Phloem tissue is well designed for long distance transport. The flow of materials takes place through sieve tubes, which are made up of long, tube-like, enucleate sieve elements arranged end to end and connected by specialized end walls known as sieve plates. The sieve plates contain large (up to 1 to 2 μm) pores that allow for passage of materials between sieve elements. Typically, each sieve element in angiosperms is accompanied by one or more companion cells, which interact intimately with the sieve element and play a crucial role in regulating phloem loading and unloading and in the turnover of sieve element proteins and other components (reviewed by Oparka and Turgeon, 1999). Sieve elements and companion cells are derived from unequal longitudinal division of a single “fusiform mother cell.” The cell that becomes the sieve element undergoes a highly regulated partial autolysis, not unlike that of programmed cell death, in which the central vacuole breaks down and the nucleus, ribosomes, cytoskeleton, and Golgi bodies are degraded. The result is a large, nearly empty cell that is well suited for conductance of a wide range of molecules. However, specific components and organelles are retained that may have functions in transport and/or cell–cell interactions within the sieve tubes (van Bel and Knoblauch, 2000).

It is possible to obtain electron micrographs from well-preserved and largely uninjured sieve tubes using gentle tissue preparation techniques. Evert (1982) showed an electron micrograph of Cucurbita maxima sieve tubes with completely unoccluded pores. Ehlers et al. (2000) presented a detailed analysis of sieve elements in Vicia faba (broad bean) and Lycopersicon esculentum (tomato). Well-preserved sieve elements clearly showed open, unoccluded sieve pores. Deposits of stacked ER cisternae, plastids, and proteins were observed along the parietal walls and along the walls of sieve plates (particularly in V. faba), but gaps that were free of materials were maintained in front of the sieve pores. These authors also showed that, after injury, the sieve plate pores became occluded by dispersed P-proteins, but the sieve element organelles remained intact and in place along the parietal walls. Numerous clamp-like structures were observed that appeared to anchor the plastids, mitochondria, and ER cisternae in place along the plasma membrane of the parietal wall. The clamps appear to be specific to organelles of the sieve elements and likely function to prevent the components of sieve elements from being carried along through the sieve tubes by the mass flow turbulence.

In addition to the “structural” sieve element proteins discussed here, a wide variety of soluble proteins have been reported from phloem exudates of many plant species. Hayashi et al. (2000) reported that more than 100 polypeptides could be detected in phloem exudates from a variety of species, including wheat and rice. Unlike the protein composition of other plant tissues, low-molecular-weight proteins appeared to be the dominant proteins in the phloem. In addition, thioredoxin h, glutaredoxin, and glutathione reductase have been found in reasonably high concentrations in phloem exudates, suggesting a redox mechanism for the regulation of sieve tube function. Interestingly, some of the principal sieve element proteins from Oryza sativa, C. maxima, and Ricinus communis can interact with plasmodesmata to increase their size exclusion limit. Two C. maxima proteins, CmPP16 and CmPP36, also appear to mediate the transport of RNAs (Xoconostle-Cázares et al., 1999, 2000). It is likely that the primary function of many of these proteins is the regulation of macromolecular trafficking via the unilaterally branched plasmodesmata located at the junction of sieve elements and companion cells (van Bel and Knoblauch, 2000). As shown by Knoblauch et al. in the present work, other phloem-specific proteins appear to function in the regulation of conductance through the sieve plate pores.

Calcium is an important component of many signal transduction pathways, and calcium regulation has been implicated in phloem function. In the current study, Knoblauch et al. show that an influx of calcium into legume sieve elements stimulates the rapid and reversible dispersal of crystalloid P-protein to occlude sieve plate pores. It will be important to determine if this phenomenon is limited strictly to the Fabaceae or if other families have similar mechanisms, perhaps involving the parietal and/or other P-proteins or plastids. The concentration of free calcium in sieve tubes of R. communis has been found to be significantly higher than that in surrounding tissue (Brauer et al., 1998), and calcium-dependent protein kinases have been detected in rice phloem sap (Nakamura et al., 1993). Volk and Franceschi (2000) showed evidence of a calcium channel in the sieve element plasma membrane of tobacco and of the aquatic plant Pistia stratiotes using immunolabeling with antibodies to a calcium channel protein. These authors proposed that calcium channels become activated during wounding or pathogen attack, facilitating calcium influx into phloem tissues. McEuen et al. (1981) detected a calcium binding protein distinct from calmodulin in phloem exudates of C. maxima and speculated that it might be associated with P-protein function. Do any of these species have a calcium-regulated reversible mechanism for sealing sieve elements? The grasses may represent an interesting case for the investigation of sieve tube sealing after injury; they appear to lack structural sieve element proteins (i.e., crystalloid or fibrous P-proteins and parietal proteins), and sealing of injured sieve elements might be achieved only by the contents of exploded plastids and/or the slower process of callose formation.