Figure 6.

A time course of mucilage secretory cell development showing the timing of TTG1, GL2, AP2, and MUM1 through 5 action as determined from their phenotypes. Mucilage secretory cell development can be divided into five stages: (1) growth, (2) amyloplast accumulation and commencement of cytoplasmic rearrangement, (3) simultaneous mucilage production/secretion and cytoplasmic column formation, (4) secondary cell wall production to form the columella, and (5) desiccation of the seed coat. The genes discussed in this paper appear to affect seed coat epidermal cell differentiation at three of these stages. AP2 is necessary to continue differentiation of the cells past the growth phase (transition of Stage 2 to Stage 3). TTG1 and GL2 reiterate their roles in cell morphology, being necessary for the completion of Stage 3 mucilage production and cytoplasmic rearrangement, with seed-specific MUM4 acting alongside or downstream, whereas MUM3 and MUM5 apparently affect Stage 3 mucilage biosynthesis with respect to sugar composition. MUM2 (and possibly MUM1) may play a role in post-deposition mucilage or cell wall modifications in late Stage 4.