Plant And Cell Physiology

Floral bicolor pigmentation is caused by naturally occurring RNA interference (RNAi) in some cultivars of petunia and dahlia. In both plants, the chalcone synthase gene is highly expressed only in the pigmented region of bicolor petals. However, it remains unknown why RNAi is induced only in the unpigmented region. To elucidate the mechanism of this clear bicolor pattern formation, we examined the dicing activity of Dicer-like 4 (DCL4), which produces small interfering RNAs essential for the induction of RNAi. We showed that the crude extract in the pigmented petal region inhibits dicing activity of DCL4, but not when flavonoids were depleted from the extract. Moreover, we showed the inhibitory activity was associated with flavonoid aglycons. The in vivo dicing activities were detected in the intact protoplasts prepared from the unpigmented region but not from the pigmented region. These results suggest that in the unpigmented region, flavonoids that inhibit DCL4 are not synthesized, and RNAi is maintained, whereas in the pigmented region, DCL4 is inhibited by flavonoids, RNAi is not induced, and anthocyanin biosynthesis is maintained, which ensures RNAi inhibition. Therefore, a clear bicolor pattern is generated by the bidirectional feedforward mechanism of antagonizing DCL4 and flavonoids.

Plants have the remarkable ability to regenerate whole organisms through formation of pluripotent cell masses from somatic cells. Cellular programs leading to fate change of somatic to pluripotent cells resembles lateral root (LR) formation and both are chiefly regulated by auxin. Brassinosteroid signalling also plays an important role during LR formation but little is known about the direct link between auxin and brassinosteroid components, such as BZR1 and BES1, in relation to pluripotency acquisition. Here we show that gain-of-function mutants bzr1-D and bes1-D exhibit altered callus formation, yet disruption of these transcription factors does not produce major changes to callus formation or de novo organogenesis. Moreover, our data reveals that BZR1 displays enhanced expression in callus tissue and directly binds to the promoters of ARF7 and ARF19, two master pluripotency regulators, leading to their enhanced transcription. Remarkably, we see abrogation of callus formation in bzr1-D upon disruption of ARF7 and ARF19, emphasizing that BZR1 callus phenotype is dependent on these two auxin signalling components. In conclusion, we depict a link between ARF7, ARF19 and BZR1 in the promotion of pluripotency acquisition, portraying BZR1 as a major supporting factor in callus formation. IMPORTANTO_LIManuscripts submitted to Review Commons are peer reviewed in a journal-agnostic way. C_LIO_LIUpon transfer of the peer reviewed preprint to a journal, the referee reports will be available in full to the handling editor. C_LIO_LIThe identity of the referees will NOT be communicated to the authors unless the reviewers choose to sign their report. C_LIO_LIThe identity of the referee will be confidentially disclosed to any affiliate journals to which the manuscript is transferred. C_LI GUIDELINESO_LIFor reviewers: https://www.reviewcommons.org/reviewers C_LIO_LIFor authors: https://www.reviewcommons.org/authors C_LI CONTACTThe Review Commons office can be contacted directly at: office@reviewcommons.org

Optimization of antenna size of photosynthetic systems is one strategy to increase plant canopy photosynthesis and crop yield potential. The relationship between antenna size and photosynthesis rate has been extensively studied recently. However, conflicting results have been obtained. Here we show that the extent of decrease in antenna size is a major factor determining the consequences of decreasing antenna on photosynthesis and growth-related parameters. Specifically, we constructed transgenic rice lines with artificial microRNA (amiRNA) targeting to Chlorophyll Synthesis (YGL1) to generate transgene heterozygous and homozygous lines with different leaf chlorophyll contents and antenna sizes. We found that canopy photosynthesis (Ac), biomass and grain yield of the heterozygote were not significantly different from those of WT while the Ac, biomass and grain yield of the homozygote were lower than those of WT. Further, when the maximal quantum yield of photosystem II (Fv/Fm) was larger than 0.8, decreasing antenna size by reducing chlorophyll biosynthesis did not affect leaf photosynthesis; but when Fv/Fm was lower than 0.8, there is a positive relationship between antenna size and leaf photosynthesis. There is large variation in both leaf chlorophyll content and antenna size in elite rice cultivars, suggesting that there is a large scope to decrease leaf chlorophyll content to increase nitrogen use efficiency as long as the quantum yield of PSII is not compromised.

