Ferric reduction by a CYBDOM protein counteracts increased iron availability in root meristems induced by phosphorus deficiency

To mobilize sparingly available phosphorus (P) in the rhizosphere, many plant species secrete malate to release P sorbed onto (hydr)oxides of aluminum and iron (Fe). In the presence of Fe, malate can provoke Fe over-accumulation in the root apoplast, triggering a series of events that inhibit root growth. Here, we identified HYPERSENSITIVE TO LOW P1 (HYP1), a CYBDOM protein constituted of a DOMON and a cytochrome b561 domain, as critical to maintain cell elongation and meristem integrity under low P. We demonstrate that HYP1 mediates ascorbate-dependent trans-plasma membrane electron transport and can reduce ferric and cupric substrates in Xenopus laevis oocytes and in planta. HYP1 expression is up-regulated in response to P deficiency in the proximal zone of the root apical meristem. Disruption of HYP1 leads to increased Fe and callose accumulation in the root meristem and causes significant transcriptional changes in roots. We further demonstrate that HYP1 activity overcomes malate-induced Fe accumulation, thereby preventing Fe-dependent root growth arrest in response to low P. Collectively, our results uncover an ascorbate-dependent metalloreductase that is critical to protect root meristems of P-deficient plants from increased Fe availability and provide insights into the physiological function of the yet poorly characterized but ubiquitous CYBDOM proteins.

Identification and functional characterization of the sugarcane (Saccharum spp.) AMT2-type ammonium transporter ScAMT3;3 revealed a presumed role in shoot ammonium remobilization

Sugarcane (Saccharum spp.) is an important crop for sugar and bioethanol production worldwide. To maintain and increase sugarcane yields in marginal areas, the use of nitrogen (N) fertilizers is essential, but N overuse may result in the leaching of reactive N to the natural environment. Despite the importance of N in sugarcane production, little is known about the molecular mechanisms involved in N homeostasis in this crop, particularly regarding ammonium (NH4 +), the sugarcane's preferred source of N. Here, using a sugarcane bacterial artificial chromosome (BAC) library and a series of in silico analyses, we identified an AMMONIUM TRANSPORTER (AMT) from the AMT2 subfamily, sugarcane AMMONIUM TRANSPORTER 3;3 (ScAMT3;3), which is constitutively and highly expressed in young and mature leaves. To characterize its biochemical function, we ectopically expressed ScAMT3;3 in heterologous systems (Saccharomyces cerevisiae and Arabidopsis thaliana). The complementation of triple mep mutant yeast demonstrated that ScAMT3;3 is functional for NH3/H+ cotransport at high availability of NH4 + and under physiological pH conditions. The ectopic expression of ScAMT3;3 in the Arabidopsis quadruple AMT knockout mutant restored the transport capacity of 15N-NH4 + in roots and plant growth under specific N availability conditions, confirming the role of ScAMT3;3 in NH4 + transport in planta. Our results indicate that ScAMT3;3 belongs to the low-affinity transport system (Km 270.9 µM; Vmax 209.3 µmol g-1 root DW h-1). We were able to infer that ScAMT3;3 plays a presumed role in NH4 + source-sink remobilization in the shoots via phloem loading. These findings help to shed light on the functionality of a novel AMT2-type protein and provide bases for future research focusing on the improvement of sugarcane yield and N use efficiency.

Functional characterization of the sugarcane (Saccharum spp.) ammonium transporter AMT2;1 suggests a role in ammonium root-to-shoot translocation

