Extended Data Fig. 3: When 11C dilution was mitigated by reducing sugar concentrations in sut1 mutant leaves by dark treatment, 11C export remained extremely low in sut1 mutants, but sap flow velocity remained high by comparison.
From: Sugar loading is not required for phloem sap flow in maize plants
Because the concentration of [12C]-sucrose was extremely high in sut1 leaves, and the specific activity was low (Fig. 1F and H, respectively; Extended Data Fig. 2), one could speculate that the reduced 11C export in sut1 might have appeared more drastic than overall 12C export due to isotopic dilution. However, it is possible that much of the sucrose was unavailable for phloem loading in sut1 mutants, for example if much of it is stored in vacuoles or exuded from hydathodes44,74,75. To measure export under more uniform cellular sugar status, we moved sut1 mutant plants into constant darkness for 42 hrs immediately prior to 11C assays, reducing the unlabeled (12C) leaf sucrose, glucose, fructose and starch concentrations (a) to near WT levels. Note the 30 fold difference in Y-axis scaling for sugar concentrations compared to Extended Data Fig. 2a (6 fold for starch). (b) 11C-labeled sucrose (Poverall model <0.0001; Pwt-vs-sut1 <0.0001; Phet-vs-sut1 = 0.0002; Pwt-vs-het = 0.89), glucose (Poverall model = 0.0051; Pwt-vs-sut1 = 0.0056; Phet-vs-sut1 = 0.042; Pwt-vs-het = 0.61), fructose (Poverall model = 0.017; Pwt-vs-sut1 = 0.018; Phet-vs-sut1 = 0.091; Pwt-vs-het = 0.71), and starch. (c) Specific activities of sucrose, glucose, fructose, and starch were similar in WT and sut1 mutant plants after dark treatment. For all panels, the central line is the median, the box indicates the first and third quartiles, and the whiskers indicate the minimum and maximum values (WT n = 5, het n = 4, and sut1 n = 7). Statistical significance according to a one-way ANOVA is indicated by *(P<0.05), **(P<0.01), ***(P<0.001). Different letters above bars in indicate which genotypes were statistically different based on a Tukey’s post hoc test. In these conditions, we confirmed that the greatly reduced 11C-export by sut1 mutant leaves (Figs. 1c and 2b) was not an artifact of isotopic dilution by high [12C]-sucrose concentrations. The reduction in phloem sap flow velocities in the low-carbohydrate, dark-treated sut1 mutant leaves relative to WT remained modest in this experiment (Fig. 2c), compared to the large reduction of 11C-photoassimilate export (Fig. 2b), similar to the experiment with no dark treatment (Fig. 1c-d).
