Fig. 12.

Interpretation of the synergistic effect of combining ectopic ground cell divisions and auxin transport inhibition on leaf vein development. Auxin transport inhibition leads to overproduction of veins in Arabidopsis leaves (right leaf; Mattsson et al., 1999, Sieburth et al., 1999), probably as a consequence of accumulation of auxin in developing leaves as indicated by the DR5::GUS auxin response marker (Mattsson et al., 2003). On its own, ectopic ground cell divisions in developing J0571>>C4 leaves result primarily in fragmentation of higher-order veins (left leaf). When combined, ectopic cell divisions and auxin transport inhibition leads to a phenotype not seen in any of the single treatments: a massive vascularization of the leaf blade (centre leaf). We hypothesize that the vascular overproduction in auxin transport-inhibited leaves is limited by simultaneous differentiation of ground cells into mesophyll cells. Ectopic ground cell divisions may negate this effect by providing undifferentiated ground cells that respond to accumulated auxin by differentiation into vascular cells.