Auxin Transport Research Articles

The regulatory role of the RUS family in plant growth and development

The chloroplast genome encodes many key proteins involved in photosynthesis and other metabolic processes, and metabolites synthesized in chloroplasts are essential for normal plant growth and development. Root-UVB (ultraviolet radiation B)-sensitive (RUS) family proteins composed of highly conserved DUF647 domain belong to chloroplast proteins. They play an important role in the regulation of various life activities such as plant morphogenesis, material transport and energy metabolism. This article summarizes the recent advances of the RUS family proteins in the growth and development of plants such as embryonic development, photomorphological construction, VB6 homeostasis, auxin transport and anther development, with the aim to facilitate further study of its molecular regulation mechanism in plant growth and development.

Genome Identification and Expression Profiling of the PIN-Formed Gene Family in Phoebe bournei under Abiotic Stresses.

PIN-formed (PIN) proteins-specific transcription factors that are widely distributed in plants-play a pivotal role in regulating polar auxin transport, thus influencing plant growth, development, and abiotic stress responses. Although the identification and functional validation of PIN genes have been extensively explored in various plant species, their understanding in woody plants-particularly the endangered species Phoebe bournei (Hemsl.) Yang-remains limited. P. bournei is an economically significant tree species that is endemic to southern China. For this study, we employed bioinformatics approaches to screen and identify 13 members of the PIN gene family in P. bournei. Through a phylogenetic analysis, we classified these genes into five sub-families: A, B, C, D, and E. Furthermore, we conducted a comprehensive analysis of the physicochemical properties, three-dimensional structures, conserved motifs, and gene structures of the PbPIN proteins. Our results demonstrate that all PbPIN genes consist of exons and introns, albeit with variations in their number and length, highlighting the conservation and evolutionary changes in PbPIN genes. The results of our collinearity analysis indicate that the expansion of the PbPIN gene family primarily occurred through segmental duplication. Additionally, by predicting cis-acting elements in their promoters, we inferred the potential involvement of PbPIN genes in plant hormone and abiotic stress responses. To investigate their expression patterns, we conducted a comprehensive expression profiling of PbPIN genes in different tissues. Notably, we observed differential expression levels of PbPINs across the various tissues. Moreover, we examined the expression profiles of five representative PbPIN genes under abiotic stress conditions, including heat, cold, salt, and drought stress. These experiments preliminarily verified their responsiveness and functional roles in mediating responses to abiotic stress. In summary, this study systematically analyzes the expression patterns of PIN genes and their response to abiotic stresses in P. bournei using whole-genome data. Our findings provide novel insights and valuable information for stress tolerance regulation in P. bournei. Moreover, the study offers significant contributions towards unraveling the functional characteristics of the PIN gene family.

Ectopic assembly of an auxin efflux control machinery shifts developmental trajectories.

Polar auxin transport in the Arabidopsis (Arabidopsis thaliana) root tip maintains high auxin levels around the stem cell niche that gradually decrease in dividing cells but increase again once they transition toward differentiation. Protophloem differentiates earlier than other proximal tissues and employs a unique auxin "canalization" machinery that is thought to balance auxin efflux with retention. It consists of a proposed activator of PIN-FORMED (PIN) auxin efflux carriers, the cAMP-, cGMP- and Calcium-dependent (AGC) kinase PROTEIN KINASE ASSOCIATED WITH BRX (PAX); its inhibitor, BREVIS RADIX (BRX); and PHOSPHATIDYLINOSITOL-4-PHOSPHATE-5-KINASE (PIP5K) enzymes, which promote polar PAX and BRX localization. Because of a dynamic PAX-BRX-PIP5K interplay, the net cellular output of this machinery remains unclear. In this study, we deciphered the dosage-sensitive regulatory interactions among PAX, BRX, and PIP5K by their ectopic expression in developing xylem vessels. The data suggest that the dominant collective output of the PAX-BRX-PIP5K module is a localized reduction in PIN abundance. This requires PAX-stimulated clathrin-mediated PIN endocytosis upon site-specific phosphorylation, which distinguishes PAX from other AGC kinases. An ectopic assembly of the PAX-BRX-PIP5K module is sufficient to cause cellular auxin retention and affects root growth vigor by accelerating the trajectory of xylem vessel development. Our data thus provide direct evidence that local manipulation of auxin efflux alters the timing of cellular differentiation in the root.

