Sucrose-Starch Conversion in Heterotrophic Tissues of Plants

The historical progress in recent years pertaining to the sucrose-starch conversion in heterotrophic tissues of plants has been described. Special attention has been focused on the enzymatic breakdown of sucrose to produce hexose units that are transported to the amyloplast compartment by means of specific translocator molecules and act as glucose donors for starch biosynthesis. Although the current prevailing view is that variable mechanisms operate in different plant tissues and organs, it is often argued that the following enzymic steps are essential in the overall step of sucrose to starch conversion: sucrose + UDP -> UDPGlc + Fru(sucrose synthase-SS) (UDPGlc UDPGlc + PPi -» G1P + UTPpyrophosphorylase-UGPase) (ADPGlc GlP + ATP -» ADPGlc + PPipyrophosphorylase-AGPase) (starch ADPGlc -> starchsynthase) The presence of an ADPGlc-specific translocator in the amyloplast envelope has been demonstrated in a number of plant sources, which indicates the potential role of ADPGlc-synthesizing machineries located in the cytosol of starch-storing cells. Although it was initially believed that AGPase is present exclusively in the amyloplast compartment, the presence of a cytosolic enzyme has been shown in some cereals. The SS has a potential to produce ADPGlc, but the general belief is that this is not a dominant reaction in the mechanism of starch biosynthesis. Numerous experimental trials have been reported by many scientists employing transgenic plants transformed with cDNAs either in antisense- or sense- orientation encoding enzymes which are presumably involved in the process of sucrose-starch conversion. Although great caution is needed to interpret the data obtained, the general picture is contradictory to the mechanism presented above. It now appears that serious reconsideration is needed for the possible mechanism of SS-catalyzed ADPGlc formation and its subsequent link to starch formation. In the newly proposed mechanistic scheme, which appears to be consistent with the results by other scientists as well, hexokinase, phosphoglucomutase (PGM), and ADPGlc formation by AGPase are components in the cyclic turnover of starch molecules in the amyloplast compartment.