Can hypodermal passage cell distribution limit root penetration by mycorrhizal fungi?

The basis for significant interspecific variability in colonization by arbuscular mycorrhizal fungi is poorly understood. Limited evidence suggests that, for species with a dimorphic hypodermis, colonization of the root cortex occurs only through hypodermal passage cells. Therefore, the hypothesis that interspecific variability in mycorrhizal colonization is accounted for by interspecific variation in passage cell distribution was tested. The arbuscular mycorrhizal colonization and distribution of fungal penetration points and hypodermal passage cells in the root systems of eight species (seven plant families) possessing a dimorphic hypodermis were characterized. Mycorrhizal fungal penetration of the hypodermis occurred exclusively through passage cells. Moreover, the proportion of root length with passage cells explained nearly 99% of the variability among the eight plant species in the proportion of root length with penetration points. In dimorphic hypodermal species, passage cells appear to be key determinants of mycorrhizal colonization because they are the cells through which fungal penetration of the hypodermis occurs. Variation among such species in mycorrhizal colonization may be at least partly determined by variation in the proportion of root length with passage cells.