Diversity of the Superfamily of Phloem Lectins (Phloem Protein 2) in Angiosperms

Abstract

Phloem protein 2 (PP2) is one of the most abundant and enigmatic proteins in the phloem sap. Although thought to be associated with structural P-protein, PP2 is translocated in the assimilate stream where its lectin activity or RNA-binding properties can exert effects over long distances. Analyzing the diversity of these proteins in vascular plants led to the identification ofPP2-like genes in species from 17 angiosperm and gymnosperm genera. This wide distribution of PP2 genes in the plant kingdom indicates that they are ancient and common in vascular plants. Their presence in cereals and gymnosperms, both of which lack structural P-protein, also supports a wider role for these proteins. Within this superfamily, PP2 proteins have considerable size polymorphism. This is attributable to variability in the length of the amino terminus that extends from a highly conserved domain. The conserved PP2 domain was identified in the proteins encoded by six genes from several cucurbits, celery (Apium graveolens), and Arabidopsis that are specifically expressed in the sieve element-companion cell complex. The acquisition of additional modular domains in the amino-terminal extensions of other PP2-like proteins could reflect divergence from its phloem function.

A Long-Distance Translocatable Phloem Protein from Cucumber Forms a Ribonucleoprotein Complex In Vivo with Hop Stunt Viroid RNA

“…The functions of this protein are not yet clear (5,17,18). Recent analyses have led to the identification of PP2-like genes in species from 17 angiosperm and gymnosperm genera, indicating that these proteins are ancient and common in vascular plants (10).…”

Binding Properties of the N-Acetylglucosamine and High-Mannose N-Glycan PP2-A1 Phloem Lectin in Arabidopsis

“…The PP2s isolated from C. maxima and Cucumis melo have lectin activity (Sabnis and Hart, 1978;Allen, 1979;Read and Northcote, 1983b;Dinant et al, 2003). These proteins bind to chitin columns, demonstrating an affinity for oligomers of GlcNAc.…”

“…In Cucurbita maxima, the P-proteins are the two most abundant sap proteins: Phloem Protein1 (PP1), which belongs to a gene family found only in cucurbits (Clark et al, 1997), and PP2, which belongs to a large gene family in angiosperms (Bostwick et al, 1992(Bostwick et al, , 1994Dinant et al, 2003). In cucurbits, PP2s display additional properties, including RNA binding Pallás, 2001, 2004;Owens et al, 2001;Gó mez et al, 2005), trafficking from cell to cell, and modification of the size exclusion limit of the plasmodesmata (Balachandran et al, 1997).…”

Functional Characterization of HFR1, a High-MannoseN-Glycan-Specific Wheat Lectin Induced by Hessian Fly Larvae

“…Lectin from garlic (Allium sativum) bulb is active against insects such as cowpea weevil (Callosobruchus maculates), Egyptian cotton leafworm (Spodoptera littoralis), and tomato moth, causing reduced larval weight gain and delayed development (Fitches et al, 1997(Fitches et al, , 2001. Some lectins, such as the garlic bulb lectin, play a dual role as storage protein and as defense proteins that are mobilized against attacking insects Smeets et al, 1997;Dinant et al, 2003). Transgenic rice (Oryza sativa) plants expressing garlic leaf agglutinin have antifeedant properties that are insecticidal against sap-sucking insect pests such as the brown planthopper, the green leafhopper (Nephotettix virescens; Saha et al, 2006), and the Egyptian cotton leafworm (Sadeghi et al, 2008).…”