The maize WRKY transcription factor ZmWRKY64 confers cadmium tolerance in Arabidopsis and maize (Zea mays L.).

ZmWRKY64 positively regulates Arabidopsis and maize Cd stress through modulating Cd uptake, translocation, and ROS scavenging genes expression. Cadmium (Cd) is a highly toxic heavy metal with severe impacts on crops growth and development. The WRKY transcription factor is a significant regulator influencing plant stress response. Nevertheless, the function of the WRKY protein in maize Cd stress response remains unclear. Here, we identified a maize WRKY gene, ZmWRKY64, the expression of which was enhanced in maize roots and leaves under Cd stress. ZmWRKY64 was localized in the nucleus and displayed transcriptional activity in yeast. Heterologous expression of ZmWRKY64 in Arabidopsis diminished Cd accumulation in plants by negatively regulating the expression of AtIRT1, AtZIP1, AtHMA2, AtNRAMP3, and AtNRAMP4, which are involved in Cd uptake and transport, resulting in Cd stress tolerance. Knockdown of ZmWRKY64 in maize led to excessive Cd accumulation in leaf cells and in the cytosol of the root cells, resulting in a Cd hypersensitive phenotype. Further analysis confirmed that ZmWRKY64 positively regulated ZmABCC4, ZmHMA3, ZmNRAMP5, ZmPIN2, ZmABCG51, ZmABCB13/32, and ZmABCB10, which may influence Cd translocation and auxin transport, thus mitigating Cd toxicity in maize. Moreover, ZmWRKY64 could directly enhance the transcription of ZmSRG7, a reported key gene regulating reactive oxygen species homeostasis under abiotic stress. Our results indicate that ZmWRKY64 is important in maize Cd stress response. This work provides new insights into the WRKY transcription factor regulatory mechanism under a Cd-polluted environment and may lead to the genetic improvement of Cd tolerance in maize.

Auxin receptor OsTIR1 mediates auxin signaling during seed filling in rice.

Cereal endosperm represents the most important source of the world's food. Nevertheless, the molecular mechanisms behind sugar import into rice (Oryza sativa) endosperm and their relationship with auxin signaling are poorly understood. Here, we report that auxin transport inhibitor response 1 (TIR1) plays an essential role in rice grain yield and quality via modulating sugar transport into endosperm. The fluctuations of OsTIR1 transcripts parallel to the early stage of grain expansion among those of the 5 TIR1/AFB (auxin-signaling F-box) auxin co-receptor proteins. OsTIR1 is abundantly expressed in ovular vascular trace, nucellar projection, nucellar epidermis, aleurone layer cells, and endosperm, providing a potential path for sugar into the endosperm. Compared to wild-type (WT) plants, starch accumulation is repressed by mutation of OsTIR1 and improved by overexpression of the gene, ultimately leading to reduced grain yield and quality in tir1 mutants but improvement in overexpression lines. Of the rice AUXIN RESPONSE FACTOR (ARF) genes, only the OsARF25 transcript is repressed in tir1 mutants and enhanced by overexpression of OsTIR1; its highest transcript is recorded at 10 d after fertilization, consistent with OsTIR1 expression. Also, OsARF25 can bind the promoter of the sugar transporter OsSWEET11 (SWEET, sugars will eventually be exported transporter) in vivo and in vitro. arf25 and arf25/sweet11 mutants exhibit reduced starch content and seed size (relative to the WTs), similar to tir1 mutants. Our data reveal that OsTIR1 mediates sugar import into endosperm via the auxin signaling component OsARF25 interacting with sugar transporter OsSWEET11. The results of this study are of great significance to further clarify the regulatory mechanism of auxin signaling on grain development in rice.

Structure and Function of Auxin Transporters.

Auxins, a group of central hormones in plant growth and development, are transported by a diverse range of transporters with distinct biochemical and structural properties. This review summarizes the current knowledge on all known auxin transporters with respect to their biochemical and biophysical properties and the methods used to characterize them. In particular, we focus on the recent advances that were made concerning the PIN-FORMED family of auxin exporters. Insights derived from solving their structures have improved our understanding of the auxin export process, and we discuss the current state of the art on PIN-mediated auxin transport, including the use of biophysical methods to examine their properties. Understanding the mechanisms of auxin transport is crucial for understanding plant growth and development, as well as for the development of more effective strategies for crop production and plant biotechnology.

