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Project
V. Effects of gravity on plant metabolism (Huber,
Muday,
and Brown).
This research effort is one of five integrated projects that in their
totality comprise the NSCORT in Gravitational Biology. Project V focuses
on the role of carbon metabolism in the gravity response, and two model
systems have been studied within the past year.
1).
Enzymes of sucrose metabolism:
Previously, changes in the intracellular distribution of the sucrose metabolizing
enzyme, sucrose synthase, were noted suggesting a mechanism regulated
by protein phosphorylation that affects the membrane and cytoskeletal
localization of the enzyme (Winter and Huber 2000). Studies have focused
on sucrose synthase, which is an important enzyme of sucrose metabolism
in growing plant tissues such as the maize leaf elongation zone and the
graviresponding maize pulvinus. Sequence analysis of the enzyme suggested
that serine-170 may be a phosphorylation site, along with the previously
identified major phosphorylation site at serine-15. To test this, antibodies
were produced that specifically recognized the sucrose synthase protein
when phosphorylated at the serine-170 site. The results indicate that
this site can be readily phosphorylated, but in vivo, the site is only
weakly phosphorylated. It appears that phosphorylation in vivo may be
restricted by other proteins that bind to sucrose synthase. Further, when
the serine-170 site is phosphorylated, sucrose synthase is "marked"
for degradation by the proteasome system. This could be one mechanism
that regulates the level of the sucrose synthase protein in various plant
tissues (Hardin et al. in press)
2).
Starch granule production:
The effect of spaceflight on starch development in soybean cotyledons
and potato tubers was compared to ground controls. In general, starch
grains from both species were 20-50% smaller than the ground controls.
The densities were similar, but the magnetic susceptibility (movement
of starch grains in a magnetic field) was altered, indicating changes
in the starch structure. There was increased amylose content and an increase
in the rate of starch degradation (from isolated grains) in the space
tissue compared to the ground controls. Higher levels of ethylene were
measured post-flight in the space exposed plants, suggesting that ethylene
is playing a role (Kuznetsov
et al. 2001)