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Linking plant processes
Daniel Wipf
To cite this version:
Daniel Wipf. Linking plant processes. International Innovation, 2013, 2 p. �hal-01004733�
Disseminating science, research and technology
INVESTIGATING NUTRIENT UPTAKE AND
EXCHANGE IN BIOGRAPHIC INTERACTIONS
Professor Daniel Wipf’s work on the mechanisms underlying mycorrhiza has given him a novel perspective on ways to improve plant productivity and ecosystem sustainability.
Here, he discusses exciting new research in the area
What is the driving force of your latest research into mycorrhizal symbiosis?
Mycorrhizae are mutualistic symbioses formed between the vast majority of terrestrial plants and soil fungi. Although they are principally based on the trophic exchanges between both partners, mycorrhizae often have further roles.
Understanding the mechanisms underlying the efficiency of high nutrient use and carbon allocation within the context of mycorrhizal interactions is critical for the sound management of both croplands and forests. In addition, it provides key information for improving the management of the ecosystem services provided by mycorrhizal fungi. Indeed, availability, uptake and exchange of nutrients in biotrophic interactions will drive plant growth and modulate biomass allocations central to plant yield. From my perspective, this kind of knowledge is one of the major outcomes we hope to gain from the TRANSMUT project.
Why is the nutritional metabolism of fungus of interest to your project?
Transport processes across the specialised and polarised membrane interfaces are of major importance in the functioning of established mycorrhizal associations, where symbiotic relation is based on a fair trade between fungus and host plant. Uptake and exchanges of nutrients or metabolites at biotrophic interfaces are controlled by the activity of transporters located in the fungal or plant membrane. Therefore, their patterns of regulation are essential in determining the outcome of plant fungal interactions and in adapting to changes in soil nutrient quantity or quality.
At the special interface between both symbiotic partners within the mycorrhiza, the fungal nutritional metabolism undergoes a unique reorientation. Despite its importance, the release of major nutrients taken up by the extraradical hyphae into the root common space, namely the apoplasm, occurs through widely unknown mechanisms including the differentiation and polarisation of the fungal membrane transport functions.
Can you explain the difference between ectomycorrhizae and endomycorrhizae?
At least seven different types of mycorrhizal associations have been recognised. The difference between ectomycorrhizae and endomycorhiza – the two major forms – is the nature of the symbionts and the structure of the mycorrhiza itself. In the endomycorrhiza, the fungus penetrates into cortical cells by passing the cell wall but not the plasma membrane, whereas it remains between the cortical cells in the ectomycorrhiza.
Bi-directional movement of nutrients characterises the mycorrhizal symbiosis.
Carbon flows to the fungus and inorganic nutrients move to the plant, thereby providing a critical linkage between the plant root and the soil. In depleted soils, nutrients taken up by the mycorrhizal fungi can lead to improved plant growth and reproduction. As a result, mycorrhizal plants are often more competitive and better able to tolerate environmental stresses than non-mycorrhizal plants.
What have you discovered about the carriers and how they determined flow?
One example is how we have identified several potassium transporter genes.
Our current working hypothesis is that the potassium channels (the most widely distributed type of ion channel) could mediate the potassium efflux from the fungal cells towards the plant root cells, and the potassium transporters could be responsible for potassium uptake from the soil.
The role of these different potassium transport systems will be analysed by the creation of transgenic fungal lines. Also, several genes coding for new phosphate transporters, whose function is not yet established, have been found to be differentially expressed in the symbiotic context compared to the fungus in pure culture.
As for potassium transport, we are studying the actual role of these new candidates by measuring the effect of transgenic fungal lines on plant phosphorous accumulation.
Could you outline the principle aims of the Medicago truncatula study in relation to sucrose? Have you ascertained any novel results?
In plants, long distance transport of sugars from photosynthetic source leaves to sink
Linking plant processes
TRANSMUT
INTERNATIONAL INNOVATION
organs comprises different crucial steps depending on the species and organ types.
Sucrose, the main carbohydrate for long distance transport, is synthesised in the mesophyll and then loaded into the phloem.
