Improved Gateway binary vectors: high-performance vectors for creation of fusion constructs in transgenic analysis of plants

Molecular and physiological characterization of brassinosteroid receptor BRI1 mutants in Sorghum bicolor

The high sequence and structural similarities between BRASSINOSTEROID INSENSITIVE 1 (BRI1) brassinosteroid (BR) receptors of Arabidopsis (AtBRI1) and sorghum (SbBRI1) prompted us to study the functionally conserved roles of BRI1 in both organisms. Introducing sorghum SbBRI1 in Arabidopsis bri1 mutants restores defective growth and developmental phenotypes to wild-type levels. Sorghum mutants for SbBRI1 show defective BR sensitivity and impaired plant growth and development throughout the entire sorghum life cycle. Embryonic analysis of sorghum primary root techniques permits to trace back root growth and development to early stages in an unprecedented way, revealing the functionally conserved roles of the SbBRI1 receptor in BR perception during meristem development. RNA-seq analysis uncovers the downstream regulation of the SbBRI1 pathway in cell wall biogenesis during cell growth. Together, these results uncover that the sorghum SbBRI1 protein plays functionally conserved roles in plant growth and development, while encouraging the study of BR pathways in sorghum and its implications for improving resilience in cereal crops.

Myosin XI is required for boron transport under boron limitation via maintenance of endocytosis and polar localization of the boric acid channel AtNIP5;1

Myosin XI plays a major role in cytoplasmic streaming and is essential for intracellular transport. Here, we investigated the physiological roles of myosin XI in nutrient transport using double (2ko) and triple (3ko) myosin XI knockout mutants of Arabidopsis thaliana. The results revealed that the mutants exhibited more severe boron deficiency phenotypes under boron-limiting conditions, and the boron concentration in the aerial parts of mutant plants was lower than that in the wild-type. Microscopic analysis demonstrated a reduction in general endocytosis and abolishment of NIP5; 1's polar localization in 2ko and 3ko plants. Overall, these results indicate that myosin XI is necessary for proper boron transport via the maintenance of the endocytic pathway and NIP5; 1's polar localization.

Live-cell RNA imaging with the inactivated endonuclease Csy4 enables new insights into plant virus transport through plasmodesmata

Plant-infecting viruses spread through their hosts by transporting their infectious genomes through intercellular nano-channels called plasmodesmata. This process is mediated by virus-encoded movement proteins. Whilst the sub-cellular localisations of movement proteins have been intensively studied, live-cell RNA imaging systems have so far not been able to detect viral genomes inside the plasmodesmata. Here, we describe a highly sensitive RNA live-cell reporter based on an enzymatically inactive form of the small bacterial endonuclease Csy4, which binds to its cognate stem-loop with picomolar affinity. This system allows imaging of plant viral RNA genomes inside plasmodesmata and shows that potato virus X RNA remains accessible within the channels and is therefore not fully encapsidated during movement. We also combine Csy4-based RNA-imaging with interspecies movement complementation to show that an unrelated movement protein from tobacco mosaic virus can recruit potato virus X replication complexes adjacent to plasmodesmata. Therefore, recruitment of potato virus X replicase is mediated non-specifically, likely by indirect coupling of movement proteins and viral replicase via the viral RNA or co-compartmentalisation, potentially contributing to transport specificity. Lastly, we show that a 'self-tracking' virus can express the Csy4-based reporter during the progress of infection. However, expression of the RNA-binding protein in cis interferes with viral movement by an unidentified mechanism when cognate stem-loops are present in the viral RNA.

The rhizobial effector NopT targets Nod factor receptors to regulate symbiosis in Lotus japonicus

It is well documented that type-III effectors are required by Gram-negative pathogens to directly target different host cellular pathways to promote bacterial infection. However, in the context of legume-rhizobium symbiosis, the role of rhizobial effectors in regulating plant symbiotic pathways remains largely unexplored. Here, we show that NopT, a YopT-type cysteine protease of Sinorhizobium fredii NGR234 directly targets the plant's symbiotic signaling pathway by associating with two Nod factor receptors (NFR1 and NFR5 of Lotus japonicus). NopT inhibits cell death triggered by co-expression of NFR1/NFR5 in Nicotiana benthamiana. Full-length NopT physically interacts with NFR1 and NFR5. NopT proteolytically cleaves NFR5 both in vitro and in vivo, but can be inactivated by NFR1 as a result of phosphorylation. NopT plays an essential role in mediating rhizobial infection in L. japonicus. Autocleaved NopT retains the ability to cleave NFR5 but no longer interacts with NFR1. Interestingly, genomes of certain Sinorhizobium species only harbor nopT genes encoding truncated proteins without the autocleavage site. These results reveal an intricate interplay between rhizobia and legumes, in which a rhizobial effector protease targets NFR5 to suppress symbiotic signaling. NFR1 appears to counteract this process by phosphorylating the effector. This discovery highlights the role of a bacterial effector in regulating a signaling pathway in plants and opens up the perspective of developing kinase-interacting proteases to fine-tune cellular signaling processes in general.

Taxus NPF transporter involved in the uptake of 10-deacetylbaccatin III facilitates the biosynthesis of taxane compounds

Paclitaxel is an anticancer diterpene derivative produced by yew trees (Taxus spp.) as a forest resource. The biosynthetic pathway in Taxus spp. consists of intricate enzyme reactions, which involve many acylation steps on the taxadiene core structure. Time course analysis of the culture medium of yew cell suspension cultures revealed the dynamics of relevant taxane compounds, suggesting the active movement of biosynthetic intermediates across the plasma membrane leading to paclitaxel formation. Here, we report the identification of a yew NPF-type transporter, NPF2.1, involved in the uptake of 10-deacetylbaccatin III as a proton symporter. Expression of NPF2.1 in yeast facilitated the in vivo acetylation of 10-deacetylbaccatin III. In YPD culture media, 10-deacetylbaccatin III (0.1 mg L-1) was effectively converted to the acetylated product within 5 days at pH 5.3. The NPF2.1-mediated yeast bioconversion system was then used for gene discovery studies, which identified a novel BAHD acyltransferase that exhibited acylation activity with broad substrate specificity for acyl donors. These results suggest that the application of yeast NPF2.1 is a powerful molecular tool for the discovery of new paclitaxel biosynthetic genes and also for the production of paclitaxel in a synthetic biology approach.

UGT73FB1 contributes to scaffold-selective biosynthesis of triterpenoid glucosyl esters in saponin-rich bark of arjuna tree

Plants make structurally diverse triterpenoids for their physiological needs, which have shown numerous therapeutic applications. Arjuna tree (Terminalia arjuna) produces bioactive oleanane (β-amyrin-derived) triterpenoids arjunic acid, arjungenin, and arjunolic acid, and the respective C28-O-glucopyranosyl esters arjunetin, arjunglucoside I, and arjunglucoside II. Arjunic acid and arjunetin are the major oleananes in bark, while arjunolic acid and arjunglucoside II are found in minor levels. Although arjungenin was detected at a considerable level, arjunglucoside I was found only at a trace level, suggesting selective biosynthesis and/or accumulation of triterpenoid glucosyl esters in bark. However, the enzyme contributing to triterpenoid C28-O-glucosylation was not characterized. We mined RNA-sequencing data and identified UDP-glucosyltransferase (UGT) transcripts that were enriched in the bark transcriptome. Further, biochemical screening of UGTs identified UGT73FB1, which catalyzed triterpenoid C28-O-glucosylation in a scaffold-selective manner. Recombinant UGT73FB1 produced in Escherichia coli or Nicotiana benthamiana formed arjunic acid and arjunolic acid C28-O-glucopyranosyl esters arjunetin and arjunglucoside II, but not arjungenin C28-O-glucopyranosyl ester (arjunglucoside I). Interestingly, UGT73FB1 showed better activity using oleananes than ursanes (α-amyrin-derived), but it did not show C28-O-glucosylation activity using various lupane triterpenoids (lupeol-derived). Overall, the spatial patterns of UGT73FB1 transcript expression and triterpenoid accumulation and scaffold-selective activity of UGT73FB1 suggested a major role of UGT73FB1 in the biosynthesis of C28-O-glucopyranosyl esters in arjuna. Moreover, UGT73FB1 co-expression with β-amyrin synthase and triterpenoid C2, C23, and C28 hydroxylases/oxidases led to complete reconstruction of the arjunglucoside II pathway in N. benthamiana, suggesting the utility of arjuna enzymes for the biosynthesis of rare triterpenoid glucopyranosyl esters in heterologous hosts.

