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Although the defect in mucilage production is readily explained by the putative
function of MUM4 as an NDP-l-Rha synthase, the connection with the columella shape change is less obvious. At least two hypotheses can explain the dependence
of cell morphogenesis on mucilage synthesis. The formation of the apoplastic space and cytoplasmic column may be driven by
the pressure of accumulating mucilage in the extracytoplasmic space and/or the formation of an osmotic gradient between the
cell and the hydrophilic pectin may lead to the loss of turgor and size reduction of the vacuole. Such passive processes may
contribute to cell morphogenesis but seem too simple to account for the apparent complexity of the shape of the cytoplasmic
column. An alternate but not mutually exclusive explanation is that the synthesis of mucilage may represent or lead to the
production of an oligosaccharide signal that stimulates the cell to undergo morphogenesis (Dumville and Fry, 2000; Ridley et al., 2001). Determination of the exact processes involved would be facilitated by the isolation of mutants affected specifically in
the shaping of the columella.
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