AMMONIUM TRANSPORTER/METHYLAMMONIUM PERMEASE/RHESUS (AMT) family members transport ammonium across membranes in all life domains. Plant AMTs can be categorized into AMT1 and AMT2 subfamilies. Functional studies of AMTs, particularly AMT1-type, have been conducted using model plants but little is known about the function of AMTs from crops. Sugarcane (Saccharum spp.) is a major bioenergy crop that requires heavy nitrogen fertilization but depends on a low carbon-footprint for competitive sustainability. Here, we identified and functionally characterized sugarcane ScAMT2;1 by complementing ammonium uptake-defective mutants of Saccharomyces cerevisiae and Arabidopsis thaliana. Reporter gene driven by the ScAMT2;1 promoter in A. thaliana revealed preferential expression in the shoot vasculature and root endodermis/pericycle according to nitrogen availability and source. Arabidopsis quadruple mutant plants expressing ScAMT2;1 driven by the CaMV35S promoter or by a sugarcane endogenous promoter produced significantly more biomass than mutant plants when grown in NH₄ ⁺ and showed more ¹⁵N-ammonium uptake by roots and nitrogen translocation to shoots. In A. thaliana, ScAMT2;1 displayed a K_m of 90.17 µM and V_max of 338.99 µmoles h^-1 g^-1 root DW. Altogether, our results suggest that ScAMT2;1 is a functional high-affinity ammonium transporter that might contribute to ammonium uptake and presumably to root-to-shoot translocation under high NH₄ ⁺ conditions.

PDX1.1-dependent biosynthesis of vitamin B6 protects roots from ammonium-induced oxidative stress

Despite serving as a major inorganic nitrogen source for plants, ammonium causes toxicity at elevated concentrations, inhibiting root elongation early on. While previous studies have shown that ammonium-inhibited root development relates to ammonium uptake and formation of reactive oxygen species (ROS) in roots, it remains unclear about the mechanisms underlying the repression of root growth and how plants cope with this inhibitory effect of ammonium. In this study, we demonstrate that ammonium-induced apoplastic acidification co-localizes with Fe precipitation and hydrogen peroxide (H₂O₂) accumulation along the stele of the elongation and differentiation zone in root tips, indicating Fe-dependent ROS formation. By screening ammonium sensitivity in T-DNA insertion lines of ammonium-responsive genes, we identified PDX1.1, which is upregulated by ammonium in the root stele and whose product catalyzes de novo biosynthesis of vitamin B₆. Root growth of pdx1.1 mutants is hypersensitive to ammonium, while chemical complementation or overexpression of PDX1.1 restores root elongation. This salvage strategy requires non-phosphorylated forms of vitamin B₆ that are able to quench ROS and rescue root growth from ammonium inhibition. Collectively, these results suggest that PDX1.1-mediated synthesis of non-phosphorylated B₆ vitamers acts as a primary strategy to protect roots from ammonium-dependent ROS formation.

Citrus carotenoid isomerase gene characterization by complementation of the "Micro-Tom" tangerine mutant

Complementation of the "Micro-Tom" tomato tangerine mutant with a Citrus CRTISO allele restores the wild-type fruit carotenoid profile, indicating that the Citrus allele encodes an authentic functional carotenoid isomerase. Citrus fruits are rich in carotenoids; the genus offers a large diversity in composition, yet to be fully explored to improve fruit nutritional quality. As perennial tree species, Citrus lack the resources for functional genetic studies, requiring the use of model plant systems. Here, we used the "Micro-Tom" (MT) tomato carrying the tangerine mutation (t), deficient for the carotenoid isomerase (CRTISO) gene, to functionally characterize the homologous C. sinensis genes. We identified four putative loci in the C. sinensis genome, named CsCRTISO, CsCRTISO-Like 1, CsCRTISO-Like 2, and CsCRTISO-Like 2B, with the latter as a presumed duplication of CRTISO-Like 2. In general, all the Citrus paralogs showed less expression specialization than the tomato ones, with CsCRTISO being the most expressed gene in all tissues analyzed. MT-t plants were successfully complemented with the CsCRTISO, and fruits showed a carotenoid profile similar to the control, indicating that the Citrus allele indeed encodes an authentic functional carotenoid isomerase and that the signal peptide is functional in tomato. MT was silenced using an inverted repeat of a fragment from the Citrus CRTISO resulting in a stronger phenotype than MT-t. MT-t and MT silenced for CRTISO presented an overall decrease in transcript accumulation of all genes from the biosynthesis pathway. The expression of the Citrus CRTISO gene is able to restore the biosynthesis of carotenoids with the appropriate regulation in MT-t. The decrease in transcript accumulation in MT-t and MT-CRTISO-suppressed lines reinforces previous suggestions that transcriptional regulation of the carotenoid biosynthesis involves regulatory loops by intermediate products.