OVEREXPRESSION OF PYROPHOSPHATASE LEADS TO INCREASED SUCROSE DEGRADATION AND STARCH SYNTHESIS, INCREASED ACTIVITIES OF ENZYMES FOR SUCROSE-STARCH INTERCONVERSIONS, AND INCREASED LEVELS OF NUCLEOTIDES IN GROWING POTATO-TUBERS
P. Geigenberger et al., OVEREXPRESSION OF PYROPHOSPHATASE LEADS TO INCREASED SUCROSE DEGRADATION AND STARCH SYNTHESIS, INCREASED ACTIVITIES OF ENZYMES FOR SUCROSE-STARCH INTERCONVERSIONS, AND INCREASED LEVELS OF NUCLEOTIDES IN GROWING POTATO-TUBERS, Planta, 205(3), 1998, pp. 428-437
Overexpression of inorganic pyrophosphatase (PPase) from Escherichia c
oli in the cytosol of plants (ppal plants) leads to a decrease of inor
ganic pyrophosphate (PPi; U. Sonnewald, 1992, Plant J 2. 571-581). The
consequences for sucrose-starch interconversions have now been studie
d in growing potato (Solanum tuberosum L. cv. Desiree) tubers. Sucrose
is degraded via sucrose synthase and UDP-glucose pyrophosphorylase in
growing tubers, and it was expected that the low PPi in the ppal tran
sformants would restrict the mobilisation of sucrose and conversion to
starch. Over-expression of PPase resulted in an accumulation of sucro
se and UDP-glucose, and decreased concentrations of hexose phosphates
and glycerate-3-phosphate in growing ppal tubers. Unexpectedly, the ra
te of degradation of [C-14] sucrose was increased by up to 30%, the ra
te of starch synthesis was increased, and the starch content was incre
ased by 20-30% in ppal tubers compared to wildtype tubers. Reasons for
this unexpectedly efficient conversion of sucrose to starch in the pp
al tubers were investigated. (i)The transformed tubers contained incre
ased activities of several enzymes required for sucrose-starch interco
nversions including two- to threefold more sucrose synthase and 60% mo
re ADP-glucose pyrophosphorylase. They also contained 30-100% increase
d activities of several glycolytic enzymes and amylase, increased prot
ein, and unaltered or slightly decreased starch phosphorylase, acid in
vertase and mannosidase. (ii) The transformants contained higher pools
of uridine nucleotides. As a result, although the UDP-glucose pool is
increased two- to threefold, this does not lead to a decrease of UTP
or UDP. (iii) The transformants contained twofold larger pools of ATP
and ADP, and ADP-glucose was increased by up to threefold. In stored p
pal tubers, there were no changes in the activities of glycolytic enzy
mes, and nucleotides did not increase. It is concluded that in growing
tubers PPi has a wider significance than just being an energy donor f
or specific reactions in the cytosol. Increased rates of PPi hydrolysi
s also affect general aspects of cell activity including the levels of
nucleotides and protein. Possible ways in which PPi hydrolysis could
affect these processes are discussed.