Auxin, actin and growth of the Arabidopsis thaliana primary root

Reverse genetics approach to characterize a function of NADH-glutamate synthase1 in rice plants

Rice plants grown in anaerobic paddy soil prefer to use ammonium ion as an inorganic nitrogen source for their growth. The ammonium ions are assimilated by the coupled reaction of glutamine synthetase (GS) and glutamate synthase (GOGAT). In rice, there is a small gene family for GOGAT: there are two NADH-dependent types and one ferredoxin (Fd)-dependent type. Fd-GOGAT is important in the re-assimilation of photorespiratorily generated ammonium ions in chloroplasts. Although cell-type and age-dependent expression of two NADH-GOGAT genes has been well characterized, metabolic function of individual gene product is not fully understood. Reverse genetics approach is a direct way to characterize functions of isoenzymes. We have isolated a knockout rice mutant lacking NADH-dependent glutamate synthase1 (NADH-GOGAT1) and our studies show that this isoenzyme is important for primary ammonium assimilation in roots at the seedling stage. NADH-GOGAT1 is also important in the development of active tiller number, when the mutant was grown in paddy field until the harvest. Expression of NADH-GOGAT2 and Fd-GOGAT in the mutant was identical with that in wild-type, suggesting that these GOGATs are not able to compensate for NADH-GOGAT1 function.

Monitoring the regulation of gene expression in a growing organ using a fluid mechanics formalism

Background

Technological advances have enabled the accurate quantification of gene expression, even within single cell types. While transcriptome analyses are routinely performed, most experimental designs only provide snapshots of gene expression. Molecular mechanisms underlying cell fate or positional signalling have been revealed through these discontinuous datasets. However, in developing multicellular structures, temporal and spatial cues, known to directly influence transcriptional networks, get entangled as the cells are displaced and expand. Access to an unbiased view of the spatiotemporal regulation of gene expression occurring during development requires a specific framework that properly quantifies the rate of change of a property in a moving and expanding element, such as a cell or an organ segment.

Results

We show how the rate of change in gene expression can be quantified by combining kinematics and real-time polymerase chain reaction data in a mechanistic model which considers any organ as a continuum. This framework was applied in order to assess the developmental regulation of the two reference genes Actin11 and Elongation Factor 1-β in the apex of poplar root. The growth field was determined by time-lapse photography and transcript density was obtained at high spatial resolution. The net accumulation rates of the transcripts of the two genes were found to display highly contrasted developmental profiles. Actin11 showed pulses of up and down regulation in the accelerating and decelerating parts of the growth zone while the dynamic of EF1β were much slower. This framework provides key information about gene regulation in a developing organ, such as the location, the duration and the intensity of gene induction/repression.

Conclusions

We demonstrated that gene expression patterns can be monitored using the continuity equation without using mutants or reporter constructions. Given the rise of imaging technologies, this framework in our view opens a new way to dissect the molecular basis of growth regulation, even in non-model species or complex structures.

A conditional mutation in Arabidopsis thaliana separase induces chromosome non-disjunction, aberrant morphogenesis and cyclin B1;1 stability

The caspase family protease, separase, is required at anaphase onset to cleave the cohesin complex, which joins sister chromatids. However, among eukaryotes, separases have acquired novel functions. Here, we show that Arabidopsis thaliana radially swollen 4 (rsw4), a temperature-sensitive mutant isolated previously on the basis of root swelling, harbors a mutation in At4g22970, the A. thaliana separase. Loss of separase function in rsw4 at the restrictive temperature is indicated by the widespread failure of replicated chromosomes to disjoin. Surprisingly, rsw4 has neither pronounced cell cycle arrest nor anomalous spindle formation, which occur in other eukaryotes upon loss of separase activity. However, rsw4 roots have disorganized cortical microtubules and accumulate the mitosis-specific cyclin, cyclin B1;1, excessive levels of which have been associated with altered microtubules and morphology. Cyclin B1;1 also accumulates in certain backgrounds in response to DNA damage, but we find no evidence for aberrant responses to DNA damage in rsw4. Our characterization of rsw4 leads us to hypothesize that plant separase, in addition to cleaving cohesin, regulates cyclin B1;1, with profound ramifications for morphogenesis.

