Rhizosphere chemical dialogues: Plant-microbe interactions

In natural ecosystems, the roots of many plants exist in association with arbuscular mycorrhizal (AM) fungi, and the resulting symbiosis has profound effects on the plant. The most frequently documented response is an increase in phosphorus nutrition; however, other effects have been noted, including increased resistance to abiotic and biotic stresses. Here we used a 16,000-feature oligonucleotide array and real-time quantitative RT-PCR to explore transcriptional changes triggered in Medicago truncatula roots and shoots as a result of AM symbiosis. By controlling the experimental conditions, phosphorus-related effects were minimized, and both local and systemic transcriptional responses to the AM fungus were revealed. The transcriptional response of the roots and shoots differed in both the magnitude of gene induction and the predicted functional categories of the mycorrhiza-regulated genes. In the roots, genes regulated in response to three different AM fungi were identified, and, through split-root experiments, an additional layer of regulation, in the colonized or non-colonized sections of the mycorrhizal root system, was uncovered. Transcript profiles of the shoots of mycorrhizal plants indicated the systemic induction of many genes predicted to be involved in stress or defense responses, and suggested that mycorrhizal plants might display enhanced disease resistance. Experimental evidence supports this prediction, and mycorrhizal M. truncatula plants showed increased resistance to a virulent bacterial pathogen, Xanthomonas campestris. Thus, the symbiosis is accompanied by a complex pattern of local and systemic changes in gene expression, including the induction of a functional defense response.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-313X.2007.03069.x

Arbuscular mycorrhizal symbiosis is accompanied by local and systemic alterations in gene expression and an increase in disease resistance in the shoots.

In response to the colonization by arbuscular mycorrhizal (AM) fungi, plants reprioritize their phosphate (Pi)-uptake strategies to take advantage of nutrient transfer via the fungus. The mechanisms underlying Pi transport are beginning to be understood, and recently, details of the regulation of plant and fungal Pi transporters in the AM symbiosis have been revealed. This review summarizes recent advances in this area and explores current data and hypotheses of how the plant Pi status affects the symbiosis. Finally, suggestions of an interrelationship of Pi and nitrogen (N) in the AM symbiosis are discussed.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-3040.2006.01617.x

Phosphate in the arbuscular mycorrhizal symbiosis: transport properties and regulatory roles

Most terrestrial plants form arbuscular mycorrhiza (AM), mutualistic associations with soil fungi of the order Glomeromycota. The obligate biotrophic fungi trade mineral nutrients, mainly phosphate (P(i) ), for carbohydrates from the plants. Under conditions of high exogenous phosphate supply, when the plant can meet its own P requirements without the fungus, AM are suppressed, an effect which could be interpreted as an active strategy of the plant to limit carbohydrate consumption of the fungus by inhibiting its proliferation in the roots. However, the mechanisms involved in fungal inhibition are poorly understood. Here, we employ a transcriptomic approach to get insight into potential shifts in metabolic activity and symbiotic signalling, and in the defence status of plants exposed to high P(i) levels. We show that in mycorrhizal roots of petunia, a similar set of symbiosis-related genes is expressed as in mycorrhizal roots of Medicago, Lotus and rice. P(i) acts systemically to repress symbiotic gene expression and AM colonization in the root. In established mycorrhizal roots, P(i) repressed symbiotic gene expression rapidly, whereas the inhibition of colonization followed with a lag of more than a week. Taken together, these results suggest that P(i) acts by repressing essential symbiotic genes, in particular genes encoding enzymes of carotenoid and strigolactone biosynthesis, and symbiosis-associated phosphate transporters. The role of these effects in the suppression of symbiosis under high P(i) conditions is discussed.

/pdf/phosphate-systemically-inhibits-development-of-arbuscular-17wij82yy3.pdf

Phosphate systemically inhibits development of arbuscular mycorrhiza in Petunia hybrida and represses genes involved in mycorrhizal functioning

Summary
• Arbuscular mycorrhizas (AMs) contribute significantly to soil nutrient uptake in plants. As a consequence of the fungal colonization and of the deep reorganization shown by arbusculated cells, important impacts on root transcriptome are expected.
• An Affymetrix GeneChip with 50 000 probe-sets and real-time RT-PCR allowed us to detect transcriptional changes triggered in Lotus japonicus by the AM fungus Gigaspora margarita, when arbuscules are at their maximum (28 d postinoculation (dpi)). An early time (4 dpi) was selected to differentiate genes potentially involved in signaling and/or in colonization of outer tissues.
• A large number (75 out of 558) of mycorrhiza-induced genes code for proteins involved in protein turnover, membrane dynamics and cell wall synthesis, while many others are involved in transport (47) or transcription (24). Induction of a subset (24 genes) of these was tested and confirmed by qRT-PCR, and transcript location in arbusculated cells was demonstrated for seven genes using laser-dissected cells.
• When compared with previously published papers, the transcript profiles indicate the presence of a core set of responsive genes (25) that seem to be conserved irrespective of the symbiotic partner identity.

