Regulating Root Elongation Research Articles

A mulberry 9-cis-epoxycarotenoid dioxygenase gene MaNCED1 is involved in plant growth regulation and confers salt and drought tolerance in transgenic tobacco.

The phytohormone abscisic acid (ABA) is vital in regulating root elongation, seed germination, and abiotic stress responses in plants. Conversely, the mechanisms of ABA in mulberry root growth, seed germination, and abiotic stress responses are poorly understood. Here, we reported that exogenous ABA and drought treatment inhibited the growth of mulberry seedlings but significantly increased the ratio of root/stem. Inhibition of ABA synthesis by fluridone and sodium tungstate resulted in the decrease of root/stem ratio. We also showed that the expression of MaNCED1 in the root was strongly induced by drought and salt stress. Increasing the expression of MaNCED1 in tobacco using overexpression leads to increased root elongation and reduced seed germination. Compared with the wild type, the accumulation of H2O2 and MDA was reduced, while the POD activity and proline content was increased in the transgenic plants after drought and salt treatment. Further studies revealed increased resistance to drought and salt stress in MaNCED1 overexpressed tobaccos. Meanwhile, the auxin and ethylene signal pathway-related gene expression levels increased in MaNCED1 overexpressed tobaccos. This study demonstrated the roles of mulberry MaNCED1 in regulating plant development and abiotic stress responses. It gave further insights into the coordinated regulation of ABA, auxin, and ethylene in seed growth and germination.

The Cotton GhWRKY91 Gene Negatively Regulates Root Elongation in Overexpressed Transgenic Arabidopsis thaliana

The Cotton GhWRKY91 Gene Negatively Regulates Root Elongation in Overexpressed Transgenic Arabidopsis thaliana

GmEIL4 enhances soybean (Glycine max) phosphorus efficiency by improving root system development.

Phosphorus (P) deficiency seriously affects plant growth and development and ultimately limits the quality and yield of crops. Here, a new P efficiency-related major quantitative trait locusgene, GmEIL4 (encoding an ethylene-insensitive 3-like 1 protein), was cloned at qP2, which was identified by linkage analysis and genome-wide association studyacross four environments. Overexpressing GmEIL4 significantly improved the P uptake efficiency by increasing the number, length and surface area of lateral roots of hairy roots in transgenic soybeans, while interfering with GmEIL4 resulted in poor root phenotypic characteristics compared with the control plants under low P conditions. Interestingly, we found that GmEIL4 interacted with EIN3-binding F box protein 1 (GmEBF1), which may regulate the root response to low P stress. We conclude that the expression of GmEIL4 was induced by low-P stress and that overexpressing GmEIL4 improved P accumulation by regulating root elongation and architecture. Analysis of allele variation of GmEIL4 in 894 soybean accessions suggested that GmEIL4 is undergoing artificial selection during soybean evolution, which will benefit soybean production. Together, this study further elucidates how plants respond to low P stress by modifying root structure and provides insight into the great potential of GmEIL4 in crop P-efficient breeding.

A novel type of phytosulfokine, PSK-ε, positively regulates root elongation and formation of lateral roots and root nodules in Medicago truncatula

ABSTRACT Phytosulfokines (PSKs) are a class of tyrosine-sulfated pentapeptides. PSK-α, PSK-γ, and PSK-δ are three reported PSK members involved in regulating plant growth, development, and resistance to biotic and abiotic stresses. Here, we reported a novel type of PSK, PSK-ε with the sequence YSO3VYSO3TN, and its precursor proteins (MtPSKε, LjPSKε, and GmPSKε), specifically from legume species. PSK-ε peptide differs from PSK-δ by one amino acid and is close to PSK-δ in the phylogenetic relationship. Expression profile analysis showed that MtPSKε was highly expressed in Medicago truncatula roots, especially in root tips and emerged lateral roots. Application of the synthetic sulfated PSK-ε peptide and overexpression of MtPSKε significantly promoted M. truncatula root elongation and increased lateral root number, probably by inducing cell division and expansion in roots. Furthermore, MtPSKε expression was induced by rhizobia infection and was detected in root nodules including nodule primordia. Both PSK-ε peptide treatment and MtPSKε overexpression significantly increased nodule number in M. truncatula. Taken together, these results demonstrate that PSK-ε, a novel type of phytosulfokine, positively regulates root elongation and formation of lateral root and root nodule in M. truncatula.

