Figure 3.

Model depicting interactions between auxin function components and key genetic pathways controlling leaf development in Arabidopsis thaliana. Convergence points of auxin flow (arrows) generated via polar PIN1 localization in epidermal cells contribute to establishing an auxin activity maximum (red dot) at the periphery of the SAM (purple). AS1 (together with the LOB domain protein AS2, not shown here) and auxin repress the KNOX gene BP, thereby contributing to leaf outgrowth at the flanks of the SAM. The KNOX gene STM prevents AS1 expression in the meristem, thus establishing a mutually repressive interaction between meristem cells and leaf initials. Expression of STM might be regulated by auxin activity gradients, but this requires further investigation. CUC proteins (gray) are expressed at the boundary between meristem and leaf cells. The coordinated differentiation of abaxial and adaxial cell fates is critical for leaf function because it underpins functional specialization of the upper side (yellow), specialized for light capture, and a lower side (orange), specialized for gas exchange. Members of the HD-ZIP III class, such as PHB, promote adaxial fate and meristem activity, and are regulated by two known pathways. First, miRNA165/166 directly repress HD-ZIP III transcripts, which results in exclusion of HD-ZIP III expression from the abaxial domain and definition of HD-ZIP III expression level in the meristem and adaxial leaf domain. Second, expression of HD-ZIP III genes is repressed by the abaxial fate-promoting KAN proteins. The auxin response factors ARF3/ETT and ARF4 are required for KAN activity. ARF4 is expressed abaxially, whereas ARF3 mRNA may be more broadly distributed throughout the meristem and leaf primordia. Both ARF3 and ARF4 are subject to negative regulation by ta-siRNAs.