let's revive the group

[Charles Delwiche]

In article <9302052250.AA21754 at esds01.es.dupont.com> scolnipa at esvax.dnet.dupont.com writes:
>
>There seems to be a consensus that the development of leaves and 
>chloroplasts follow separate and independent pathways.  Some recent 
>observations in our lab, however, suggest that there is "cross-talk"
>between leaves and chloroplasts during development.  Any useful 
>observations out there?
>
>Pablo A. Scolnik

Hmmm...  This is just off the top of my head, but how has the
"consensus that the development of leaves and chloroplasts follow
separate and independent pathways" emerged?  I find that a surprising
assertion.  I suppose that it would be difficult to demonstrate
interactions between development at two very different levels of
organization (subcellular vs whole-organ), but I personally would be
surprised if there were not at least some cross-talk between the
various levels of organization.  There might not be direct
communication, but I would expect at least associative coordination.
I say this because I am familiar with the close synchronization of
organellar and organismal developmental pathways in simpler organisms
than plants.

I do much of my work on Charophycean (sensu Mattox and Stewart) green
algae -- the Charophyceae are a class of fresh water green algae that
have a number of interesting ultrastructural, biochemical, and
molecular characteristics shared with land plants.  By "land plant" I
mean "bryophytes" and drier plants like ferns, "vascular cryptogams",
and seed plants.  What we understand of photosynthetic eukaryotic
phylogeny suggests that charophytes + plants constitute a monophyletic
group.  But there is much more between-taxon diversity of chloroplast
form in the charophytes than there is plants, and this lets one make
some interesting observations.

For example in one charophycean order, the Coleochaetales, there is
normally only one plastid per cell (this is also the condition for one
group of land plants, the "hornworts", or Anthocerotae, which are
informally lumped as "bryophytes).  This plastid is a large,
frisbee-shaped structure which is typically appressed to the dorsal
cell wall.  When the alga, which is multicellular, grows, it is
obviously imperative that cell division and plastid division be well
synchronized.  It is easy to see this synchronization if you study a
living organism; the whole operation is meticulously choreographed.

Even more elaborate is the plastid division that accompanies zygotic
division.  I don't want to torture you all with the gorey details of
algal life histories, but in the genus _Coleochaete_ when the zygote
germinates it undergoes a poorly understood set of divisions that
ultimately produces 8, 16 or even 32 cells from the zygote.  The
zygote initially has a single plastid like a (very large) vegetative
cell, and this plastid undergoes a fantastically complex division that
partitions the plastid and its sub-organellar components, and also
associates the progeny plastids with the appropriate regions of the
cell for subsequent placement in the multiple cells that are produced
from the zygote.  The meiotic divisions, plastid divisions, and
cytokinesis are all ridiculously complex, and equally well coordinated.

OK, so you would probably say "that's all very well and good, but all
that happens in a single cell, which is a far cry from coordination of
organellar and organ development".  But for me it provides a vivid
demonstration of the fact that algal plastids are not just randomly
partitioned during cell division and development.  Their development
is in fact quite closely coordinated with the development of the alga
as a whole.  It would not surprise me to learn that plant plastids are
similarly coordinated with the overall maturation of the cell.  Is it
asking too much to jump from there to coordination with maturation of
the plant organ?  I don't know.  Maybe.  I do believe that at higher
levels of organization (the leaf) there are emergent properties that
one cannot easily predict from properties at lower levels of
organization (at the level of the cell, or at the subcellular level).

What do you think?  Is this a complete red herring?

Chuck

P.S.  I like this newsgroup, but haven't had the time to post lately.
-- 
Charles F. Delwiche       |  'O Oysters, come and walk with us!'
Dept. Biology, I.U.       |      The Walrus did beseech.
Bloomington, IN 47405     |  'A pleasant walk, a pleasant talk,
[delwiche at bio.indiana.edu]|      Along the briny beach'. -- L. Carrol