However, leaf Zn was correlated poorly to visual deficiency symptoms. Some cultivars with no visual deficiency symptoms had leaf levels in the lowest range, whereas some of these had high leaf Zn concentration.
In an effort to develop a molecular understanding for these zinc nutritional observations, efforts have been initiated to identify zinc transporter genes in this species. Zinc transport across cellular and intracellular membranes is facilitated by several types of membrane-localized proteins, especially the recently characterized Zip transporter family. The name Zip stands for zrt-like, irt-like protein, with zrt (zinc-regulated transporter) and irt (iron-regulated transporter) referring to metal transporter genes identified in yeast (94). Several plant genes from various species (e.g., Arabidopsis thaliana, pea, tomato, soybean) have now been identified whose translation products demonstrate high homology with the Zip family (95). Functional analysis of several of these proteins has demonstrated them to be divalent metal transporters, with some having high selectivity for Zn2+ (96). Recent work in Grusak's laboratory (M.A. Grusak, USDA-ARS Baylor College of Medicine, Weslaco, TX, U.S.A., personal communication) has led to the identification of six new Zip genes in the model legume, annual or barrel medic (Medicago truncatula Gaertn.), with some of the genes showing differential expression in leaves versus roots, or in response to Zn-replete versus Zn-deficient conditions (Grusak, personal communication). With the assistance of Grauke (USDA-ARS, Somerville, TX, U.S.A.), Grusak's group has used polymerase chain reaction (PCR) approaches to attempt to clone Zip genes in pecan. Primers developed from the Medicago truncatula Zip sequences were used to perform PCRs with mRNA isolated from pecan leaves. Leaf samples were collected from a cultivar with low leaf zinc concentration and severe deficiency from a cultivar with low leaf zinc and no apparent deficiency, and from a cultivar with high leaf Zn and no apparent deficiency. Current results have yielded at least three different PCR products from the pecans, whose predicted translations indicate high amino acid sequence homology to Zip proteins from M. truncatula and other species (see (97,98) and L�pez-Mill�n, Grusak, and Grauke, unpublished results). Preliminary qualitative PCR analysis also suggests that a putative pecan Zip shows higher levels of mRNA expression in the pecan cultivars with no apparent leaf Zn deficiency (i.e., those with either high or low leaf Zn concentration). This Zip could be localized to a subcellular membrane and might influence or improve the intracellular partitioning of zinc. These results are exciting because they suggest that whole-plant zinc efficiency may be influenced by scion characteristics. For maximum benefit to cultivated pecan, therefore, appropriate root-mediated uptake mechanisms (e.g., root vigor) may need to be compatibly combined with scion-mediated uptake mechanisms (e.g., the expression or regulation of Zn transport proteins). Further characterization of the pecan Zip genes, including analysis of possible polymorphisms between genotypes of diverse geographic origin, should enhance our understanding of zinc nutrition in this crop, and possibly provide tools for breeding new zinc-efficient cultivars.
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