Role of PIN-mediated auxin efflux in apical hook development of Arabidopsis thaliana

Here comes the sun: integration of light, temperature, and auxin during herbaceous plant grafting

MAIN CONCLUSION

Light and temperature can regulate auxin production which has been recently shown to be key during graft healing, suggesting that abiotic factors may be vital variables for future graft studies. Grafting is an important horticultural tool used to combine advantageous plant traits. Despite its broad usage, the mechanisms that underlie graft healing remain poorly understood. Recent work has highlighted the influence of high temperature-mediated auxin flow on graft success. Light and temperature sensing utilize partially overlapping mechanisms to regulate auxin biosynthesis, signaling, and transport. In this review, we explore the sensors and transcriptional regulators that modulate auxin response, specifically emphasizing how these components regulate graft success and vascular reconnection. We also discuss areas of graft biology regulated by auxin and underexplored areas of photobiology that may be key to a better understanding of graft mechanisms. This review underscores the importance of translating genetic findings from model systems into horticultural crops to expand our knowledge of economically valuable techniques like grafting.

BTB/POZ-MATH proteins regulate Arabidopsis seedling development by promoting auxin-independent degradation of the Aux/IAA protein IAA10

After germination, seedlings undergo etiolated development (skotomorphogenesis), enabling them to grow toward the soil surface. In Arabidopsis (Arabidopsis thaliana), etiolated seedlings exhibit rapid hypocotyl elongation, apical hook formation, and closed cotyledons to protect the meristem. In this study, we found that high-order mutants in the BPM (BTB/POZ-MATH) gene family displayed defects in seedling development, characterized by a shorter hypocotyl, early apical hook opening, and opened cotyledons in the dark. BPM1, BPM2, BPM4, and BPM5 exhibited distinct expression patterns and subcellular localization in etiolated seedlings. In a hypocotyl segment assay, the bpm mutants showed defects in auxin response, indicating impaired auxin signaling in the hypocotyl. Expression of the auxin reporter DR5:GFP was also altered in the bpm1,4,5 mutant in various tissues compared with the wild type. Furthermore, yeast 2-hybrid, bimolecular fluorescence complementation, and co-immunoprecipitation assay analyses showed that BPM1 interacts with IAA10. Experiments in protoplasts indicated that BPM1 promotes IAA10 ubiquitylation and degradation, which was supported by greater IAA10 protein accumulation in the bpm1,4,5 mutant background. In addition, IAA10 overexpression resulted in phenotypes similar to those of the bpm mutants, indicating that the BPMs may target the Aux/IAA proteins for ubiquitylation and degradation. Overall, our findings shed light on the key roles of the BPMs in auxin signaling during seedling development.

The protein kinases KIPK and KIPK-LIKE1 suppress overbending during negative hypocotyl gravitropic growth in Arabidopsis

Plants use environmental cues to orient organ and plant growth, such as the direction of gravity or the direction, quantity, and quality of light. During the germination of Arabidopsis thaliana seeds in soil, negative gravitropism responses direct hypocotyl elongation such that the seedling can reach the light for photosynthesis and autotrophic growth. Similarly, hypocotyl elongation in the soil also requires mechanisms to efficiently grow around obstacles such as soil particles. Here, we identify KIPK (KINESIN-LIKE CALMODULIN-BINDING PROTEIN-INTERACTING PROTEIN KINASE) and the paralogous KIPKL1 (KIPK-LIKE1) as genetically redundant regulators of gravitropic hypocotyl bending. Moreover, we demonstrate that the homologous KIPKL2 (KIPK-LIKE2), which shows strong sequence similarity, must be functionally distinct. KIPK and KIPKL1 are polarly localized plasma membrane-associated proteins that can activate PIN-FORMED auxin transporters. KIPK and KIPKL1 are required to efficiently align hypocotyl growth with the gravity vector when seedling hypocotyls are grown on media plates or in soil, where contact with soil particles and obstacle avoidance impede direct negative gravitropic growth. Therefore, the polar KIPK and KIPKL1 kinases have different biological functions from the related AGC1 family kinases D6PK (D6 PROTEIN KINASE) or PAX (PROTEIN KINASE ASSOCIATED WITH BRX).

Identification and characterization of auxin response factor (ARF) gene family in five Bambusoideae species reveals the role of PedARF 23 in regulating lignin synthesis through auxin signaling

Auxin plays a critical role in plant growth and development, mediating responses to environmental stimuli and directing organ growth by establishing and maintaining auxin concentration gradients. Auxin Response Factors (ARFs) are key transcription factors that regulate the expression of auxin-responsive genes. Different ARF genes can act as transcriptional activators or repressors, functioning through various auxin signaling pathways at different developmental stages and under various environmental conditions. Studying the ARF gene family in bamboo is essential for uncovering the molecular mechanisms underlying rapid growth, maintenance of apical dominance, and root development in bamboo. However, research on the function of ARF genes in bamboo is still limited. In this study, we utilized the latest bamboo genome data to comprehensively analyze the genomes of five representative bamboo species, identifying a total of 216 ARF genes and classifying them into 12 subfamilies based on phylogenetic relationships. Through a combination of transcriptome date and RT-qPCR analysis, we observed that the PedARF23 gene in Moso bamboo was significantly upregulated following NAA treatment. To further explore the mechanisms by which PedARF23 modulates plant growth and development via auxin signaling, we developed PedARF23 overexpression lines. By measuring auxin and lignin levels, the expression of key genes (4CL3, 4CL7, and CCoAOMT1) in the lignin biosynthesis pathway in both Moso bamboo and transgenic Arabidopsis, as well as the Yeast one-hybrid assay and LUC activation assay, study discovered that PedARF23 could enhances auxin synthesis and activates the auxin signaling pathway, thereby regulating lignin biosynthesis. Overall, this study provides a foundation for understanding the classification, characteristics, and functions of the ARF gene family in the Bambusoideae. It elucidates the role of ARFs in regulating lignin synthesis, offering valuable insights into the regulation of lignin content in bamboo.

