Journal of Integrative Plant Biology

flg22 is a 22-amino peptide released from bacterial flagellin, a Microbe-Associated Molecular Pattern (MAMP) that is recognized by the plant cell as a signal indicating that bacteria are present. On its own, flg22 initiates a rapid increase in cytoplasmic calcium, extracellular reactive oxygen species, and activation of a Mitogen Activated Protein Kinase (MAPK) cascade, all of which are activated within 15 minutes after the cell perceives flg22. Here we show a massive change in protein abundance and phosphorylation state of the Arabidopsis root cell proteome within this 15-minute duration in wildtype and a mutant deficient in G-protein coupled signaling. Integration of phosphoproteome with protein-protein interactome data followed by network topology analyses discovered that many of the flg22-induced phosphoproteome changes fall on proteins that comprise the G protein interactome and on the most highly populated hubs of the immunity network. Approximately 95% of the phosphorylation changes in the G-protein interactome depend on a functional heterotrimeric G protein complex, some occur on proteins that interact directly with components of G-coupled signal transduction. One of these is ATB, a substrate-recognition sub-unit of the PP2A Ser/Thr phosphatase and an interactor to Arabidopsis thaliana REGULATOR OF G SIGNALING 1 protein (AtRGS1), a 7-transmembrane spanning modulator of the nucleotide-binding state of the core G protein complex. AtRGS1 is phosphorylated by BAK1, a component of the flg22 receptor, to initiate AtRGS1 endocytosis. A null mutation of ATB confers high basal endocytosis of AtRGS1, suggesting sustained phosphorylated status. Loss of ATB confers traits associated with loss of AtRGS1. Because the basal level of AtRGS1 is lower in the atb null mutant in a proteasome-dependent manner, we propose that phosphorylation-dependent endocytosis of AtRGS1 is part of a mechanism to degrade AtRGS1 which then sustains activation of the G protein complex. Thus, the role of ATB is now established as a central component of phosphorylation-dependent regulation of system dynamics in innate immunity.

O-glycosylation of nucleocytosolic proteins by the Arabidopsis enzymes SPINDLY (SPY; O-fucosyltransferase) and SECRET AGENT (SEC; O-GlcNAc transferase) is essential for plant growth and development, yet the scope of their substrates and regulatory impact remains poorly defined. Here, we combined TurboID-based proximity labeling with quantitative proteomics to systematically map the SPY interactome and determine how SPY- and SEC-dependent modifications influence protein abundance. A functional SPY-TD enriched 221 proxiome proteins, including 80 known O-fucosylated substrates and 141 new interactors. The SPY-TD proxiome is enriched in nuclear pore components, chromatin regulators, transcription factors, and RNA-processing proteins. Integration with O-fucose and O-GlcNAc datasets yielded a comprehensive Arabidopsis SPY/SEC (At-S/S) protein list of 886 candidates. We quantified proteome-wide changes in spy single mutants and inducible spy sec double mutants. Loss of SPY alone caused selective stabilization or destabilization of targets, whereas combined SPY/SEC depletion triggered widespread, synergistic protein abundance changes, particularly affecting nucleoporins, transcriptional regulators, and RNA-binding proteins. Integration with ubiquitination datasets revealed extensive overlap, supporting potential crosstalk between O-fucosylation, O-GlcNAcylation, and ubiquitin-mediated protein turnover. Together, our study establishes proximity labeling as a powerful strategy to define plant O-glycosylation networks and reveals dual, context-dependent roles of SPY and SEC in controlling protein homeostasis and stress-responsive pathways.

