Elongation Response Research Articles

Brassinosteroids in Plants: Crosstalk with Small-Molecule Compounds.

Brassinosteroids (BRs) are known as the sixth type of plant hormone participating in various physiological and biochemical activities and play an irreplaceable role in plants. Small-molecule compounds (SMCs) such as nitric oxide (NO), ethylene, hydrogen peroxide (H2O2), and hydrogen sulfide (H2S) are involved in plant growth and development as signaling messengers. Recently, the involvement of SMCs in BR-mediated growth and stress responses is gradually being discovered in plants, including seed germination, adventitious rooting, stem elongation, fruit ripening, and stress responses. The crosstalk between BRs and SMCs promotes plant development and alleviates stress damage by modulating the antioxidant system, photosynthetic capacity, and carbohydrate metabolism, as well as osmotic adjustment. In the present review, we try to explain the function of BRs and SMCs and their crosstalk in the growth, development, and stress resistance of plants.

BZIP72 promotes submerged rice seed germination and coleoptile elongation by activating ADH1

Seed germination and coleoptile elongation in response to flooding stress is an important trait for the direct seeding of rice. However, the genes regulating this process and the underlying mechanisms are little understood. In this study, bZIP72 was identified as a positive regulator of seed germination under submergence. Transcription of bZIP72 was submergence induced. Over-expression of bZIP72 enhanced submerged seed germination and coleoptile elongation, while bzip72 mutants exhibited the opposite tendency. Using biochemical interaction assays, we showed that bZIP72 directly binds to the promoter of alcohol dehydrogenase 1 (ADH1), enhances its activity, and subsequently produces more NAD+, NADH and ATP involved in the alcoholic fermentation and glycolysis pathway, ultimately providing necessary energy reserves thus conferring tolerance to submergence. In summary, this research provides novel insights into bZIP72 participation in submerged rice seed germination and coleoptile elongation.

An AFM study of the nanostructural response of New Zealand white rabbit Achilles tendons to cyclic loading.

The nanostructural response of New Zealand white rabbit Achilles tendons to a fatigue damage model was assessed quantitatively and qualitatively using the endpoint of dose assessments of each tendon from our previous study. The change in mechanical properties was assessed concurrently with nanostructural change in the same non-viable intact tendon. Atomic force microscopy was used to study the elongation of D-periodicities, and the changes were compared both within the same fibril bundle and between fibril bundles. D-periodicities increased due to both increased strain and increasing numbers of fatigue cycles. Although no significant difference in D-periodicity lengthening was found between fibril bundles, the lengthening of D-periodicity correlated strongly with the overall tendon mechanical changes. The accurate quantification of fibril elongation in response to macroscopic applied strain assisted in assessing the complex structure-function relationship in Achilles tendons.

A nascent polypeptide sequence modulates DnaA translation elongation in response to nutrient availability.

The ability to regulate DNA replication initiation in response to changing nutrient conditions is an important feature of most cell types. In bacteria, DNA replication is triggered by the initiator protein DnaA, which has long been suggested to respond to nutritional changes; nevertheless, the underlying mechanisms remain poorly understood. Here, we report a novel mechanism that adjusts DnaA synthesis in response to nutrient availability in Caulobacter crescentus. By performing a detailed biochemical and genetic analysis of the dnaA mRNA, we identified a sequence downstream of the dnaA start codon that inhibits DnaA translation elongation upon carbon exhaustion. Our data show that the corresponding peptide sequence, but not the mRNA secondary structure or the codon choice, is critical for this response, suggesting that specific amino acids in the growing DnaA nascent chain tune translational efficiency. Our study provides new insights into DnaA regulation and highlights the importance of translation elongation as a regulatory target. We propose that translation regulation by nascent chain sequences, like the one described, might constitute a general strategy for modulating the synthesis rate of specific proteins under changing conditions.

Green light reduces elongation when partially replacing sole blue light independently from cryptochrome 1a.

