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Fusarium wilt in tobacco caused by the fungus Fusarium oxysporum f. sp. nicotianae is a disease‑management challenge worldwide, as there are few effective and environmentally benign chemical agents for its control. This challenge results in substantial losses in both the quality and yield of tobacco products. Based on an in vitro analysis of the effects of different phenylpropanoid intermediates, we found that the early intermediates trans‑cinnamic acid and para‑coumaric acid effectively inhibit the mycelial growth of F. oxysporum f. sp. nicotianae strain FW316F, whereas the downstream intermediates quercetin and caffeic acid exhibit no fungicidal properties. Therefore, our in vitro screen suggests that trans‑cinnamic acid and para‑coumaric acid are promising chemical agents and natural lead compounds for the suppression of F. oxysporum f. sp. nicotianae growth.

Cytokinins control critical aspects of plant development and environmental responses. Perception of cytokinin ultimately leads to the activation of proteins belonging to the type-B Response Regulator family of cytokinin response activators. In Arabidopsis thaliana, ARR1 is one of the most abundantly expressed type-B Response Regulators. We investigated the link between cytokinin signaling, protein synthesis, plant growth and osmotic stress tolerance. We show that the increased cytokinin signaling in ARR1 gain-of-function transgenic lines is associated with increased rates of protein synthesis, which lead to growth inhibition and hypersensitivity to osmotic stress. Cytokinin-induced growth inhibition and osmotic stress hypersensitivity were rescued by treatments with ABA, a hormone known to inhibit protein synthesis. We also demonstrate that cytokinin-induced protein synthesis requires isoforms of the ribosomal protein L4 encoded by the cytokinin-inducible genes RPL4A and RPL4D, and that RPL4 loss-of-function increases osmotic stress tolerance and decreases sensitivity to cytokinin-induced growth inhibition. These findings reveal that an increase in protein synthesis negatively impacts growth and osmotic stress tolerance and explain some of the adverse effects of elevated cytokinin action on plant development and stress physiology.

Summary: The chloroplast is the main organelle for stress-induced production of reactive oxygen species (ROS). However, how chloroplastic ROS homeostasis is maintained under salt stress is largely unknown. We show that EGY3, a gene encoding a chloroplast-localized protein, is induced by salt and oxidative stresses. The loss of EGY3 function causes stress hypersensitivity while EGY3 overexpression increases the tolerance to both salt and chloroplastic oxidative stresses. EGY3 interacts with chloroplastic Cu/Zn-SOD2 (CSD2) and promotes CSD2 stability under stress conditions. In egy3-1 mutant plants, the stress-induced CSD2 degradation limits H2O2 production in chloroplasts and impairs H2O2-mediated retrograde signaling, as indicated by the decreased expression of retrograde-signal-responsive genes required for stress tolerance. Both exogenous application of H2O2 (or APX inhibitor) and CSD2 overexpression can rescue the salt-stress hypersensitivity of egy3-1 mutants. Our findings reveal that EGY3 enhances the tolerance to salt stress by promoting the CSD2 stability and H2O2-mediated chloroplastic retrograde signaling.

Duckweeds, quick-growing aquatic plants, have been recently recognized as promising hosts for the large-scale production of recombinant proteins and as an ideal biomass feedstock for biofuel production. These possible wide-spread industrial uses of duckweeds intensified research aimed at understanding the mechanisms that control duckweed growth. Here, we describe how the hormone cytokinin affects growth. We performed a number of standard cytokinin growth- and physiological-response assays using sterile-grown colonies of Lemna gibba and Spirodela polyrhiza. Similar to land plants, cytokinin inhibited root elongation in duckweeds. Surprisingly, and in contrast to land plants, cytokinin promoted growth of aerial organs in both duckweed species, suggesting that the cytokinin growth response fundamentally differs between aquatic and land plants.

The hormones auxin and cytokinin regulate numerous aspects of plant development and often act as an antagonistic hormone pair. One of the more striking examples of the auxin/cytokinin antagonism involves regulation of the shoot/root growth ratio in which cytokinin promotes shoot and inhibits root growth, whereas auxin does the opposite. Control of the shoot/root growth ratio is essential for the survival of terrestrial plants because it allows growth adaptations to water and mineral nutrient availability in the soil. Because a decrease in shoot growth combined with an increase in root growth leads to survival under drought stress and nutrient limiting conditions, it was not surprising to find that auxin promotes, while cytokinin reduces, drought stress tolerance and nutrient uptake. Recent data show that drought stress and nutrient availability also alter the cytokinin and auxin signaling and biosynthesis pathways and that this stress-induced regulation affects cytokinin and auxin in the opposite manner. These antagonistic effects of cytokinin and auxin suggested that each hormone directly and negatively regulates biosynthesis or signaling of the other. However, a growing body of evidence supports unidirectional regulation, with auxin emerging as the primary regulatory component. This master regulatory role of auxin may not come as a surprise when viewed from an evolutionary perspective.

