The Leaf Adaxial-Abaxial Boundary and Lamina Growth

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.

TheTrans-Acting Short Interfering RNA3 Pathway and NO APICAL MERISTEM Antagonistically Regulate Leaf Margin Development and Lateral Organ Separation, as Revealed by Analysis of anargonaute7/lobed leaflet1Mutant inMedicago truncatula

“…Leaf margin cells are a type of boundary cell that forms a specific structure to separate the adaxial and abaxial epidermis of the leaf blade. Such structures are required for proper leaf blade growth and are also regulated by leaf adaxialabaxial polarity (Sarojam et al, 2010;Szakonyi et al, 2010;Wang et al, 2011;Yamaguchi et al, 2012;Nakata and Okada, 2013). In this study, the dorsiventral polarity of the leaf is altered in ago7 as a result of the ectopic expression of ARF3.…”

“…Therefore, by regulating the establishment of leaf polarity, ARF3 is probably indirectly involved in the formation of leaf margin cells. On the other hand, the pattern of cell proliferation and expansion is a major factor affecting organ shape (Nakata and Okada, 2013). Accompanying the defects in margin cell identity in ago7, the density of margin cells was increased and the cell size was decreased in the sinus of the leaf margin, suggesting that both cell proliferation and expansion are changed.…”

“…Based on the expression of DR5 reporter constructs, auxin responsiveness was altered along the leaf margin of ago7. It has been suggested that auxin distribution is dependent on the dorsiventral pattern (Zgurski et al, 2005;Nakata and Okada, 2013). Therefore, the increased auxin response in ago7 is likely a result of the changed leaf polarity induced by ectopic expression of ARF3.…”

Axillary Meristem Formation in Rice Requires the WUSCHEL Ortholog TILLERS ABSENT1

“…Apart from maintenance of the SAM, WOX genes play diverse roles in plant development, such as embryogenesis, vascular differentiation, lateral organ patterning, and maintenance of the root apical meristem (reviewed in Hirakawa et al, 2011;Lau et al, 2012;Nakata and Okada, 2013), in both Arabidopsis and rice. In addition, phylogenetic and evolutionary studies have added to our understanding of the function of WOX genes not only in angiosperms but also in gymnosperms (Nardmann et al, 2007(Nardmann et al, , 2009Lin et al, 2013;Nardmann and Werr, 2013) Although much progress has been made in elucidating the role of the WOX genes, the function of WUS orthologs is still unclear in monocots.…”

Leaf development and morphogenesis

“…We also touch briefly on environmental factors that can impact leaf development (see Box 2), and on recently developed quantitative approaches (see Box 3), which can serve to further characterize and understand leaf development. We chose not to discuss adaxial-abaxial, vascular, trichome or stomatal patterning, as several recent reviews have discussed these topics (Grebe, 2012;Kidner and Timmermans, 2010;Lau and Bergmann, 2012;Nakata and Okada, 2013;Sack and Scoffoni, 2013).…”