Translocation of Structural P Proteins in the Phloem

Abstract

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...

Proteomics analysis reveals novel host molecular mechanisms associated with thermotherapy of ‘Ca. Liberibacter asiaticus’-infected citrus plants

“…Kim et al [48] and Achor et al [49] showed that the accumulation of a lectin-like protein, at the sieve plates is associated with blockage of the translocation stream in HLB-affected citrus plants. Additionally, studies have demonstrated that lectin-like proteins interact with RNA molecules and are involved in long-distance trafficking, suggesting a role for these proteins in long-distance signaling response in HLB-affected citrus plants [39, 50]. …”

The effect of ‘Candidatus Liberibacter asiaticus’ infection on the proteomic profiles and nutritional status of pre-symptomatic and symptomatic grapefruit (Citrus paradisi) plants

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The extrafascicular phloem is made for fighting

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