OsSPL14 is involved in nitrogen-deficiency-induced root elongation in rice

Nitrogen (N) is an essential element for rice (Oryza sativa L.) growth. Rice responds to N-limiting conditions by changing root morphology. Strigolactones (SLs) and auxin modulate rice root elongation under N-deficient conditions. However, the interactions between SLs and auxin in the regulation of root elongation in response to N deficiency remain needing further study. Here, we analysed the function of SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 14 (SPL14, a downstream transcription factor of strigolactone signaling) in regulating root elongation under low N (LN) supply. Compared with sufficient N (SN) condition, LN and SN+GR24 (SL analogue) reduced protein interaction between SLs signaling repressor DWARF 53 (D53) and SPL14. The expression of SPL14 was induced by LN supply. In contrasts to WT plants, knockdown of SPL14 resulted in less extensive changes in root length under LN condition. Overexpression of SPL14 under SN supply enhanced root length to the same extent as WT plants under LN condition, suggesting that SPL14 was induced by LN condition and led to changes in root elongation. The transcription of PIN-FORMED 2 and 10b (auxin efflux carriers) were negatively and positively regulated by SPL14, respectively. Under SN and LN conditions, similar root morphology was observed in overexpressing PIN10b and SPL14 lines, and opposite root phenotypes were recorded between pin2 mutants and SPL14-RNAi lines.These results indicated that PIN2 and PIN10b were involved in SPL14-induced root elongation under LN condition. Therefore, we presented that, LN inhibits D53 and SPL14 interaction, which releases SPL14 to modulate the transcription of PIN2 and PIN10b, resulting in root elongation in rice.

A novel mitochondrial protein is required for cell wall integrity, auxin accumulation and root elongation in Arabidopsis under salt stress

Maintenance of root elongation is beneficial for the growth and survival of plants under salt stress, but currently the cellular components involved in the regulation of root growth under high salinity are not fully understood. In this study, we identified an Arabidopsis mutant, rres1, which exhibited reduced root elongation under treatment of a variety of salts, including NaCl, NaNO3, KCl, and KNO3. RRES1 encodes a novel mitochondrial protein and its molecular function is still unknown. Under salt stress, the root meristem length was shorter in the rres1 mutant compared to the wild type, which was correlated with a reduced auxin accumulation in the mutant. Reactive oxygen species (ROS), as important signals that regulate root elongation, were accumulated to higher levels in the rres1 mutant than the wild type after salt treatment. Measurement of monosaccharides in the cell wall showed that arabinose and xylose contents were decreased in the rres1 mutant under salt stress, and application of boric acid, which is required for the crosslinking of pectic polysaccharide rhamnogalacturonan-II (RG-II), largely rescued the root growth arrest of the rres1 mutant, suggesting that RRES1 participates in the maintenance of cell wall integrity under salt stress. GUS staining assay indicated that the RRES1 gene was expressed in leaves and weakly in root tip under normal conditions, but its expression was dramatically increased in leaves and roots after salt treatment. Together, our study reveals a novel mitochondrial protein that regulates root elongation under salt stress via the modulation of cell wall integrity, auxin accumulation, and ROS homeostasis.

CALMODULIN-LIKE-38 and PEP1 RECEPTOR 2 integrate nitrate and brassinosteroid signals to regulate root growth.

