Abstract
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.
Highlights
The ―boundaries‖ between two different cell populations play a role as the center of many developmental events
Adaxial-abaxial polarity is required for lateral organ morphogenesis, especially lamina growth and margin formation of the leaf
Ectopic expression of WOX1 in the abaxial region induces adventitious outgrowth with margin-like structures on the abaxial surface of the leaf similar to kan1 kan2 [113]. These findings strongly suggest that PRESSED FLOWER (PRS) and WOX1 play key roles in lamina growth and margin formation at the adaxial-abaxial boundary downstream of the adaxial and abaxial regulators
Summary
The ―boundaries‖ between two different cell populations play a role as the center of many developmental events. Plants 2013, 2 formation, brain development and Drosophila wing development), boundary formation is triggered by the juxtaposition of two different cell populations expressing different sets of regulatory genes at an early developmental stage, and cells located at the boundary between two different cell populations obtain special characteristics and express specific regulatory genes during boundary formation [1,2,3,4,5,6,7,8] These special characteristics and/or specific regulators contribute to both organizing morphogenesis and preventing the intermingling of the different cell populations to maintain a sharp and straight boundary, despite perturbations due to cell proliferation and tissue deformation [3,5,7,9,10,11,12,13]. The difference in gene expression level between the adaxial and abaxial sides plays an important role in cell differentiation pattern and lamina growth. Recent studies have revealed the mechanism that maintains the adaxial-abaxial pattern and the boundary during lamina growth; we highlight the roles of lamina-growth regulators, especially the WUSCHEL-RELATED HOMEOBOX (WOX) family genes, in preventing the mixing of adaxial and abaxial features to maintain a proper boundary
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