Sieve Tubes in Action

Abstract

A method was designed for in vivo observation of sieve element/companion complexes by using confocal laser scanning microscopy. A leaf attached to an intact fava bean plant was mounted upside down on the stage of a confocal microscope. Two shallow paradermal cortical cuts were made in the major vein. The basal cortical window allowed us to observe the phloem intact. The apical window at 3 cm from the site of observation was used to apply phloem-mobile fluorochromes, which identified living sieve elements at the observation site. In intact sieve tubes, the sieve plates did not present a barrier to mass flow, because the translocation of fluorochromes appeared to be unhindered. Two major occlusion mechanisms were distinguished. In response to intense laser light, the parietal proteins detached from the plasma membrane and formed a network of minute strands and clustered material that aggregated and pressed against the sieve plate. In response to mechanical damage, the evenly distributed P plastids exploded, giving rise to the formation of a massive plug against the sieve plate. In case of mechanical damage, the parietal proteins transformed into elastic threads (strands) that extended throughout the sieve element lumen. Our observations cover the phenomena encountered in previous microscopic and electron microscopic studies and provide a temporal disentanglement of the events giving rise to the confusing mass of structures observed thus far. INTRODUCTIONSince the discovery of sieve elements (SEs) by Theodor Hartig (1837), countless attempts have been made to elucidate their structure and mode of action. It was understood that SEs are involved in long-distance transport of photoassimilates (Hartig, 1860), but the mechanism of translocation had not yet been determined. Diffusion and protoplasmic streaming were discarded as potential mechanisms as soon as the transport velocities proved to be typically 50 to 100 cm hr Ϫ 1 (Canny, 1975). Currently, the most widely favored mechanism for long-distance transport through SEs is the pressure flow hypothesis (Münch, 1930).One of the stumbling blocks to full acceptance of pressure flow as the mechanism of phloem translocation is the typical components of the SEs. In particular, those directly behind the sieve plates may obstruct mass flow. These bodies, originally designated as Schleim (slime) by Hartig (1854), were later rebaptized as P proteins due to their proteinaceous character. Another hindrance may be presented by transcellular strands observed by light microscopy ( Thaine, 1961). Initially, the strands were considered to be membranous structures ( Thaine, 1962) but later respected as P protein filaments enclosing "endoplasmic tubules" (Thaine, 1969). Their rhythmic contraction produced peristaltic waves to facilitate long-distance transport (Thaine, 1969).In electron microscopic images, a parietal filamentous network that fully or partly plugged the sieve pores was observed (Robidoux et al., 1973;Johnson et al., 1976). These filaments sometimes transformed into lon...

Evidence for Apoplasmic Phloem Unloading in Developing Apple Fruit

“…The secondary phloem is clearly transport phloem, unlike the typical terminal release phloem that is mostly of primary nature (Kempers et al, 1998). Transport phloem SEs are often symplasmically isolated (Kempers et al, 1998;Knoblauch and van Bel, 1998). The mechanism of photosynthate release from the sieve tubes of transport phloem is currently unknown.…”

A Plasma Membrane-Anchored Fluorescent Protein Fusion Illuminates Sieve Element Plasma Membranes in Arabidopsis and Tobacco

“…3C and 4, A, D, E, G, and L-O). The increased brightness often observed in one face of a sieve plate relative to the other could be due to surging (Knoblauch and van Bel, 1998) of these free mCitrine-labeled membrane structures following fixation or hand sectioning (Fig. 4, A, C, and M).…”

“…Transmission electron microscopy is a potential alternative (Kü hn et al, 1997;Ehlers et al, 2000;Turgeon et al, 2001) but lacks the three-dimensional perspective needed for good quantitative measurements, it requires more time to implement than light microscopy, and it is inappropriate for statistical sampling of large amounts of tissue. Likewise, exogenously applied fluorochromes (Oparka et al, 1995;Knoblauch and van Bel, 1998;Knoblauch et al, 2001;Martens et al, 2006;Bauby et al, 2007) and sieve element (SE)-specific antibodies, like RS6 (Khan et al, 2007), have proven their utility, but none of the fluorochromes is specific to the phloem and the use of antibodies is expensive and time consuming, lessening the opportunity for fast and direct observation of phloem cells alone.…”

Plant- and stimulus-specific variations in remote-controlled sieve-tube occlusion

“…Upon phloem injury, sieve elements (SEs) are occluded by combined callose-collar formation around sieve pores 1,2 and protein plugging [3][4][5][6] to prevent leakage of nutrients and invasion of phytopathogens. 7 Given the fact that sieve pores are modified plasmodesmata (PDs), 8 callose synthesis around PDs and sieve pores may strongly resemble.…”