The hormones auxin and cytokinin are essential for plant growth and development. Because of the central importance of root and shoot apical meristems in plant growth, auxin/cytokinin interactions have been predominantly analyzed in relation to apical meristem formation and function. In contrast, the auxin/cytokinin interactions during organ growth have remained largely unexplored. Here, we show that a specific interaction between auxin and cytokinin operates in both the root and the shoot where it serves as an additional determinant of plant development. We found that auxin at low concentrations limits the action of cytokinin. An increase in cytokinin level counteracts this inhibitory effect and leads to an inhibition of auxin signaling. At higher concentrations of both hormones, these antagonistic interactions between cytokinin and auxin are absent. Thus, our results reveal a bidirectional and asymmetrical interaction of auxin and cytokinin beyond the bounds of apical meristems. The relation is bidirectional in that both hormones exert inhibitory effects on each other's signaling mechanisms. However, this relation is also asymmetrical because under controlled growth conditions, auxin present in nontreated plants suppresses cytokinin signaling, whereas the reverse is not the case.

Background The phenylpropanoid pathway is responsible for the synthesis of numerous compounds important for plant growth and responses to the environment. In the first committed step of phenylpropanoid biosynthesis, the enzyme phenylalanine ammonia-lyase (PAL) deaminates L-phenylalanine into trans-cinnamic acid that is then converted into p-coumaric acid by cinnamate-4-hydroxylase (C4H). Recent studies showed that the Kelch repeat F-box (KFB) protein family of ubiquitin ligases control phenylpropanoid biosynthesis by promoting the proteolysis of PAL. However, this ubiquitin ligase family, alternatively named Kiss Me Deadly (KMD), was also implicated in cytokinin signaling as it was shown to promote the degradation of type-B ARRs, including the key response activator ARR1. Considering that ubiquitin ligases typically have narrow target specificity, this dual targeting of structurally and functionally unrelated proteins appeared unusual. Results Here we show that the KFBs indeed target PAL but not ARR1. Moreover, we show that changes in early phenylpropanoid biosynthesis alter cytokinin sensitivity – as reported earlier - but that the previously documented cytokinin growth response changes are primarily the result of altered auxin signaling. We found that reduced PAL accumulation decreased, whereas the loss of C4H function increased the strength of the auxin response. The combined loss of function of both enzymes led to a decrease in auxin sensitivity, indicating that metabolic events upstream of C4H control auxin sensitivity. This auxin/phenylpropanoid interaction impacts both shoot and root development and revealed an auxin-dependent stimulatory effect of trans-cinnamic acid feeding on leaf expansion and thus biomass accumulation. Conclusions Collectively, our results show that auxin-regulated plant growth is fine-tuned by early steps in phenylpropanoid biosynthesis and suggest that metabolites accumulating upstream of the C4H step impact the auxin response mechanism. Electronic supplementary material The online version of this article (10.1186/s12870-018-1477-0) contains supplementary material, which is available to authorized users.

The multicatalytic ubiquitin-proteasome system (UPS) carries out proteolysis in a highly orchestrated way and regulates a large number of cellular processes. Deregulation of the UPS in many disorders has been documented. In some cases, such as carcinogenesis, elevated proteasome activity has been implicated in disease development, while the etiology of other diseases, such as neurodegeneration, includes decreased UPS activity. Therefore, agents that alter proteasome activity could suppress as well as enhance a multitude of diseases. Metal oxide nanoparticles, often developed as diagnostic tools, have not previously been tested as modulators of proteasome activity. Here, several types of metal oxide nanoparticles were found to adsorb to the proteasome and show variable preferential binding for particular proteasome subunits with several peptide binding "hotspots" possible. These interactions depend on the size, charge, and concentration of the nanoparticles and affect proteasome activity in a time-dependent manner. Should metal oxide nanoparticles increase proteasome activity in cells, as they do in vitro, unintended effects related to changes in proteasome function can be expected.

In this review, we summarize the evidence pointing at the important role of 26S proteasome-dependent proteolysis in the regulation of microtubule synthesis and microtubule dynamics. Because most of the advances in this relatively unexplored research field originate from yeast and animal studies, we have considered those studies that describe the role of proteolysis in processes that are evolutionarily conserved and known to exist in plants. In addition, we place particular emphasis on the proteasome-dependent degradation of plant-specific microtubule-associated protein SPIRAL1 and its function in MT rearrangements associated with salt stress.

