Figure 9.

Comparison of strigolactone (SL) exudation into the soil and SL-dependent guidance of arbuscular mycorrhizal fungi to the host root between Petunia axillaris and Medicago truncatula. In P. axillaris, SLs are secreted into the rhizosphere by PaPDR1 that is localized in the hypodermal passage cells (HPCs) within the exodermis. A steep concentration gradient of SLs, created by PaPDR1, guides arbuscular mycorrhizal fungi (AMF) to access penetration sites – i.e. unsuberized HPCs. In the M. truncatula, due to a lack of exodermis, SLs could passively enter the rhizosphere because they do not encounter the apoplastic diffusion barrier. However, we suggest that to achieve the full extent of AMF colonization, the active export of SLs, mediated by MtABCG59, is required. Its action ensures sufficiently high concentrations of SLs around the roots to attract AM fungi, which together with the characteristic root anatomical traits in M. truncatula, enables the colonization of the whole root surface.