Light Modulation and Localization of Sucrose Phosphate Synthase Activity between Mesophyll Cells and Bundle Sheath Cells in C₄ Species

Abstract

Experiments were conducted with several Panicum species (representing the different C4 subtypes) to examine the light modulation of sucrose phosphate synthase (SPS) activity and the effect of illumination on the distribution of SPS activity between mesophyll cells (MC) and bundle sheath cells (BSC). Activity of SPS in the light decreased in the order: C4> C3-C4 intermediate > C3. In illuminated leaves, SPS activities were similar among the three C4 subtypes, but SPS activity was higher for NAD-malic enzyme (NAD-ME) species with centripetal chloroplasts in BSC (NAD-ME(P) species) than for NAD-ME species with centrifugal chloroplasts in BSC (NAD-ME(F) species). Transfer of plants into darkness for 30 minutes resulted in decreased SPS activity for all species tested except Panicum bisukatum (C3 species) and Panicum virgatum (NAD-ME(P) species) which showed little or no change. All C4 subtypes had some SPS activity both in MC and BSC. In the light, SPS activity was mainly in the MC for NADP-ME, NAD-ME(F) and phosphoenolpyruvate carboxykinase species, while it was mainly in the BSC for NAD-ME(P) species. In the dark, for all C4 subtypes, SPS activity in the MC was decreased to a greater extent than that in the BSC. It is intriguing that NAD-ME(F) and NAD-ME(P) species differed in the activity and distribution of SPS activity between MC and BSC, although they are otherwise identical in the photosynthetic carbon assimilation pathway. Diurnal changes in SPS activity in the MC and BSC were also examined in maize leaves. SPS activity in the MC in maize leaves was high and relatively constant throughout the middle of the light period, dropped rapidly after sunset and increased again prior to the light period. On the other hand, SPS activity in the BSC was lower and changed more coincidently with light intensity than that in the MC. The results suggested that light activation of SPS activity located in the BSC may require higher irradiance for saturation than the SPS in the MC. We conclude that SPS may function in both MC and BSC for sucrose synthesis in the light, particularly at high light intensity, while in the dark, the major function may be in the BSC during starch degradation. sucrose-P from fructose-6-P and UDP-glucose, plays a key role in the regulation of sucrose biosynthesis (10). In C3 species, sucrose biosynthesis occurs in the cytoplasm of mesophyll cells and the biochemical mechanism of sucrose formation has been relatively well established (28). In C4 species, however, research has concentrated almost exclusively on maize which is an NADP-ME species. The biochemical mechanism involved in the regulation of photosynthetic carbohydrate formation is considered to be more complex than in C3 species since two different cell types (i.e. MC and BSC) are involved in the photosynthetic carbon assimilation.In maize leaves, the MC is considered to be the main site of sucrose biosynthesis since SPS and other enzymes involved in sucrose formation are thought to be located largely, if not exclusively, in the MC (5...

Sucrose Phosphate Synthase Expression at the Cell and Tissue Level Is Coordinated with Sucrose Sink-to-Source Transitions in Maize Leaf

“…The immunohistological data show that the SPS protein was localized in both BS and M in mature leaves, as shown previously by Ohsugi and Huber (1987), based on enzyme activity from extractable saps from BS and M. In contrast, Furbank et al (1985) observed an average of 10-fold excess SPS activity in M as compared with BS. There is, however, a large range of variability in these estimates, and the lowest ratio of M:BS was 2.0.…”

“…In addition, we show here that cell-type-specific alterations also occur with light and dark treatments in response to sink-source alterations, as was shown previously in mature leaves (Ohsugi and Huber, 1987). Our studies also show that the Sps gene is regulated at the RNA level of expression by light/dark treatments.…”

“…In addition, we suggest that the enzyme in BS may play a major role in the early biosynthesis of sucrose i n 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 [19871 Plant Physiol 84: 1096-1101). In contrast, western blot analyses did not show any significant changes i n the levels of SPS protein in either young or mature leaves subsequent to similar dark treatments.…”

“…38) Despite such compartmentalization of functions, there are conflicting reports of cellular level specificity of SPS enzyme/protein in maize leaf. Whereas Furbank et al (1985) showed that the predominant site of Suc biosynthesis is in the M, Ohsugi and Huber (1987) demonstrated that SPS is localized in both M and BS. However, both of the studies were limited to mature leaves and were based only on enzyme activity assays.…”

Elements Required for an Efficient NADP-Malic Enzyme Type C4 Photosynthesis

“…1), as indicated by observed localization of relevant substrates, products, and enzymes (Furbank et al, 1985;Lunn and Furbank, 1997). Although immunohistological (Cheng et al, 1996), activity (Ohsugi and Huber, 1987), and proteomic analyses have indicated that Suc-P-synthase, a key enzyme in Suc synthesis, was also found in the BSC cytosol (Majeran et al, 2010), the Suc-P-synthase is assumed to be involved in Suc synthesis during starch degradation at night rather than participating in photosynthesis during the day. Following Stitt and Heldt (1981), starch synthesis and degradation are assumed to occur simultaneously.…”