The metabolic impact of altering the amount of an enzyme depends on the physiological state of the system. Extensive analysis of transgenic tobacco lines possessing decreased amounts of Rubisco has established that the flux control coefficient of the enzyme on photosynthesis varies in response to both the immediate conditions and the conditions under which the plant developed (Stitt and Schulze, 1994). For plants grown and analyzed under moderate irradiance, photosynthesis was only slightly inhibited when Rubisco was decreased to about 60% of the wild-type amount. However, stimulation of photosynthesis by an immediate increase in light intensity resulted in a near-proportional relationship between the amount of Rubisco and the rate of photosynthesis. In contrast, when photosynthesis was measured at saturating CO2 levels, Rubisco content could be decreased by as much as 80% without any appreciable effect on the rate of assimilation. Thus, the metabolic impact of modifying the amount of Rubisco depended on the conditions under which the flux was measured. Moreover, the response to reduced Rubisco also depended on the conditions under which the plants were grown: a moderate decrease in Rubisco had a relatively minor effect on photosynthesis in plants grown at high irradiance, in contrast to the near-proportional decrease in photosynthesis for plants grown at low irradiance prior to transfer to a higher light intensity. Similarly, growth of plants on low nitrogen fertilizer increased the extent to which photosynthesis was impaired by a decrease in the amount of Rubisco. This extensively investigated example emphasizes that any assessment of the potential of a specific enzyme as a target for metabolic manipulation must take into consideration both the conditions in which flux is being measured and the conditions in which the plant is grown (Stitt and Schulze, 1994).
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