Food Biochemistry and Food Processing

11 Starch Synthesis in the Potato Tuber 257

sive body of evidence has been provided that there is
a strong correlation between the 3PGA and ADP-
glucose levels and the rate of starch synthesis under
a wide variety of environmental conditions (Geigen-
berger et al. 1998, Geigenberger 2003a).
More recently, an important physiological role for
posttranslational redox regulation of AGPase has
been established (Tiessen et al. 2002). In this case,
reduction of an intermolecular cysteine bridge be-
tween the two small subunits of the heterotetrameric
enzyme leads to a dramatic increase of activity, due
to increased substrate affinities and sensitivity to
allosteric activation by 3PGA (Fig. 11.4). Redox
activation of AGPase in plantacorrelated closely
with the potato tuber sucrose content across a range
of physiological and genetic manipulations (Tiessen

et al. 2002), indicating that redox modulation is part

of a novel regulatory loop that directs incoming

sucrose towards storage starch synthesis (Tiessen et

al. 2003). Crucially, it allows the rate of starch syn-

thesis to be increased in response to sucrose supply

and independently of any increase in metabolite lev-

els (Fig. 11.4), and it is therefore an interesting tar-

get for approaches to improving starch yield (see

below). There are at least two separate sugar signal-

ing pathways leading to posttranslational redox acti-

vation of AGPase, one involving an SNF1-like pro-

tein kinase (SnRK1), the other involving hexokinase

(Tiessen et al. 2003). Both hexokinase and SnRK1

have previously been shown to be involved in the

transcriptional regulation of many plant genes.

Obviously, the transduction pathway that regulates

Figure 11.4.Regulation of starch synthesis in potato tubers: ADPGlc-pyrophosphorylase (AGPase) is a key regula-
tory enzyme of starch biosynthesis. It is regulated at different levels of control, involving allosteric regulation, regula-
tion by posttranslational redox modification and transcriptional regulation. Redox regulation of AGPase represents a
novel mechanism regulating starch synthesis in response to changes in sucrose supply (Tiessen et al. 2002). There
are at least two separate sugar-sensing pathways leading to posttranslational redox activation of AGPase, one
involving an SNF1-like protein kinase (SnRK1), the other involving hexokinase (HK) (Tiessen et al. 2003).