Investigation of stimulated growth effect by application of L-aspartic acid on poplar

Poplar is an important resource for the production of wood-based products. L-aspartic acid (Asp) is a proteinogenesis amino acid that plays a critical role in cell growth modulation and stress response. Also, Asp is widespread in the commonly used amino acid-based biostimulant products. However, its effects on poplars remain largely unknown. To address this issue, we applied different concentrations of Asp to poplar seedlings under N reduction conditions (simulating low N availability status), and assessed the effects of Asp on poplar growth and development. Our results showed that supplement of Asp at an appropriate concentration, that is, 0.5 mM in the present study, had the best growth-promoting effect on poplars. It significantly increased poplar biomass and height, and simultaneously elevated the photosynthetic parameters, the chlorophyll content and total free amino acid content, as well as the activity of N metabolism-related enzymes in poplars-treated with Asp in comparision with the control plants without Asp addition. Furthermore, the expression of Asp-metabolism pathway genes (AspAT1, AspAT6, AspAT10, AK1, ASS1 and ASS2), key chlorophyll synthesis gene (LHCB3) and cell division gene (CYCD3) was all dramatically upregulated in response to Asp application, suggesting that Asp-induced effect invoked photosynthesis activation and metabolism reprogramming. More interestingly, our results showed that the levels of various phytohormones, such as auxin and cytokinins were remarkably altered, and key auxin transport gene (PIN1c-2), auxin conjugation gene (GH3.6–2) were induced by Asp treatment, indicative of the possible involvement of plant hormones under the effect of Asp, especially fine-tuning auxin homeostasis by forming IAA-Asp conjugates to store auxin and catabolizing IAA-Asp to release auxin, may account for Asp-mediated growth modulation to an extent in poplars. Our study provides a novo insight into the growth promotion effect of Asp on poplars, especially, when the N source is limited, even though the underlying mechanism of Asp-induced stimulation action needs further in-depth investigation.

DNA methylome analysis reveals novel insights into active hypomethylated regulatory mechanisms of temperature-dependent flower opening in Osmanthus fragrans.

Short-term ambient low temperature (ALT) stimulation is necessary for Osmanthus fragrans to facilitate continued flower opening after floral bud development reaches maturity. DNA methylation, a vital epigenetic modification, regulates various biological processes in response to temperature fluctuations. However, its role in temperature-driven flower opening remains elusive. In this study, we identified the pivotal timeframe during which O. fragrans promptly detected temperature cues. Using whole-genome bisulfite sequencing, we explored global DNA hypomethylation during this phase, with the most significant changes occurring in CHH sequence contexts. Auxin transport inhibitor (TIBA) application revealed that ALT-induced endogenous auxin accumulation promoted peduncle elongation. In our mRNA-seq analysis, we discovered that the differentially expressed genes (DEGs) with hypo-differentially methylated regions (hypo-DMRs) were mainly enriched in auxin and temperature response, RNA processing, and carbohydrate and lipid metabolism. Transcripts of three DNA demethylase genes (OfROS1a, OfDML3, OfDME) showed upregulation. Furthermore, all DNA methylase genes, except OfCMT2b, also displayed increased expression, specifically with two of them, OfCMT3a and OfCMT1, being associated with hypo-DMRs. Promoter assays showed that OfROS1a, with promoters containing low-temperature- and auxin-responsive elements, were activated by ALT and exogenous IAA at low concentrations but inhibited at high concentrations. Overexpression of OfROS1 reduced endogenous auxin levels but enhanced the expression of genes related to auxin response and spliceosome in petunia. Furthermore, OfROS1 promoted sucrose synthesis in petunia corollas. Our data characterized the rapid response of active DNA hypomethylation to ALT and suggested a possible epiregulation of temperature-dependent flower opening in O. fragrans. This study revealed the pivotal role of DNA hypomethylation in O. fragrans during the ALT-responsive phase before flower opening, involving dynamic DNA demethylation, auxin signaling modulation, and a potential feedback loop between hypomethylation and methylation.

