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Review
. 2011 Jul;156(3):1050-7.
doi: 10.1104/pp.111.174581. Epub 2011 Apr 5.

Roles of arbuscular mycorrhizas in plant phosphorus nutrition: interactions between pathways of phosphorus uptake in arbuscular mycorrhizal roots have important implications for understanding and manipulating plant phosphorus acquisition

Sally E Smith  1 , Iver Jakobsen, Mette Grønlund, F Andrew Smith
Affiliations
Review

Roles of arbuscular mycorrhizas in plant phosphorus nutrition: interactions between pathways of phosphorus uptake in arbuscular mycorrhizal roots have important implications for understanding and manipulating plant phosphorus acquisition

Sally E Smith et al. Plant Physiol. 2011 Jul.
No abstract available

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Figures

Figure 1.
Figure 1.
The two pathways of P uptake in an AM root involve different regions of the root, different cell types, and different Pi transporters. In the direct pathway (DP), Pi is absorbed from the rhizosphere by plant Pi transporters in epidermis and root hairs (green circles) close to the root surface. Uptake is normally faster than replacement by diffusion from the bulk soil, resulting in reduced Pi concentrations (depletion) close to the roots (callout 1). In the mycorrhizal pathway, Pi is taken up into AM fungal hyphae by fungal Pi transporters (blue circles) several centimeters from the root and translocated to intracellular fungal structures (arbuscules and hyphal coils) in root cortical cells (callout 2). Plant Pi transporters, induced in colonized cells (yellow circle), transfer Pi from the interfacial apoplast to plant cortical cells (callout 3).
Figure 2.
Figure 2.
Possible signaling events in AM roots based on studies of Pi starvation in nonmycorrhizal plants and miR399 expression in AM medic. In NM plants, low P increases the activity of the transcription factor (TF) PHR1, which binds to the P1BS element in promoters of several Pi starvation-induced genes (A) and increases their expression. PHR1 also increases the expression of miR399s. miR399s are probably largely synthesized in shoots, where they accumulate more in AM than in NM plants (callout 1); this implies the transfer of (unknown) MYC signals from root to shoot in AM plants (callout 2). miR399s are transferred from shoots to roots. Accumulation in roots is influenced by PHR1 and by Suc transport from shoots (callout 3). High miR399 levels under low P reduce the activity of the enzyme encoded by PHO2 and hence increase PHO2-dependent Pi-starvation responses, including increased expression of PiTs (callout 4). Effects of miR399s in reducing PHO2 activity can be quenched by noncoding RNAs such as IPS1 (callout 5). PHO2 might then inhibit Pi-starvation responses and reduce the expression of PiTs. (Modified from Branscheid et al. [2010].)
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