The cell is the fundamental unit of construction from which all living things are built, whether it is a microscopic unicellular amoeba or an enormous multicellular giant redwood. Internally, each cell is furnished with a range of smaller bodies – termed organelles – that perform various essential functions; e.g. chloroplasts in plant cells that are involved in photosynthesis, mitochondria in both animal and plant cells that engage in respiration, and ribosomes that facilitate protein synthesis. Given the importance of cells and cell structure you might be forgiven for thinking that we know all there is to know – well, at least are aware of all of the organelles that exist. Not so, as work by Jean-Marc Brillouet and colleagues demonstrate. The team, based in Montpellier, Le Mans (both in France) and Budapest (Hungary), have identified a new organelle in plant cells. They have called it the tannosome because it is involved in the formation of condensed tannins (compounds present in most land plants and which are thought to provide defence against herbivores and pathogens). Although tannins have been recognised as present within the vacuoles in plant cells, their site of synthesis has hitherto not been determined. Using a wide range of techniques – including light and electron microscopy, antibody-localisation, chemical analysis and cell fractionation – the team concludes that the tannins are formed within a new type of plastid (a group of plant organelles that includes the chloroplast and the amyloplast), the aforementioned tannosome. These are derived from the thylakoids within the chloroplast and once formed, small protrusions develop from the latter’s surface and the tannin-packed tannosomes are transported as tiny membrane-bound spheres to the vacuole, in which organelle they accumulate. Examining a range of plants from the groups in the plant kingdom, including cycads, ginkgo, horse-tails, ferns, conifers and flowering plants, they conclude that tannosomes are likely to be universal amongst those so-called vascular plants. Aside from their protective roles in plants, tannins are also important in making tea and red wine taste the way they do. It isn’t every day that a new organelle is identified, but this goes to show that something as commonplace and seemingly familiar as the cell still has secrets to be discovered. I wonder how long it will take for tannosome to be rechristened tannoplast (or will that cause too much confusion with tonoplast…)?
[And if you’re wondering when the previous new plant organelle was identified, my suggestion is plastid stromules (in 1997…). And for more on plant tannin research, check out this work by Dr Irene Müller-Harvey at the University of Reading (UK) – Ed.]