Perturbation of auxin homeostasis caused by mitochondrial FtSH4 gene-mediated peroxidase accumulation regulates arabidopsis architecture

RNA editing factor SlORRM2 regulates the formation of fruit pointed tips in tomato

Domestication of tomato (Solanum lycopersicum) has led to large variation in fruit size and morphology. The development of the distal end of the fruit is a critical factor in determining its overall shape. However, the intricate mechanisms underlying distal fruit development require further exploration. This study aimed to investigate the regulatory role of an organelle RNA recognition motif (RRM)-containing protein SlORRM2 in tomato fruit morphology development. Mutant plants lacking SlORRM2 exhibited fruits with pointed tips at the distal end. However, this phenotype could be successfully restored through the implementation of a "functional complementation" strategy. Our findings suggest that the formation of pointed tips in the fruits of the CR-slorrm2 mutants is linked to alterations in the development of the ovary and style. We observed a substantial decrease in the levels of indole-3-acetic acid (IAA) and altered expression of IAA-related response genes in the ovary and style tissues of CR-slorrm2. Moreover, our data demonstrated that SlORRM2 plays a role in regulating mitochondrial RNA editing sites, particularly within genes encoding various respiratory chain subunits. Additionally, the CR-slorrm2 mutants exhibited modified organellar morphology and increased levels of reactive oxygen species. These findings provide valuable insights into the mechanisms underlying the formation of fruit pointed tips in tomato and offer genetic resources for tomato breeding.

Decrypting Molecular Mechanisms Involved in Counteracting Copper and Nickel Toxicity in Jack Pine (Pinus banksiana) Based on Transcriptomic Analysis

The remediation of copper and nickel-afflicted sites is challenged by the different physiological effects imposed by each metal on a given plant system. Pinus banksiana is resilient against copper and nickel, providing an opportunity to build a valuable resource to investigate the responding gene expression toward each metal. The objectives of this study were to (1) extend the analysis of the Pinus banksiana transcriptome exposed to nickel and copper, (2) assess the differential gene expression in nickel-resistant compared to copper-resistant genotypes, and (3) identify mechanisms specific to each metal. The Illumina platform was used to sequence RNA that was extracted from seedlings treated with each of the metals. There were 449 differentially expressed genes (DEGs) between copper-resistant genotypes (RGs) and nickel-resistant genotypes (RGs) at a high stringency cut-off, indicating a distinct pattern of gene expression toward each metal. For biological processes, 19.8% of DEGs were associated with the DNA metabolic process, followed by the response to stress (13.15%) and the response to chemicals (8.59%). For metabolic function, 27.9% of DEGs were associated with nuclease activity, followed by nucleotide binding (27.64%) and kinase activity (10.16%). Overall, 21.49% of DEGs were localized to the plasma membrane, followed by the cytosol (16.26%) and chloroplast (12.43%). Annotation of the top upregulated genes in copper RG compared to nickel RG identified genes and mechanisms that were specific to copper and not to nickel. NtPDR, AtHIPP10, and YSL1 were identified as genes associated with copper resistance. Various genes related to cell wall metabolism were identified, and they included genes encoding for HCT, CslE6, MPG, and polygalacturonase. Annotation of the top downregulated genes in copper RG compared to nickel RG revealed genes and mechanisms that were specific to nickel and not copper. Various regulatory and signaling-related genes associated with the stress response were identified. They included UGT, TIFY, ACC, dirigent protein, peroxidase, and glyoxyalase I. Additional research is needed to determine the specific functions of signaling and stress response mechanisms in nickel-resistant plants.

Cloning and functional analysis prohibitins protein-coding gene EuPHB1 in Eucommia ulmoides Oliver

As multifunctional proteins, prohibitins(PHBs) participate in many cellular processes and play essential roles in organisms. In this study, using rapid amplification of cDNA end (RACE) technology, EuPHB1 was cloned from Eucommia ulmoides Oliver (E. ulmoides). A subcellular localization assay preliminarily located EuPHB1 in mitochondria. Then EuPHB1 was transformed into tobacco, and phenotype analyses showed that overexpression of EuPHB1 caused leaves to become chlorotic and shrivel. Furthermore, genes related to hormone and auxin signal transduction, auxin binding, and transport, such as ethylene-responsive transcription factor CRF4-like and ABC transporter B family member 11-like, were significantly inhibited in response to EuPHB1 overexpression. Its overexpression disturbs the original signal transduction pathway, thus causing the corresponding phenotypic changes in transgenic tobacco. Indeed, such overexpression caused fading of palisade tissue and an increase in the number of certain mesophyll cells. It also increased adenosine triphosphate (ATP) synthase activity, mitochondrial membrane potential, ATP content, and reactive oxygen species (ROS) levels in cells. Our results suggest that EuPHB1 expression promotes cellular energy metabolism by accelerating the oxidative phosphorylation of the mitochondrial respiratory chain. Elevated levels of EuPHB1 in the mitochondria, which helps supply the extra energy required to support rapid rates of cell division.

Robust organ size in Arabidopsis is primarily governed by cell growth rather than cell division patterns

Organ sizes and shapes are highly reproducible, or robust, within a species and individuals. Arabidopsis thaliana sepals, which are the leaf-like organs that enclose flower buds, have consistent size and shape, which indicates robust development. Counterintuitively, variability in cell growth rate over time and between cells facilitates robust development because cumulative cell growth averages to a uniform rate. Here we investigate how sepal morphogenesis is robust to changes in cell division but not robust to changes in cell growth variability. We live image and quantitatively compare the development of sepals with increased or decreased cell division rate ( lgo mutant and LGO overexpression, respectively), a mutant with altered cell growth variability ( ftsh4 ), and double mutants combining these. We find that robustness is preserved when cell division rate changes because there is no change in the spatial pattern of growth. Meanwhile when robustness is lost in ftsh4 mutants, cell growth accumulates unevenly, and cells have disorganized growth directions. Thus, we demonstrate in vivo that both cell growth rate and direction average in robust development, preserving robustness despite changes in cell division.

Summary statement

Robust sepal development is preserved despite changes in cell division rate and is characterized by spatiotemporal averaging of heterogeneity in cell growth rate and direction.