After long distance transport through the phloem vessels, sucrose is finally unloaded towards sink organs. Alternatively, sugar can also be transferred to non-plant sinks, and plant colonisation by heterotrophic organisms increases the sink strength and creates an additional sugar demand for the host plant.
These sugar fluxes are coordinated by transport systems. We identified de novo sucrose transporter (SUT) genes involved in long distance transport of sucrose from photosynthetic source leaves towards sink organs in the model leguminous species Medicago truncatula. The identification and functional analysis of sugar transporters provides key information on mechanisms that underlie carbon partitioning in plant–
microorganism interactions. Sucrose represents the main sugar transport form in M. truncatula and the expression profiles of MtSUT1-1, MtSUT2, and MtSUT4-1 highlight a fine tuning regulation for beneficial sugar fluxes towards the fungal symbiont. Taken together, our results suggest distinct functions for proteins from the SUT1, SUT2 and SUT4 clades in plant and in biotrophic interactions.
Do genetics contribute to symbiotic relationships? How have you been able to determine candidate genes?
Genetics is certainly a key part of a symbiotic relationship. For example, it is worthwhile to note that different maize varieties answer differently to mycorrhization and thus benefit the plant in a variety of ways. Different poplar genotypes display contrasted mycorrhization ability, showing that the plant is an active
partner. It is essential for the future to identify genetic markers of efficient mycorrhization.
To identify the candidate genes, we first categorised complete transporter sets for the different elements by both targeted and untargeted approaches, such as genome analysis and polymerase chain reaction (PCR) with degenerated primers. Subsequently the putative regulation of these genes during the mycorrhization was investigated.
Have you been able to identify ways that forest and cropland management practice would be changed?
Mycorrhizal fungi improve the growth of plants through increased uptake of available essential soil minerals. Their beneficial effects on plant performance and soil health are essential for the sustainable management of agricultural ecosystems. Nevertheless, since the first ‘green revolution’ less attention has been given to beneficial soil microorganisms in general, and to mycorrhizal fungi in particular.
Our proposed project will provide molecular and biochemical markers useful for facilitation of the first steps towards manipulation of crops with greater nutrient use efficiency and mycorrhizal ability, poplar, Pinus and Medicago. In the long term, these will be extremely useful in the development and implementation of environmentally sound crop production systems. Tree-based crop rotations can sustain high levels of biomass production without the use of fertilisers under reduced pest pressure. Similarly, development of superior legume cultivars for use in low applied fertiliser crop production systems will benefit from this fundamental research.
Is it possible to genetically modify plants to conduct beneficial symbiotic
relationships which would reduce the need for fertilisers?
The genetic markers for efficient symbiosis are still to be identified. Without genetically modifying plants, it would be more promising to take care of the mycorrhiza efficiency in breeding programmes, so having no question about safety of the resulting crop. It is worthwhile to note that the arbuscular mycorrhiza – the most widespread form of mycorrhiza – is the most ancient symbiosis for land plants. Therefore, the mycorrhization of plants is the rule in Nature rather than the exception.
In what capacity will the results of this project inform future investigations? Do you have any future studies in the pipeline?
From a fundamental point of view, the expected results will significantly contribute to understanding of mycorrhizal symbiosis. Our results regarding functional characterisation of genes will improve understanding of the specificity of transport occurring at the polarised membrane sites of the major form of mycorrhiza. Analysis of gene expression regulation by transcript profiling as well as spatio-temporal analysis of gene expression for genes of interest for different mycorrhizal model species will add a great value to functional data, thus uncovering their physiological function within the symbiotic association.
Of course, we have future studies in the pipeline. The generation of the transportome blue print will just be the first step in unravelling the functioning of the biotrophic interfaces in the mycorrhizal associations. I would very much like to see the development of a project to follow up this investigation.
