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PLANTS INTERACTING WITH OTHER ORGANISMS

The Mycorrhizal Fungus Gigaspora margarita Possesses a CuZn Superoxide Dismutase That Is Up-Regulated during Symbiosis with Legume Hosts¹

Dipartimento di Biologia Vegetale, Università di Torino, 10125 Turin, Italy (L.L., M.N., P.B.); and Istituto per la Protezione delle Piante, Sezione di Torino, CNR, 10125 Turin, Italy (P.B.)

A full-length cDNA showing high similarity to previously described CuZn superoxide dismutases (SODs) was identified in an expressed sequence tag collection from germinated spores of the arbuscular mycorrhizal fungus Gigaspora margarita (BEG 34). The corresponding gene sequence, named GmarCuZnSOD, is composed of four exons. As revealed by heterologous complementation assays in a yeast mutant, GmarCuZnSOD encodes a functional polypeptide able to confer increased tolerance to oxidative stress. The GmarCuZnSOD RNA was differentially expressed during the fungal life cycle; highest transcript levels were found in fungal structures inside the roots as observed on two host plants, Lotus japonicus and Medicago truncatula. These structures also reacted positively to 3,3'-diaminobenzidine, used to localize H₂O₂ accumulation. This H₂O₂ is likely to be produced by CuZnSOD activity since treatment with a chelator of copper ions, generally used to inhibit CuZnSODs, strongly reduced the 3,3'-diaminobenzidine deposits. A slight induction of GmarCuZnSOD gene expression was also observed in germinated spores exposed to L. japonicus root exudates, although the response showed variation in independent samples. These results provide evidence of the occurrence, in an arbuscular mycorrhizal fungus, of a functional SOD gene that is modulated during the life cycle and may offer protection as a reactive oxygen species-inactivating system against localized host defense responses raised in arbuscule-containing cells.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.050435.

^* Corresponding author; e-mail p.bonfante{at}ipp.cnr.it; fax 39–011–6705962.

Received July 29, 2004; returned for revision December 1, 2004; accepted December 20, 2004.

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