Long Distance Mobile mRNA CAX3 Modulates Iron Uptake and Zinc Compartmentalization

Fe deficiency in plants can lead to excessive absorption of Zn, however importat 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 Fe-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 Zn vacuole compartmentalization. We also show that MdCAX3 activated by MdCXIP1 is not only involved in Fe uptake but also in regulating Zn detoxification by compartmentalizing Zn in vacuoles to avoid the Fe starvation-induced Zn toxicity. Thus, mobile MdCAX3 mRNA is involved in the regulation of Fe and Zn homeostasis in response to Fe starvation.