The complete Glomus intraradices mitochondrial genome sequence – a milestone in mycorrhizal research

摘要

Arbuscular mycorrhizal fungi (AMF) belong to the Zygomycota, a phylogenetically inhomogeneous (paraphyletic) taxon. Arbuscular mycorrhizal fungi are ubiquitous and most common in soil, forming symbioses with the roots of approx. 80% of all vascular plant species (Smith & Read, 1997). They offer a variety of benefits to their host, including improved phosphate and water uptake, drought tolerance and resistance to root pathogens. Arbuscular mycorrhizal fungi reproduce by asexual spores that contain hundreds of nuclei and mitochondria (Fig. 1, Supporting Information Video S1). Arbuscular mycorrhizal fungi cannot be cultivated without a host plant (i.e. they are obligate biotrophs) and are therefore notoriously elusive to molecular biology and geneticsresearch. A common culturing technique is to grow AMF together with T-DNA-transformed carrot roots on solid synthetic media (Bécard & Fortin, 1988). This technique usually permits the purification of AMF spores en masse, but collecting adequate amountsof the fragile vegetative cells (hyphae) remains challenging. Therefore, many fundamental questions bearing on AMF biology remain difficult to investigate. In similar cases, nuclear genome sequencing has been used to kick-start the development of molecular genetics and biochemical research. Not so in AMF. The nuclear genome project has turned into a sequence assembly nightmare (Martin et al., 2008). Evidently, and contrary to previous views (e.g. Pawlowska & Taylor, 2004), gene sequences differ substantially within these populations of nuclei (Hijri & Sanders, 2005; Croll & Sanders, 2009; Croll et al., 2009). This implies that nuclear genome sequencing is not a simple matter of deciphering 15 Mbp (the estimated average genome size within Glomus intraradices nuclear populations; Hijri & Sanders, 2004), but two to three orders of magnitude more. Despite all these obstacles, it has been shown that genetic exchange between distinct isolates of G. intraradices does in fact occur via cellular connections,or anastomoses (Croll et al., 2009). There is also early evidence for limited recombination among nuclear genes (Croll & Sanders, 2009).