The Lotus japonicus Genome

Major advances in understanding starch deg-

radation have come from research inArabidopsis

(Fig.10.1b) (Kötting et al. 2010 ; Smith et al.

2005 ; Smith 2012 ; Stitt and Zeeman 2012 ). The

pathway inArabidopsisleaves is very different

from the well-established pathway in the endo-

sperm of germinating cereal grains. Briefly, this

pathway is composed of three major steps. The

first is a reduction in the organisation of the

starch granule surface, making it more accessible

to hydrolytic attack. This is brought about by

phosphorylation of small numbers of glucosyl

residues within amylopectin molecules by the

enzymes glucan, water dikinase (GWD) and

phosphoglucan, water dikinase (PWD, also

called GWD3). The second step is the hydrolysis

of the starch polymers byß-amylases (BAM) and

isoamylase 3 (ISA3). Hydrolysis is facilitated by

dephosphorylation of amylopectin chains by the

phosphoglucan phosphatases SEX4 and LSF2

(Zeeman et al. 2010 ; Santelia et al. 2012 ; Smith

2012 ). The products of hydrolysis are maltose

and short linear glucans. These glucans are fur-

ther metabolised via a glucanotransferase (dis-

proportionating enzyme) DPE1, yielding

glucose. Finally, maltose and glucose are

exported via specific transporters (MEX1 and

GlcT, respectively) to the cytosol, where they are

metabolised to sucrose via a complex pathway

involving a second glucanotransferase DPE2 and

a glucan phosphorylase PHS2 (Ruzanski et al.

2013 ).

10.3 Genes Encoding Enzymes

of Starch and Sucrose

Metabolism inLotus japonicus

Candidate genes encoding enzymes involved in

sucrose catabolism and in the synthesis and

degradation of starch were identified by Vriet

et al. ( 2010 ) by searching the Miyakogusa gen-

ome database (http://www.kazusa.jp/lotus/index.

html) using the gene names as keywords and/or

by performing Blastp searches using the protein

Fig. 10.1 Pathways of starch synthesis (a) and degrada-
tion (b) in the leaf. Much of this information has been
obtained through studies inArabidopsis.aPathway of
starch synthesis during the day. Intermediates from the
Calvin–Benson cycle are used for the synthesis of sucrose
and starch. Starch is synthesised inside the chloroplast;
sucrose is synthesised in the cytosol from triose phos-
phates, exported from the chloroplast via the triose
phosphate transporter.SSstarch synthase;SBEstarch-
branching enzyme;ISAisoamylase.bPathway of starch
degradation at night. Phosphorylation by glucan water
dikinase (GWD, also called GWD1) and phosphoglucan
water dikinase (PWD, also called GWD3) makes the
granule surface accessible to β-amylases (BAM) and
isoamylases (ISA) following removal of the phosphate by
the enzyme SEX4 (a glucan phosphate phosphatase). The
hydrolysis products maltose, and some longer malto-
oligosaccharides are further metabolised by the glucano-
transferase, DPE1, to glucose. Maltose is exported from
the chloroplast via the maltose transporter, MEX1, and
glucose via the GlcT transporter. In the cytosol, maltose is
converted to hexose phosphates via the glucanotransfer-
ase, DPE2. The next reactions involve a putative heter-
oglycan as a glucosyl acceptor and the glucan
phosphorylase, PHS2. The size of the arrows indicates
approximately the importance of each pathway; for
example, greater export is via MEX1 than GlcT

10 Sucrose and Starch Metabolism 99