Pentatricopeptide repeat (PPR) proteins belong to a family of approximately 450 members in Arabidopsis, of which few have been characterized. We identified loss of function alleles of SLO2, defective in a PPR protein belonging to the E+ subclass of the P-L-S subfamily. slo2 mutants are characterized by retarded leaf emergence, restricted root growth, and late flowering. This phenotype is enhanced in the absence of sucrose, suggesting a defect in energy metabolism. The slo2 growth retardation phenotypes are largely suppressed by supplying sugars or increasing light dosage or the concentration of CO2. The SLO2 protein is localized in mitochondria. We identified four RNA editing defects and reduced editing at three sites in slo2 mutants. The resulting amino acid changes occur in four mitochondrial proteins belonging to complex I of the electron transport chain. Both the abundance and activity of complex I are highly reduced in the slo2 mutants, as well as the abundance of complexes III and IV. Moreover, ATP, NAD+, and sugar contents were much lower in the mutants. In contrast, the abundance of alternative oxidase was significantly enhanced. We propose that SLO2 is required for carbon energy balance in Arabidopsis by maintaining the abundance and/or activity of complexes I, III, and IV of the mitochondrial electron transport chain.

In spite of the mounting concerns, current understanding of the extent and mechanisms of phytotoxicity of manufactured nanomaterials remains limited. Here we show that in Arabidopsis thaliana, ultra-small anatase TiO(2) nanoparticles cause reorganization and elimination of microtubules followed by the accelerated and 26S proteasome-dependent degradation of tubulin monomers. Similar to other microtubule-disrupting agents, TiO(2) nanoparticles induce isotropic growth of root cells. Because microtubules are essential for the normal function of all eukaryotic cells, these results reveal a potentially important consequence of environmental pollution by this widely used nanomaterial.

Plant cells contain a mixture of 26S and 20S proteasomes that mediate ubiquitin-dependent and ubiquitin-independent proteolysis, respectively. The 26S proteasome contains the 20S proteasome and one or two regulatory particles that are required for ubiquitin-dependent degradation. Comparative analyses of Arabidopsis proteasome mutants revealed that a decrease in 26S proteasome biogenesis causes heat shock hypersensitivity and reduced cell division rates that are compensated by increased cell expansion. Loss of 26S proteasome function also leads to an increased 20S proteasome biogenesis, which in turn enhances the cellular capacity to degrade oxidized proteins and thus increases oxidative stress tolerance. These findings suggest the intriguing possibility that 26S and 20S proteasome activities are regulated to control plant development and stress responses. This mini-review highlights some of the recent studies on proteasome regulation in plants.

Although the final size of plant organs is influenced by environmental cues, it is generally accepted that the primary size determinants are intrinsic factors that regulate and coordinate cell proliferation and cell expansion. Here, we show that optimal proteasome function is required to maintain final shoot organ size in Arabidopsis (Arabidopsis thaliana). Loss of function of the subunit regulatory particle AAA ATPase (RPT2a) causes a weak defect in 26S proteasome activity and leads to an enlargement of leaves, stems, flowers, fruits, seeds, and embryos. These size increases are a result of increased cell expansion that compensates for a reduction in cell number. Increased ploidy levels were found in some but not all enlarged organs, indicating that the cell size increases are not caused by a higher nuclear DNA content. Partial loss of function of the regulatory particle non-ATPase (RPN) subunits RPN10 and RPN12a causes a stronger defect in proteasome function and also results in cell enlargement and decreased cell proliferation. However, the increased cell volumes in rpn10-1 and rpn12a-1 mutants translated into the enlargement of only some, but not all, shoot organs. Collectively, these data show that during Arabidopsis shoot development, the maintenance of optimal proteasome activity levels is important for balancing cell expansion with cell proliferation rates.