Arabidopsis SGS3 is recruited to chromatin by CHR11 to select RNA that initiate siRNA production

In plants, aberrant RNAs produced by endogenous genes or transgenes are normally degraded by the nuclear and cytosolic RNA quality control (RQC) pathways. Under certain biotic or abiotic stresses, RQC is impaired, and aberrant RNAs are converted into siRNAs that initiate post-transcriptional gene silencing (PTGS) in the cytosol. How aberrant RNAs are selected and brought to the cytoplasm is not known. Here we show that the RNA-binding protein SUPPRESSOR OF GENE SILENCING (SGS)3 shuttles between the cytosol and the nucleus where it associates with the ISWI-like CHROMATIN REMODELER (CHR)11 and with RNAs transcribed from PTGS-sensitive transgene loci binding CHR11. Knocking down CHR11 and its paralog CHR17 strongly reduces transgene PTGS, suggesting that SGS3 recruitment by CHR11/17 facilitates PTGS initiation. CHR11 is also enriched at endogenous protein-coding genes (PCGs) producing nat-siRNAs and va-siRNAs under biotic or abiotic stresses, and this production is reduced in chr11 chr17 double mutants at genome-wide level. Moreover, impairing CHR11 and CHR17 rescues the lethal phenotype caused by the massive production of siRNAs from PCGs in RQC-deficient mutants. We propose that SGS3 recruitment by CHR11/17 allows exporting RNAs to the cytosol to initiate the production of siRNAs.

Plant cell-cycle regulators control the nuclear environment for viral pathogenesis

The proper regulation of cell-cycle regulators is curial for both viral replication and host-plant adaptive growth during the viral pathogenesis. Mechanisms on reorchestrating RETINOBLASTOMA-RELATED 1 (RBR1), repressor of E2F transcription factor, and downstream genes in host-virus interactions are unclear. Here, we discover that anaphase-promoting complex/cyclosome (APC/C) E3 ligase activator cell division cycle 20 (CDC20) in tomato binds RBR1 or mediates cyclin D1 depletion to preserve RBR1-E2F complexes, while geminivirus or crinivirus repurposes APC/CCDC20 activities to liberate E2Fs in two ways: activating APC/CCDC20 to deplete RBR1 or blocking APC/CCDC20 to stimulate cyclin-D1-mediated RBR1 depletion. The liberated E2Fs activate DNA polymerase or heat shock protein 70 gene transcription to favor virus propagation. The improper disruption of RBR1-E2F complexes via hijacking APC/CCDC20 causes the host growth repression. We uncover a scenario in which the virus co-opts host APC/CCDC20 to reprogram RBR1-E2F complex to favor its propagation while dampening host vitality.

Gibberellin and cytokinin signaling antagonistically control female-germline cell specification in Arabidopsis

How do growth hormones interact to specify female-germline cell types in flowering plants and control production of the first female-germline cell? Here, we find that gibberellin (GA) biosynthesis and signaling are restricted in ovule primordia, with overexpression of receptors and biosynthetic enzymes resulting in multiple and enlarged megaspore mother cells (MMCs) in Arabidopsis. GA signaling machinery interacts with and promotes the degradation of cytokinin (CK) type-B Arabidopsis response regulators (ARR1/10/12), which also directly interact with DELLA proteins. CK biosynthesis and signaling components are expressed in both MMCs and sporophytic cells, with signaling negatively controlled by GA in ovule primordia, and perturbations leading to the induction of multiple, enlarged MMC-like cells. The vacuolar sorting protein SHRUBBY (SHBY) interacts with GA and CK signaling components to block GA-induced degradation. CK signaling restricts multiple sub-epidermal cells in distal ovule primordia from acquiring MMC identity. By balancing degradation activity, GA and CK signaling antagonistically control female-germline cell specification.

Arabidopsis MEB3 functions as a vacuolar metal transporter to regulate iron accumulation in roots

Iron is an essential nutrient for plant photosynthesis and development, but excess iron leads to stress. After absorption from the soil, plants store iron in roots and distribute it to shoots via long-distance transport. The vacuole is involved in iron storage and the maintenance of cellular iron homeostasis, and vacuolar iron transporter (VIT) family proteins have been identified as plant vacuolar iron transporters. However, the contribution of vacuolar iron transporters to overall iron homeostasis in plants is not fully understood. Here, we show that MEMBRANE PROTEIN OF ER BODY 3 (MEB3), a VIT family member, functions as a vacuolar metal transporter for iron distribution in Arabidopsis thaliana. Heterologous expression of Arabidopsis MEB3 in yeast vacuolar iron or zinc transporter mutants restored the iron- and zinc-resistance phenotypes of the respective mutants, indicating that MEB3 regulates iron and zinc transport. In Arabidopsis, MEB3 was expressed in almost all tissues, albeit to higher levels in roots and seedlings, and MEB3 protein localized to the tonoplast. Iron but not zinc levels were reduced in meb3 knockout mutant roots, suggesting that the knockout reduced iron storage capacity in roots. At high iron concentration, meb3 mutants accumulated more iron in shoots and less iron in roots than the wild type, indicating impairment of proper iron distribution in meb3 mutants. These findings demonstrate that MEB3 is a vacuolar transporter involved in the homeostasis of iron and other metals in plants.

Maize mutants in miR394-regulated genes show improved drought tolerance

Water limitation represents one of the major threats to agricultural production, which often leads to drought stress and results in compromised growth, development and yield of crop species. Drought tolerance has been intensively studied in search of potential targets for molecular approaches to crop improvement. However, drought adaptive traits are complex, and our understanding of the physiological and genetic basis of drought tolerance is still incomplete. The miR394-LCR pathway is a conserved regulatory module shown to participate in several aspects of plant growth and development, including stress response. Here, we characterized the miR394 pathway in maize, which harbours two genetic loci producing an evolutionarily conserved mature zma-miR394 targeting two transcripts coding for F-Box proteins, named hereby ZmLCR1 and ZmLCR2. Arabidopsis plants overexpressing the zma-MIR394B gene showed high tolerance to drought conditions compared to control plants. Moreover, analysis of the growth and development of single and double maize mutant plants in ZmLCR genes indicate that these mutations do not affect plant fitness when they grow in normal watering conditions, but mutants showed better survival than wild-type plants under water deprivation conditions. This increased drought tolerance is based on more efficient intrinsic water use, changes in root architecture and increased epicuticular wax content under water-limiting conditions. Our results indicate that the miR394-regulated ZmLCR genes are involved in drought stress tolerance and are remarkable candidates for maize crop improvement.

Control of plasma membrane-associated actin polymerization specifies the pattern of the cell wall in xylem vessels

Cell wall patterning is central to determining the shape and function of plant cells. Protoxylem and metaxylem vessel cells deposit banded and pitted cell walls, respectively, which enable their distinctive water transport capabilities. Here, we show that the pitted cell wall pattern in metaxylem vessels is specified by transcriptional control of actin polymerization. A newly isolated allele of KNOTTED-LIKE HOMEOBOX TRANSCRIPTION FACTOR 7 (KNAT7) was associated with the formation of banded cell walls in metaxylem vessels. Loss of KNAT7 caused misexpression of FORMIN HOMOLOGY DOMAIN CONTAINING PROTEIN11 (FH11) in the metaxylem, which in turn caused rearrangements of ROP GTPases and microtubules in banded patterns. FH11 function required its plasma membrane anchoring and actin polymerization activity. These results suggest that excessive actin polymerization at the plasma membrane abolishes the pitted cell wall formation and promotes banded cell wall formation in metaxylem vessels. This study unveils the importance of proper control of actin polymerization for cell wall pattern determination.

Florigen-producing cells express FPF1-LIKE PROTEIN 1 to accelerate flowering and stem growth in Arabidopsis

Plants induce the expression of the florigen FLOWERING LOCUS T (FT) in response to seasonal changes. FT is expressed in a distinct subset of phloem companion cells in Arabidopsis. Using tissue-specific translatome analysis, we discovered that the FT-expressing cells also express FLOWERING PROMOTING FACTOR 1 (FPF1)-LIKE PROTEIN 1 (FLP1), specifically under long-day conditions with the red/far-red ratio of natural sunlight. The master regulator of FT, CONSTANS (CO), is essential for FLP1 expression, suggesting that FLP1 is involved in the photoperiod pathway. We show that FLP1 promotes early flowering independently of FT, is active in the shoot apical meristem, and induces the expression of SEPALLATA3 (SEP3), a key E-class homeotic gene. Unlike FT, FLP1 also facilitates inflorescence stem elongation. Our cumulative evidence suggests that the small FLP1 protein acts as a mobile signal like FT. Taken together, FLP1 accelerates flowering in parallel with FT and orchestrates flowering and stem elongation during the reproductive transition.

