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Researchers from the Swammerdam Institute for Life Sciences (UvA) and the Universities of Lecce and Perugia have discovered a new organelle in plant cells. The so-called vacuolino is a tiny type of vacuole involved in transportation in the epidermis of flower petals. Their findings are published as cover article in Cell Reports.

A cell from the epidermis of a petal from a petunia flower.

Cells in the epidermis of petals in flowers from different plant species contain a large central vacuole filled with pigments. Francesca Quattrocchio, Ronald Koes, Yanbang Li, Shuang-Jiang Li, Kees Spelt and their Italian colleagues show that these cells also contain smaller vacuoles (vacuolinos) whose presence is regulated together with the acquisition of colour. Any protein that travels from its location of synthesis on the ribosomes, to the central vacuole, briefly localises on vacuolinos. Proteins finally reach their vacuolar destination when vacuolinos fuse to the vacuole itself and deliver their content and membranes.

SNARE proteins

Like any other process of membrane fusion, this event depends on the presence of the so-called SNARE proteins, which regulate the approach and recognition of membranes. However, unexpectedly, the fusion of the vacuolinos to the vacuole also requires the interaction of the SNAREs with a vacuolar transporter (PH1). In the petals of plants in which PH1 is affected by mutations, vacuolinos do not fuse to the central vacuole and instead keep on accumulating, increasing in number and dimensions.

Coexistence of vacuoles

Thorough understanding of the genetics regulating the formation of vacuolinos can provide insight into how these organelles are made, identifying the genes required for their genesis and physiology. This opens the way to study how distinct vacuole with different functions can coexist in one cell and exchange membranes and material.

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A cell from the epidermis of a petal from a petunia flower. The outer cell membrane is labelled in red, the central vacuole is filled wih anthocyanins resulting in the blue colour and vacuolinos are marked in green (photo: UvA).