Plants exhibit remarkable developmental plasticity, enabling them to adapt to adverse environmental conditions such as low nitrogen (N) in the soil. Brassinosteroids (BRs) promote root foraging for nutrients under mild N deficiency, but the crosstalk between the BR- and N-signaling pathways in the regulation of root growth remains largely unknown. Here, we show that CALMODULIN-LIKE-38 (CML38), a calmodulin-like protein, specifically interacts with the PEP1 RECEPTOR 2 (PEPR2), and negatively regulates root elongation in Arabidopsis (Arabidopsis thaliana) in response to low nitrate (LN). CML38 and PEPR2 are transcriptionally induced by treatments of exogenous nitrate and BR. Compared with Col-0, the single mutants cml38 and pepr2 and the double mutant cml38 pepr2 displayed enhanced primary root growth and produced more lateral roots under LN. This is consistent with their higher nitrate absorption abilities, and their stronger expression of nitrate assimilation genes. Furthermore, CML38 and PEPR2 regulate common downstream genes related to BR signaling, and they have positive roles in BR signaling. Low N facilitated BR signal transmission in Col-0 and CML38- or PEPR2-overexpressing plants, but not in the cml38 and pepr2 mutants. Taken together, our results illustrate a mechanism by which CML38 interacts with PEPR2 to integrate LN and BR signals for coordinating root development to prevent quick depletion of N resources in Arabidopsis.

Screening of Rice Varieties based on Remodeling of Root Architecture Linked to Enhanced Phosphorus Transporters and Ethylene Signaling for Better Phosphorous Acquisition under Limiting Conditions

Root architectural modifications in response to altered nutrient level can be used as selection marker for better adapted rice varieties. In this study, we screened six local rice varieties commonly grown in Pakistan, using their unique root architecture and several molecular markers to identify best adapted local variety under phosphorus limiting conditions. Our data showed that rice variety with significant changes in its three-dimensional root architecture system (RSA) and enhanced expression of phosphorus transporters (OsPT2, OsPT4 and OsPT6) is the best variety to handle stress as compared to other varieties. Along with development of screening strategy/method, our data provided evidence that phosphorus starvation leads to upregulation of stress hormone ethylene, which regulates root elongation and root hair development therefore facilitating root architecture modification. We then further checked, how to mitigate or enhance phosphorus starvation responses by application of hormones exogenously, our results showed that ethylene application/treatment enhances phosphorus starvation responses, whereas cytokinin on the other hand reverses deficiency effects which implicates hormonal cross talk is key to modulate P-deficiency responses in rice. This study provides an easy and quick method of analysis of root architecture as physiological marker for rice screening and improve crop yield by selecting best adapted variety for P deficient soils. In future, detail study for understanding phytohormone mediated transcriptomic changes in response to nutrient deficiency and in correlation with physiological response will help to select better adapted varieties that will eventually result in increase of rice yield.

FASCICLIN-LIKE 18 Is a New Player Regulating Root Elongation in Arabidopsis thaliana.

The plasticity of root development represents a key trait that enables plants to adapt to diverse environmental cues. The pattern of cell wall deposition, alongside other parameters, affects the extent, and direction of root growth. In this study, we report that FASCICLIN-LIKE ARABINOGALACTAN PROTEIN 18 (FLA18) plays a role during root elongation in Arabidopsis thaliana. Using root-specific co-expression analysis, we identified FLA18 to be co-expressed with a sub-set of genes required for root elongation. FLA18 encodes for a putative extra-cellular arabinogalactan protein from the FLA-gene family. Two independent T-DNA insertion lines, named fla18-1 and fla18-2, display short and swollen lateral roots (LRs) when grown on sensitizing condition of high-sucrose containing medium. Unlike fla4/salt overly sensitive 5 (sos5), previously shown to display short and swollen primary root (PR) and LRs under these conditions, the PR of the fla18 mutants is slightly longer compared to the wild-type. Overexpression of the FLA18 CDS complemented the fla18 root phenotype. Genetic interaction between either of the fla18 alleles and sos5 reveals a more severe perturbation of anisotropic growth in both PR and LRs, as compared to the single mutants and the wild-type under restrictive conditions of high sucrose or high-salt containing medium. Additionally, under salt-stress conditions, fla18sos5 had a small, chlorotic shoot phenotype, that was not observed in any of the single mutants or the wild type. As previously shown for sos5, the fla18-1 and fla18-1sos5 root-elongation phenotype is suppressed by abscisic acid (ABA) and display hypersensitivity to the ABA synthesis inhibitor, Fluridon. Last, similar to other cell wall mutants, fla18 root elongation is hypersensitive to the cellulose synthase inhibitor, Isoxaben. Altogether, the presented data assign a new role for FLA18 in the regulation of root elongation. Future studies of the unique vs. redundant roles of FLA proteins during root elongation is anticipated to shed a new light on the regulation of root architecture during plant adaptation to different growth conditions.

