Abstract
Arbuscular mycorrhizal fungi (AMF) are obligate biotrophs that supply mineral nutrients to the host plant in exchange for carbon derived from photosynthesis. Sucrose is the end-product of photosynthesis and the main compound used by plants to translocate photosynthates to non-photosynthetic tissues. AMF alter carbon distribution in plants by modifying the expression and activity of key enzymes of sucrose biosynthesis, transport, and/or catabolism. Since sucrose is essential for the maintenance of all metabolic and physiological processes, the modifications addressed by AMF can significantly affect plant development and stress responses. AMF also modulate plant lipid biosynthesis to acquire storage reserves, generate biomass, and fulfill its life cycle. In this review we address the most relevant aspects of the influence of AMF on sucrose and lipid metabolism in plants, including its effects on sucrose biosynthesis both in photosynthetic and heterotrophic tissues, and the influence of sucrose on lipid biosynthesis in the context of the symbiosis. We present a hypothetical model of carbon partitioning between plants and AMF in which the coordinated action of sucrose biosynthesis, transport, and catabolism plays a role in the generation of hexose gradients to supply carbon to AMF, and to control the amount of carbon assigned to the fungus.
Highlights
Arbuscular mycorrhiza (AM) is a mutualistic association between fungi from the Glomeromycotina group and plants from most phylogenetic clades [1,2]
We propose that the ability of AM to impact plant carbon allocation and establish itself as a sink depends in part on the ability of arbuscular mycorrhizal fungi (AMF) to stimulate the expression and activity of plant carbohydrate transporters, resulting in the amount of carbon received from the host
The plant lines with higher invertase activity showed lower hexoses content in the roots, together with the up-regulation of the pathogenesis-related (PR) genes PAR1, PR-Q, and PR-1b in the leaves, and reduced levels of root mycorrhization. These results indicated that hexoses accumulated in the leaves activated defense mechanisms potentially with a negative effect on AMF; they suggest that changes in carbohydrate metabolism in shoots influence the establishment of mycorrhizal interaction in the roots, and that this influence is not exclusively determined by the carbon supply to roots [109]
Summary
Arbuscular mycorrhiza (AM) is a mutualistic association between fungi from the Glomeromycotina group and plants from most phylogenetic clades [1,2]. Significant increases in photosynthesis have been reported in mycorrhizal plants, and this increase has been associated with the fungal symbiont acting as an additional carbon sink in the root system [16]. Glucose (Glc) and Fru [25] In mycorrhizal roots, these hexoses are transported across the periarbuscular and fungal plasma membranes in arbusculated cells to reach the fungal partner [26]. The recent characterization of the lipid acquisition by AMF in the mycorrhizae, established with bryophytes and vascular plants, revealed that the mechanisms of lipid biosynthesis and transport are conserved among land plants This supports the hypothesis that land colonization by plants during the Ordovician was favored by mycorrhizal interactions reminiscent of the arbuscular type and primitive plants [31,32,33]. The goal of this review is to analyze the current state of the art on the role of carbon metabolism in mycorrhizae interactions, and to propose an integrated model of carbon partitioning during this symbiotic association
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