Dynamic coordination of cytoskeletal and cell wall systems during plant cell morphogenesis

Underlying the architectural complexity of plants are diverse cell types that, under the microscope, easily reveal relationships between cell structure and specialized functions. Much less obvious are the mechanisms by which the cellular growth machinery and mechanical properties of the cell interact to dictate cell shape. The recent combined use of mutants, genomic analyses, sophisticated spectroscopies, and live cell imaging is providing new insight into how cytoskeletal systems and cell wall biosynthetic activities are integrated during morphogenesis. The purpose of this review is to discuss the unique geometric properties and physical processes that regulate plant cell expansion, then to overlay on this mechanical system some of the recent discoveries about the protein machines and cellular polymers that regulate cell shape. In the end, we hope to make clear that there are many interesting opportunities to develop testable mathematical models that improve our understanding of how subcellular structures, protein motors, and extracellular polymers can exert effects at spatial scales that span cells, tissues, and organs.

Probing the actin-auxin oscillator

The directional transport of the plant hormone auxin depends on transcellular gradients of auxin-efflux carriers that continuously cycle between plasma membrane and intracellular compartments. This cycling has been proposed to depend on actin filaments. However, the role of actin for the polarity of auxin transport has been disputed. To get insight into this question, actin bundling was induced by overexpression of the actin-binding domain of talin in tobacco BY-2 cells and in rice plants. This bundling can be reverted by addition of auxins, which allows to address the role of actin organization on the flux of auxin. In both systems, the reversion of a normal actin configuration can be restored by addition of exogenous auxins and this fully restores the respective auxin-dependent functions. These findings lead to a model of a self-referring regulatory circuit between polar auxin transport and actin organization. To further dissect the actin-auxin oscillator, we used photoactivated release of caged auxin in tobacco cells to demonstrate that auxin gradients can be manipulated at a subcellular level.

A mutation in GDP-mannose pyrophosphorylase causes conditional hypersensitivity to ammonium, resulting in Arabidopsis root growth inhibition, altered ammonium metabolism, and hormone homeostasis

Ascorbic acid (AA) is an antioxidant fulfilling a multitude of cellular functions. Given its pivotal role in maintaining the rate of cell growth and division in the quiescent centre of the root, it was hypothesized that the AA-deficient Arabidopsis thaliana mutants vtc1-1, vtc2-1, vtc3-1, and vtc4-1 have altered root growth. To test this hypothesis, root development was studied in the wild type and vtc mutants grown on Murashige and Skoog medium. It was discovered, however, that only the vtc1-1 mutant has strongly retarded root growth, while the other vtc mutants exhibit a wild-type root phenotype. It is demonstrated that the short-root phenotype in vtc1-1 is independent of AA deficiency and oxidative stress. Instead, vtc1-1 is conditionally hypersensitive to ammonium (NH(4)(+)). To provide new insights into the mechanism of NH(4)(+) sensitivity in vtc1-1, root development, NH(4)(+) content, glutamine synthetase (GS) activity, glutamate dehydrogenase activity, and glutamine content were assessed in wild-type and vtc1-1 mutant plants grown in the presence and absence of high NH(4)(+) and the GS inhibitor MSO. Since VTC1 encodes a GDP-mannose pyrophosphorylase, an enzyme generating GDP-mannose for AA biosynthesis and protein N-glycosylation, it was also tested whether protein N-glycosylation is affected in vtc1-1. Furthermore, since root development requires the action of a variety of hormones, it was investigated whether hormone homeostasis is linked to NH(4)(+) sensitivity in vtc1-1. Our data suggest that NH(4)(+) hypersensitivity in vtc1-1 is caused by disturbed N-glycosylation and that it is associated with auxin and ethylene homeostasis and/or nitric oxide signalling.