Gossypium hirsutum gene of unknown function Gohir.A02G161000 encodes a potential transmembrane Root UVB Sensitive 4 Protein with a putative protein-protein interaction interface.

A gene of unknown function, Gohir.A02G161000.1, identified in Gossypium hirsutum was studied using computational sequence and structure bioinformatics tools. The associated protein GhRUS4-A0A1U8JPV7 (UniProt A0A1U8JPV7) is predicted to be a plastid-localized, transmembrane root UVB-sensitive 4 (RUS4) protein with a newly identified potential dimerization surface. Evidence from homology and sequence conservation suggest involvement in auxin transport and pollen maturation.

Gibberellin promotes cambium reestablishment during secondary vascular tissue regeneration after girdling in an auxin-dependent manner in Populus.

Secondary vascular tissue (SVT) development and regeneration are regulated by phytohormones. In this study, we used an in vitro SVT regeneration system to demonstrate that gibberellin (GA) treatment significantly promotes auxin-induced cambium reestablishment. Altering GA content by overexpressing or knocking down ent-kaurene synthase (KS) affected secondary growth and SVT regeneration in poplar. The poplar DELLA gene GIBBERELLIC ACID INSENSITIVE (PtoGAI) is expressed in a specific pattern during secondary growth and cambium regeneration after girdling. Overexpression of PtoGAI disrupted poplar growth and inhibited cambium regeneration, and the inhibition of cambium regeneration could be partially restored by GA application. Further analysis of the PtaDR5:GUS transgenic plants, the localization of PIN-FORMED 1 (PIN1) and the expression of auxin-related genes found that an additional GA treatment could enhance the auxin response as well as the expression of PIN1, which mediates auxin transport during SVT regeneration. Taken together, these findings suggest that GA promotes cambium regeneration by stimulating auxin signal transduction.

Carbon Monoxide Modulates Auxin Transport and Nitric Oxide Signaling in Plants under Iron Deficiency Stress

Carbon Monoxide Modulates Auxin Transport and Nitric Oxide Signaling in Plants under Iron Deficiency Stress

The plant growth promoting rhizobacterium Achromobacter sp. 5B1, rescues Arabidopsis seedlings from alkaline stress by enhancing root organogenesis and hormonal responses

Soil alkalinity is a critical environmental factor for plant growth and distribution in ecosystems. An alkaline condition (pH > 7) is imposed by the rising concentration of hydroxides and cations, and prevails in semiarid and arid environments, which represent more than 25% of the total arable land of the world. Despite the great pressure exerted by alkalinity for root viability and plant survival, scarce information is available to understand how root microbes contribute to alkaline pH adaptation. Here, we assessed the effects of alkalinity on shoot and root biomass production, chlorophyll content, root growth and branching, lateral root primordia formation, and the expression of CYCB1, TOR kinase, and auxin and cytokinin-inducible trangenes in shoots and roots of Arabidopsis seedlings grown in Petri plates with agar-nutrient medium at pH values of 7.0, 7.5, 8.0, 8.5, and 9.0. The results showed an inverse correlation between the rise of pH and most growth, hormonal and genetic traits analyzed. Noteworthy, root inoculation with Achromobacter sp. 5B1, a beneficial rhizospheric bacterium, with plant growth promoting and salt tolerance features, increased biomass production, restored root growth and branching and enhanced auxin responses in WT seedlings and auxin-related mutants aux1–7 and eir1, indicating that stress adaptation operates independently of canonical auxin transporter proteins. Sequencing of the Achromobacter sp. 5B1 genome unveiled 5244 protein-coding genes, including genes possibly involved in auxin biosynthesis, quorum-sensing regulation and stress adaptation, which may account for its plant growth promotion attributes. These data highlight the critical role of rhizobacteria to increase plant resilience under high soil pH conditions potentially through genes for adaptation to an extreme environment and bacteria-plant communication.