Although green light is sometimes neglected, it can have several effects on plant growth and development. Green light is probably sensed by cryptochromes (crys), one of the blue light photoreceptor families. The aim of this study is to investigate the possible interaction between green and blue light and the involvement of crys in the green light response of plant photomorphogenesis. We hypothesize that green light effects on morphology only occur when crys are activated by the presence of blue light. Wild‐type Moneymaker (MM), cry1a mutant (cry1a), and two CRY2 overexpressing transgenic lines (CRY2‐OX3 and CRY2‐OX8) of tomato (Solanum lycopersicum) were grown in a climate chamber without or with green light (30 μmol m−2 s−1) on backgrounds of sole red, sole blue and red/blue mixture, with all treatments having the same photosynthetic photon flux density of 150 μmol m−2 s−1. Green light showed no significant effects on biomass accumulation, nor on leaf characteristics such as leaf area, specific leaf area, and chlorophyll content. However, in all genotypes, green light significantly decreased stem length on a sole blue background, whereas green light hardly affected stem length on sole red and red/blue mixture background. MM, cry1a, and CRY2‐OX3/8 plants all exhibited similar responses of stem elongation to green light, indicating that cry1a, and probably cry2, is not involved in this green light effect. We conclude that partially replacing blue light by green light reduces elongation and that this is independent of cry1a.

Composite polymer hydrogels with high and reversible elongation under magnetic stimuli

The field of soft actuators is dominated by elastomers that experience mechanical deformations in response to external stimuli. In this context, magnetic stimuli attract considerable interest because of their easy application, tunability, fast response, remote actuation, and safe penetration in biological environments. Since very recently, research interests in the field are being redirected towards hydrogels, which could virtually replace elastomers, overcoming their limitations and expanding the field of application of soft actuators. The mechanical actuation of hydrogels is a nascent field full of challenges, such as achieving reliable and significant responsiveness. Here we demonstrate that the combination of a physical polymer hydrogel with a dispersed phase consisting of clusters of magnetic particles, results in magnetic hydrogel composites that exhibit high and reversible elongation in response to magnetic stimuli. Our analyses show that this response is strongly dependent on the matrix elasticity, the concentration of magnetic particles, and the particle distribution within the network of polymer nanofibres. Our strategy for the maximization of the response of magnetic hydrogels should be a catalyst for the development of novel applications of composite hydrogels, such as a valve remotely actuated by a magnetic field that we also present here as a proof-of-concept.

The canonical smooth muscle cell marker TAGLN is present in endothelial cells and is involved in angiogenesis.

Elongation of vascular endothelial cells (ECs) is an important process in angiogenesis; however, the molecular mechanisms remain unknown. The actin-crosslinking protein TAGLN (transgelin, also known as SM22 or SM22α) is abundantly expressed in smooth muscle cells (SMCs) and is widely used as a canonical marker for this cell type. In the course of studies using mouse embryonic stem cells (ESCs) carrying an Tagln promoter-driven fluorescence marker, we noticed activation of the Tagln promoter during EC elongation. Tagln promoter activation co-occurred with EC elongation in response to vascular endothelial growth factor A (VEGF-A). Inhibition of phosphoinositide 3-kinase (PI3K)-Akt signaling and mTORC1 also induced EC elongation and Tagln promoter activation. Human umbilical vein endothelial cells (HUVECs) elongated, activated the TAGLN promoter and increased TAGLN transcripts in an angiogenesis model. Genetic disruption of TAGLN augmented angiogenic behaviors of HUVECs, as did the disruption of TAGLN2 and TAGLN3 genes. Tagln expression was found in ECs in mouse embryos. Our results identify TAGLN as a putative regulator of angiogenesis whose expression is activated in elongating ECs. This finding provides insight into the cytoskeletal regulation of EC elongation and an improved understanding of the molecular mechanisms underlying the regulation of angiogenesis.