The 26S proteasome is an essential multicatalytic protease complex that degrades a wide range of intracellular proteins, especially those modified with ubiquitin. Arabidopsis thaliana and other plants use pairs of genes to encode most of the core subunits, with both of the isoforms often incorporated into the mature complex. Here, we show that the gene pair encoding the regulatory particle non-ATPase subunit (RPN5) has a unique role in proteasome function and Arabidopsis development. Homozygous rpn5a rpn5b mutants could not be generated due to a defect in male gametogenesis. While single rpn5b mutants appear wild-type, single rpn5a mutants display a host of morphogenic defects, including abnormal embryogenesis, partially deetiolated development in the dark, a severely dwarfed phenotype when grown in the light, and infertility. Proteasome complexes missing RPN5a are less stable in vitro, suggesting that some of the rpn5a defects are caused by altered complex integrity. The rpn5a phenotype could be rescued by expression of either RPN5a or RPN5b, indicating functional redundancy. However, abnormal phenotypes generated by overexpression implied that paralog-specific functions also exist. Collectively, the data point to a specific role for RPN5 in the plant 26S proteasome and suggest that its two paralogous genes in Arabidopsis have both redundant and unique roles in development.

The 26S proteasome (26SP) is a multi-subunit, multi-catalytic protease that is responsible for most of the cytosolic and nuclear protein turnover. The 26SP is composed of two sub-particles, the 19S regulatory particle (RP) that binds and unfolds protein targets, and the 20S core particle (20SP) that degrades proteins into small peptides. Most 26SP targets are conjugated to a poly-ubiquitin (Ub) chain that serves as a degradation signal. However, some targets, such as oxidized proteins, do not require a poly-Ub tag for proteasomal degradation, and recent studies have shown that the main protease in this Ub-independent pathway is free 20SP. It is currently unknown how the ratio of 26SP- to 20SP-dependent proteolysis is controlled. Here we show that loss of function of the Arabidopsis RP subunits RPT2a, RPN10 and RPN12a leads to decreased 26SP accumulation, resulting in reduced rates of Ub-dependent proteolysis. In contrast, all three RP mutants have increased 20SP levels and thus enhanced Ub-independent protein degradation. As a consequence of this shift in proteolytic activity, mutant seedlings are hypersensitive to stresses that cause protein misfolding, and have increased tolerance to treatments that promote protein oxidation. Taken together, the data show that plant cells increase 20SP-dependent proteolysis when 26SP activity is impaired.

*† ‡ Summary Ubiquitin C-terminal hydrolases (UCHs) are a subset of de-ubiquitinating proteases that release covalently linked ubiquitin (Ub), and as such play essential roles in recycling Ub and reversing the action of Ub conjugation. We show here that two related Arabidopsis UCHs, UCH1, and UCH2, are important for shoot development. The UCH1 and 2 genes are ubiquitously expressed, with the corresponding proteins present in both the cytoplasm and nucleus. Unlike their animal and fungal counterparts, we found no evidence that the Arabidopsis UCH1 and 2 proteins stably associate with the 26S proteasome. Altering the levels of UCH1 and 2 has substantial effects on Arabidopsis shoot development, especially with respect to inflorescence architecture, with over-expression and double mutants enhancing and suppressing the outgrowth of cauline branches, respectively. Neither UCH1-over-expressing nor uch1-1 uch2-1 plants have detectably altered sensitivity to cytokinins or auxins individually, but exhibit an altered sensitivity to the ratio of the two hormones. UCH1over-expressing plants show dramatically enhanced phenotypes when combined with auxin-insensitive mutants axr1-3 and axr2-1, suggesting that one or more aspects of auxin signaling are affected by this enzyme pair. Previous studies revealed that the ubiquitination and degradation of the AUX/IAA family of repressors is a key step in auxin signaling. Here, we show that turnover of a reporter fused to a representative AUX/IAA protein AXR3 is faster in the uch1-1 uch2-1 double mutant but slower in the UCH1 over-expression backgrounds. Taken together, our results indicate that de-ubiquitination helps to modify plant shoot architecture, possibly via its ability to directly or indirectly protect upstream target proteins involved in auxin/ cytokinin signaling from Ub-mediated degradation.

By screening for ethylene response mutants in Arabidopsis, a novel mutant, eer2, was isolated which displays enhanced ethylene responses. On a low nutrient medium (LNM) light-grown eer2 seedlings showed a significant hypocotyl elongation in response to low levels of 1-amino-cyclopropane-1-carboxylate (ACC), the precursor of ethylene, compared with the wild type, indicating that eer2 is hypersensitive to ethylene. Treatment with 1-MCP (1-methylcyclopropene), a competitive inhibitor of ethylene signalling, suppressed this hypersensitive response, demonstrating that it is a bona fide ethylene effect. By contrast, roots of eer2 were less sensitive than the wild type to low concentrations of ACC. The ethylene levels in eer2 did not differ from the wild type, indicating that ethylene overproduction is not the primary cause of the eer2 phenotype. In addition to its enhanced ethylene response of hypocotyls, eer2 is also affected in the pattern of senescence and its phenotype depends on the nutritional status of the growth medium. Furthermore, linkage analysis of eer2 suggests that this mutant defines a new locus in ethylene signalling.

