Phloem-specific proteins (P proteins) are particularly useful markers to investigate long-distance trafficking of macromolecules in plants. In this study, genus-specific molecular probes were used in combination with intergeneric grafts to reveal the presence of a pool of translocatable P protein subunits. Immunoblot analyses demonstrated that Cucurbita spp P proteins PP1 and PP2 are translocated from Cucurbita maxima stocks and accumulate in Cucumis sativus scions. Cucurbita maxima or Cucurbita ficifolia PP1 and PP2 mRNAs were not detected in Cucumis sativus scions by either RNA gel blot analysis or reverse transcription-polymerase chain reaction, indicating that the proteins, rather than transcripts, are translocated. Tissue prints of the Cucumis sativus scion, using antibodies raised against Cucurbita maxima PP1 or PP2, detected both proteins in the fascicular phloem of the stem at points distal to the graft union and in the petiole of a developing leaf, suggesting that the proteins move within the assimilate stream toward sink tissues. Cucurbita maxima PP1 was immunolocalized by light microscopy in sieve elements of the extrafascicular phloem of Cucumis sativus scions, whereas Cucurbita maxima PP2 was detected in both sieve elements and companion cells. INTRODUCTIONThe long-distance movement of macromolecules in vascular tissues can impact profoundly normal plant growth and development. The importance of long-distance signaling in response to wounding as well as systemic infections by plant pathogens, such as viruses, has been well documented (Narváez-Vásquez et al., 1995;Schaller and Ryan, 1995;Nelson and Van Bel, 1998). However, little is known about the mechanisms or effects of translocating the numerous proteins that are known to be expressed specifically within the phloem tissue. The phloem of most angiosperms contains proteinaceous structures, collectively called P proteins (phloem proteins), that accumulate in differentiating sieve elements and persist in translocating sieve elements. The P protein is deposited initially into ultrastructurally distinct polymorphous or crystalline bodies during sieve element differentiation (reviewed in Cronshaw, 1975;Cronshaw and Sabnis, 1990;Sabnis and Sabnis, 1995). P protein bodies either persist or more often disperse, forming a filamentous network in the parietal cytoplasm that is thought to be immobilized through interactions with the appressed endomembrane system (Smith et al., 1987). Disruption of sieve elements that occurs during wounding results in the accumulation of P protein filaments at the sieve plate, ostensibly blocking translocation by forming P protein plugs.P protein filaments in Cucurbita maxima (pumpkin) are composed of two very abundant proteins: phloem protein 1 (PP1), a 96-kD phloem filament protein, and phloem protein 2 (PP2), a 48-kD dimeric lectin that specifically binds poly(  -1,4-N -acetylglucosamine) (Beyenbach et al., 1974;Sabnis and Hart, 1978;Allen, 1979;Read and Northcote, 1983b). Analysis of soluble phloem filaments present in phl...
Sequence analysis has shown that diseased wheat plants in Northern Germany were infected with the New York strain of soil‐borne wheat mosaic virus (SBWMV). This is in contrast to the only other confirmed site of SBWMV occurring in Germany, where a variant closely related to the Nebraska‐type strain of SBWMV was found. The results indicate that there have been at least two separate introductions of SBWMV strains to Germany. A survey is required to study the actual distribution of SBWMV in Germany.
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