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  • Author or Editor: Danielle R. Ellis x
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A 22-kDa Zn-binding protein (ZBP) was isolated from the phloem tissue and evacuated xylem sap of `Valencia' sweet orange [Citrus sinensis (L.) Osbeck] on rough lemon [C. jambhiri (L.)], as well as Valencia on Rangpur lime [Citrus limonia Osbeck]. Phloem and xylem Zn was associated with the 22 kDa ZBP. The Mr value of this ZBP was estimated to be 19,500 by size exclusion chromatography and 22,800 by SDS-PAGE. This protein was isolated with an isoelectric point of 7.5. Ion exchange chromatography demonstrated that 22-kDa ZBP was highly anionic, requiring 0.43 M NaCl for elution from QAE Sepharose. The 22-kDa ZBP appears unique to citrus, having no cross reaction with protein from several tissues from a range of plant species. Accumulation decreased under Zn-deficient conditions, was enhanced by osmotic stress, and the protein completely disappeared with wounding. Amino acid composition demonstrated that the protein was rich in aspartate, and glutamate; and contained 6 cysteine, and 4 histidine residues. These amino acids may be involved in metal binding. N-terminal amino acid sequencing demonstrated that the 22-kDa ZBP had identity with sporamin A&B precursors, Kunitz-type trypsin inhibitors, and miraculin. It is suggested that the genes that encode these proteins are derived from a common ancestral gene.

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Trees of `Redhaven' peach [Prunus persica (L.) Batsch] budded to `Lovell', `Bailey', and `Nemaguard' rootstocks were grown with bahiagrass or cultivated orchard middles. Terminal shoots were collected once a month through the dormant season. `Redhaven' on `Lovell' had significantly higher levels of sucrose, sorbitol, total soluble sugars, starch and total non-structural carbohydrates than `Redhaven' on `Nemaguard'. However, there were no significant differences in any carbohydrate fraction between `Redhaven' on `Bailey' and the other rootstocks. Orchard floor management system had no significant effect on carbohydrate levels.

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A partial cDNA (cvzbp-1) was cloned based on the N-terminal sequence of a citrus (Citrus L.) vascular Zn-binding protein (CVZBP) previously isolated from vascular tissue (Taylor et al., 2002). CVZBP has homology to the Kunitz soybean proteinase inhibitor (KSPI) family. Recombinant protein produced using the cDNA clone inhibited the cysteine proteinase, papain. Metal binding capacity has not been reported for any other member of this family. CVZBP was present in leaves, stems, and roots but not seeds of all citrus species examined. However, CVZBP was present in germinating seeds after the cotyledons had turned green. Within four hrs after wounding, CVZBP was undetectable in the wounded leaf and adjacent leaves. It has been suggested that many members of the KSPI family serve a function in defense. However, the expression of the CVZBP is in direct contrast with those of KSPI members that were implicated in defense response. Though systemically regulated during wounding, we suggest that CVZBP is not a defense protein but rather may function in vascular development.

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Zinc in xylem and phloem of the citrus rootstock, rough lemon [Citrus jambhiri (L.)] was associated with a Zn-binding protein, designated citrus vascular Zn-binding protein (CVZBP). The apparent molecular mass of the CVZBP was 19.5 kDa after nondenaturing size exclusion chromatography and 21.8 kDa after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Ion exchange chromatography demonstrated that CVZBP was anionic, requiring 0.43 n NaCl for elution from quaternary aminoethyl Sepharose. Antiserum to the protein cross-reacted more with total protein extracts from leaf midveins than with total protein from the rest of the leaf lamina, further suggesting a vascular location of the Zn-binding protein. Quantitative analysis indicated that ≈2 to 3 mol of Zn were associated with 1 mol of native protein. Binding studies with the partially purified CVZBP demonstrated a capacity to bind several divalent cations: Cd, Ni, Pb, and Zn. Reaction with Ellman's reagent suggested that the protein has significant sulfhydryl group content that may be involved in metal binding. N-terminal sequencing demonstrates identity with papaya latex trypsin inhibitor, sporamin, or other Kunitz soybean proteinase inhibitors.

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