AMF-induced tolerance to drought stress in citrus: A review

• Citrus is often exposed to drought stress, which restricts tree growth and yield.
• Citrus rhizosphere inhabits AMF, which can form arbuscular mycorrhizas with roots.
• AM-inoculated citrus has shown greater tolerance to drought stress.
• An overview of possible AM-mediated mechanisms to drought tolerance was provided.

Citrus is one of the most widely cultivated fruit crops, whose rhizosphere inhabits a class of beneficial fungi, popularly known as arbuscular mycorrhizal fungi (AMF). Different species of AMF viz., Acaulospora, Entrophospora, Gigaspora, Glomus, Pacispora, Sclerocystis, and Scutellospora have been observed to colonize citrus roots for the formation of arbuscular mycorrhizal (AM) symbiosis, where both the symbiotic partners are mutually benefited (up to 20% of photosynthetic carbohydrates from the host plant is diverted toward the growth of AM, in the exchange of water and nutrient uptake from the fungal partner to the host plant). AM symbiosis can usually confer better plant growth, higher nutrient uptake, greater tolerance to abiotic and biotic stresses, and soil structure improvement in the host plant. Meanwhile, AM-inoculated citrus plants have shown greater tolerance to drought stress (DS). Drought stress strongly restricted both the development of non-AM-citrus and the mycorrhizal development of AM-citrus, but AM colonization produced a positive effect on plant growth and photosynthesis, even under DS. This review provides an overview of possible mechanisms involved in DS tolerance through improved water and nutrient uptake (especially P nutrition) using extraradical hyphal growth; effective spatial configuration of root system; elevated concentration of tetramine spermine; osmotic adjustment through non-structural carbohydrates, K+, Ca2+, and Mg2+, but not proline; scavenging reactive oxygen species through antioxidant enzymes and antioxidants; and glomalin-bound soil structural improvements, besides, some new exciting perspectives including water transport by mycorrhizal hyphae and molecular analysis are suggested.