Developmental regulation of pectic epitopes during potato tuberisation

We show, by immunogold labelling, that potato (Solanum tuberosum L. cv Karn ico) pectic epitopes are developmentally regulated within regions of the st olon, in addition to showing tissue-specific differences in abundance and l ocalisation. The (1 -->4)-beta -D-galactan and (1 -->5)-alpha -arabinan epi topes demarcate two distinct zones within stolons; galactans are enriched i n primary walls of elongating cells proximal to the stolon hook, whilst ara binans predominate in younger cells distal to the hook. Low-methoxyl homoga lacturonan epitopes are concentrated in the middle lamella and show a proxi mo-distal gradient in stolons similar to that of galactans, whilst high-met hoxyl homogalacturonan is uniformly abundant. Calcium pectate is restricted to the middle lamella at cell corners and pit fields. Calcium-binding site s are uniformly present in stolon cell walls, but their total density is re duced and they become localised to a few cell corners in mature tubers, as determined by image-electron energy loss spectroscopy. During the transitio n from elongation growth to ISO-diametric expansion during tuberisation of the stolon hook, there were no detectable changes in pectic epitope abundan ce or localisation. As tubers matured, all epitopes increased in abundance in parenchymal cell walls, except for calcium pectate. We conclude that pot entially significant changes in pectic composition occur as young cells dis tal to the stolon hook move into the zone of cell elongation proximal to th e hook.