The Wheat NLR Protein PM3b Localises to Endoplasmic Reticulum-Plasma Membrane Contact Sites and Interacts With AVRPM3b2/c2 Through Its LRR Domain

Plant nucleotide-binding leucine-rich repeat (NLR) proteins are intracellular immune receptors that directly or indirectly perceive pathogen-derived effector proteins to induce an immune response. NLRs display diverse subcellular localisations, which are associated with the capacity of the immune receptor to confer disease resistance and recognise its corresponding avirulence effector. In wheat, the NLR PM3b recognises the wheat powdery mildew effector AVRPM3b2/c2 and we examined the molecular mechanism underlying this recognition. We show that PM3b and other PM3 variants localise to endoplasmic reticulum (ER)-plasma membrane (PM) contact sites (EPCS), while AVRPM3b2/c2 localises to the nucleocytoplasmic space. Additionally, we found that PM3b interacts in planta with AVRPM3b2/c2 through its LRR domain. We further demonstrate that full-length PM3b interaction with AVRPM3b2/c2 is considerably weaker than for the isolated PM3b LRR domain or the susceptible PM3 variant PM3CS, indicating that activation of PM3b leads to dissociation of the complex. In line with this, we observed a strong interaction between PM3b and AVRPM3b2/c2 in a P-loop mutant of PM3b that was unable to initiate a cell death response, or when an inactive variant of AVRPM3b2/c2 was used. We propose that PM3b transiently interacts with AVRPM3b2/c2 through residues in the LRR that are conserved among PM3 variants, while the amino acids necessary for full activation and cell death signalling are unique to PM3b. Our data suggests that PM3b localisation and interaction with AVRPM3b2/c2 differ from other well-studied NLRs and further highlights the mechanistic diversity in NLR-mediated responses against pathogens in plants.

A barley MLA immune receptor is activated by a fungal nonribosomal peptide effector for disease susceptibility

The barley Mla locus contains functionally diversified genes that encode intracellular nucleotide-binding leucine-rich repeat receptors (NLRs) and confer strain-specific immunity to biotrophic and hemibiotrophic fungal pathogens. In this study, we isolated a barley gene Scs6, which is an allelic variant of Mla genes but confers susceptibility to the isolate ND90Pr (BsND90Pr) of the necrotrophic fungus Bipolaris sorokiniana. We generated Scs6 transgenic barley lines and showed that Scs6 is sufficient to confer susceptibility to BsND90Pr in barley genotypes naturally lacking the receptor. The Scs6-encoded NLR (SCS6) is activated by a nonribosomal peptide (NRP) effector produced by BsND90Pr to induce cell death in barley and Nicotiana benthamiana. Domain swaps between MLAs and SCS6 reveal that the SCS6 leucine-rich repeat domain is a specificity determinant for receptor activation by the NRP effector. Scs6 is maintained in both wild and domesticated barley populations. Our phylogenetic analysis suggests that Scs6 is a Hordeum-specific innovation. We infer that SCS6 is a bona fide immune receptor that is likely directly activated by the nonribosomal peptide effector of BsND90Pr for disease susceptibility in barley. Our study provides a stepping stone for the future development of synthetic NLR receptors in crops that are less vulnerable to modification by necrotrophic pathogens.

Catalytic mechanism underlying the regiospecificity of coumarin-substrate transmembrane prenyltransferases in Apiaceae

Plant membrane-bound prenyltransferases (PTs) catalyze the transfer of prenyl groups to acceptor substrates, phenols, using prenyl diphosphates as the donor substrate. The presence of prenyl residues in the reaction products, prenylated phenols, is key to the expression of a variety of physiological activities. Plant PTs generally exhibit high specificities for both substrate recognition and prenylation sites, while the molecular mechanism involved in these enzymatic properties is largely unknown. In this study, we performed a systematic biochemical analysis to elucidate the catalytic mechanism responsible for the reaction specificity of plant PTs. Using two representative PTs, PsPT1 and PsPT2, from parsnip (Pastinaca sativa, Apiaceae), which differ only in the regiospecificity of the prenylation site, we performed domain swapping and site-directed mutagenesis of these PTs, followed by detailed enzymatic analysis combined with 3D modeling. As a result, we discovered the domains that control prenylation site specificity and further defined key amino acid residues responsible for the catalytic mechanism. In addition, we showed that the control mechanism of prenylation specificity revealed here is also highly conserved among coumarin-substrate PTs. These data suggest that the regulatory domain revealed here is commonly involved in prenylation regiospecificity in Apiaceae PTs.

Evolution of interorganismal strigolactone biosynthesis in seed plants

Strigolactones (SLs) are methylbutenolide molecules derived from β-carotene through an intermediate carlactonoic acid (CLA). Canonical SLs act as signals to microbes and plants, whereas noncanonical SLs are primarily plant hormones. The cytochrome P450 CYP722C catalyzes a critical step, converting CLA to canonical SLs in most angiosperms. Using synthetic biology, we investigated the function of CYP722A, an evolutionary predecessor of CYP722C. CYP722A converts CLA into 16-hydroxy-CLA (16-OH-CLA), a noncanonical SL detected exclusively in the shoots of various flowering plants. 16-OH-CLA application restores control of shoot branching to SL-deficient mutants in Arabidopsis thaliana and is perceived by the SL signaling pathway. We hypothesize that biosynthesis of 16-OH-CLA by CYP722A was a metabolic stepping stone in the evolution of canonical SLs that mediate rhizospheric signaling in many flowering plants.

Identification and functional analyses of drought stress resistance genes by transcriptomics of the Mongolian grassland plant Chloris virgata

BACKGROUND

Mongolian grasslands, including the Gobi Desert, have been exposed to drought conditions with few rains. In such harsh environments, plants with highly resistant abilities against drought stress survive over long periods. We hypothesized that these plants could harbor novel and valuable genes for enhancing drought stress resistance.

RESULTS

In this study, we identified Chloris virgata, a Mongolian grassland plant with strong drought resistance. RNA-seq-based transcriptome analysis was performed to uncover genes associated with drought stress resistance in C. virgata. De novo transcriptome assembly revealed 25,469 protein-coding transcripts and 1,219 upregulated genes after 3- and 6-hr drought stress treatments. Analysis by homology search and Gene Ontology (GO) enrichment indicated that abscisic acid (ABA)- and drought stress-related GO terms were enriched. Among the highly induced genes, ten candidate cDNAs were selected and overexpressed in Arabidopsis. When subjected to drought stress, three of these genes conferred strong drought resistance in the transgenic plants. We named these genes Mongolian Grassland plant Drought-stress resistance genes 1, 2, and 3 (MGD1, MGD2, and MGD3). Gene expression analyses in the transformants suggested that MGD1, MGD2, and MGD3 may activate drought stress-related signalling pathways.

CONCLUSION

This study highlighted the drought resistance of C. virgata and identified three novel genes that enhance drought stress resistance.

ATG8 delipidation is not universally critical for autophagy in plants

Intracellular recycling via autophagy is governed by post-translational modifications of the autophagy-related (ATG) proteins. One notable example is ATG4-dependent delipidation of ATG8, a process that plays critical but distinct roles in autophagosome formation in yeast and mammals. Here, we aim to elucidate the specific contribution of this process to autophagosome formation in species representative of evolutionarily distant green plant lineages: unicellular green alga Chlamydomonas reinhardtii, with a relatively simple set of ATG genes, and a vascular plant Arabidopsis thaliana, harboring expanded ATG gene families. Remarkably, the more complex autophagy machinery of Arabidopsis renders ATG8 delipidation entirely dispensable for the maturation of autophagosomes, autophagic flux, and related stress tolerance; whereas autophagy in Chlamydomonas strictly depends on the ATG4-mediated delipidation of ATG8. Importantly, we also demonstrate the distinct impact of different Arabidopsis ATG8 orthologs on autophagosome formation, especially prevalent under nitrogen depletion, providing new insight into potential drivers behind the expansion of the ATG8 family in higher plants. Our findings underscore the evolutionary diversification of the molecular mechanism governing the maturation of autophagosomes in eukaryotic lineages and highlight how this conserved pathway is tailored to diverse organisms.