Auxin response in Arabidopsis under cold stress: underlying molecular mechanisms

To understand the mechanistic basis of cold temperature stress and the role of the auxin response, we characterized root growth and gravity response of Arabidopsis thaliana after cold stress, finding that 8 to 12 h at 4 degrees C inhibited root growth and gravity response by approximately 50%. The auxin-signaling mutants axr1 and tir1, which show a reduced gravity response, responded to cold treatment like the wild type, suggesting that cold stress affects auxin transport rather than auxin signaling. Consistently, expression analyses of an auxin-responsive marker, IAA2-GUS, and a direct transport assay confirmed that cold inhibits root basipetal (shootward) auxin transport. Microscopy of living cells revealed that trafficking of the auxin efflux carrier PIN2, which acts in basipetal auxin transport, was dramatically reduced by cold. The lateral relocalization of PIN3, which has been suggested to mediate the early phase of root gravity response, was also inhibited by cold stress. Additionally, cold differentially affected various protein trafficking pathways. Furthermore, the inhibition of protein trafficking by cold is independent of cellular actin organization and membrane fluidity. Taken together, these results suggest that the effect of cold stress on auxin is linked to the inhibition of intracellular trafficking of auxin efflux carriers.

Light can rescue auxin-dependent synchrony of cell division in a tobacco cell line

Pattern formation in plants has to cope with ambient variability and therefore must integrate environmental cues such as light. Synchrony of cell divisions was previously observed in cell files of tobacco suspension cultures, which represents a simple case of pattern formation. To develop cellular approaches for light-dependent patterning, light-responsive tobacco cell lines were screened from the cell line Nicotiana tabacum L. cv. Virginia Bright Italia 0 (VBI-0). The light responsive and auxin-autonomous cell line VBI-3 was isolated. As in the progenitor line VBI-0, cell divisions are synchronized in VBI-3 during exponential growth phase. This synchrony can be inhibited by 1-N-naphthylphthalamic acid, an auxin transport inhibitor, and this process was accompanied by the disassembly of actin filaments. However, the synchrony could be rescued when the cells were cultured under white light or with exogenous indolyl-3-acetic acid. The rescue was most efficient for continuous far-red light followed by continuous blue light, whereas continuous red light was least effective. These findings are discussed in the context of phytochrome-induced auxin biosynthesis and auxin-dependent synchrony of cell division.

Auxin stimulates its own transport by shaping actin filaments

The directional transport of the plant hormone auxin has been identified as central element of axis formation and patterning in plants. This directionality of transport depends on gradients, across the cell, of auxin-efflux carriers that continuously cycle between plasma membrane and intracellular compartments. This cycling has been proposed to depend on actin filaments. However, the role of actin for the polarity of auxin transport has been disputed. The organization of actin, in turn, has been shown to be under control of auxin. By overexpression of the actin-binding protein talin, we have generated transgenic rice (Oryza sativa) lines, where actin filaments are bundled to variable extent and, in consequence, display a reduced dynamics. We show that this bundling of actin filaments correlates with impaired gravitropism and reduced longitudinal transport of auxin. We can restore a normal actin configuration by addition of exogenous auxins and restore gravitropism as well as polar auxin transport. This rescue is mediated by indole-3-acetic acid and 1-naphthyl acetic acid but not by 2,4-dichlorophenoxyacetic acid. We interpret these findings in the context of a self-referring regulatory circuit between polar auxin transport and actin organization. This circuit might contribute to the self-amplification of auxin transport that is a central element in current models of auxin-dependent patterning.

PINOID kinase regulates root gravitropism through modulation of PIN2-dependent basipetal auxin transport in Arabidopsis

Reversible protein phosphorylation is a key regulatory mechanism governing polar auxin transport. We characterized the auxin transport and gravitropic phenotypes of the pinoid-9 (pid-9) mutant of Arabidopsis (Arabidopsis thaliana) and tested the hypothesis that phosphorylation mediated by PID kinase and dephosphorylation regulated by the ROOTS CURL IN NAPHTHYLPHTHALAMIC ACID1 (RCN1) protein might antagonistically regulate root auxin transport and gravity response. Basipetal indole-3-acetic acid transport and gravitropism are reduced in pid-9 seedlings, while acropetal transport and lateral root development are unchanged. Treatment of wild-type seedlings with the protein kinase inhibitor staurosporine phenocopies the reduced auxin transport and gravity response of pid-9, while pid-9 is resistant to inhibition by staurosporine. Staurosporine and the phosphatase inhibitor, cantharidin, delay the asymmetric expression of DR5revGFP (green fluorescent protein) at the root tip after gravistimulation. Gravity response defects of rcn1 and pid-9 are partially rescued by treatment with staurosporine and cantharidin, respectively. The pid-9 rcn1 double mutant has a more rapid gravitropic response than rcn1. These data are consistent with a reciprocal regulation of gravitropism by RCN1 and PID. Furthermore, the effect of staurosporine is lost in pinformed2 (pin2). Our data suggest that reduced PID kinase function inhibits gravitropism and basipetal indole-3-acetic acid transport. However, in contrast to PID overexpression studies, we observed wild-type asymmetric membrane distribution of the PIN2 protein in both pid-9 and wild-type root tips, although PIN2 accumulates in endomembrane structures in pid-9 roots. Similarly, staurosporine-treated plants expressing a PIN2GFP fusion exhibit endomembrane accumulation of PIN2GFP, but no changes in membrane asymmetries were detected. Our data suggest that PID plays a limited role in root development; loss of PID activity alters auxin transport and gravitropism without causing an obvious change in cellular polarity.

