Unveiling Molecular Signatures in Light-Induced Seed Germination: Insights from PIN3, PIN7, and AUX1 in Arabidopsis thaliana

Light provides seeds with information that is essential for the adjustment of their germination to the conditions that are most favorable for the successful establishment of the future seedling. The promotion of germination depends mainly on environmental factors, like temperature and light, as well as internal factors associated with the hormonal balance between gibberellins (GA) and abscisic acid (ABA), although other hormones such as auxins may act secondarily. While transcriptomic studies of light-germinating Arabidopsis thaliana seeds suggest that auxins and auxin transporters are necessary, there are still no functional studies connecting the activity of the auxin transporters in light-induced seed germination. In this study, we investigated the roles of two auxin efflux carrier (PIN3 and PIN7) proteins and one auxin influx (AUX1) carrier protein during Arabidopsis thaliana seed germination. By using next-generation sequencing (RNAseq), gene expression analyses, hormonal sensitivity assays, and the quantification of indole-3-acetic acid (IAA) levels, we assessed the functional roles of PIN3, PIN7, and AUX1 during light-induced seed germination. We showed that auxin levels are increased 24 h after a red-pulse (Rp). Additionally, we evaluated the germination responses of pin3, pin7, and aux1 mutant seeds and showed that PIN3, PIN7, and AUX1 auxin carriers are important players in the regulation of seed germination. By using gene expression analysis in water, fluridone (F), and ABA+F treated seeds, we confirmed that Rp-induced seed germination is associated with auxin transport, and ABA controls the function of PIN3, PIN7, and AUX1 during this process. Overall, our results highlight the relevant and positive role of auxin transporters in germinating the seeds of Arabidopsis thaliana.

The MADS-box family gene PtrANR1 encodes a transcription activator promoting root growth and enhancing plant tolerance to drought stress

KEY MESSAGE

PtrANR1 positively regulates plant drought tolerance by increasing proline level and reducing ROS accumulation. PtrANR1 directly activates PtrAUX1 expression to promote root growth and improve plant drought tolerance. Citrus quality and yield are severely declined under drought stress. To date, the effects of MADS-box family transcription factors (TFs) on plant drought resistance have made some progress. However, whether MADS-box family TFs are associated with citrus drought response has remained unclear. The current paper identified a MADS-box family gene PtrANR1 encoding anthocyanidin reductase from trifoliate orange. PtrANR1 exhibits high identities with ANR1 proteins found in various plants. PtrANR1 possesses two conserved domains known as MADS and kertanin-like domains. PtrANR1 is a nuclear protein which has transactivation activity. A significant induction of PtrANR1 transcript was detected in leaves and roots of trifoliate orange treated with PEG6000 and ABA. Under drought stress, Arabidopsis ectopic overexpressing PtrANR1 exhibited obviously elevated contents of proline, ABA and IAA, better developed root, enhanced antioxidant enzyme activities, as well as notably reduced accumulation of malondialdehyde (MDA) and reactive oxygen species (ROS) compared with WT plants. However, opposite change trends of these physiological indices were detected in PtrANR1 homolog silencing lemon. Furthermore, transgenic Arabidopsis displayed significantly increased expression levels in genes associated with ABA, IAA and proline production, IAA polar transport, ROS elimination and drought response. However, these genes exhibited noticeably decreased transcript levels in PtrANR1 homolog silencing lemon. Moreover, PtrANR1 could increase IAA content and promote root growth by binding to GArG-box in the promoter of PtrAUX1 to activate its transcript. These findings indicated that PtrANR1 had a beneficial impact on plant drought resistance through promoting root development, increasing proline accumulation and scavenging of ROS.

ERF1 inhibits lateral root emergence by promoting local auxin accumulation and repressing ARF7 expression

Lateral roots (LRs) are crucial for plants to sense environmental signals in addition to water and nutrient absorption. Auxin is key for LR formation, but the underlying mechanisms are not fully understood. Here, we report that Arabidopsis ERF1 inhibits LR emergence by promoting local auxin accumulation with altered distribution and regulating auxin signaling. Loss of ERF1 increases LR density compared with the wild type, whereas ERF1 overexpression causes the opposite phenotype. ERF1 enhances auxin transport by upregulating PIN1 and AUX1, resulting in excessive auxin accumulation in the endodermal, cortical, and epidermal cells surrounding LR primordia. Furthermore, ERF1 represses ARF7 transcription, thereby downregulating the expression of cell-wall remodeling genes that facilitate LR emergence. Together, our study reveals that ERF1 integrates environmental signals to promote local auxin accumulation with altered distribution and repress ARF7, consequently inhibiting LR emergence in adaptation to fluctuating environments.

