The Phloem Sieve Element: A River Runs through It.

Abstract

31 9-335-3620. the sieve tube, which is formed by a series of connected phloem sieve elements. This fundamental difference imposes requirements on the development of the sieve element that are unique to these cells, and the differentiation of sieve elements follows a course that is unparalleled in the biological world. Essentially, a series of differentiating sieve elements form a syncytium, a single compartment bound by a plasma membrane (Murphy and Aikman, 1989). Within the developing syncytium, the larger cellular organelles, which could impede assimilate flow, are degraded and removed, and the mature sieve tube lacks nuclei, vacuoles, Golgi bodies, or ribosomes. The plasmodesmata in the walls between the individual sieve elements are converted into large openings (sieve pores) that facilitate flow from cell to cell through the sieve tube. Significantly, this differentiation process, including the formation of sieve pores, is accomplished while maintaining the continuity of the plasma membrane around the individual cells and through the pores formed between the cells. The result of this unique differentiation process is the formation of a continuous, membrane-lined compartment, the sieve tube, that provides a pathway for nutrient flow and for signaling activities (Ryals et al., 1996) throughout the plant body.