Ubiquitin proteases play a crucial role in protein degradation and turnover by regulating the cleavage of polyubiquitin chains. TARANI/UBIQUITIN SPECIFIC PROTEASE-14 (TNI/UBP14) specifically cleaves Lys-48-linked and linear polyubiquitin chains into mono-ubiquitins. The tni mutant exhibits pleiotropic phenotypes, including cup-shaped leaves, tri-cotyledons, reduced lateral roots, and increased petal number, though the underlying mechanisms driving these phenotypes remain unclear. In this study, we generated TNI transgenic lines and employed immunoprecipitation mass spectrometry, proximity labelling, and yeast two-hybrid screening to identify TNIs interacting proteins. These analyses revealed 92 interactors involved in diverse biological processes, including protein and carbohydrate metabolism, light signalling, and intracellular transport. Subcellular localization analysis showed that many of the interacting proteins are located in the nucleus and cytoplasm, suggesting that TNIs nuclear localization may regulate gene function. We further validated the in planta biological significance of ULTRAPETALA 2 and HASPIN KINASE as key interacting partners of TNI. These findings uncover previously uncharacterized functions of TNI/UBP14, shedding light on its central role in cellular processes and providing insights into its regulatory mechanisms--an area that has remained largely unexplored until now. Summary statementThe proteins that interact with the TARANI/ Ubiquitin protease 14 in vivo have been identified using immunoprecipitation mass-spectrometry methods. Identification of non-overlapping targets highlight the importance of using diverse protein identification methods.

Transposon is a mobile and ubiquitous DNA that can vastly causes genomic alterations. In plants, it is well documented that transposon mobilization is strongly repressed by DNA methylation; however, the roles of RNA methylation in transposon control remain unknown. Here we suggest that transposon RNA is marked by m6A RNA methylation and is sequestered in stress granule (SG) in m6A-dependent manner. Intriguingly, a SG-localized AtALKBH9B selectively demethylates a heat-activated retroelement Onsen, and thereby releases from spatial confinement allowing for its mobilization. In addition, we show evidence that m6A RNA methylation contributes to transpositional suppression by inhibiting the virus-like particles assembly and extrachromosomal DNA production. In summary, this study unveils a hidden role for m6A in the suppression of transposon mobility and provides an insight into how transposon counteracts the hosts epitranscriptomic control by hitchhiking RNA demethylase.

SUMOylation is a highly dynamic posttranslational modification that plays a critical role in regulating plant stress responses. The global SUMOylation is quickly induced by dehydration and hyperosmotic stresses in plants, while the detailed mechanism underlying such SUMOylation dynamics is largely unknown. Here, we report that the SNF1-related protein kinase 2 (SnRK2) and SUMO E3 ligase SIZ1 module is crucial for the stress-induced increment of SUMOylation in Arabidopsis. Under osmotic stress, or application of phytohormone Abscisic Acid (ABA), the rapidly activated SnRK2s physically interact with and phosphorylate SIZ1, enhancing its stability. The Ser820 residue in C-terminal region of SIZ1 proteins is a functional SnRK2 phosphosite, whose phosphorylation is abolished in the high-order mutant of SnRK2s. The non-phosphorylatable SIZ1S820A is unstable both in vivo and in vitro. We also noticed the degradation of SIZ1 is largely darkness-dependent, interestingly, independent of COP1, a key ubiquitin E3 ligase regulating photomorphogenesis. Multiple SUMOylation, Ubiquitination, and phosphorylation sites in SIZ1 proteins, which may coordinate the dynamics of SIZ1 proteins and global SUMOylation upon environmental changes. Our findings highlight the critical role of the SnRK2-SIZ1 module in regulating SUMOylation dynamics during plant stress responses and provide new insights into the regulatory mechanisms underlying this essential posttranslational modification.

Switch defective/sucrose non-fermentable (SWI/SNF) complexes are evolutionarily conserved multi-subunit machines that play vital roles in chromatin architecture regulation for modulating gene expression via sliding or ejection of nucleosomes in eukaryotes. In plants, perturbations of SWI/SNF subunits often result in severe developmental disorders. However, the subunit composition, pathways of assembly, and genomic targeting of the plant SWI/SNF complexes remain undefined. Here, we reveal the organization, genomic targeting and assembly of three distinct Arabidopsis SWI/SNF complexes: BRAHMA-Associated SWI/SNF complexes (BAS), SPLAYED-Associated SWI/SNF complexes (SAS) and MINUSCULE-Associated SWI/SNF complexes (MAS). We show that BAS complexes are equivalent to human ncBAF, whereas SAS and MAS complexes evolve in multiple subunits unique to plants, suggesting a plant-specific functional evolution of SWI/SNF complexes. We further demonstrate overlapping and specific genomic targeting of the three plant SWI/SNF complexes on chromatin and reveal that SAS complexes are necessary for the correct genomic localization of the BAS complexes. Finally, we define the role of core module subunit in the assembly of the plant SWI/SNF complexes and highlight that ATPase module subunit is required for global complex stability and the interaction of core module subunits in SAS and BAS complexes in Arabidopsis. Together, our work highlights the divergence of SWI/SNF chromatin remodelers during the eukaryote evolution and provides a comprehensive landscape for understanding the plant SWI/SNF complexes organization, assembly, genomic targeting, and function. One-sentence summaryComprehensively define the organization, genomic targeting and assembly of three distinct SWI/SNF chromatin remodeling complexes in Arabidopsis