Sinomonas gamaensis NEAU-HV1 remodels the IAA14-ARF7/19 interaction to promote plant growth

Sinomonas species typically reside in soils or the rhizosphere and can promote plant growth. Sinomonas enrichment in rhizospheric soils is positively correlated with increases in plant biomass. However, the growth promotion mechanisms regulated by Sinomonas remain unclear. By using soil systems, we studied the growth-promoting effects of Sinomonas gamaensis NEAU-HV1 on various plants. Through a combination of phenotypic analyses and microscopic observations, the effects of NEAU-HV1 on root development were evaluated. We subsequently conducted molecular and genetic experiments to reveal the mechanism promoting lateral root (LR) development. We demonstrated that NEAU-HV1 significantly promoted the growth of lettuce, wheat, maize, peanut and Arabidopsis. This effect was associated with multiple beneficial traits, including phosphate solubilization, indole-3-acetic acid and 1-aminocyclopropane-1-carboxylic acid deaminase production and survival ability in the rhizosphere and within the inner tissue of roots. In addition, NEAU-HV1 could secrete metabolites to promote LR development by affecting auxin transport and signaling. Importantly, we found that the influence of auxin signaling may be attributed to the remodeling interaction between SOLITARY-ROOT (SLR)/IAA14 and ARF7/19, occurring independently of the auxin receptor TIR1/AFB2. Our results indicate that NEAU-HV1-induced LR formation is dependent on direct remodeling interactions between transcription factors, providing novel insights into plant-microbe interactions.

Reduced RG-II pectin dimerization disrupts differential growth by attenuating hormonal regulation

Defects in cell wall integrity (CWI) profoundly affect plant growth, although, underlying mechanisms are not well understood. We show that in Arabidopsis mur1 mutant, CWI defects from compromising dimerization of RG-II pectin, a key component of cell wall, attenuate the expression of auxin response factors ARF7-ARF19. As a result, polar auxin transport components are misexpressed, disrupting auxin response asymmetry, leading to defective apical hook development. Accordingly, mur1 hook defects are suppressed by enhancing ARF7 expression. In addition, expression of brassinosteroid biosynthesis genes is down-regulated in mur1 mutant, and supplementing brassinosteroid or enhancing brassinosteroid signaling suppresses mur1 hook defects. Intriguingly, brassinosteroid enhances RG-II dimerization, showing hormonal feedback to the cell wall. Our results thus reveal a previously unrecognized link between cell wall defects from reduced RG-II dimerization and growth regulation mediated via modulation of auxin-brassinosteroid pathways in early seedling development.

Phenylmercury stress induces root tip swelling through auxin homeostasis disruption

We previously reported that in Arabidopsis, the phytochelatin-mediated metal-detoxification machinery is also essential for organomercurial phenylmercury (PheHg) tolerance. PheHg treatment causes severe root growth inhibition in cad1-3, an Arabidopsis phytochelatin-deficient mutant, frequently accompanied by abnormal root tip swelling. Here, we examine morphological and physiological characteristics of PheHg-induced abnormal root tip swelling in comparison to Hg(II) stress and demonstrate that auxin homeostasis disorder in the root is associated with the PheHg-induced root tip swelling. Both Hg(II) and PheHg treatments severely inhibited root growth in cad1-3 and simultaneously induced the disappearance of starch-containing plastid amyloplasts in columella cells. However, further confocal imaging of the root tip revealed distinct effects of Hg(II) and PheHg toxicity on root cell morphology. PheHg treatment suppressed most major genes involved in auxin homeostasis, whereas these expression levels were up-regulated after 24 h of Hg(II) treatment. PheHg-triggered suppression of auxin transporters PIN1, PIN2, and PIN3 as GFP-fusion proteins was observed in the root tip, accompanied by an auxin reporter DR5rev::GFP signal reduction. Supplementation of indole-3-acetic acid (IAA) drastically canceled the PheHg-induced root swelling, however, Hg(II) toxicity was not mitigated by IAA. The presented results show that the collapse of auxin homeostasis especially in root tips is a cause for the abnormal root tip swelling under PheHg stress conditions.

Differential growth is an emergent property of mechanochemical feedback mechanisms in curved plant organs

Differential growth is central to eukaryotic morphogenesis. We showed using cellular imaging, simulations, and perturbations that light-induced differential growth in a curved organ, the Arabidopsis thaliana apical hook, emerges from the longitudinal expansion of subepidermal cells, acting in parallel with a differential in the material properties of epidermal cell walls that resist expansion. The greater expansion of inner hook cells that results in apical hook opening is gated by wall alkalinity and auxin, both of which are depleted upon illumination. We further identified mechanochemical feedback from wall mechanics to light stimulated auxin depletion, which may contribute to gating hook opening under mechanical restraint. These results highlight how plant cells coordinate growth among tissue layers by linking mechanics and hormonal gradients with the cell wall remodeling required for differential growth.

BPMs regulate Arabidopsis seedling development by promoting auxin-independent degradation of the Aux/IAA protein IAA10

After germination, seedlings undergo etiolated development (skotomorphogenesis), enabling them to grow towards the soil surface. In Arabidopsis, etiolated seedlings exhibit rapid hypocotyl elongation, apical hook formation and closed cotyledons to protect the meristem. In this study, we found that high-order mutants in the BPM gene family displayed defects in seedling development, characterized by a shorter hypocotyl, early apical hook opening, and opened cotyledons in the dark. BPM1, BPM2, BPM4, and BPM5 exhibit distinct expression patterns and subcellular localization in etiolated seedlings. In a hypocotyl segment assay the bpm mutants showed defects in auxin response indicating impaired auxin signaling in the hypocotyl. Expression of the auxin reporter DR5:GFP was also altered in the bpm1,4,5 mutant in various tissues compared to the wild type. Furthermore, we showed that BPM1 and IAA10 interact in yeast two-hybrid, BiFC, and Co-IP assays. Experiments in protoplasts indicated that BPM1 promotes ubiquitylation and degradation of IAA10, and the level of IAA10 protein is greater in the bpm1,4,5 mutant. In addition, IAA10 over-expression resulted in phenotypes similar to the bpm mutants. These results indicate that the BPMs target the Aux/IAA proteins for ubiquitylation and degradation. Overall, our findings shed light on the key roles of the BPMs in auxin signaling during seedling development.