In competition for limited resources, many plants release allelochemicals to inhibit the growth of neighboring plants. Momilactone B (MB) is a major allelochemical produced by rice (Oryza sativa), however its mode of action is currently unknown. We used Arabidopsis (Arabidopsis thaliana) as a model system to evaluate potential mechanisms underlying the inhibitory effects of MB on seed germination, seedling establishment and root growth through the use of confocal microscopy and the examination of transcriptional responses in MB-treated seedlings. In response to MB treatment, transcript levels for genes encoding several key ABA biosynthetic enzymes and signaling components, including the transcription factor ABA-INSENSITIVE 4 (ABI4), were dramatically increased. Additionally, ABA insensitive 4 (abi4) mutant seedlings exhibited reduced susceptibility to exogenously-provided MB. Although the transcript levels of DELLA genes, which negatively regulate GA signaling, were significantly increased upon MB exposure, exogenous GA application did not reverse the inhibitory effects of MB on Arabidopsis germination and seedling development. Moreover, a reduction in seedling root meristematic activity, associated with reduced expression of auxin biosynthetic genes and efflux transporters, and apparent lowered auxin content, was observed in MB-treated root tips. Exogenous auxin applications partially rescued the inhibitory effects of MB observed in root growth. Our results indicate that MB suppresses Arabidopsis seed germination and root growth primarily via disruption of ABA and auxin signaling. These findings underscore the crucial roles played by phytohormones in mediating responses to allelochemical exposure. One-sentence summaryMomilactone B, the key allelochemical of rice, inhibits Arabidopsis growth and development via disruption of ABA and auxin signaling, suggesting the crucial roles of phytohormones in plant allelopathy

Small molecules, peptides and miRNAs, are the crucial regulators of plant growth. Here, we show the importance of cross-talk between miPEP858a/miR858a and Phytosulfokine (PSK4) in regulating plant growth and development in Arabidopsis. Genome-wide expression analysis suggested modulated expression of PSK4 in miR858 mutant and overexpression, miR858OX, plants. The silencing of PSK4 in miR858OX plants compromised the growth, whereas over-expression of PSK4 in miR858 mutant rescued the developmental defects. The exogenous application of synthetic PSK4 further complemented the plant development in mutant plants. Exogenous treatment of synthetic miPEP858a in PSK4 mutant led to clathrin-mediated internalization of the peptide however did not enhance growth as in the case of wild-type plants. We also demonstrate that the MYB3 is an important molecular component participating in miPEP858a/miR858a-PSK4 module. Finally, our work highlights the signalling between miR858/miPEP858-MYB3-PSK4 in modulating the expression of key elements involved in auxin responses leading to the regulation of growth. One-sentence summarySignaling network between small molecules, miPEP858a/miR858a and phytosulfokine, regulates plant growth in Arabidopsis.

Plants possess remarkable regenerative abilities to form de novo vasculature after damage and in response to pathogens that invade and withdraw nutrients. To look for common factors that affect vascular formation upon stress, we searched for Arabidopsis thaliana genes differentially expressed during Agrobacterium infection, nematode infection and plant grafting. One such gene was cell-wall associated and highly induced by all three stresses. Mutations in it enhanced ectopic xylem formation in Vascular cell Induction culture System Using Arabidopsis Leaves (VISUAL) and enhanced graft formation and was thus named ENHANCER OF VISUAL AND GRAFTING 1 (EVG1). Mutated evg1 inhibited cambium development and callus formation yet promoted tissue attachment, syncytium size, phloem reconnection and xylem formation. evg1 affected abscisic acid and cell wall responses and was itself down regulated by ABA. We found mutations in a receptor-like gene, RLP44, had the same regeneration phenotype as EVG1 mutations including enhancing VISUAL and grafting. evg1 and rlp44 mutants affected the expression of many genes in common including those important for successful regeneration and vascular formation. We propose that EVG1 integrates information from cutting, wounding or parasitism stresses and functions with RLP44 to suppress vascular differentiation during regeneration.