Effect of Low Light Stress on Distribution of Auxin (Indole-3-acetic Acid) between Shoot and Roots and Development of Lateral Roots in Barley Plants

Depending on their habitat conditions, plants can greatly change the growth rate of their roots. However, the mechanisms of such responses remain insufficiently clear. The influence of a low level of illumination on the content of endogenous auxins, their localization in leaves and transport from shoots to roots were studied and related to the lateral root branching of barley plants. Following two days' reduction in illumination, a 10-fold reduction in the emergence of lateral roots was found. Auxin (IAA, indole-3-acetic acid) content decreased by 84% in roots and by 30% in shoots, and immunolocalization revealed lowered IAA levels in phloem cells of leaf sections. The reduced content of IAA found in the plants under low light suggests an inhibition of production of this hormone under these conditions. At the same time, two-fold downregulation of the LAX3 gene expression, facilitating IAA influx into the cells, was detected in the roots, as well as a decline in auxin diffusion from shoots through the phloem by about 60%. It was suggested that the reduced emergence of lateral roots in barley under a low level of illumination was due to a disturbance of auxin transport through the phloem and down-regulation of the genes responsible for auxin transport in plant roots. The results confirm the importance of the long distance transport of auxins for the control of the growth of roots under conditions of low light. Further study of the mechanisms that control the transport of auxins from shoots to roots in other plant species is required.

Mitochondrial HSC70-1 Regulates Polar Auxin Transport through ROS Homeostasis in Arabidopsis Roots

Arabidopsis mitochondrial-localized heat shock protein 70-1 (mtHSC70-1) modulates vegetative growth by assisting mitochondrial complex IV assembly and maintaining reactive oxygen species (ROS) homeostasis. In addition, mtHSC70-1 affects embryo development, and this effect is mediated by auxin. However, whether mtHSC70-1 regulates vegetative growth through auxin and knowledge of the link between ROS homeostasis and auxin distribution remain unclear. Here, we found that mtHSC70-1 knockout seedlings (mthsc70-1a) displayed shortened roots, decreased fresh root weight and lateral root number, increased root width and abnormal root morphology. The introduction of the mtHSC70-1 gene into mthsc70-1a restored the growth and development of roots to the level of the wild type. However, sugar and auxin supplementation could not help the mutant roots restore to normal. Moreover, mthsc70-1a seedlings showed a decrease in meristem length and activity, auxin transport carrier (PINs and AUX1) and auxin abundances in root tips. The application of exogenous reducing agents upregulated the levels of PINs in the mutant roots. The introduction of antioxidant enzyme genes (MSD1 or CAT1) into the mthsc70-1a mutant rescued the PIN and local auxin abundances and root growth and development. Taken together, our data suggest that mtHSC70-1 regulates polar auxin transport through ROS homeostasis in Arabidopsis roots.

The role of AUX1 during lateral root development in the domestication of the model C4 grass Setaria italica

C4 photosynthesis increases the efficiency of carbon fixation by spatially separating high concentrations of molecular oxygen from Rubisco. The specialized leaf anatomy required for this separation evolved independently many times. The morphology of C4 root systems is also distinctive and adapted to support high rates of photosynthesis; however, little is known about the molecular mechanisms that have driven the evolution of C4 root system architecture. Using a mutant screen in the C4 model plant Setaria italica, we identify Siaux1-1 and Siaux1-2 as root system architecture mutants. Unlike in S. viridis, AUX1 promotes lateral root development in S. italica. A cell by cell analysis of the Siaux1-1 root apical meristem revealed changes in the distribution of cell volumes in all cell layers and a dependence of the frequency of protophloem and protoxylem strands on SiAUX1. We explore the molecular basis of the role of SiAUX1 in seedling development using an RNAseq analysis of wild-type and Siaux1-1 plants and present novel targets for SiAUX1-dependent gene regulation. Using a selection sweep and haplotype analysis of SiAUX1, we show that Hap-2412TT in the promoter region of SiAUX1 is an allele which is associated with lateral root number and has been strongly selected for during Setaria domestication.

