Arbuscular mycorrhizal fungi induce sucrose cleavage for carbon supply of arbuscular mycorrhizas in citrus genotypes

• The study evaluated sucrose cleavage induced by AMs in three citrus genotypes.
• AMs induced more glucose accumulation in leaf, root, and total plant.
• AM colonization notably positively correlated with glucose but not sucrose.
• AMs induced different responses of AI, NI, or SS between leaf and root.
• Sucrose cleavage lied on sucrose-cleaving enzymes and citrus genotypes for AMs.

Arbuscular mycorrhizas (AMs) must gain carbohydrates (mainly glucose) through sucrose cleavage from their plant hosts. Using the AM fungus Funneliformis mosseae inoculated or non-inoculated citrus genotypes [red tangerine (Citrus tangerina Hort. ex Tanaka), kumquat (Fortunella margarita Lour. Swingle), and trifoliate orange (Poncirus trifoliata L. Raf.)], we tested the hypothesis that the sucrose cleavage would depend on sucrose-cleaving enzyme types and/or host genotypes for carbon supply to the AM development. Mycorrhizal inoculation significantly increased biomass production and glucose contents of leaf, root, and total plant, irrespectively of citrus genotypes. In contrast, under mycorrhization similar leaf sucrose contents were observed between these three citrus genotypes, whereas significantly higher sucrose contents were in roots of red tangerine and kumquat, and in total plant (leaf plus root) of red tangerine and trifoliate orange. Root AM colonization was significantly positively correlated with leaf and total plant glucose but not with root glucose or with sucrose in either leaf and/or root. Root AM colonization increased the activity of leaf acid invertase (AI) and sucrose synthase (SS) and root neutral invertase (NI) in trifoliate orange, leaf SS and root AI in kumquat, and root NI in red tangerine, but decreased leaf NI in all three citrus genotypes, leaf SS in red tangerine, root AI in red tangerine and trifoliate orange, and root SS in kumquat and trifoliate orange. These results support our hypothesis that the sucrose cleavage highly depended on both citrus genotypes and sucrose-cleaving enzyme types/activity, and thus may provide higher glucose as carbon source to support their fungal partners.