Pectin esterification is spatially regulated both within cell walls and between developing tissues of root apices

Monoclonal antibodies recognizing un-esterified (JIM5) and methyl-esterified (JIM7) epitopes of pectin have been used to locate these epitopes by indirect immunofluorescence and immunogold electron microscopy in the root apex of carrot (Daucus carota L.). Both antibodies labelled the walls of cells in all tissues of the developing root apex. Immunogold labelling observed at the level of the electron microscope indicated differential location of the pectin epitopes within the cell walls. The un-esterified epitope was located to the inner surface of the primary cell walls adjacent to the plasma membrane, in the middle lamella and abundantly to the outer surface at intercellular spaces. In contrast, the epitope containing methyl-esterified pectin was located evenly throughout the cell wall. In root apices of certain other species the JIM5 and JIM7 epitopes were found to be restricted to distinct tissues of the developing roots. In the root apex of oat (Avena sativa L.), JIM5 was most abundantly reactive with cell walls at the region of intercellular spaces of the cortical cells. JIM7 was reactive with cells of the cortex and the stele. Neither epitope occurred in walls of the epidermal or root-cap cells. These pattern of expression were observed to derive from the very earliest stages of the development of these tissues in the oat root meristem and were maintained in the mature root. In the coleoptile and leaf tissues of oat seedlings, JIM5 labelled all cells abundantly whereas JIM7 was unreactive. Other members of the Gramineae and also the Chenopodiaceae are shown to express similar restricted spatial patterns of distribution of these pectin epitopes in root apices.