Ubiquitination of various intracellular proteins by ubiquitin-protein ligases (or E3s) plays an essential role in eukaryotic cell regulation primarily through its ability to selectively target proteins for degradation by the 26S proteasome. Skp1, Cullin, F-box (SCF) complexes are one influential E3 class that use F-box proteins to deliver targets to a core ligase activity provided by the Skp1, Cullin, and Rbx1 subunits. Almost 700 F-box proteins can be found in Arabidopsis , indicating that SCF E3s likely play a pervasive role in plant physiology and development. Here, we describe the reverse genetic analysis of two F-box proteins, EBF1 and -2, that work coordinately in SCF complexes to repress ethylene action. Mutations in either gene cause hypersensitivity to exogenous ethylene and its precursor 1-aminocyclopropane-1-carboxylic acid. EBF1 and -2 interact directly with ethylene insensitive 3 (EIN3), a transcriptional regulator important for ethylene signaling. Levels of EIN3 are increased in mutants affecting either EBF1 or -2 , suggesting that the corresponding SCF complexes work together in EIN3 breakdown. Surprisingly, double ebf1 ebf2 mutants display a substantial arrest of seedling growth and have elevated EIN3 levels, even in the absence of exogenous ethylene. Collectively, our results show that the SCF EBF1/EBF2 -dependent ubiquitination and subsequent removal of EIN3 is critical not only for proper ethylene signaling but also for growth in plants.

The 26 S proteasome is a multisubunit protease complex responsible for degrading a wide range of intracelular proteins in eukaryotes, especially those modified with polyubiquitin chains. It is composed of a self-compartmentalized core protease (CP) that houses the peptidase active sites appended on either or both ends by a regulatory particle (RP) that identifies appropriate substrates and translocates them into the lumen of the CP for breakdown. Here, we describe the molecular and biochemical properties of the 26 S proteasome from the plant Arabidopsis thaliana. Like the CP and the ATPase ring of the RP, the RP non-ATPase subunits are often encoded by two transcriptionally active genes with some pairs displaying sufficient sequence divergence to suggest functional differences. Most RPN subunits could functionally replace their yeast counterparts, implying that they have retained their positions and activities within the complex. A method was developed to purify the 26 S proteasome intact from whole Arabidopsis seedlings. These preparations are biochemically indistinguishable from those from yeast and mammals, including the need for ATP to maintain integrity and a strong sensitivity to the inhibitors MG115, MG132, lactacystin, and epoxomicin. Mass spectrometric analysis of the complex detected the presence of almost all CP and RP subunits. In many cases, both products of paralogous genes were detected, demonstrating that each isoform assembles into the mature particle. As with the yeast and animal 26 S proteasomes, attenuation of individual RP genes induces a coordinated up-regulation of many of the other 26 S proteasome genes, suggesting that plants contain a negative feedback mechanism to regulate the 26 S proteasome levels. The incorporation of paralogous subunits into the Arabidopsis holoprotease raises the intriguing possibility that plants synthesize multiple 26 S proteasome types with unique properties and/or target specificities.

The 26S proteasome is an essential protease complex responsible for removing most short-lived intracellular proteins, especially those modified with polyubiquitin chains. We show here that an Arabidopsis mutant expressing an altered RPN10 subunit exhibited a pleiotropic phenotype consistent with specific changes in 26S proteasome function. rpn10-1 plants displayed reduced seed germination, growth rate, stamen number, genetic transmission through the male gamete, and hormone-induced cell division, which can be explained partially by a constitutive downregulation of the key cell cycle gene CDKA;1. rpn10-1 also was more sensitive to abscisic acid (ABA), salt, and sucrose stress and to DNA-damaging agents and had decreased sensitivity to cytokinin and auxin. Most of the phenotypes can be explained by a hypersensitivity to ABA, which is reflected at the molecular level by the selective stabilization of the short-lived ABA-signaling protein ABI5. Collectively, these results indicate that RPN10 affects a number of regulatory processes in Arabidopsis likely by directing specific proteins to the 26S proteasome for degradation. A particularly important role may be in regulating the responses to signals promulgated by ABA.

