Fig. 2: ssNMR reveals that callose is close to water, is not close to other secondary cell wall polymers and increases the lignin to cellulose distance.

a, Left: 1.5 s mixing time ¹³C CP PDSD NMR spectrum of an induced poplar sample (inducible L11 following estradiol induction for 14 weeks). Right: 1D slices taken from the 1.5 s mixing time PDSD spectrum. A slice taken through the callose Cal4a shift at 68.5 ppm (pink) is compared with slices taken at different cellulose environments. The cellulose slices are C4¹ at 88.7 ppm (light blue), C4² at 84.3 ppm (green) and a further C4² at 83.7 ppm (purple). The additional peaks, marked with ‘*’, seen in the callose slice come from galacturonic acid whose C2 carbon has a shift of ~68.8 ppm very similar to callose C4. The barely resolvable peaks at ~73.0 and 75.7 ppm in this slice come from the second callose environment. The slices show that the callose is further than 10 Å from both cellulose and hemicellulose. b, Slices from the 1.5 s PDSD through the lignin peak at 56 ppm of the inducible L11 following estradiol induction (red) vs its DMSO mock control (blue) and the difference (green). The difference reveals intensity at all cellulose shifts as shown when overlaying a CP MAS spectrum over the difference spectrum (mauve) indicating that the lignin is closer to cellulose in the DMSO mock control sample. c, A comparison of the water-edited ¹³C CP NMR spectrum of induced (L11) poplar with a diffusion time of 4 ms (red) and the normal CP spectrum (blue) normalized to the cellulose C4 at 89 ppm. The increased relative intensity of the callose peaks shows that it is close to water.