Rhodotorulic acid enhances root colonization of tomato plants by arbuscular mycorrhizal (AM) fungi due to its stimulatory effect on the pre-symbiotic stages of the AM fungi

Exudates of Rhodotorula mucilaginosa, a yeast commonly found in the rhizosphere, increased hyphal length of the arbuscular mycorrhizal (AM) fungi Gigaspora rosea and Gigaspora margarita. Rhodotorulic acid (RA), a siderophore compound obtained from R. mucilaginosa exudates, increased hyphal length and branching. Thus, the increase in the number of entry points and the higher AM root colonization of tomato plants in the presence of RA can at least partially be explained by the positive effect of RA on the pre-symbiotic stages of the AM fungi. 2008 Elsevier Ltd. All rights reserved. Arbuscular mycorrhizal (AM) fungi are known to interact with other soil microorganisms (Andrade et al., 1997). Yeasts belonging to the genera Rhodotorula are common components of the soil rhizosphere (Slavikova and Vadkertiova, 2000). Recently, it has been observed that yeasts, mainly through their exudates, can have a positive effect on AM fungi (Fracchia et al., 2003; Sampedro et al., 2004). Siderophores are low-molecular weight (<1500 Da) ferricspecific chelates, produced under iron limitation. Iron(III) is solubilized by binding to siderophores and thus, can enter microbial cells via specific siderophores transport systems (Winkelmann, 2007). Siderophores are also involved in microbial interactions in the rhizosphere and in the colonization of roots by microorganisms (Neilands, 1981). There are some indications that siderophores affect phosphorous (P) cycling by releasing P from iron phosphates (Zaı̈d et al., 2003), or by regulating the production of phosphatases (Monds et al., 2006). This can be important in the AM symbiosis as AM fungi apparently are not able to produce siderophores (Caris et al., 1998), but possibly, similar to other microorganisms, interact with siderophores (Winkelmann, 2007). Rhodotorulic acid (RA) is a hydroxamate siderophore produced by Rhodotorula mucilaginosa and found in its exudates (Andersen et al., 2003). No data on its effect on AM fungi are available yet. In the present work we studied the effect of exudates and RA produced by R. mucilaginosa (BAFC 1769) on several stages of the development of the AM fungi, Gigaspora rosea (BEG no. 9) and Giigaspora margarita (J7). The exudates were obtained by growing the yeast in Y liquid medium (Fracchia et al., 2003). After 6, 12, 18, 24, 32, 40 and 48 h the yeast growth was measured at 600 nm. When OD1⁄4 1 was equivalent to 1 10 cellsml 1 (Andersen et al., 2003), the culture medium was centrifuged at 3000 rpm and the supernatant was sterilized (0.22 mm Millipore). The effect of yeast exudates on hyphal length of AM spores was tested by adding 1, 2.5, 5, 8, 10, 15, 20 or 40 ml of the liquid culture medium after 6, 12, 18, 24, 32, 40 and 48 h of yeast growth to Petri dishes with 10 ml of 4% Gel-Gro in 10 mM of MES buffer (pH 7) and with 10 surfacesterilized spores (Mosse, 1962). Ten replicates were used for each treatment. The plates were incubated at 25 C for 15 days and the hyphal length was determined (Marsh, 1971). The RA was obtained from R. mucilaginosa cultured in 500 ml medium (Fracchia et al., 2003) and the greatest production of RA was found after 24 h of culture (Andersen et al., 2003). At this time * Corresponding author. Tel.: þ34 58 181600; fax: þ34 58 129600. E-mail address: [email protected] (J.A. Ocampo).