Why do Mitochondria but not Plastids of Arabidopsis SAM
Cells Reticulate?
Two of the principal architectural differences between SAM cell
mitochondria and chloroplasts are their size and their numbers.
In interphase cells, the mitochondria contribute ~10% of the cell
volume,1 whereas the volume contribution of plastids is closer to
20%. Since the number of plastids is also smaller than the total
number of mitochondria, the average plastid is much larger in
volume and contains a greater number of copies of plastid DNA.
Thus, one explanation for the absence of plastid reticulation is that
the individual
plastids are large enough to permit efficient intermixing
of chloroplast DNA and therefore no reticulation of the plastids is
needed. To what extent the plastids fuse and fission in SAM cells is
unknown. However, since the chloroplast DNA always seems to be
anchored to thylakoids, and stromules do not contain any thylakoids,
it is unlikely that significant amounts of DNA intermixing occurs via
stromule-type bridges.24 In conclusion, we postulate that organelle
size might explain why mitochondria but not plastids form reticulate
structures in SAM cells.