Calcium Related Fungal Genes Implicated In Arbuscular Mycorrhiza

Fluctuations in intracellular (Ca2+) calcium levels generate signalling events and regulate different cellular processes. Whilst the implication of Ca2+in plant cell responses during arbuscular mycorrhiza (AM) interactions is well documented, nothing is known about the regulation or role of this secondary meesenger in the fungal symbiont. The molecular basis of fungal calcium homeostasis in the AM symbiosis was analyzed by investigating the expression of Ca2+-related fungal genes. In a first study, G. mosseae genes putatively encoding a MAP3k-like protein kinase (Gm2) and a P-type ATPase (Gm152) were investigated. Both Ca2+-related genes were up-regulated by A. sinicum root exudates, suggesting a role in early interactions prior to symbiosis establishment. The full-length cDNA sequence of Gm152obtained from germinating spores of G mosseae confirmed its identity. The role of Ca2+in fungal processes leading to establishment of an AM symbiosis was investigated in more detail in G intraradices-M. truncatula interactions. Enhanced expression of genes encoding six membrane transport proteins and one nuclear protein kinase, selected from the G intraradices transcriptome database, was related to colonization of wild-type M. truncatula (line J5) roots and not observed with the mycorrhiza-resistant mutant dmi3/Mtsym13. Laser microdissection mapping of transcripts indicated that the Ca2+-related G intraradices genes were differentially up-regulated in arbuscules and/or in intercellular hyphae. The tempo-spatial variations in fungal gene expression suggest different roles in the development or functioning of the AM symbiosis. Full-length cDNA of three G intraradices genes putatively encoding a PMR-like endoplasmic reticulum P-type ATPase, a VCX1-like vacuolar Ca2+ion transporter and a nuclear CCaMK were obtained for functional analyses in yeast mutants to gain insight into their role in the mycorrhizal symbiosis. Possible mechanisms are discussed in which Ca2+-related proteins of G intraradices may play a role in the mobilization and perception of the intracellular messenger by the AM fungus during symbiotic interactions with host roots.