Primary Root and Mesocotyl Elongation in Maize Seedlings: Two Organs with Antagonistic Growth below the Soil Surface.

Maize illustrates one of the most complex cases of embryogenesis in higher plants that results in the development of early embryo with distinctive organs such as the mesocotyl, seminal and primary roots, coleoptile, and plumule. After seed germination, the elongation of root and mesocotyl follows opposite directions in response to specific tropisms (positive and negative gravitropism and hydrotropism). Tropisms represent the differential growth of an organ directed toward several stimuli. Although the life cycle of roots and mesocotyl takes place in darkness, their growth and functions are controlled by different mechanisms. Roots ramify through the soil following the direction of the gravity vector, spreading their tips into new territories looking for water; when water availability is low, the root hydrotropic response is triggered toward the zone with higher moisture. Nonetheless, there is a high range of hydrotropic curvatures (angles) in maize. The processes that control root hydrotropism and mesocotyl elongation remain unclear; however, they are influenced by genetic and environmental cues to guide their growth for optimizing early seedling vigor. Roots and mesocotyls are crucial for the establishment, growth, and development of the plant since both help to forage water in the soil. Mesocotyl elongation is associated with an ancient agriculture practice known as deep planting. This tradition takes advantage of residual soil humidity and continues to be used in semiarid regions of Mexico and USA. Due to the genetic diversity of maize, some lines have developed long mesocotyls capable of deep planting while others are unable to do it. Hence, the genetic and phenetic interaction of maize lines with a robust hydrotropic response and higher mesocotyl elongation in response to water scarcity in time of global heating might be used for developing more resilient maize plants.

A mechanism coordinating root elongation, endodermal differentiation, redox homeostasis and stress response.

Root growth relies on both cell division and cell elongation, which occur in the meristem and elongation zones, respectively. SCARECROW (SCR) and SHORT-ROOT (SHR) are GRAS family genes essential for root growth and radial patterning in the Arabidopsis root. Previous studies showed that SCR and SHR promote root growth by suppressing cytokinin response in the meristem, but there is evidence that SCR expressed beyond the meristem is also required for root growth. Here we report a previously unknown role for SCR in promoting cell elongation. Consistent with this, we found that the scr mutant accumulated a higher level of reactive oxygen species (ROS) in the elongation zone, which is probably due to decreased expression of peroxidase gene 3, which consumes hydrogen peroxide in a reaction leading to Casparian strip formation. When the oxidative stress response was blocked in the scr mutant by mutation in ABSCISIC ACID 2 (ABA2) or when the redox status was ameliorated by the upbeat 1 (upb1) mutant, the root became significantly longer, with longer cells and a larger and more mitotically active meristem. Remarkably, however, the stem cell and radial patterning defects in the double mutants still persisted. Since ROS and peroxidases are essential for endodermal differentiation, these results suggest that SCR plays a role in coordinating cell elongation, endodermal differentiation, redox homeostasis and oxidative stress response in the root. We also provide evidence that this role of SCR is independent of SHR, even though they function similarly in other aspects of root growth and development.

The chemical compound 'Heatin' stimulates hypocotyl elongation and interferes with the Arabidopsis NIT1-subfamily of nitrilases.

