Background: The biochemical mechanisms that determine the molecular architecture of
amylopectin are central in plant biology because they allow long-term storage of reduced carbon.
Amylopectin structure imparts the ability to form semi-crystalline starch granules, which in turn
provides its glucose storage function. The enzymatic steps of amylopectin biosynthesis resemble
those of the soluble polymer glycogen, however, the reasons for amylopectin's architectural
distinctions are not clearly understood. The multiplicity of starch biosynthetic enzymes conserved
in plants likely is involved. For example, amylopectin chain elongation in plants involves five
conserved classes of starch synthase (SS), whereas glycogen biosynthesis typically requires only one
class of glycogen synthase.
Results: Null mutations were characterized in AtSS2, which codes for SSII, and mutant lines were
compared to lines lacking SSIII and to an Atss2, Atss3 double mutant. Loss of SSII did not affect
growth rate or starch quantity, but caused increased amylose/amylopectin ratio, increased total
amylose, and deficiency in amylopectin chains with degree of polymerization (DP) 12 to DP28. In
contrast, loss of both SSII and SSIII caused slower plant growth and dramatically reduced starch
content. Extreme deficiency in DP12 to DP28 chains occurred in the double mutant, far more
severe than the summed changes in SSII- or SSIII-deficient plants lacking only one of the two
enzymes.
Conclusion: SSII and SSIII have partially redundant functions in determination of amylopectin
structure, and these roles cannot be substituted by any other conserved SS, specifically SSI, GBSSI,
or SSIV. Even though SSIII is not required for the normal abundance of glucan chains of DP12 to
DP18, the enzyme clearly is capable of functioning in production such chains. The role of SSIII in
producing these chains cannot be detected simply by analysis of an individual mutation.
Competition between different SSs for binding to substrate could in part explain the specific
distribution of glucan chains within amylopectin