Inter cell-later communication during shoot growth. (a) Scheme of the shoot apical meristem and its cell-layers. L1 and L2 are single layers of cells which divide anticlinal to the plane and L3 is the remaining cells which divide both anticlinal and periclinal. L1 will form the epidermis, L2- the subepidermal mesophyll and L3 the inner mesophyll and the ground tissue. (b-e) Summary of autonomous and non-autonomous signals generated and perceived by the epidermis. A schematic organization of the cells in the leaf and the leaf phenotype are shown. (b) Wild-type. (c) Expression of the cell-cycle inhibitor KRP1/ICK1 under the ubiquitous 35S promoter (35S::KRP1/ICK1, upper panel) and the L1 specific promoter AtML1 (AtML1:: KRP1/ICK1, lower panel). Cells with KRP1/ICK1 expression are marked in red. Note that when KRP1/ICK1 is confined to the epidermis, cells in the inner layers have the same total cell number as in wild-type. (d) Deletion sectors of HBT in hbt²³¹¹. Sectors are marked in grey. Note the morphology of the leaf, which is more similar to wild type when sectors are derived solely from the epidermis. In this case, cell cycle, but not cell morphology is restored. (e) BR effects on cell expansion. BR deficient of insensitive mutant (cpd and bri1 respectively) cells are marked in yellow (upper panel). CPD and BRI1 fused to the yellow or green fluorescence protein are expressed under AtML1 promoter (AtML1::CPD:YFP and AtML1::BRI1:GFP respectively, middle panel, blue cells). Note the full rescue of the cell size in inner layers. Lowe panel, BR biosynthetic enzyme BAS1 fused to YFP expression (purple cells) in wild-type background. Cell morphology is as predicted, but has not been confirmed experimentally.

Arrow represents unknown non-autonomous signal. Arrow with a cross represents autonomous behavior.