Characterization of field pea (Pisum sativum) resistance against Peyronellaea pinodes and Didymella pinodella that cause ascochyta blight

Ascochyta blight is one of the most destructive diseases in field pea and is caused by either individual or combined infections by the necrotrophic pathogens Peyronellaea pinodes, Didymella pinodella, Ascochyta pisi and Ascochyta koolunga. Knowledge of disease epidemiology will help in understanding the resistance mechanisms, which, in turn, is beneficial in breeding for disease resistance. A pool of breeding lines and cultivars were inoculated with P. pinodes and D. pinodella to study the resistance responses and to characterize the underlying resistance reactions. In general, phenotypic analysis of controlled environment disease assays showed clear differential responses among genotypes against the two pathogens. The released variety PBA Wharton and the breeding line 11HP302-12HO-1 showed high levels of resistance against both pathogens whereas PBA Twilight and 10HP249-11HO-7 showed differential responses between the two pathogens, showing higher resistance against D. pinodella as compared to P. pinodes. OZP1604 had high infection levels against both pathogens. Histochemical analysis of leaves using diamino benzidine (DAB) showed the more resistant genotypes had lower accumulation of hydrogen peroxide compared to susceptible genotypes. The digital images of DAB staining were analyzed using ImageJ, an image analysis software. The image analysis results showed that quantification of leaf disease infection through image analysis is a useful tool in estimating the level of cell death in biotic stress studies. The qRT-PCR analysis of defense related genes showed that partially resistant genotypes had significantly higher expression of PsOXII and Pshmm6 in the P. pinodes treated plants, whereas expression of PsOXII, PsAPX1, PsCHS3 and PsOPR1 increased in partially resistant plants inoculated with D. pinodella. The differential timing and intensity of expression of a range of genes between resistant lines challenged with the same pathogen, or challenged with different pathogens, suggests that there are multiple pathways that restrict infection in this complex pathogen-host interaction. The combination of phenotypic, histochemical and molecular approaches provide a comprehensive picture of the infection process and resistance mechanism of pea plants against these pathogens.

Mitochondrial HSC70-1 Regulates Polar Auxin Transport through ROS Homeostasis in Arabidopsis Roots

Arabidopsis mitochondrial-localized heat shock protein 70-1 (mtHSC70-1) modulates vegetative growth by assisting mitochondrial complex IV assembly and maintaining reactive oxygen species (ROS) homeostasis. In addition, mtHSC70-1 affects embryo development, and this effect is mediated by auxin. However, whether mtHSC70-1 regulates vegetative growth through auxin and knowledge of the link between ROS homeostasis and auxin distribution remain unclear. Here, we found that mtHSC70-1 knockout seedlings (mthsc70-1a) displayed shortened roots, decreased fresh root weight and lateral root number, increased root width and abnormal root morphology. The introduction of the mtHSC70-1 gene into mthsc70-1a restored the growth and development of roots to the level of the wild type. However, sugar and auxin supplementation could not help the mutant roots restore to normal. Moreover, mthsc70-1a seedlings showed a decrease in meristem length and activity, auxin transport carrier (PINs and AUX1) and auxin abundances in root tips. The application of exogenous reducing agents upregulated the levels of PINs in the mutant roots. The introduction of antioxidant enzyme genes (MSD1 or CAT1) into the mthsc70-1a mutant rescued the PIN and local auxin abundances and root growth and development. Taken together, our data suggest that mtHSC70-1 regulates polar auxin transport through ROS homeostasis in Arabidopsis roots.

How is auxin linked with cellular energy pathways to promote growth?

Auxin is the 'growth hormone' and modulation of its concentration correlates with changes in photosynthesis and respiration, influencing the cellular energy budget for biosynthesis and proliferation. However, the relative importance of mechanisms by which auxin directly influences photosynthesis and respiration, or vice versa, are unclear. Here we bring together recent evidence linking auxin with photosynthesis, plastid biogenesis, mitochondrial metabolism and retrograde signalling and through it we propose three hypotheses to test to unify current findings. These require delving into the control of auxin conjugation to primary metabolic intermediates, translational control under auxin regulation and post-translational influences of auxin on primary metabolic processes.

RNA N6-methyladenosine modification promotes auxin biosynthesis required for male meiosis in rice

N⁶-methyladenosine (m⁶A) RNA modification confers an essential layer of gene regulation in living organisms, including plants; yet, the underlying mechanisms of its deposition on specific target mRNAs involved in key plant developmental processes are so far unknown. Here, we show that a core component of the rice m⁶A methyltransferase complex, OsFIP37, is recruited by an RNA-binding protein, OsFIP37-associated protein 1 (OsFAP1), to mediate m⁶A RNA modification on an auxin biosynthesis gene, OsYUCCA3, during microsporogenesis. This stabilizes OsYUCCA3 mRNA and promotes local auxin biosynthesis in anthers during male meiosis, which is essential for meiotic division and subsequent pollen development in rice. Loss of function of OsFAP1 causes dissociation of OsFIP37 with OsYUCCA3 and the resulting abolished m⁶A deposition on OsYUCCA3. Our findings reveal that OsFAP1-dependent m⁶A deposition on OsYUCCA3 by OsFIP37 constitutes a hitherto unknown link between RNA modification and hormonal control of male meiosis in plant reproductive development.

Plant protease as regulator and signaling molecule for enhancing environmental stress-tolerance

Proteases are ubiquitous in prokaryotes and eukaryotes. Plant proteases are key regulators of various physiological processes, including protein homeostasis, organelle development, senescence, seed germination, protein processing, environmental stress response, and programmed cell death. Proteases are involved in the breakdown of peptide bonds resulting in irreversible posttranslational modification of the protein. Proteases act as signaling molecules that specifically regulate cellular function by cleaving and triggering receptor molecules. Peptides derived from proteolysis regulate ROS signaling under oxidative stress in the plant. It degrades misfolded and abnormal proteins into amino acids to repair the cell damage and regulates the biological process in response to environmental stress. Proteases modulate the biogenesis of phytohormones which control plant growth, development, and environmental stresses. Protein homeostasis, the overall balance between protein synthesis and proteolysis, is required for plant growth and development. Abiotic and biotic stresses are major factors that negatively impact cellular survivability, biomass production, and reduced crop yield potentials. Therefore, the identification of various stress-responsive proteases and their molecular functions may elucidate valuable information for the development of stress-resilient crops with higher yield potentials. However, the understanding of molecular mechanisms of plant protease remains unexplored. This review provides an overview of proteases related to development, signaling, and growth regulation to acclimatize environmental stress in plants.

