Cell organells always had a deep poetic appeal to my mind. Not only did the chloroplasts or even the mitochondria bell into fantastic and colourful visions of ballooning bubbles, refreshing the cytoplasmic protoplasmic soup, but so were all the other pet-like plastids populating rather specialized and exotic cells. Just like the chromoplasts of petals, the amyloplasts of potato tubercules, or the fatty elaioplasts.
Today, I want to tell you about a very recent brother of all: the bizonoplast*. It really has a lovely name, though its magic decreases a little bit as soon as you cut it into its proper roots. It’s simply made of bi (two), zono (just zone or place), and what the object simply is, another plast. Well, as chloroplast stands for “green shape”, bizonoplast represents a structure characterized by “two distinct zones”.
Actually, bizonoplasts are chloroplasts (that’s pure bizarroplastic!). But they are special chloroplasts. It’s not only because of their typical schizotopic nature, it’s also that they are huge plastids, as illustrated aside (the bizonoplast is the big dark half of the cell, modified after C-R Sheue et al. 2007*). It has a typical chloroplastic lower side, and an upper zone with lots of thylakoids (and that’s for the bizono, for the morphological reason still needs to be worked out).
A cell with a bizonoplast apparently just have one. Sure, it also counts a bunch of mitochondria, but it can only host a single bizonoplast. To date, bizonoplasts are only described from a unique species (Selaginella erythropus, a pike-moss — which isn’t a moss!), but other description may follow in other species. Furthermore, bizonoplasts are not present in each cell of this species, but only in epidermic and some cells. Precisely, you’ll find them in abaxial (i.e. lower) cells of dorsal microphylls and adaxial (i.e. upper) cells of ventral microphylls.
Needless to say, this very specific location makes it easier to have a guess as to what function bizonoplasts have: if you add that the plant cited above is adapted to very deep shades from tropical forests, it follows that bizonoplasts certainly evolved as a mean to keep photosynthesis efficient enough even though the plant is living in poor light places… This needs further investigation but should be promising to understand plant adaptations to shade.
* C-R Sheue et al. 2007. Bizonoplast, a unique chloroplast in the epidermal cells of microphylls in the shade plant Selaginella erythropus (Selaginaceae). American Journal of Botany 94(12): 1922–1929.