Enhanced salt stress tolerance in plants without growth penalty through increased photosynthesis activity by plastocyanin from Antarctic moss

Salinity poses a significant challenge to plant growth and crop productivity by adversely affecting crucial processes, including photosynthesis. Efforts to enhance abiotic stress tolerance in crops have been hindered by the trade-off effect, where increased stress resistance is accompanied by growth reduction. In this study, we identified and characterized a plastocyanin gene (PaPC) from the Antarctic moss Polytrichastrum alpinum, which enhanced photosynthesis and salt stress tolerance in Arabidopsis thaliana without compromising growth. While there were no differences in growth and salt tolerance between the wild type and Arabidopsis plastocyanin genes (AtPC1 and AtPC2)-overexpressing plants, PaPC-overexpressing plants demonstrated superior photosynthetic efficiency, increased biomass, and enhanced salt tolerance. Similarly, PaPC-overexpressing rice plants exhibited improved yield potential and photosynthetic efficiency under both normal and salt stress conditions. Key amino acid residues in PaPC responsible for this enhanced functionality were identified, and their substitution into AtPC2 conferred improved photosynthetic performance and stress tolerance in Arabidopsis, tobacco, and tomato. These findings not only highlight the potential of extremophiles as valuable genetic resources but also suggest a photosynthesis-based strategy for developing stress-resilient crops without a growth penalty.

Viral condensates formed by Pea enation mosaic virus 2 sequester ribosomal components and suppress translation

Viral proteins with intrinsic disorder, such as the p26 movement protein from Pea enation mosaic virus 2 (PEMV2), can phase separate and form condensates that aid specific stages of virus replication. However, little is known about the impact of viral condensate formation on essential cellular processes, like translation. In this study, we performed mass spectrometry on affinity-purified p26 condensates and found an enrichment of RNA-binding proteins involved in translation and ribosome biogenesis. Puromycin assays and polysome profiling show that ectopic p26 expression suppresses ribosome assembly and translation in Nicotiana benthamiana, mirroring defects in late-stage PEMV2 infection. Despite interactions with the 2'-O-methyltransferase fibrillarin, p26 does not inhibit translation by altering rRNA methylation but instead binds directly to rRNAs and decreases their solubility. Disruption of ribosome assembly and translation by p26 during late PEMV2 infection may promote stages of the virus replication cycle that are incompatible with translation, including systemic movement.

BIL7 enhances plant growth by regulating the transcription factor BIL1/BZR1 during brassinosteroid signaling

Brassinosteroids (BRs) are plant steroid hormones that regulate plant development and environmental responses. BIL1/BZR1, a master transcription factor that regulates approximately 3000 genes in the BR signaling pathway, is transported to the nucleus from the cytosol in response to BR signaling; however, the molecular mechanism underlying this process is unknown. Here, we identify a novel BR signaling factor, BIL7, that enhances plant growth and positively regulates the nuclear accumulation of BIL1/BZR1 in Arabidopsis thaliana. BIL7-overexpressing plants were resistant to the BR biosynthesis inhibitor Brz and taller than wild-type (WT) plants were due to increased cell division. BIL7 is mainly localized to the plasma membrane, but during the early stages of cell growth, it was also localized to the nucleus. BIL7 was directly phosphorylated by the kinase BIN2, and nuclear localization of BIL7 was enhanced by the BIN2 inhibitor bikinin. BIL7 was found to bind to BIL1/BZR1, and nuclear accumulation of BIL1/BZR1 was strongly enhanced by BIL7 overexpression. Finally, double overexpression of BIL1/BZR1 and BIL7 led to greatly elongated hypocotyls in the presence of Brz. These findings suggest that BIL7 mediates nuclear accumulation of BIL1/BZR1, which activates inflorescence elongation in plants via BR signaling.

Temperature-driven changes in membrane fluidity differentially impact FILAMENTATION TEMPERATURE-SENSITIVE H2-mediated photosystem II repair

The Arabidopsis (Arabidopsis thaliana) yellow variegated2 (var2) mutant, lacking functional FILAMENTATION TEMPERATURE-SENSITIVE H2 (FtsH2), an ATP-dependent zinc metalloprotease, is a powerful tool for studying the photosystem II (PSII) repair process in plants. FtsH2, forming hetero-hexamers with FtsH1, FtsH5, and FtsH8, plays an indispensable role in PSII proteostasis. Although abiotic stresses like cold and heat increase chloroplast reactive oxygen species (ROS) and PSII damage, var2 mutants behave like wild-type plants under heat stress but collapse under cold stress. Our study on transgenic var2 lines expressing FtsH2 variants, defective in either substrate extraction or proteolysis, reveals that cold stress causes an increase in membrane viscosity, demanding more substrate extraction power than proteolysis by FtsH2. Overexpression of FtsH2 lacking substrate extraction activity does not rescue the cold-sensitive phenotype, while overexpression of FtsH2 lacking protease activity does in var2, with other FtsH isomers present. This indicates that FtsH2's substrate extraction activity is indispensable under cold stress when membranes become more viscous. As temperatures rise and membrane fluidity increases, substrate extraction activity from other isomers suffices, explaining the var2 mutant's heat stress resilience. These findings underscore the direct effect of membrane fluidity on the functionality of the thylakoid FtsH complex under stress. Future research should explore how membrane fluidity impacts proteostasis, potentially uncovering strategies to modulate thermosensitivity.

The nuclear exosome subunit HEN2 acts independently of the core exosome to assist transcription in Arabidopsis

Regulation of gene expression is at the frontier of plant responses to various external stimuli including stress. RNA polymerase-based transcription and post-transcriptional degradation of RNA play vital roles in this regulation. Here, we show that HUA ENHANCER 2 (HEN2), a co-factor of the nuclear exosome complex, influences RNAPII transcription elongation in Arabidopsis (Arabidopsis thaliana) under cold conditions. Our results demonstrate that a hen2 mutant is cold sensitive and undergoes substantial transcriptional changes compared to wild type when exposed to cold conditions. We found an accumulation of 5' fragments from a subset of genes (including C-repeat binding factors 1-3 [CBF1-3]) that do not carry over to their 3' ends. In fact, hen2 mutants have lower levels of full-length mRNA for a subset of genes. This distinct 5'-end accumulation and 3'-end depletion was not observed in other NEXT complex members or core exosome mutants, highlighting HEN2's distinctive role. We further used RNAPII-associated nascent RNA to confirm that the transcriptional phenotype is a result of lower active transcription specifically at the 3' end of these genes in a hen2 mutant. Taken together, our data point to the unique role of HEN2 in maintaining RNAPII transcription dynamics especially highlighted under cold stress.

AtDGCR14L contributes to salt-stress tolerance via regulating pre-mRNA splicing in Arabidopsis

In plants, pre-mRNA alternative splicing has been demonstrated to be a crucial tier that regulates gene expression in response to salt stress. However, the underlying mechanisms remain elusive. Here, we studied the roles of DIGEORGE-SYNDROME CRITICAL REGION 14-like (AtDGCR14L) in regulating pre-mRNA splicing and salt stress tolerance. We discovered that Arabidopsis AtDGCR14L is required for maintaining plant salt stress tolerance and the constitutively spliced and active isoforms of important stress- and/or abscisic acid (ABA)-responsive genes. We also identified the interaction between AtDGCR14L and splicing factor U1-70k, which needs a highly conserved three amino acid (TWG) motif in DGCR14. Different from wild-type AtDGCR14L, the overexpression of the TWG-substituted AtDGCR14L mutant did not change salt stress tolerance or pre-mRNA splicing of stress/ABA-responsive genes. Additionally, SWITCH3A (SWI3A) is a core subunit of the SWI/SUCROSE NONFERMENTING (SWI/SNF) chromatin-remodeling complexes. We found that SWI3A, whose splicing depends on AtDGCR14L, actively enhances salt stress tolerance. These results revealed that AtDGCR14L may play an essential role in crosstalk between plant salt-stress response and pre-mRNA splicing mechanisms. We also unveiled the potential role of SWI3A in controlling salt stress tolerance. The TWG motif in the intrinsically disordered region of AtDGCR14L is highly conserved and crucial for DGCR14 functions.

