Primary vascular pattern and expression of ATHB-8 in shoots of Arabidopsis.

•  Primary vascular pattern determines pathways of long distance transport for water, nutrients and signalling molecules within plant shoot systems. •  Expression of an Arabidopsis thaliana homeobox gene 8::GUS construct is restricted to the procambium and provides a molecular marker for vascular pattern at early developmental stages. •  Primary vascular pattern and phyllotaxis are highly coordinated, with vascular sympodia corresponding to phyllotactic parastichies. During vegetative development, primary vasculature forms a reticulate pattern, with each leaf trace derived from two vascular sympodia. Shoot phase change is marked by alterations of this fundamental pattern. Formation of leaf trace procambial strands is temporally coordinated with primordium initiation, but ATHB-8::GUS expression is discontinuous in these strands at early stages. •  The interconnection of vascular sympodia provides alternate pathways for long distance transport in shoots of A. thaliana and reflects the limited secondary growth in this species. ATHB-8::GUS expression identifies a prepattern that precedes anatomical definition of procambium. The longitudinal discontinuity in ATHB-8::GUS expression indicates that one function of this gene is to define the xylem components of vascular radial pattern.