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).

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Rice seeds, a rich reserve of starch and protein, are a major food source in many countries. Unlike the seeds of other plants, which typically accumulate one major type of storage protein, rice seeds use two major classes, prolamines and globulin-like glutelins. Both storage proteins are synthesized on the endoplasmic reticulum (ER) and translocated to the ER lumen, but are then sorted into separate intracellular compartments. Prolamines are retained in the ER lumen as protein bodies whereas glutelins are transported and stored in protein storage vacuoles. Mechanisms responsible for the retention of prolamines within the ER lumen and their assembly into intracisternal inclusion granules are unknown, but the involvement of RNA localization has been suggested. Here we show that the storage protein RNAs are localized to distinct ER membranes and that prolamine RNAs are targeted to the prolamine protein bodies by a mechanism based on RNA signal(s), a process that also requires a translation initiation codon. Our results indicate that the ER may be composed of subdomains that specialize in the synthesis of proteins directed to different compartments of the plant endomembrane system.

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