The RETINOBLASTOMA-RELATED (RBR) protein in plants functions as a cell-cycle inhibitor, regulating cell numbers in developing organs and establishing cellular quiescence during growth. Although the role of RBR counterparts in animals also involves regulating cell size, this potential function remains unexplored in plants. We investigated transgenic Arabidopsis plants with altered RBR levels and observed corresponding changes in cell size from embryogenesis through organ development. In addition, stomatal meristemoid cells with reduced RBR levels divided beyond the size threshold, whereas elevated RBR levels increased their size. RBR stimulated terminal differentiation in the stomatal lineage by inducing MUTE and CYCLIN D5;1 expression, whereas reduced RBR levels maintained asymmetric divisions through high SPEECHLESS and CYCLIN D3;1 expression. Interestingly, the cell proliferation-dependent phosphorylation of RBR at the conserved 911Ser site positively correlated with RBR protein levels in the transgenic lines and aligned with the effect of RBR on cell size. This study discusses the potential link between RBRs control of cell proliferation and cell size, providing new insights into the coordinated regulation of plant development.

Brassinosteroid (BR) hormones regulate various physiological and developmental processes in plants. BR signaling is primarily influenced by the plasma membrane abundance of the BR receptor BR INSENSITIVE1 (BRI1), a process regulated by ubiquitination, endocytosis, and protein degradation. Despite extensive research, only a few negative regulators of BRI1 internalization and ubiquitination have been identified. In this study, we show that the conserved eukaryotic regulatory proteins 14-3-3 directly interact with BRI1 at Threonine 872 (T872) within its juxtamembrane domain. Furthermore, phosphorylation at Serine 858 (S858) in BRI1s juxtamembrane domain enhances T872 phosphorylation, facilitating 14-3-3 protein binding. Consequently, by inhibiting BRI1 ubiquitination without affecting its kinase activity or BAK1 interaction, 14-3-3 binding increased BRI1 plasma membrane abundance and enhanced BR signaling. Both non-epsilon and epsilon isoforms of 14-3-3 proteins contribute to the regulation of BRI1 and, consequently, to plant responsiveness to BRs. Our results revealed a previously undescribed function of 14-3-3 proteins in regulating BRI1 stability. One Sentence Summary14-3-3s stabilize BRI1 by antagonizing its ubiquitination

Genome editing and in vitro based-plant propagation require efficient plant regeneration system. Somatic embryogenesis (SE) or de novo shoot regeneration are two major systems that widely used for plant in vitro regeneration. Most SE or shoot regeneration protocols rely on the exogenous application of the synthetic auxin analog 2,4-dichlorophenoxyacetic acid (2,4-D) and naphthylene acetic acid (NAA), whereas the natural auxins indole-3-acetic acid (IAA), 4-chloroindole-3-acetic acid (4-Cl-IAA) or indole-3-butyric acid (IBA) are not or less effective for plant regeneration. Although these synthetic auxins mimics the physiological activity of the main natural auxin IAA in many aspects, there are also clear differences that have been attributed to differences in stability or to different affinities for certain TIR1/AFB-Aux/IAA auxin co-receptor pairs. Here we show that the success of 2,4-D in inducing SE from Arabidopsis is related to ineffectiveness as substrate for auxin efflux, resulting in its intracellular 2,4-D accumulation. Reducing auxin efflux by addition of the auxin transport inhibitor naphthylphthalamic acid (NPA) also allowed natural auxins and other synthetic analogs to induce SE in Arabidopsis with similar efficiencies as 2,4-D. The PIN-FORMED auxin efflux carriers PIN1, PIN2 and the ATP-binding cassette-B auxin transporters ABCB1 and ABCB19 were shown to be partially responsible for the efflux of natural auxins during SE induction. Importantly, all somatic embryos induced in Arabidopsis by IAA in the presence of NPA showed a normal embryo to seedling conversion and subsequent plant development, whereas for the 2,4-D system this was limited to 50-60% of the embryos. We showed that the auxin transport inhibition promotes de novo shoot regeneration capacity from callus induced by 4-Cl-IAA in Brassica napus. In addition, we observed a obvious acceleration in shoot bud emerging from callus induced by 4-Cl-IAA than 2,4-D. Based on our data we conclude, that the efficiency of plant propagation can be significantly improved by applying the natural auxins in the presence of the auxin transport inhibitor NPA.