Macroautophagy (hereafter autophagy) is essential for cells to respond to nutrient stress by delivering cytosolic contents to vacuoles for degradation via the formation of a multi-layer vesicle named autophagosome. A set of autophagy-related (ATG) regulators are recruited to the phagophore assembly site for the initiation of phagophore, as well as its expansion and closure and subsequent delivery into the vacuole. However, it remains elusive that how the phagophore assembly is regulated under different stress conditions. Here, we described an unknown Arabidopsis (Arabidopsis thaliana) cytosolic ATG8-interaction protein family (ERC1/2), that binds ATG8 and NBR1 to promote autophagy. ERC1 proteins translocate to the phagophore membrane and develop into classical ring-like autophagosomes upon autophagic induction. However, ERC1 proteins form large droplets together with ATG8e proteins when in the absence of ATG8 lipidation activity. We described the property of these structures as phase-separated membraneless condensates by solving the in vivo organization with spatial and temporal resolution. Moreover, ERC1 condensates elicits a strong recruitment of the autophagic receptor NBR1. Loss of ERC1 suppressed NBR1 turnover and attenuated plant tolerance to heat stress condition. This work provides novel insights into the mechanical principle of phagophore initiation via an unreported ERC1-mediated biomolecular condensation for heat tolerance in Arabidopsis.

DNA methylation is an important epigenetic mark that regulates the expression of genes and transposons. RNA-directed DNA methylation (RdDM) is the main molecular pathway responsible for de novo DNA methylation in plants. In Arabidopsis, however, mutations in RdDM genes cause no visible developmental defects, which raising the question of the biological significance of RdDM in plant development. Here, we isolated and cloned Five Elements Mountain 1 (FEM1), which encodes an RNA-dependent RNA polymerase. Mutation in FEM1 substantially decreased genome-wide CHH methylation levels and abolished the accumulation of 24-nt small interfering RNAs. Moreover, male and female reproductive development was disturbed, which led to the sterility of fem1 mutants. In wild-type (WT) plants but not in fem1 mutants, genome-wide CHH DNA methylation levels were greater in panicles, stamens, and pistils than in seedlings. The global increase of methylation in reproductive organs of the WT was attributed to enhancement of RdDM activity including FEM1 activity. More than half of all encoding genes in the rice genome overlapped with hypermethylated regions in the sexual organs of the WT, and many of them appear to be directly regulated by an increase in DNA methylation.Our results demonstrate that a global increase of DNA methylation through enhancement of RdDM activity in reproductive organs ensures sexual reproduction of rice.View Full Text

Plants have the ability to perceive a slight upsurge in ambient temperature and respond by undergoing morphological changes, such as elongated hypocotyls and early flowering. The dynamic functioning of PHYTOCHROME INTERACTING FACTOR4 (PIF4) in thermomorphogenesis has been well established, although the regulatory pathway involved in thermosensing is not deciphered completely. In our study, we demonstrate that an increase in temperature from 22 to 28 induces the phosphorylation of PIF4 by MITOGEN-ACTIVATED PROTEIN KINASE 4 (MPK4) which shows high expression and activation at 28. Apparently, phosphorylated PIF4 represses the expression of ACTIN-RELATED PROTEIN 6 (ARP6) that is required for mediating histone variant H2A.Z deposition at its target gene loci. We demonstrate that variation of ARP6 expression in PIF4 phosphor -null and phosphor-mimetic seedlings affects hypocotyl growth and flowering at 22 and 28. Further, we show that change in ARP6 expression affects H2A.Z deposition at the loci of genes involved in hypocotyl elongation using PIF4 phosphor -null and phosphor-mimetic seedlings. Interestingly, the expression of MPK4 is also controlled by H2A.Z deposition in temperature dependent manner. Taken together, our findings highlight the cumulative molecular interplay between MPK4, PIF4, and chromatin modification by ARP6-mediated H2A.Z deposition as a regulatory mechanism of thermosensing.

