TLWT and TLUp1-9 had similar amounts of AGPase activity, which
was
∼2-fold more than WT (results not shown) and they fixed CO2
at similar rates (Table 1). TLWT, however, accumulated less than
70% of the amount of leaf starch as TLUp1-9. These differences are
likely due to the regulatory properties of AGPase activities in these
plant lines because TLWT expresses a WT enzyme while TLUp1-9
expresses an enzyme with up-regulatory properties, i.e. more sensitive
to activation by 3-PGA and less sensitive to inhibition by Pi
[14,20]. Consistent with this view, TLUp1-9 contained up to twice as
much AGPase enzyme activity as WT plants when assayed under
3-PGA activating conditions (Fig. 1A). Although the AGPase activity
in TLUp1-9 was sensitive to Pi inhibition, it required more than
twice the amount of this effector to obtain the same degree of
inhibition as the WT plants (Fig. 1B). Hence, when measured at
0.25 mM 3-PGA, TLUp1-9 displayed two- to four-fold higher AGPase
activity levels than WT over a range of Pi concentrations. The upregulatory
properties of the AGPase activity in TLUp2-1 may also
account for the enhancement of starch accumulation compared to
TL46. This transgenic line, which expresses an AGP-L1 equivalent
to the potato UpReg2 [14,20], accumulated similar levels of starch
as TLWT despite having a lower CO2 assimilation rate.