Pentatricopeptide repeat protein CNS1 regulates maize mitochondrial complex III assembly and seed development

The maize mTERF18 regulates transcriptional termination of the mitochondrial nad6 gene and is essential for kernel development

Mitochondria are semi-autonomous organelles present in eukaryotic cells, containing their own genome and transcriptional machinery. However, their functions are intricately linked to proteins encoded by the nuclear genome. Mitochondrial transcription termination factors (mTERFs) are nucleic acid-binding proteins involved in RNA splicing and transcription termination within plant mitochondria and chloroplasts. Despite their recognized importance, the specific roles of mTERF proteins in maize remain largely unexplored. Here, we clone and functionally characterize the maize mTERF18 gene. Our findings reveal that mTERF18 mutations lead to severely undifferentiated embryos, resulting in abortive phenotypes. Early kernel exhibits abnormal basal endosperm transfer layer and a significant reduction in both starch and protein accumulation in mterf18. We identify the mTERF18 gene through mapping-based cloning and validate this gene through allelic tests. mTERF18 is widely expressed across various maize tissues and encodes a highly conserved mitochondrial protein. Transcriptome data reveal that mTERF18 mutations disrupt transcriptional termination of the nad6 gene, leading to undetectable levels of Nad6 protein and reduced complex I assembly and activity. Furthermore, transmission electron microscopy observation of mterf18 endosperm uncover severe mitochondrial defects. Collectively, these findings highlight the critical role of mTERF18 in mitochondrial gene transcription termination and its pivotal impact on maize kernel development.

Maize unstable factor for orange1 encodes a nuclear protein that affects redox accumulation during kernel development

The basal endosperm transfer layer (BETL) of the maize (Zea mays L.) kernel is composed of transfer cells for nutrient transport to nourish the developing kernel. To understand the spatiotemporal processes required for BETL development, we characterized 2 unstable factor for orange1 (Zmufo1) mutant alleles. The BETL defects in these mutants were associated with high levels of reactive oxygen species, oxidative DNA damage, and cell death. Interestingly, antioxidant supplementation in in vitro cultured kernels alleviated the cellular defects in mutants. Transcriptome analysis of the loss-of-function Zmufo1 allele showed differential expression of tricarboxylic acid cycle, redox homeostasis, and BETL-related genes. The basal endosperms of the mutant alleles had high levels of acetyl-CoA and elevated histone acetyltransferase activity. The BETL cell nuclei showed reduced electron-dense regions, indicating sparse heterochromatin distribution in the mutants compared with wild-type. Zmufo1 overexpression further reduced histone methylation marks in the enhancer and gene body regions of the pericarp color1 (Zmp1) reporter gene. Zmufo1 encodes an intrinsically disordered nuclear protein with very low sequence similarity to known proteins. Yeast two-hybrid and luciferase complementation assays established that ZmUFO1 interacts with proteins that play a role in chromatin remodeling, nuclear transport, and transcriptional regulation. This study establishes the critical function of Zmufo1 during basal endosperm development in maize kernels.

A weak allele of AtECB2, a member of the pentatricopeptide repeat motif superfamily, causes leaf virescence in Arabidopsis

KEY MESSAGE

Through the study of a point mutation of AtECB2, it is reconfirmed that AtECB2 plays an important role in the early development of chloroplast. AtECB2(EARLY CHLOROPLAST BIOGENESIS 2, At1g15510), a member of the pentatricopeptide repeat motif proteins (PPR) superfamily, and its loss of function mutation ecb2-1causes seedling lethal, while a point mutation ecb2-2 causes delayed chloroplast development. Finding more AtECB2 weak alleles helps to understand the molecular mechanisms of AtECB2. In this study, a leaf virescence mutant was identified from ethylmethane sulfonate (EMS) treated Arabidopsis Col-0 M2 mutants library. The mutation of this mutant was first confirmed as a recessive mutation of one gene through the phenotype of F1 and its F2 phenotype segregation of this mutant crossed with Col-0. The mutation of G1931A of AtECB2 is identified as the cause of this leaf virescence phenotype sequentially through positional cloning, whole genome resequencing, Sanger sequencing and complementation. Therefore, we named this weak allele of AtECB2 as ecb2-3. The chlorophyll content and photosystem II maximum photochemical efficiency of ecb2-3 are obviously lower than that of Col-0 and its complementation lines, respectively. The chloroplast development of ecb2-3 is also inferior to that of Col-0 and its complementation line at the observed time points using the transmission electron microscope. The RNA editing efficiency of three chloroplast gene sites (accD C794 and C1568, ndhF C290) was observed much lower compared with that of Col-0 and its complementation line. In summary, AtECB2 plays an important role in early chloroplast biogenesis through related chloroplast gene editing regulation, and this weak mutant ecb2-3 may be useful material in dissecting the function of AtECB2 in the near future.

The translocase of the inner mitochondrial membrane 22-2 is required for mitochondrial membrane function during Arabidopsis seed development

The carrier translocase (also known as translocase of the inner membrane 22; TIM22 complex) is an important component of the mitochondrial protein import apparatus. However, the biological functions of AtTIM22-2 in Arabidopsis remain poorly defined. Here, we report studies on two tim22-2 mutants that exhibit defects in embryo and endosperm development, leading to seed abortion. AtTIM22-2, which was localized in mitochondria, was widely expressed in embryos and in various seedling organs. Loss of AtTIM22-2 function resulted in irregular mitochondrial cristae, decreased respiratory activity, and a lower membrane potential, together with changes in gene expression and enzyme activity related to reactive oxygen species (ROS) metabolism, leading to increased accumulation of ROS in the embryo. The levels of transcripts encoding mitochondrial protein import components were also altered in the tim22-2 mutants. Furthermore, mass spectrometry, bimolecular fluorescence complementation and co-immunoprecipitation assays revealed that AtTIM22-2 interacted with AtTIM23-2, AtB14.7 (a member of Arabidopsis OEP16 family encoded by At2G42210), and AT5G27395 (mitochondrial inner membrane translocase complex, subunit TIM44-related protein). Taken together, these results demonstrate that AtTIM22-2 is essential for maintaining mitochondrial membrane functions during seed development. These findings lay the foundations for a new model of the composition and functions of the TIM22 complex in higher plants.

The biogenesis and regulation of the plant oxidative phosphorylation system

Mitochondria are central organelles for respiration in plants. At the heart of this process is oxidative phosphorylation (OXPHOS) system, which generates ATP required for cellular energetic needs. OXPHOS complexes comprise of multiple subunits that originated from both mitochondrial and nuclear genome, which requires careful orchestration of expression, translation, import, and assembly. Constant exposure to reactive oxygen species due to redox activity also renders OXPHOS subunits to be more prone to oxidative damage, which requires coordination of disassembly and degradation. In this review, we highlight the composition, assembly, and activity of OXPHOS complexes in plants based on recent biochemical and structural studies. We also discuss how plants regulate the biogenesis and turnover of OXPHOS subunits and the importance of OXPHOS in overall plant respiration. Further studies in determining the regulation of biogenesis and activity of OXPHOS will advances the field, especially in understanding plant respiration and its role to plant growth and development.