Control of Vein Network Formation by Auxin Signaling

Abstract

Multicellular organisms solve the problem of long-distance transport of signals and nutrients by means of tissue networks such as the vascular system of vertebrate embryos and the vein networks of plant leaves; therefore, how vascular networks form is a key question in biology. In vertebrates, the formation of the embryonic vascular system relies on direct cell-cell interaction and at least in part on cell migration. Both direct cell-cell interaction and cell migration are precluded in plants by a cell wall that keeps cells apart and in place; therefore, vascular networks form differently in plant leaves. How vein networks form in plant leaves is unclear, but available evidence suggests that signal transduction of the plant hormone auxin is nonredundantly required for vein network formation. Nonredundant functions of auxin signaling in vein network formation in turn depend on nonredundant functions of the MONOPTEROS (MP) transcription factor. MP is expressed in all the cells of the leaf at early stages of tissue development, but over time, epidermal expression becomes restricted to the basalmost cells and inner-tissue expression becomes restricted to developing veins. However, it is currently unknown what the function of MP expression in the leaf epidermis and vascular tissue is in auxin-signaling-dependent vein-network formation. Here we identify and characterize GAL4/GFP enhancer-trap lines for the targeted expression of genes of interest in specific cells and tissues of developing leaves. We combine GAL4-driven tissue-specific gene expression with cellular imaging and molecular genetic analysis to address the question what the function of MP expression in the leaf epidermis and vascular tissue is in auxin-signaling-dependent vein-network formation. We find that MP expression in the leaf epidermis is dispensable and that MP expression in the vascular tissue is sufficient for auxin-signaling-dependent vein-network formation. Moreover, we show that constitutively active auxin signaling in the epidermis is insufficient for vascular differentiation anywhere in the leaf, whereas constitutively active auxin signaling in the vascular tissue is sufficient for supernumerary vein formation. In the inner tissue of the developing leaf, broadly expressed MP activates expression of its target gene ARABIDOPSIS THALIANA HOMEOBOX8 (ATHB8) in narrow domains that presage sites of vein formation. Activation of ATHB8 expression in narrow domains depends on binding of MP to a low-affinity MP-binding site in the ATHB8 promoter. However, the biological relevance of the activation of ATHB8 expression by MP is unclear: whereas MP promotes vein formation, ATHB8 seems to have only transient and conditional functions in vein network formation. Furthermore, whereas both ATHB8 and MP are expressed in files of vascular cell precursors, MP is additionally expressed in surrounding nonvascular cells, which fail to activate ATHB8 expression. However, it is unclear why ATHB8 expression is only activated in a subset of MP-expressing cells. Here we address this question by combining cellular imaging and molecular genetic analysis. We show that ATHB8 promotes vein formation and that both levels of ATHB8 expression and width of ATHB8 expression domains are relevant to vein formation. Finally, we show that ATHB8 expression is restricted to narrow preprocambial domains by a combination of (1) activation of ATHB8 expression through binding of peak levels of MP to a low-affinity MP-binding site in the ATHB8 promoter and (2) repression of ATHB8 expression by MP target genes of the INDOLE-3-ACETIC-ACID-INDUCIBLE family. The very same regulatory logic that underlies activation of ATHB8 expression in files of vascular cell precursors is most frequently used in animals to generate stripes of gene expression, suggesting unexpected conservation of regulatory logic of striped gene expression in plants and animals in spite of the independent evolution of their multicellularity.