Development of the vascular system in the inflorescence stem of Arabidopsis

The development of the vasculature in the inflorescence stem and pedicel of two common Arabidopsis thaliana ecotypes is reported, describing the tissues and events in primary and secondary growth of the stem vasculature. Light and transmission electron microscopy was used to investigate the vascular system during plant growth; cellulose, lignin and callose were detected by cytochemistry. The innermost cortical layer differentiated as a starch sheath during primary growth. The outermost parenchymatous layers of the interfascicular region lignified the walls at the onset of fascicular cambium activity. The secondary structure formed during silique production. In the internodes, the interfascicular cambium was mainly produced by starch sheath cells. In the interfascicular arcs of the internode, the secondary xylem mainly consists of xylem parenchyma whereas in the bundle region, it comprises vessels. Secondary phloem and xylary fibres were produced in limited quantities. Medullary sheath and phloem cap cells also became lignified. The results show that the secondary vasculature develops in the stem and pedicel, and that, simultaneously, some primary tissues lignify, enhancing stem rigidity. Secondary vasculature is compared with the vascular alterations reported for some mutants.