Different effects of aluminum on the actin cytoskeleton and brefeldin A-sensitive vesicle recycling in root apex cells of two maize varieties differing in root elongation rate and aluminum tolerance

A relationship between aluminum (Al) toxicity, endocytosis, endosomes and vesicle recycling in the root transition zone has recently been demonstrated. Here the importance of filamentous actin (F-actin)-based vesicle trafficking for Al tolerance has been investigating in maize varieties differing in their Al sensitivities. More Al was internalized into root tip cells of the Al-sensitive variety 16x36 than in the Al-tolerant variety Cateto. The actin cytoskeleton and vesicle trafficking were primary targets for Al toxicity in the root tips of the sensitive variety. Visualization of boron-cross-linked rhamnogalacturonan II (RGII)-containing brefeldin A (BFA) compartments revealed that Al inhibited the formation of these compartments, especially in variety 16x36. The time sequence of Al effects on pectin recycling matches the growth effects of Al in this sensitive variety. These results support the hypothesis that Al binding to pectin-rich cell walls can contribute to reversible inhibition of root elongation. Al-induced alterations on F-actin were most evident in the central part of the transition zone of Al-sensitive 16x36, where Al was localized inside the nucleoli. In relation to this observation, a role for symplastic Al in both irreversible growth inhibition and amelioration of BFA-induced inhibition of root elongation is discussed.

Hormone interactions at the root apical meristem

Plants exhibit an amazing developmental flexibility. Plant embryogenesis results in the establishment of a simple apical-basal axis represented by apical shoot and basal root meristems. Later, during postembryonic growth, shaping of the plant body continues by the formation and activation of numerous adjacent meristems that give rise to lateral shoot branches, leaves, flowers, or lateral roots. This developmental plasticity reflects an important feature of the plant's life strategy based on the rapid reaction to different environmental stimuli, such as temperature fluctuations, availability of nutrients, light or water and response resulting in modulation of developmental programs. Plant hormones are important endogenous factors for the integration of these environmental inputs and regulation of plant development. After a period of studies focused primarily on single hormonal pathways that enabled us to understand the hormone perception and signal transduction mechanisms, it became obvious that the developmental output mediated by a single hormonal pathway is largely modified through a whole network of interactions with other hormonal pathways. In this review, we will summarize recent knowledge on hormonal networks that regulate the development and growth of root with focus on the hormonal interactions that shape the root apical meristem.

Cytokinin regulates root meristem activity via modulation of the polar auxin transport

Plant development is governed by signaling molecules called phytohormones. Typically, in certain developmental processes more than 1 hormone is implicated and, thus, coordination of their overlapping activities is crucial for correct plant development. However, molecular mechanisms underlying the hormonal crosstalk are only poorly understood. Multiple hormones including cytokinin and auxin have been implicated in the regulation of root development. Here we dissect the roles of cytokinin in modulating growth of the primary root. We show that cytokinin effect on root elongation occurs through ethylene signaling whereas cytokinin effect on the root meristem size involves ethylene-independent modulation of transport-dependent asymmetric auxin distribution. Exogenous or endogenous modification of cytokinin levels and cytokinin signaling lead to specific changes in transcription of several auxin efflux carrier genes from the PIN family having a direct impact on auxin efflux from cultured cells and on auxin distribution in the root apex. We propose a novel model for cytokinin action in regulating root growth: Cytokinin influences cell-to-cell auxin transport by modification of expression of several auxin transport components and thus modulates auxin distribution important for regulation of activity and size of the root meristem.