ABSTRUCTA phytohormone abscisic acid (ABA) has a major role in abiotic stress responses in plants, and subclass III SNF1-related protein kinase 2 (SnRK2) mediates ABA signaling. In this study, we identified Raf36, a group C Raf-like protein kinase in Arabidopsis, as an interacting protein with SnRK2. A series of reverse genetic and biochemical analyses revealed that Raf36 negatively regulates ABA responses and is directly phosphorylated by SnRK2s. In addition, we found that Raf22, another C-type Raf-like kinase, functions partially redundantly with Raf36 to regulate ABA responses. Comparative phosphoproteomic analysis using Arabidopsis wild-type and raf22raf36-1 plants identified proteins that are phosphorylated downstream of Raf36 and Raf22 in planta. Together, these results reveal a novel subsection of ABA-responsive phosphosignaling pathways branching from SnRK2.

Sensing of pathogen- or microbe-associated molecular patterns (PAMPs/MAMPs) by pattern recognition receptors (PRRs) at the cell surface induces the first layer of host immunity against invading microbial pathogens. The immune receptor FLS2 perceives bacterial flagellin to initiate host immune signaling upon pathogen infections. It has been known that the FLS2 abundance is crucial for plant pattern-triggered immunity. Nevertheless, the underpinning regulatory mechanisms remain largely unclear. In this study, we demonstrate that XBAT35.2 positively modulates the protein level of FLS2. In addition to the Golgi, XBAT35.2 localizes at the plasma membrane and constitutively associates with FLS2, BAK1, and BIK1. Flg22 treatment increases the association of XBAT35.2 with FLS2 and BAK1 but reduces the interaction with BIK1. XBAT35.2 ubiquitinates two key components of the ESCRT-I complex, VPS37-1 and VPS28-2 with K48 and K63-linked polyubiquitin chains respectively, leading to degradation of VPS37-1 and diminished interaction of VPS28-2 with FLS2. Additionally, VPS37-1 and VPS28-2 play redundant, negative roles in FLS2-mediated immunity by promoting vacuolar breakdown of FLS2. Thus, by intercepting the function of VPS28-2 and VPS37-1, XBAT35.2 stabilizes FLS2 for host immunity. Our findings reveal a new regulatory circuit for modulating the FLS2 abundance and deepen our understanding of controlling the homeostasis of cell surface receptors. One-sentence summaryTargeting the ESCRT-I subunits VPS28-2 and VPS37-1 with distinct ubiquitin chains by the E3 ligase XBT35.2 stabilizes FLS2 and positively modulates plant immunity.

Protein S-acylation is the addition of fatty acids to the cysteine residues in a protein, catalyzed by protein S-acyltransferases (PATs). Despite extensive research on protein S-acylation in animals, our understanding of this process in plants remains limited. In this study, we sought to characterize the S-acylproteome of membrane proteins in Arabidopsis and identify potential substrates for two important plant immunity-related PATs (PAT5 and PAT9). To achieve this, S-acylated membrane proteins were first enriched via our optimized acyl-biotinyl exchange strategies at both the protein-level and peptide-level. The enriched samples were then analyzed by label-free quantitative liquid chromatography-mass spectrometry. The results from the two enrichment methods demonstrated that they were complementary in identifying S-acylated proteins and S-acylation sites. Using these methods, over 2500 S-acylation sites in more than 2000 putative S-acylated proteins were identified. Proteins involved in vesicle trafficking, plant phosphorylation, immune responses, and signal transduction pathways were significantly enriched. Additionally, certain amino acid patterns surrounding the S-acylation sites were identified. Comparisons of the S-acylproteomes between the wild type and the PAT5 and PAT9 mutants revealed over 100 potential substrates for both S-acyltransferases. The high quality of our data was supported by the significant overlap with the previously reported data and successful experimental verification of selected candidate proteins. Overall, our study revealed a well-represented S-acylproteome for Arabidopsis (especially its membrane proteins) and identified potential substrates for PAT5 and PAT9. These findings will facilitate the functional characterization of S-acylated proteins in plants.

