Arbuscular mycorrhizal fungi affect root morphology locally but not systemically through altering nutrient- and phytohormone-related gene expressions at low soil P level

Arbuscular mycorrhizal fungi (AMF) can affect plant root morphology. However, whether the impact is localized or systemic is still unclear.

A root-splitting microcosm experiment was conducted to examine AMF effects on the root morphology of their host plants at two soil phosphorus (P) levels (5.7 and 25 mg kg⁻¹). The main treatment (MT) had one root chamber with AMF inoculation and the other without, while the arbuscular mycorrhizal (AC) or non-inoculation control (NC) included the active or sterilized fungal inoculum in both root chambers, respectively.

In the MT, root length and surface area were larger in the AMF infected than non-infected chambers, particularly for the secondary lateral root development. Notably, mycorrhizal promotion was stronger at low than high P level. Transcriptome sequencing revealed that AMF infection upregulated gene expressions particularly in pathways related to starch and sucrose metabolism, phenylpropanoid metabolism, terpenoid biosynthesis, fatty acid synthesis, and hormone metabolism and transport. Moreover, the number of upregulated genes was higher at low than high P level. Redundancy analysis demonstrated that the key differential genes were related to nutrients, phytohormones and root morphological traits.

These findings suggest that AMF can induce a localized effect on root morphology, potentially through the upregulation of nutrient- and hormone-related gene expressions, but the magnitude depends on soil P levels.