KAI2 promotes Arabidopsis root hair elongation at low external phosphate by controlling local accumulation of AUX1 and PIN2

Root hair (RH) growth to increase the absorptive root surface area is a key adaptation of plants to limiting phosphate availability in soils. Despite the importance of this trait, especially for seedling survival, little is known about the molecular events connecting phosphate starvation sensing and RH growth regulation. KARRIKIN INSENSITIVE2 (KAI2), an α/β-hydrolase receptor of a yet-unknown plant hormone ("KAI2-ligand" [KL]), is required for RH elongation.¹ KAI2 interacts with the F-box protein MORE AXILLIARY BRANCHING2 (MAX2) to target regulatory proteins of the SUPPRESSOR of MAX2 1 (SMAX1) family for degradation.² Here, we demonstrate that P_i starvation increases KL signaling in Arabidopsis roots through transcriptional activation of KAI2 and MAX2. Both genes are required for RH elongation under these conditions, while smax1 smxl2 mutants have constitutively long RHs, even at high P_i availability. Attenuated RH elongation in kai2 mutants is explained by reduced shootward auxin transport from the root tip resulting in reduced auxin signaling in the RH zone, caused by an inability to increase localized accumulation of the auxin importer AUXIN TRANSPORTER PROTEIN1 (AUX1) and the auxin exporter PIN-FORMED2 (PIN2) upon P_i starvation. Consistent with AUX1 and PIN2 accumulation being mediated via ethylene signaling,³ expression of 1-AMINOCYCLOPROPANE-1-CARBOXYLATE SYNTHASE 7 (ACS7) is increased at low P_i in a KAI2-dependent manner, and treatment with an ethylene precursor restores RH elongation of acs7, but not of aux1 and pin2. Thus, KAI2 signaling is increased by phosphate starvation to trigger an ethylene- AUX1/PIN2-auxin cascade required for RH elongation.

Auxin-induced actin cytoskeleton rearrangements require AUX1

The actin cytoskeleton is required for cell expansion and implicated in cellular responses to the phytohormone auxin. However, the mechanisms that coordinate auxin signaling, cytoskeletal remodeling and cell expansion are poorly understood. Previous studies examined long-term actin cytoskeleton responses to auxin, but plants respond to auxin within minutes. Before this work, an extracellular auxin receptor - rather than the auxin transporter AUXIN RESISTANT 1 (AUX1) - was considered to precede auxin-induced cytoskeleton reorganization. In order to correlate actin array organization and dynamics with degree of cell expansion, quantitative imaging tools established baseline actin organization and illuminated individual filament behaviors in root epidermal cells under control conditions and after indole-3-acetic acid (IAA) application. We evaluated aux1 mutant actin organization responses to IAA and the membrane-permeable auxin 1-naphthylacetic acid (NAA). Cell length predicted actin organization and dynamics in control roots; short-term IAA treatments stimulated denser and more parallel, longitudinal arrays by inducing filament unbundling within minutes. Although AUX1 is necessary for full actin rearrangements in response to auxin, cytoplasmic auxin (i.e. NAA) stimulated a lesser response. Actin filaments became more 'organized' after IAA stopped elongation, refuting the hypothesis that 'more organized' actin arrays universally correlate with rapid growth. Short-term actin cytoskeleton response to auxin requires AUX1 and/or cytoplasmic auxin.

NRT1.1-Mediated Nitrate Suppression of Root Coiling Relies on PIN2- and AUX1-Mediated Auxin Transport

Asymmetric root growth (ARG) on tilted plates, or root coiling on horizontally placed plates, is proposed to be a combination of gravitropism, mechanical sensing, and "circumnutation," a word designated by Charles Darwin to describe the helical movement that all plant organs make around the growth direction. ARG is developmentally controlled in which microtubule-regulating proteins and the phytohormone auxin participates. Nutrient deficiency influences ARG. However, it is unclear which nutrients play key roles in regulating ARG, what endogenous components are involved in responding to nutrient deficiency for ARG, and how nutrient deficiency is translated into endogenous responses. We report here that nitrate deficiency resulted in a strong ARG in Arabidopsis. Nitrate deficiency caused root coiling on horizontal plates, which is inhibited by an auxin transport inhibitor, and by mutations in PIN-FORMED2 (PIN2) and AUXIN RESISTANT 1 (AUX1). We further show that suppression of ARG by nitrate is mediated by the nitrate transporter/sensor NRT1.1. In addition, PIN2- and AUX1-mediated auxin transports are epistatic to NRT1.1 in nitrate deficiency-induced ARG. This study reveals a signaling pathway in root growth by responding to exogenous nitrate and relaying it into altered auxin transport.

