Efficient phloem remobilisation of Zn protects apple trees during the early stages of Zn deficiency.

Abstract

Apple trees are extensively cultivated worldwide but are often affected by zinc (Zn) deficiency. Limited knowledge regarding Zn remobilisation within fruit crops has hampered the development of efficient strategies for providing adequate amounts of Zn. In the present study, Zn distribution and remobilisation were compared among apple trees cultivated under different Zn conditions. Without Zn application, plants showed visible symptoms of Zn deficiency at the shoot tips after 1 year, but appeared to grow normally during the first 6 months (early stage of Zn deficiency). Compared with apple plants under sufficient Zn treatment, plants suffering from early-stage Zn deficiency showed preferential Zn distribution to young leaves and higher Zn levels in phloem, demonstrating that hidden Zn deficiency triggers a highly efficient remobilisation of Zn in this species. The in-vivo Zn-nicotianamine complex in phloem tissues, combined with the significant enhanced expression of MdNAS3 and MdYSL6, suggested a positive role for nicotianamine in the phloem remobilisation of Zn. These results strongly suggest that a proportion of Zn in the old leaves of apple trees can be efficiently remobilised by phloem transport to the shoot tips, partially in the form of Zn-nicotianamine; thus, protecting apple trees against the early stages of Zn deficiency.