Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 7, 1996
Publication Date: N/A
Interpretive Summary: Sucrose, the end-product of photosynthesis, is of central importance to plant growth, development and ultimately agronomic productivity. The enzyme sucrose phosphate synthase (SPS) plays a key role in both synthesis as well as in metabolic utilization of sucrose in leaves. Tropical grasses, such as corn, sorghum and sugarcane are well known for the maximal use of the light energy into its net fixation in the form of chemical energy (sucrose). The leaf anatomy of these plants is also marked by the two distinct types of photosynthetic cells, bundle sheath (BS) and mesophyll (M) cells. In this study, we report the following observations based on the use of molecular and cellular approaches. (1) In young leaves, the earliest expression of the SPS gene is restricted to the BS cells. (2) The mature leaves, fully capable of photosynthesis, show SPS protein in both the cell types in daytime; however, after a dark phase of about 12 hours the expression is limited mainly to the BS cells. These results suggest that both BS and M cells are associated with light dependent biosynthesis of sucrose; whereas, only the BS cells are engaged in sucrose synthesis (through a break-down of reserve starch) in dark environment. Such a division of labor in these two cell types is probably critical to the high rate of photosynthesis in corn leaves.
Immunohistological analyses for sucrose phosphate synthase (SPS) protein show that the protein is localized in both bundle sheath (BS) and mesophyll (M) cells in maize leaves. In young juvenile leaves, SPS protein was predominantly in the BS cells; whereas, mature leaves showed nearly equal levels of signal in both BS and M cells. Cell type specific response was also seen to light and dark treatments; dark treatments led to reduced signal in M cells, however, little or no change was detected in BS cells. We suggest that SPS in BS cells is engaged in sucrose biosynthesis by both photoassimilatory as well as by starch turnover reactions in maize leaf. In addition, we suggest that the enzyme in BS cells may play a major role in the early biosynthesis of sucrose in young leaves. The Northern blot analyses indicate that the steady state levels of SPS transcripts were markedly reduced after dark treatments of >12 hours. Overall, our results indicate that Sps gene expression in maize leaf is modulated at multiple levels of controls by both developmental as well as environmental factors.