Sucrose phosphate synthase expression at the cell and tissue level is coordinated with sucrose sink-to-source transitions in maize leaf

Immunohistological analyses for sucrose phosphate synthase (SPS) show that the protein is localized in both bundle-sheath cells (BS) and mesophyll cells (M) in maize (Zea mays) leaves. In young leaves, SPS protein was predominantly in the BS, whereas mature leaves showed nearly equal levels of signal in both BS and M. A cell-type-specific response was also seen in light and dark treatments. Dark treatments led to reduced signal in M; however, little or no change was detected in BS. We suggest that SPS in BS is engaged in sucrose biosynthesis by both photoassimilatory and starch turnover reactions in maize leaves. In addition, we suggest that the enzyme in BS may play a major role in the early biosynthesis of sucrose in young leaves. These cell-specific changes in expression in situ were in agreement with the estimates of extractable enzyme activity from isolated BS and M of mature leaves (R. Ohsugi, S.C. Huber [1987] Plant Physiol 84: 1096-1101). In contrast, western blot analyses did not show any significant changes in the levels of SPS protein in either young or mature leaves subsequent to similar dark treatments. It is interesting that the northern blot analyses indicate that the steady-state levels of SPS transcripts were markedly reduced after dark treatments of >12 h. Overall, our results indicate that Sps gene expression in maize leaf is modulated at multiple levels of controls by both developmental and environmental factors.