and development and for yield-related traits of
crop species will require research on a broader
range of species (Stitt and Zeeman 2012 ). Such
studies will be increasingly straightforward now
that genomic resources are being developed for
many plants, including cultivated species.
Although the starch metabolism pathway
appears well conserved, the extent to which
growth and development is dependent on this
pathway differs between species (Stitt and Zee-
man 2012 ). By virtue of the genomic resources
and the numerous mutants already available,
Lotus japonicusis a useful model plant in which
to uncover some of these differences. Here, we
outline the available resources and review pro-
gress in identifying and functionally character-
ising genes of starch and sucrose metabolism in
this model legume.
10.2 The Pathways of Sucrose
Breakdown and Starch
Metabolism
The entry of sucrose into cellular metabolism is
catalysed by two enzymes—sucrose synthase
and invertase. Sucrose synthases form a small
family of cytosolic enzymes, whereas invertases
constitute a larger family that is divided into two
classes based on properties and cellular location:
acid (vacuolar or cell wall localised) invertases
and neutral/alkaline invertases. The neutral/
alkaline forms can be either cytosolic or organ-
ellar, located either in the plastid or the mito-
chondrion (Vargas and Salerno 2010 ). Sucrose
synthases and invertases catalyse different reac-
tions. The former convert sucrose and UDP to
UDP–glucose and fructose in a physiologically
reversible reaction, whereas the latter convert
sucrose directly to its monomers, glucose and
fructose, in a reaction that is physiologically
irreversible. Entry of sucrose into metabolism via
sucrose synthase requires less consumption of
ATP than entry via invertases and offers more
opportunities for coordination of this process
with demand for carbon by cellular metabolism
(Barratt et al. 2009 ). In this section, we will focus
on sucrose synthases and neutral/alkaline inver-
tases since these enzymes are present in the
cytosol where sucrose enters general cellular
metabolism.
The pathway of starch synthesis was defined
by genetic and biochemical research on specia-
lised starch-storing organs, including the cotyle-
dons of legume seeds (Wang et al. 1998 ). The
enzymes that convert glucose-6-phosphate into
starch are essentially conserved among plant
species (Streb et al. 2009 ; Stitt and Zeeman
2012 ) and simplified synthesis and breakdown
pathways are shown in Fig. 10.1. Glucose 6-
phosphate is converted to glucose 1-phosphate
via phosphoglucomutase, and then, glucose 1-
phosphate is converted to ADP glucose via ADP
glucose pyrophosphorylase with the consump-
tion of ATP. Four different classes of starch
synthase isoforms use the glucosyl moiety of
ADP glucose to elongateα-1,4-linked glucose
chains, into which branch points are introduced
by two classes of isoforms of starch-branching
enzyme. Debranching enzymes called isoamy-
lases subsequently cleave some of the α-1,6
linkages to create a branched polymer—amylo-
pectin—that becomes organised to form the
matrix of the starch granule. Within the matrix, a
fifth class of isoform of starch synthase called
granule-bound starch synthase generates amy-
lose, an essentially linearα-1,4-linked polymer
(Fig.10.1a). Amylose makes up ca. 25 % of the
starch granule in non-photosynthetic organs, and
typically less than 15 % of the starch granule in
leaves (Wang et al. 1998 ; Zeeman et al. 2010 ).
The pathway of starch synthesis occurs inside
plastids in almost all plant organs. In chloroplasts,
glucose 6-phosphate is synthesised from the
Calvin–Benson cycle intermediate fructose 6-
phosphate (Fig.10.1a), whereas in non-photo-
synthetic plastids, it is imported via a phosphate-
exchange translocator from the cytosol. The
exception to this picture is the developing endo-
sperm of cereals, where the conversion of glucose
6-phosphate to the starch synthase substrate ADP
glucose occurs largely in the cytosol, and ADP
glucose is subsequently imported into the plastid
for starch synthesis (James et al. 2003 ).98 C. Vriet et al.