Turnover of Soluble Proteins in the Wheat Sieve Tube

Abstract

Although the enucleate conducting cells of the phloem are incapable of protein synthesis, phloem exudates characteristically contain low concentrations of soluble proteins. The role of these proteins and their movement into and out of the sieve tubes poses important questions for phloem physiology and for cell-to-cell protein movement via plasmodesmata. Because mature sieve elements lack both a nucleus and ribosomes (4), they are incapable of protein synthesis. Clearly, the ongoing presence of proteins in the translocation stream requires their continual replacement by movement from adjacent nucleate cells. Companion cells are the most likely origin of such proteins and characteristically possess an active-appearing cytoplasm, including abundant ribosomes. Recently, Nakamura et al. (16) (17) showed that even 'structural' P-proteins are involved in rapid turnover and presented microautoradiographic evidence for their synthesis in companion cells.The occurrence of protein turnover in sieve tubes raises a number of intriguing and physiologically significant questions. Movement of proteins into and out of the sieve tube presumably occurs via plasmodesmata, which, except for pathological conditions, appear to provide passageways too small for intercellular protein movement (21). Although their functions are unknown, the proteins presumably have some role in source-sink and/or sieve tube-companion cell relations. As Raven (20) recently emphasized, the striking longevity of sieve elements as enucleate cells poses ongoing maintenance problems that almost certainly require intercellular protein transport. Finally, the synthesis and movement of phloem proteins in healthy plants may provide insight into the replication and movement of phloem-limited viruses and Mycoplasma-like organisms.The following experiments were undertaken to investigate some of the overall characteristics of soluble sieve tube proteins, especially their number and variability along the transport pathway, and their pattems of synthesis, transport, and turnover. MATERIALS AND METHODS Plant MaterialWheat plants (Triticum aestivum L. cv SUN 9E) were grown in a growth chamber as described previously (9). Experiments were performed with plants in the middle portion of the grain-filling stage (approximately 15-25 d after anthesis).

S-Methylmethionine Plays a Major Role in Phloem Sulfur Transport and Is Synthesized by a Novel Type of Methyltransferase

“…The identity of 35 S-SMM was established by three criteria: comigration with authentic SMM in three separation systems; complete destruction by hot 1 M NaOH; and conversion to the corresponding ␣ -hydroxy acid by nitrous acid treatment ( Figure 2C). The proportion (80%) of the total sulfur-35 in the exudate that was accounted for by 35 S-SMM matched that reported for the unidentified 35 S-Met metabolite (i.e., SMM) by Fisher et al (1992). …”

“…Proposed general roles for SMM and its cycle include Met storage, methyl donation, and regulation of the AdoMet/Met ratio, all of which are reasonable but unsupported by experimental evidence (Giovanelli et al, 1980;Mudd and Datko, 1990). In all of these proposed roles, SMM would exert its functions without exiting the cells that produce it.Data reported in a study of sieve tube protein turnover by Fisher et al (1992) led us to suspect that SMM might have a quite different role-in phloem sulfur transport. Fisher et al (1992) found that when wheat flag leaves were given 35 SMet, most of the sulfur-35 in the phloem sap moving to the ear was in an unidentified metabolite with low mobility in thin-layer chromatography (TLC).…”

Plant intercellular communication via plasmodesmata.

“…More than 100 proteins, ranging in molecular weight from 10 to 90 kD, apparently move from CCs into the enucleate SE of the phloem, the long-distance photoassimilate transport system (Fisher et al, 1992;Nakamura et al, 1993;Sakuth et al, 1993). Because the mature SEs of the phloem lack nuclei and ribosomes and thus are incapable of protein synthesis, all macromolecules must be transported into SEs from neighboring CCs, presumably through PD (see Sjolund, 1997, in this issue).…”

Ways of signal transmission and physiological role of electrical potentials in plants

“…Its transport takes place in the phloem and in this way, as systemic signal, reaches distant plant parts in which it activates genes of proteinase inhibitors. The same way of transmission is proposed in the case of other polypeptides (elicitin, cryptogein), which can be recognized as signal substances participating in systemic responses of the plant to damages (Fisher 1992;Ebel and Mithöfer 1998).…”

“…The main transmission route of chemical compounds participating in signaling, besides volatile substances, is phloem (Fisher et al 1992;Ishiwatari et al 1995). The important feature of the phloem transmission of the compounds playing a role in signaling is that it can be directed both basi-and acropetally.…”