First indications for the involvement of strigolactones on nodule formation in alfalfa (Medicago sativa)

Strigolactones have recently been suggested to be phytohormones that are present in all plants. Strigolactones are released by roots into the rhizosphere, stimulating the seed germination of parasitic plants such as Striga spp. and Orobanche spp. and play a crucial role in the interaction between plants and symbiotic arbuscular mycorrhizal fungi. By applying different concentrations of the synthetic strigolactone analogue GR24 to alfalfa (Medicago sativa) inoculated with Sinorhizobium meliloti we could show that in alfalfa nodulation is positively affected by the presence of the strigolactone analogue GR24. Moreover, we could show that this increased nodulation cannot be linked with a stimulatory effect of GR24 on the growth or the expression of nod genes of S. meliloti. Putative mechanisms operating in the plant in response to the addition of GR24 and leading to increased nodule formation by rhizobia are discussed. 2009 Elsevier Ltd. All rights reserved. Strigolactones have been identified as phytohormones involved in the regulation of shoot branching in plants and thus have been suggested to be ubiquitous in the plant kingdom (Gómez-Roldán et al., 2008; Umehara et al., 2008). Since long, strigolactones e that are released by roots into the rhizosphere e have been known as seed germination stimulants for parasitic plants such as Striga spp. and Orobanche spp. (Bouwmeester et al., 2007; López-Ráez et al., 2008a) and recently, their role as key signalling compounds in the interaction between plants and soil-borne symbiotic arbuscular mycorrhizal fungi has been suggested (Akiyama et al., 2005; Besserer et al., 2006). Seed germination stimulants e including strigolactones e are exuded, although at varying levels, by the roots of all plants investigated so far (Berner andWilliams,1998; Fernández-Aparicio et al., 2009; Lendzemo et al., 2009; Steinkellner et al., 2007; Yoneyama et al., 2008), and thus a more general signalling role of these compounds in plant microbe interactions in the soil has been suggested (García-Garrido et al., 2009). Scarce data are available on strigolactones and other fungi apart from AMF (García-Garrido et al., 2009; Martinez et al., 2001; Sabbagh, 2008; Steinkellner et al., 2007), however, nothing is known yet whether strigolactones affect the Rhizobium-legume symbiosis,