Sugar-induced increase in cytosolic Ca²⁺ in Arabidopsis thaliana whole plants

Using Ca²⁺-dependent photoprotein aequorin-transformed Arabidopsis thaliana, sugar-induced increase in cytosolic free Ca²⁺ concentration ([Ca²⁺]_cyt) was investigated by luminescence imaging technique. When 0.1 M sucrose was fed to roots of autotrophically grown intact whole plants whose roots had been incubated overnight with coelenterazine to reconstitute aequorin systemically, strong and transient (within 20 s) luminescence was observed in the roots; that luminescence was followed by weak luminescence moving from the lower leaves to the upper leaves. The moving rate of luminescence was roughly comparable to that of [¹⁴C]sucrose. Application of 0.1 M glucose or fructose induced transient luminescence in excised leaves. No such luminescence was observed in heterotrophically grown (with sucrose) whole plants or in excised tissues. mRNA levels of sucrose-H⁺ symporter genes AtSUC1 and AtSUC2 were higher in autotrophic plants than in heterotrophic plants. These results indicate that influx of transported sucrose together with H⁺ into the mesophyll cells of autotrophic plants may depolarize the membrane potential, and subsequently activate a voltage-gated Ca²⁺ channel on the plasma membrane, resulting in a [Ca²⁺]_cyt increase. The [Ca²⁺]_cyt increase might initiate Ca²⁺ signaling leading to the expression of genes related to biosynthesis of storage carbohydrates. Hexoses, when applied, might also be involved in the [Ca²⁺]_cyt increase mediated by monosaccharide-H⁺ co-transporters.