Arbuscular mycorrhizal fungi differentially affect expression of genes coding for sucrose synthases in maize roots

•   The effects of three arbuscular mycorrhizal (AM) fungal isolates on the expression of genes coding for sucrose synthase and on the nonstructural carbohydrate (CHO) status of maize roots were evaluated. Gene expression and CHOs were compared with their status in nonmycorrhizal plants grown at three soil phosphorus (P) concentrations. •   The AM fungi and soil P supply influenced expression of genes coding for sucrose synthase in maize roots. In general, up to 18 d after plant emergence, AM colonization increased the expression of genes coding for sucrose synthase in maize roots, whereas increasing soil P decreased this gene expression. The responses in gene expression were detected earlier than other effects of AM fungal colonization, such as increased leaf P status and plant growth response under limiting P supply. •   Higher sucrose-synthase gene expression was not related to the concentration of sucrose, reducing sugars or starch in the root tissue. •   Higher gene expression in AM roots confirms that there is greater allocation of sucrose from nonstructural CHO pools in roots for the AM fungus during the earliest phase of colonization than in nonmycorrhizal roots.