TRANSMUT
A special symbiosis
By using advanced analytical tools to investigate the functioning of plant-fungi interactions, the ANR-funded TRANSMUT project aims to facilitate the development of novel land management techniques
THE USE OF agricultural land throughout the world has intensified and there is a great deal of research focused on investigating how the exchange of nutrients between fungi and flora can impact on soil health, and subsequently affect land management practices – particularly the use of fertilisers and pesticides. A collaborative research effort led by the Agroécologie group at INRA Dijon, France is exploring the availability, uptake and exchange of nutrients in biotrophic interactions. The impact of the biotrophic transportome on mutualistic plant-fungal interactions (TRANSMUT) project, is investigating the transportome role at biotrophic interfaces – or the membrane transporters and channels that control cellular influx and efflux of nutrients – within the mutualistic interactions of plant and fungi, the mycorrhiza. The objective is to elucidate ways in which these mutualistic processes affect plant growth and biomass production.
The aims of this research include using tree and legume plant models to determine biotrophic transportome blueprints of the mutual exchange boundary, assessing the main transporters responsible for controlling the nutrient fluxes that are occurring between the partners and improving knowledge about the underlying mechanisms of the fungal plasma membrane. The team’s interest in this latter process is driven by a desire to better understand how nutrient uptake and secretion impact on the host’s mineral nutrition balance, and ultimately on the management of land.
Project Coordinator Professor Daniel Wipf
from University of Burgundy, France explains that substantial evidence has been gathered to demonstrate how the balanced use of the microsymbiont properties could “significantly contribute to decreasing fertiliser and pesticide use in agriculture and forestry”. International alliances are absolutely essential to the success of this work. The project is financed by ANR and the French consortium strongly benefits from international collaborations. In particular, TRANSMUT has forged a productive partnership with a team from the University of Basel, Switzerland, under the guidance of Dr Pierre-Emmanuel Courty, which is currently supported by a Germaine de Stael project from the Swiss Academy of Engineering Sciences and the French Ministry of Higher Education and Research.
ADVANCED ANALYTICAL TECHNIQUES Many vascular plants enjoy a mycorrhizal association, or a symbiotic relationship, with fungi through their roots. The processes that transport the key nutrients are of critical importance to these associations, in terms of a fair trade between the two. Transporters are known to be responsible for the long distance transport of different nutrients from the source to the sink. For example, sucrose and monosaccharide transporters facilitate the long distance movement of sugar through a plant. TRANSMUT comprises scientists with expertise in nitrogen, phosphate, potassium and carbon nutrition in mycorrhiza, as well as those specialised in movement proteins in the symbiotic context. Some of the researchers are
also members of other genome sequencing projects investigating mycorrhizal fungi and their plant partners. This means that the group has access to important information about unknown transporter genes. The consortium is using a combination of complementary investigative approaches, such as bioinformatics analyses and transcriptomic methodologies using laser capture microdissection microscopy to identify the complete sets of regulated genes encoding transporters responsible for the movement of nitrogen, phosphorous, potassium and sugar at the biotrophic interface.
The researchers have been attempting to identify genetic markers of efficient mycorrhization. Following this, they investigated the putative regulation of these genes during the mycorrhization. In addition, functional analysis of selected genes is being used to establish the transporter properties and functions. “We are employing cellular imaging techniques to precisely identify the spatio-temporal patterns of expression of candidate genes and proteins by using in situ techniques, such as hybridisation and immuno-localisation,” Wipf reveals. Lastly, by completing the genetic transformation of plants and fungi combined with novel RNAi technology the researchers are able to better comprehend the importance of transport proteins from a symbiotic perspective.
MAPPING INTERFACE EXCHANGES
One area the TRANSMUT consortium is particularly keen to advance is the establishment of biotrophic transportome blueprints at symbiotic exchange interfaces.
To achieve this, the researchers are using tree and legume plant models where they can focus on the exchanges of nitrogen, phosphate, potassium and calcium in fungi.
This entails dissecting part of the molecular mechanisms believed to be responsible for the functional ectomycorrhizal symbiosis, with a focus on the Hebeloma cylindrosporum/
Pinus pinaster model, because it is the most advanced model available. Through this effort the team has been able to identify and clone new transporters for each element. The work on the actual function of the first candidate genes chosen by the consortium has been advanced by creation of overexpressor and RNAi lines, and then expanded to a more complete set of potassium transport systems by using Hebeloma genome information.
