SEQUESTRATION OF CYTOPLASMIC ORTHOPHOSPHATE BY MANNOSE AND ITS DIFFERENTIAL EFFECT ON PHOTOSYNTHETIC STARCH SYNTHESIS IN C₃ AND C4 SPECIES

Abstract

SUMMARYLeaf tissues from many C3 species respond to exogenous mannose in the light by a marked stimulation of starch synthesis. This is attributed to a decreased movement of triose phosphate from the chloroplast following sequestration of cytoplasmic orthophosphate as mannose phosphate and interference with the phosphate translocator. By contrast, in most C4 species, feeding of mannose inhibits starch synthesis. It is concluded that, in these plants, the principal effect of sequestration of orthophosphate is interference with the regeneration of the COjacceptor.

The role of UDP‐glucose epimerase in carbohydrate metabolism of Arabidopsis

“…A similar effect, the increase of mannose-6-phosphate with a concomitant decrease in inorganic phosphate, was observed for plants incubated on mannose (Loughman et al, 1989). Sheu-Hwa et al (1975) and Herold et al (1976) found an accumulation of starch in some plant species incubated on mannose. These authors speculated that the sequestration of inorganic phosphate by mannose-6-phosphate accumulation caused a reduction in the activity of the triose-phosphate translocator, thereby reducing the export of carbon from the chloroplast and inhibiting starch degradation.…”

“…In contrast to glucose, mannose is known to be toxic for plant growth, supposedly due to a sequestration of inorganic phosphate as mannose-6-phosphate (Herold et al, 1976;Sheu-Hwa et al, 1975). Similarly, galactose is highly toxic when supplied exogenously to plants in concentrations of 1-10 mM (Loughman et al, 1989;Maretzki and Thom, 1978).…”

Influence of Mannose on the Apoplasmic Retrieval Systems of Source Leaves

“…In sugar beet, mannose did not reduce the sucrose level in the manner expected from reports in the literature (5,10,13,14); however, it did cause a reduction in exogenous ['4C]sucrose influx. In addition, mannose pretreatment prevented, in some way, the change in partitioning of exogenously supplied ['4C]sucrose, which was found to occur following the removal of sink activity (31).…”

“…Since the transport of Pi between the vacuole and cytosol is thought to be slow (23, 27; but see Ref. 16), the addition of mannose effects a sequestration ofcytosolic Pi (5,13; but cf Ref. 16).…”

“…Furthermore, the inhibitory aspects of mannose cannot be explained by a reduction in adenine nucleotide levels alone since, after a 24 h mannose pretreatment, these levels recovered to near-control values. Since tobacco has been reported to be able to metabolize exogenously supplied mannose (5,12,13,20), we selected two species of Nicotiana, N. rustica and N. tabacum, to form a comparative basis for this study. We found that whereas sucrose influx into N. rustica was completely insensitive to mannose pretreatment, transport into N. tabacum was inhibited.…”