Defense-related callose synthase PMR4 promotes root hair callose deposition and adaptation to phosphate deficiency in Arabidopsis thaliana

Plants acquire phosphorus (P) primarily as inorganic phosphate (Pi) from the soil. Under Pi deficiency, plants induce an array of physiological and morphological responses, termed phosphate starvation response (PSR), thereby increasing Pi acquisition and use efficiency. However, the mechanisms by which plants adapt to Pi deficiency remain to be elucidated. Here, we report that deposition of a β-1,3-glucan polymer called callose is induced in Arabidopsis thaliana root hairs under Pi deficiency, in a manner independent of PSR-regulating PHR1/PHL1 transcription factors and LPR1/LPR2 ferroxidases. Genetic studies revealed PMR4 (GSL5) callose synthase being required for the callose deposition in Pi-depleted root hairs. Loss of PMR4 also reduces Pi acquisition in shoots and plant growth under low Pi conditions. The defects are not recovered by simultaneous disruption of SID2, mediating defense-associated salicylic acid (SA) biosynthesis, excluding SA defense activation from the cause of the observed pmr4 phenotypes. Grafting experiments and characterization of plants expressing PMR4 specifically in root hair cells suggest that a PMR4 pool in the cell type contributes to shoot growth under Pi deficiency. Our findings thus suggest an important role for PMR4 in plant adaptation to Pi deficiency.

BIL9 Promotes Both Plant Growth via BR Signaling and Drought Stress Resistance by Binding with the Transcription Factor HDG11

Drought stress is a major threat leading to global plant and crop losses in the context of the climate change crisis. Brassinosteroids (BRs) are plant steroid hormones, and the BR signaling mechanism in plant development has been well elucidated. Nevertheless, the specific mechanisms of BR signaling in drought stress are still unclear. Here, we identify a novel Arabidopsis gene, BRZ INSENSITIVE LONG HYPOCOTYL 9 (BIL9), which promotes plant growth via BR signaling. Overexpression of BIL9 enhances drought and mannitol stress resistance and increases the expression of drought-responsive genes. BIL9 protein is induced by dehydration and interacts with the HD-Zip IV transcription factor HOMEODOMAIN GLABROUS 11 (HDG11), which is known to promote plant resistance to drought stress, in vitro and in vivo. BIL9 enhanced the transcriptional activity of HDG11 for drought-stress-resistant genes. BIL9 is a novel BR signaling factor that enhances both plant growth and plant drought resistance.

Nuclear interactions between the Pseudo-Response Regulator clock proteins and the Multi-Step Phosphorelay mediator Histidine-containing phosphotransfers in the moss Physcomitrium patens

Pseudo-Response Regulator (PRR) proteins constitute a fundamental set of circadian clock components in plants. PRRs have an amino acid sequence stretch with similarity to the receiver (REC) domain of response regulators (RRs) in the Multi-Step Phosphorelay (MSP). However, it has never been elucidated whether PRRs interact with Histidine-containing Phosphotransfer (HPt) proteins, which transfer a phosphate to RRs. Here, we studied whether PRRs interact with HPts in the moss Physcomitrium patens by the Yeast Two-Hybrid system and Bimolecular Fluorescence Complementation. P. patens PRR1/2/3 interacted with HPt1/2 in the nucleus, but not with HPt3, suggesting that P. patens PRRs function as authentic RRs. We discuss these results in relation to the evolution and diversity of the plant circadian clocks.

Autophagosome development and chloroplast segmentation occur synchronously for piecemeal degradation of chloroplasts

Plants distribute many nutrients to chloroplasts during leaf development and maturation. When leaves senesce or experience sugar starvation, the autophagy machinery degrades chloroplast proteins to facilitate efficient nutrient reuse. Here, we report on the intracellular dynamics of an autophagy pathway responsible for piecemeal degradation of chloroplast components. Through live-cell monitoring of chloroplast morphology, we observed the formation of chloroplast budding structures in sugar-starved leaves. These buds were then released and incorporated into the vacuolar lumen as an autophagic cargo termed a Rubisco-containing body. The budding structures did not accumulate in mutants of core autophagy machinery, suggesting that autophagosome creation is required for forming chloroplast buds. Simultaneous tracking of chloroplast morphology and autophagosome development revealed that the isolation membranes of autophagosomes interact closely with part of the chloroplast surface before forming chloroplast buds. Chloroplasts then protrude at the site associated with the isolation membranes, which divide synchronously with autophagosome maturation. This autophagy-related division does not require DYNAMIN-RELATED PROTEIN 5B, which constitutes the division ring for chloroplast proliferation in growing leaves. An unidentified division machinery may thus fragment chloroplasts for degradation in coordination with the development of the chloroplast-associated isolation membrane.

Related type 2C protein phosphatases Pic3 and Pic12 negatively regulate immunity in tomato to Pseudomonas syringae

Type 2C protein phosphatases (PP2Cs) constitute a large family in most plant species, but relatively few of them have been implicated in immunity. To identify and characterize PP2C phosphatases that affect tomato (Solanum lycopersicum) immunity, we generated loss-of-function mutations in 11 PP2C-encoding genes whose expression is altered in response to immune elicitors or pathogens. We report that 2 closely related PP2C phosphatases, PP2C immunity-associated candidate 3 (Pic3) and Pic12, are involved in regulating resistance to the bacterial pathogen Pseudomonas syringae pv. tomato (Pst). Loss-of-function mutations in Pic3 led to enhanced resistance to Pst in older but not younger leaves, whereas such mutations in Pic12 resulted in enhanced resistance in both older and younger leaves. Overexpression of Pic3 and Pic12 proteins in leaves of Nicotiana benthamiana inhibited resistance to Pst, and this effect was dependent on Pic3/12 phosphatase activity and an N-terminal palmitoylation motif associated with localization to the cell periphery. Pic3, but not Pic12, had a slight negative effect on flagellin-associated reactive oxygen species generation, although their involvement in the response to Pst appeared independent of flagellin. RNA-sequencing analysis of Rio Grande (RG)-PtoR wild-type plants and 2 independent RG-pic3 mutants revealed that the enhanced disease resistance in RG-pic3 older leaves is associated with increased transcript abundance of multiple defense-related genes. RG-pic3/RG-pic12 double-mutant plants exhibited stronger disease resistance than RG-pic3 or RG-pic12 single mutants. Together, our results reveal that Pic3 and Pic12 negatively regulate tomato immunity in an additive manner through flagellin-independent pathways.

Use of the Puccinia sorghi haustorial transcriptome to identify and characterize AvrRp1-D recognized by the maize Rp1-D resistance protein

The common rust disease of maize is caused by the obligate biotrophic fungus Puccinia sorghi. The maize Rp1-D allele imparts resistance against the P. sorghi IN2 isolate by initiating a defense response that includes a rapid localized programmed cell death process, the hypersensitive response (HR). In this study, to identify AvrRp1-D from P. sorghi IN2, we employed the isolation of haustoria, facilitated by a biotin-streptavidin interaction, as a powerful approach. This method proves particularly advantageous in cases where the genome information for the fungal pathogen is unavailable, enhancing our ability to explore and understand the molecular interactions between maize and P. sorghi. The haustorial transcriptome generated through this technique, in combination with bioinformatic analyses such as SignalP and TMHMM, enabled the identification of 251 candidate effectors. We ultimately identified two closely related genes, AvrRp1-D.1 and AvrRp1-D.2, which triggered an Rp1-D-dependent defense response in Nicotiana benthamiana. AvrRp1-D-induced Rp1-D-dependent HR was further confirmed in maize protoplasts. We demonstrated that AvrRp1-D.1 interacts directly and specifically with the leucine-rich repeat (LRR) domain of Rp1-D through yeast two-hybrid assay. We also provide evidence that, in the absence of Rp1-D, AvrRp1-D.1 plays a role in suppressing the plant immune response. Our research provides valuable insights into the molecular interactions driving resistance against common rust in maize.

