Sucrose-starch conversion in heterotrophic tissues of plants

The historical progress in recent years pertaining to the sucrose-starch co nversion in heterotrophic tissues of plants has been described. Special att ention has been focused on the enzymatic breakdown of sucrose to produce he xose units that are transported to the amyloplast compartment by means of s pecific translocator molecules and act as glucose donors for starch biosynt hesis. Although the current prevailing view is that variable mechanisms ope rate in different plant tissues and organs, it is often argued that the fol lowing enzymic steps are essential in the overall step of sucrose to starch conversion: sucrose + UDP --> UDPGlc + Fru (sucrose synthase-SS) UDPGlc + PPi --> GlP + UTP (UDPGlc pyrophosphorylase-UGPase) GlP + ATP --> ADPGlc + PPi (ADPGlc pyrophosphorylase-AGPase) ADPGlc --> starch (starch synthase) The presence of an ADPGlc-specific translocator in the amyloplast envelope has been demonstrated in a number of plant sources, which indicates the pot ential role of ADPGlc-synthesizing machineries located in the cytosol of st arch-storing cells. Although it was initially believed that AGPase is prese nt exclusively in the amyloplast compartment, the presence of a cytosolic e nzyme has been shown in some cereals. The SS has a potential to produce ADP Glc, but the general belief is that this is not a dominant reaction in the mechanism of starch biosynthesis. Numerous experimental trials have been re ported by many scientists employing transgenic plants transformed with cDNA s either in antisense- or sense- orientation encoding enzymes which are pre sumably involved in the process of sucrose-starch conversion. Although grea t 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 ADPGl c formation and its subsequent link to starch formation. In the newly propo sed 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 molecul es in the amyloplast compartment.