Mycorrhiza-induced resistance: more than the sum of its parts?

TLDR: This opinion article proposes that MIR is a cumulative effect of direct plant responses to mycorrhizal infection and indirect immune responses to ISR-eliciting rhizobacteria in the myCorrhizosphere.

About: This article is published in Trends in Plant Science.The article was published on 2013-10-01 and is currently open access. It has received 359 citations till now. The article focuses on the topics: Systemic acquired resistance & Mycorrhizosphere.

Figures

Figure 1. Spatiotemporal model of mycorrhiza-induced resistance (MIR). Phase I: Root exudation of strigolactones (blue arrows) induces hyphal branching in arbuscular mycorrhizal fungi (AMF) and stimulates infection. Phase II: AMF initiate infection of the root cortex. Microbe-associated molecular patterns (MAMPs) from the fungus are recognised by the plant innate immune system. This leads to transient expression of MAMP-triggered immunity (red cells) and generation of long-distance signals in the vascular tissues (red arrow), which induce long-lasting priming of salicylic acid (SA)-dependent defences and systemic acquired resistance (SAR). Phase III: AMF employ specific effector molecules and stimulate production of abscisic acid (ABA) to suppress MAMP-triggered immunity locally. ABA can be transported through the xylem to the shoot (brown arrow), where it can prime cell wall defences. Formation of intracellular arbuscules increases sugar import from the shoot and delivers phosphorous and other nutrients from the soil, thereby altering root metabolism and exudates. Moreover, metabolically active hyphae can alter the chemical composition of root exudates. The combined impact of plant immune modulation, enhanced sugar allocation, increased nutrient uptake, and fungal modification of root exudates leads to changes in root exudation chemistry (green arrows) and recruitment/selection of specific mycorrhizosphere bacteria. Phase IV: Establishment of the mycorrhizosphere is associated with dense colonisation by selected bacteria that metabolise mycorrhizal root exudates and deliver ISR-eliciting signals at the root surface and/or fungal hyphae (purple arrows). After perception of these signals by the host plant, long-distance signals (blue arrow) are generated that prime jasmonate- and ethylene-dependent plant defences and cause induced systemic resistance (ISR).

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Mycorrhiza-induced resistance: more than the sum of its parts?" ?

This ‘ mycorrhiza-induced resistance ’ ( MIR ) provides systemic protection against a wide range of attackers and shares characteristics with systemic acquired resistance ( SAR ) after pathogen infection and induced systemic resistance ( ISR ) following root colonisation by non-pathogenic rhizobacteria. In this opinion article, the authors present a novel model of MIR that integrates different aspects of the induced resistance phenomenon. The authors propose that MIR is a cumulative effect of direct plant responses to mycorrhizal infection and indirect immune responses to ISR-eliciting rhizobacteria in the mycorrhizosphere. 

Q2. What are the consequences of the mycorrhizosphere effect?

The consequences of the mycorrhizosphere effect, including recruitment of PGPRs, may not only boost nutrient mobilisation by AMF but could also provide non-nutritional benefits, such as disease suppression via antibiosis and/or competitive exclusion. 

Q3. What is the common example of defenceeliciting MAMPs from bacteria?

Well-known examples of defenceeliciting MAMPs from bacteria are rhamnolipids, the elongation factor Tu, flagellin, and cell-wall lipopolysaccharides [53]. 

Q4. What is the effect of MAMPs on plant immunity?

Recognition of MAMPs by pattern-recognition receptors elicits a series of signalling cascades resulting in enhanced production of the plant defence hormone SA and expression of MAMP-triggered immunity [21]. 

Q5. How much of the plant’s photosynthesis is derived from roots?

Estimates suggest plants can exude up to 40% of their photosynthates from roots, representing a rich source of energy for soil microbes [15]. 

Q6. What is the role of afm in the mycorrhizosphere?

increased densities of selected rhizobacteria in the mycorrhizosphere have the potential to suppress pests and diseases in systemic plant tissues through priming of inducible defences. 

Q7. What is the role of the mycorrhizosphere in bacterial resistance?

The spatially confined structure of the mycorrhizosphere allows rhizobacterial strains to reach exceptionally high cell densities [5]. 

Q8. What is the alternative strategy to decipher the contribution of mycorrhizosphere bacteria?

A global inventory of microbial diversity through 16S RNA gene sequence analysis, coupled to temporal profiling of metabolites in mycorrhizal root exudates would be an alternative strategy to decipher the contribution of mycorrhizosphere bacteria in MIR. 

Q9. How can the authors verify the role of plant metabolites in MIR?

Involvement of candidate plant metabolites as regulators of resistance-inducing activities by mycorrhizosphere bacteria can be verified by genetic manipulation of the corresponding biosynthetic pathways in the host plant. 

Q10. Why is it possible that AMF stimulate ABA production in the roots?

Because ABA can suppress SA-dependent defences against biotrophic pathogens [36,37], it is plausible that AMF stimulate ABA production in the roots to promote theirTable 1. 

Q11. What is the effect of the mycorrhizal roots on soil?

The enhanced microbial activity surrounding mycorrhizal roots compared with non-mycorrhizal roots is called the ‘mycorrhizosphere effect’ [5]. 

Q12. What is the funding source for S.C.M.W.?

Rothamsted Research receives grant aid from the Biotechnology and Biological Science Research Council of the U.K. S.C.M.W.’s research is supported by the Dutch Technology Foundation STW (VIDI grant no. 

Q13. What is the role of the microbial autoinducer in triggering ISR?

the potential for mycorrhizosphere bacteria to elicit ISR is not only determined by their presence, but also depends on their metabolic activity in relation to chemical signals from mycorrhizal root exudates, their cell density, and the presence of competing microbes. 

Q14. What is the main focus of research on plant–mycorrhiza interactions?

Research on plant–mycorrhiza interactions has mostly focussed on the physiology of nutrient-for-carbon exchange and plant signal-transduction pathways controlling the interaction. 

Q15. What is the significance of the relationship between plant and fungi?

It has been estimated that 80% of plant species retain these ancient arbuscular mycorrhizal associations [1], illustrating the importance of this mutualism to both partners. 

Q16. What is the role of the malate transporter gene in the rhizosphere?

In non-mycorrhizal Arabidopsis, mutation in the malate transporter gene ALMT1 affects recruitment of ISR-eliciting Bacillus subtilis FB17 after treatment of the leaves with MAMPs [48], indicating thatlittle was known about their additional beneficial role in the rhizosphere, despite the fact that non-hosts for parasitic Orobanchaceae produce strigolactones profusely. 

Q17. What is the role of QS in bacterial resistance?

This autoinduction process, known as quorum sensing (QS), allows bacteria to adjust community gene expression in accordance with their environment [54]. 

Q18. What is the nature of the systemic signals controlling rhizobia-induced SAR?

It is thus possible that jasmonates function as complementary long-distance signals of MIR, which may be the result of systemic signalling processes similar to autoregulation of nodulation during rhizobia–legume interactions [65].