COORDINATOR: GRAZIELLA RANGHINO

The objective of the WP4 is to analyze a plant bioactive molecule (a strigolactone) in order i) to investigate its biological impact on plants/fungi/ bacteria; ii) to identify the crucial steps of the metabolic pathways, iii) to define the receptor-binding sites in both plant and fungi by using bioinformatics approaches and iv) to simulate the conditions for ligand site optimisation by computational biology.

The WP represents the starting point of the second part of the project (Fig.2 attached to the paper copy) which has the main aim to develop new molecules of interest for white and green biotechnologies. Strigolactones are a group of sesquiterpene lactones, previously isolated as seed-germination stimulants for the parasitic weeds Striga and Orobanche8. The natural strigolactones 5-deoxy-strigol, sorgolactone and strigol, and a synthetic analogue, GR24, induced extensive hyphal branching in germinating spores of the AM fungus Gigaspora margarita at very low concentrations.

The project starts from this remarkable study demonstrating that plants as Lotus japonicus release an active molecule, (a strigolactone) which is perceived by the AM fungus G. margarita and causes an extensive branching which is essential for the successful colonization events (Akyama et al., Nature, 2005). Enhanced hyphal growth is a prerequisite for the colonization, since it increases the number of potential contact points with the host surface. Interestingly, strigolactones are known to stimulate the seed germination of parasitic plants. Besserer et al (2006) found that a strigolactone from a monocotyledonous plant, Sorghum, strongly and rapidly stimulated the proliferation of the AM fungus Gigaspora rosea, at concentrations as low as 10−13 M. Within one h of treatment, the density of mitochondria in the fungal cells increased, and their shape and movement changed dramatically. Strigolactones derive from the carotenoid pathway and belong to the group of apocaroteinoids, which have been identified as the yellow pigments often present in mature mycorrhizas (Walter et al., 2007): these new results suggest that apocarotenoids play important and distinct roles in mycorrhizas.

Strigolactons therefore are active
1. on seed germination of parasitic plants (and can be used as biopest control)
2. on AM fungi (and can be used as stimulators in microbial inocula)
3. on endobacteria (our unpublished results)

However, there is no information on the receptor/s which bind such active molecule. This avoids any current applicative use of this strigolactone in green biotechnologies. In the context of projects dealing with the detection of differential gene expression during AMplant interactions by analyzing microarrays in model plants, the activation of genes involved in the carotenoid has been demonstrated. However, a direct relationship with the strigolacton pathways has not yet been determined.

In conclusion, the main goals of the WP4 are twofold : first is the definition of the metabolism and activity of strigolactons as a powerful bioactive molecule, with identification of candidate biosynthetic genes through oligochips analysis; second is the characterization by means of biocomputational tools of the putative enzymatic function and structure of the genetically derived receptors which will give insights into the receptorbinding sites. This last result will guide the bioinformatic definition of analogs suitable to synthetic approach.