Figure 4.

Suc stimulates an endogenous carbon-sensing pathway. A, CO₂ assimilation rates of wild-type (WT), pifQ, and PIF5ox seedlings were quantified over a range of light intensities (photosynthetically active radiation [PAR]). Overexpression of PIF5 reduced net carbon uptake. CO₂ assimilation rates of wild-type seedlings on medium with and without Suc at the standard growth conditions of this study (60 μmol m⁻² s⁻¹; gray shading) are shown in the inset. The black arrow indicates the photosynthetic rate under the conditions for the high-light experiment shown in Figure 2D. Error bars represent se. B, Wild-type seedlings grown in elevated CO₂ (approximately 800 μL L⁻¹; black) showed a small but significant increase in height compared with seedlings grown in ambient CO₂ (approximately 400 μL L⁻¹; gray). This response was completely abolished in pifQ mutants. Overexpression of PIF5 (PIF5ox) greatly enhanced the growth promotion effect of elevated CO₂. In max2 mutants, where auxin transport is constitutively increased, response to increased CO₂ was reduced. Error bars represent se for four independent experiments. Asterisks indicate significant differences between ambient and elevated CO₂ treatments for each genotype using an ANOVA with Tukey’s pairwise comparisons (***P < 0.001). C, Suc supplementation led to bleaching in low-nitrogen conditions by 8 dpg. Seedlings were grown on low-nitrogen medium supplemented with either 0.5 mm KNO₃ or equimolar KCl. PIF5ox seedlings remained green in all medium conditions. Bar = 5 mm. D, Suc strongly induces the gene encoding NRT1.1, a nitrate-sensitive auxin transporter. The induction of NRT1.1 can be detected in whole seedlings (W) but appears stronger in dissected roots (R) when compared with dissected shoots (S). This effect of Suc is not PIF dependent, as it is still detectable in pifQ mutants. Error bars represent se. [See online article for color version of this figure.]