Decreased mitochondrial activities of malate dehydrogenase and fumarase in tomato lead to altered root growth and architecture via diverse mechanisms

Transgenic tomato (Solanum lycopersicum) plants in which either mitochondrial malate dehydrogenase or fumarase was antisense inhibited have previously been characterized to exhibit altered photosynthetic metabolism. Here, we demonstrate that these manipulations also resulted in differences in root growth, with both transgenics being characterized by a dramatic reduction of root dry matter deposition and respiratory activity but opposite changes with respect to root area. A range of physiological, molecular, and biochemical experiments were carried out in order to determine whether changes in root morphology were due to altered metabolism within the root itself, alterations in the nature of the transformants' root exudation, consequences of alteration in the efficiency of photoassimilate delivery to the root, or a combination of these factors. Grafting experiments in which the transformants were reciprocally grafted to wild-type controls suggested that root length and area were determined by the aerial part of the plant but that biomass was not. Despite the transgenic roots displaying alteration in the expression of phytohormone-associated genes, evaluation of the levels of the hormones themselves revealed that, with the exception of gibberellins, they were largely unaltered. When taken together, these combined experiments suggest that root biomass and growth are retarded by root-specific alterations in metabolism and gibberellin contents. These data are discussed in the context of current models of root growth and biomass partitioning.

Tobacco Arp3 is localized to actin-nucleating sites in vivo

The polarity of actin is a central determinant of intracellular transport in plant cells. To visualize actin polarity in living plant cells, the tobacco homologue of the actin-related protein 3 (ARP3) was cloned and a fusion with the red fluorescent protein (RFP) was generated. Upon transient expression of these fusions in the tobacco cell line BY-2 (Nicotiana tabacum L. cv. Bright Yellow 2), punctate structures were observed near the nuclear envelope and in the cortical plasma. These dots could be shown to decorate actin filaments by expressing RFP-ARP3 in a marker line, where actin was tagged by GFP (green fluorescent protein)-FABD (fimbrin actin-binding domain 2). When actin filaments were disrupted by latrunculin B or by prolonged cold treatment, and subsequently allowed to recover, the actin filaments reformed from the RFP-ARP3 structures, that therefore represented actin nucleation sites. The intracellular distribution of these sites was followed during the formation of pluricellular files, and it was observed that the density of RFP-ARP3 increased in the apex of the polarized, terminal cells of a file, whereas it was equally distributed in the central cells of a file. These findings are interpreted in terms of position-dependent differences of actin organization.

Environmental effects on spatial and temporal patterns of leaf and root growth

Leaves and roots live in dramatically different habitats, but are parts of the same organism. Automated image processing of time-lapse records of these organs has led to understanding of spatial and temporal patterns of growth on time scales from minutes to weeks. Growth zones in roots and leaves show distinct patterns during a diel cycle (24 h period). In dicot leaves under nonstressful conditions these patterns are characterized by endogenous rhythms, sometimes superimposed upon morphogenesis driven by environmental variation. In roots and monocot leaves the growth patterns depend more strongly on environmental fluctuations. Because the impact of spatial variations and temporal fluctuations of above- and belowground environmental parameters must be processed by the plant body as an entire system whose individual modules interact on different levels, growth reactions of individual modules are often highly nonlinear. A mechanistic understanding of plant resource use efficiency and performance in a dynamically fluctuating environment therefore requires an accurate analysis of leaf and root growth patterns in conjunction with knowledge of major intraplant communication systems and metabolic pathways.

Cell polarity signaling: focus on polar auxin transport

Polar auxin transport, which is required for the formation of auxin gradients and directional auxin flows that are critical for plant pattern formation, morphogenesis, and directional growth response to vectorial cues, is mediated by polarized sub-cellular distribution of PIN-FORMED Proteins (PINs, auxin efflux carriers), AUX1/AUX1-like proteins (auxin influx facilitators), and multidrug resistance P-glycoproteins (MDR/PGP). Polar localization of these proteins is controlled by both developmental and environmental cues. Recent studies have revealed cellular (endocytosis, transcytosis, and endosomal sorting and recycling) and molecular (PINOID kinase, protein phosphatase 2A) mechanisms underlying the polar distribution of these auxin transport proteins. Both TIR1-mediated auxin signaling and TIR1-independent auxin-mediated endocytosis have been shown to regulate polar PIN localization and auxin flow, implicating auxin as a self-organizing signal in directing polar transport and directional flows.