Auxin and Cell Wall Crosstalk as Revealed by the Arabidopsis thaliana Cellulose Synthase Mutant Radially Swollen 1

Plant cells sheath themselves in a complex lattice of polysaccharides, proteins and enzymes forming an integral matrix known as the cell wall. Cellulose microfibrils, the primary component of cell walls, are synthesized at the plasma membrane by CELLULOSE SYNTHASE A (CESA) proteins throughout cellular growth and are responsible for turgor-driven anisotropic expansion. Associations between hormone signaling and cell wall biosynthesis have long been suggested, but recently direct links have been found revealing hormones play key regulatory roles in cellulose biosynthesis. The radially swollen 1 (rsw1) allele of Arabidopsis thaliana CESA1 harbors a single amino acid change that renders the protein unstable at high temperatures. We used the conditional nature of rsw1 to investigate how auxin contributes to isotropic growth. We found that exogenous auxin treatment reduces isotropic swelling in rsw1 roots at the restrictive temperature of 30�C. We also discovered decreases in auxin influx between rsw1 and wild-type roots via confocal imaging of AUX1-YFP, even at the permissive temperature of 19�C. Moreover, rsw1 displayed mis-expression of auxin-responsive and CESA genes. Additionally, we found altered auxin maxima in rsw1 mutant roots at the onset of swelling using DII-VENUS and DR5:vYFP auxin reporters. Overall, we conclude disrupted cell wall biosynthesis perturbs auxin transport leading to altered auxin homeostasis impacting both anisotropic and isotropic growth that affects overall root morphology.

AUX1 acts upstream of PIN2 in regulating root gravitropism

AUX1 and PIN2 auxin transporter are required for the asymmetric distribution of auxin for root gravitropic response. However, the relationship between AUX1 and PIN2 in root gravitropism is unclear. Here, we report that aux1-T mutant show stronger defects in root gravitropism than pin2-T, and aux1-T pin2-T double mutants display similar agravitropic phenotype to aux1-T. The gravity-induced asymmetric distribution of auxin responses could not be established in pin2-T, aux1-T and aux1-T pin2-T mutants; whereas aux1-T pin2-T double mutants showed similar auxin responses to aux1-T mutant. These findings support AUX1 plays a role in root gravitropism upstream of PIN2.

Broad spectrum developmental role of Brachypodium AUX1

Targeted cellular auxin distribution is required for morphogenesis and adaptive responses of plant organs. In Arabidopsis thaliana (Arabidopsis), this involves the prototypical auxin influx facilitator AUX1 and its LIKE-AUX1 (LAX) homologs, which act partially redundantly in various developmental processes. Interestingly, AUX1 and its homologs are not strictly essential for the Arabidopsis life cycle. Indeed, aux1 lax1 lax2 lax3 quadruple knock-outs are mostly viable and fertile, and strong phenotypes are only observed at low penetrance. Here we investigated the Brachypodium distachyon (Brachypodium) AUX1 homolog BdAUX1 by genetic, cell biological and physiological analyses. We report that BdAUX1 is essential for Brachypodium development. Bdaux1 loss-of-function mutants are dwarfs with aberrant flower development, and consequently infertile. Moreover, they display a counter-intuitive root phenotype. Although Bdaux1 roots are agravitropic as expected, in contrast to Arabidopsis aux1 mutants they are dramatically longer than wild type roots because of exaggerated cell elongation. Interestingly, this correlates with higher free auxin content in Bdaux1 roots. Consistently, their cell wall characteristics and transcriptome signature largely phenocopy other Brachypodium mutants with increased root auxin content. Our results imply fundamentally different wiring of auxin transport in Brachypodium roots and reveal an essential role of BdAUX1 in a broad spectrum of developmental processes, suggesting a central role for AUX1 in pooideae.

Ethylene mediates dichromate-induced inhibition of primary root growth by altering AUX1 expression and auxin accumulation in Arabidopsis thaliana

The hexavalent form of chromium [Cr(VI)] causes a major reduction in yield and quality of crops worldwide. The root is the first plant organ that interacts with Cr(VI) toxicity, which inhibits primary root elongation, but the underlying mechanisms of this inhibition remain elusive. In this study, we investigate the possibility that Cr(VI) reduces primary root growth of Arabidopsis by modulating the cell cycle-related genes and that ethylene signalling contributes to this process. We show that Cr(VI)-mediated inhibition of primary root elongation was alleviated by the ethylene perception and biosynthesis antagonists silver and cobalt, respectively. Furthermore, the ethylene signalling defective mutants (ein2-1 and etr1-3) were insensitive, whereas the overproducer mutant (eto1-1) was hypersensitive to Cr(VI). We also report that high levels of Cr(VI) significantly induce the distribution and accumulation of auxin in the primary root tips, but this increase was significantly suppressed in seedlings exposed to silver or cobalt. In addition, genetic and physiological investigations show that AUXIN-RESISTANT1 (AUX1) participates in Cr(VI)-induced inhibition of primary root growth. Taken together, our results indicate that ethylene mediates Cr(VI)-induced inhibition of primary root elongation by increasing auxin accumulation and polar transport by stimulating the expression of AUX1.

