EXPRESSION OF ADP-GLUCOSE PYROPHOSPHORYLASE IN MAIZE (ZEA-MAYS L) GRAIN AND SOURCE LEAF DURING GRAIN FILLING

The time course of ADP-glucose pyrophosphorylase activity and of starc h accumulation rate measured in grain, from pollination to maturity, i n Zea mays L. plants grown outdoors, was coincident for 2 years. No su ch correlation was observed in the adjacent leaf, which, furthermore, presented large year-to-year differences in starch accumulation patter n. Analysis of the expression of ADP-glucose synthase at the protein l evel, using antibodies directed against the Bt2 or Sh2 subunits, estab lished that the variation of activity in the grain was explained by pa rallel changes in the content of both subunits. The cDNA for Bt2 and S h2 subunits were used as probes to quantify the corresponding messenge r. In grain, the time course of Bt2 and Sh2 mRNA accumulation anticipa ted, with a similar pattern, the specific peptide variations, which su ggests a transcriptional control of expression. By contrast, the contr ol of leaf activity by protein content was less obvious than in the gr ain, and changes in leaf enzyme specific activity were suggested durin g the first 20 d after pollination. A clone homologous to the grain Bt 2 subunit cDNA was isolated from a maize leaf cDNA library, and a sequ ence comparison showed that the leaf clone (L2) was a partial cDNA rep resenting one-third of the mature peptide. A 97% homology was observed between Bt2 and L2 in their coding region, but homology was poor in t he 3' noncoding border. This result demonstrates that Bt2 and L2 arise from different genes presenting a tissue-specific expression pattern and provides an explanation for the earlier reported differences betwe en leaf and grain in the size of peptide and mRNA for the Bt2-homologo us subunit.