Vein pattern development in adult leaves of Arabidopsis thaliana

Vein pattern development in the leaves of higher plants requires that a continuous system with hierarchical vein size classes and regular spacing be formed de novo from ground meristem precursors. In this study, we use a molecular marker of procambium identity, AtHB-8::GUS, to investigate the procambial stage of vein pattern formation in adult rosette leaves of Arabidopsis thaliana and to compare the elaboration of AtHB-8-marked vein pattern with that expressed by xylem differentiation. While the well-studied juvenile leaves of Arabidopsis have a relatively simple brochidodromous vein pattern with the simultaneous appearance of the looped secondary veins, adult leaves have a complex semicraspedodromous vein pattern, and a majority of the straight secondary veins develop progressively. Late-formed secondary veins either arise in the basal portions of the leaf or are intercalated between earlier-formed secondary veins. Smaller "connector'' veins become enhanced during development to form the subterminal loops of the semicraspedodromous secondary vein pattern. Higher-order veins, especially freely ending veinlets, are formed throughout leaf expansion, maintaining a stable vein density. These unique features of adult leaf vein pattern are strongly correlated with the presence of marginal serrations and a protracted period of leaf expansion. In early leaf development, AtHB-8::GUS expression precedes any of the hallmark anatomical features of procambial cells in presumptive procambial strands, defining a "preprocambial'' stage. In contrast, AtHB-8::GUS expression was not detected during the late formation of higher-order veins, indicating that functionally redundant mechanisms guide the development of leaf vascular pattern.