An early proposal suggested that extensin weaves cellulose microfibrils together in the primary wall, and breakages of these cross-links allowed microfibrils to slide apart during elongation growth, but direct evidence for this is still lacking. More recently expansins and xyloglucan endotransglycosylases have been suggested as important determinants in elongation. Both PRP and HRGP are proposed to have a role in lignification because of the affinity of phenolic substrates for prolyl residues in the protein. The plant cell wall varies in composition and architecture in accordance with developmental stages of different cell types and in response to environmental factors and stresses. The data on plant cell wall proteins is similar in that these proteins are not equally abundant in all cell types.
Abundance varies in accord with different functions of different cell types and tissues.
The Ye and Varner paper which was mentioned above shows that HRGP and GRP are expressed in a developmental and tissue specific manner. HRGPs are highly expressed in cambium cells of the soybean stem. It is also expressed in parenchyma cells surrounding primary xylem and in a few layers of cortical cells adjacent to primary phloem that are under compression. GRP are localized in all lignified cells including primary xylem, secondary xylem, and primary phloem. The highest expression is in protoxylem cells.