Abstract text: How are the polymers of the pectic network interconnected? Is rhamnogalacturonan-I (RG-I) invariably linked to homogalacturonan (HG)? Do free galactans exist, or are they always attached as side chains to the RG-I backbone? These questions are central to understanding the higher-order organisation of primary walls.

The pith of the swollen stem of Brassica oleracea var. gongylodes provides a simple experimental system: a glassy, highly hydrated tissue composed of nearly uniform parenchyma. Alcohol-insoluble residues (AIR) were prepared and subjected to gentle sequential extraction. Extracts were spotted onto microarrays and probed with a panel of monoclonal antibodies (CoMPP).

Baseline separation was acheived between molecules recognised by different HG-specific antibodies — an unexpected result. Likewise, galactan and arabinan did not co-extract, indicating the presence of distinct RG-I populations.

Particle size also proved critical. A simple water extraction of AIR yielded RG-I backbone epitopes but no galactan. Moderate dry milling released galactans into the water extract, whereas wet milling — reducing particle size from ~100 µm to ~10 µm — solubilised all major pectic polymers.

It is relevant to consider whether these pectic populations coexist uniformly within primary walls, form distinct microdomains, or even reside in different cells despite the homogeneity of the tissue.