Auxin-Induced Plasma Membrane Depolarization Is Regulated by Auxin Transport and Not by AUXIN BINDING PROTEIN1

Auxin is a molecule, which controls many aspects of plant development through both transcriptional and non-transcriptional signaling responses. AUXIN BINDING PROTEIN1 (ABP1) is a putative receptor for rapid non-transcriptional auxin-induced changes in plasma membrane depolarization and endocytosis rates. However, the mechanism of ABP1-mediated signaling is poorly understood. Here we show that membrane depolarization and endocytosis inhibition are ABP1-independent responses and that auxin-induced plasma membrane depolarization is instead dependent on the auxin influx carrier AUX1. AUX1 was itself not involved in the regulation of endocytosis. Auxin-dependent depolarization of the plasma membrane was also modulated by the auxin efflux carrier PIN2. These data establish a new connection between auxin transport and non-transcriptional auxin signaling.

Jasmonoyl-L-Tryptophan Disrupts IAA Activity through the AUX1 Auxin Permease

Amide-linked conjugates between tryptophan (Trp) and jasmonic (JA) or indole-3-acetic (IAA) acids interfered with gravitropism and other auxin-dependent activities in Arabidopsis, but the mechanism was unclear. To identify structural features necessary for activity several additional Trp conjugates were synthesized. The phenylacetic acid (PAA) conjugate was active, while several others were not. Common features of active conjugates is that they have ring structures that are linked to Trp through an acetic acid side chain, while longer or shorter linkages are inactive or less active. A dominant mutant, called tryptophan conjugate response1-D that is insensitive to JA-Trp, but still sensitive to other active conjugates, was identified and the defect was found to be a substitution of Asn for Asp⁴⁵⁶ in the C-terminal domain of the IAA cellular permease AUX1. Mutant seedling primary root growth in the absence of added conjugate was 15% less than WT, but otherwise plant phenotype appeared normal. These results suggest that JA-Trp disrupts AUX1 activity, but that endogenous JA-Trp has only a minor role in regulating plant growth. In contrast with IAA- and JA-Trp, which are present at <2 pmole g^-1 FW, PAA-Trp was found at about 30 pmole g^-1 FW. The latter, or other undiscovered Trp conjugates, may still have important endogenous roles, possibly helping to coordinate other pathways with auxin response.

PETAL LOSS and ROXY1 Interact to Limit Growth Within and between Sepals But to Promote Petal Initiation in Arabidopsis thaliana

The activity of genes controlling organ development may be associated with the redox state of subregions within the meristem. Glutaredoxins react to the level of oxidative potential and can reduce cysteine dithiols, in some cases to activate specific transcription factors. In Arabidopsis, loss of function of the glutaredoxin ROXY1 or the trihelix transcription factor PETAL LOSS (PTL) each results in reduced numbers of petals. Here, genetic studies have revealed that loss of petals in ptl mutant plants depends on ROXY1 function. The two genes also act together to restrain stamen-identifying C function from entering the outer whorls. On the other hand, they suppress growth between sepals and in sepal margins, with ROXY1 action partially redundant to that of PTL. Genetic interactions with aux1 mutations indicate that auxin activity is reduced in the petal whorl of roxy1 mutants as in ptl mutants. However, it is apparently increased in the sepal whorl of triple mutants associated with the ectopic outgrowth of sepal margins, and of finger-like extensions of inter-sepal zones that in 20% of cases are topped with bunches of ectopic sepals. These interactions may be indirect, although PTL and ROXY1 proteins can interact directly when co-expressed in a transient assay. Changes of conserved cysteines within PTL to similar amino acids that cannot be oxidized did not block its function. It may be in some cases that under reducing conditions ROXY1 binds PTL and activates it by reducing specific conserved cysteines, thus resulting in growth suppression.

Cytokinin acts through the auxin influx carrier AUX1 to regulate cell elongation in the root

Hormonal interactions are crucial for plant development. In Arabidopsis, cytokinins inhibit root growth through effects on cell proliferation and cell elongation. Here, we define key mechanistic elements in a regulatory network by which cytokinin inhibits root cell elongation in concert with the hormones auxin and ethylene. The auxin importer AUX1 functions as a positive regulator of cytokinin responses in the root; mutation of AUX1 specifically affects the ability of cytokinin to inhibit cell elongation but not cell proliferation. AUX1 is required for cytokinin-dependent changes of auxin activity in the lateral root cap associated with the control of cell elongation. Cytokinin regulates root cell elongation through ethylene-dependent and -independent mechanisms, both hormonal signals converging on AUX1 as a regulatory hub. An autoregulatory circuit is identified involving the control of ARR10 and AUX1 expression by cytokinin and auxin, this circuit potentially functioning as an oscillator to integrate the effects of these two hormones. Taken together, our results uncover several regulatory circuits controlling interactions of cytokinin with auxin and ethylene, and support a model in which cytokinin regulates shootward auxin transport to control cell elongation and root growth.

