Is genetic manipulation of arbuscular mycorrhizal fungi possible?

Unlike many fungi, arbuscular mycorrhizal (AM) fungi have proven recalcitrant to genetic manipulation due to their obligate biotrophic lifestyle and multinucleate, coenocytic cellular structure. In this review, we examine past attempts at AM fungal transformation, we identify key biological and technical barriers and explore recent advances to overcome them. We focus on techniques never before applied in AM fungi, including CRISPR/Cas9, microinjection, and protoplast-based transformation, and we explore how they provide viable strategies for achieving this elusive goal. We conclude by outlining guidelines for future research, distinguishing between established approaches that are readily applicable to AM fungi and others that first require addressing key outstanding questions in AM fungal cell biology and genetics to ensure success.