Vascular Differentiation and Transition in the Seedling of Arabidopsis thaliana (Brassicaceae)

Abstract

Seedlings of Arabidopsis thaliana (L.) Heynh. (Landsberg erecta) were examined by light and electron microscopy to study vascular development through the transition region. In addition, the primary vasculature of the rosette and its relation to that of the root‐hypocotyl‐cotyledon unit were examined. The transition region is restricted to a very short portion of the upper hypocotyl, the cotyledonary node, and the cotyledons. For most of its length, the hypocotyl is rootlike in vascular arrangement, with a diarch xylem oriented in the exarch condition and an alternate arrangement of the xylem and phloem. The appearance of a pith interrupting the primary xylem plate marks the beginning of vascular transition. Slightly above this level, the phloem strands on either side of the primary xylem plate bifurcate, forming four phloem strands. Moving upward through the cotyledonary node, two phloem strands become spatially associated with each of the xylem groups, forming the traces of the cotyledons. The transition from exarch to endarch xylem is completed in the midveins in the lower portion of the cotyledons. As they ascend the midveins, the two phloem strands assume a position opposite the endarch xylem; the bundles now are almost collateral. About a third of the length into the midveins the phloem strands come to lie side by side, completing transition. By the sixth day, a vascular cambium is initiated from periclinal divisions of procambial cells between the phloem strands and primary xylem plate in the upper hypocotyl and corresponding portion of the cotyledonary node. With interpolation of secondary vascular tissues between the primary phloem and primary xylem, the primary vasculature is disrupted. The vascular system of the rosette develops after the primary vascular system of the root‐hypocotyl‐cotyledon unit and is superimposed on the latter. Each rosette leaf trace is connected to traces of two other leaves, including the cotyledons, lower on the axis.