SUMMARYTemperature passively affects biological processes involved in plant growth. Therefore, it is challenging to study the dedicated temperature signalling pathways that orchestrate thermomorphogenesis, a suite of elongation growth‐based adaptations that enhance leaf‐cooling capacity. We screened a chemical library for compounds that restored hypocotyl elongation in the pif4‐2–deficient mutant background at warm temperature conditions in Arabidopsis thaliana to identify modulators of thermomorphogenesis. The small aromatic compound ‘Heatin’, containing 1‐iminomethyl‐2‐naphthol as a pharmacophore, was selected as an enhancer of elongation growth. We show that ARABIDOPSIS ALDEHYDE OXIDASES redundantly contribute to Heatin‐mediated hypocotyl elongation. Following a chemical proteomics approach, the members of the NITRILASE1‐subfamily of auxin biosynthesis enzymes were identified among the molecular targets of Heatin. Our data reveal that nitrilases are involved in promotion of hypocotyl elongation in response to high temperature and Heatin‐mediated hypocotyl elongation requires the NITRILASE1‐subfamily members, NIT1 and NIT2. Heatin inhibits NIT1‐subfamily enzymatic activity in vitro and the application of Heatin accordingly results in the accumulation of NIT1‐subfamily substrate indole‐3‐acetonitrile in vivo. However, levels of the NIT1‐subfamily product, bioactive auxin (indole‐3‐acetic acid), were also significantly increased. It is likely that the stimulation of hypocotyl elongation by Heatin might be independent of its observed interaction with NITRILASE1‐subfamily members. However, nitrilases may contribute to the Heatin response by stimulating indole‐3‐acetic acid biosynthesis in an indirect way. Heatin and its functional analogues present novel chemical entities for studying auxin biology.

Agrobacterium Mediated Transformation Optimizations for Sugarcane (Saccharum Officinarum L.) Cultivar SPF-234 with Direct Organogenesis

Sugarcane (Saccharum officinarum L.) is the most important food and energy crop worldwide. In the present study, an efficient Agrobacterium mediated transformation and regeneration system for sugarcane cultivar SPF-234 was established. Agrobacterium tumefaciens strains EHA101and LBA4404 using vector pIG121 Hm, having GUS, HPTII and NPTII genes were used. Polymerase chain reaction (PCR) and histochemical assays confirmed the GUS gene expression. A 620 bp fragment from GUS positive plants was amplified. The GUS expressing putative transformants were 35% of the total plants formed under 30 minute immersion time and 72 hr of incubation period. The co-cultivation media having 60 µM acetosyringone produced 66% GUS expressing plants for LBA4404 and 58% for EHA101. The maximum average number of directly produced shoot (59.5%) from leaf explant was in M6 media having 1.00 mg/l 6-Benzylaminopurine (BAP) and 2.5 mg/l Naphthaleneacetic acid (NAA). A significant decrease (17%) was observed when auxin (NAA) concentration was increased to 4.0 mg/l. The best response of shoot elongation was observed in SE4 media having equal concentration (2.00 mg/l) of both kinetin and BAP. Increased concentrations of kinetin significantly decreased shoot elongation of the subject cultivar. Agrobacterium strain LBA4404 performed better for genetic transformation of the said sugarcane cultivar.This quick and less expensive transformation and direct regeneration system could be exploited for sugarcane on commercial scale in general, and for this elite cultivar in particular.

SPL14/17 act downstream of strigolactone signalling to modulate rice root elongation in response to nitrate supply

Nitrogen (N) is an essential major nutrient for food crops. Although ammonium (NH4 + ) is the primary N source of rice, nitrate (NO3 - ) can also be absorbed and utilized. Rice responds to NO3 - application by altering its root morphology, such as root elongation. Strigolactones (SLs) are important modulators of root length. However, the roles of SLs and their downstream genes in NO3 - -induced root elongation remain unclear. Here, the levels of total N and SL (4-deoxyorobanchol), and the responses of seminal root (SR) lengths to NH4 + and NO3 - were investigated in rice plants. NO3 - -promoted SR elongation, possibly due to short-term signal perception and long-term nutrient function. Compared with NH4 + condition, higher SL signalling/levels and less D53 protein were recorded in roots of NO3 - -treated rice plants. In contrast to wild-type (WT) plants, SR lengths of d mutants were less responsive to NO3 - condition, and application of rac-GR24 (SL analogue) restored SR length in d10 (SL-biosynthesis mutant) but not in d3,d14 and d53 (SL-responsive mutants), suggesting that higher SL signalling/levels participated in NO3 - -induced root elongation. D53 interacted with SPL17, and inhibited SPL17-mediated transactivation from PIN1b promoter. Mutation of SPL14/17 and PIN1b caused insensitivity of root elongation response to NO3 - and rac-GR24 applications. Therefore, we presented that perception of SLs by D14 led to degradation of D53 via the proteasome system, which released the suppression of SPL14/17-modulated the transcription of PIN1b, and resultedin root elongation under NO3 - supply.

