Pattern of differentiation of the first vascular elements in the embryo and seedling of Arabidopsis thaliana

Seeds and young seedlings of Arabidopsis thaliana it.) Heynh. (Landsberg erecta) were examined with light and electron microscopes to determine the pattern of differentiation of the first protoxylem and protophloem elements. In the mature embryo, a continuous procambial system extended the length of the hypocotyl-root axis, through the cotyledonary node, and the length of the cotyledons. No mature or differentiating vascular elements were found in the mature embryo; however, the precursor elements for the first protophloem sieve tube elements and the first protoxylem vessel element could be identified. Following imbibition, precursor vascular elements showed early removal of storage products with respect to neighboring procambial cells. Nearly simultaneous sieve tube differentiation was found 2.5 d after imbibition along the entire seedling axis, while vessel differentiation had begun at two loci in the lower portion of each cotyledon and in the upper portion of the hypocotyl. Mature or nearly mature sieve tubes extended from the lower portion of the hypocotyl to the distal tips of both cotyledon midveins in 2.75-d-old seedlings. Mature vessels were first found in 3-d-old seedlings. With the exception of a small interruption in the lower portion of the inner cotyledon, mature vessels extended from the distal portions of both cotyledons to the basal part of the hypocotyl. At the same time, sieve tubes extended from the distal portions of both cotyledons to the upper portion of the root. Differentiation of the first conducting elements in the lateral veins of the cotyledons was delayed from that of the midveins, with sieve tube differentiation preceding that of the vessels.