Phloem iron remodels root development in response to ammonium as the major nitrogen source

Abstract

Plants use nitrate and ammonium as major nitrogen (N) sources, each affecting root development through different mechanisms. However, the exact signaling pathways involved in root development are poorly understood. Here, we show that, in Arabidopsis thaliana, either disruption of the cell wall-localized ferroxidase LPR2 or a decrease in iron supplementation efficiently alleviates the growth inhibition of primary roots in response to NH4+ as the N source. Further study revealed that, compared with nitrate, ammonium led to excess iron accumulation in the apoplast of phloem in an LPR2-dependent manner. Such an aberrant iron accumulation subsequently causes massive callose deposition in the phloem from a resulting burst of reactive oxygen species, which impairs the function of the phloem. Therefore, ammonium attenuates primary root development by insufficiently allocating sucrose to the growth zone. Our results link phloem iron to root morphology in response to environmental cues.

The ferroxidases are critical for Fe(II) oxidation in xylem to ensure a healthy Fe allocation in Arabidopsis thaliana

“…To have a close-up view of Fe accumulation in the vascular bundles, the cross section of old leaves was examined. As shown in Figure 5C , we were surprised to find that Fe was mainly deposited at the periphery of xylem vessel, which is a cell wall matrix and thus coincides with the cell wall localization feature of either LPR1 or LPR2 ( Muller et al, 2015 ; Liu et al, 2022 ). However, the cells adjacent to the xylem vessel of lpr1lpr2 also had deeper Fe staining than that of wild type.…”

“…Under our growth condition with sufficient Fe [50 μM Fe(III)–EDTA], the total Fe concentration in the xylem and phloem saps was much higher in lpr1lpr2 than in wild type ( Figures 2C,G ), which was consistent with the Fe concentration of shoots ( Figures 1D,E ). Given that LPRs could oxidize Fe(II) to Fe(III) in vitro ( Muller et al, 2015 ; Liu et al, 2022 ), we subsequently investigated the Fe valence states in the xylem sap based on the ferrozine method ( Verschoor and Molot, 2013 ). Both Fe(II) and Fe(III) concentrations in the xylem saps were markedly increased in lpr1lpr2 compared with wild type ( Figures 2D,E ), and the proportion of Fe(II) in the total Fe of xylem sap of lpr1lpr2 was 2.2-fold the wild type ( Figure 2F ).…”

“…Therefore, Fe(II) oxidation process is essential after Fe(II) is loaded into the xylem. Previously, cell wall-localized LPR1 and LPR2 were identified as multicopper oxidases (MCO) with Fe(II) oxidation activity ( Muller et al, 2015 ; Liu et al, 2022 ; Naumann et al, 2022 ). In this study, GUS staining of transgenic pLPR1:GUS and pLPR2:GUS plants showed that both LPR1 and LPR2 were specifically expressed in the vascular tissues ( Figures 2A,B ).…”

“…All plant lines used in this study were in A. thaliana ecotype Col-0 background. The mutant seeds of lpr1-1, lpr2-1 , and lpr1-1lpr2-1 were previously described ( Dong et al, 2017 ; Liu et al, 2022 ). The pLPR1:GUS /Col-0 transgenic line was a kind gift from Jian-Li Yang ( Xu et al, 2020 ).…”

“…Low phosphate root 2 (LPR2) is the only homologous gene of LPR1 in A. thaliana . Our recent study found that the LPR2 protein is also resided in the cell wall matrix and has ferroxidase activity to catalyze the oxidation of Fe(II) to Fe(III), both of which are very similar to LPR1 ( Liu et al, 2022 ). In view of the Fe(II) oxidation activity of LPR1 and LPR2 (LPRs), these two ferroxidases are likely to play a vital role in plant Fe nutrition, but related studies have not yet reported.…”