Promoter Architecture and Transcriptional Regulation of Genes Upregulated in Germination and Coleoptile Elongation of Diverse Rice Genotypes Tolerant to Submergence.

Rice has the natural morphological adaptation to germinate and elongate its coleoptile under submerged flooding conditions. The phenotypic deviation associated with the tolerance to submergence at the germination stage could be due to natural variation. However, the molecular basis of this variation is still largely unknown. A comprehensive understanding of gene regulation of different genotypes that have diverse rates of coleoptile elongation can provide significant insights into improved rice varieties. To do so, publicly available transcriptome data of five rice genotypes, which have different lengths of coleoptile elongation under submergence tolerance, were analyzed. The aim was to identify the correlation between promoter architecture, associated with transcriptional and hormonal regulation, in diverse genotype groups of rice that have different rates of coleoptile elongation. This was achieved by identifying the putative cis-elements present in the promoter sequences of genes upregulated in each group of genotypes (tolerant, highly tolerant, and extremely tolerant genotypes). Promoter analysis identified transcription factors (TFs) that are common and unique to each group of genotypes. The candidate TFs that are common in all genotypes are MYB, bZIP, AP2/ERF, ARF, WRKY, ZnF, MADS-box, NAC, AS2, DOF, E2F, ARR-B, and HSF. However, the highly tolerant genotypes interestingly possess binding sites associated with HY5 (bZIP), GBF3, GBF4 and GBF5 (bZIP), DPBF-3 (bZIP), ABF2, ABI5, bHLH, and BES/BZR, in addition to the common TFs. Besides, the extremely tolerant genotypes possess binding sites associated with bHLH TFs such as BEE2, BIM1, BIM3, BM8 and BAM8, and ABF1, in addition to the TFs identified in the tolerant and highly tolerant genotypes. The transcriptional regulation of these TFs could be linked to phenotypic variation in coleoptile elongation in response to submergence tolerance. Moreover, the results indicate a cross-talk between the key TFs and phytohormones such as gibberellic acid, abscisic acid, ethylene, auxin, jasmonic acid, and brassinosteroids, for an altered transcriptional regulation leading to differences in germination and coleoptile elongation under submergence. The information derived from the current in silico analysis can potentially assist in developing new rice breeding targets for direct seeding.

Optimization of mechanical properties of bioplastics with the addition of ZnO and glycerol plasticizer

Bioplastics are an alternative to conventional plastics that have been used in society. The advantage of bioplastics is that they are easily degraded in nature because they come from easily renewable materials such as starch which comes from plants. However, biodegradable plastics have shortcomings in terms of tensile strength so that additional supporting materials are needed in their manufacture. Optimization using the Response Surface Methodology (RSM) central composite design (CCD) in this study aims to obtain optimal mechanical properties (tensile strength). The independent variables used in this study include ZnO concentrations (3%, 6%, and 9%) and glycerol concentrations (2 ml, 5 ml, and 8 ml) as well as mechanical properties such as tensile strength and elongation in response. The optimum conditions were obtained at a concentration of ZnO 6.99% and glycerol 5.32 ml with a tensile strength value of 1.58456 MPa and 9.082201% elongation.

