Plant mitochondria are typically portrayed as being small, oval
organelles. However, a recent study has demonstrated that the
chondriome of shoot apical meristem (SAM) cells of Arabidopsis
thaliana is unique in having two types of mitochondria, a large,
central, tentaculate mitochondrion and variable numbers of small,
oval mitochondria in the cell cortex that fuse with and fission
from the tentaculate mitochondrion.1 The tentaculate mitochondrion
wraps around the nucleus and persists throughout the cell
cycle, undergoing distinct changes in morphology and size in a
cell cycle-dependent manner. Here we demonstrate that SAM cell
plastids, which also contain DNA, do not reticulate, and address
the question as to why SAM cell mitochondria but not plastids
form reticulate structures. We postulate that the presence of a
large, tentaculate mitochondrion in SAM cells provides an efficient
means for homogenizing the mitochondrial DNA and proteins
during vegetative life prior to gamete production, and that this
mitochondrial architecture prevents speciation. The lack of plastid
reticulation in the same cells most likely reflects on the fact that
the individual plastids are much larger than the small mitochondria
and therefore do not need to fuse to achieve efficient intermixing
of their genomes.
Introduction
Mitochondria are highly dynamic organelles that can rapidly
change their form and their distribution in response to changing
cellular energy needs. In plants they have been reported to be oval
or sausage-like organelles capable of undergoing rounds of fusion
and fission.2 With some exceptions,1,3 the observation of reticulartype
morphologies has been restricted to mitochondrial mutants
or to cells subjected to experimental perturbations.2,4-10 Although
mitochondria contain their own DNA, mtDNA, and can synthesize
proteins, they are only semi-autonomous organelles, since most of