Vacuolino, a Novel Endomebrane Compartment Involved in the Trafficking of Proteins to the Vacuole in Epidermal Petal Cells of Petunia Flowers

To be submitted SUMMARY The lytic vacuole is the main vacuole of the plant cell and is usually indicated as the central vacuole (CV). This compartment occupies up to over 90% of the cell volume and its lumen is more acidic than the surrounding cytoplasm. In the epidermal cells of flower petals the CV specialized to display the color due to anthocyanin pigments accumulation. Here we show that these cells contain, besides the central vacuole, several smaller vacuoles on which tonoplast proteins are sorted before reaching the CV. The small vacuole, here named vacuolino, is absent in mutants for the transcription factors PH3 and PH4 (involved in vacuolar pH regulation). The vacuolino requires the presence of the PH1 P 3B-ATPase to fuse to the CV and is altered by the overexpression of vacuolar SNAREs SYP51 and SYP22. We show that the vacuolino is specific for petal epidermal cells, is not required for correct vacuolar acidification and is part of a novel protein sorting pathway. The availability of mutants that control the biogenesis and/or the fusion of vacuolinos with the CV, provides a formidable chance to study the mechanisms of vacuolar dynamics.