During leaf development in flowering plants, adaxial (upper) and abaxial (lower) side-specific genes are responsible for blade outgrowth, which takes places predominantly in the lateral direction, and for margin development as well as differentiation of adaxial and abaxial tissues. However, the underlying mechanisms are poorly understood. Here, we show that two WUSCHEL-RELATED HOMEOBOX (WOX) genes, PRESSED FLOWER (PRS)/WOX3 and WOX1, encoding homeobox transcription factors, act in blade outgrowth and margin development downstream of adaxial/abaxial polarity establishment. The expression of PRS and WOX1 defines a hitherto undescribed middle domain, including two middle mesophyll layers and the margin, as a center that organizes the outgrowth of leaf blades. The expression of PRS and WOX1 is repressed in the abaxial leaf domain by the abaxial-specific transcription factor KANADI. Furthermore, PRS and WOX1 coordinate adaxial/abaxial patterning together with adaxial-and abaxial-specific genes. Our data suggest a model of blade outgrowth and adaxial/abaxial patterning via the middle domain-specific WOX genes in Arabidopsis thaliana leaves.

In multicellular organisms, boundaries have a role in preventing the intermingling of two different cell populations and in organizing the morphogenesis of organs and the entire organism. Plant leaves have two different cell populations, the adaxial (or upper) and abaxial (or lower) cell populations, and the boundary is considered to be important for lamina growth. At the boundary between the adaxial and abaxial epidermis, corresponding to the margin, margin-specific structures are developed and structurally separate the adaxial and abaxial epidermis from each other. The adaxial and abaxial cells are determined by the adaxial and abaxial regulatory genes (including transcription factors and small RNAs), respectively. Among many lamina-growth regulators identified by recent genetic analyses, it has been revealed that the phytohormone, auxin, and the WOX family transcription factors act at the adaxial-abaxial boundary downstream of the adaxial-abaxial pattern. Furthermore, mutant analyses of the WOX genes shed light on the role of the adaxial-abaxial boundary in preventing the mixing of the adaxial and abaxial features during lamina growth. In this review, we highlight the recent studies on the dual role of the adaxial-abaxial boundary.

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