The transcription factor OsSPL9 endows rice with copper deficiency resilience.

Copper (Cu) is a crucial micronutrient essential for the growth and development of plants. Rice exhibits remarkable resistance to Cu deficiency, but the underlying molecular mechanisms are not well understood. In this study, we reveal that the plant's ability to withstand Cu deficiency is orchestrated by a transcription factor known as OsSPL9. We have demonstrated that OsSPL9 functions as a central regulator of Cu homeostasis. Disrupting OsSPL9 through knockout significantly reduces the plant's tolerance to Cu deficiency. As a result, the spl9 mutants exhibit reduced Cu accumulation in their shoots when compared to wild-type plants. This reduction is linked to a disruption in the transport of Cu from older leaves to younger ones. Furthermore, we show that OsSPL9 directly binds to GTAC motifs in the promoters of key genes involved in Cu uptake and transport, as well as Cu-miRNAs, and enhances their transcription under Cu-deficient conditions. Overall, our findings shed light on the molecular basis of rice resilience to Cu deficiency stress and place the transcription factor OsSPL9 as a master regulator of this response.

The Splicing Factor OsSCL26 Regulates Phosphorus Homeostasis in Rice.

Phosphorus (P) is an essential nutrient for plant growth. However, its deficiency poses a significant challenge for crop production. To overcome the low P availability, plants have developed various strategies to regulate their P uptake and usage. In this study, we identified a splicing factor, OsSCL26, belonging to the Serine/arginine-rich (SR) proteins, that plays a crucial role in regulating P homeostasis in rice. OsSCL26 is expressed in the roots, leaves, and base nodes, with higher expression levels observed in the leaf blades during the vegetative growth stage. The OsSCL26 protein is localized in the nucleus. Mutation of OsSCL26 resulted in the accumulation of P in the shoot compared to the wild-type, and the dwarf phenotype of the osscl26 mutant was alleviated under low P conditions. Further analysis revealed that the accumulated P concentrations in the osscl26 mutant were higher in the old leaves and lower in the new leaves. Furthermore, the P-related genes, including the PHT and SPX family genes, were upregulated in the osscl26 mutant, and the exclusion/inclusion ratio of the two genes, OsSPX-MFS2 and OsNLA2, was increased compared to wild-type rice. These findings suggest that the splicing factor OsSCL26 plays a pivotal role in maintaining P homeostasis in rice by influencing the absorption and distribution of P through the regulation of the transcription and splicing of the P transport genes.

The bHLH Transcription Factor OsbHLH057 Regulates Iron Homeostasis in Rice.

Many basic Helix-Loop-Helix (bHLH) transcription factors precisely regulate the expression of Fe uptake and translocation genes to control iron (Fe) homeostasis, as both Fe deficiency and toxicity impair plant growth and development. In rice, three clade IVc bHLH transcription factors have been characterised as positively regulating Fe-deficiency response genes. However, the function of OsbHLH057, another clade IVc bHLH transcription factor, in regulating Fe homeostasis is unknown. Here, we report that OsbHLH057 is involved in regulating Fe homeostasis in rice. OsbHLH057 was highly expressed in the leaf blades and lowly expressed in the roots; it was mainly expressed in the stele and highly expressed in the lateral roots. In addition, OsbHLH057 was slightly induced by Fe deficiency in the shoots on the first day but was not affected by Fe availability in the roots. OsbHLH057 localised in the nucleus exhibited transcriptional activation activity. Under Fe-sufficient conditions, OsbHLH057 knockout or overexpression lines increased or decreased the shoot Fe concentration and the expression of several Fe homeostasis-related genes, respectively. Under Fe-deficient conditions, plants with an OsbHLH057 mutation showed susceptibility to Fe deficiency and accumulated lower Fe concentrations in the shoot compared with the wild type. Unexpectedly, the OsbHLH057-overexpressing lines had reduced tolerance to Fe deficiency. These results indicate that OsbHLH057 plays a positive role in regulating Fe homeostasis, at least under Fe-sufficient conditions.

A tale of two players: the role of phosphate in iron and zinc homeostatic interactions.

MAIN CONCLUSION: This minireview details the impact of iron-phosphate and zinc-phosphate interactions in plants and provides perspectives for further areas of research regarding nutrient homeostasis. Iron (Fe) and zinc (Zn) are among the most important micronutrients for plant growth and have numerous implications for human health and agriculture. While plants have developed efficient uptake and transport mechanisms for Fe and Zn, emerging research has shown that the availability of other nutrients in the environment influences the homeostasis of Fe and Zn within plants. In this minireview, we present the current knowledge regarding homeostatic interactions of Fe and Zn with the macronutrient phosphorous (P) and the resulting physiological responses to combined deficiencies of these nutrients. Fe and P interactions have been shown to influence root development, photosynthesis, and biological processes aiding Fe uptake. Zn and P interactions also influence root growth, and coordination of Zn-dependent transcriptional regulation contributes to phosphate (Pi) transport in the plant. Understanding homeostatic interactions among these different nutrients is of critical importance to obtain a more complete understanding of plant nutrition in complex soil environments.