Cellular mechanism of polarized auxin transport on fruit shape determination revealed by time-lapse live imaging

KEY MESSAGE

Polarized auxin transport regulates fruit shape determination by promoting anisotropic cell growth. Angiosperms produce organs with distinct shape resultant from adaptive evolution. Understanding the cellular basis underlying the development of plant organ has been a central topic in plant biology as it is key to unlock the mechanisms leading to the diversification of plants. Variations in the location of synthesis, polarized auxin transport (PAT) have been proposed to account for the development of diverse organ shapes, but the exact cellular mechanism has yet to be elucidated. The Capsella rubella develops a perfect heart-shaped fruit from an ovate shape gynoecium that is tightly linked to the localized auxin synthesis in the valve tips and provides a unique opportunity to address this question. In this study, we studied auxin movement in the fruits and the cellular effect of N-1-Naphthylphthalamic Acid (NPA) on the fruit shape determination by constructing the pCrPIN3:PIN3:GFP reporter and live-imaging. We found PAT in the valve epidermis is in congruent with fruit shape development and NPA treatment disrupts the heat-shaped fruit development mainly by repressing cell anisotropic growth with minor effect on division. As the Capsella fruit is unusually big in size, we also included a detailed step-by-step protocol on how to conduct live-imaging experiment. We further test the utility of this protocol by conducting a live-imaging analysis of the gynophore in Arachis hypogaea. Collectively, the results of this study elucidated the mechanism on how auxin signal was translated into instructions guiding cell growth during organ shape determination. In addition, the description of the detailed live-imaging protocol will encourage further studies of the cellular mechanisms underlying shape diversification in angiosperms.

Over 25 years of decrypting PIN-mediated plant development

Identification of PIN exporters for auxin, the major coordinative signal in plants, some 25 years ago, signifies a landmark in our understanding of plant-specific mechanisms underlying development and adaptation. Auxin is directionally transported throughout the plant body; a unique feature already envisioned by Darwin and solidified by PINs' discovery and characterization. The PIN-based auxin distribution network with its complex regulations of PIN expression, localization and activity turned out to underlie a remarkable multitude of developmental processes and represents means to integrate endogenous and environmental signals. Given the recent anniversary, we here summarize past and current developments in this exciting field.

Modulation of cell differentiation and growth underlies the shift from bud protection to light capture in cauline leaves

Plant organs have evolved into diverse shapes for specialized functions despite emerging as simple protrusions at the shoot apex. Cauline leaves serve as photosynthetic organs and protective structures for emerging floral buds. However, the growth patterns underlying this dual function remain unknown. Here, we investigate the developmental dynamics shaping Arabidopsis (Arabidopsis thaliana) cauline leaves underlying their functional diversification from other laminar organs. We show that cauline leaves display a significant delay in overall elongation compared with rosette leaves. Using live imaging, we reveal that their functional divergence hinges on early modulation of the timing of cell differentiation and cellular growth rates. In contrast to rosette leaves and sepals, cell differentiation is delayed in cauline leaves, fostering extended proliferation, prolonged morphogenetic activity, and growth redistribution within the organ. Notably, cauline leaf growth is transiently suppressed during the early stages, keeping the leaf small and unfolded during the initiation of the first flowers. Our findings highlight the unique developmental timing of cauline leaves, underlying their shift from an early protective role to a later photosynthetic function.

The endophytic fungus Serendipita indica affects auxin distribution in Arabidopsis thaliana roots through alteration of auxin transport and conjugation to promote plant growth

Plants share their habitats with a multitude of different microbes. This close vicinity promoted the evolution of interorganismic interactions between plants and many different microorganisms that provide mutual growth benefits both to the plant and the microbial partner. The symbiosis of Arabidopsis thaliana with the beneficial root colonizing endophyte Serendipita indica represents a well-studied system. Colonization of Arabidopsis roots with S. indica promotes plant growth and stress tolerance of the host plant. However, until now, the molecular mechanism by which S. indica reprograms plant growth remains largely unknown. This study used comprehensive transcriptomics, metabolomics, reverse genetics, and life cell imaging to reveal the intricacies of auxin-related processes that affect root growth in the symbiosis between A. thaliana and S. indica. Our experiments revealed the sustained stimulation of auxin signalling in fungus infected Arabidopsis roots and disclosed the essential role of tightly controlled auxin conjugation in the plant-fungus interaction. It particularly highlighted the importance of two GRETCHEN HAGEN 3 (GH3) genes, GH3.5 and GH3.17, for the fungus infection-triggered stimulation of biomass production, thus broadening our knowledge about the function of GH3s in plants. Furthermore, we provide evidence for the transcriptional alteration of the PIN2 auxin transporter gene in roots of Arabidopsis seedlings infected with S. indica and demonstrate that this transcriptional adjustment affects auxin signalling in roots, which results in increased plant growth.

Polarly localized Bro1 domain proteins regulate PIN-FORMED abundance and root gravitropic growth in Arabidopsis

The developmental plasticity of the root system plays an essential role in the adaptation of plants to the environment. Among many other signals, auxin and its directional, intercellular transport are critical in regulating root growth and development. In particular, the PIN-FORMED2 (PIN2) auxin exporter acts as a key regulator of root gravitropic growth. Multiple regulators have been reported to be involved in PIN2-mediated root growth; however, our information remains incomplete. Here, we identified ROWY Bro1-domain proteins as important regulators of PIN2 sorting control. Genetic analysis revealed that Arabidopsis rowy1 single mutants and higher-order rowy1 rowy2 rowy3 triple mutants presented a wavy root growth phenotype. Cell biological experiments revealed that ROWY1 and PIN2 colocalized to the apical side of the plasma membrane in the root epidermis and that ROWYs are required for correct PM targeting of PIN2. In addition, ROWYs also affected PIN3 protein abundance in the stele, suggesting the potential involvement of additional PIN transporters as well as other proteins. A global transcriptome analysis revealed that ROWY genes are involved in the Fe2+ availability perception pathway. This work establishes ROWYs as important novel regulators of root gravitropic growth by connecting micronutrient availability to the proper subcellular targeting of PIN auxin transporters.

Light-stabilized GIL1 suppresses PIN3 activity to inhibit hypocotyl gravitropism

Light and gravity coordinately regulate the directional growth of plants. Arabidopsis Gravitropic in the Light 1 (GIL1) inhibits the negative gravitropism of hypocotyls in red and far-red light, but the underlying molecular mechanisms remain elusive. Our study found that GIL1 is a plasma membrane-localized protein. In endodermal cells of the upper part of hypocotyls, GIL1 controls the negative gravitropism of hypocotyls. GIL1 directly interacts with PIN3 and inhibits the auxin transport activity of PIN3. Mutation of PIN3 suppresses the abnormal gravitropic response of gil1 mutant. The GIL1 protein is unstable in darkness but it is stabilized by red and far-red light. Together, our data suggest that light-stabilized GIL1 inhibits the negative gravitropism of hypocotyls by suppressing the activity of the auxin transporter PIN3, thereby enhancing the emergence of young seedlings from the soil.