Plasma membrane-associated SCAR complex subunits promote cortical F-actin accumulation and normal growth characteristics in Arabidopsis roots

The ARP2/3 complex, a highly conserved nucleator of F-actin polymerization, and its activator, the SCAR complex, have been shown to play important roles in leaf epidermal cell morphogenesis in Arabidopsis. However, the intracellular site(s) and function(s) of SCAR and ARP2/3 complex-dependent actin polymerization in plant cells remain unclear. We demonstrate that putative SCAR complex subunits BRK1 and SCAR1 are localized to the plasma membrane at sites of cell growth and wall deposition in expanding cells of leaves and roots. BRK1 localization is SCAR-dependent, providing further evidence of an association between these proteins in vivo. Consistent with plasma membrane localization of SCAR complex subunits, cortical F-actin accumulation in root tip cells is reduced in brk1 mutants. Moreover, mutations disrupting the SCAR or ARP2/3 complex reduce the growth rate of roots and their ability to penetrate semi-solid medium, suggesting reduced rigidity. Cell walls of mutant roots exhibit abnormal structure and composition at intercellular junctions where BRK1 and SCAR1 are enriched in the adjacent plasma membrane. Taken together, our results suggest that SCAR and ARP2/3 complex-dependent actin polymerization promotes processes at the plasma membrane that are important for normal growth and wall assembly.

The role of auxins and cytokinins in the mutualistic interaction between Arabidopsis and Piriformospora indica

Arabidopsis growth and reproduction are stimulated by the endophytic fungus Piriformospora indica. The fungus produces low amounts of auxins, but the auxin levels and the expression of auxin-regulated genes are not altered in colonized roots. Also, mutants with reduced auxin levels (ilr1-1, nit1-3, tfl2, cyp79 b2b3) respond to P. indica. However, the fungus rescues the dwarf phenotype of the auxin overproducer sur1-1 by converting free auxin into conjugates, which also results in the downregulation of the auxin-induced IAA6 and the upregulation of the P. indica-induced LRR1 gene. The fungus produces relatively high levels of cytokinins, and the cytokinin levels are higher in colonized roots compared with the uncolonized controls. trans-Zeatin cytokinin biosynthesis and the CRE1/AHK2 receptor combination are crucial for P. indica-mediated growth stimulation, while mutants lacking cis-zeatin, impaired in other cytokinin receptor combinations, or containing reduced cytokinin levels respond to the fungus. Since root colonization is not affected in the cytokinin mutants, we propose that cytokinins are required for P. indica-induced growth promotion. Finally, a comparative analysis of the phytohormone mutants allows the conclusion that the response to P. indica is independent of the architecture and size of the roots.

D'orenone blocks polarized tip growth of root hairs by interfering with the PIN2-mediated auxin transport network in the root apex

The C(18) ketone (5E,7E)-6-methyl-8-(2,6,6-trimethylcyclohex-1-enyl)octa-5,7-dien-2-one (D'orenone) has been postulated to be an early cleavage product of beta-carotene en route to trisporic acids; these act as morphogenetic factors during the sexual reproduction of zygomycetes. Here we report that D'orenone blocks the highly polarized tip growth of root hairs, causing tip growth to stop completely within a few minutes. Importantly, external auxin reverses the effects of D'orenone on root hairs. Further analysis revealed that D'orenone lowers the auxin concentration in trichoblasts via PIN2-mediated auxin efflux to below the critical levels essential for root hair growth. D'orenone specifically increases PIN2 protein abundance without affecting PIN2 transcripts, and the PIN2 expression domain enlarges and shifts basipetally, resulting in more active auxin transport. The observation that D'orenone does not interfere with the root hair growth in roots of null mutant lines provides additional evidence that PIN2 is its specific target.