Cytokinins influence root gravitropism via differential regulation of auxin transporter expression and localization in Arabidopsis

Redirection of intercellular auxin fluxes via relocalization of the PIN-FORMED 3 (PIN3) and PIN7 auxin efflux carriers has been suggested to be necessary for the root gravitropic response. Cytokinins have also been proposed to play a role in controlling root gravitropism, but conclusive evidence is lacking. We present a detailed study of the dynamics of root bending early after gravistimulation, which revealed a delayed gravitropic response in transgenic lines with depleted endogenous cytokinins (Pro35S:AtCKX) and cytokinin signaling mutants. Pro35S:AtCKX lines, as well as a cytokinin receptor mutant ahk3, showed aberrations in the auxin response distribution in columella cells consistent with defects in the auxin transport machinery. Using in vivo real-time imaging of PIN3-GFP and PIN7-GFP in AtCKX3 overexpression and ahk3 backgrounds, we observed wild-type-like relocalization of PIN proteins in the columella early after gravistimulation, with gravity-induced relocalization of PIN7 faster than that of PIN3. Nonetheless, the cellular distribution of PIN3 and PIN7 and expression of PIN7 and the auxin influx carrier AUX1 was affected in AtCKX overexpression lines. Based on the retained cytokinin sensitivity in pin3 pin4 pin7 mutant, we propose the AUX1-mediated auxin transport rather than columella-located PIN proteins as a target of endogenous cytokinins in the control of root gravitropism.

Auxin Influx Carrier AUX1 Confers Acid Resistance for Arabidopsis Root Elongation Through the Regulation of Plasma Membrane H+-ATPase

The plant plasma membrane (PM) H+-ATPase regulates pH homeostasis and cell elongation in roots through the formation of an electrochemical H+ gradient across the PM and a decrease in apoplastic pH; however, the detailed signaling for the regulation of PM H+-ATPases remains unclear. Here, we show that an auxin influx carrier, AUXIN RESISTANT1 (AUX1), is required for the maintenance of PM H+-ATPase activity and proper root elongation. We isolated a low pH-hypersensitive 1 (loph1) mutant by a genetic screen of Arabidopsis thaliana on low pH agar plates. The loph1 mutant is a loss-of-function mutant of the AUX1 gene and exhibits a root growth retardation restricted to the low pH condition. The ATP hydrolysis and H+ extrusion activities of the PM H+-ATPase were reduced in loph1 roots. Furthermore, the phosphorylation of the penultimate threonine of the PM H+-ATPase was reduced in loph1 roots under both normal and low pH conditions without reduction of the amount of PM H+-ATPase. Expression of the DR5:GUS reporter gene and auxin-responsive genes suggested that endogenous auxin levels were lower in loph1 roots than in the wild type. The aux1-7 mutant roots also exhibited root growth retardation in the low pH condition like the loph1 roots. These results indicate that AUX1 positively regulates the PM H+-ATPase activity through maintenance of the auxin accumulation in root tips, and this process may serve to maintain root elongation especially under low pH conditions.

Redirection of auxin flow in Arabidopsis thaliana roots after infection by root-knot nematodes

Plant-parasitic root-knot nematodes induce the formation of giant cells within the plant root, and it has been recognized that auxin accumulates in these feeding sites. Here, we studied the role of the auxin transport system governed by AUX1/LAX3 influx proteins and different PIN efflux proteins during feeding site development in Arabidopsis thaliana roots. Data generated via promoter-reporter line and protein localization analyses evoke a model in which auxin is being imported at the basipetal side of the feeding site by the concerted action of the influx proteins AUX1 and LAX3, and the efflux protein PIN3. Mutants in auxin influx proteins AUX1 and LAX3 bear significantly fewer and smaller galls, revealing that auxin import into the feeding sites is needed for their development and expansion. The feeding site development in auxin export (PIN) mutants was only slightly hampered. Expression of some PINs appears to be suppressed in galls, probably to prevent auxin drainage. Nevertheless, a functional PIN4 gene seems to be a prerequisite for proper nematode development and gall expansion, most likely by removing excessive auxin to stabilize the hormone level in the feeding site. Our data also indicate a role of local auxin peaks in nematode attraction towards the root.