Physiological and transcriptome analysis reveal the nitrogen preference and regulatory pathways of nitrogen metabolism in an epiphytic orchid, Cymbidium tracyanum

Epiphytic orchids acquire nutrients elements, including nitrogen, from the atmosphere. However, the nitrogen form preferred by epiphytic orchids Cymbidium tracyanum and adaptive mechanisms that underlie nitrogen metabolism remain unclear. In this study, we determined which nitrogen form the C. tracyanum prefers, and identified adaptive strategies and molecular pathways activated during nitrogen metabolism. For this purpose, we measured the physiological and morphological responses to different ratios of nitrate and ammonium (NO3−/NH4+ = 1:0, 3:1, 1:1, 1:3, 0:1) and used transcriptome analysis to identify genes differentially expressed in response to these nitrogen forms. Physiological and morphological measures of plant growth (i.e., net photosynthetic rate, biomass accumulation, leaf area, root elongation and branching) were highest in plants treated with nitrate. Furthermore, C. tracyanum plants treated with nitrate had a higher root/mass ratio and a lower leaf/mass ratio. Transcriptome analysis identified that several key genes (NRT1.1–2, NRT1.4–4, NRT1.7–2, and CLC-G-2) highly expressed in plants treated with pure nitrate. These genes may be involved in the absorption and transportation of nitrate, and in auxin transportation from leaf to root in C. tracyanum. Our findings suggest that the nitrogen form preferred by C. tracyanum is nitrate and that nitrate alters both root and leaf traits via several molecular pathways, including the plant hormone signal transduction pathway and MAPK signaling pathway. We speculate that the preference of C. tracyanum for nitrate as a nitrogen form may be a result of long-term adaptation to epiphytic habitat via the regulation of leaf and root traits.

Aluminium stress-induced modulation of root gravitropism in pea (Pisum sativum) via auxin signalling

Aluminium (Al) toxicity stands out as a primary cause of crop failure in acidic soils. The root gravity setpoint angle (GSA), one of the important traits of the root system architecture (RSA), plays a pivotal role in enabling plants to adapt to abiotic stress. This study explored the correlation between GSA and Al stress using hydroponic culture with pea (Pisum sativum) plants. The findings revealed that under Al stress, GSA increased in newly developed lateral roots. Notably, this response remained consistent regardless of the treatment duration, extending for at least 3 days during the experiment. Furthermore, exposure to Al led to a reduction in both the size and quantity of starch granules, pivotal components linked to gravity perception. The accumulation of auxin in root transition zone increased. This variation was mirrored in the expression of genes linked to granule formation and auxin efflux, particularly those in the PIN-formed family. This developmental framework suggested a unique role for the root gravitropic response that hinges on starch granules and auxin transport, acting as mediators in the modulation of GSA under Al stress. Exogenous application of indole-3-acetic acid (IAA) and the auxin efflux inhibitor N-1-naphthylphthalamic acid (NPA) had an impact on the root gravitropic response to Al stress. The outcomes indicate that Al stress inhibited polar auxin transport and starch granule formation, the two processes crucial for gravitropism. This impairment led to an elevation in GSA and a reconfiguration of RSA. This study introduces a novel perspective on how plant roots react to Al toxicity, culminating in RSA modification in the context of acidic soil with elevated Al concentrations.

Parallel auxin transport via PINs and plasmodesmata during the Arabidopsis leaf hyponasty response

Key messageThe leaf hyponasty response depends on tip-to-petiole auxin transport. This transport can happen through two parallel pathways: active trans-membrane transport mediated by PIN proteins and passive diffusion through plasmodesmata.A plant’s ability to counteract potential shading by neighboring plants depends on transport of the hormone auxin. Neighbor sensing at the leaf tip triggers auxin production. Once this auxin reaches the abaxial petiole epidermis, it causes cell elongation, which leads to leaf hyponasty. Two pathways are known to contribute to this intercellular tip-to-petiole auxin movement: (i) transport facilitated by plasma membrane-localized PIN auxin transporters and (ii) diffusion enabled by plasmodesmata. We tested if these two modes of transport are arranged sequentially or in parallel. Moreover, we investigated if they are functionally linked. Mutants in which one of the two pathways is disrupted indicated that both pathways are necessary for a full hyponasty response. Visualization of PIN3-GFP and PIN7-GFP localization indicated PIN-mediated transport in parallel to plasmodesmata-mediated transport along abaxial midrib epidermis cells. We found plasmodesmata-mediated cell coupling in the pin3pin4pin7 mutant to match wild-type levels, indicating no redundancy between pathways. Similarly, PIN3, PIN4 and PIN7 mRNA levels were unaffected in a mutant with disrupted plasmodesmata pathway. Our results provide mechanistic insight on leaf hyponasty, which might facilitate the manipulation of the shade avoidance response in crops.