Knockdown of Succinate Dehydrogenase Assembly Factor 2 Induces Reactive Oxygen Species-Mediated Auxin Hypersensitivity Causing pH-Dependent Root Elongation

Metabolism, auxin signaling and reactive oxygen species (ROS) all contribute to plant growth, and each is linked to plant mitochondria and the process of respiration. Knockdown of mitochondrial succinate dehydrogenase assembly factor 2 (SDHAF2) in Arabidopsis thaliana lowered succinate dehydrogenase activity and led to pH-inducible root inhibition when the growth medium pH was poised at different points between 7.0 and 5.0, but this phenomenon was not observed in wildtype (WT). Roots of sdhaf2 mutants showed high accumulation of succinate, depletion of citrate and malate and up-regulation of ROS-related and stress-inducible genes at pH 5.5. A change of oxidative status in sdhaf2 roots at low pH was also evidenced by low ROS staining in root tips and altered root sensitivity to H2O2. sdhaf2 had low auxin activity in root tips via DR5-GUS staining but displayed increased indole-3-acetic acid (IAA, auxin) abundance and IAA hypersensitivity, which is most likely caused by the change in ROS levels. On this basis, we conclude that knockdown of SDHAF2 induces pH-related root elongation and auxin hyperaccumulation and hypersensitivity, mediated by altered ROS homeostasis. This observation extends the existing evidence of associations between mitochondrial function and auxin by establishing a cascade of cellular events that link them through ROS formation, metabolism and root growth at different pH values.

FTSH4 and OMA1 mitochondrial proteases reduce moderate heat stress-induced protein aggregation

The threat of global warming makes uncovering mechanisms of plant tolerance to long-term moderate heat stress particularly important. We previously reported that Arabidopsis (Arabidopsis thaliana) plants lacking mitochondrial proteases FTSH4 or OMA1 suffer phenotypic changes under long-term stress of 30°C, while their growth at 22°C is not affected. Here we found that these morphological and developmental changes are associated with increased accumulation of insoluble mitochondrial protein aggregates that consist mainly of small heat-shock proteins (sHSPs). Greater accumulation of sHSPs in ftsh4 than oma1 corresponds with more severe phenotypic abnormalities. We showed that the proteolytic activity of FTSH4, and to a lesser extent of OMA1, as well as the chaperone function of FTSH4, is crucial for protecting mitochondrial proteins against aggregation. We demonstrated that HSP23.6 and NADH dehydrogenase subunit 9 present in aggregates are proteolytic substrates of FTSH4, and this form of HSP23.6 is also a substrate of OMA1 protease. In addition, we found that the activity of FTSH4 plays an important role during recovery from elevated to optimal temperatures. Isobaric tags for relative and absolute quantification (iTRAQ)-based proteomic analyses, along with identification of aggregation-prone proteins, implicated mitochondrial pathways affected by protein aggregation (e.g. assembly of complex I) and revealed that the mitochondrial proteomes of ftsh4 and oma1 plants are similarly adapted to long-term moderate heat stress. Overall, our data indicate that both FTSH4 and OMA1 increase the tolerance of plants to long-term moderate heat stress by reducing detergent-tolerant mitochondrial protein aggregation.

The microRNA476a-RFL module regulates adventitious root formation through a mitochondria-dependent pathway in Populus

For woody plants, clonal propagation efficiency is largely determined by adventitious root (AR) formation at the bases of stem cuttings. However, our understanding of the molecular mechanisms contributing to AR morphogenesis in trees remains limited, despite the importance of vegetative propagation, currently the most common practice for tree breeding and commercialization. Here, we identified Populus-specific miR476a as a regulator of wound-induced adventitious rooting that acts by orchestrating mitochondrial homeostasis. MiR476a exhibited inducible expression during AR formation and directly targeted several Restorer of Fertility like (RFL) genes encoding mitochondrion-localized pentatricopeptide repeat proteins. Genetic modification of miR476a-RFL expression revealed that miR476a/RFL-mediated dynamic regulation of mitochondrial homeostasis influences AR formation in poplar. Mitochondrial perturbation via exogenous application of a chemical inhibitor indicated that miR476a/RFL-directed AR formation depends on mitochondrial regulation that acts via auxin signaling. Our results thus establish a microRNA-directed mitochondrion-auxin signaling cascade required for AR development, providing insights into the role of mitochondrial regulation in the developmental plasticity of plants.

The mitochondrial AAA protease FTSH3 regulates Complex I abundance by promoting its disassembly

ATP is generated in mitochondria by oxidative phosphorylation. Complex I (NADH:ubiquinone oxidoreductase or NADH dehydrogenase) is the first multisubunit protein complex of this pathway, oxidizing NADH and transferring electrons to the ubiquinone pool. Typically, Complex I mutants display a slow growth rate compared to wild-type plants. Here, using a forward genetic screen approach for restored growth of a Complex I mutant, we have identified the mitochondrial ATP-dependent metalloprotease, Filamentous Temperature Sensitive H 3 (FTSH3), as a factor that is required for the disassembly of Complex I. An ethyl methanesulfonate-induced mutation in FTSH3, named as rmb1 (restoration of mitochondrial biogenesis 1), restored Complex I abundance and plant growth. Complementation could be achieved with FTSH3 lacking proteolytic activity, suggesting the unfoldase function of FTSH3 has a role in Complex I disassembly. The introduction of the rmb1 to an additional, independent, and extensively characterized Complex I mutant, ndufs4, resulted in similar increases to Complex I abundance and a partial restoration of growth. These results show that disassembly or degradation of Complex I plays a role in determining its steady-state abundance and thus turnover may vary under different conditions.

Cross-talk between mitochondrial function, growth, and stress signalling pathways in plants

Plant mitochondria harbour complex metabolic routes that are interconnected with those of other cell compartments, and changes in mitochondrial function remotely influence processes in different parts of the cell. This implies the existence of signals that convey information about mitochondrial function to the rest of the cell. Increasing evidence indicates that metabolic and redox signals are important for this process, but changes in ion fluxes, protein relocalization, and physical contacts with other organelles are probably also involved. Besides possible direct effects of these signalling molecules on cellular functions, changes in mitochondrial physiology also affect the activity of different signalling pathways that modulate plant growth and stress responses. As a consequence, mitochondria influence the responses to internal and external factors that modify the activity of these pathways and associated biological processes. Acting through the activity of hormonal signalling pathways, mitochondria may also exert remote control over distant organs or plant tissues. In addition, an intimate cross-talk of mitochondria with energy signalling pathways, such as those represented by TARGET OF RAPAMYCIN and SUCROSE NON-FERMENTING1-RELATED PROTEIN KINASE 1, can be envisaged. This review discusses available evidence on the role of mitochondria in shaping plant growth and stress responses through various signalling pathways.

