Agriculture Reference
In-Depth Information
mainly those involved in glycolysis such as UDP-glucose
dehydrogenase, fructokinase, phosphoglucomutase, and
pyrurate decarboxylase.
Prior to the synthesis of amylose and amylopectin, sucrose
loaded into endosperm cells by sucrose transporters and
converted to Glc-6-P via several reaction steps catalyzed
by enzymes such as Suc synthase and then into ADP-G1C,
the substrate for Į-1, 4-polyglucan synthesis, by ADP-G1C
pyrophosphorylase. Import of ADP-G1C and G1C-6-P into
amyloplasts is conducted by distinct transporters. For each
enzyme and transporter involved, several isoforms are known
to be differentially regulated. Although amylose synthesis
is exclusively governed by granule bound starch synthase,
amylopectin is synthesized via concerted reactions catalyzed
by soluble starch synthase, branching enzyme and starch
debranching enzyme (Nakamura 2002). In particular, the
relative balance of Į-1, 6 branch formation and the subsequent
Į-1, 4-chain elongation which are catalyzed by distinct
branching enzyme and soluble starch synthase isoforms,
respectively, are important for determining the amylopectin
fi ne structure (Nakamura 2002) (Fig. 36).
A comprehensive analysis of the transcript levels of genes
which encode starch synthesis enzymes is fundamental for the
assessment of the function of each enzymes and the regulatory
mechanism for starch biosynthesis in source and sink organs.
Using quantitative real-time RT-PCR, an examination was made
of the expression profi les of 27 rice genes encoding six classes
of enzymes, i.e., ADP glucose pyrophosphorylase (AG Pase),
starch synthase, starch branching enzyme in developing seeds
and leaves. The modes of gene expression were tissue and
developmental stage specifi c, four patterns of expression in the
seed were identifi ed: group I genes, which are expressed very