WAV E3 ubiquitin ligases mediate degradation of IAA32/34 in the TMK1-mediated auxin signaling pathway during apical hook development

Auxin regulates plant growth and development through downstream signaling pathways, including the best-known SCFTIR1/AFB-Aux/IAA-ARF pathway and several other less characterized "noncanonical" pathways. Recently, one SCFTIR1/AFB-independent noncanonical pathway, mediated by Transmembrane Kinase 1 (TMK1), was discovered through the analyses of its functions in Arabidopsis apical hook development. Asymmetric accumulation of auxin on the concave side of the apical hook triggers DAR1-catalyzed release of the C-terminal of TMK1, which migrates into the nucleus, where it phosphorylates and stabilizes IAA32/34 to inhibit cell elongation, which is essential for full apical hook formation. However, the molecular factors mediating IAA32/34 degradation have not been identified. Here, we show that proteins in the CYTOKININ INDUCED ROOT WAVING 1 (CKRW1)/WAVY GROWTH 3 (WAV3) subfamily act as E3 ubiquitin ligases to target IAA32/34 for ubiquitination and degradation, which is inhibited by TMK1c-mediated phosphorylation. This antagonistic interaction between TMK1c and CKRW1/WAV3 subfamily E3 ubiquitin ligases regulates IAA32/34 levels to control differential cell elongation along opposite sides of the apical hook.

Two orthogonal differentiation gradients locally coordinate fruit morphogenesis

Morphogenesis requires the coordination of cellular behaviors along developmental axes. In plants, gradients of growth and differentiation are typically established along a single longitudinal primordium axis to control global organ shape. Yet, it remains unclear how these gradients are locally adjusted to regulate the formation of complex organs that consist of diverse tissue types. Here we combine quantitative live imaging at cellular resolution with genetics, and chemical treatments to understand the formation of Arabidopsis thaliana female reproductive organ (gynoecium). We show that, contrary to other aerial organs, gynoecium shape is determined by two orthogonal, time-shifted differentiation gradients. An early mediolateral gradient controls valve morphogenesis while a late, longitudinal gradient regulates style differentiation. Local, tissue-dependent action of these gradients serves to fine-tune the common developmental program governing organ morphogenesis to ensure the specialized function of the gynoecium.

The emerging roles of clathrin-mediated endocytosis in plant development and stress responses

Clathrin-mediated endocytosis (CME) is a highly conserved pathway that plays a crucial role in the endocytosis of plasma membrane proteins in eukaryotic cells. The pathway is initiated when the adaptor protein complex 2 (AP2) and TPLATE complex (TPC) work together to recognize cargo proteins and recruit clathrin. This review provides a concise overview of the functions of each subunit of AP2 and TPC, and highlights the involvement of CME in various biological processes, such as pollen development, root development, nutrient transport, extracellular signal transduction, auxin polar transport, hyperosmotic stress, salinity stress, high ammonium stress, and disease resistance. Additionally, the review explores the regulation of CME by phytohormones, clathrin-mediated exocytosis (CMX), and AP2M phosphorylation. It also suggests potential future research directions for CME.

The mechanism underlying asymmetric bending of lateral petals in Delphinium (Ranunculaceae)

During the process of flower opening, most petals move downward in the direction of the pedicel (i.e., epinastic movement). In most Delphinium flowers, however, their two lateral petals display a very peculiar movement, the mirrored helical rotation, which requires the twist of the petal stalk. However, in some lineages, their lateral petals also exhibit asymmetric bending that increases the degree of mirrored helical rotation, facilitating the formation of a 3D final shape. Notably, petal asymmetric bending is a novel trait that has not been noticed yet, so its morphological nature, developmental process, and molecular mechanisms remain largely unknown. Here, by using D. anthriscifolium as a model, we determined that petal asymmetric bending was caused by the localized expansion of cell width, accompanied by the specialized array of cell wall nano-structure, on the adaxial epidermis. Digital gene analyses, gene expression, and functional studies revealed that a class I homeodomain-leucine zipper family transcription factor gene, DeanLATE MERISTEM IDENTITY1 (DeanLMI1), contributes to petal asymmetric bending; knockdown of it led to the formation of explanate 2D petals. Specifically, DeanLMI1 promotes cell expansion in width and influences the arrangement of cell wall nano-structure on the localized adaxial epidermis. These results not only provide a comprehensive portrait of petal asymmetric bending for the first time but also shed some new insights into the mechanisms of flower opening and helical movement in plants.

Exploring fine tuning between phytohormones and ROS signaling cascade in regulation of seed dormancy, germination and seedling development

In higher plants, seed is a propagule which ensures dissemination and survival of species. Developmental phases of a seed comprise embryogenesis, maturation and germination paving a way to its final fate i.e. seedling establishment. The final stage of seed maturation is marked by dehydration, acquisition of dessication tolerance and induction of dormancy. A precise Abscisic acid (ABA) to Gibberellins (GA) ratio, accumulation of miRNA 156, low level of reactive oxygen species (ROS) and enzyme inactivity govern seed dormancy. This also prevent pre harvest sprouting of the seeds. Overtime, stored seed mRNAs and proteins are degraded through oxidation of specific nucleotides in response to ROS accumulation. This degradation alleviates seed dormancy and transforms a dormant seed into a germinating seed. At this stage, ABA catabolism and degradation accompanied by GA synthesis contribute to low ABA to GA ratio. GA as well as ROS acts downstream, to mobilize reserve food materials, rupture testa, enhance imbibition and protrude radicle. All these events mark seed germination. Further, seedling is established under the governance of auxin and light. ABA and GA are master regulators while auxin, cytokinins, ethylene, jasmonic acid, brassinosteroids act through interdependent pathways to tightly regulate seed dormancy, germination and seedling establishment. In this review, the role of phytohormones and ROS in accordance with environmental factors in governing seed dormancy, promoting seed germination and thus, establishing a seedling is discussed in detail.

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

KEY MESSAGE

The 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.