Decreased Levels of Thioredoxin o1 Influences Stomatal Development and Aperture but Not Photosynthesis under Non-Stress and Saline Conditions

Salinity has a negative impact on plant growth, with photosynthesis being downregulated partially due to osmotic effect and enhanced cellular oxidation. Redox signaling contributes to the plant response playing thioredoxins (TRXs) a central role. In this work we explore the potential contribution of Arabidopsis TRXo1 to the photosynthetic response under salinity analyzing Arabidopsis wild-type (WT) and two Attrxo1 mutant lines in their growth under short photoperiod and higher light intensity than previous reported works. Stomatal development and apertures and the antioxidant, hormonal and metabolic acclimation are also analyzed. In control conditions mutant plants displayed less and larger developed stomata and higher pore size which could underlie their higher stomatal conductance, without being affected in other photosynthetic parameters. Under salinity, all genotypes displayed a general decrease in photosynthesis and the oxidative status in the Attrxo1 mutant lines was altered, with higher levels of H₂O₂ and NO but also higher ascorbate/glutathione (ASC/GSH) redox states than WT plants. Finally, sugar changes and increases in abscisic acid (ABA) and NO may be involved in the observed higher stomatal response of the TRXo1-altered plants. Therefore, the lack of AtTRXo1 affected stomata development and opening and the mutants modulate their antioxidant, metabolic and hormonal responses to optimize their adaptation to salinity.

Arabidopsis thaliana SURFEIT1-like genes link mitochondrial function to early plant development and hormonal growth responses

Mutations in SURFEIT1 (SURF1) genes affect cytochrome c oxidase (COX) levels in different prokaryotic and eukaryotic organisms. In this work, we report that Arabidopsis thaliana has two genes that potentially encode SURF1 proteins, as a result of a duplication that took place in Brassicaceae. Both genes encode mitochondrial proteins and mutation in AtSURF1a causes embryonic lethality. Mutation in AtSURF1b, instead, causes defects in hypocotyl elongation under growth-stimulating conditions, such as low light intensity, increased ambient temperature and incubation with glucose. Mutants in AtSURF1b show reduced expression of the auxin reporter DR5:GUS and increased levels of the gibberellin reporter GFP-RGA, suggesting that auxin and gibberellin homeostasis are affected. In agreement, growth defects caused by AtSURF1b mutation can be overcome by treatment with indole-3-acetic acid and gibberellin A₃ , and also by increasing expression of the auxin biosynthesis gene YUC8 or the transcription factor PIF4, which shows lower abundance in AtSURF1b-deficient plants. Mutants in AtSURF1b display lower COX levels, higher alternative oxidase and superoxide levels, and increased expression of genes that respond to mitochondrial dysfunction. Decreased hypocotyl growth and DR5:GUS expression can be reversed by treatment with reduced glutathione, suggesting that redox changes, probably related to mitochondrial dysfunction, are responsible for the effect of AtSURF1b deficiency on hormone responses. The results indicate that changes in AtSURF1b affect mitochondrial function and the production of reactive oxygen species, which, in turn, impinges on a growth regulatory circuit that involves auxin, gibberellins and the transcription factor PIF4.

Direct comparison of Arabidopsis gene expression reveals different responses to melatonin versus auxin

BACKGROUND

Melatonin (N-acetyl-5-methoxytryptamine) in plants, regulates shoot and root growth and alleviates environmental stresses. Melatonin and the phyto-hormone auxin are tryptophan-derived compounds. However, it largely remains controversial as to whether melatonin and auxin act through similar or overlapping signalling and regulatory pathways.

RESULTS

Here, we have used a promoter-activation study to demonstrate that, unlike auxin (1-naphthalene acetic acid, NAA), melatonin neither induces Direct repeat 5 DR5 expression in Arabidopsis thaliana roots under normal growth conditions nor suppresses the induction of Alternative oxidase 1a AOX1a in leaves upon Antimycin A treatment, both of which are the hallmarks of auxin action. Additionally, comparative global transcriptome analysis conducted on Arabidopsis treated with melatonin or NAA revealed differences in the number and types of differentially expressed genes. Auxin (4.5 μM) altered the expression of a diverse and large number of genes whereas melatonin at 5 μM had no significant effect but melatonin at 100 μM had a modest effect on transcriptome compared to solvent-treated control. Interestingly, the prominent category of genes differentially expressed upon exposure to melatonin trended towards biotic stress defence pathways while downregulation of key genes related to photosynthesis was observed.

CONCLUSION

Together these findings indicate that though they are both indolic compounds, melatonin and auxin act through different pathways to alter gene expression in Arabidopsis thaliana. Furthermore, it appears that effects of melatonin enable Arabidopsis thaliana to prioritize biotic stress defence signalling rather than growth. These findings clear the current confusion in the literature regarding the relationship of melatonin and auxin and also have greater implications of utilizing melatonin for improved plant protection.

Dissection of Phenotypic and Genetic Variation of Drought-Related Traits in Diverse Chinese Wheat Landraces

Variations in 16 seedling traits under normal and drought conditions were investigated. Extremely resistant and sensitive accessions were identified for future analyses. Under normal and drought conditions, 57 and 29 QTL were identified, respectively. A total of 77 candidate genes were identified, and four were validated by qRT-PCR. Drought is one of the most important abiotic stressors affecting wheat (Triticum aestivum L.) production. To improve wheat yield, a better understanding of the genetic control of traits governing drought resistance is paramount. Here, using 645 wheat landraces, we evaluated 16 seedling traits related to root and shoot growth and water content under normal and drought (induced by polyethylene glycol) conditions. Extremely resistant and sensitive accessions were identified for future drought-resistance breeding and further genetic analyses. A genome-wide association study was performed for the 16 traits using 52,118 diversity arrays technology sequencing (DArT-seq) markers. A total of 57 quantitative trait loci (QTL) were detected for seven traits under normal conditions, whereas 29 QTL were detected for eight traits under drought conditions. On the basis of these markers, we identified 56 candidate genes for six seedling traits under normal conditions, and 21 candidate genes for seven seedling traits under drought conditions. Four candidate genes were validated under normal and drought conditions using quantitative reverse transcription polymerase chain reaction (qRT-PCR) data. The co-localization of the flowering date and drought-related traits indicates that the regulatory networks of flowering may also respond to drought stress or are associated with the correlated responses of these traits. The phenotypic and genetic elucidation of drought-related traits will assist future gene discovery efforts and provide a basis for breeding drought-resistant wheat cultivars.

