LAM1 is required for dorsoventrality and lateral growth of the leaf blade in Nicotiana.

The role of LAM1 in dorsoventrality and lateral growth of the leaf blade was investigated in the 'bladeless' lam1 mutant of Nicotiana sylvestris and in periclinal chimeras with lam1 and wild-type (N. glauca) cell layers. Mutant lam1 primordia show normal dorsoventrality at emergence, but produce blade tissue that lacks dorsal cell types and fails to expand in the lateral plane. In leaves of a lam1-glauca-glauca (L1-L2-L3) chimera, we observed restoration of dorsal identity in the lam1 upper epidermis, suggesting non-cell-autonomous movement of a dorsalizing factor between cell layers of the blade. A lam1-lam1-glauca chimera generated a leaf blade with lam1 cells in the L1-derived epidermis and the L2-derived upper and lower mesophyll. An in situ lineage analysis revealed that N. glauca cells in the L3-derived middle mesophyll restore palisade differentiation in the adjoining lam1 upper mesophyll. Movement of dorsalizing information appears short-range, however, having no effect on the upper lam1 epidermis in lam1-lam1-glauca. Clusters of lam1 mesophyll in distal or proximal positions show a localized default to radial growth, indicating that the LAM1 function is required for dorsoventrality and lateral growth throughout blade expansion.