Arbuscular Mycorrhizal Symbiosis: From Infection Signaling to Bidirectional Nutrient Exchanges

Arbuscular mycorrhizal symbiosis evolved between the first land plants and the ancestor of arbuscular mycorrhizal fungi 400 million years ago; however, due to mandatory biotrophy and the lack of protocols for genetic transformations, we still lack a lot of information about this symbiosis in the literature. For example: Do mycorrhizal factors also cause calcium influx into the epidermis—as do Nod factors? Which carbon source is most critical for arbuscular mycorrhizal fungus: lipids or hexoses? Would the SUT2 transporter and abscisic acid control the expansion of the intraradical mycelium? These and other questions are discussed here to help fill in the above gaps. And that is why, in this review, we present a vast research on the asymbiotic, pre-symbiotic, and symbiotic phases of arbuscular mycorrhizal interaction: from infection signaling (host plant and fungus signaling) to bidirectional nutrient exchanges (phosphorus, nitrogen, potassium, sulfate, and carbon: lipids, sucrose, and hexoses), passing through the phytohormones and transporters involved in the formation and development of the arbuscule.