Arabidopsis SHR and SCR transcription factors and AUX1 auxin influx carrier control the switch between adventitious rooting and xylogenesis in planta and in in vitro cultured thin cell layers

BACKGROUND AND AIMS

Adventitious roots (ARs) are essential for vegetative propagation. The Arabidopsis thaliana transcription factors SHORT ROOT (SHR) and SCARECROW (SCR) affect primary/lateral root development, but their involvement in AR formation is uncertain. LAX3 and AUX1 auxin influx carriers contribute to primary/lateral root development. LAX3 expression is regulated by SHR, and LAX3 contributes to AR tip auxin maximum. In contrast, AUX1 involvement in AR development is unknown. Xylogenesis is induced by auxin plus cytokinin as is AR formation, but the genes involved are largely unknown. Stem thin cell layers (TCLs) form ARs and undergo xylogenesis under the same auxin plus cytokinin input. The aim of this research was to investigate SHR, SCR, AUX1 and LAX3 involvement in AR formation and xylogenesis in intact hypocotyls and stem TCLs in arabidopsis.

METHODS

Hypocotyls of scr-1, shr-1, lax3, aux1-21 and lax3/aux1-21 Arabidopsis thaliana null mutant seedlings grown with or without auxin plus cytokinin were examined histologically, as were stem TCLs cultured with auxin plus cytokinin. SCR and AUX1 expression was monitored using pSCR::GFP and AUX1::GUS lines, and LAX3 expression and auxin localization during xylogenesis were monitored by using LAX3::GUS and DR5::GUS lines.

KEY RESULTS

AR formation was inhibited in all mutants, except lax3. SCR was expressed in pericycle anticlinally derived AR-forming cells of intact hypocotyls, and in cell clumps forming AR meristemoids of TCLs. The apex was anomalous in shr and scr ARs. In all mutant hypocotyls, the pericycle divided periclinally to produce xylogenesis. Xylary element maturation was favoured by auxin plus cytokinin in shr and aux1-21. Xylogenesis was enhanced in TCLs, and in aux1-21 and shr in particular. AUX1 was expressed before LAX3, i.e. in the early derivatives leading to either ARs or xylogenesis.

CONCLUSIONS

AR formation and xylogenesis are developmental programmes that are inversely related, but they involve fine-tuning by the same proteins, namely SHR, SCR and AUX1. Pericycle activity is central for the equilibrium between xylary development and AR formation in the hypocotyl, with a role for AUX1 in switching between, and balancing of, the two developmental programmes.

Plant embryogenesis requires AUX/LAX-mediated auxin influx

The plant hormone auxin and its directional transport are known to play a crucial role in defining the embryonic axis and subsequent development of the body plan. Although the role of PIN auxin efflux transporters has been clearly assigned during embryonic shoot and root specification, the role of the auxin influx carriers AUX1 and LIKE-AUX1 (LAX) proteins is not well established. Here, we used chemical and genetic tools on Brassica napus microspore-derived embryos and Arabidopsis thaliana zygotic embryos, and demonstrate that AUX1, LAX1 and LAX2 are required for both shoot and root pole formation, in concert with PIN efflux carriers. Furthermore, we uncovered a positive-feedback loop between MONOPTEROS (ARF5)-dependent auxin signalling and auxin transport. This MONOPTEROS-dependent transcriptional regulation of auxin influx (AUX1, LAX1 and LAX2) and auxin efflux (PIN1 and PIN4) carriers by MONOPTEROS helps to maintain proper auxin transport to the root tip. These results indicate that auxin-dependent cell specification during embryo development requires balanced auxin transport involving both influx and efflux mechanisms, and that this transport is maintained by a positive transcriptional feedback on auxin signalling.

Role of auxin during intercellular infection of Discaria trinervis by Frankia

Nitrogen-fixing nodules induced by Frankia in the actinorhizal plant Discaria trinervis result from a primitive intercellular root invasion pathway that does not involve root hair deformation and infection threads. Here, we analyzed the role of auxin in this intercellular infection pathway at the molecular level and compared it with our previous work in the intracellular infected actinorhizal plant Casuarina glauca. Immunolocalisation experiments showed that auxin accumulated in Frankia-infected cells in both systems. We then characterized the expression of auxin transporters in D. trinervis nodules. No activation of the heterologous CgAUX1 promoter was detected in infected cells in D. trinervis. These results were confirmed with the endogenous D. trinervis gene, DtAUX1. However, DtAUX1 was expressed in the nodule meristem. Consistently, transgenic D. trinervis plants containing the auxin response marker DR5:VENUS showed expression of the reporter gene in the meristem. Immunolocalisation experiments using an antibody against the auxin efflux carrier PIN1, revealed the presence of this transporter in the plasma membrane of infected cells. Finally, we used in silico cellular models to analyse auxin fluxes in D. trinervis nodules. Our results point to the existence of divergent roles of auxin in intercellularly- and intracellularly-infected actinorhizal plants, an ancestral infection pathways leading to root nodule symbioses.

