SAM meristematic cells of Arabidopsis and most likely other plants,
because these cells give rise to the floral meristem (Fig. 1C), the germ
lines, and ultimately—and most importantly—the female gametes.
In Arabidopsis and many other species, the female gametes are
responsible for the exclusive transmission of all of their chondriome
to the newly formed zygote and thus to the new generations.16,17
In this context, the presence of such a homogenizing mechanism in
SAM cells is critically important for preserving the stability of the
mitochondrial traits that have to be passed on to future gametes and
embryos, i.e., to prevent speciation.
Thus, our hypothesis (Fig. 1C) postulates that mitochondrial
reticulation starts as soon as the embryo germinates and produces a applies to floral organs. During the vegetative to floral meristem
transition, reticulation might be retained, but at the latest would
disappear when the floral organs are formed, since comparatively
few rounds of division are needed to generate the gametophytes
and subsequently the gametes. Finally, after meiosis and microspore
compartmentalization, the developmental need for a mechanism
for generating a homogeneous mtDNA pool to be transmitted to
gametes would no longer be needed. Thus, during gamete, embryo
and seed development, this characteristic mitochondrial feature
would be silenced and would only be reactivated during germination
of the new seedling when a new SAM is formed and vegetative
development starts.
At the molecular level, the different mitochondrial morphologies
in vegetative, reproductive and embryo tissues might be governed
by different sets of tissue-specific machineries for mitochondrial
fusion and fission. In plants, there are many examples of alternative
tissue or organ-specific protein expression patterns. For example, in
Arabidopsis there are eight actin isovariants, five reproductive and
eight vegetative,18 that perform specific functions in each type of
tissue. Since actin is also involved in mitochondrial motility and
shape,19,20 it is tempting to speculate that one set of actin and actinrelated
protein isoforms might help maintain the reticulated type of
mitochondria in SAM cells during vegetative development. On the
other hand, a different set would serve to allow for the maintenance
of a homogeneous population of small, oval mitochondria in the
rest of vegetative tissues as well as in the reproductive and embryo
tissues.