SummaryIron (Fe) is an essential plant nutrient and its deficiency typically limits plant growth. Long non‐coding (lnc) RNAs are involved in adaptive responses to nutrient stress; however, it is not known whether they function in the regulation of the canonical Fe‐deficiency response. The expression of Malus domestica (apple) lncRNA MSTRG.85814 is induced by Fe deficiency, as identified by high‐throughput strand‐specific RNA‐seq analysis of an apple homograft system. MSTRG.85814 has a complex structure, with 13 predicted RNA sequence variants, four of which are upregulated in the roots of plants experiencing Fe deficiency. We found that one MSTRG.85814 splice variant (MSTRG.85814.11) positively modulated its cis target mRNA derived from the small auxin upregulated gene SAUR32. This in turn promoted the expression of SAUR32 and caused an increase in the expression of a plasma membrane proton ATPase, AHA10. Using a pH imaging technique, a significant decrease in the apoplastic pH was observed to occur in the root tips of MSTRG.85814.11 or SAUR32‐overexpressing apple plants. Thus MSTRG.85814.11 was shown to positively promote SAUR32 expression, which then activated proton extrusion involved in the Fe‐deficiency response. These results reveal a mechanism by which lncRNA promotes environmental Fe‐deficiency stress adaption.

Iron deficiency in plants can lead to excessive absorption of zinc; however, important details of this mechanism have yet to be elucidated. Here, we report that MdCAX3 mRNA is transported from the leaf to the root, and that MdCAX3 is then activated by MdCXIP1. Suppression of MdCAX3 expression leads to an increase in the root apoplastic pH, which is associated with the iron deficiency response. Notably, overexpression of MdCAX3 does not affect the apoplastic pH in a MdCXIP1 loss‐of‐function Malus baccata (Mb) mutant that has a deletion in the MdCXIP1 promoter. This deletion in Mb weakens MdCXIP1 expression. Co‐expression of MdCAX3 and MdCXIP1 in Mb causes a decrease in the root apoplastic pH. Furthermore, suppressing MdCAX3 in Malus significantly reduces zinc vacuole compartmentalization. We also show that MdCAX3 activated by MdCXIP1 is not only involved in iron uptake, but also in regulating zinc detoxification by compartmentalizing zinc in vacuoles to avoid iron starvation‐induced zinc toxicity. Thus, mobile MdCAX3 mRNA is involved in the regulation of iron and zinc homeostasis in response to iron starvation.

Long-distance mobile mRNAs play key roles in gene regulatory networks that control plant development and stress tolerance. However, the mechanisms underlying species-specific delivery of mRNA still need to be elucidated. Here, the use of grafts involving highly heterozygous apple (Malus) genotypes allowed us to demonstrate that apple (Malus domestica) oligopeptide transporter3 (MdOPT3) mRNA can be transported over a long distance, from the leaf to the root, to regulate Fe uptake; however, the mRNA of Arabidopsis (Arabidopsis thaliana) oligopeptide transporter 3 (AtOPT3), the MdOPT3 homolog from A. thaliana, does not move from shoot to root. Reciprocal heterologous expression of the two types of mRNAs showed that the immobile AtOPT3 became mobile and moved from the shoot to the root in two woody species, Malus and Populus, while the mobile MdOPT3 became immobile in two herbaceous species, A. thaliana and tomato (Solanum lycopersicum). Furthermore, we demonstrated that the different transmissibility of OPT3 in A. thaliana and Malus might be caused by divergence in RNA-binding proteins (RBPs) between herbaceous and woody plants. This study provides insights into mechanisms underlying differences in mRNA mobility and validates the important physiological functions associated with this process.

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