GSA-1/ARG1 protects root gravitropism in Arabidopsis under ammonium stress

Gravitropism plays a critical role in plant growth and development, plant stability and acclimation to changes in water and nutrient availability. Ammonium (NH4(+)) is well known to have profound effects on root growth, but its impacts on gravitropism are poorly understood. To determine which genes are essential for the maintenance of root gravitropism under NH4(+) stress, we isolated and identified an NH4 (+)-sensitive mutant, gsa-1 (gravitropism sensitive to ammonium-1), in Arabidopsis thaliana, using an agar plate root reorientation assay. We found that, under NH4(+) stress, gsa-1 displayed increased loss of root gravitropism. Gene cloning and sequencing revealed that gsa-1 contains a G to C transversion mutation at the highly conserved 5'-GT splice position of intron 10 of ARG1 (ALTERED RESPONSE TO GRAVITY1), known to participate in the transduction of the root gravity signal. Genetic complement tests established the locus of GSA-1/ARG1 and its role in resistance to NH4 (+) inhibition on root gravitropism. GSA-1/ARG1 is required for normal AUX1 expression and basipetal auxin transport in root apices. In addition, PIN-FORMED2 (PIN2) is proposed as a target in the reduction of root gravitropism under NH4(+) stress, a response which can be antagonized by the GSA-1/ARG1-dependent pathway. These results suggest that GSA-1/ARG1 protects root gravitropism in Arabidopsis thaliana under ammonium stress.

AUX/LAX family of auxin influx carriers-an overview

Auxin regulates several aspects of plant growth and development. Auxin is unique among plant hormones for exhibiting polar transport. Indole-3-acetic acid (IAA), the major form of auxin in higher plants, is a weak acid and its intercellular movement is facilitated by auxin influx and efflux carriers. Polarity of auxin movement is provided by asymmetric localization of auxin carriers (mainly PIN efflux carriers). PIN-FORMED (PIN) and P-GLYCOPROTEIN (PGP) family of proteins are major auxin efflux carriers whereas AUXIN1/LIKE-AUX1 (AUX/LAX) are major auxin influx carriers. Genetic and biochemical evidence show that each member of the AUX/LAX family is a functional auxin influx carrier and mediate auxin related developmental programmes in different organs and tissues. Of the four AUX/LAX genes, AUX1 regulates root gravitropism, root hair development and leaf phyllotaxy whereas LAX2 regulates vascular development in cotyledons. Both AUX1 and LAX3 have been implicated in lateral root (LR) development as well as apical hook formation whereas both AUX1 and LAX1 and possibly LAX2 are required for leaf phyllotactic patterning.

The role of AUX1 gene and auxin content to the branching phenotype of Kenaf (Hibiscus cannabinus L.)

The objectives of this research were to identify auxin gene, AUX1, and to determine the plant auxin content and their role in conferring branching on Kenaf. PCR analysis using AUX1 primer capable to amplify the DNA of non branching (KR11) and branching kenaf mutant, resulting in 800 bp PCR product. The sequence of the PCR product showed high degree of homology with the sequence of AUX1 gene of other plants in the NCBI GenBank database, confirming kenaf possession of the gene AUX1. However, some variation on the DNA sequence was found between branching and non branching phenotype indicated allele differences of the same gene which were responsible for the variation in the type of branching. Identification of auxin content in the roots, apical shoot, and axillary branches using spectrophotometry method showed that the branching plant has higher auxin content in the apical shoot compared to the content in the branches. This indicate that AUX1 controls the formation of branches by controlling either the content of auxin in the apical shoot and branches, or the ratio of auxin content in the shoot and branches.

A Role for auxin during actinorhizal symbioses formation?

The symbiotic interaction between the soil bacteria Frankia and actinorhizal plants leads to the formation of nitrogen-fixing nodules resembling modified lateral roots. Little is known about the signals exchanged between the two partners during the establishment of these endosymbioses. However, a role for plant hormones has been suggested.Recently, we studied the role of auxin influx activity during actinorhizal symbioses. An inhibitor of auxin influx was shown to perturb nodule formation. Moreover we identified a functional auxin influx carrier that is produced specifically in Frankia-infected cells. These results together with previous data showing auxin production by Frankia lead us to propose a model of auxin action during the symbiotic infection process.