Chemical inhibition of Arabidopsis PIN-FORMED auxin transporters by the anti-inflammatory drug naproxen

The phytohormone auxin plays central roles in many growth and developmental processes in plants. Development of chemical tools targeting the auxin pathway is useful for both plant biology and agriculture. Here we reveal that naproxen, a synthetic compound with anti-inflammatory activity in humans, acts as an auxin transport inhibitor targeting PIN-FORMED (PIN) transporters in plants. Physiological experiments indicate that exogenous naproxen treatment affects pleiotropic auxin-regulated developmental processes. Additional cellular and biochemical evidence indicates that naproxen suppresses auxin transport, specifically PIN-mediated auxin efflux. Moreover, biochemical and structural analyses confirm that naproxen binds directly to PIN1 protein via the same binding cavity as the indole-3-acetic acid substrate. Thus, by combining cellular, biochemical, and structural approaches, this study clearly establishes that naproxen is a PIN inhibitor and elucidates the underlying mechanisms. Further use of this compound may advance our understanding of the molecular mechanisms of PIN-mediated auxin transport and expand our toolkit in auxin biology and agriculture.

Nyctinastic movement in legumes: Developmental mechanisms, factors and biological significance

In legumes, a common phenomenon known as nyctinastic movement is observed. This movement involves the horizontal expansion of leaves during the day and relative vertical closure at night. Nyctinastic movement is driven by the pulvinus, which consists of flexor and extensor motor cells. The turgor pressure difference between these two cell types generates a driving force for the bending and deformation of the pulvinus. This review focuses on the developmental mechanisms of the pulvinus, the factors affecting nyctinastic movement, and the biological significance of this phenomenon in legumes, thus providing a reference for further research on nyctinastic movement.

Amyloplast sedimentation repolarizes LAZYs to achieve gravity sensing in plants

Gravity controls directional growth of plants, and the classical starch-statolith hypothesis proposed more than a century ago postulates that amyloplast sedimentation in specialized cells initiates gravity sensing, but the molecular mechanism remains uncharacterized. The LAZY proteins are known as key regulators of gravitropism, and lazy mutants show striking gravitropic defects. Here, we report that gravistimulation by reorientation triggers mitogen-activated protein kinase (MAPK) signaling-mediated phosphorylation of Arabidopsis LAZY proteins basally polarized in root columella cells. Phosphorylation of LAZY increases its interaction with several translocons at the outer envelope membrane of chloroplasts (TOC) proteins on the surface of amyloplasts, facilitating enrichment of LAZY proteins on amyloplasts. Amyloplast sedimentation subsequently guides LAZY to relocate to the new lower side of the plasma membrane in columella cells, where LAZY induces asymmetrical auxin distribution and root differential growth. Together, this study provides a molecular interpretation for the starch-statolith hypothesis: the organelle-movement-triggered molecular polarity formation.

Genome-wide analysis of PIN genes in cultivated peanuts (Arachis hypogaea L.): identification, subcellular localization, evolution, and expression patterns

BACKGROUND

Auxin is an important hormone in plants and the PIN-FORMED (PIN) genes are essential to auxin distribution in growth and developmental processes of plants. Peanut is an influential cash crop, but research into PIN genes in peanuts remains limited.

RESULTS

In this study, 16 PIN genes were identified in the genome of cultivated peanut, resolving into four subfamilies. All PIN genes were predicted to be located in the plasma membrane and a subcellular location experiment confirmed this prediction for eight of them. The gene structure, cis-elements in the promoter, and evolutionary relationships were elucidated, facilitating our understanding of peanut PINs and their evolution. In addition, the expression patterns of these PINs in various tissues were analyzed according to a previously published transcriptome dataset and qRT-PCR, which gave us a clear understanding of the temporal and spatial expression of PIN genes in different growth stages and different tissues. The expression trend of homologous genes was similar. AhPIN2A and AhPIN2B exhibited predominant expression in roots. AhPIN1A-1 and AhPIN1B-1 displayed significant upregulation following peg penetration, suggesting a potential close association with peanut pod development. Furthermore, we presented the gene network and gene ontology enrichment of these PINs. Notably, AhABCB19 exhibited a co-expression relationship with AhPIN1A and AhPIN1B-1, with all three genes displaying higher expression levels in peanut pegs and pods. These findings reinforce their potential role in peanut pod development.

CONCLUSIONS

This study details a comprehensive analysis of PIN genes in cultivated peanuts and lays the foundation for subsequent studies of peanut gene function and phenotype.

Jasmonates regulate apical hook development by repressing brassinosteroid biosynthesis and signaling

An apical hook is a special structure formed during skotomorphogenesis in dicotyledonous plant species. It is critical for protecting the shoot apical meristem from mechanical damage during seed germination and hypocotyl elongation in soil. Brassinosteroid (BR) and jasmonate (JA) phytohormones antagonistically regulate apical hook formation. However, the interrelationship between BRs and JAs in this process has not been well elucidated. Here, we reveal that JAs repress BRs to regulate apical hook development in Arabidopsis (Arabidopsis thaliana). Exogenous application of methyl jasmonate (MeJA) repressed the expression of the rate-limiting BR biosynthetic gene DWARF4 (DWF4) in a process relying on 3 key JA-dependent transcription factors, MYC2, MYC3, and MYC4. We demonstrated that MYC2 interacts with the critical BR-activated transcription factor BRASSINAZOLE RESISTANT 1 (BZR1), disrupting the association of BZR1 with its partner transcription factors, such as those of the PHYTOCHROME INTERACTING FACTOR (PIF) family and downregulating the expression of their target genes, such as WAVY ROOT GROWTH 2 (WAG2), encoding a protein kinase essential for apical hook development. Our results indicate that JAs not only repress the expression of BR biosynthetic gene DWF4 but, more importantly, attenuate BR signaling by inhibiting the transcriptional activation of BZR1 by MYC2 during apical hook development.

Antigravitropic PIN polarization maintains non-vertical growth in lateral roots

Lateral roots are typically maintained at non-vertical angles with respect to gravity. These gravitropic setpoint angles are intriguing because their maintenance requires that roots are able to effect growth response both with and against the gravity vector, a phenomenon previously attributed to gravitropism acting against an antigravitropic offset mechanism. Here we show how the components mediating gravitropism in the vertical primary root-PINs and phosphatases acting upon them-are reconfigured in their regulation such that lateral root growth at a range of angles can be maintained. We show that the ability of Arabidopsis lateral roots to bend both downward and upward requires the generation of auxin asymmetries and is driven by angle-dependent variation in downward gravitropic auxin flux acting against angle-independent upward, antigravitropic flux. Further, we demonstrate a symmetry in auxin distribution in lateral roots at gravitropic setpoint angle that can be traced back to a net, balanced polarization of PIN3 and PIN7 auxin transporters in the columella. These auxin fluxes are shifted by altering PIN protein phosphoregulation in the columella, either by introducing PIN3 phosphovariant versions or via manipulation of levels of the phosphatase subunit PP2A/RCN1. Finally, we show that auxin, in addition to driving lateral root directional growth, acts within the lateral root columella to induce more vertical growth by increasing RCN1 levels, causing a downward shift in PIN3 localization, thereby diminishing the magnitude of the upward, antigravitropic auxin flux.

