Fig. 5.

Examples of conserved responses of CBC metabolite levels in Arabidopsis (blue, left-hand panels) and rice (red, right-hand panels). (A) FBP/F6P and SBP/S7P ratios peak at low irradiance and then decline. These are the substrate/product ratios of the two irreversible reactions FBPase and SBPase, respectively. A high substrate/product ratio reveals that the enzyme is restricting flux, even though flux will be very low at these low irradiances close to the compensation point. The implication is that FBPase and SBPase are inhibited, probably because they are not or only weakly activated by post-translational redox activation. (B) DHAP levels are already relatively high at the light compensation point and only rise another 3- to 4-fold at higher irradiance as increasingly fast rates of photosynthesis are achieved. This implies that there is a restriction on the use of triose-P (and other CBC metabolites) for end-product synthesis at very low light. The results are shown as the mean ±SD (n=4 in almost all cases). One-way ANOVA with FDR was performed on log-transformed data as in Fig. 4. This was followed by a Tukey’s HSD post-hoc test. The display is modified from Borghi et al. (2019). Metabolite abbreviations are as in Fig. 1.