Metabolic Repression of Transcription in Higher Plants

Abstract

Using freshly isolated maize mesophyll protoplasts and a transient expression method, I showed that the transcriptional activity of seven maize photosynthetic gene promoters is specifically and coordinately repressed by the photosynthetic end products sucrose and glucose and by the exogenous carbon source acetate. Analysis of deleted, mutated, and hybrid promoters showed that sugars and acetate inhibit the activity of distinct positive upstream regulatory elements without a common consensus. The metabolic repression of photosynthetic genes overrides other forms of regulation, e.g., light, tissue type, and developmental stage. Repression by sugars and repression by acetate are mediated by different mechanisms. The identification of conditions that avoid sugar repression overcomes a major obstacle to the study of photosynthetic gene regulation in higher plants.

An Arabidopsis mutant showing reduced feedback inhibition of photosynthesis

“…A number of studies showed a loss of Rubisco total activity in leaves which accumulated soluble carbohydrates (Krapp etaL, 1991(Krapp etaL, , 1993Stitt, 1991;Van Oosten and Besford, 1995). This reduction in Rubisco activity was the result of a similar loss of Rubisco protein (Krapp etaL, 1993;Van Oosten and Besford, 1995) which was concomitant with a reduction in RBCS transcript (Criqui etaL, 1992;Krapp et aL, 1993;Van Oosten and Besford, 1995) and a repression of the transcriptional activity of a RBCS promoter (Krapp et aL, 1993;Sheen, 1989). The repression of RBCSand other genes by carbohydrates suggests a common mechanism of control of nuclear genes coding for photosynthetic proteins (Jang and Sheen, 1994;Jang etaL, 1997;Sheen, 1989Sheen, , 1994.…”

“…In particular, photosynthesis is inhibited when the production of photosynthates exceeds the rate of utilization by sinks, a sink being an organ which utilizes or stores carbohydrate generated by other parts of the plant. This was clearly shown in experiments in which the source-sink status of the plant was manipulated by reducing export out of the leaf (AzconBieto, t983; Blechmidt-Schneider et aL, 1989;Krapp et aL, 1991;Paul etaL, 1991;Plaut etaL, 1987). Tobacco plants expressing a yeast invertase in the apoplastic space had stunted growth, a clear carbohydrate increase and a strong inhibition of photosynthesis (von Schaewen etaL, 1991).…”

“…This glucose analog is phosphorylated by hexokinase and is partially metabolized (Guy and Reinhold, 1974). At a relatively low concentration, it mimics the effect of glucose on photosynthetic gene expression (Sheen, 1989;Jang and Sheen, 1994) and on genes involved in the glyoxylate cycle (Graham etaL, 1994). The advantage of using 2DG is that there is no contamination by micro-organisms for up to two weeks, whereas the use of sucrose and glucose rapidly results in massive contamination.…”

“…It has been demonstrated that the transcriptional activity of seven promoters of maize photosynthetic genes was specifically repressed by sucrose, glucose and fructose using a protoplast transient assay system (Sheen, 1989). The repression of nuclear-encoded genes by sucrose and glucose was also observed in tobacco protoplast (Criqui et al, 1992), Chenopodium cell culture (Krapp et aL, 1993), detached leaves supplied with various sugars through the transpiration stream (Krapp etaL, 1993;Van Oosten etal., 1994), whole plants expressing a yeast invertase in the apoplast (Krapp etaL, 1993;von Schaewen et al, 1990) or whole plants exposed to elevated CO2 (Van Oosten and Besford, 1995).…”

“…After the phosphorylation by hexokinase, no further metabolism of glucose was required to induce repression (Graham etal., 1994;Jang and Sheen, 1994). 2-deoxy-g-glucose (2DG) which is not rapidly metabolized in plants could mimic the control by glucose of gene expression (Graham etal., 1994;Jang and Sheen, 1994;Sheen, 1989).…”