Carbon metabolism in leaves of transgenic tobacco (Nicotiana tabacum L.) containing elevated fructose 2,6‐bisphosphate levels

Abstract

SummaryThe aim of this work was to investigate the role of fructose 2,6-bisphosphete (Fru 2,6-P z) during photosynthesis. The level of Fru 2,6-P2 in tobacco plants was elevated by the introduction of a modified mammalian gene encoding 6-phosphofructo-2-kinase (6-PF-2-K). Estimates of the metabolite control coefficient (C) for Fru 2,6-P 2 levels in response to increased 6-PF-2-K activity, suggest that small increases in 6-PF-2-K activity have little effect upon steadystate Fru 2,6-P 2 levels (C = +0.08 for a 0-58% increase in 6-PF-2-K activity). However, larger changes resulted in dramatic rises in Fru 2,6-P2 levels (C = +3.35 for 206-268% increase in 6-PF-2-K activity). Transgenic plants contained Fru 2,6-Pz levels in the dark that ranged from 104 to 230% of the level in wild-type tobacco. Plants with altered levels of Fru 2,6-P 2 were used to determine the effects of this signal metabolite upon carbohydrate metabolism during the initial phase of the light period. Here we provide direct evidence that Fru 2,6-P 2 contributes to the regulation of carbon partitioning in tobacco leaves by inhibiting sucrose synthesis.

Influence of Elevated Fructose-2,6-Bisphosphate Levels on Starch Mobilization in Transgenic Tobacco Leaves in the Dark

“…The transgenic tobacco lines used were generated as described by Scott et al (1995). Plants were grown under illumination of 200 pmol PAR m-' s-' at 25°C.…”

“…A large range of plant lines was used to demonstrate that our observations were consistent over a wide range of transformants. However, our study has concentrated on the plant lines A(31) and D(86), in which photosynthetic metabolism was studied in earlier work (Scott et al, 1995). The principal metabolic characteristics of the transgenic plants used in this study were described by Scott et al (1995).…”

“…It is through inhibition of FBPase that Fru-2,6-bisP is thought to contribute to the regulation of the partitioning of photosynthetically fixed carbon between Suc and starch. Work on spinach, Clarkia, and transgenic tobacco (Nicotiana tabacum L.) has confirmed that Fru-2,6-bisP m&es an important contribution to the control of this partitioning during the early stages of the light period (Neuhaus et al, , 1990Scott et al, 1995). Plant Physiol.…”

“…To address this problem we have studied carbohydrate metabolism in darkened leaves of transgenic plants containing a modified mammalian gene for 6-PF-2-K (EC 2.7.1.105) (Scott et al, 1995). In these plants we have observed up to a 4-fold increase in the extractable 6-PF-2-K activity in leaves.…”

N-terminal truncation affects the kinetics and structure of fructose-6-phosphate 2-kinase/fructose-2,6-bisphosphatase from Arabidopsis thaliana

“…In plants Fru-2,6-P # is involved primarily in the regulation of photosynthetic carbon partitioning [2]. Studies of transgenic plants have confirmed that a decrease in the level of Fru-2,6-P # in leaves increases the flux towards sucrose [3,4], whereas an increase in the level of Fru-2,6-P # increases the flux towards starch [5,6]. The synthesis and degradation of Fru-2,6-P # are catalysed by two specific enzyme activities, fructose-6-phosphate 2-kinase (6-phosphofructo-2-kinase ; F6P,2K ; EC 2.7.1.105) and fructose-2,6-bisphosphatase (F26BPase ; EC 3.1.3.46).…”