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SUMMARY: The plant serine/arginine‐rich (SR) splicing factor SR45 plays important roles in several biological processes, such as splicing, DNA methylation, innate immunity, glucose regulation, and abscisic acid signaling. A homozygous Arabidopsis sr45‐1 null mutant is viable, but exhibits diverse phenotypic alterations, including delayed root development, late flowering, shorter siliques with fewer seeds, narrower leaves and petals, and unusual numbers of floral organs. Here, we report that the sr45‐1 mutant presents an unexpected constitutive iron deficiency phenotype characterized by altered metal distribution in the plant. RNA‐Sequencing highlighted severe perturbations in metal homeostasis, the phenylpropanoid pathway, oxidative stress responses, and reproductive development. Ionomic quantification and histochemical staining revealed strong iron accumulation in the sr45‐1 root tissues accompanied by iron starvation in aerial parts. Mis‐splicing of several key iron homeostasis genes, including BTS, bHLH104, PYE, FRD3, and ZIF1, was observed in sr45‐1 roots. We showed that some sr45‐1 developmental abnormalities can be complemented by exogenous iron supply. Our findings provide new insight into the molecular mechanisms governing the phenotypes of the sr45‐1 mutant. Significance Statement: This study shows that some of the sr45‐1 mutant phenotypes result from a severe alteration in metal mobilization, localization and transport and that the exogenous application of iron can rescue the reproductive defects of the mutant. Our data fine‐tune our understanding of the physiological responses impaired in the mutant, such as the oxidative stress response, metal homeostasis, and the phenylpropanoid pathway. [ABSTRACT FROM AUTHOR]

Summary: FRD3 (FERRIC REDUCTASE DEFECTIVE 3) plays a major role in iron (Fe) and zinc (Zn) homeostasis in Arabidopsis. It transports citrate, which enables metal distribution in the plant. An frd3 mutant is dwarf and chlorotic and displays a constitutive Fe‐deficiency response and strongly altered metal distribution in tissues. Here, we have examined the interaction between Fe and Zn homeostasis in an frd3 mutant exposed to varying Zn supply. Detailed phenotyping using transcriptomic, ionomic, histochemical and spectroscopic approaches revealed the full complexity of the frd3 mutant phenotype, which resulted from altered transition metal homeostasis, manganese toxicity, and oxidative and biotic stress responses. The cell wall played a key role in these processes, as a site for Fe and hydrogen peroxide accumulation, and displayed modified structure in the mutant. Finally, we showed that Zn excess interfered with these mechanisms and partially restored root growth of the mutant, without reverting the Fe‐deficiency response. In conclusion, the frd3 mutant molecular phenotype is more complex than previously described and illustrates how the response to metal imbalance depends on multiple signaling pathways. Significance Statement: This study shows that the phenotype of an frd3 (ferric reductase defective 3) mutant not only results from altered iron, zinc and manganese homeostasis networks, but also from their interconnection with oxidative and biotic stress responses. These observations highlight that the response to transition metal homeostasis imbalance in Arabidopsis is intertwined with multiple signaling pathways. [ABSTRACT FROM AUTHOR]

SUMMARY The qualitative screening method used to select complex I mutants in the microalga Chlamydomonas, based on reduced growth under heterotrophic conditions, is not suitable for high-throughput screening. In order to develop a fast screening method based on measurements of chlorophyll fluorescence, we first demonstrated that complex I mutants displayed decreased photosystem II efficiency in the genetic background of a photosynthetic mutation leading to reduced formation of the electrochemical proton gradient in the chloroplast ( pgrl1 mutation). In contrast, single mutants ( complex I and pgrl1 mutants) could not be distinguished from the wild type by their photosystem II efficiency under the conditions tested. We next performed insertional mutagenesis on the pgrl1 mutant. Out of about 3000 hygromycin-resistant insertional transformants, 46 had decreased photosystem II efficiency and three were complex I mutants. One of the mutants was tagged and whole genome sequencing identified the resistance cassette in NDUFAF3, a homolog of the human NDUFAF3 gene, encoding for an assembly factor involved in complex I assembly. Complemented strains showed restored complex I activity and assembly. Overall, we describe here a screening method which is fast and particularly suited for the identification of Chlamydomonas complex I mutants. [ABSTRACT FROM AUTHOR]