Fig. 6.

Proposed model for the cytokinin (CK) regulation of bidirectional exchange of carbon and phosphorus in arbuscular mycorrhizal (AM) symbiosis. A normal CK status in shoots and in roots contributes to balance the bidirectional flow of carbon (C) and phosphorus (P) between symbionts (balanced symbiosis), which is critical for the growth promotion of AM plants (Johnson 2010). A normal CK status of the shoots combined with reduced CK status in the roots maintain a strong source of C from the shoots into the roots (Werner et al. 2008) but may reduce the sink capacity of the roots in relation to that of the AM fungi, irrespective of P supply, causing an unbalanced C for P exchange between symbionts (unbalanced symbiosis). This can lead to fungal parasitism and consequently to reduced growth of AM plants (Johnson 2010). A strongly reduced CK status of the shoots negatively regulates the source of C from the shoots by reducing the availability of sugars (Werner et al. 2008), which may reduce the AM pathway for P uptake, irrespective of the root CK status (reduced symbiosis). Arrow thickness illustrates the relative flow strength of C or P