Arbuscular mycorrhizal fungi (AM) are indeed mysterious and still surprising organisms. Plenty of papers have recently provided new insights on these organisms. The classification of AM fungi has been reviewed even if it's still under debate. The phosphate transporters in AM fungi and their impact on the phosphate transfer in plant is giving new exciting glances to a crucial step of plant physiology. Other fundamental aspects, such as lipids and carbohydrates metabolism and C translocation, have been clarified by means of NMR experiments. However, the mother of all the questions is still unsolved: what about the genome in AM fungi? There are two levels hidden behind this same question: i) the organization and the complexity of the fungal genome (genome size, ploidy, repeated sequences and so on) and ii) the affordability of a genome sequencing project like those already available for Saccharomyces cerevisiae or Neurospora crassa. The first level is a priority to understand the accomplishment of the AM life cycle, the genetic polymorphisms, as well as the variability which has already been reported at least in ribosomal genes. All these data have so far supported the concept that heterokaryosis, the coexistence of different nuclei in cells, occurs troughout the AM fungal life. Very recent results opened the way to the alternative hypothesis of homokaryosis. The second level will be crucial to compare AM fungi with saprotrophic or pathogenic fungi and to understand how they impact on plant development. Recently it was demonstrated that Glomus intraradices is haploid and has a small genome size in the lower limit for eukaryotes. It will be very important to see whether low values are confirmed also for other AM species: this will allow us to understand whether there is a relationship between biotrophism and genome size. The genomics and functional genomics of mycorrhizal plants are currently important tools to investigate the influence of the fungus in comparison to non-mycorrhizal plants. It has been successfully demonstrated that EST-based approaches are effective in monitoring the interaction between symbionts, but information is mostly about the plant or the mycorrhiza taken as a whole, rather than about the fungus itself. Fungal transcripts coming from mycorrhizal roots are evaluated as not more than the 5%. Therefore, a more direct way is to study the fungus alone, by sequencing the genome and collecting ESTs. In the last months the proposal to sequencing G. intraradices genome has been approved, and this project will be carried out during 2004. In addition to the extraordinary possibility of having a global genetic blueprint for an AM fungus, we can expect to have some answers to one of the basic questions we have: is there a genetic basis for biotrophy? By comparing the genome of G. intraradices with that of a saprobic fungus, we will be able to see whether some genes, crucial for an independent life, are missing and whether the host plant complements the missing functions.
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