Strigolactones are plant hormones that stimulate the branching and growth of symbiotic arbuscular mycorrhizal fungi, increasing the probability of contact and establishment of a symbiotic association between the plant and fungus. Strigolactones also inhibit plant shoot branching, and trigger germination of parasitic plant seeds (for example Striga, from which they gained their name).
Strigolactones are carotenoid-derived and contain a labile ether bond that is easily hydrolyzed in the rhizosphere. In 2009, strigolactone biosynthesis was found to be DWARF27-dependent.
Plant roots exude strigolactones which induce spore germination and hyphal branching and increase physiological activity in fungal spores and hyphae. Strigolactones also induce seed germination in parasitic plants, such as Striga124. Fungi produce mycorrhiza (Myc) factors that are operationally defined through their ability to induce calcium oscillations in root epidermal cells34 and to activate plant symbiosis-related genes32. AM fungi form special types of appressoria called hyphopodia, which by definition develop from mature hyphae and not from germination tubes125. As a consequence of sequential chemical and mechanical stimulation, plant cells produce a prepenetration apparatus (PPA). Subsequently, a fungal hypha that extends from the hyphopodium enters the PPA, which guides the fungus through root cells towards the cortex. Here, the fungus leaves the plant cell and enters the apoplast, where it branches and grows laterally along the root axis. These hyphae induce the development of PPA-like structures in inner cortical cells45, subsequently enter these cells and branch to form arbuscules. Vesicles, which are proposed to function as storage organs of the fungus, are sometimes, but not always, formed in AM and are present in the apoplast (not shown). New spores are typically synthesized outside of the plant root at the leading tip of individual fungal hyphae (Arbuscular mycorrhiza: the mother of plant root endosymbioses, 2008)