The 26S proteasome is an ATP-dependent eukaryotic protease responsible for degrading many important cell regulators, especially those conjugated with multiple ubiquitins. Bound on both ends of the 20S core protease is a multisubunit regulatory particle that plays a crucial role in substrate selection by an as yet unknown mechanism(s). Here, we show that the RPN12 subunit of the Arabidopsis regulatory particle is involved in cytokinin responses. A T-DNA insertion mutant that affects RPN12a has a decreased rate of leaf formation, reduced root elongation, delayed skotomorphogenesis, and altered growth responses to exogenous cytokinins, suggesting that the mutant has decreased sensitivity to the hormone. The cytokinin-inducible genes CYCD3 and NIA1 are upregulated constitutively in rpn12a-1, indicating that feedback-inhibitory mechanisms also may be altered. rpn12a-1 seedlings also showed changes in auxin-induced growth responses, further illustrating the close interaction between auxin and cytokinin regulation. In yeast, RPN12 is necessary for the G1/S and G2/M transitions of the cell cycle, phases that have been shown to be under cytokinin control in plants. We propose that RPN12a is part of the Arabidopsis 26S proteasome that controls the stability of one or more of the factors involved in cytokinin regulation.

Aggregation of proteins damaged by stress is often a causal factor of cell death. To prevent aggregation, eukaryotic cells rapidly degrade damaged proteins by engaging two types of proteasomes. The first type is the 26S proteasome (26SP) which is composed of a cylindrical proteolytic core—the 20S proteasome (20SP)—and one or two regulatory particles (RPs) that interact with ubiquitinated proteins. The second type is the free 20SP which mediates ubiquitin-independent proteolysis. We have recently shown that loss of RP function in Arabidopsis leads to an expected decrease in 26SP-dependent protein degradation and hypersensitivity to stresses that induce protein misfolding. Surprisingly, RP mutants have increased 20SP activity and tolerance to oxidative stress. This finding suggests that misfolded proteins carry one type of degradation signal that steers them to ubiquitination enzymes and the 26SP, while oxidatively damaged proteins carry another that guides them directly to the 20SP for degradation. Here we suggest that protein oxidation induces the formation of unstructured regions that serve as targeting signals for 20SP-dependent proteolysis.

GT-2 is a plant transcriptional activator that contains two separate, but similar, trihelix DNA-binding domains. GT-1 is similar to GT-2, although it contains only one of such domains. cDNAs that encode GT-2 were isolated from rice ( OS-GT2 ) and Arabidopsis ( AT-GT2 ). Evidence is presented for the existence of an Arabidopsis gene family that is structurally related to AT-GT2 . Two members of this GT2 -like family, AT-GTL1 and AT-GTL2 , have been isolated and characterized. Their sequences suggest that they evolved by a recent gene duplication event. Both AT-GT2 and AT-GTL genes contain an intron in the amino-terminal trihelix motif, indicating that this DNA-binding domain resulted from exon shuffling. RNA gel blot analysis using AT-GTL1 as a probe revealed four transcripts in the aerial part of the plant. All mRNA levels were significantly higher in siliques, suggesting that this gene family may function in fruit and/or seed development. To date, DNA-binding proteins characterized by the trihelix motif have been described only in plants, and may therefore be involved in plant-specific processes. Our results show that in Arabidopsis thaliana , the trihelix motif is not restricted to the GT-1 and GT-2 DNA-binding proteins.

Mutations at the GI locus in Arabidopsis are pleiotropic: gi mutants are late-flowering, tolerant to the toxicity of the herbicide paraquat, and have an increased starch content. We tested the effects of exogenous sucrose supply on the level of paraquat tolerance and on growth and development of the gi-3 mutant. Paraquat tolerance was the highest in gi-3 seedlings grown on medium containing 1% sucrose. As expected, all measured growth parameters (root length, fresh weight, anthocyanin, and sucrose content) were influenced by the sucrose dose, but in a number of assays (effect on fresh weight and developmental characteristics) the sucrose-dependent response in gi-3 was heterochronic. Additionally, the late-flowering phenotype of the gi-3 mutant was reverted to wild type after prolonged growth in darkness on sucrose-containing media.

Ethylene inhibits hypocotyl elongation in etiolated Arabidopsis seedlings. However, when Arabidopsis was grown in the light in the presence of ethylene or its precursor 1-aminocyclopropane-1-carboxylic acid (ACC), a marked induction of hypocotyl elongation occurred. This resulted from an increase in cell expansion rather than cell division. The effects of ethylene and ACC were antagonized by the ethylene action inhibitor Ag + . The elongation response was absent or weakened in a set of ethylene-insensitive mutants ( etr1-3 , ein2-1 , ein3-1 , ein4 , ain1-10 , ein7 ). With the exception of ein4 , the degree of inhibition of hypocotyl elongation was correlated with the strength of the ethylene-insensitive phenotype based on the triple response assay. In addition, the constitutive ethylene response mutant ctr1-1 , grown in the light, had a longer hypocotyl than the wild type. Exogenous auxin also induced hypocotyl elongation in light-grown Arabidopsis . Again, the response was abolished by treatment with Ag + , suggesting that ethylene might be a mediator. The results showed that, depending on light conditions, ethylene can induce opposite effects on cell expansion in Arabidopsis hypocotyls.