Arabidopsis mtHSC70-1 plays important roles in the establishment of COX-dependent respiration and redox homeostasis

The 70 kDa heat shock proteins function as molecular chaperones and are involved in diverse cellular processes. However, the functions of the plant mitochondrial HSP70s (mtHSC70s) remain unclear. Severe growth defects were observed in the Arabidopsis thaliana mtHSC70-1 knockout lines, mthsc70-1a and mthsc70-1b. Conversely, the introduction of the mtHSC70-1 gene into the mthsc70-1a background fully reversed the phenotypes, indicating that mtHSC70-1 is essential for plant growth. The loss of mtHSC70-1 functions resulted in abnormal mitochondria and alterations to respiration because of an inhibition of the cytochrome c oxidase (COX) pathway and the activation of the alternative respiratory pathway. Defects in COX assembly were observed in the mtHSC70-1 knockout lines, leading to decreased COX activity. The mtHSC70-1 knockout plants have increased levels of reactive oxygen species (ROS). The introduction of the Mn-superoxide dismutase 1 (MSD1) or the catalase 1 (CAT1) gene into the mthsc70-1a plants decreased ROS levels, reduced the expression of alternative oxidase, and partially rescued growth. Taken together, our data suggest that mtHSC70-1 plays important roles in the establishment of COX-dependent respiration.

DEG10 contributes to mitochondrial proteostasis, root growth, and seed yield in Arabidopsis

Maintaining mitochondrial proteome integrity is especially important under stress conditions to ensure a continued ATP supply for protection and adaptation responses in plants. Deg/HtrA proteases are important factors in the cellular protein quality control system, but little is known about their function in mitochondria. Here we analyzed the expression pattern and physiological function of Arabidopsis thaliana DEG10, which has homologs in all photosynthetic eukaryotes. Both expression of DEG10:GFP fusion proteins and immunoblotting after cell fractionation showed an unambiguous subcellular localization exclusively in mitochondria. DEG10 promoter:GUS fusion constructs showed that DEG10 is expressed in trichomes but also in the vascular tissue of roots and aboveground organs. DEG10 loss-of-function mutants were impaired in root elongation, especially at elevated temperature. Quantitative proteome analysis revealed concomitant changes in the abundance of mitochondrial respiratory chain components and assembly factors, which partially appeared to depend on altered mitochondrial retrograde signaling. Under field conditions, lack of DEG10 caused a decrease in seed production. Taken together, our findings demonstrate that DEG10 affects mitochondrial proteostasis, is required for optimal root development and seed set under challenging environmental conditions, and thus contributes to stress tolerance of plants.

NEEDLE1 encodes a mitochondria localized ATP-dependent metalloprotease required for thermotolerant maize growth

Meristems are highly regulated structures ultimately responsible for the formation of branches, lateral organs, and stems, and thus directly affect plant architecture and crop yield. In meristems, genetic networks, hormones, and signaling molecules are tightly integrated to establish robust systems that can adapt growth to continuous inputs from the environment. Here we characterized needle1 (ndl1), a temperature-sensitive maize mutant that displays severe reproductive defects and strong genetic interactions with known mutants affected in the regulation of the plant hormone auxin. NDL1 encodes a mitochondria-localized ATP-dependent metalloprotease belonging to the FILAMENTATION TEMPERATURE-SENSITIVE H (FTSH) family. Together with the hyperaccumulation of reactive oxygen species (ROS), ndl1 inflorescences show up-regulation of a plethora of stress-response genes. We provide evidence that these conditions alter endogenous auxin levels and disrupt primordia initiation in meristems. These findings connect meristem redox status and auxin in the control of maize growth.

The Arabidopsis PHB3 is a pleiotropic regulator for plant development

Prohibitins (PHBs) are composed of an obviously conserved protein family in eukaryotic cells. Despite the extensive and in-depth research of mammalian PHB1 and PHB2, the plant prohibitins functions are not completely elucidated and little is known about their mechanism of action. This review focuses on the current knowledge about the protein subcellular localization, interaction proteins and target genes of PHB3. Furthermore, we discussed the roles of PHB3 protein in plant growth and development, plant responses to abiotic or biotic stresses and its participation in phytohormonal signaling.

Hydrogen Peroxide: Its Role in Plant Biology and Crosstalk with Signalling Networks

Hydrogen peroxide (H₂O₂) is steadily gaining more attention in the field of molecular biology research. It is a major REDOX (reduction⁻oxidation reaction) metabolite and at high concentrations induces oxidative damage to biomolecules, which can culminate in cell death. However, at concentrations in the low nanomolar range, H₂O₂ acts as a signalling molecule and in many aspects, resembles phytohormones. Though its signalling network in plants is much less well characterized than are those of its counterparts in yeast or mammals, accumulating evidence indicates that the role of H₂O₂-mediated signalling in plant cells is possibly even more indispensable. In this review, we summarize hydrogen peroxide metabolism in plants, the sources and sinks of this compound and its transport via peroxiporins. We outline H₂O₂ perception, its direct and indirect effects and known targets in the transcriptional machinery. We focus on the role of H₂O₂ in plant growth and development and discuss the crosstalk between it and phytohormones. In addition to a literature review, we performed a meta-analysis of available transcriptomics data which provided further evidence for crosstalk between H₂O₂ and light, nutrient signalling, temperature stress, drought stress and hormonal pathways.

Targeted Proteomics Approach Toward Understanding the Role of the Mitochondrial Protease FTSH4 in the Biogenesis of OXPHOS During Arabidopsis Seed Germination

Seed germination provides an excellent model to study the process of mitochondrial biogenesis. It is a complex and strictly regulated process which requires a proper biogenesis of fully active organelles from existing promitochondrial structures. We have previously reported that the lack of the inner mitochondrial membrane protease FTSH4 delayed Arabidopsis seed germination. Here, we implemented a targeted mass spectrometry-based approach, Multiple Reaction Monitoring (MRM), with stable-isotope-labeled standard peptides for increased sensitivity, to quantify mitochondrial proteins in dry and germinating wild-type and ftsh4 mutant seeds, lacking the FTSH4 protease. Using total seed protein extracts we measured the abundance of the peptide targets belonging to the OXPHOS complexes, AOX1A, transport, and inner membrane scaffold as well as mitochondrial proteins that are highly specific to dry and germinating seeds. The MRM assay showed that the abundance of these proteins in ftsh4 did not differ substantially from that observed in wild-type at the level of dry seed and after stratification, but we observed a reduction in protein abundance in most of the examined OXPHOS subunits in the later stages of germination. These changes in OXPHOS protein levels in ftsh4 mutants were accompanied by a lower cytochrome pathway activity as well as an increased AOX1A amount at the transcript and protein level and alternative pathway activity. The analyses of the steady-state transcript levels of mitochondrial and nuclear genes encoding OXPHOS subunits did not show significant difference in their amount, indicating that the observed changes in the OXPHOS occurred at the post-transcriptional level. At the time when ftsh4 seeds were fully germinated, the abundance of the OXPHOS proteins in the mutant was either slightly lowered or comparable to these amounts in wild-type seeds at the similar developmental stage. By the implementation of an integrative approach combining targeted proteomics, quantitative transcriptomics, and physiological studies we have shown that the FTSH4 protease has an important role in the biogenesis of OXPHOS and thus biogenesis of mitochondria during germination of Arabidopsis seeds.

