Freeze-fracture analysis of phloem structure in plant tissue cultures

The Phloem-Delivered RNA Pool Contains Small Noncoding RNAs and Interferes with Translation

“…Thus, functional aminoacylated tRNAs capable of facilitating translation seem to exist in the phloem. However, ultrastructural observations (Sjolund and Shih, 1983), amino acid loading experiments (Fisher et al, 1992), and proteomic analysis of the PS content from several species such as Cucumis sativus (Walz et al, 2004), Triticum aestivum (Fukuda et al, 2005), or B. napus (Giavalisco et al, 2006) showed that a translatory system based on ribosomes using tRNAs is not present in sieve elements. In addition, supporting the notion that peptide synthesis cannot occur in the sieve tube system, we observed that native PS RNA and truncated tRNAs effectively inhibit translation.…”

The Phloem Sieve Element: A River Runs through It.

“…One possibility is that the ATP is produced by the companion cells, which have more abundant mitochondria than do mature sieve elements, and is then transferred to sieve elements by means of trafficking through the branched plasmodesmata (van Bel, 1996). However, sieve elements also contain mitochondria ( Figures 1A and 1 B), even at maturity (Esau and Cronshaw, 1968), and the possibility that sieve element mitochondria could provide the ATP required by an H+-ATPase (Sjolund and Shih, 1983a) remains to be established. Nevertheless, evidente that sieve element mitochondria are metabolically active comes from experiments showing that they take up the dye Janus green B (McGivern, 1957;Lee et al, 1971) and that they accumulate the lipophilic cation Rhodamine 123 (Moniger et al, 1993).…”

“…An elaborate system of membranes, the sieve element reticulum (SER) (Srivastava and O'Brien, 1966;Sjolund and Shih, 1983a), is a prominent feature of the mature sieve element in virtually all plants studied to date ( Figures 1A and 1 B). The SER lacks ribosomes and is often described on the basis of its morphology as stacked, anastomosing, or parietal endoplasmic reticulum.…”

“…companion cells. The SER may play a role in the directed trafficking of ATP or other molecules from the companion cells to sites on the sieve element plasma membrane (Thorsch and Esau, 1981a;Sjolund and Shih, 1983a).…”

Aphid watery saliva counteracts sieve-tube occlusion: a universal phenomenon?

“…SEs are elongated cells that lack a nucleus and vacuole and contain only an intact plasma membrane, phloem plastids, a SE endoplasmic reticulum (Sjölund and Shih, 1983), a few mitochondria, and an extensive set of phloem-specific proteins (reviewed by Hayashi et al, 2000). During SE development, the terminal walls transform into sieve plates, perforated by sieve pores (Evert, 1990) to connect adjacent SEs end to end.…”