The function of the 30-kilodalton movement protein (MP) of tobacco mosaic virus is to facilitate cell-to-cell movement of viral progeny in an infected plant. A novel method for delivering non-plasmalemma-permeable fluorescent probes to the cytosol of spongy mesophyll cells of tobacco leaves was used to study plasmodesmatal size exclusion limits in transgenic plants that express the MP gene. Movement of fluorescein isothiocyanate-labeled dextran (F-dextran) with an average molecular mass of 9400 daltons and an approximate Stokes radius of 2.4 nanometers was detected between cells of the transgenic plants, whereas the size exclusion limit for the control plants was 700 to 800 daltons. No evidence of F-dextran metabolism in the leaves of the transgenic plants was found. Thus, the tobacco mosaic virus movement protein has a direct effect on a plasmodesmatal function.

The phloem transports nutrients, defensive compounds, and informational signals throughout vascular plants. Sampling the complex components of mobile phloem sap is difficult because of the damage incurred when the pressurized sieve tubes are breached. In this review we discuss sampling methods, the artifacts that can be introduced by different sampling procedures, the intricate pathways by which materials enter and exit the phloem, and the major types of compounds transported. Loading and unloading patterns are largely determined by the conductivity and number of plasmodesmata and the position-dependent function of solute-specific, plasma membrane transport proteins. Recent evidence indicates that mobile proteins and RNA are part of the plant's long-distance communication signaling system. Evidence also exists for the directed transport and sorting of macromolecules as they pass through plasmodesmata. A future challenge is to dissect the molecular and cellular aspects of long-distance macromolecular trafficking in the signal transduction pathways of the whole plant.

Expresslon of the tobacco mosalc virus 30-kD movement protein (TMV MP) gene in tobacco plants increases the plasmodesmatal size exclusion limlt (SEL) 10-fold between mesophyll cells in mature leaves. In the present study, we examined the structure of plasmodesmata as a functlon of leaf development. In young leaves of 30-kD TMV MP transgenic (line 274) and vector control ( l h e 306) plants, almost all plasmodesmata were primary in nature. In both plant lines, secondary plasmodesmata were formed, in a basipetal pattern, as the leaves underwent expansion growth. Ultrastructural and immunolabellng studies demonstrated that ln line 274 the TMV MP accumulated predominantly in secondary plasmodesmata of nonvascular tissues and was associated with a filamentous material. A developmental progression was detected in terms of the presence of TMV MP; all secondary plasmodesmata in the tip of the fourth leaf contained TMV MP in association with the filamentous material. Dye-coupling experiments demonstrated that the TMV MP-induced increase in plasmodesmatal SEL could be routinely detected in the tip of the fourth leaf, but was restricted to mesophyll and bundle sheath cells. These findings are dlscussed with respect to the structure and function of plasmodesmata, particularly those aspects related to virus movement.

The phloem's role as a tissue responsible for the distribution of photoassimilates and nutrients among the various organs of higher plants has long been recognized. Recent studies have established that numerous proteins and mRNA molecules are also present in the phloem translocation stream; however, limited information is available on the identity of transcripts present within the phloem sap. In this study, a genomic approach was taken to produce a transcription profile of melon phloem sap. A cDNA library was constructed from mRNAs extracted from melon phloem sap and 1900 clones were randomly selected for sequencing. Selection of high-quality sequences resulted in 986 unique transcripts corresponding to 1830 ESTs. A comparison between the phloem-sap library and publicly available libraries from leaves and fruits indicated that the transcript profile of phloem sap is unique, with a substantially higher proportion of genes associated with biotic stimulus, response to stress, and metal-ion binding. Manual functional analyses revealed that over 40% of the transcripts are related to stress and defence responses, while over 15% of them are related to signal transduction. Out of the 1830 ESTs, only three were characterized as coding for chlorophyll-binding protein or ribulose bisphosphate carboxylase. Heterografting experiments established that six out of 43 examined transcripts are capable of long-distance trafficking from melon stocks to pumpkin scions. Annotation of these six transcripts revealed that three of them are associated with auxin-signal transduction while the other three were not identified. The potential role of the expressed transcripts in the phloem sap is discussed.

Expression of the tobacco mosaic virus 30-kD movement protein (TMV MP) gene in tobacco plants increases the plasmodesmatal size exclusion limit (SEL) 10-fold between mesophyll cells in mature leaves. In the present study, we examined the structure of plasmodesmata as a function of leaf development. In young leaves of 30-kD TMV MP transgenic (line 274) and vector control (line 306) plants, almost all plasmodesmata were primary in nature. In both plant lines, secondary plasmodesmata were formed, in a basipetal pattern, as the leaves underwent expansion growth. Ultrastructural and immunolabeling studies demonstrated that in line 274 the TMV MP accumulated predominantly in secondary plasmodesmata of nonvascular tissues and was associated with a filamentous material. A developmental progression was detected in terms of the presence of TMV MP; all secondary plasmodesmata in the tip of the fourth leaf contained TMV MP in association with the filamentous material. Dye-coupling experiments demonstrated that the TMV MP-induced increase in plasmodesmatal SEL could be routinely detected in the tip of the fourth leaf, but was restricted to mesophyll and bundle sheath cells. These findings are discussed with respect to the structure and function of plasmodesmata, particularly those aspects related to virus movement.