Seed maturation is an important plant developmental process that follows embryo development. It is associated with a series of physiological changes such as the establishment of desiccation tolerance, seed longevity and seed dormancy. However, the translational dynamics associated with seed maturation, especially its connection with seed germination remains largely elusive. Here transcriptome and translatome profiling were performed during seed maturation. During seed maturation we observed a gradual disappearance of polysomes and a relative increase of monosomes, indicating a gradual reduction of global translation. Comparing the levels of polysomal associated mRNAs with total mRNA levels showed that thousands of genes are translationally regulated at early sates of maturation, as judged by dramatic changes in polysomal occupancy. By including previous published data from germination and seedling establishment, a translational regulatory network: SeedTransNet was constructed. Network analysis identified hundreds of gene modules with distinct functions and transcript sequence features indicating the existence of separate translational regulatory circuits possibly acting through specific regulatory elements. The regulatory potential of one such element was confirmed in vivo. The network identified several seed maturation associated genes as central nodes, and we could confirm the importance of many of these hub genes with a maturation associated seed phenotype by mutant analysis. One of the identified regulators an AWPM19 family protein PM19-Like1 (PM19L1) was shown to regulate seed dormancy and longevity. This putative RBP also affects the transitional regulation of one its, by the SeedTransNet identified, target mRNAs. Our data shows the usefulness of SeedTransNet in identifying regulatory pathways during seed phase transitions.

O_LIDinucleoside polyphosphates (NpnNs) are considered novel signalling molecules involved in the induction of plant defence mechanisms. However, the NpnNs signal recognition and transduction are still enigmatic. Here we report, for the first time, that diadenosine tetraphosphate (Ap4A) is recognized by the Arabidopsis thaliana purinoreceptor P2K1/DORN1 (Does Not Respond to Nucleotides 1) and causes stomatal closure. C_LIO_LIExtracellular Ap4A- and dicytidine tetraphosphate (Cp4C)-induced stomatal closure was observed using a microscope. Reactive oxygen species (ROS) accumulation was determined by staining with nitroblue tetrazolium (NBT) and 3,3'-diaminobenzidine tetrahydrochloride (DAB). Transcriptional changes were determined by quantitative real-time PCR. Wild-type Col-0 and the dorn1-3 A. thaliana knockout mutant were used. C_LIO_LIExamination of the leaf epidermis dorn1-3 mutant provided evidence that P2K1/DORN1 recognizes extracellular Ap4A but not Cp4C. ROS are involved in signal transduction caused by Ap4A and Cp4C, leading to stomatal closure. Ap4A induced and Cp4C suppressed the transcriptional response in wild-type plants. Moreover, in dorn1-3 leaves, the effect of Ap4A on gene expression was impaired. C_LIO_LIOur research demonstrated, for the first time, that P2K1/DORN1 is a plant purinoreceptor for Ap4A. This interaction leads to changes in the transcription of signalling hubs in signal transduction pathways. C_LI