Cytokinins negatively regulate the root iron uptake machinery in Arabidopsis through a growth-dependent pathway

Plants display a number of biochemical and developmental responses to low iron availability in order to increase iron uptake from the soil. The ferric-chelate reductase FRO2 and the ferrous iron transporter IRT1 control iron entry from the soil into the root epidermis. In Arabidopsis, expression of IRT1 and FRO2 is tightly controlled to maintain iron homeostasis, and involves local and long-distance signals, as well as transcriptional and post-transcriptional events. FIT encodes a putative basic helix-loop-helix (bHLH) transcription factor that regulates iron uptake responses in Arabidopsis. Here, we uncover a new regulation of the root iron uptake genes. We show that IRT1, FRO2 and FIT are repressed by the exogenous addition of cytokinins (CKs), and that this repression acts at the level of transcript accumulation, and depends on the AHK3 and CRE1 CK receptors. The CKs and iron-deficiency signals act through distinct pathways to regulate the soil iron uptake genes, as (i) CK repression is independent of the iron status, (ii) IRT1 and FRO2 downregulation is unchanged in a fit loss-of-function mutant, indicating that FIT does not mediate CK repression, and (iii) the iron-regulated genes AtNRAMP3 and AtNRAMP4 are not downregulated by CKs. We show that root growth-inhibitory conditions, such as abiotic stresses (mannitol, NaCl) and hormonal treatments (auxin, abscissic acid), repress the iron starvation response genes. We propose that CKs control the root iron uptake machinery through a root growth dependent pathway in order to adapt nutrient uptake to the demand of the plant.

Ethylene-auxin interactions regulate lateral root initiation and emergence in Arabidopsis thaliana

Plant root systems display considerable plasticity in response to endogenous and environmental signals. Auxin stimulates pericycle cells within elongating primary roots to enter de novo organogenesis, leading to the establishment of new lateral root meristems. Crosstalk between auxin and ethylene in root elongation has been demonstrated, but interactions between these hormones in root branching are not well characterized. We find that enhanced ethylene synthesis, resulting from the application of low concentrations of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC), promotes the initiation of lateral root primordia. Treatment with higher doses of ACC strongly inhibits the ability of pericycle cells to initiate new lateral root primordia, but promotes the emergence of existing lateral root primordia: behaviour that is also seen in the eto1 mutation. These effects are correlated with decreased pericycle cell length and increased lateral root primordia cell width. When auxin is applied simultaneously with ACC, ACC is unable to prevent the auxin stimulation of lateral root formation in the root tissues formed prior to ACC exposure. However, in root tissues formed after transfer to ACC, in which elongation is reduced, auxin does not rescue the ethylene inhibition of primordia initiation, but instead increases it by several fold. Mutations that block auxin responses, slr1 and arf7 arf19, render initiation of lateral root primordia insensitive to the promoting effect of low ethylene levels, and mutations that inhibit ethylene-stimulated auxin biosynthesis, wei2 and wei7, reduce the inhibitory effect of higher ethylene levels, consistent with ethylene regulating root branching through interactions with auxin.

Auxin transport inhibitors impair vesicle motility and actin cytoskeleton dynamics in diverse eukaryotes

Many aspects of plant development, including patterning and tropisms, are largely dependent on the asymmetric distribution of the plant signaling molecule auxin. Auxin transport inhibitors (ATIs), which interfere with directional auxin transport, have been essential tools in formulating this concept. However, despite the use of ATIs in plant research for many decades, the mechanism of ATI action has remained largely elusive. Using real-time live-cell microscopy, we show here that prominent ATIs such as 2,3,5-triiodobenzoic acid (TIBA) and 2-(1-pyrenoyl) benzoic acid (PBA) inhibit vesicle trafficking in plant, yeast, and mammalian cells. Effects on micropinocytosis, rab5-labeled endosomal motility at the periphery of HeLa cells and on fibroblast mobility indicate that ATIs influence actin cytoskeleton. Visualization of actin cytoskeleton dynamics in plants, yeast, and mammalian cells show that ATIs stabilize actin. Conversely, stabilizing actin by chemical or genetic means interferes with endocytosis, vesicle motility, auxin transport, and plant development, including auxin transport-dependent processes. Our results show that a class of ATIs act as actin stabilizers and advocate that actin-dependent trafficking of auxin transport components participates in the mechanism of auxin transport. These studies also provide an example of how the common eukaryotic process of actin-based vesicle motility can fulfill a plant-specific physiological role.

Different subcellular localizations and functions of Arabidopsis myosin VIII

BACKGROUND

Myosins are actin-activated ATPases that use energy to generate force and move along actin filaments, dragging with their tails different cargos. Plant myosins belong to the group of unconventional myosins and Arabidopsis myosin VIII gene family contains four members: ATM1, ATM2, myosin VIIIA and myosin VIIIB.

