THE EFFECT OF TRANSPORTER GENES ON ZINC STRESS IN APPLE (MALUS × DOMESTICA BORKH.)

Abstract

Zinc deficiencies are commonly encountered on apple and are associated with poor tree growth, yields and fruit quality. Corrective foliar sprays and/or soil applications of Zn are only partially effective and may elevate Zn in the soil to levels of regulatory concern. The goal of this work is to elucidate the feasibility of increasing the uptake and transport of Zn to the aboveground parts by constitutively overexpressing Zn transporter genes in 'M.26' apple rootstock. Clones containing the coding regions of Zn transporter genes ZNT1 or ZIP4 were ligated along with a dual 35S promoter into a pBIN+ARS plasmid vector. Agrobacterium-mediated transformation was used to introduce the gene construct into in vitro apple explants of 'M.26' rootstock. The ability of three transgenic lines, T4 (ZNT1), T5 (Vector Control), T6 (ZIP4) and untransformed 'M.26', to absorb Zn were evaluated in a greenhouse, using diethylenetriamine pentaacetic acid (DTPA) chelator-buffered nutrient solutions containing low (2 μM) or optimal (24 μM) Zn concentrations. Plants grown under the low Zn level showed Zn deficiency symptoms and had reduced growth. The over-expression of ZIP4 and ZNT1 transporter genes in 'M.26' apple rootstock had very limited and no effect on the plants' Zn nutritional status, respectively. The over-expression of ZIP4, but not the ZNT1 transporter gene, consistently increased the concentration of Ca and Cu in all roots and aboveground tissues, thus indicating enhanced uptake and transport of these elements. The fact that ZIP4 was expressed to a greater extent than ZNT1, corroborates the increased tissue nutrient concentrations of the T6 (ZIP4) line.