Root-specific expression of chickpea cytokinin oxidase/dehydrogenase 6 leads to enhanced root growth, drought tolerance and yield without compromising nodulation.

SummaryCytokinin group of phytohormones regulate root elongation and branching during post‐embryonic development. Cytokinin‐degrading enzymes cytokinin oxidases/dehydrogenases (CKXs) have been deployed to investigate biological activities of cytokinin and to engineer root growth. We expressed chickpea cytokinin oxidase 6 (CaCKX6) under the control of a chickpea root‐specific promoter of CaWRKY31 in Arabidopsis thaliana and chickpea having determinate and indeterminate growth patterns, respectively, to study the effect of cytokinin depletion on root growth and drought tolerance. Root‐specific expression of CaCKX6 led to a significant increase in lateral root number and root biomass in Arabidopsis and chickpea without any penalty to vegetative and reproductive growth of shoot. Transgenic chickpea lines showed increased CKX activity in root. Soil‐grown advanced chickpea transgenic lines exhibited higher root‐to‐shoot biomass ratio and enhanced long‐term drought tolerance. These chickpea lines were not compromised in root nodulation and nitrogen fixation. The seed yield in some lines was up to 25% higher with no penalty in protein content. Transgenic chickpea seeds possessed higher levels of zinc, iron, potassium and copper. Our results demonstrated the potential of cytokinin level manipulation in increasing lateral root number and root biomass for agronomic trait improvement in an edible legume crop with indeterminate growth habit.

Gibberellins modulate local auxin biosynthesis and polar auxin transport by negatively affecting flavonoid biosynthesis in the root tips of rice

As critical signalling molecules, both gibberellin (GA) and auxin play essential roles in regulating root elongation, and many studies have been shown that auxin influences GA biosynthesis and signalling. However, the mechanism by which GA affects auxin in root elongation is still unknown. In this study, root elongation and DR5-GUS activity were analyzed in rice seedlings. Paclobutrazol-induced short root phenotypes could be partially reversed by co-treatment with IAA, and the inhibition of root elongation caused by naphthylphthalamic acid could be partially reversed when plants were co-treated with GA. DR5-GUS activity was increased in the presence of GA and was reduced at the root tip of paclobutrazol-treated seedlings, indicating that GA could regulate local auxin biosynthesis and polar auxin transport (PAT) in rice root tips. Our RNA-seq analysis showed that GA was involved in the regulation of flavonoid biosynthesis. Flavonoid accumulation level in ks1 root tips was significantly increased and negatively correlated with GA content in GA- and PAC-treated seedlings. GA also rescued the decreased DR5-GUS activity induced by quercetin in rice root tips, confirming that flavonoids act as an intermediary in GA-mediated auxin biosynthesis and PAT. Based on RNA-seq and qPCR analyses, we determined that GA regulates local auxin biosynthesis and polar auxin transport by modulating the expression of OsYUCCA6 and PIN. Our findings provide valuable new insights into the interactions between GA and auxin in the root tips of rice.

The physiological mechanism underlying root elongation in response to nitrogen deficiency in crop plants.

In response to low nitrogen stress, multiple hormones together with nitric oxide signaling pathways work synergistically and antagonistically in crop root elongation. Changing root morphology allows plants to adapt to soil nutrient availability. Nitrogen is the most important essential nutrient for plant growth. An important adaptive strategy for crops responding to nitrogen deficiency is root elongation, thereby accessing increased soil space and nitrogen resources. Multiple signaling pathways are involved in this regulatory network, working together to fine-tune root elongation in response to soil nitrogen availability. Based on existing research, we propose a model to explain how different signaling pathways interact to regulate root elongation in response to low nitrogen stress. In response to a low shoot nitrogen status signal, auxin transport from the shoot to the root increases. High auxin levels in the root tip stimulate the production of nitric oxide, which promotes the synthesis of strigolactones to accelerate cell division. In this process, cytokinin, ethylene, and abscisic acid play an antagonistic role, while brassinosteroids and auxin play a synergistic role in regulating root elongation. Further study is required to identify the QTLs, genes, and favorable alleles which control the root elongation response to low nitrogen stress in crops.