Chloroplast Auxin Efflux Mediated by ABCB28 and ABCB29 Fine-Tunes Salt and Drought Stress Responses in Arabidopsis.

Photosynthesis is among the first processes negatively affected by environmental cues and its performance directly determines plant cell fitness and ultimately crop yield. Primarily sites of photosynthesis, chloroplasts are unique sites also for the biosynthesis of precursors of the growth regulator auxin and for sensing environmental stress, but their role in intracellular auxin homeostasis, vital for plant growth and survival in changing environments, remains poorly understood. Here, we identified two ATP-binding cassette (ABC) subfamily B transporters, ABCB28 and ABCB29, which export auxin across the chloroplast envelope to the cytosol in a concerted action in vivo. Moreover, we provide evidence for an auxin biosynthesis pathway in Arabidopsis thaliana chloroplasts. The overexpression of ABCB28 and ABCB29 influenced stomatal regulation and resulted in significantly improved water use efficiency and survival rates during salt and drought stresses. Our results suggest that chloroplast auxin production and transport contribute to stomata regulation for conserving water upon salt stress. ABCB28 and ABCB29 integrate photosynthesis and auxin signals and as such hold great potential to improve the adaptation potential of crops to environmental cues.

Local auxin biosynthesis promotes shoot patterning and stem cell differentiation in Arabidopsis shoot apex.

The shoot apical meristem (SAM) of higher plants comprises distinct functional zones. The central zone (CZ) is located at the meristem summit and harbors pluripotent stem cells. Stem cells undergo cell division within the CZ and give rise to descendants, which enter the peripheral zone (PZ) and become recruited into lateral organs. Stem cell daughters that are pushed underneath the CZ form rib meristem (RM). To unravel the mechanism of meristem development, it is essential to know how stem cells adopt distinct cell fates in the SAM. Here, we show that meristem patterning and floral organ primordia formation, besides auxin transport, are regulated by auxin biosynthesis mediated by two closely related genes of the TRYPTOPHAN AMINOTRANSFERASE family. In Arabidopsis SAM, TAA1 and TAR2 played a role in maintaining auxin responses and the identity of PZ cell types. In the absence of auxin biosynthesis and transport, the expression pattern of the marker genes linked to the patterning of the SAM is perturbed. Our results prove that local auxin biosynthesis, in concert with transport, controls the patterning of the SAM into the CZ, PZ and RM.

Cytokinin and indole-3-acetic acid crosstalk is indispensable for silicon mediated chromium stress tolerance in roots of wheat seedlings

The rising heavy metal contamination of soils imposes toxic impacts on plants as well as other life forms. One such highly toxic and carcinogenic heavy metal is hexavalent chromium [Cr(VI)] that has been reported to prominently retard the plant growth. The present study investigated the potential of silicon (Si, 10 µM) to alleviate the toxicity of Cr(VI) (25 µM) on roots of wheat (Triticum aestivum L.) seedlings. Application of Si to Cr(VI)-stressed wheat seedlings improved their overall growth parameters. This study also reveals the involvement of two phytohormones, namely auxin and cytokinin and their crosstalk in Si-mediated mitigation of the toxic impacts of Cr(VI) in wheat seedlings. The application of cytokinin alone to wheat seedlings under Cr(VI) stress reduced the intensity of toxic effects of Cr(VI). In combination with Si, cytokinin application to Cr(VI)-stressed wheat seedlings significantly minimized the decrease induced by Cr(VI) in different parameters such as root-shoot length (10.8% and 13%, respectively), root-shoot fresh mass (11.3% and 10.1%, respectively), and total chlorophyll and carotenoids content (13.4% and 6.8%, respectively) with respect to the control. This treatment also maintained the regulation of proline metabolism (proline content, and P5CS and PDH activities), ascorbate-glutathione (AsA-GSH) cycle and nutrient homeostasis. The protective effect of Si and cytokinin against Cr(VI) stress was minimized upon supplementation of an inhibitor of polar auxin transport- 2,3,5-triiodobenzoic acid (TIBA) which suggested a potential involvement of auxin in Si and cytokinin-mediated mitigation of Cr(VI) toxicity. The exogenous addition of a natural auxin – indole-3-acetic acid (IAA) confirmed auxin is an active member of a signaling cascade along with cytokinin that aids in Si-mediated Cr(VI) toxicity alleviation as IAA application reversed the negative impacts of TIBA on wheat roots treated with Cr(VI), cytokinin and Si. The results of this research are also confirmed by the gene expression analysis conducted for nutrient transporters (Lsi1, CCaMK, MHX, SULT1 and ZIP1) and enzymes involved in the AsA-GSH cycle (APX, GR, DHAR and MDHAR). The overall results of this research indicate towards possible induction of a crosstalk between cytokinin and IAA upon Si supplementation which in turn stimulates physiological, biochemical and molecular changes to exhibit protective effects against Cr(VI) stress. Further, the information obtained suggests probable employment of Si, cytokinin and IAA alone or combined in agriculture to maintain plant productivity under Cr(VI) stress and data regarding expression of key genes can be used to develop new crop varieties with enhanced resistance against Cr(VI) stress together with its reduced load in seedlings.

