Significant cellulose pectin interactions in plant cell walls have been reported recently based on 2D C-13 solid-state NMR spectra of intact cell walls, but how these interactions affect cell growth has not been probed. Here, we characterize two Arabidopsis thaliana lines with altered expression of the POLYGALACTURONASE INVOLVED IN EXPANSION1 (PGX1) gene, which encodes a polygalacturonase that cleaves homogalacturonan (HG). PGX1(AT) plants overexpress PGX1, have HG with lower molecular weight, and grow larger, whereas pgx1-2 knockout plants have HG with higher molecular weight and grow smaller. Quantitative C-13 solid-state NMR spectra show that PGX1AT cell walls have lower galacturonic acid and xylose contents and higher HG methyl esterification than controls, whereas high molecular weight pgx1-2 walls have similar galacturonic acid content and methyl esterification as controls. H-1-transferred C-13 INEPT spectra indicate that the interfibrillar HG backbones are more aggregated whereas the RG-I side chains are more dispersed in PGX1(AT) cell walls than in pgx1-2 walls. In contrast, the pectins that are close to cellulose become more mobile and have weaker cross peaks with cellulose in PGX1(AT) walls than in pgx1-2 walls. Together, these results show that polygalacturonase-mediated plant growth is accompanied by increased esterification and decreased cross-linking of HG, increased aggregation of interfibrillar HG, and weaker HG cellulose interactions. These structural and dynamical differences give molecular insights into how pectins influence wall dynamics during cell growth.