GmEXLB1, a Soybean Expansin-Like B Gene, Alters Root Architecture to Improve Phosphorus Acquisition in Arabidopsis.

Expansins comprise four subfamilies, α-expansin (EXPA), β-expansin (EXPB), expansin-like A (EXLA), and expansin-like B (EXLB), which are involved in the regulation of root development and growth under abiotic stress. To date, few EXLB genes have been shown to respond to low phosphorus (P) in plants. In this study, we identified an EXLB gene, GmEXLB1, by analyzing the transcription profiles of GmEXLBs in soybean. Quantitative analysis showed that GmEXLB1 was expressed and induced in the lateral roots of soybean under low P conditions. The observation of β-glucuronidase staining in transgenic Arabidopsis suggested that GmEXLB1 might be associated with lateral root emergence. GmEXLB1 overexpression altered the root architecture of transgenic Arabidopsis by increasing the number and length of lateral roots and the length of primary roots under low P conditions. Additionally, the length of the elongation zone and the average cell length in the elongation zone were increased in transgenic Arabidopsis. Increases in biomass and P content suggested that GmEXLB1 overexpression enhanced P acquisition in Arabidopsis. Overall, we conclude that GmEXLB1 expression is induced in soybean under low P conditions, and the overexpression of GmEXLB1 improves P acquisition by regulating root elongation and architecture in Arabidopsis, which provides a possible direction for research of the function of this gene in soybean.

Genome-wide association study reveals genomic regions controlling root and shoot traits at late growth stages in wheat.

Background and AimsRoot system morphology is important for sustainable agriculture, but the genetic basis of root traits and their relationship to shoot traits remain to be elucidated. The aim of the present study was to dissect the genetic basis of root traits at late growth stages and its implications on shoot traits in wheat.MethodsAmong 323 wheat accessions, we investigated phenotypic differences in root traits at booting and mid-grain fill stages in PVC tubes, shoot traits including plant height (PH), canopy temperature (CT) and grain yield per plant (YPP) in a field experiment, and performed a genome-wide association study with a Wheat 660K SNP Array.Key ResultsDeep-rooted accessions had lower CT and higher YPP than those with shallow roots, but no significant relationship was identified between root dry weight and shoot traits. Ninety-three significantly associated loci (SALs) were detected by the mixed linear model, among which three were hub SALs (Co-6A, Co-6B and Co-6D) associated with root depth at both booting and mid-grain fill stages, as well as CT and YPP. Minirhizotron system scanning results suggested that the causal genes in the three SALs may regulate root elongation in the field. The heritable independence between root depth and PH was demonstrated by linkage disequilibrium analysis. The YPP was significantly higher in genotypes which combined favourable marker alleles (FMAs) for root depth and PH, suggesting that a deep root and shorter plant height are suitable traits for pyramiding target alleles by molecular marker-assisted breeding.ConclusionsThese results uncovered promising genomic regions for functional gene discovery of root traits in the late growth period, enhanced understanding of correlation between root and shoot traits, and will facilitate intensive study on root morphology and breeding through molecular design.

Strigolactones and interaction with auxin regulating root elongation in tall fescue under different temperature regimes

Strigolactones (SL) have recently been found to play roles in regulating root development. However, it remains unclear how SL may mediate root elongation of perennial grass species under different temperatures that differentially affect root growth. The objectives of this study were to examine effects of SL on root elongation of tall fescue and to examine the interactive effects of SL and auxin in regulating root growth under both non-stress and heat stress conditions. Tall fescue (cv. ‘Kentuck-31′) plants were treated with GR24 (a synthetic strigolactones), NAA (α-naphthylacetic acid), or NPA (auxin transport inhibitor N-1-naphthylphalamic acid) or their combination under non-stress control and heat stress (35/30 °C) in growth chamber. Crown root elongation was evaluated by measuring root length. Cell number and length in root tips were measured under confocal microscope. Expression levels of genes related to cell growth, SL signaling and auxin transport were determined. SL promoted crown root elongation in tall fescue under normal temperature and heat stress, and alleviated heat-inhibition of root growth. GR24-enhanced root elongation was accompanied with the increase in cell numbers, up-regulation of cell cycle-related genes, and down-regulation auxin transport-related genes in crown root tips of tall fescue. The positive effects of SL for promoting crown root elongation in tall fescue under both non-stress and heat stress could be mainly due to its regulation of cell division and involve the interference of auxin transport.

