amyloplasts, statolith & mutant

[caspart at esvax.dnet.dupont.com]

Peter Gogarten asked:

>Until last week I thought that the perception of the 
>gravitational field by statocytes in the root cap was one of 
>the few solved puzzles in the gravitropic response.  
>However, I learned during a discussion that starch free 
>Arabidopsis mutants (which, I assume cannot use amyloplasts 
>as statoliths) do have a gravitropic response (although I 
>read in the Salisbury and Ross' plant physiology textbook 
>that the response is slower). Has anyone looked at the 
>ultrastructure of the root cap in the mutant?  Does anyone 
>have a convincing idea to save amyloplasts as statoliths?
>
>Peter Gogarten
>Peter at carrot.mcb.uconn.edu                                                 

	I have isolated and studied two classes of starchless 
mutants of Arabidopsis.  One class affects the plastid 
isozyme of phosphoglucomutase and the other affects 
ADPglucose pyrophosphorylase.  The PGM mutant has been better 
studied, but where the ADP glucose pyrophosphorylase mutant 
has been studied, its responses seem similar to the PGM 
mutant.  As Peter notes, both mutants are strongly 
gravitropic, although the response is slower than the WT 
(about 70% of the WT rate for a simple induction experiment).  
At least 4 groups have looked at the ultrastructure of the 
PGM mutant.  Three of these groups (Caspar and Pickard, 
Planta 177:185; Sack and Kiss, Am. J. of Bot. 76:454; Moore, 
Ann. Bot. 64:271) have found that the root cap of the mutant 
lacks starch and as a result the plastids are much smaller 
than those in the WT.  Otherwise, however, the root cap of 
the mutant is similar to the WT.  Surprisingly, a fourth 
group (Saether and Iversen, Planta 184:491) has found that 
the mutant contains large amounts of starch in the root cap 
(about 14% of WT levels).  I know of no explanation for this 
discrepancy.
	Peter also asks about what these results mean for the 
starch statolith theory.  At a minimum, the theory must be 
modified to account for these results.  Starch clearly is not 
required for gravitropic sensing.  However, since the 
starchless mutants are gravitropically slower than the WT, 
there appears to be some role for starch in gravitropism.  
One possibility is that the starchless plastids have a 
sufficient mass and density that they can produce an 
attenuated but still sufficient gravity-induced signal to 
trigger the gravitropic response.  Supporting this 
explanation are results (Caspar and Pickard, Planta 177:185; 
Kiss et al., Planta 177:198) which show the starchless 
plastids do not sediment at 1 g but do after centrifugation 
equivalent to 4.5 g.   Alternatively, the starch content of 
the plastid may be required for gravitropism as a source of 
metabolic energy, carbon skeletons, etc.  Supporting this 
explanation is the fact that the starch in the WT root cap 
cells is rapidly broken down as the columella cells slough 
off the root cap indicating it is metabolically accessible.  
In addition, there are many more arguments in each direction 
on this issue which I will not go into here.
	Finally, I have heard reports (although I have not seen 
any publications) that Ken Poff has evidence that there may 
be a second mutation in several independently-isolated PGM 
mutant lines which reduces their gravitropism.  If this is 
true, then the reduced gravitropism in the starchless mutants 
may be unrelated to the absence of starch and the role of 
starch in gravitropism will have to be further re-evaluated.

Tim Caspar
Dupont Central Research and Development