Chrysanthemum lavandulifolium homolog CYCLIN A2;1 modulates cell division in ray florets

The morphology of ray florets in chrysanthemums is tightly associated with cell division and expansion, both of which require proper progression of the cell cycle. Here, we identified a Chrysanthemum lavandulifolium homolog, CYCLIN A2;1 (CYCA2;1), the expression of which in ray florets is negatively correlated with petal width. We found that CYC2a, a TEOSINTE BRANCHED1/CYCLOIDEA/PROLIFERATING CELL FACTOR (TCP) transcription factor in the CYCLOIDEA2 (CYC2) family, interacts with and stabilizes CYC2b, and the latter can bind to the promoter of CYCA2;1 to activate its transcription. Overexpression of CYCA2;1 in C. lavandulifolium reduced the size of capitula and ray florets. Cytological analysis revealed that CYCA2;1 overexpression inhibited both cell division and expansion via repression of the mitotic cell cycle in ray florets, the latitudinal development of which was more relatively negatively influenced, thereby leading to increased ratios of petal length to width at later developmental stages. Yeast two-hybrid library screening revealed multiple proteins that interacted with CYCA2;1 including ACTIN-RELATED PROTEIN7 (ARP7), and silencing ARP7 inhibited the development of ray florets. Co-immunoprecipitation assays confirmed that CYCA2;1 could induce the degradation of ARP7 to inhibit the development of ray florets. Taken together, our results indicate the presence of a regulatory network in ray floret development in chrysanthemum consisting of CYC2b-CYCA2;1-ARP7 that acts via governing mitosis. The identification of this network has the potential to facilitate breeding efforts targeted at producing novel ornamental traits in the flowers.

The endophytic fungus Serendipita indica affects auxin distribution in Arabidopsis thaliana roots through alteration of auxin transport and conjugation to promote plant growth

Plants share their habitats with a multitude of different microbes. This close vicinity promoted the evolution of interorganismic interactions between plants and many different microorganisms that provide mutual growth benefits both to the plant and the microbial partner. The symbiosis of Arabidopsis thaliana with the beneficial root colonizing endophyte Serendipita indica represents a well-studied system. Colonization of Arabidopsis roots with S. indica promotes plant growth and stress tolerance of the host plant. However, until now, the molecular mechanism by which S. indica reprograms plant growth remains largely unknown. This study used comprehensive transcriptomics, metabolomics, reverse genetics, and life cell imaging to reveal the intricacies of auxin-related processes that affect root growth in the symbiosis between A. thaliana and S. indica. Our experiments revealed the sustained stimulation of auxin signalling in fungus infected Arabidopsis roots and disclosed the essential role of tightly controlled auxin conjugation in the plant-fungus interaction. It particularly highlighted the importance of two GRETCHEN HAGEN 3 (GH3) genes, GH3.5 and GH3.17, for the fungus infection-triggered stimulation of biomass production, thus broadening our knowledge about the function of GH3s in plants. Furthermore, we provide evidence for the transcriptional alteration of the PIN2 auxin transporter gene in roots of Arabidopsis seedlings infected with S. indica and demonstrate that this transcriptional adjustment affects auxin signalling in roots, which results in increased plant growth.

The Arabidopsis U1 snRNP regulates mRNA 3'-end processing

The removal of introns by the spliceosome is a key gene regulatory mechanism in eukaryotes, with the U1 snRNP subunit playing a crucial role in the early stages of splicing. Studies in metazoans show that the U1 snRNP also conducts splicing-independent functions, but the lack of genetic tools and knowledge about U1 snRNP-associated proteins have limited the study of such splicing-independent functions in plants. Here we describe an RNA-centric approach that identified more than 200 proteins associated with the Arabidopsis U1 snRNP and revealed a tight link to mRNA cleavage and polyadenylation factors. Interestingly, we found that the U1 snRNP protects mRNAs against premature cleavage and polyadenylation within introns-a mechanism known as telescripting in metazoans-while also influencing alternative polyadenylation site selection in 3'-UTRs. Overall, our work provides a comprehensive view of U1 snRNP interactors and reveals novel functions in regulating mRNA 3'-end processing in Arabidopsis, laying the groundwork for understanding non-canonical functions of plant U1 snRNPs.

The StPti5 ethylene response factor acts as a susceptibility factor by negatively regulating the potato immune response to pathogens

Ethylene response factors (ERFs) have been associated with biotic stress in Arabidopsis, while their function in non-model plants is still poorly understood. Here we investigated the role of potato ERF StPti5 in plant immunity. We show that StPti5 acts as a susceptibility factor. It negatively regulates potato immunity against potato virus Y and Ralstonia solanacearum, pathogens with completely different modes of action, and thereby has a different role than its orthologue in tomato. Remarkably, StPti5 is destabilised in healthy plants via the autophagy pathway and accumulates exclusively in the nucleus upon infection. We demonstrate that StEIN3 and StEIL1 directly bind the StPti5 promoter and activate its expression, while synergistic activity of the ethylene and salicylic acid pathways is required for regulated StPti expression. To gain further insight into the mode of StPti5 action in attenuating potato defence responses, we investigated transcriptional changes in salicylic acid deficient potato lines with silenced StPti5 expression. We show that StPti5 regulates the expression of other ERFs and downregulates the ubiquitin-proteasome pathway as well as several proteases involved in directed proteolysis. This study adds a novel element to the complex puzzle of immune regulation, by deciphering a two-level regulation of ERF transcription factor activity in response to pathogens.

Pepper RING-Type E3 Ligase CaFIRF1 Negatively Regulates the Protein Stability of Pepper Stress-Associated Protein, CaSAP14, in the Dehydration Stress Response

As part of the cellular stress response in plants, the ubiquitin-proteasome system (UPS) plays a crucial role in regulating the protein stability of stress-related transcription factors. Previous study has indicated that CaSAP14 is functionally involved in enhancing pepper plant tolerance to dehydration stress by modulating the expression of downstream genes. However, the comprehensive regulatory mechanism underlying CaSAP14 remains incompletely understood. Here, we identified a RING-type E3 ligase, CaFIRF1, which interacts with and ubiquitinates CaSAP14. Pepper plants with silenced CaFIRF1 exhibited a dehydration-tolerant phenotype when subjected to dehydration stress, while overexpression of CaFIRF1 in pepper and Arabidopsis resulted in reduced dehydration tolerance. Co-silencing of CaFIRF1 and CaSAP14 in pepper increased sensitivity to dehydration, suggesting that CaFIRF1 acts upstream of CaSAP14. A cell-free degradation analysis demonstrated that silencing of CaFIRF1 led to decreased CaSAP14 protein degradation, implicating CaFIRF1 in the regulation of CaSAP14 protein via the 26S proteasomal degradation pathway. Our findings suggest a mechanism by which CaFIRF1 mediates the ubiquitin-dependent proteasomal degradation of CaSAP14, thereby influencing the response of pepper plants to dehydration stress.

H2O2 sulfenylates CHE, linking local infection to the establishment of systemic acquired resistance

In plants, a local infection can lead to systemic acquired resistance (SAR) through increased production of salicylic acid (SA). For many years, the identity of the mobile signal and its direct transduction mechanism for systemic SA synthesis in initiating SAR have been debated. We found that in Arabidopsis thaliana, after a local infection, the conserved cysteine residue of the transcription factor CCA1 HIKING EXPEDITION (CHE) undergoes sulfenylation in systemic tissues, which enhances its binding to the promoter of the SA-synthesis gene ISOCHORISMATE SYNTHASE1 (ICS1) and increases SA production. Furthermore, hydrogen peroxide (H2O2) produced through NADPH oxidases is the mobile signal that sulfenylates CHE in a concentration-dependent manner. Accumulation of SA and the previously reported signal molecules, such as N-hydroxypipecolic acid (NHP), then form a signal amplification loop to establish SAR.

Protein complexes from edible mushrooms as a sustainable potato protection against coleopteran pests

Protein complexes from edible oyster mushrooms (Pleurotus sp.) composed of pleurotolysin A2 (PlyA2) and pleurotolysin B (PlyB) exert toxicity in feeding tests against Colorado potato beetle (CPB) larvae, acting through the interaction with insect-specific membrane sphingolipid. Here we present a new strategy for crop protection, based on in planta production of PlyA2/PlyB protein complexes, and we exemplify this strategy in construction of transgenic potato plants of cv Désirée. The transgenics in which PlyA2 was directed to the vacuole and PlyB to the endoplasmic reticulum are effectively protected from infestation by CPB larvae without impacting plant performance. These transgenic plants showed a pronounced effect on larval feeding rate, the larvae feeding on transgenic plants being on average five to six folds lighter than larvae feeding on controls. Further, only a fraction (11%-37%) of the larvae that fed on transgenic potato plants completed their life cycle and developed into adult beetles. Moreover, gene expression analysis of CPB larvae exposed to PlyA2/PlyB complexes revealed the response indicative of a general stress status of larvae and no evidence of possibility of developing resistance due to the functional inactivation of PlyA2/PlyB sphingolipid receptors.