Arbuscular mycorrhizal fungal spores. © A Colombet INTERNATIONAL INNOVATION
TRANSMUT
Mycorrhizal effect on clover (left) and Tephrosia (right) growth (NM = non-mycorrhized; M = mycorrhized). © A Colombet
The effect of these first transgenic fungal lines is now being analysed by looking at mycorrhized pine seedlings.
INVESTIGATING SUGAR EXCHANGES IN FUNGI
The important role of fungi in facilitating the long distance transport of sugar through a plant is only just starting to emerge. Recent work has helped to define the function of such transporters in the mutualistic and pathogenic interactions between fungi and plants. TRANSMUT has been looking into how sugar transporters influence the distribution of carbohydrates within plant cells. Additionally, the consortium has been investigating how they impact on the functioning of plant-fungal interactions. The study has been made possible by new work on transcriptomic databases which have enabled the scientists to develop an inventory of the sugar transporter genes. Wipf expounds that the uptake and exchanges of
nutrient and metabolites at biotrophic interfaces are controlled by the activity of transporters located in the fungal or the plant membrane.
“Therefore, their patterns of regulation are essential in determining the outcome of plant fungal interactions and in adapting to changes in soil nutrient quantity and quality.”
The researchers have discovered that the mycorrhiza is subjected to a ‘unique reorientation’ of the fungal nutritional metabolism. It is the mechanisms controlling this process, such as the differentiation and polarisation of the fungal membrane transport functions, that the team is keen to explore.
The findings from this part of the project have been reported in a number of international journals, including Trends in Plant Science. The group has also successfully identified de novo sucrose transporter (SUT) genes involved in long-distance transport of sugar in the model species Medicago truncatula. Wipf notes that when considered in tandem, their findings highlight that there are distinct functions for proteins from different SUT clades in plant and in biotrophic interactions. There is still much work to be done in this particular field, including learning more about the system of cellular efflux at the biotrophic interfaces and developing a deeper understanding about how nutrient exchanges are regulated, both within and between organisms. New advances in molecular and post-genomic techniques will help support further TRANSMUT research work into the key nutrient transporters that are a crucial component the mutually- beneficial interactions.
TRANSMUT
OBJECTIVES
To investigate the potential of the availability, uptake and exchange of nutrients in biotrophic interactions to drive plant growth and modulate biomass allocation, in the context of high biomass production.
PARTNERS
UMR Agroécologie, Dijon • UMR interactions Arbres Microorganismes, Nancy • UMR Ecologie Fonctionnelle et Biogéochimie des Sols et
Agroécosystèmes, Montpellier • UMR Biochemistry and Plant Molecular Physiology, Montpellier
KEY COLLABORATOR
Dr Pierre-Emmanuel Courty, University of Basel, Switzerland
FUNDING
Blanc Programme from the French National Agency for Research (ANR), commission SVSE 6 : (SVSE 6 - Génomique, génomique fonctionnelle, bioinformatique, biologie systémique) – contract no. ANR-10- BLAN-1604-01
CONTACT
Professor Daniel Wipf Project Coordinator
UMR 1347 Agroécologie AgroSup/INRA/
University of Burgundy Pôle IPM - ERL CNRS 6300 BP 86510
17 rue Sully 21065 Dijon Cedex France
T + 33 3 80 69 34 52 F + 33 3 80 69 37 53 E daniel.wipf@dijon.inra.fr
www6.dijon.inra.fr/umragroecologie_
eng/Departments/IPM
PROFESSOR DANIEL WIPF is co-leader of the mycorrhizal team of the Agroecology Unit (Dijon). The research group has outstanding expertise in investigations of the development of the Medicago/Glomus symbiosis and has focused on early cell processes involved in fungal host recognition mechanisms as well as mechanisms for sugar transfer at a biotrophic interface. Wipf and his team have been involved in several national and international projects related to molecular genetics, agricultural relevance and functional genomics of plant-microbe interactions.
Plant root colonised by an arbuscular mycorrhizal fungus. © A Colombet