O_LIReactive oxygen species (ROS) regulate plant growth, development, and responses to the environment. ROS production by the RESPIRATORY BURST OXIDASE PROTEIN D (RBOHD) protein was recently shown to be regulated by PHYTOCHROME B (phyB), and phyB was found to be phosphorylated by FERONIA, highlighting the possibility that these three proteins interact to regulate ROS levels during stress. C_LIO_LIImmunoprecipitation and proximity labelling, followed by split-luciferase and functional validation assays, were used to study the interactions between FERONIA, phyB, and RBOHD during excess light (EL) stress in Arabidopsis thaliana. C_LIO_LIWe reveal that phyB and FERONIA interact with RBOHD, that phosphorylation of phyB by FERONIA, as well as the kinase activity of FERONIA, are required for RBOHD-driven ROS production in response to EL stress, and that CYSTEINE-RICH RECEPTOR LIKE KINASE 10 (CRK10) and PLASMA MEMBRANE INTRINSIC PROTEIN 2;6 (PIP2;6) interact with RBOHD and phyB and are also required for EL-driven RBOHD ROS production. C_LIO_LIOur findings uncover the existence of a putative plasma membrane complex between FERONIA, RBOHD, CRK10, and PIP2;6 that interacts with phyB to regulate ROS production in Arabidopsis in response to stress. This complex could play a canonical role in the integration and regulation of multiple signaling pathways in plants. C_LI Plain Language SummaryWe identified a complex between several different proteins at the plasma membrane that interacts with the light and temperature receptor protein phytochrome B to regulate reactive oxygen species formation during stress in plants. This complex could be involved in the regulation and integration of multiple abiotic and biotic signals in plants. tenten

Brassinosteroids (BRs) are steroid hormones identified in plants. Besides promoting cell elongation and division, BRs facilitate the development of both male and female reproductive tissues. In animals, reproductive steroid hormones play an essential role in reproductive tissue development by regulating gene expression. Here, we focused on the function of BRs during fertilization. We measured the content of biologically active BRs, brassinolide (BL) and castasterone (CS), in the reproductive tissues of Arabidopsis thaliana. Both BL and CS accumulated abundantly in pollen grains and in larger amounts in pistils than in leaves. To evaluate BL function during fertilization, we used an in vitro guidance assay with exogenously applied BL. Although pollen tubes need to be elongated through the pistils for efficient capacitation, BL treatment promoted pollen tube capacitation and improved attraction to ovules in vitro. Transcriptome analysis demonstrated that BL treatment induced the expression of half of the genes expressed in pollen tubes that elongated through the pistils. These results indicated that BL supplied from pistils is a key factor for pollen tube capacitation. However, using the bri1 mutant for the guidance assay resulted in reduced pollen tube capacitation, suggesting that BRI1-signaling in pistils is also important. Furthermore, BRs act on ovules. Exogenous BL application to ovules maintained guidance capacity by promoting the expression of small secreted proteins involved in pollen tube attraction and gamete fusion. Overall, BRs play a significant role as male and female reproductive hormones throughout the plant fertilization process.

The transcription factor ERF109 acts as a crosstalk node between jasmonic acid signaling and auxin biosynthesis by directly regulating YUC2 and ASA1 during lateral root formation in Arabidopsis. However, whether ERF109 regulates the auxin transport remains unclear. Here we report a mechanism of ERF109-mediated auxin transport in root system. Through root transcriptome comparison between erf109, wild type, and 35S:ERF109, we found that the genes PIN2 and PIN4, encoding the major membrane-based efflux carriers of auxin, were enriched in the overexpression line. In the promoters of these auxin transport genes, GCC-box cis elements were found and potentially bound by ERF109. Moreover, PID, encoding a key regulator in polar auxin transport, was found upregulated in 35S:ERF109 and down regulated in erf109. Yeast-one-hybrid and chromatin immunoprecipitation assays showed that ERF109 directly bound to the GCC-box of PIN2, PIN4, and PID. Genetic analyses with double mutants confirmed the function of ERF109 in the regulation of auxin transport in Arabidopsis roots. Taken together, our results show that ERF109 modulates auxin transport by directly regulating PIN2, PIN4 and PID. This ERF109-mediated auxin transport likely works together with ERF109-mediated auxin synthesis to establish auxin maxima for lateral root initiation.