In-vitro propagation of Pluchea lanceolata (DC) C.B.Clarke a potent antiarthritic medicinal herb through axillary bud

Pluchea lanceolata, commonly known as Rasana, is highly valued for its anti-inflammatory, antiarthritic, and analgesic properties and is extensively employed in treating rheumatoid arthritis, cough, neurological diseases, edema, sciatica, psoriasis, bronchitis, dyspepsia, and piles. Shrinking populations and insufficient variation due to rootstock propagation methods warrant efficient mass multiplication and conservation protocols. In the present study, explant (axillaries bud was cultured on Murashige and Skoog (MS) medium supplemented with different concentrations of cytokinins and auxin (PGRs) individually, as well as in combination. The highest mean shoot number (2.06 ± 1.43) and second highest mean shoot length (2.62 ± 1.49 cm) were obtained in MS media enriched with 4 mg/L Benzyl amino purine (BAP) + 1mg/L Naphthalene acetic acid (NAA). Whereas, the best shoots multiplication (2.62 ± 1.41) and shoots elongation (5.88 ± 2.44) responses were displayed when MS medium supplemented with 0.25 mg/L (BAP) +0.50 mg/L Kinetin (KIN) were used.

The identification of critical time windows of postnatal root elongation in response to Wnt/β-catenin signaling.

In this study, we attempted to define the precise window of time for molar root elongation using a gain-of-function mutation of β-catenin model. Both the control and constitutively activated β-catenin (CA-β-cat) mice received a one-time tamoxifen administration (for activation of β-catenin at newborn, postnatal day 3, or 5, or 7, or 9) and were harvested at the same stage of P21. Multiple approaches were used to define the window of time of postnatal tooth root formation. In the early activation groups (tamoxifen induction at newborn, or P3 or P5), there was a lack of molar root elongation in the CA-β-cat mice. When induced at P7, the root length was slightly reduced at P21. However, the root length was essentially the same as that in the control when β-cat activated at P9. This study indicates that root elongation occurs in a narrow time of window, which is highly sensitive to a change of β-catenin levels. Molecular studies showed a drastic decrease in the levels of nuclear factor I-C (NFIC) and osterix (OSX), plus sharp reductions of odontoblast differentiation markers, including Nestin, dentin sialoprotein (DSP), and dentin matrix protein 1 (DMP1) at both mRNA and protein levels. Murine molar root elongation is precisely regulated by the Wnt/β-catenin signaling within a narrow window of time (newborn to day 5).

Regulation of cytoskeleton-associated protein activities: Linking cellular signals to plant cytoskeletal function.

The plant cytoskeleton undergoes dynamic remodeling in response to diverse developmental and environmental cues. Remodeling of the cytoskeleton coordinates growth in plant cells, including trafficking and exocytosis of membrane and wall components during cell expansion, and regulation of hypocotyl elongation in response to light. Cytoskeletal remodeling also has key functions in disease resistance and abiotic stress responses. Many stimuli result in altered activity of cytoskeleton-associated proteins, microtubule-associated proteins (MAPs) and actin-binding proteins (ABPs). MAPs and ABPs are the main players determining the spatiotemporally dynamic nature of the cytoskeleton, functioning in a sensory hub that decodes signals to modulate plant cytoskeletal behavior. Moreover, MAP and ABP activities and levels are precisely regulated during development and environmental responses, but our understanding of this process remains limited. In this review, we summarize the evidence linking multiple signaling pathways, MAP and ABP activities and levels, and cytoskeletal rearrangements in plant cells. We highlight advances in elucidating the multiple mechanisms that regulate MAP and ABP activities and levels, including calcium and calmodulin signaling, ROP GTPase activity, phospholipid signaling, and post-translational modifications.