Differential influence of Bacillus subtilis strains on Arabidopsis root architecture through common and distinct plant hormonal pathways

Plant growth-promoting microbes (PGPM) can enhance crop yield and health, but knowledge of their mode-of-action is limited. We studied the influence of two Bacillus subtilis strains, the natural isolate ALC_02 and the domesticated 168 Gö, on Arabidopsis and hypothesized that they modify the root architecture by modulating hormone transport or signaling. Both bacteria promoted increase of shoot and root surface area in vitro, but through different root anatomical traits. Mutant plants deficient in auxin transport or signaling responded less to the bacterial strains than the wild-type, and application of the auxin transport inhibitor NPA strongly reduced the influence of the strains. Both bacteria produced auxin and enhanced shoot auxin levels in DR5::GUS reporter plants. Accordingly, most of the beneficial effects of the strains were dependent on functional auxin transport and signaling, while only 168 Gö depended on functional ethylene signaling. As expected, only ALC_02 stimulated plant growth in soil, unlike 168 Gö that was previously reported to have reduced biofilms. Collectively, the results highlight that B. subtilis strains can have strikingly different plant growth-promoting properties, dependent on what experimental setup they are tested in, and the importance of choosing the right PGPM for a desired root phenotype.

Fluctuations in auxin levels depend upon synchronicity of cell divisions in a one-dimensional model of auxin transport.

Auxin is a well-studied plant hormone, the spatial distribution of which remains incompletely understood. Here, we investigate the effects of cell growth and divisions on the dynamics of auxin patterning, using a combination of mathematical modelling and experimental observations. In contrast to most prior work, models are not designed or tuned with the aim to produce a specific auxin pattern. Instead, we use well-established techniques from dynamical systems theory to uncover and classify ranges of auxin patterns as exhaustively as possible as parameters are varied. Previous work using these techniques has shown how a multitude of stable auxin patterns may coexist, each attainable from a specific ensemble of initial conditions. When a key parameter spans a range of values, these steady patterns form a geometric curve with successive folds, often nicknamed a snaking diagram. As we introduce growth and cell division into a one-dimensional model of auxin distribution, we observe new behaviour which can be explained in terms of this diagram. Cell growth changes the shape of the snaking diagram, and this corresponds in turn to deformations in the patterns of auxin distribution. As divisions occur this can lead to abrupt creation or annihilation of auxin peaks. We term this phenomenon 'snake-jumping'. Under rhythmic cell divisions, we show how this can lead to stable oscillations of auxin. We also show that this requires a high level of synchronisation between cell divisions. Using 18 hour time-lapse imaging of the auxin reporter DII:Venus in roots of Arabidopsis thaliana, we show auxin fluctuates greatly, both in terms of amplitude and periodicity, consistent with the snake-jumping events observed with non-synchronised cell divisions. Periodic signals downstream of the auxin signalling pathway have previously been recorded in plant roots. The present work shows that auxin alone is unlikely to play the role of a pacemaker in this context.