Tissue level localization of acid invertase in leaves: an hypothesis for the regulation of carbon export

Photoassimilate produced by mature leaves is usually loaded into the phloem as sucrose. Transport of sucrose from the mesophyll cells to the phloem might involve an apoplastic step or be entirely symplastic, depending on species. Acid invertase occurs as both soluble and cell-wall-bound forms and has been detected in mature leaves. However, the ability of this enzyme to act as a regulator of carbon export and storage depends on whether it occurs either in the sucrose transport path (which could decrease export), or is located within the mesophyll cells (potentially resulting in intracellular futile cycling between sucrose synthesis in the cytoplasm and hydrolysis in the vacuole). Tissue-level localization of acid invertase was investigated by tissue printing, which establishes protein distribution within a leaf. Antiserum was raised against a synthetic peptide corresponding to a region close to the active site of acid invertase. Tissue prints from Pisum sativum L. and Hordeum vulgare L. showed invertase to be located in the vascular regions, which was not observed in Fuchsia hybrida L. This corresponds to differences in the presence of oligosaccharides in leaf extracts and in the pattern of plasmodesmatal connections between mesophyll cells and the vasculature. We hypothesize that the location of acid invertase activity is implicated in species-specific differences in sucrose and oligosaccharide metabolism.