Long non-coding RNAs (lncRNAs), representing the non-coding RNA regions, constitute a significant portion of the genomes in complex organisms. Recent studies suggest that some lncRNAs have the capability to encode peptides. However, the presence of the lncRNA-derived sORFs-encoded polypeotides (LSEPs) in plants is not well understood. In this study, we developed a multi-omics approach that encompasses transcriptomics, translatomics (Ribo-seq), and proteomics to identify of LSEPs in rice. Among the 2764 identified lncRNAs, 42.69% were found to be bound by the ribosome, indicating a potential for encoding. Optimized small peptide extraction protocol was further developed, and the small peptides from rice leaves were extracted and subjected to LC-MS/MS analysis, leading to the identification of a total of 403 LSEPs across four constructed search databases. This work confirms the peptide-coding ability of lncRNAs in plants. Collectively, our study establishes an efficient multi-omics method for identifiying small peptides encoded by lncRNAs, which may be valuable for large-scare screening of LSEPs in plants.

Brassinosteroids (BR) and Target of Rapamycin Complex (TORC) are two major processes coordinating plant growth and stress responses. BRs function through a signaling pathway to extensively regulate gene expression and TORC is known to regulate translation and autophagy. Recent studies revealed that these two pathways crosstalk, but a system-wide view of their interplay is still missing. Thus, we performed transcriptome, proteome, and phosphoproteome profiling of Arabidopsis mutants with altered levels of either BIN2 or RAPTOR1B, two key players in BR and TORC signaling, respectively. We found that perturbation of BIN2 or RAPTOR1B levels affects a common set of gene-products involved in growth and stress responses. Additionally, we performed Multiplexed Assay for Kinase Specificity (MAKS), which provided a system-wide view of direct BIN2 substrates. Furthermore, phosphoproteomic data was used to reconstruct a kinase-signaling network and to identify novel proteins dependent on BR and/or TORC signaling pathways. Loss of function mutants of many of these proteins led to an altered BR response and/or modulated autophagy activity. Altogether, these results provide genome-wide evidence for crosstalk between BR and TORC signaling and established a kinase signaling network that defines the molecular mechanisms of BR and TORC interactions in the regulation of plant growth/stress balance.

As much as 10% of plant immune receptors from the nucleotide-binding domain leucine-rich repeat (NLR) family carry integrated domains (IDs) that can directly bind pathogen effectors. However, it remains unclear whether direct binding to effectors is a universal feature of ID-containing NLRs given that only a few NLR-IDs have been functionally characterized. Here we show that the rice (Oryza sativa) sensor NLR-ID Pii2 confers resistance to strains of the rice blast fungus Magnaporthe oryzae that carry the effector AVR-Pii without directly binding this protein. First, we show that AVR-Pii binds the exocyst subunit OsExo70F2 in rice (Oryza sativa) to dissociate preformed complexes of OsExo70F2 with host RPM1 INTERACTING PROTEIN4 (RIN4) at the conserved NOI motif, facilitating a possible virulence function. Second, we show that in its resting state, Pii2 binds OsExo70F2 and OsExo70F3, essential components of Pii-mediated resistance, through its integrated NOI domain. Remarkably, AVR-Pii binding to OsExo70F2/F3 leads to dissociation of the Pii2-OsExo70F2 and Pii2-OsExo70F3 complexes, destabilization of Pii2, and activation of immunity. These findings support a novel conceptual model in which an NLR-ID monitors alterations of tethered host proteins targeted by pathogen effectors, providing insight into pathogen recognition mechanisms. Significance statementPlant diseases diminish crop yields by over 20% each year, and deploying resistant crops is the most effective way to combat them. Nucleotide-binding domain leucine-rich repeat (NLR)-type receptors are the major player in plant resistance against pathogens, with a subset of NLRs containing unconventional domains called integrated domains (ID) derived from host proteins. Previous studies suggest that pathogen avirulence (AVR) effectors directly bind or modify NLR-IDs before they are recognized by the host. Here, we reveal that the rice NLR-ID receptor Pii2 indirectly recognizes AVR-Pii when the effector dissociates Pii2 from the host Exo70 proteins tethered to Pii2. We propose a new model of how NLRs can recognize pathogens, expanding our understanding of plant immunity.