Localization of the MTP4 transporter to trans-Golgi network in pollen tubes of Arabidopsis thaliana

Zinc (Zn) is an essential element for plants. Numerous proteins in different cellular compartments require Zn for their structure and function. Zn can be toxic when it accumulates in high levels in the cytoplasm. Therefore, Zn homeostasis at tissue, cell, and organelle levels is vital for plant growth. A part of the metal tolerance protein (MTP) / Cation Diffusion Facilitator (CDF) transporters functions as Zn transporters, exporting Zn from the cytosol to various membrane compartments. In Arabidopsis thaliana, MTP1, MTP2, MTP3, MTP4, MTP5, and MTP12 are classified as Zn transporters (Zn-CDF). In this study, we systematically analyzed the localization of GFP-fused Zn-CDFs in the leaf epidermal cells of Nicotiana benthamiana. As previously reported, MTP1 and MTP3 were localized to tonoplast, MTP2 to endoplasmic reticulum, and MTP5 to Golgi. In addition, we identified the localization of MTP4 to trans-Golgi Network (TGN). Since MTP4 is specifically expressed in pollen, we analyzed the localization of MTP4-GFP in the Arabidopsis pollen tubes and confirmed that it is in the TGN. We also showed the Zn transport capability of MTP4 in yeast cells. We then analyzed the phenotype of an mtp4 T-DNA insertion mutant under both limited and excess Zn conditions. We found that their growth and fertility were not largely different from the wild-type. Our study has paved the way for investigating the possible roles of MTP4 in metallating proteins in the secretory pathway or in exporting excess Zn through exocytosis. In addition, our system of GFP-fused MTPs will help study the mechanisms for targeting transporters to specific membrane compartments.

An Arabidopsis Kinesin-14D motor is associated with midzone microtubules for spindle morphogenesis

The acentrosomal spindle apparatus has kinetochore fibers organized and converged toward opposite poles; however, mechanisms underlying the organization of these microtubule fibers into an orchestrated bipolar array were largely unknown. Kinesin-14D is one of the four classes of Kinesin-14 motors that are conserved from green algae to flowering plants. In Arabidopsis thaliana, three Kinesin-14D members displayed distinct cell cycle-dependent localization patterns on spindle microtubules in mitosis. Notably, Kinesin-14D1 was enriched on the midzone microtubules of prophase and mitotic spindles and later persisted in the spindle and phragmoplast midzones. The kinesin-14d1 mutant had kinetochore fibers disengaged from each other during mitosis and exhibited hypersensitivity to the microtubule-depolymerizing herbicide oryzalin. Oryzalin-treated kinesin-14d1 mutant cells had kinetochore fibers tangled together in collapsed spindle microtubule arrays. Kinesin-14D1, unlike other Kinesin-14 motors, showed slow microtubule plus end-directed motility, and its localization and function were dependent on its motor activity and the novel malectin-like domain. Our findings revealed a Kinesin-14D1-dependent mechanism that employs interpolar microtubules to regulate the organization of kinetochore fibers for acentrosomal spindle morphogenesis.

Arabidopsis VQ motif-containing proteins VQ1 and VQ10 interact with plastidial 1-deoxy-D-xylulose-5-phosphate synthase

VQ1 and VQ10 are largely unstructured homologous proteins with a significant potential for protein-protein interactions. Yeast two-hybrid (Y2H) analysis confirmed that both proteins interact not only with themselves and each other but also with other VQ and WRKY proteins. Screening an Arabidopsis Y2H library with VQ1 as bait identified 287 interacting proteins. Validation of the screening confirmed that interactions with VQ1 also occurred with VQ10, supporting their functional homology. Although VQ1 or VQ10 proteins do not localize in plastids, 47 VQ1-targets were found to be plastidial proteins. In planta interaction with the isoprenoid biosynthetic enzyme 1-deoxy-D-xylulose-5-phosphate synthase (DXS) was confirmed by co-immunoprecipitation. DXS oligomerizes through redox-regulated intermolecular disulfide bond formation, and the interaction with VQ1 or VQ10 do not involve their unique C residues. The VQ-DXS protein interaction did not alter plastid DXS localization or its oligomerization state. Although plants with enhanced or reduced VQ1 and VQ10 expression did not exhibit significantly altered levels of isoprenoids compared to wild-type plants, they did display significantly improved or diminished photosynthesis efficiency, respectively.

Single-cell transcriptome atlases of soybean root and mature nodule reveal new regulatory programs that control the nodulation process

The soybean root system is complex. In addition to being composed of various cell types, the soybean root system includes the primary root, the lateral roots, and the nodule, an organ in which mutualistic symbiosis with N-fixing rhizobia occurs. A mature soybean root nodule is characterized by a central infection zone where atmospheric nitrogen is fixed and assimilated by the symbiont, resulting from the close cooperation between the plant cell and the bacteria. To date, the transcriptome of individual cells isolated from developing soybean nodules has been established, but the transcriptomic signatures of cells from the mature soybean nodule have not yet been characterized. Using single-nucleus RNA-seq and Molecular Cartography technologies, we precisely characterized the transcriptomic signature of soybean root and mature nodule cell types and revealed the co-existence of different sub-populations of B. diazoefficiens-infected cells in the mature soybean nodule, including those actively involved in nitrogen fixation and those engaged in senescence. Mining of the single-cell-resolution nodule transcriptome atlas and the associated gene co-expression network confirmed the role of known nodulation-related genes and identified new genes that control the nodulation process. For instance, we functionally characterized the role of GmFWL3, a plasma membrane microdomain-associated protein that controls rhizobial infection. Our study reveals the unique cellular complexity of the mature soybean nodule and helps redefine the concept of cell types when considering the infection zone of the soybean nodule.

Seed longevity is controlled by metacaspases

To survive extreme desiccation, seeds enter a period of quiescence that can last millennia. Seed quiescence involves the accumulation of protective storage proteins and lipids through unknown adjustments in protein homeostasis (proteostasis). Here, we show that mutation of all six type-II metacaspase (MCA-II) proteases in Arabidopsis thaliana disturbs proteostasis in seeds. MCA-II mutant seeds fail to restrict the AAA ATPase CELL DIVISION CYCLE 48 (CDC48) at the endoplasmic reticulum to discard misfolded proteins, compromising seed storability. Endoplasmic reticulum (ER) localization of CDC48 relies on the MCA-IIs-dependent cleavage of PUX10 (ubiquitination regulatory X domain-containing 10), the adaptor protein responsible for titrating CDC48 to lipid droplets. PUX10 cleavage enables the shuttling of CDC48 between lipid droplets and the ER, providing an important regulatory mechanism sustaining spatiotemporal proteolysis, lipid droplet dynamics, and protein homeostasis. In turn, the removal of the PUX10 adaptor in MCA-II mutant seeds partially restores proteostasis, CDC48 localization, and lipid droplet dynamics prolonging seed lifespan. Taken together, we uncover a proteolytic module conferring seed longevity.

FvUVI4 inhibits cell division and cell expansion to modulate fruit development in Fragaria vesca

Fruit development is mainly regulated by cell division and expansion. As a negative regulator of the anaphase-promoting complex/cyclosome, UVI4 plays important roles in plant growth and development via coordinating cell cycle. However, currently there is no report on UVI4's functions in regulating fruit development in strawberry. Here, Fragaria vesca homolog FvUVI4 is identified and localizes in the nucleus. FvUVI4 has high gene expression in roots, leaves, flower, buds and green fruits, and low expression in petiole, stem, white and yellow fruit. Fruit development of F. vesca 'Hawaii4' is regulated by endoreduplication, and the expression of FvUVI4 is negatively correlated with fruit cell size. Overexpression of FvUVI4 inhibits endoreduplication of leaves, flowers and fruits in both Arabidopsis and F. vesca 'Hawaii4', thereby limiting cell expansion and decreasing cell area. Overexpression of FvUVI4 also inhibits mitotic cell cycle leading to decreased cell number, and ultimately affects the growth of leaves, petals and seeds or fruits. Arabidopsis uvi4 mutants obtained via CRISPR-Cas9 technology display opposite growth phenotypes to Arabidopsis and F. vesca 'Hawaii4' overexpression lines, which can be restored by overexpression of FvUVI4 in Arabidopsis uvi4 mutants. In conclusion, our study indicates that FvUVI4 inhibits cell expansion and cell division to modulate receptacle development in woodland strawberry.