Proteomic analysis reveals that auxin homeostasis influences the eighth internode length heterosis in maize (Zea mays)

Ear height is an important maize morphological trait that influences plant lodging resistance in the field, and is based on the number and length of internodes under the ear. To explore the effect of internodes on ear height, the internodes under the ear were analysed in four commercial hybrids (Jinsai6850, Zhengdan958, Xundan20, and Yuyu22) from different heterotic groups in China. The eighth internode, which is the third aboveground extended internode, exhibited high-parent or over high-parent heterosis and contributed considerably to ear height. Thus, the proteome of the eighth internode was examined. Sixty-six protein spots with >1.5-fold differences in accumulation (P < 0.05) among the four hybrids were identified by mass spectrometry and data analyses. Most of the differentially accumulated proteins exhibited additive accumulation patterns, but with epistatic effects on heterosis performance. Proteins involved in phenylpropanoid and benzoxazinoid metabolic pathways were observed to influence indole-3-acetic acid biosynthesis and polar auxin transport during internode development. Moreover, indole-3-acetic acid content was positively correlated with the eighth internode length, but negatively correlated with the extent of the heterosis of the eighth internode length.

Impairment of Meristem Proliferation in Plants Lacking the Mitochondrial Protease AtFTSH4

Shoot and root apical meristems (SAM and RAM, respectively) are crucial to provide cells for growth and organogenesis and therefore need to be maintained throughout the life of a plant. However, plants lacking the mitochondrial protease AtFTSH4 exhibit an intriguing phenotype of precocious cessation of growth at both the shoot and root apices when grown at elevated temperatures. This is due to the accumulation of internal oxidative stress and progressive mitochondria dysfunction. To explore the impacts of the internal oxidative stress on SAM and RAM functioning, we study the expression of selected meristem-specific (STM, CLV3, WOX5) and cell cycle-related (e.g., CYCB1, CYCD3;1) genes at the level of the promoter activity and/or transcript abundance in wild-type and loss-of-function ftsh4-1 mutant plants grown at 30 °C. In addition, we monitor cell cycle progression directly in apical meristems and analyze the responsiveness of SAM and RAM to plant hormones. We show that growth arrest in the ftsh4-1 mutant is caused by cell cycle dysregulation in addition to the loss of stem cell identity. Both the SAM and RAM gradually lose their proliferative activity, but with different timing relative to CYCB1 transcriptional activity (a marker of G2-M transition), which cannot be compensated by exogenous hormones.

The Complexity of Mitochondrial Complex IV: An Update of Cytochrome c Oxidase Biogenesis in Plants

Mitochondrial respiration is an energy producing process that involves the coordinated action of several protein complexes embedded in the inner membrane to finally produce ATP. Complex IV or Cytochrome c Oxidase (COX) is the last electron acceptor of the respiratory chain, involved in the reduction of O₂ to H₂O. COX is a multimeric complex formed by multiple structural subunits encoded in two different genomes, prosthetic groups (heme a and heme a₃), and metallic centers (Cu_A and Cu_B). Tens of accessory proteins are required for mitochondrial RNA processing, synthesis and delivery of prosthetic groups and metallic centers, and for the final assembly of subunits to build a functional complex. In this review, we perform a comparative analysis of COX composition and biogenesis factors in yeast, mammals and plants. We also describe possible external and internal factors controlling the expression of structural proteins and assembly factors at the transcriptional and post-translational levels, and the effect of deficiencies in different steps of COX biogenesis to infer the role of COX in different aspects of plant development. We conclude that COX assembly in plants has conserved and specific features, probably due to the incorporation of a different set of subunits during evolution.

Auxin homeostasis: the DAO of catabolism

Nearly all programmed and plastic plant growth responses are at least partially regulated by auxins, such as indole-3-acetic acid (IAA). Although vectorial, long distance auxin transport is essential to its regulatory function, all auxin responses are ultimately localized in individual target cells. As a consequence, cellular auxin concentrations are tightly regulated via coordinated biosynthesis, transport, conjugation, and oxidation. The primary auxin oxidative product across species is 2-oxindole-3-acetic acid (oxIAA), followed by glucose and amino acid conjugation to oxIAA. Recently, the enzymes catalyzing the oxidative reaction were characterized in Arabidopsis thaliana. DIOXYGENASE OF AUXIN OXIDATION (DAO) comprises a small subfamily of the 2-oxoglutarate and Fe(II) [2-OG Fe(II)] dependent dioxygenase superfamily. Biochemical and genetic studies have revealed critical physiological functions of DAO during plant growth and development. Thus far, DAO has been identified in three species by homology. Here, we review historical and recent studies and discuss future perspectives regarding DAO and IAA oxidation.

AtOMA1 Affects the OXPHOS System and Plant Growth in Contrast to Other Newly Identified ATP-Independent Proteases in Arabidopsis Mitochondria

Compared with yeast, our knowledge on members of the ATP-independent plant mitochondrial proteolytic machinery is rather poor. In the present study, using confocal microscopy and immunoblotting, we proved that homologs of yeast Oma1, Atp23, Imp1, Imp2, and Oct1 proteases are localized in Arabidopsis mitochondria. We characterized these components of the ATP-independent proteolytic system as well as the earlier identified protease, AtICP55, with an emphasis on their significance in plant growth and functionality in the OXPHOS system. A functional complementation assay demonstrated that out of all the analyzed proteases, only AtOMA1 and AtICP55 could substitute for a lack of their yeast counterparts. We did not observe any significant developmental or morphological changes in plants lacking the studied proteases, either under optimal growth conditions or after exposure to stress, with the only exception being retarded root growth in oma1-1, thus implying that the absence of a single mitochondrial ATP-independent protease is not critical for Arabidopsis growth and development. We did not find any evidence indicating a clear functional complementation of the missing protease by any other protease at the transcript or protein level. Studies on the impact of the analyzed proteases on mitochondrial bioenergetic function revealed that out of all the studied mutants, only oma1-1 showed differences in activities and amounts of OXPHOS proteins. Among all the OXPHOS disorders found in oma1-1, the complex V deficiency is distinctive because it is mainly associated with decreased catalytic activity and not correlated with complex abundance, which has been observed in the case of supercomplex I + III₂ and complex I deficiencies. Altogether, our study indicates that despite the presence of highly conservative homologs, the mitochondrial ATP-independent proteolytic system is not functionally conserved in plants as compared with yeast. Our findings also highlight the importance of AtOMA1 in maintenance of proper function of the OXPHOS system as well as in growth and development of Arabidopsis thaliana.

