998; Pozo et al. 1999). Mycorrhizal protection of grapevine against
Meloidogyne incognita has also been associated with primed systemic expression
of a chitinase gene in response to the nematode (Li et al. 2006). But the primed
response is not restricted to the root system. Recently, we have shown priming of
defences also in shoots of mycorrhizal plants (Pozo et al. 2009).
Evidence is accumulating that priming associated to systemic resistance induced
by beneficial organisms is regulated by similar jasmonate signalling pathways (Van
Wees et al. 2008). Indeed, studies on rhizobacteria induced systemic resistance
(ISR) in Arabidopsis revealed the requirement of a functional JA signalling path-
way for the efficient induction of resistance (Pieterse et al. 1998; Pozo et al. 2008).
The JA signalling pathway is also required for rhizobacteria ISR in tomato (Yan
et al. 2002) and for the induction of resistance by the beneficial fungi Trichoderma
and Piriformospora (Shoresh et al. 2005; Stein et al. 2008). Interestingly, JA accu-
mulation have been proposed to mediate plant “memory” of previous challenges
(Galis et al. 2009), a possible basis for the primed state.
Jasmonates are key regulators in the AM symbiosis, and elevated endogenous
levels of JA have been confirmed in mycorrhizal roots (reviewed in Hause et al.
2007 and Hause and Schaarschmidt, 2009). We have found a significant increase in
JAs in mycorrhizal tomato roots (López-Ráez et al. 2010), but the levels were not
altered in the shoots (López-Ráez and Pozo, unpublished). However, we found
small, yet significant, increases in the expression of marker genes for JA responses,
a result that may indicate an enhanced sensitivity to the hormone.
To confirm whether AM leads to priming of JA-dependent responses in the
shoots, we compared the response of non-mycorrhizal and AM plants to foliar
application of different defence-related stimuli. Transcript profiling of leaves 24 h