Carbohydrate metabolism and production of phytohormones of Serendipita indica and Serendipita herbamans in interactions with their plant partner

Carbohydrate allocation in plants plays a primary role in economy and human food consumption. The storage of sugars in specific organs is an important driver to yield plant products with high nutritional value. Serendipita indica (former Piriformospora indica) and Serendipita herbamans are root endophytic fungi of the order Sebacinales; they can live in symbiosis with different species of plants without negative interference with plant health. S. indica possess growth promoting effects by changing the host metabolism probably through the secretion of particular proteins and metabolites including phytohormones. The resulting holosymbiont shows metabolic behaviours different from both partners alone. The current project is aimed to understand the relation between carbohydrate metabolism and phytohormone balance in order to reveal the mechanisms underlying the plant growth-promoting effects.

S. indica and S. herbamans show particular carbon source preferences and modify the carbohydrate metabolism accordingly. Genes for invertase and phosphoglucose-isomerase are present in their genomes and the corresponding protein activities have been detected in liquid culture of both fungi. In addition, they secrete different enzymes into liquid medium apparently related with the sugars used to feed the culture. Sugars seem to be a modulator of gene expression of carbohydrate related enzymes. Molecular data concerning expression of invertase gene will be presented.

On the plant side, secondary metabolites of both fungi can boost carbohydrate metabolism in tomato not-transformed root-organ culture, with a significant increase of cell-wall and cytosolic invertase activities.

The presence of fungal genes for auxin biosynthesis have been already shown; in addition, putative genes encoding enzymes involved in the biosynthesis of cytokinins, gibberellins, brassinosteroids and ABA have been detected in S. indica and S. herbamans genomes.

Interaction with plant mutants show that the fungi can produce compounds which complement phytohormone deficiencies of their plant partner. Level of cytokinins and biosynthetic genes (tRNA-IPT) data will be presented.