Abstract:
Mechanisms by which plant organs accumulate
photoassimilate are important in determining final sink size
and may involve invertase activity. Because strawberry
fruit are strong sinks for sugars, changes in invertase
levels during development and mechanisms of sugar uptake
within the fruit were examined.
Fruit from greenhouse-grown, day-neutral strawberries,
(Fragaria X ananassa Duch. cv Brighton), were used for all
studies. Fruit invertase levels were analyzed from 0 to 2 5
days post anthesis (DPA). Soluble and insoluble acid
invertases were found, both with optimal activity at pH 4.6.
Soluble invertase activity was highest at anthesis and
decreased during development. Insoluble invertase activity
was low at anthesis, but increased by 5 DPA where it
remained for the following 20 days. Insoluble invertase was
solubilized in 1 M NaCl and is presumably ionically bound to
the cell wall. At 15 DPA, histochemical staining showed
invertase activity (mostly insoluble) throughout the tissue,
but denser around vascular bundles; cortex tissue stained
predominantly in the cell walls. Location of invertase near
vascular bundles suggests a function in phloem unloading.
Sugar uptake kinetics of cortex protoplasts from 16-18
DPA fruit were similar to those found previously in tissue
disks. Glucose uptake was higher than sucrose or fructose
and exhibited simple saturation kinetics. Sucrose and
fructose had similar uptake rates with biphasic curves;
saturable kinetics was observed at low sugar levels (below
10 mM) and first-order (linear) kinetics at higher levels.
Apoplastic sucrose, glucose and fructose concentrations
were estimated to be 80, 80, and 120 mM, respectively.
Higher fructose levels may result from sucrose hydrolysis by
cell wall invertase followed by more rapid uptake of
glucose. The high apoplastic sugar concentrations indicate
that the linear component of sucrose and fructose uptake is
important in sugar uptake in vivo. No competition was seen
among the three sugars, suggesting separate uptake pathways.
1'-Fluorosucrose, a sucrose analog, was not hydrolyzed
by cell wall invertase and showed uptake kinetics similar to
sucrose in cortex tissue. This, coupled with a lack of
competition in sucrose uptake and demonstration of sucrose
uptake by protoplasts, suggests that hydrolysis of sucrose
by cell wall invertase is not a prerequisite for its uptake
in strawberry fruit.