Loss-of-function of gynoecium-expressed phospholipase pPLAIIγ triggers maternal haploid induction in Arabidopsis

Production of in planta haploid embryos that inherit chromosomes from only one parent can greatly increase breeding efficiency via quickly generating homozygous plants, called doubled haploid. One of the main players of in planta haploid induction is a pollen-specific phospholipase A, which is able, when mutated, to induce in vivo haploid induction in numerous monocots. However, no functional orthologous gene has been identified in dicots plants. Here, we show that loss-of-function of gynoecium-expressed phospholipase AII (pPLAIIγ) triggers maternal haploid plants in Arabidopsis, at an average rate of 1.07%. Reciprocal crosses demonstrate that haploid plants are triggered from the female side and not from the pollen, and the haploid plants carry the maternal genome. Promoter activity of pPLAIIγ shows enriched expression in the funiculus of flower development stages 13 and 18, and pPLAIIγ fused to yellow fluorescent protein reveals a plasma-membrane localization Interestingly, the polar localized PIN1 at the basal plasma membrane of the funiculus was all internalized in pplaIIγ mutants, suggesting that altered PIN1 localization in female organ could play a role in maternal haploid induction.

Endoreplication controls cell size via mechanochemical signaling

During hypocotyl development, an asymmetric auxin gradient causes differential cell elongation, leading to tissue bending and apical hook formation. Recently, Ma et al. identified a molecular pathway that links auxin with endoreplication and cell size through cell wall integrity sensing, cell wall remodeling, and regulation of cell wall stiffness.

Dynamic GOLVEN-ROOT GROWTH FACTOR 1 INSENSITIVE signaling in the root cap mediates root gravitropism

Throughout the exploration of the soil, roots interact with their environment and adapt to different conditions. Directional root growth is guided by asymmetric molecular patterns but how these become established or are dynamically regulated is poorly understood. Asymmetric gradients of the phytohormone auxin are established during root gravitropism, mainly through directional transport mediated by polarized auxin transporters. Upon gravistimulation, PIN-FORMED2 (PIN2) is differentially distributed and accumulates at the lower root side to facilitate asymmetric auxin transport up to the elongation zone where it inhibits cell elongation. GOLVEN (GLV) peptides function in gravitropism by affecting PIN2 abundance in epidermal cells. In addition, GLV signaling through ROOT GROWTH FACTOR 1 INSENSITIVE (RGI) receptors regulates root apical meristem maintenance. Here, we show that GLV-RGI signaling in these 2 processes in Arabidopsis (Arabidopsis thaliana) can be mapped to different cells in the root tip and that, in the case of gravitropism, it operates mainly in the lateral root cap (LRC) to maintain PIN2 levels at the plasma membrane (PM). Furthermore, we found that GLV signaling upregulates the phosphorylation level of PIN2 in an RGI-dependent manner. In addition, we demonstrated that the RGI5 receptor is asymmetrically distributed in the LRC and accumulates in the lower side of the LRC after gravistimulation. Asymmetric GLV-RGI signaling in the root cap likely accounts for differential PIN2 abundance at the PM to temporarily support auxin transport up to the elongation zone, thereby representing an additional level of control on the asymmetrical auxin flux to mediate differential growth of the root.

Save your TIRs - more to auxin than meets the eye

Auxin has long been known as an important regulator of plant growth and development. Classical studies in auxin biology have uncovered a 'canonical' transcriptional auxin-signalling pathway involving the TRANSPORT INHIBITOR RESPONSE1/AUXIN SIGNALING F-BOX (TIR1/AFB) receptors. TIR1/AFB perception of auxin triggers the degradation of repressors and the derepression of auxin-responsive genes. Nevertheless, the canonical pathway cannot account for all aspects of auxin biology, such as physiological responses that are too rapid for transcriptional regulation. This Tansley insight will explore several 'non-canonical' pathways that have been described in recent years mediating fast auxin responses. We focus on the interplay between a nontranscriptional branch of TIR1/AFB signalling and a TRANSMEMBRANE KINASE1 (TMK1)-mediated pathway in root acid growth. Other developmental aspects involving the TMKs and their association with the controversial AUXIN-BINDING PROTEIN 1 (ABP1) will be discussed. Finally, we provide an updated overview of the ETTIN (ETT)-mediated pathway in contexts outside of gynoecium development.

Genome-wide identification and characterization of PIN-FORMED (PIN) and PIN-LIKES (PILS) gene family reveals their role in adventitious root development in tea nodal cutting (Camellia Sinensis)

Auxin affects all aspects of plant growth and development, including morphogenesis and adaptive responses. Auxin transmembrane transport is promoted by PIN formation (PIN) and a structurally similar PIN-like (PILS) gene family, which jointly controls the directional transport of the auxin between plant cells, and the accumulation of intracellular auxin. At present, there is no study investigating the roles of CslPIN and CslPILS gene family in root development in the tea plant (Camellia sinensis). In this study, 8 CslPIN and 10 CslPILS genes were identified in the tea plant, and their evolutionary relationships, physical and chemical properties, conserved motifs, cis-acting elements, chromosome location, collinearity, and expression characteristics were analyzed. The mechanism of CslPIN and CslPILS in the formation of tea adventitious roots (ARs) was studied by the AR induction system. Through functional verification, the regulation of CslPIN3 gene on root growth and development of tea plant was studied by over-expression of CslPIN3 in Arabidopsis thaliana and in situ hybridization in Camellia sinensis. The results confirmed CslPIN3 was involved in the regulation of root growth and development as well as auxin accumulation. This study provides a better insight into the regulatory mechanism of CslPIN and CslPILS gene family on the formation of AR in tea plant.

