Iron (Fe) is an essential micronutrient that is frequently inaccessible to plants. Rice (Oryza sativa L.) plants employ the Combined Strategy for Fe uptake, which is composed by all features of Strategy II, common to all Poaceae species, and some features of Strategy I, common to non-Poaceae species. To understand the evolution of Fe uptake mechanisms, we analyzed the root transcriptomic response to Fe deficiency in O. sativa and its wild progenitor O. rufipogon. We identified 622 and 2,017 differentially expressed genes in O. sativa and O. rufipogon, respectively. Among the genes up-regulated in both species, we found Fe transporters associated with Strategy I, such as IRT1, IRT2 and NRAMP1; and genes associated with Strategy II, such as YSL15 and IRO2. In order to evaluate the conservation of these Strategies among other Poaceae, we identified the orthologs of these genes in nine species from the Oryza genus, maize and sorghum, and evaluated their expression profile in response to low Fe condition. Our results indicate that the Combined Strategy is not specific to O. sativa as previously proposed, but also present in species of the Oryza genus closely related to domesticated rice, and originated around the same time the AA genome lineage within Oryza diversified. Therefore, adaptation to Fe²⁺ acquisition via IRT1 in flooded soils precedes O. sativa domestication.

Copper (Cu) is essential to plants but can also be harmful due to Fenton chemistry. Because of that, it is necessary to keep Cu within a narrow concentration limit. Plants evolved mechanisms to sense Cu and precisely regulate uptake and accumulation to avoid both deficiency and toxicity. However, plants experience variable Cu levels in soils, both due to natural environments as well as human’s-based farming practices that can lead to Cu accumulation in soils. Therefore, we need to understand Cu homeostasis. Here, we review how Cu is found in soils; Cu toxicity responses in plants; the role of Cu transporters, described mainly in model species such as Arabidopsis thaliana and Oryza sativa; the crosstalk between Cu and iron (Fe) homeostasis; Cu hyperaccumulator plants; and discuss some gaps and future directions, which can improve our understanding in the field.

The ionome is defined as the inorganic composition of an organism. In plants, the ionome has been shown to be integrated, as the concentrations of different elements affect one another, with complex regulatory mechanisms to keep nutrients, trace and toxic elements balanced. Iron (Fe) is an essential micronutrient that is necessary for photosynthesis, mitochondrial respiration, and redox metabolism; its concentration in plant tissues is finely regulated to avoid deficiency and excess stress. It has been known that varying concentrations of Fe affect other components of the ionome, while variations in the concentrations of other elements also perturb Fe homeostasis. Recently, molecular mechanisms that regulate the crosstalk of Fe homeostasis with that of zinc, copper, phosphorus, and nitrogen have been uncovered. Here we review these regulatory circuits, demonstrating that the ionome should be balanced and that micronutrients are important for nutrient use efficiency and to avoid nutrient deficiency as well as excess. We focus mainly on the model plant Arabidopsis thaliana and rice, for which mechanistic models have been proposed. Our review will help to integrate models to understand how plants balance the ionome.

NAC transcription factors are plant-specific proteins involved in many processes during the plant life cycle and responses to biotic and abiotic stresses. Previous studies have shown that stress-induced OsNAC5 from rice (Oryza sativa L.) is up-regulated by senescence and might be involved in control of iron (Fe) and zinc (Zn) concentrations in rice seeds. Aiming a better understanding of the role of OsNAC5 in rice plants, we investigated a mutant line carrying a T-DNA insertion in the promoter of OsNAC5, which resulted in enhanced expression of the transcription factor. Plants with OsNAC5 enhanced expression were shorter at the seedling stage and had reduced yield at maturity. In addition, we evaluated the expression level of OsNAC6, which is co-expressed with OsNAC5, and found that enhanced expression of OsNAC5 leads to increased expression of OsNAC6, suggesting that OsNAC5 might regulate OsNAC6 expression. Ionomic analysis of leaves and seeds from the OsNAC5 enhanced expression line revealed lower Fe and Zn concentrations in leaves and higher Fe concentrations in seeds than in WT plants, further suggesting that OsNAC5 may be involved in regulating the ionome in rice plants. Our work shows that fine-tuning of transcription factors is key when aiming at crop improvement.