Summary The cytokinins play essential roles in the development and environmental responses of higher plants. Cytokinin signaling leads to the phosphorylation-dependent activation of two classes of Arabidopsis response regulators (RRs): the type-B RR (RRB) transcriptional activators that promote the expression of cytokinin response genes and the type-A RRs (RRAs) that are encoded by primary cytokinin response genes and function as response inhibitors. We show that cytokinin signaling increases the abundance of ARR1, a ubiquitously expressed RRB, by preventing its degradation by the 26S proteasome. We also show that the RRAs act to suppress ARR1 accumulation, thus providing an explanation for their inhibitory action in cytokinin signaling. Collectively, our results reveal an additional regulatory mechanism in the cytokinin response pathway that involves the cytokinin-dependent stability control of a major RRB response activator.

Background: In the Arabidopsis 26S proteasome mutant rpn12a-1, an exon-trap T-DNA is inserted 531 base pairs downstream of the RPN12a STOP codon. We have previously shown that this insertion activates a STOP codon-associated latent 5' splice site that competes with the polyadenylation signal during processing of the pre-mRNA. As a result of this dual input from splicing and polyadenylation in the rpn12a-1 mutant, two RPN12a transcripts are produced and they encode the wild-type RPN12a and a chimeric RPN12a-NPTII protein. Both proteins form complexes with other proteasome subunits leading to the formation of wild-type and mutant proteasome versions. The net result of this heterogeneity of proteasome particles is a reduction of total cellular proteasome activity. One of the consequences of reduced proteasomal activity is decreased sensitivity to the major plant hormone cytokinin. Methods: We performed ethyl methanesulfonate mutagenesis of rpn12a-1 and isolated revertants with wild-type cytokinin sensitivity. Results: We describe the isolation and analyses of suppressor of rpn12a-1 (sor1). The sor1 mutation is intragenic and located at the fifth position of the chimeric intron. This mutation weakens the activated 5' splice site associated with the STOP codon and tilts the processing of the RPN12a mRNA back towards polyadenylation. Conclusions: These results validate our earlier interpretation of the unusual nature of the rpn12a-1 mutation. Furthermore, the data show that optimal 26S proteasome activity requires RPN12a accumulation beyond a critical threshold. Finally, this finding reinforces our previous conclusion that proteasome function is critical for the cytokinin-dependent regulation of plant growth.

Surface functionalization of nanoparticles has become an important tool for the in vivo delivery of bioactive agents to their target sites. Here we describe the reverse strategy, nanoharvesting, in which nanoparticles are used as a tool to isolate and enrich bioactive compounds from living cells. Anatase TiO2 nanoparticles smaller than 20 nm form strong bonds with molecules carrying enediol and especially catechol groups. We show that these nanoparticles can enter plant cells, conjugate enediol and catechol group-rich flavonoids in situ, and exit plant cells as flavonoid-nanoparticle conjugates. The source plant tissues remain viable after treatment. As predicted by the surface chemistry of anatase TiO2 nanoparticles, the quercetin-based flavonoids were enriched amongst the nanoharvested flavonoid species. Nanoharvesting eliminates the use of organic solvents, allows spectral identification of the isolated compounds, and offers a new avenue for the use of nanomaterials for the coupled isolation and testing of bioactive properties of plant-made compounds.

The ubiquitin/26S proteasome-dependent proteolysis of response regulators is a critical element of many plant hormone signaling pathways. We have recently shown that cytokinin signaling requires the AXR1 component of the related to ubiquitin (RUB) protein modification pathway to promote the proteasome-dependent degradation of the cytokinin response inhibitor ARR5. Here, we show that ARR5 also accumulates in the 26S proteasome mutant rpn12a-1, and leads to a marked resistance to cytokinins. Collectively, these results suggest that proteasome-dependent proteolysis of feedback inhibitors such as ARR5 is essential for the maintenance of optimal responsivity and plasticity in cytokinin signaling.