- Plant architecture is controlled by several endogenous signals including hormones and sugars. However, only little is known about the nature and roles of the sugar signalling pathways in this process. Here we test whether the sugar pathway mediated by HEXOKINASE1 (HXK1) is involved in the control of shoot branching. - To test the involvement of HXK1 in the control of shoot architecture we modulated the HXK1 pathway using physiological and genetic approaches in diverse plants, rose, arabidopsis and pea and evaluated impacts of hormonal pathways. - We show that triggering a hexokinase-dependent pathway was able to promote bud outgrowth in pea and rose. In arabidopsis, both HXK1 deficiency and defoliation led to decreased shoot branching and conferred hypersensitivity to auxin. HXK1 expression was positively correlated with sugar availability. HXK1-deficient plants displayed decreased cytokinin levels and increased expression of MAX2 which is required for strigolactone signalling. The branching phenotype of HXK1-deficient plants could be partly restored by cytokinin treatment and strigolactone deficiency could override the negative impact of HXK1 deficiency on shoot branching. - Our observations demonstrate that a HXK1-dependent pathway contributes to the regulation of shoot branching and interact with hormones to modulate plant architecture.

Actin dynamic is critical for cell morphogenesis in plants, but the signaling mechanisms underlying its regulation are not well understood. Here we found PRL1 (Pleiotropic Regulatory Locus1) modulates leaf pavement cell (PC) morphogenesis in Arabidopsis by maintaining the dynamic homeostasis of actin microfilaments (MF). Our previous studies indicated PC shape formation was mediated by the counteracting ROP2 and ROP6 signaling pathways that promote the organization of cortical MF and microtubules (MT), respectively. Our genetic screen for ROP6 enhancers identified prl1 alleles. Genetic analysis suggested that prl1 acted synergistically with ROP2 and ROP6 in regulation of PC morphogenesis. We further found that the activities of ROP2 and ROP6 were increased and decreased in prl1 mutants, respectively. Interestingly prl1 was found to prefer to depolymerize MF independent of ROP2 and ROP6. Stress (high salinity and low temperature) induced similar changes of ROP activities as do prl1 mutations. Together our findings provided evidence that PRL1 governed two signaling pathways that counteractively maintain actin dynamics and resultant cell morphogenesis.

O_LIIn leaves of C4 plants the reactions of photosynthesis become restricted between two compartments. Typically, this allows accumulation of C4 acids in mesophyll cells and subsequent decarboxylation in the bundle sheath. In C4 grasses proliferation of plasmodesmata between these cell types is thought to increase cell-to-cell connectivity to allow efficient metabolite movement. However, it is not known if C4 dicotyledons also show this enhanced plasmodesmal connectivity and so whether this is a general requirement for C4 photosynthesis is not clear. How mesophyll and bundle sheath cells in C4 leaves become highly connected is also not known. C_LIO_LIWe investigated these questions using 3D- and 2D- electron microscopy on the C4 dicotyledon Gynandropsis gynandra as well as phylogenetically close C3 relatives. C_LIO_LIThe mesophyll-bundle sheath interface of C4 G. gynandra showed higher plasmodesmal frequency compared with closely related C3 species. Formation of these plasmodesmata was induced by light. Pharmacological agents that perturbed chloroplast development or photosynthesis reduced the number of plasmodesmata, but this inhibitory effect could be reversed by the provision of exogenous sucrose. C_LIO_LIWe conclude that enhanced formation of plasmodesmata between mesophyll and bundle sheath cells is wired to the induction of photosynthesis in C4 G. gynandra. C_LI