OsbHLH061 links TOPLESS/TOPLESS-RELATED repressor proteins with POSITIVE REGULATOR OF IRON HOMEOSTASIS 1 to maintain iron homeostasis in rice.

As excess iron (Fe) is toxic, uptake of this essential micronutrient must be tightly controlled. Previous studies have shown that Oryza sativa (rice) POSITIVE REGULATOR OF IRON HOMEOSTASIS1 (OsPRI1) acts upstream of the iron-related transcription factor 2 (OsIRO2) and OsIRO3 to positively regulate root-to-shoot Fe translocation. However, as expression of OsPRI1 is constitutive it is unclear how the Fe-deficiency response is turned off to prevent toxicity when Fe is sufficient. The bHLH transcription factor OsbHLH061 interacts with OsPRI1, and this study used molecular, genetics, biochemical and physiological approaches to functionally characterise OsbHLH061 and how it affects Fe homeostasis. OsbHLH061 knockout or overexpression lines increase or decrease Fe accumulation in shoots respectively. Mechanistically, OsbHLH061 expression is upregulated by high Fe, and physically interacts with OsPRI1, the OsbHLH061-OsPRI1 complex recruits TOPLESS/TOPLESS-RELATED (OsTPL/TPR) co-repressors to repress OsIRO2 and OsIRO3 expression. The OsbHLH061 ethylene-responsive element-binding factor-associated amphiphilic repression (EAR) motif is required for this transcriptional repression activity. These results define a functional OsTPL/TPR-OsbHLH061-OsPRI1-OsIRO2/3 module that negatively controls long-distance transport of Fe in plants for adaptation to changing Fe environments and maintain Fe homeostasis in rice.

OsIRO3 Plays an Essential Role in Iron Deficiency Responses and Regulates Iron Homeostasis in Rice.

Iron (Fe) homeostasis is essential for plant growth and development, and it is strictly regulated by a group of transcriptional factors. Iron-related transcription factor 3 (OsIRO3) was previously identified as a negative regulator for Fe deficiency response in rice. However, the molecular mechanisms by which OsIRO3 regulate Fe homeostasis is unclear. Here, we report that OsIRO3 is essential for responding to Fe deficiency and maintaining Fe homeostasis in rice. OsIRO3 is expressed in the roots, leaves, and base nodes, with a higher level in leaf blades at the vegetative growth stage. Knockout of OsIRO3 resulted in a hypersensitivity to Fe deficiency, with severe necrosis on young leaves and defective root development. The iro3 mutants accumulated higher levels of Fe in the shoot under Fe-deficient conditions, associated with upregulating the expression of OsNAS3, which lead to increased accumulation of nicotianamine (NA) in the roots. Further analysis indicated that OsIRO3 can directly bind to the E-box in the promoter of OsNAS3. Moreover, the expression of typical Fe-related genes was significantly up-regulated in iro3 mutants under Fe-sufficient conditions. Thus, we conclude that OsIRO3 plays a key role in responding to Fe deficiency and regulates NA levels by directly, negatively regulating the OsNAS3 expression.

Alternative Splicing Plays a Critical Role in Maintaining Mineral Nutrient Homeostasis in Rice (Oryza sativa).

Alternative splicing (AS) of pre-mRNAs promotes transcriptome and proteome diversity and plays important roles in a wide range of biological processes. However, the role of AS in maintaining mineral nutrient homeostasis in plants is largely unknown. To clarify this role, we obtained whole transcriptome RNA sequencing data from rice (Oryza sativa) roots grown in the presence or absence of several mineral nutrients (Fe, Zn, Cu, Mn, and P). Our systematic analysis revealed 13,291 alternatively spliced genes, representing ∼53.3% of the multiexon genes in the rice genome. As the overlap between differentially expressed genes and differentially alternatively spliced genes is small, a molecular understanding of the plant's response to mineral deficiency is limited by analyzing differentially expressed genes alone. We found that the targets of AS are highly nutrient-specific. To verify the role of AS in mineral nutrition, we characterized mutants in genes encoding Ser/Arg (SR) proteins that function in AS. We identified several SR proteins as critical regulators of Zn, Mn, and P nutrition and showed that three SR protein-encoding genes regulate P uptake and remobilization between leaves and shoots of rice, demonstrating that AS has a key role in regulating mineral nutrient homeostasis in rice.