To curve for survival: Apical hook development

Apical hook is a simple curved structure formed at the upper part of hypocotyls when dicot seeds germinate in darkness. The hook structure is transient but essential for seedlings' survival during soil emergence due to its efficient protection of the delicate shoot apex from mechanical injury. As a superb model system for studying plant differential growth, apical hook has fascinated botanists as early as the Darwin age, and significant advances have been achieved at both the morphological and molecular levels to understand how apical hook development is regulated. Here, we will mainly summarize the research progress at these two levels. We will also briefly compare the growth dynamics between apical hook and hypocotyl gravitropic bending at early seed germination phase, with the aim to deduce a certain consensus on their connections. Finally, we will outline the remaining questions and future research perspectives for apical hook development.

Low-Speed Clinorotation of Brachypodium distachyon and Arabidopsis thaliana Seedlings Triggers Root Tip Curvatures That Are Reminiscent of Gravitropism

Clinostats are instruments that continuously rotate biological specimens along an axis, thereby averaging their orientation relative to gravity over time. Our previous experiments indicated that low-speed clinorotation may itself trigger directional root tip curvature. In this project, we have investigated the root curvature response to low-speed clinorotation using Arabidopsis thaliana and Brachypodium distachyon seedlings as models. We show that low-speed clinorotation triggers root tip curvature in which direction is dictated by gravitropism during the first half-turn of clinorotation. We also show that the angle of root tip curvature is modulated by the speed of clinorotation. Arabidopsis mutations affecting gravity susception (pgm) or gravity signal transduction (arg1, toc132) are shown to affect the root tip curvature response to low-speed clinorotation. Furthermore, low-speed vertical clinorotation triggers relocalization of the PIN3 auxin efflux facilitator to the lateral membrane of Arabidopsis root cap statocytes, and creates a lateral gradient of auxin across the root tip. Together, these observations support a role for gravitropism in modulating root curvature responses to clinorotation. Interestingly, distinct Brachypodium distachyon accessions display different abilities to develop root tip curvature responses to low-speed vertical clinorotation, suggesting the possibility of using genome-wide association studies to further investigate this process.

Local light signaling at the leaf tip drives remote differential petiole growth through auxin-gibberellin dynamics

Although plants are immobile, many of their organs are flexible to move in response to environmental cues. In dense vegetation, plants detect neighbors through far-red light perception with their leaf tip. They respond remotely, with asymmetrical growth between the abaxial and adaxial sides of the leafstalk, the petiole. This results in upward movement that brings the leaf blades into better lit zones of the canopy. The plant hormone auxin is required for this response, but it is not understood how non-differential leaf tip-derived auxin can remotely regulate movement. Here, we show that remote signaling of far-red light promotes auxin accumulation in the abaxial petiole. This local auxin accumulation is facilitated by reinforcing an intrinsic directionality of the auxin transport protein PIN3 on the petiole endodermis, as visualized with a PIN3-GFP line. Using an auxin biosensor, we show that auxin accumulates in all cell layers from endodermis to epidermis in the abaxial petiole, upon far-red light signaling in the remote leaf tip. In the petiole, auxin elicits a response to both auxin itself as well as a second growth promoter; gibberellin. We show that this dual regulation is necessary for hyponastic leaf movement in response to light. Our data indicate that gibberellin is required to permit cell growth, whereas differential auxin accumulation determines which cells can grow. Our results reveal how plants can spatially relay information about neighbor proximity from their sensory leaf tips to the petiole base, thus driving adaptive growth.

Multiple mechanisms behind plant bending

To survive, plants constantly adapt their body shape to their environment. This often involves remarkably rapid bending of their organs such as stems, leaves and roots. Since plant cells are enclosed by stiff cell walls, they use various strategies for bending their organs, which differ from bending mechanisms of soft animal tissues and involve larger physical forces. Here we attempt to summarize and link different viewpoints on bending mechanisms: genes and signalling, mathematical modelling and biomechanics. We argue that quantifying cell growth and physical forces could open a new level in our understanding of bending and resolve some of its paradoxes.

Endoreplication mediates cell size control via mechanochemical signaling from cell wall

Endoreplication is an evolutionarily conserved mechanism for increasing nuclear DNA content (ploidy). Ploidy frequently scales with final cell and organ size, suggesting a key role for endoreplication in these processes. However, exceptions exist, and, consequently, the endoreplication-size nexus remains enigmatic. Here, we show that prolonged tissue folding at the apical hook in Arabidopsis requires endoreplication asymmetry under the control of an auxin gradient. We identify a molecular pathway linking endoreplication levels to cell size through cell wall remodeling and stiffness modulation. We find that endoreplication is not only permissive for growth: Endoreplication reduction enhances wall stiffening, actively reducing cell size. The cell wall integrity kinase THESEUS plays a key role in this feedback loop. Our data thus explain the nonlinearity between ploidy levels and size while also providing a molecular mechanism linking mechanochemical signaling with endoreplication-mediated dynamic control of cell growth.

Karrikin signaling regulates hypocotyl shade avoidance response by modulating auxin homeostasis in Arabidopsis

Shade affects all aspects of plant growth and development, including seed germination, hypocotyl elongation, petiole growth, leaf hyponasty, and flowering time. Here, we found that mutations in the key Arabidopsis karrikins signal perception-associated KARRIKIN INSENSITIVE 2 (KAI2) gene, encoding an α/β-fold hydrolase, and the MORE AXILLARY GROWTH 2 (MAX2) gene, encoding an F-box protein, led to greater hypocotyl elongation under shade avoidance conditions. We further verified that these phenotypes were caused by perception of the endogenous KAI2-ligands (KLs), and that this phenotype is independent of strigolactone biosynthetic or signaling pathways. Upon perception of a KL, it is probable that the target protein forms a complex with the KAI2/MAX2 proteins, which are degraded through the action of the 26S proteasome. We demonstrated that SUPPRESSOR OF MAX2-1 (SMAX1) is the degradation target for the KAI2/MAX2 complex in the context of shade avoidance. KAI2 and MAX2 require SMAX1 to limit the hypocotyl growth associated with shade avoidance. Treatment with l-kynurenine, an inhibitor of auxin accumulation, partially restored elongation of kai2 mutant hypocotyls under simulated shade. Furthermore, KAI2 is involved in regulating auxin accumulation and polar auxin transport, which may contribute to the hypocotyl shade response. In addition, SMAX1 gene overexpression promoted the hypocotyl shade response. RNA-sequencing analysis revealed that SMAX1-overexpression affected the expression of many auxin homeostasis genes, especially under simulated shade. Altogether, our data support the conclusion that KL signaling regulates shade avoidance by modulating auxin homeostasis in the hypocotyl.

