Joan Doidy, doctorant à l’UMR INRA 1347 d’Agrosup (Université de Bourgogne), soutiendra publiquement sa thèse

le 23 mai à 14h, salle de conférences INRA Dijon (17 rue Sully).

The Medicago truncatula sucrose transporter family:
sugar transport from plant source leaves towards the arbuscular mycorrhizal fungus.

 
Thèse en cotutelle Université de Bourgogne- LMU Munich (Allemagne)

Résumé: In plants, long distance transport of sugars from photosynthetic source leaves to sink organs comprises different crucial steps depending on the species and organ types. Sucrose, the main carbohydrate for long distance transport is synthesized 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 colonization by heterotrophic organisms increase the sink strength and creates an additional sugar demand for the host plant. These sugar fluxes are coordinated by transport systems. Main sugar transporters in plants comprise sucrose (SUTs) and monosaccharide (MSTs) transporters which constitute key components for carbon partitioning at the whole plant level and in interactions with fungi. Although complete SUTs and MSTs gene families have been identified from the reference Dicots Arabidopsis thaliana and Monocots rice (Oriza sativa), sugar transporter families of the leguminous plant Medicago truncatula, which represents a widely used model for studying plant-fungal interactions in arbuscular mycorrhiza (AM), have not yet been investigated.
With the recent completion of the M. truncatula genome sequencing as well as the release of transcriptomic databases, monosaccharide and sucrose transporter families of M. truncatula were identified and now comprise repectively 62 MtMSTs and 6 MtSUTs. I focused on the study of the newly identified MtSUTs at a full family scale; phylogenetic analyses showed that the 6 members of the MtSUT family distributed in all three Dicotyledonous SUT clades and were named upon phylogenetic grouping into particular clades: MtSUT1-1, MtSUT1-2, MtSUT1-3, MtSUT2, MtSUT4-1 and MtSUT4-2. Functional analyses by yeast complementation and expression profiles by quantitative RT-PCR revealed that MtSUT1-1 and MtSUT4-1 are H+/sucrose symporters and represent key members of the MtSUT family. Conservation of transport capacity between orthologous leguminous protein, expression profiles and subcellular localization compared to previously characterized plant SUTs suggest that MtSUT1-1 is the main protein involved in phloem loading in source leaves whilst MtSUT4-1 mediate vacuolar sucrose export for remobilization of intracellular reserve.
The AM symbiosis between plant and fungi from the Glomeromycota phylum is characterized by trophic exchanges between the two partners. The fungi supply the autotrophic host with nutrient and thereby promotes plant growth. In return, the host plant provides sugar photosynthates to the heterotrophic symbiont. Here, sugar fluxes from plant source leaves towards colonized sink roots in the association between the model leguminous plant M. truncatula and the reference arbuscular mycorrhiza fungus (AMF) Glomus intraradices were investigated. Sugar transporters candidates from both the plant and fungal partners presenting differential expression profiles using available transcriptomic tools were pinpointed. Gene expression profiles of MtSUTs and sugar quantification analyses upon high and low phosphorus nutrient supply and inoculation by the AMF suggest a mycorrhizal-driven stronger sink in AM roots with a fine-tuning regulation of MtSUT gene expression. Conserved regulation patterns were observed for orthologous SUTs in response to colonization by glomeromycotan fungi.
In parallel, a non-targeted strategy consisting in the development of a M. truncatula – G. intraradices  expression library suitable for yeast functional complementation and screening of symbiotic marker genes, similar to the approach that led to the identification of the first glomeromycotan hexose transporter (GpMST1), has been developed in this study.
Taken together, with the identification, functional characterization and gene expression pattern of sugar transporter families, a more complete picture of sugar fluxes in the AM symbiosis has begun to emerge. This study opens new perspectives by identifying interesting candidate genes involved in sugar partitioning at both the plant and fungal levels and at the symbiotic interface in the widely used AM symbiosis model between M. truncatula and G. intraradices.