Cytokinins control key aspects of plant growth, including shoot and root meristem development and the timing of senescence of leaves and stems. Cytokinin perception triggers a 2-component signaling mechanism that ultimately leads to phosphorylation-dependent activation of a class of transcriptional regulators called type-B ARRs (RRBs). We have recently shown that the stability of the RRB family member ARR1 is increased in response to elevated cytokinin concentrations. In contrast, cytokinin decreases the stability of the closely related RRB member ARR2. The molecular mechanism governing the differential stability regulation of these 2 closely related RRBs remains unknown.

The dynamic instability of cortical microtubules (MTs) (i.e., their ability to rapidly alternate between phases of growth and shrinkage) plays an essential role in plant growth and development. In addition, recent studies have revealed a pivotal role for dynamic instability in the response to salt stress conditions. The salt stress response includes a rapid depolymerization of MTs followed by the formation of a new MT network that is believed to be better suited for surviving high salinity. Although this initial depolymerization response is essential for the adaptation to salt stress, the underlying molecular mechanism has remained largely unknown. Here, we show that the MT-associated protein SPIRAL1 (SPR1) plays a key role in salt stress–induced MT disassembly. SPR1, a microtubule stabilizing protein, is degraded by the 26S proteasome, and its degradation rate is accelerated in response to high salinity. We show that accelerated SPR1 degradation is required for a fast MT disassembly response to salt stress and for salt stress tolerance.

While few publications have documented the uptake of nanoparticles in plants, this is the first study describing uptake and distribution of the ultra-small anatase TiO2 in the plant model system Arabidopsis. We modified the nanoparticle surface with Alizarin red S and sucrose, and demonstrated that nanoconjugates traversed cell walls, entered into plant cells, and accumulated in specific subcellular locations. Optical and X-ray fluorescence microscopy co-registered the nanoconjugates in cell vacuoles and nuclei.

The current literature offers contradictory results regarding the role of the proteasome subunit RPN1a in Arabidopsis development. Here we show that plants lacking RPN1a are viable and have increased cell sizes, decreased heat shock tolerance, increased oxidative stress tolerance and other phenotypes characteristic for 26S proteasome subunit mutants. These results strengthen our contention that most of the phenotypes of 26S proteasome mutants in Arabidopsis described to date reflect a general impairment in 26S proteasome function rather than a specific defect of a single subunit, and suggest that the role of the RPN1a subunit during embryogenesis needs to be reconsidered.

Aggregation of proteins damaged by stress is often a causal factor of cell death. To prevent aggregation, eukaryotic cells rapidly degrade damaged proteins by engaging two types of proteasomes. The first type is the 26S proteasome (26SP) which is composed of a cylindrical proteolytic core—the 20S proteasome (20SP)—and one or two regulatory particles (RPs) that interact with ubiquitinated proteins. The second type is the free 20SP which mediates ubiquitin-independent proteolysis. We have recently shown that loss of RP function in Arabidopsis leads to an expected decrease in 26SP-dependent protein degradation and hypersensitivity to stresses that induce protein misfolding. Surprisingly, RP mutants have increased 20SP activity and tolerance to oxidative stress. This finding suggests that misfolded proteins carry one type of degradation signal that steers them to ubiquitination enzymes and the 26SP, while oxidatively damaged proteins carry another that guides them directly to the 20SP for degradation. Here we suggest that protein oxidation induces the formation of unstructured regions that serve as targeting signals for 20SP-dependent proteolysis.

Ethylene or its precursor 1-aminocyclopropane-1-carboxylic acid (ACC) can stimulate hypocotyl elongation in light-grown Arabidopsis seedlings. A mutant, designated ACC-related long hypocotyl 1 (alh1), that displayed a long hypocotyl in the light in the absence of the hormone was characterized. Etiolatedalh1 seedlings overproduced ethylene and had an exaggerated apical hook and a thicker hypocotyl, although no difference in hypocotyl length was observed when compared with wild type.Alh1 plants were less sensitive to ethylene, as reflected by reduction of ACC-mediated inhibition of hypocotyl growth in the dark and delay in flowering and leaf senescence.Alh1 also had an altered response to auxin, whereas auxin levels in whole alh1 seedlings remained unaffected. In contrast to wild type, alh1 seedlings showed a limited hypocotyl elongation when treated with indole-3-acetic acid. Alh1 roots had a faster response to gravity. Furthermore, the hypocotyl elongation of alh1 and of ACC-treated wild type was reverted by auxin transport inhibitors. In addition, auxin up-regulated genes were ectopically expressed in hypocotyls upon ACC treatment, suggesting that the ethylene response is mediated by auxins. Together, these data indicate thatalh1 is altered in the cross talk between ethylene and auxins, probably at the level of auxin transport.