Annexins - calcium- and membrane-binding proteins in the plant kingdom: potential role in nodulation and mycorrhization in Medicago truncatula.

Annexins belong to a family of multi-functional membrane- and Ca(2+)-binding proteins. The characteristic feature of these proteins is that they can bind membrane phospholipids in a reversible, Ca(2+)-dependent manner. While animal annexins have been known for a long time and are fairly well characterized, their plant counterparts were discovered only in 1989, in tomato, and have not been thoroughly studied yet. In the present review, we discuss the available information about plant annexins with special emphasis on biochemical and functional properties of some of them. In addition, we propose a link between annexins and symbiosis and Nod factor signal transduction in the legume plant, Medicago truncatula. A specific calcium response, calcium spiking, is an essential component of the Nod factor signal transduction pathway in legume plants. The potential role of annexins in the generation and propagation of this calcium signal is considered in this review. M. truncatula annexin 1 (MtAnn1) is a typical member of the plant annexin family, structurally similar to other members of the family. Expression of the MtAnn1 gene is specifically induced during symbiotic associations with both Sinorhizobium meliloti and the mycorrhizal fungus Glomus intraradices. Furthermore, it has been reported that the MtAnn1 protein is preferentially localized at the nuclear periphery of rhizobial-activated cortical cells, suggesting a possible role of this annexin in the calcium response signal elicited by symbiotic signals from rhizobia and mycorrhizal fungi.