Apoplastic H2 O2 plays a critical role in axillary bud outgrowth by altering auxin and cytokinin homeostasis in tomato plants

Although phytohormones such as indole-3-acetic acid (IAA), cytokinin (CK) and strigolactone are important modulators of plant architecture, it remains unclear whether reactive oxygen species are involved in the regulation of phytohormone-dependent axillary bud outgrowth in plants. We used diverse techniques, including transcriptional suppression, HPLC-MS, biochemical methodologies and gene transcript analysis to investigate the signaling pathway for apoplastic hydrogen peroxide (H2 O2 )-induced axillary bud outgrowth. Silencing of tomato RESPIRATORY BURST OXIDASE HOMOLOG 1 (RBOH1) and WHITEFLY INDUCED 1 (WFI1), two important genes involved in H2 O2 production in the apoplast, enhanced bud outgrowth, decreased transcript of FZY - a rate-limiting gene in IAA biosynthesis and IAA accumulation in the apex - and increased the transcript of IPT2 involved in CK biosynthesis and CK accumulation in the stem node. These effects were fully abolished by the application of exogenous H2 O2 . Both decapitation and the silencing of FZY promoted bud outgrowth, and downregulated and upregulated the transcripts for IAA3 and IAA15, and IPT2, respectively. However, these effects were not blocked by treatment with exogenous H2 O2 but by napthaleneacetic acid (NAA) treatment. These results suggest that RBOHs-dependent apoplastic H2 O2 promotes IAA biosynthesis in the apex, which, in turn, inhibits CK biosynthesis and subsequent bud outgrowth in tomato plants.

YUCCA-mediated auxin biogenesis is required for cell fate transition occurring during de novo root organogenesis in Arabidopsis

Many plant organs have the ability to regenerate a new plant after detachment or wounding via de novo organogenesis. During de novo root organogenesis from Arabidopsis thaliana leaf explants, endogenic auxin is essential for the fate transition of regeneration-competent cells to become root founder cells via activation of WUSCHEL-RELATED HOMEOBOX 11 (WOX11). However, the molecular events from leaf explant detachment to auxin-mediated cell fate transition are poorly understood. In this study, we used an assay to determine the concentration of indole-3-acetic acid (IAA) to provide direct evidence that auxin is produced after leaf explant detachment, a process that involves YUCCA (YUC)-mediated auxin biogenesis. Inhibition of YUC prevents expression of WOX11 and fate transition of competent cells, resulting in the blocking of rooting. Further analysis showed that YUC1 and YUC4 act quickly (within 4 hours) in response to wounding after detachment in both light and dark conditions and promote auxin biogenesis in both mesophyll and competent cells, whereas YUC5, YUC8, and YUC9 primarily respond in dark conditions. In addition, YUC2 and YUC6 contribute to rooting by providing a basal auxin level in the leaf. Overall, our study indicates that YUC genes exhibit a division of labour during de novo root organogenesis from leaf explants in response to multiple signals.

The mitochondrial protease AtFTSH4 safeguards Arabidopsis shoot apical meristem function

The shoot apical meristem (SAM) ensures continuous plant growth and organogenesis. In LD 30 °C, plants lacking AtFTSH4, an ATP-dependent mitochondrial protease that counteracts accumulation of internal oxidative stress, exhibit a puzzling phenotype of premature SAM termination. We aimed to elucidate the underlying cellular and molecular processes that link AtFTSH4 with SAM arrest. We studied AtFTSH4 expression, internal oxidative stress accumulation, and SAM morphology. Directly in the SAM we analysed H₂O₂ accumulation, mitochondria behaviour, and identity of stem cells using WUS/CLV3 expression. AtFTSH4 was expressed in proliferating tissues, particularly during the reproductive phase. In the mutant, SAM, in which internal oxidative stress accumulates predominantly at 30 °C, lost its meristematic fate. This process was progressive and stage-specific. Premature meristem termination was associated with an expansion in SAM area, where mitochondria lost their functionality. All these effects destabilised the identity of the stem cells. SAM termination in ftsh4 mutants is caused both by internal oxidative stress accumulation with time/age and by the tissue-specific role of AtFTSH4 around the flowering transition. Maintaining mitochondria functionality within the SAM, dependent on AtFTSH4, is vital to preserving stem cell activity throughout development.

Interaction between hormonal and mitochondrial signalling during growth, development and in plant defence responses

Mitochondria play a central role in plant metabolism as they are a major source of ATP through synthesis by the oxidative phosphorylation pathway and harbour key metabolic reactions such as the TCA cycle. The energy and building blocks produced by mitochondria are essential to drive plant growth and development as well as to provide fuel for responses to abiotic and biotic stresses. The majority of mitochondrial proteins are encoded in the nuclear genome and have to be imported into the organelle. For the regulation of the corresponding genes intricate signalling pathways exist to adjust their expression. Signals directly regulate nuclear gene expression (anterograde signalling) to adjust the protein composition of the mitochondria to the needs of the cell. In parallel, mitochondria communicate back their functional status to the nucleus (retrograde signalling) to prompt transcriptional regulation of responsive genes via largely unknown signalling mechanisms. Plant hormones are the major signalling components regulating all layers of plant development and cellular functions. Increasing evidence is now becoming available that plant hormones are also part of signalling networks controlling mitochondrial function and their biogenesis. This review summarizes recent advances in understanding the interaction of mitochondrial and hormonal signalling pathways.

The J-protein AtDjB1 is required for mitochondrial complex I activity and regulates growth and development through ROS-mediated auxin signalling

AtDjB1 is a mitochondria-located J-protein in Arabidopsis thaliana It is involved in the regulation of plant growth and development; however, the exact mechanisms remain to be determined. We performed comparison analyses of phenotypes, auxin signalling, redox status, mitochondrial structure and function using wild-type plants, AtDjB1 mutants, rescued AtDjB1 mutants by AtDjB1 or YUCCA2 (an auxin synthesis gene), and AtDjB1 overexpression plants. AtDjB1 mutants (atj1-1 or atj1-4) exhibited inhibition of growth and development and reductions in the level of IAA and the expression of YUCCA genes compared to wild-type plants. The introduction of AtDjB1 or YUCCA2 into atj1-1 largely rescued phenotypic defects and the IAA level, indicating that AtDjB1 probably regulates growth and development via auxin. Furthermore, atj1-1 plants displayed a significant reduction in amount/activity of mitochondrial complex I compared to wild-type plants; this resulted in the accumulation of reactive oxygen species (ROS). Moreover, exogenous H2O2 markedly inhibited the expression of YUCCA genes in wild-type plants. In contrast, the reducing agent ascorbate increased the expression of YUCCA genes and IAA level in atj1-1 plants, indicating that the low auxin level observed in atj1-1 was probably due to the high oxidation status. Overall, the data presented here suggest that AtDjB1 is required for mitochondrial complex I activity and regulates growth and development through ROS-mediated auxin signalling in Arabidopsis.