Peroxisomal 4-coumaroyl-CoA ligases participate in shikonin production in Lithospermum erythrorhizon

4-Coumaroyl-CoA ligase (4CL) is a key enzyme in the phenylpropanoid pathway, which is involved in the biosynthesis of various specialized metabolites such as flavonoids, coumarins, lignans, and lignin. Plants have several 4CLs showing divergence in sequence: Class I 4CLs involved in lignin metabolism, Class II 4CLs associated with flavonoid metabolism, and atypical 4CLs and 4CL-like proteins of unknown function. Shikonin, a Boraginaceae-specific specialized metabolite in red gromwell (Lithospermum erythrorhizon), is biosynthesized from p-hydroxybenzoic acid, and the involvement of 4CL in its biosynthesis has long been debated. In this study, we demonstrated the requirement of 4CL for shikonin biosynthesis using a 4CL-specific inhibitor. In silico analysis of the L. erythrorhizon genome revealed the presence of at least 8 4CL genes, among which the expression of 3 (Le4CL3, Le4CL4, and Le4CL5) showed a positive association with shikonin production. Phylogenetic analysis indicated that Le4CL5 belongs to Class I 4CLs, while Le4CL3 and Le4CL4 belong to clades that are distant from Class I and Class II. Interestingly, both Le4CL3 and Le4CL4 have peroxisome targeting signal 1 in their C-terminal region, and subcellular localization analysis revealed that both localize to the peroxisome. We targeted each of the 3 Le4CL genes by CRISPR/Cas9-mediated mutagenesis and observed remarkably lower shikonin production in Le4CL3-ge and Le4CL4-ge genome-edited lines compared with the vector control. We, therefore, conclude that peroxisomal Le4CL3 and Le4CL4 are responsible for shikonin production and propose a model for metabolite-specific 4CL distribution in L. erythrorhizon.

A multifaceted kinase axis regulates plant organ abscission through conserved signaling mechanisms

Plants have evolved mechanisms to abscise organs as they develop or when exposed to unfavorable conditions.1 Uncontrolled abscission of petals, fruits, or leaves can impair agricultural productivity.2,3,4,5 Despite its importance for abscission progression, our understanding of the IDA signaling pathway and its regulation remains incomplete. IDA is secreted to the apoplast, where it is perceived by the receptors HAESA (HAE) and HAESA-LIKE2 (HSL2) and somatic embryogenesis receptor kinase (SERK) co-receptors.6,7,8,9 These plasma membrane receptors activate an intracellular cascade of mitogen-activated protein kinases (MAPKs) by an unknown mechanism.10,11,12 Here, we characterize brassinosteroid signaling kinases (BSKs) as regulators of floral organ abscission in Arabidopsis. BSK1 localizes to the plasma membrane of abscission zone cells, where it interacts with HAESA receptors to regulate abscission. Furthermore, we demonstrate that YODA (YDA) has a leading role among other MAPKKKs in controlling abscission downstream of the HAESA/BSK complex. This kinase axis, comprising a leucine-rich repeat receptor kinase, a BSK, and an MAPKKK, is known to regulate stomatal patterning, early embryo development, and immunity.10,13,14,15,16 How specific cellular responses are obtained despite signaling through common effectors is not well understood. We show that the identified abscission-promoting allele of BSK1 also enhances receptor signaling in other BSK-mediated pathways, suggesting conservation of signaling mechanisms. Furthermore, we provide genetic evidence supporting independence of BSK1 function from its kinase activity in several developmental processes. Together, our findings suggest that BSK1 facilitates signaling between plasma membrane receptor kinases and MAPKKKs via conserved mechanisms across multiple facets of plant development.

Knockout of endoplasmic reticulum-localized molecular chaperone HSP90.7 impairs seedling development and cellular auxin homeostasis in Arabidopsis

The Arabidopsis endoplasmic reticulum-localized heat shock protein HSP90.7 modulates tissue differentiation and stress responses; however, complete knockout lines have not been previously reported. In this study, we identified and analyzed a mutant allele, hsp90.7-1, which was unable to accumulate the HSP90.7 full-length protein and showed seedling lethality. Microscopic analyses revealed its essential role in male and female fertility, trichomes and root hair development, proper chloroplast function, and apical meristem maintenance and differentiation. Comparative transcriptome and proteome analyses also revealed the role of the protein in a multitude of cellular processes. Particularly, the auxin-responsive pathway was specifically downregulated in the hsp90.7-1 mutant seedlings. We measured a much-reduced auxin content in both root and shoot tissues. Through comprehensive histological and molecular analyses, we confirmed PIN1 and PIN5 accumulations were dependent on the HSP90 function, and the TAA-YUCCA primary auxin biosynthesis pathway was also downregulated in the mutant seedlings. This study therefore not only fulfilled a gap in understanding the essential role of HSP90 paralogs in eukaryotes but also provided a mechanistic insight on the ER-localized chaperone in regulating plant growth and development via modulating cellular auxin homeostasis.

BRZ-INSENSITIVE-LONG HYPOCOTYL8 inhibits kinase-mediated phosphorylation to regulate brassinosteroid signaling

Glycogen synthase kinase 3 (GSK3) is an evolutionarily conserved serine/threonine protein kinase in eukaryotes. In plants, the GSK3-like kinase BRASSINOSTEROID-INSENSITIVE 2 (BIN2) functions as a central signaling node through which hormonal and environmental signals are integrated to regulate plant development and stress adaptation. BIN2 plays a major regulatory role in brassinosteroid (BR) signaling and is critical for phosphorylating/inactivating BRASSINAZOLE-RESISTANT 1 (BZR1), also known as BRZ-INSENSITIVE-LONG HYPOCOTYL 1 (BIL1), a master transcription factor of BR signaling, but the detailed regulatory mechanism of BIN2 action has not been fully revealed. In this study, we identified BIL8 as a positive regulator of BR signaling and plant growth in Arabidopsis (Arabidopsis thaliana). Genetic and biochemical analyses showed that BIL8 is downstream of the BR receptor BRASSINOSTEROID-INSENSITIVE 1 (BRI1) and promotes the dephosphorylation of BIL1/BZR1. BIL8 interacts with and inhibits the activity of the BIN2 kinase, leading to the accumulation of dephosphorylated BIL1/BZR1. BIL8 suppresses the cytoplasmic localization of BIL1/BZR1, which is induced via BIN2-mediated phosphorylation. Our study reveals a regulatory factor, BIL8, that positively regulates BR signaling by inhibiting BIN2 activity.

Development of pENTR-NeCo-lacZα vectors for the preparation of negative control constructs in Gateway cloning

Gateway cloning is a useful technology for the simple and reliable preparation of various construct in many organisms. However, there is a problem regarding the negative control construct in the Gateway cloning system. In this study, we developed the pENTR-NeCo-lacZα vector system to create an empty vector that can be used as a negative control construct in Gateway cloning.

Duplication and neofunctionalization of a horizontally transferred xyloglucanase as a facet of the Red Queen coevolutionary dynamic

Oomycete protists share phenotypic similarities with fungi, including the ability to cause plant diseases, but branch in a distant region of the tree of life. It has been suggested that multiple horizontal gene transfers (HGTs) from fungi-to-oomycetes contributed to the evolution of plant-pathogenic traits. These HGTs are predicted to include secreted proteins that degrade plant cell walls, a barrier to pathogen invasion and a rich source of carbohydrates. Using a combination of phylogenomics and functional assays, we investigate the diversification of a horizontally transferred xyloglucanase gene family in the model oomycete species Phytophthora sojae. Our analyses detect 11 xyloglucanase paralogs retained in P. sojae. Using heterologous expression in yeast, we show consistent evidence that eight of these paralogs have xyloglucanase function, including variants with distinct protein characteristics, such as a long-disordered C-terminal extension that can increase xyloglucanase activity. The functional variants analyzed subtend a phylogenetic node close to the fungi-to-oomycete transfer, suggesting the horizontally transferred gene was a bona fide xyloglucanase. Expression of three xyloglucanase paralogs in Nicotiana benthamiana triggers high-reactive oxygen species (ROS) generation, while others inhibit ROS responses to bacterial immunogens, demonstrating that the paralogs differentially stimulate pattern-triggered immunity. Mass spectrometry of detectable enzymatic products demonstrates that some paralogs catalyze the production of variant breakdown profiles, suggesting that secretion of variant xyloglucanases increases efficiency of xyloglucan breakdown as well as diversifying the damage-associated molecular patterns released. We suggest that this pattern of neofunctionalization and the variant host responses represent an aspect of the Red Queen host-pathogen coevolutionary dynamic.