Feedbacks between soil penetration resistance, root architecture and water uptake limit water accessibility and crop growth – A vicious circle

Water is the most limiting resource for global crop production. The projected increase of dry spells due to climate change will further increase the problem of water limited crop yields. Besides low water abundance and availability, water limitations also occur due to restricted water accessibility. Soil penetration resistance, which is largely influenced by soil moisture, is the major soil property regulating root elongation and water accessibility. Until now the interactions between soil penetration resistance, root system properties, water uptake and crop productivity are rarely investigated. In the current study we quantified how interactive effects between soil penetration resistance, root architecture and water uptake affect water accessibility and crop productivity in the field. Maize was grown on compacted and uncompacted soil that was either tilled or remained untilled after compaction, which resulted in four treatments with different topsoil penetration resistance. Higher topsoil penetration resistance caused root systems to be shallower. This resulted in increased water uptake from the topsoil and hence topsoil drying, which further increased the penetration resistance in the uppermost soil layer. As a consequence of this feedback, root growth into deeper soil layers, where water would have been available, was reduced and plant growth decreased. Our results demonstrate that soil penetration resistance, root architecture and water uptake are closely interrelated and thereby determine the potential of plants to access soil water pools. Hence, these interactions and their feedbacks on water accessibility and crop productivity have to be accounted for when developing strategies to alleviate water limitations in cropping systems.

ER-Anchored Transcription Factors bZIP17 and bZIP28 Regulate Root Elongation.

The unfolded protein response (UPR) is a eukaryotic transcriptional regulatory network that is activated upon the accumulation of malformed proteins in the endoplasmic reticulum (ER). In Arabidopsis (Arabidopsis thaliana), three bZIP transcription factors modulate the UPR: bZIP17, bZIP28, and bZIP60. Although bZIP28 and bZIP60 have been relatively well studied, the physiological and transcriptional roles of bZIP17 remain largely unknown. Here, we generated a double knockout mutant of bZIP17 and bZIP28 to elucidate the function of bZIP17. The mutant plant exhibited multiple developmental defects, including markedly reduced root elongation and constantly overinduced bZIP60 activity, indicating the essential roles of bZIP17 and bZIP28 in plant development and UPR modulation. Extended analysis of the transcriptomes of three double knockout mutants of bZIP17, bZIP28, and bZIP60 revealed that bZIP28 and bZIP60 are the major activators of the canonical induced UPR. By contrast, bZIP17 functions with bZIP28 to mediate the noninducible expression of multiple genes involved in cell growth, particularly to sustain their expression under stress conditions. Our study reveals pivotal roles of bZIP17 in the plant UPR and vegetative development, with functional redundancy to bZIP28.

Characterization and Identification of a woody lesion mimic mutant lmd, showing defence response and resistance to Alternaria alternate in birch

Lesion mimic mutants (LMM) usually show spontaneous cell death and enhanced defence responses similar to hypersensitive response (HR) in plants. Many LMM have been reported in rice, wheat, maize, barley, Arabidopsis, etc., but little was reported in xylophyta. BpGH3.5 is an early auxin-response factor which regulates root elongation in birch. Here, we found a T-DNA insertion mutant in a BpGH3.5 transgenic line named lmd showing typical LMM characters and early leaf senescence in Betula platyphylla × B. pendula. lmd showed H2O2 accumulation, increased SA level and enhanced resistance to Alternaria alternate, compared with oe21 (another BpGH3.5 transgenic line) and NT (non-transgenic line). Cellular structure observation showed that programmed cell death occurred in lmd leaves. Stereomicroscope observation and Evans’ blue staining indicated that lmd is a member of initiation class of LMM. Transcriptome analysis indicated that defence response-related pathways were enriched. Southern-blot indicated that there were two insertion sites in lmd genome. Genome re-sequencing and thermal asymmetric interlaced PCR (TAIL-PCR) confirmed the two insertion sites, one of which is a T-DNA insertion in the promoter of BpEIL1 that may account for the lesion mimic phenotype. This study will benefit future research on programmed cell death, HR and disease resistance in woody plants.