Arabidopsis PROTEASOME REGULATOR1 is required for auxin-mediated suppression of proteasome activity and regulates auxin signalling

The plant hormone auxin is perceived by the nuclear F-box protein TIR1 receptor family and regulates gene expression through degradation of Aux/IAA transcriptional repressors. Several studies have revealed the importance of the proteasome in auxin signalling, but details on how the proteolytic machinery is regulated and how this relates to degradation of Aux/IAA proteins remains unclear. Here we show that an Arabidopsis homologue of the proteasome inhibitor PI31, which we name PROTEASOME REGULATOR1 (PTRE1), is a positive regulator of the 26S proteasome. Loss-of-function ptre1 mutants are insensitive to auxin-mediated suppression of proteasome activity, show diminished auxin-induced degradation of Aux/IAA proteins and display auxin-related phenotypes. We found that auxin alters the subcellular localization of PTRE1, suggesting this may be part of the mechanism by which it reduces proteasome activity. Based on these results, we propose that auxin regulates proteasome activity via PTRE1 to fine-tune the homoeostasis of Aux/IAA repressor proteins thus modifying auxin activity.

Plant responses to auxin require proteasome-mediated degradation of Aux/IAA transcriptional repressor proteins. Here, Yang et al. show that auxin suppresses proteasome activity in a manner dependent on the proteasome regulator PTRE1 and propose a mechanism for fine tuning Aux/IAA homoeostasis.

ROS Homeostasis Regulates Somatic Embryogenesis via the Regulation of Auxin Signaling in Cotton

Somatic embryogenesis (S.E.) is a versatile model for understanding the mechanisms of plant embryogenesis and a useful tool for plant propagation. To decipher the intricate molecular program and potentially to control the parameters affecting the frequency of S.E., a proteomics approach based on two-dimensional gel electrophoresis (2-DE) combined with MALDI-TOF/TOF was used. A total of 149 unique differentially expressed proteins (DEPs) were identified at different stages of cotton S.E. compared with the initial control (0 h explants). The expression profile and functional annotation of these DEPs revealed that S.E. activated stress-related proteins, including several reactive oxygen species (ROS)-scavenging enzymes. Proteins implicated in metabolic, developmental, and reproductive processes were also identified. Further experiments were performed to confirm the role of ROS-scavenging enzymes, suggesting the involvement of ROS homeostasis during S.E. in cotton. Suppressing the expression of specifically identified GhAPX proteins resulted in the inhibition of dedifferentiation. Accelerated redifferentiation was observed in the suppression lines of GhAPXs or GhGSTL3 in parallel with the alteration of endogenous ascorbate metabolism and accumulation of endogenous H₂O₂ content. Moreover, disrupting endogenous redox homeostasis through the application of high concentrations of DPI, H₂O₂, BSO, or GSH inhibited the dedifferentiation of cotton explants. Mild oxidation induced through BSO treatment facilitated the transition from embryogenic calluses (ECs) to somatic embryos. Meanwhile, auxin homeostasis was altered through the perturbation of ROS homeostasis by chemical treatments or suppression of ROS-scavenging proteins, along with the activating/suppressing the transcription of genes related to auxin transportation and signaling. These results show that stress responses are activated during S.E. and may regulate the ROS homeostasis by interacting with auxin signaling.

A tetratricopeptide repeat domain-containing protein SSR1 located in mitochondria is involved in root development and auxin polar transport in Arabidopsis

Auxin polar transport mediated by a group of Pin-formed (PIN) transporters plays important roles in plant root development. However, the mechanism underlying the PIN expression and targeting in response to different developmental and environmental stimuli is still not fully understood. Here, we report a previously uncharacterized gene SSR1, which encodes a mitochondrial protein with tetratricopeptide repeat (TPR) domains, and show its function in root development in Arabidopsis thaliana. In ssr1-2, a SSR1 knock-out mutant, the primary root growth was dramatically inhibited due to severely impaired cell proliferation and cell elongation. Significantly lowered level of auxin was found in ssr1-2 roots by auxin measurement and was further supported by reduced expression of DR5-driven reporter gene. As a result, the maintenance of the root stem cell niche is compromised in ssr1-2. It is further revealed that the expression level of several PIN proteins, namely, PIN1, PIN2, PIN3, PIN4 and PIN7, were markedly reduced in ssr1-2 roots. In particular, we showed that the reduced protein level of PIN2 on cell membrane in ssr1-2 is due to impaired retrograde trafficking, possibly resulting from a defect in retromer sorting system, which destines PIN2 for degradation in vacuoles. In conclusion, our results indicated that SSR1 is functioning in root development in Arabidopsis, possibly by affecting PIN protein expression and subcellular targeting.

A Functional Antagonistic Relationship between Auxin and Mitochondrial Retrograde Signaling Regulates Alternative Oxidase1a Expression in Arabidopsis

The perception and integration of stress stimuli with that of mitochondrion function are important during periods of perturbed cellular homeostasis. In a continuous effort to delineate these mitochondrial/stress-interacting networks, forward genetic screens using the mitochondrial stress response marker alternative oxidase 1a (AOX1a) provide a useful molecular tool to identify and characterize regulators of mitochondrial stress signaling (referred to as regulators of alternative oxidase 1a [RAOs] components). In this study, we reveal that mutations in genes coding for proteins associated with auxin transport and distribution resulted in a greater induction of AOX1a in terms of magnitude and longevity. Three independent mutants for polarized auxin transport, rao3/big, rao4/pin-formed1, and rao5/multidrug-resistance1/abcb19, as well as the Myb transcription factor rao6/asymmetric leaves1 (that displays altered auxin patterns) were identified and resulted in an acute sensitivity toward mitochondrial dysfunction. Induction of the AOX1a reporter system could be inhibited by the application of auxin analogs or reciprocally potentiated by blocking auxin transport. Promoter activation studies with AOX1a::GUS and DR5::GUS lines further confirmed a clear antagonistic relationship between the spatial distribution of mitochondrial stress and auxin response kinetics, respectively. Genome-wide transcriptome analyses revealed that mitochondrial stress stimuli, such as antimycin A, caused a transient suppression of auxin signaling and conversely, that auxin treatment repressed a part of the response to antimycin A treatment, including AOX1a induction. We conclude that mitochondrial stress signaling and auxin signaling are reciprocally regulated, balancing growth and stress response(s).