As a versatile tool for genome engineering, CRISPR-Cas9 has been revolutionizing the field of molecular biology, biotechnology, and crop improvement. By precisely targeting pre-selected genomic sites, CRISPR-Cas9 primarily induces insertions or deletions (indels) of variable size. Despite the significant advance in the technology per se, detecting these indels is the major and difficult part of the CRISPR program in polyploid species, like wheat, with relatively low mutation rates. A plethora of methods are available for detecting mutations, but no method is perfect for all mutation types. In this case study, we demonstrated a new, protocol for capturing length polymorphism from small indels using a nested PCR approach. This new method is tractable, efficient, and cost-effective in detecting and genotyping indels >3-bp. We also discussed the major genotyping platforms used in our wheat CRISPR projects, such as mismatch cleavage assay, restriction enzyme assay, ribonucleoprotein assay, and Sanger sequencing, for their advantages and pitfalls in wheat CRISPR mutation detection.

Jasmonates (JAs) are plant hormones which regulate biosynthesis of many secondary metabolites, such as glucosinolates (GLSs), through JAs-responsive transcription factors (TFs). The JAs-responsive CYP83B1 gene, has been shown to catalyze the conversion of indole-3-acetaldoxime (IAOx) to indolic glucosinolates (IGLSs). However, little is known about the regulatory mechanism of CYP83B1 gene expression by JAs. In yeast one-hybrid screens using the CYP83B1 promoter as bait we isolated two JAs-responsive TFs ERF109 and MYB51 that are involved in JAs-regulated IGLS biosynthesis. Furthermore, using a yeast two-hybrid assay, we identified ERF109 as an interacting partner of MYB51, and Jasmonate ZIM-domain (JAZ) proteins as interactors of MYB51, and BTB/POZ-MATH (BPM) proteins as interactors of ERF109. Both JAZ and BPM proteins are necessary for the full repression of the ERF109-MYB51-MYC3 ternary complex activity on CYP83B1 gene expression and JA-regulated IGLS biosynthesis. Biochemical analysis showed that the 26S proteasome-mediated degradation of ERF109 protein is mediated by a CRL3BPM E3 ligase independently of JA signaling. Genetic and physiological evidence shows that MYB51 acts as an adaptor and activator to bridge the interaction with the co-activators MYC3 and ERF109, for synergistically activating the CYP83B1 gene expression, and all three factors are essential and exert a coordinated control in JAs-induced IGLS biosynthesis. Overall, this study provides insights into the molecular mechanisms of JAs-responsive ERF109-MYB51-MYC3 ternary complexes in controlling JAs-regulated GLSs biosynthesis, which provides a better understanding of plant secondary metabolism. One-sentence summaryThe JA-responsive ERF109-MYB51-MYC3 ternary complex controls JAs-regulated GLSs biosynthesis.