/pdf/genome-wide-reprogramming-of-regulatory-networks-transport-rw3khthay8.pdf

Genome-wide reprogramming of regulatory networks, transport, cell wall and membrane biogenesis during arbuscular mycorrhizal symbiosis in Lotus japonicus.

The large majority of plants are capable of undergoing a tight symbiosis with arbuscular mycorrhizal (AM) fungi. During this symbiosis, highly specialized new structures called arbuscules are formed within the host cells, indicating that, during interaction with AM fungi, plants express AM-specific genetic programs. Despite increasing efforts, the number of genes known to be induced in the AM symbiosis is still low. In order to identify novel AM-induced genes which have not been listed before, 5,646 expressed sequence tags (ESTs) were generated from two Medicago truncatula cDNA libraries: a random cDNA library (MtAmp) and a suppression subtractive hybridization (SSH) library (MtGim), the latter being designed to enhance the cloning of mycorrhiza-upregulated genes. In silico expression analysis was applied to identify those tentative consensus sequences (TCs) of The Institute for Genomic Research M. truncatula gene index (MtGI) that are composed exclusively of ESTs deriving from the MtGim or MtAmp library, but not from any other cDNA library of the MtGI. This search revealed 115 MtAmp- or MTGim-specific TCs. For the majority of these TCs with sequence similarities to plant genes, the AM-specific expression was verified by quantitative reverse-transcription polymerase chain reaction. Annotation of the novel genes induced in mycorrhizal roots suggested their involvement in different transport as well as signaling processes and revealed a novel family of AM-specific lectin genes. The expression of reporter gene fusions in transgenic roots revealed an arbuscule-related expression of two members of the lectin gene family, indicating a role for AM-specific lectins during arbuscule formation or functioning.

https://apsjournals.apsnet.org/doi/pdf/10.1094/MPMI-18-0771

Combined transcriptome profiling reveals a novel family of arbuscular mycorrhizal-specific Medicago truncatula lectin genes.

In Medicago truncatula a family of mycorrhiza-specific expressed lectins has been identified recently, but the function and regulation of these lectins during the arbuscular mycorrhiza symbiosis are still unknown. In order to characterize a first member of this protein family, MtLec5 was analyzed concerning its localization and regulation. Confocal laser scanning microscopy showed that MtLec5 is a secretory protein indicating a role as a vegetative storage protein, which is specifically expressed in mycorrhizal root systems. To study the molecular mechanisms leading to the mycorrhiza-specific transcription, deletion studies of pMtLec5 were done using reporter gene fusions. Potential cis-acting elements could be narrowed down to a 150 bp fragment that was located approximately at −300/−150 according to the transcription start, suggesting the binding of positive regulators to this area. Similar expression pattern of the reporter gene was found after transforming roots of the non-legume Nicotiana tabacum with the heterologous promoter–reporter fusions. This indicated that the observed mycorrhiza-specific transcriptional induction is not legume-specific. Electrophoretic mobility shift assays showed that several factors which were exclusively present in mycorrhizal roots bind within the 150 bp promoter area. This strengthens the hypothesis of positive regulators mediating the AM-specific gene expression.

The conserved arbuscular mycorrhiza-specific transcription of the secretory lectin MtLec5 is mediated by a short upstream sequence containing specific protein binding sites

Roots of most plants form a mutualistic interaction with arbuscular mycorrhiza (AM) fungi. During the symbiosis, drastic morphological and physiological changes occur in the host plant root system. These changes are likely to be controlled by a specific genetic program of the plant. The legume Medicago truncatula is a model system widely used to elucidate this program. A number of loci required for AM-development have been identified by the analysis of M. truncatula mutants; however, the genes identified are also required for the Legume-Rhizobium symbiosis, since all M. truncatula mycorrhizal mutants so far reported are also nodulation mutants. We have focused on the identification and analysis of AM-specific genes as a further means to gain insight into the molecular background of the AM symbiosis and the molecular regulation of this tight symbiosis. Here, we describe the identification of AM-specific genes and the analysis of their transcriptional regulation. The identification of promoters and regulatory elements mediating the mycorrhiza-specific transcription provides a starting point to identify the corresponding specific transcription factors. This enables the identification of further upstream elements of the regulation cascade, and thus the elucidation of the molecular mechanisms controlling the AM-development within plant roots.

Towards the Elucidation of AM-Specific Transcription in Medicago truncatula

Towards the elucidation of AM-specific transcription in Medicago truncatula.

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Combined transcriptome profiling reveals a novel family of arbuscular mycorrhizal-specific Medicago truncatula lectin genes.

The conserved arbuscular mycorrhiza-specific transcription of the secretory lectin MtLec5 is mediated by a short upstream sequence containing specific protein binding sites

Towards the Elucidation of AM-Specific Transcription in Medicago truncatula

Towards the elucidation of AM-specific transcription in Medicago truncatula.