gravitropism. A mutation in this gene resulted in plants displaying altered kinetics of root
graviresponse and longer presentation time relative to wild type. Root cap morphology
was also altered, probably as a consequence of altered auxin accumulation in the root cap.
Upon gravistimulation, adk1-1mutant seedlings failed to relocalize the auxin efflux fa-
cilitator PIN3 to the lower membrane of their root statocytes, confirming a role for this
gene in early phases of gravity signal transduction (Young et al. 2006).
Surprisingly, adk1-1roots displayed wild-type cytoplasmic alkalinization of their sta-
tocytes in response to gravistimulation. Furthermore, when expression of three of the five
fast gravity-responsive genes described above was analyzed, one (At4g23670) still in-
creased in expression in adk1-1root tips upon gravistimulation as it did in wild type,
whereas the two other genes (At5g38020andAt5g48010) did not respond to gravistimu-
lation in adk1-1(Yester et al. 2006). Hence, adk1-1affected differently the expression
response to gravistimulation of three fast gravity-responsive genes, even though arg1-2
andarl2-1obliterated completely the response of all three, as discussed above (Yester et
al. 2006). It is interesting to note here that At5g38020,whose expression response to
gravistimulation is obliterated by adk1-1, encodes an enzyme whose predicted biochem-
ical function (AdoMet-dependent methyltransferase activity) is directly associated with
the AdoMet cycle (Kimbrough et al. 2004; Schoor and Moffatt 2004; Yester et al. 2006).
Together, these fascinating results can be interpreted in several ways. For instance,
gravity signal transduction could involve a single linear pathway in which ADK1func-
tions downstream of ARG1to mediate gravity-induced PIN3 relocalization and differen-
tial expression of At5g38020andAt5g48010in the statocytes, with gravity-induced cy-
toplasmic alkalinization and differential expression of At4g23670requiring only the
presence of functional ARG1(Figure 2.2A). Alternatively, it is possible that the gravity
signal transduction pathway is bifurcated, with the ADK1-dependent branch of the path-
way leading to PIN3 relocalization and up-regulation of At5g38020andAt5g48010
whereas the other branch would lead to cytoplasmic alkalinization and activation of
At4g23670. In this case, ARG1would function upstream of ADK1, before the point of
pathway bifurcation (Figure 2.2B). Further genetic analysis of double and multiple mu-
tants should help resolve this ambiguity.
The data discussed above support a role for ADK1in early phases of gravity signal
transduction in roots. However, the molecular mechanisms underlying its contribution re-
main uncharacterized. Does adenosine, the main substrate of ADK that feedback inhibits
the AdoMet cycle (Schoor and Moffatt 2004), function in cellular signaling like it does
in animal systems (Nishizaki 2004)? Or do other AdoMet cycle-derived regulatory com-
pounds modulate gravity signal transduction? A systematic study on the contribution of
distinct biochemical branches derived from the AdoMet pathway in different phases of
the root gravitropic response will undoubtedly yield new insights into these long-ranging
questions. Preliminary results suggest a role for spermine, an AdoMet-derived
polyamine, in the signal-transmission or curvature-response phases of root gravitropism
(Young et al. 2006).
In conclusion, the genes and proteins identified through the genomic and proteomic
screens described above constitute good candidates as gravity signal transducers. They
belong to only a few predicted functional categories, thereby identifying specific bio-
chemical pathways (such as the AdoMet cycle) as potential contributors to the process.
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