O_LIPlant nucleotide-binding leucine-rich repeat receptors (NLRs) initiate immune responses and the hypersensitive response by recognizing pathogen effectors. Xa1 encodes an NLR with an N-terminal BED domain, and recognizes transcription activator-like (TAL) effectors of Xanthomonas oryzae pv. oryzae (Xoo). The molecular mechanisms controlling the recognition of TAL effectors by Xa1 and the subsequent induction of immunity remain poorly understood. C_LIO_LIXa1 interacts in the nucleus with two TAL effectors via the BED domain. We identified the AP2/ERF-type transcription factor OsERF101/OsRAP2.6 as an interactor with Xa1, and found that it also interacts with the TAL effectors. Overexpression of OsERF101 exhibited an enhanced resistance to an incompatible Xoo strain only in the presence of Xa1, indicating that OsERF101 functions as a positive regulator of Xa1-mediated immunity. Unexpectedly, oserf101 mutants also showed enhanced Xa1-dependent resistance, but in a different manner from the overexpressing plants. This result revealed an additional Xa1-mediated immune pathway that is negatively regulated by OsERF101. Furthermore, OsERF101 directly interacted with the TAL effectors. C_LIO_LIOur results show that OsERF101 regulates the recognition of TAL effectors and the Xa1-mediated activation of the immune response. These data provide new insights into the molecular mechanism of NLR-mediated immunity in plants. C_LI

Proteins containing polyglutamine (polyQ) repeats are prone to aggregation and can lead to distinct human pathologies. For instance, Huntingtons disease is caused by an abnormal expansion of the polyQ stretch (> Q35) of Huntingtin (HTT) protein. However, plants express hundreds of proteins containing polyQ regions, but no pathologies arising from these factors have been reported to date. Here, we ask how plants maintain the proteostasis of polyQ-containing proteins, which are intrinsically enriched in the plant proteomes. To this end, we overexpressed an aggregation-prone fragment of human HTT (Q69) in plant cells. In contrast to invertebrate and mammalian transgenic models, we find that Arabidopsis thaliana plants suppress Q69 aggregation. This elevated proteostasis ability is mediated through the import and degradation of Q69 in chloroplasts. Conversely, inhibition of chloroplast protein import either genetically or pharmacologically reduces the capacity of plant cells to prevent Q69 aggregation. We find that Q69 interacts with the chloroplast stromal processing peptidase (SPP). Notably, expression of synthetic Arabidopsis SPP is sufficient to suppress aggregation of polyQ-expanded HTT in human cells. Beyond ectopically expressed Q69-HTT, endogenous polyQ-containing proteins also aggregate in Arabidopsis upon inhibition of chloroplast import. Among them, the plastid casein kinase 2 (pCK2), which contains a polyQ region next to the chloroplast targeting sequence motif, can also be localized into the nucleus. Upon inhibition of chloroplast import, pCK2 accumulates at higher levels in the nucleus and forms diamond-shaped amyloid-like fibrils surrounding the chloroplasts. These results indicate that the differential conformation and redistribution of pCK2 to the nucleus depends on chloroplast import efficiency, providing a role of polyQ repeats in chloroplast to nucleus communication (i.e. retrograde signaling). Together, our findings establish chloroplast protein import and proteases as determinants of polyQ proteostasis, with important implications for plant biology that can also lead to therapeutic approaches for human diseases that involve protein aggregation.

In eukaryotic cells, linker histone H1 anchors in and out ends of nucleosome DNA to promote chromatin to fold into the 30 nm fiber. However, if H1 plays a role in coordinating the three-dimensional (3D) chromatin architecture, DNA methylation, and transcriptional regulation is not clear. We engineered H1 knockout mutants in Arabidopsis thaliana which shows pleiotropic phenotypes. Using High-throughput Chromosome Conformation Capture (Hi-C), we found that H1 complete depletion dampens inter- and intra-chromosomal interactions, as well as intra- and inter-chromosomal arm interactions. MNase accessibility assays followed by sequencing (MNase-seq) showed that the nucleosome density decreases in centromeric regions and increases in chromosome arms. In contrast, DNA methylation level in CHG and CHH contexts increases in centromeric regions and decreases in chromosome arms as revealed by whole genome bisulfite sequencing (WGBS) in h1 mutant. Importantly, the functional link between DNA methylation and gene transcription is defected, and the extensive switches between chromatin compartment A and B are uncoupled from genome-wide DNA methylation and most of gene transcriptions upon H1 depletion. These results suggested that linker histone H1 works as linkers among chromatin compartmentalization, DNA methylation and transcription.