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Alexander Schulz, University of Copenhagen, Copenhagen,
Denmark
(September 2021)
When mature and functional, sieve elements of all plant groups form cell chains that are well-connected by specialized plasmodesmata and contain a somewhat reduced cytoplasm, but an intact plasma membrane. Phloem translocation uses these chains of cells as a low-resistance highway for an osmotically generated pressure flow and, thus, high transport rates of photo-assimilates from source to sink.
A need for an efficient transport of assimilates over long distances is already found in brown algae where Laminariales can reach 60 m in length. Their sieve elements are indeed connected via perforated end walls that can have pores of more than 400 nm diameter. The ultrastructure of the conducting elements in algae, mosses, seed-less vascular and seed plants is well documented in individual chapters of the monograph titled Sieve elements: Comparative structure, Induction and development (Behnke and Sjolund, 1990). Comparing sieve elements of the major plant taxa shows that the size of pores between the sieve element increases and the cytoplasmic contents of sieve elements decrease with the evolution of land plants.
Reduction of the cytoplasmic contents is by loss of the nucleus, vacuoles and protein biosynthesis machinery and attachment of sieve-element plastids, mitochondria, and ER system to the plasma membrane, thus keeping the cell lumen free and reducing intracellular flow resistance dramatically. Intercellular resistance is at the same time reduced by the widening of ordinary plasmodesmata to wide sieve pores.
Both evolutionary trends and the fact that sieve elements are only functional when alive are unifying characters across the plant taxa with an assimilate transporting system. However, the features facilitating long-distance transport come with a challenge: the longer time sieve elements are alive, the more they depend on other cells to help them with lipid and protein turnover and supply them with energy (e.g., in the form of ATP).
Interestingly, alongside with the evolutionary increased efficiency of long-distance transport, the sieve elements get a more and more intimate relationship to neighboring cells in the transport phloem (Web Figure 12.2.A). Brown algae do not seem to have such relationship; plasmodesmata between cortex cells and sieve elements have only been found in sieve-element loading regions. Sieve elements of the transport pathway have thick longitudinal cell walls, isolating them from the surrounding tissue (Schmitz, 1990).
Moss gametophytes contain sieve elements that have ordinary plasmodesma connections to neighboring cells. These are not ontogenetically related and have the normal cellular equipment of parenchyma cells (Scheirer, 1990).
In seedless vascular plants such as ferns, symplasmic coupling between sieve elements and neighboring parenchyma cells becomes closer and consists of numerous sieve-pore plasmodesma contacts with a sieve pore on the sieve-element side and a single plasmodesma on the neighbor-cell side (Evert, 1990). Sieve-pore plasmodesma contacts are accordingly a common denominator for this interface in vascular plants, emphasizing the importance of cell communication between sieve elements and neighboring cells.
Finally, the highest degree of connectivity between sieve element and neighboring cells is achieved in seed plants. Here, sieve pores contacts branch out in numerous plasmodesmata on the neighboring cell side, that is Strasburger cell and companion cells in gymnosperms and angiosperms, respectively. While Strasburger cells are not ontogenetically related to sieve elements and often derive from ray initials rather than fusiform cambium initials, companion cells of angiosperms are sister cells of sieve elements and generally the product of an unequal division of their mother cell in regular as well as in wound- or graft-induced phloem (see other chapters in Behnke and Sjolund, 1990.)
Strasburger cells of gymnosperms are, like companion cells, rich in mitochondria and ribosomes and show a high activity of different enzymes (Schulz, 1990). Companion cells and their role in collection and transport phloem are described in much more detail in the textbook Chapter 12.
In conclusion, ultrastructure and enzymatic activities are very similar in Strasburger and companion cells. They contrast in the developmental process that allows them to “find together.” In gymnosperms this is just good neighborship, while sieve elements of angiosperms already come with their companion cell, guaranteeing that protein, lipid and other turnover processes are taken care of and can occur via the pore-plasmodesma contacts. We still do not know how the need for new proteins is communicated from the sieve element to its neighbor, and how metabolic processes in sieve elements are controlled by the neighboring cells but that is an active research area.
References
Behnke HD, Sjolund RD, eds. (1990) Sieve Elements - Comparative structure, induction and development. Springer Berlin Heidelberg.
Evert RF (1990) Seedless vascular plants. In HD Behnke, RD Sjolund, eds, Sieve Elements - Comparative structure, induction and development. Springer Berlin Heidelberg, pp 35–62.
Scheirer DC (1990) Mosses. In HD Behnke, RD Sjolund, eds, Sieve Elements - Comparative structure, induction and development. Springer Berlin Heidelberg, pp 19–33.
Schmitz K (1990) Mosses. In HD Behnke, RD Sjolund, eds, Sieve Elements - Comparative structure, induction and development. Springer Berlin Heidelberg, pp 1–18.
Schulz A (1990) Conifers. In HD Behnke, RD Sjolund, eds, Sieve Elements - Comparative structure, induction and development. Springer Berlin Heidelberg, pp 63–88.