Differential Regulation of ADP-Glucose Pyrophosphorylase in the Sink and Source Tissues of Potato

Expression of potato (Solanum tuberosum L.) ADP-Glc pyrophosphorylase (AGP) was analyzed to assess whether the expression patterns of the individual subunit genes play a role in effectuating AGP activity and hence starch biosynthesis. Temporal analysis revealed that the coordinate expression of the large (IAGP) and small (sAGP) subunits, which collectively make up the heterotetrameric AGP holoenzyme, is primarily under transcriptional control during tuber development. In contrast, noncoordinate expression of the subunit transcripts was evident in leaves in which the relative level of the sAGP mRNA was present at severalfold excess compared to the level of IAGP mRNA. Immunoblot analysis, however, revealed that the levels of sAGP and IAGP polypeptides were present at near equimolar amounts, indicating that a posttranscriptional event co-ordinates subunit polypeptide levels. This posttranscriptional control of subunit abundance was also evident in leaves subjected to a photoperiod regime and during sucrose-induced starch synthesis. The predominant role of transcriptional and posttranscriptional regulation of AGP in tubers and leaves, respectively, is consistent with the distinct pathways of carbon partitioning and with the type and function of starch synthesis that occurs within each tissue.