The phytohormone JA-Ile regulates many stress responses and developmental processes in plants. A co-receptor complex formed by the F-box protein COI1 (Coronatine Insensitive 1) and a JAZ (Jasmonate ZIM-domain) repressor perceives the hormone. JA-Ile antagonists are invaluable tools for exploring the role of JA-Ile in specific tissues and developmental stages, and for identifying regulatory processes of the signalling pathway. Using two complementary chemical screens, we identified three compounds that exhibit a robust inhibitory effect on both the hormone-mediated COI-JAZ interaction and degradation of JAZ1 and JAZ9 in vivo. One molecule, J4, also restrains specific JA-induced physiological responses in different angiosperm plants, including JA-mediated gene expression, growth inhibition, chlorophyll degradation and anthocyanin accumulation. Interaction experiments with purified proteins indicate that J4 directly interferes with the formation of the Arabidopsis (Arabidopsis thaliana) COI1-JAZ complex otherwise induced by JA. The antagonistic effect of J4 on COI1- JAZ also occurs in the liverwort Marchantia polymorpha, suggesting the mode of action is conserved in land plants. Besides JA signalling, J4 works as an antagonist of the closely-related auxin signalling pathway, preventing TIR1/Aux-IAA interaction and auxin responses in planta, including hormone-mediated degradation of an auxin repressor, gene expression and gravitropic response. However, J4 does not affect other hormonal pathways. Altogether, our results show that this dual antagonist competes with JA-Ile and auxin, preventing the formation of phylogenetically related receptor complexes. J4 may be a useful tool to dissect both the JA-Ile and auxin pathways in particular tissues and developmental stages since it reversibly inhibits these pathways. One sentence summaryA chemical screen identified a molecule that antagonizes jasmonate perception by directly interfering with receptor complex formation in phylogenetically distant vascular and non-vascular plants.

Triterpene saponins (TS) are a structurally diverse group of metabolites that are widely distributed in plants. They primarily serve as defense compounds and their production is often triggered by biotic stresses through signaling cascades that are modulated by phytohormones such as the jasmonates (JA). Two JA-modulated basic helix-loop-helix (bHLH) transcription factors (TFs), TRITERPENE SAPONIN BIOSYNTHESIS ACTIVATING REGULATOR 1 (TSAR1) and TSAR2, have been previously identified as direct activators of TS biosynthesis in the model legume Medicago truncatula. Here, we report on the involvement of the core endoplasmic reticulum (ER) stress basic leucine zipper (bZIP) TFs bZIP17 and bZIP60 in the regulation of TS biosynthesis. Expression and processing of M. truncatula bZIP17 and bZIP60 proteins was altered in roots with perturbed TS biosynthesis or treated with JA. Accordingly, such roots displayed an altered ER network structure. M. truncatula bZIP17 and bZIP60 proteins were shown to be capable of interfering with the TSAR-mediated transactivation of TS biosynthesis genes, particularly under ER stress conditions, when they translocate from the ER to the nucleus. Furthermore, the inhibitory role of ER stress bZIP TFs in the regulation of JA-dependent terpene biosynthetic pathways appears to be widespread in the plant kingdom, as we demonstrate that it also occurs in the regulation of monoterpene indole alkaloid biosynthesis in the medicinal plant Catharanthus roseus. We postulate that activation of ER stress bZIP TFs provides the plant with a mechanism to balance metabolic activities and thereby adequately govern modulation of growth, development and defense processes in defined stress situations. One sentence summaryER stress bZIP transcription factors can interfere with the activity of jasmonate-inducible bHLH transcription factors to modulate the elicitation of plant specialized metabolism in stress conditions.

O_LIThe plant hormone auxin and its directional intercellular transport plays a major role in diverse aspects of plant growth and development. The establishment of auxin gradients in plants requires asymmetric distribution of members of the auxin efflux carrier PIN-FORMED (PIN) protein family to the plasma membrane. An endocytic pathway regulates the recycling of PIN proteins between the plasma membrane and endosomes, providing a mechanism for dynamic localization. C_LIO_LIN-ethylmaleimide-sensitive factor adaptor protein receptors (SNAP receptors, SNAREs) mediate fusion between vesicles and target membranes and are classed as Q- or R-SNAREs based on their sequence. We analysed gain- and loss-of-function mutants, dominant negative transgenics and protein localization of the Arabidopsis R-SNARE VAMP714 to understand its function. C_LIO_LIWe demonstrate that VAMP714 is essential for the insertion of PINs into the plasmamembrane, for polar auxin transport, and for root gravitropism and morphogenesis. VAMP714 gene expression is upregulated by auxin, and the VAMP714 protein co-localizes with ER and Golgi vesicles and with PIN proteins at the plasma membrane. C_LIO_LIIt is proposed that VAMP714 mediates the delivery of PIN-carrying vesicles to the plasma membrane, and that this forms part of a positive regulatory loop in which auxin activates a VAMP714-dependent PIN/auxin transport system to control development. C_LI