Decrease of phosphoribulokinase activity by antisense RNA in transgenic tobacco: definition of the light environment under which phosphoribulokinase is not in large excess

Abstract

To test the hypothesis that the contribution of phosphoribulokinase (PRK) to the control of photosynthesis changes depending on the light environment of the plant, the response of transgenic tobacco (Nicotiana tabacum L.) transformed with antisense PRK constructs to irradiance was determined. In plants grown under low irradiance (330 micromol m(-2) s(-1)) steady-state photosynthesis was limited in plants with decreased PRK activity upon exposure to higher irradiance, with a control coefficient of PRK for CO2 assimilation of 0.25 at and above 800 micromol m(-2) s(-1). The flux control coefficient of PRK for steady-state CO2 assimilation was zero, however, at all irradiances in plant material grown at 800 micromol m(-2) s(-1) and in plants grown in a glasshouse during mid-summer (alternating shade and sun 300-1600 micromol m(-2) s(-1)). To explain these differences between plants grown under low and high irradiances, Calvin cycle enzyme activities and metabolite content were determined. Activities of PRK and other non-equilibrium Calvin cycle enzymes fructose-1,6-bisphosphatase, sedoheptulose-1,7-bisphosphatase and ribulose-1,5-bisphosphate carboxylase-oxygenase were twofold higher in plants grown at 800 micromol m(-2) s(-1) or in the glasshouse than in plants grown at 330 micromol m(-2) s(-1). Activities of equilibrium enzymes transketolase, aldolase, ribulose-5-phosphate epimerase and isomerase were very similar under all growth irradiances. The flux control coefficient of 0.25 in plants grown at 330 micromol m(-2) s(-1) can be explained because low ribulose-5-phosphate content in combination with low PRK activity limits the synthesis of ribulose-1,5-bisphosphate. This limitation is overcome in high-light-grown plants because of the large relative increase in activities of sedoheptulose-1,7-bisphosphatase and fructose-1,6-bisphosphatase under these conditions, which facilitates the synthesis of larger amounts of ribulose-5-phosphate. This potential limitation will have maintained evolutionary selection pressure for high concentrations of PRK within the chloroplast.

Systems Analysis of the Response of Photosynthesis, Metabolism, and Growth to an Increase in Irradiance in the Photosynthetic Model OrganismChlamydomonas reinhardtii

“…The large increase in the levels of CBC intermediates in between low and higher irradiance in C. reinhardtii contrasts with previous studies in vascular plants where CBC intermediates were mostly unchanged upon an increase in light intensity (Perchorowicz et al, 1981;Perchorowicz and Jensen, 1983;Badger et al, 1984;Heber, 1984, 1986;Paul et al, 2000). The reason for the quite stable levels of phosphorylated CBC intermediates in vascular plants is thought to be due to a tight regulation of P homeostasis, which is required to prevent P i limitation of photosynthesis (Sharkey et al, 1986;Paul and Foyer, 2001).…”

Enhanced plant tolerance to iron starvation by functional substitution of chloroplast ferredoxin with a bacterial flavodoxin

“…These results were later challenged by Winder and Nishio (12), who reported that photosynthesis inhibition correlated with Rubisco decline in Fe-restricted Beta plants, and by Paul et al (32), who observed that CO 2 fixation rates were not significantly affected until PRK activity declined below 10% of the WT levels in antisense tobacco plants. However, more detailed investigations on the same lines showed that PRK did exert significant control on photosynthesis when the transformants were grown under a low-irradiation regime similar to that used in our experiments (33). It is thus likely that both the decrease in Rubisco levels (Fig.…”

A qualitative analysis of the regulation of cyclic electron flow around photosystem I from the post-illumination chlorophyll fluorescence transient in Arabidopsis: a new platform for the in vivo investigation of the chloroplast redox state

“…Using the antisense potato, it was shown that aldolase is not generally accumulated in excess in chloroplasts for CO 2 assimilation, in contrast to other enzymes that are highly regulated and are in considerable excess like Rubisco (Stitt and Schulze 1994), fructose-1,6-bisphosphatase (Koßmann et al 1994), and phosphoribulokinase (Paul et al 2000). As a result, decreased expression of aldolase induced strong suppression of photosynthesis in both low and high light.…”