Long‐distance mobile mRNA CAX3 modulates iron uptake and zinc compartmentalization

Abstract

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.

MbHY5-MbYSL7 mediates chlorophyll synthesis and iron transport under iron deficiency in Malus baccata

“…Also, we observed a reduction of active Fe content in the leaves and roots under iron deficiency. Perls staining is a reliable chemical method to stain the iron trivalent in tissues; ferric iron reacts with potassium ferrocyanide and generates blue insoluble compounds ( Lv et al., 2021 ; Hao et al., 2022 ). Under Fe deficiency, a lower ferric iron content was observed compared to that of the Fe-sufficient treatment ( Figure 1 ).…”

“…Plant material and growth conditions M. baccata in vitro shoots were cultured on MS medium (0.5 mg/l 6-BA and 0.5 mg/l IBA) for 30 days (Hao et al, 2022). Next, seedlings (with a height ~5 cm) were transported to the rooting medium (0.5 mg/l IBA) and cultured for 30 days.…”

“…The full length of the MbYSL7 -coding sequence was amplified without the stop codon using the specific primer pairs ( Supplementary Table 5 ) and was inserted into the PRI101 (AN) vector with the 35S promoter. In order to repress the expression of MbYSL7 , the pTRV -MbYSL7 vector was constructed as previously described ( Sun et al., 2020 ; Hao et al., 2022 ). The MbYSL7- overexpression and VIGS vectors were transformed into Agrobacterium tumefaciens cells (GV3101).…”

“…Fe deficiency can cause a series of problems in fruit production ( Tagliavini et al., 1995 ; Alvarez-Fernandez et al., 2003 ; Hao et al., 2022 ). Therefore, revealing the sophisticated mechanism of Fe 2+ uptake, transport, and homeostasis in fruit plants is important for fruit yield and quality.…”

“…Fe deficiency affects a variety of physiological and biochemical reactions in the leaves and roots of fruit plants. One of the most prominent symptoms in plant is interveinal chlorosis, or veins yellowing, which leads to a reduced photosynthetic performance of fruit trees ( Curie and Briat, 2003 ; Hao et al., 2022 ). About 80% of the total iron was stored in chloroplasts; although iron is not a component of chlorophyll, it is an indispensable catalyst for chlorophyll synthesis ( Yang et al., 2022 ).…”