Seedling elongation responses to gibberellin seed treatments in wheat

AbstractThe wheat (Triticum aestivum L.) Reduced height (Rht) alleles are widely used to prevent lodging through semi‐dwarfism. Seedling elongation and coleoptile length can be significantly decreased by some of these alleles, leading to reduced soil emergence after deep sowing in certain semi‐arid environments. Application of the elongation‐promoting hormone gibberellin A3 (GA3) as a seed treatment has been used as an alternative to improve seedling emergence. Seedling responses to GA3 seed treatment were investigated under controlled conditions in a collection of varieties differing for Rht dwarfing alleles and the ability to emerge from deep planting. Data between treated and untreated seed were collected on overall coleoptile length and emergence from deep planting in simulated pot studies. Gibberellin‐sensitive varieties, carrying either no dwarfing gene or the Rht8 dwarfing gene, responded to the GA3 seed treatment with increased coleoptile and subcrown internode elongation. Comparison of near‐isogenic lines carrying no dwarfing allele or the GA‐insensitive Rht‐B1b and/or Rht‐D1b semi‐dwarfing alleles showed that GA insensitivity was associated with reduced seedling response to GA3 seed treatment. However, there was variation in seedling elongation and GA3 response in a collection of GA‐insensitive varieties. Thus, it cannot be assumed that all Rht‐B1b and Rht‐D1b varieties will fail to respond to GA3 seed treatment. Interestingly, some better emerging GA‐insensitive varieties had longer coleoptiles after treatment, suggesting that selection for better emergence in semi‐arid regions of the U.S. Pacific Northwest may have led to responses in GA3 application independent of the dwarfing gene used.

Leaf elongation response to blue light is mediated by stomatal-induced variations in transpiration in Festuca arundinacea.

Reduced blue light irradiance is known to enhance leaf elongation rate (LER) in grasses, but the mechanisms involved have not yet been elucidated. We investigated whether leaf elongation response to reduced blue light could be mediated by stomata-induced variations of plant transpiration. Two experiments were carried out on tall fescue in order to monitor LER and transpiration under reduced blue light irradiance. Additionally, LER dynamics were compared with those observed in the response to vapour pressure deficit (VPD)-induced variations of transpiration. Finally, we developed a model of water flow within a tiller to simulate the observed short-term response of LER to various transpiration regimes. LER dramatically increased in response to blue light reduction and then reached new steady states, which remained higher than the control. Reduced blue light triggered a simultaneous stomatal closure which induced an immediate decrease of leaf transpiration. The hydraulic model of leaf elongation accurately predicted the LER response to blue light and VPD, resulting from an increase in the growth-induced water potential gradient in the leaf growth zone. Our results suggest that the blue light signal is sensed by stomata of expanded leaves and transduced to the leaf growth zone through the hydraulic architecture of the tiller.

The CIB1 transcription factor regulates light- and heat-inducible cell elongation via a two-step HLH/bHLH system.

Light and high temperature promote plant cell elongation. PHYTOCHROME INTERACTING FACTOR4 (PIF4, a typical basic helix-loop-helix [bHLH] transcriptional activator) and the non-DNA binding atypical HLH inhibitors PHYTOCHROME RAPIDLY REGULATED1 (PAR1) and LONG HYPOCOTYL IN FAR-RED 1 (HFR1) competitively regulate cell elongation in response to light conditions and high temperature. However, the underlying mechanisms have not been fully clarified. Here, we show that in Arabidopsis thaliana, the bHLH transcription factor CRYPTOCHROME-INTERACTING BASIC HELIX-LOOP-HELIX 1 (CIB1) positively regulates cell elongation under the control of PIF4, PAR1, and HFR1. Furthermore, PIF4 directly regulates CIB1 expression by interacting with its promoter, and PAR1 and HFR1 interfere with PIF4 binding to the CIB1 promoter. CIB1 activates genes that function in cell elongation, and PAR1 interferes with the DNA binding activity of CIB1, thus suppressing cell elongation. Hence, two antagonistic HLH/bHLH systems, the PIF4-PAR1/HFR1 and CIB1-PAR1 systems, regulate cell elongation in response to light and high temperature. We thus demonstrate the important role of non-DNA binding small HLH proteins in the transcriptional regulation of cell elongation in plants.