RESULTS

In transgenic plants expressing GFP fusions with ATM1 (IQ-tail truncation, lacking the head domain), fluorescence was differentially distributed: while in epidermis cells at the root cap GFP-ATM1 equally distributed all over the cell, in epidermal cells right above this region it accumulated in dots. Further up, in cells of the elongation zone, GFP-ATM1 was preferentially positioned at the sides of transversal cell walls. Interestingly, the punctate pattern was insensitive to brefeldin A (BFA) while in some cells closer to the root cap, ATM1 was found in BFA bodies. With the use of different markers and transient expression in Nicotiana benthamiana leaves, it was found that myosin VIII co-localized to the plasmodesmata and ER, colocalized with internalized FM4-64, and partially overlapped with the endosomal markers ARA6, and rarely with ARA7 and FYVE. Motility of ARA6 labeled organelles was inhibited whenever associated with truncated ATM1 but motility of FYVE labeled organelles was inhibited only when associated with large excess of ATM1. Furthermore, GFP-ATM1 and RFP-ATM2 (IQ-tail domain) co-localized to the same spots on the plasma membrane, indicating a specific composition at these sites for myosin binding.

CONCLUSION

Taken together, our data suggest that myosin VIII functions differently in different root cells and can be involved in different steps of endocytosis, BFA-sensitive and insensitive pathways, ER tethering and plasmodesmatal activity.

Improved imaging of actin filaments in transgenic Arabidopsis plants expressing a green fluorescent protein fusion to the C- and N-termini of the fimbrin actin-binding domain 2

The role of the actin cytoskeleton in plant development is intimately linked to its dynamic behavior. Therefore it is essential to continue refining methods for studying actin organization in living plant cells. The discovery of green fluorescent protein (GFP) has popularized the use of translational fusions of GFP with actin filament (F-actin) side-binding proteins to visualize in vivo actin organization in plants. The most recent of these live cell F-actin reporters are GFP fusions to the actin-binding domain 2 (ABD2) of Arabidopsis fimbrin 1 (ABD2-GFP). To improve ABD2-GFP fluorescence for enhanced in vivo F-actin imaging, transgenic Arabidopsis plants were generated expressing a construct with GFP fused to both the C- and N-termini of ABD2 under the control of the CaMV 35S promoter (35S::GFP-ABD2-GFP). The 35S::GFP-ABD2-GFP lines had significantly increased fluorescence compared with the original 35S::ABD2-GFP lines. The enhanced fluorescence of the 35S::GFP-ABD2-GFP-expressing lines allowed the acquisition of highly resolved images of F-actin in different plant organs and stages of development because of the reduced confocal microscope excitation settings needed for data collection. This simple modification to the ABD2-GFP construct presents an important tool for studying actin function during plant development.

The Arabidopsis CLASP gene encodes a microtubule-associated protein involved in cell expansion and division

Controlling microtubule dynamics and spatial organization is a fundamental requirement of eukaryotic cell function. Members of the ORBIT/MAST/CLASP family of microtubule-associated proteins associate with the plus ends of microtubules, where they promote the addition of tubulin subunits into attached kinetochore fibers during mitosis and stabilize microtubules in the vicinity of the plasma membrane during interphase. To date, nothing is known about their function in plants. Here, we show that the Arabidopsis thaliana CLASP protein is a microtubule-associated protein that is involved in both cell division and cell expansion. Green fluorescent protein-CLASP localizes along the full length of microtubules and shows enrichment at growing plus ends. Our analysis suggests that CLASP promotes microtubule stability. clasp-1 T-DNA insertion mutants are hypersensitive to microtubule-destabilizing drugs and exhibit more sparsely populated, yet well ordered, root cortical microtubule arrays. Overexpression of CLASP promotes microtubule bundles that are resistant to depolymerization with oryzalin. Furthermore, clasp-1 mutants have aberrant microtubule preprophase bands, mitotic spindles, and phragmoplasts, indicating a role for At CLASP in stabilizing mitotic arrays. clasp-1 plants are dwarf, have significantly reduced cell numbers in the root division zone, and have defects in directional cell expansion. We discuss possible mechanisms of CLASP function in higher plants.