Expression of sucrose-phosphate synthase (SPS) in non-photosynthetic tissues of maize

Sucrose phosphate synthase (SPS) is responsible for sucrose synthesis in photosynthetic tissues. We have detected SPS expression in non-photosynthetic tissues. It was predominantly expressed in the basal region of developing endosperm, suggesting that sucrose is re-synthesized in this region, where sucrose is unloaded from the phloem and hydrolyzed into glucose and fructose. The SPS transcript in endosperm was approximately 300 nt smaller than in leaf. However, the size of the SPS protein was similar to that of leaf but had higher activity. SPS expression was also detected in developing and germinating embryos, indicating that sucrose resynthesis also occurs in embryos. Although the level of SPS mRNA and protein was lower in embryos than in leaf, enzymatic activity was higher. Similarly, the level of SPS transcript was 10-fold lower in endosperm than in leaf but the level of SPS protein was comparable, and activity was 2 fold higher. Thus, SPS expression was evident in maize kernels, and its expression and regulation were different from the SPS in leaf.