Identification of Ice Plant (Mesembryanthemum crystallinum L.) MicroRNAs Using RNA-Seq and Their Putative Roles in High Salinity Responses in Seedlings.

The halophyte Mesembryanthemum crystallinum (common or crystalline ice plant) is a useful model for studying molecular mechanisms of salt tolerance. The morphology, physiology, metabolism, and gene expression of ice plant have been studied and large-scale analyses of gene expression profiling have drawn an outline of salt tolerance in ice plant. A rapid root growth to a sudden increase in salinity was observed in ice plant seedlings. Using a fluorescent dye to detect Na+, we found that ice plant roots respond to an increased flux of Na+ by either secreting or storing Na+ in specialized cells. High-throughput sequencing was used to identify small RNA profiles in 3-day-old seedlings treated with or without 200 mM NaCl. In total, 135 conserved miRNAs belonging to 21 families were found. The hairpin precursor of 19 conserved mcr-miRNAs and 12 novel mcr-miRNAs were identified. After 6 h of salt stress, the expression of most mcr-miRNAs showed decreased relative abundance, whereas the expression of their corresponding target genes showed increased mRNA relative abundance. The cognate target genes are involved in a broad range of biological processes: transcription factors that regulate growth and development, enzymes that catalyze miRNA biogenesis for the most conserved mcr-miRNA, and proteins that are involved in ion homeostasis and drought-stress responses for some novel mcr-miRNAs. Analyses of the functions of target genes revealed that cellular processes, including growth and development, metabolism, and ion transport activity are likely to be enhanced in roots under salt stress. The expression of eleven conserved miRNAs and two novel miRNAs were correlated reciprocally with predicted targets within hours after salt stress exposure. Several conserved miRNAs have been known to regulate root elongation, root apical meristem activity, and lateral root formation. Based upon the expression pattern of miRNA and target genes in combination with the observation of Na+ distribution, ice plant likely responds to increased salinity by using Na+ as an osmoticum for cell expansion and guard cell opening. Excessive Na+ could either be secreted through the root epidermis or stored in specialized leaf epidermal cells. These responses are regulated in part at the miRNA-mediated post-transcriptional level.

An ABA down-regulated bHLH transcription repressor gene, bHLH129 regulates root elongation and ABA response when overexpressed in Arabidopsis.

Plant hormone abscisic acid (ABA) plays a crucial role in modulating plant responses to environmental stresses. Basic helix-loop-helix (bHLH) transcription factors are one of the largest transcription factor families that regulate multiple aspects of plant growth and development, as well as of plant metabolism in Arabidopsis. Several bHLH transcription factors have been shown to be involved in the regulation of ABA signaling. We report here the characterization of bHLH129, a bHLH transcription factor in Arabidopsis. We found that the expression level of bHLH129 was reduced in response to exogenously applied ABA, and elevated in the ABA biosynthesis mutant aba1-5. Florescence observation of transgenic plants expressing bHLH129-GFP showed that bHLH129 was localized in the nucleus, and transient expression of bHLH129 in protoplasts inhibited reporter gene expression. When expressed in Arabidopsis under the control of the 35S promoter, bHLH129 promoted root elongation, and the transgenic plants were less sensitivity to ABA in root elongation assays. Quantitative RT-PCR results showed that ABA response of several genes involved in ABA signaling, including ABI1, SnRK2.2, SnRK2.3 and SnRK2.6 were altered in the transgenic plants overexpressing bHLH129. Taken together, our study suggests that bHLH129 is a transcription repressor that negatively regulates ABA response in Arabidopsis.