Modulation of receptor-like transmembrane kinase 1 nuclear localization by DA1 peptidases in Arabidopsis

Signals are often perceived by proteins in one cellular location and transduced to other locations such as the nucleus. Signaling proteins can be cleaved by peptidases to facilitate this movement, but the peptidases involved in this are poorly understood despite their widespread role. We describe a role for the ubiquitin-activated peptidase DA1 in cleaving the membrane-localized receptor-like kinase transmembrane kinase 1 (TMK1) in Arabidopsis. TMK1 is phosphorylated in response to auxin and mediates several auxin responses including growth induction by cell expansion. DA1-mediated cleavage of TMK1 facilitates nuclear localization of its intracellular kinase domain to repress auxin-mediated gene expression, facilitating differential cell expansion during growth. These analyses establish a wider role for DA1 family activities in cell growth.

The cleavage of intracellular domains of receptor-like kinases (RLKs) has an important functional role in the transduction of signals from the cell surface to the nucleus in many organisms. However, the peptidases that catalyze protein cleavage during signal transduction remain poorly understood despite their crucial roles in diverse signaling processes. Here, we report in the flowering plant Arabidopsis thaliana that members of the DA1 family of ubiquitin-regulated Zn metallopeptidases cleave the cytoplasmic kinase domain of transmembrane kinase 1 (TMK1), releasing it for nuclear localization where it represses auxin-responsive cell growth during apical hook formation by phosphorylation and stabilization of the transcriptional repressors IAA32 and IAA34. Mutations in DA1 family members exhibited reduced apical hook formation, and DA1 family-mediated cleavage of TMK1 was promoted by auxin treatment. Expression of the DA1 family-generated intracellular kinase domain of TMK1 by an auxin-responsive promoter fully restored apical hook formation in a tmk1 mutant, establishing the function of DA1 family peptidase activities in TMK1-mediated differential cell growth and apical hook formation. DA1 family peptidase activity therefore modulates TMK1 kinase activity between a membrane location where it stimulates acid cell growth and initiates an auxin-dependent kinase cascade controlling cell proliferation in lateral roots and a nuclear localization where it represses auxin-mediated gene expression and growth.

Euryale Small Auxin Up RNA62 promotes cell elongation and seed size by altering the distribution of indole-3-acetic acid under the light

Euryale (Euryale ferox Salisb.) is an aquatic crop used as both food and drug in Asia, but its utilization is seriously limited due to low yield. Previously, we hypothesized that Euryale small auxin up RNAs (EuSAURs) regulate seed size, but the underlying biological functions and molecular mechanisms remain unclear. Here, we observed that the hybrid Euryale lines (HL) generate larger seeds with higher indole-3-acetic acid (IAA) concentrations than those in the North Gordon Euryale (WT). Histological analysis suggested that a larger ovary in HL is attributed to longer cells around. Overexpression of EuSAUR62 in rice (Oryza sativa L.) resulted in larger glumes and grains and increased the length of glume cells. Immunofluorescence and protein interaction assays revealed that EuSAUR62 modulates IAA accumulation around the rice ovary by interacting with the rice PIN-FORMED 9, an auxin efflux carrier protein. Euryale basic region/leucine zipper 55 (EubZIP55), which was highly expressed in HL, directly binds to the EuSAUR62 promoter and activated the expression of EuSAUR62. Constant light increased the expression of both EubZIP55 and EuSAUR62 with auxin-mediated hook curvature in HL seedlings. Overall, we proposed that EuSAUR62 is a molecular bridge between light and IAA and plays a crucial role in regulating the size of the Euryale seed.

Initiation of aboveground organ primordia depends on combined action of auxin, ERECTA family genes, and PINOID

Leaves and flowers are produced by the shoot apical meristem (SAM) at a certain distance from its center, a process that requires the hormone auxin. The amount of auxin and the pattern of its distribution in the initiation zone determine the size and spatial arrangement of organ primordia. Auxin gradients in the SAM are formed by PIN-FORMED (PIN) auxin efflux carriers whose polar localization in the plasma membrane depends on the protein kinase PINOID (PID). Previous work determined that ERECTA (ER) family genes (ERfs) control initiation of leaves. ERfs are plasma membrane receptors that enable cell-to-cell communication by sensing extracellular small proteins from the EPIDERMAL PATTERNING FACTOR/EPF-LIKE (EPF/EPFL) family. Here, we investigated whether ERfs regulate initiation of organs by altering auxin distribution or signaling in Arabidopsis (Arabidopsis thaliana). Genetic and pharmacological data suggested that ERfs do not regulate organogenesis through PINs while transcriptomics data showed that ERfs do not alter primary transcriptional responses to auxin. Our results indicated that in the absence of ERf signaling the peripheral zone cells inefficiently initiate leaves in response to auxin signals and that increased accumulation of auxin in the er erecta-like1 (erl1) erl2 SAM can partially rescue organ initiation defects. We propose that both auxin and ERfs are essential for leaf initiation and that they have common downstream targets. Genetic data also indicated that the role of PID in initiation of cotyledons and leaves cannot be attributed solely to regulation of PIN polarity and PID is likely to have other functions in addition to regulation of auxin distribution.

On the trail of auxin: Reporters and sensors

The phytohormone auxin is a master regulator of plant growth and development in response to many endogenous and environmental signals. The underlying coordination of growth is mediated by the formation of auxin maxima and concentration gradients. The visualization of auxin dynamics and distribution can therefore provide essential information to increase our understanding of the mechanisms by which auxin orchestrates these growth and developmental processes. Several auxin reporters have been developed to better perceive the auxin distribution and signaling machinery in vivo. This review focuses on different types of auxin reporters and biosensors used to monitor auxin distribution and its dynamics, as well as auxin signaling, at the cellular and tissue levels in different plant species. We provide a brief history of each reporter and biosensor group and explain their principles and utilities.

New Wine in an Old Bottle: Utilizing Chemical Genetics to Dissect Apical Hook Development

The apical hook is formed by dicot seedlings to protect the tender shoot apical meristem during soil emergence. Regulated by many phytohormones, the apical hook has been taken as a model to study the crosstalk between individual signaling pathways. Over recent decades, the roles of different phytohormones and environmental signals in apical hook development have been illustrated. However, key regulators downstream of canonical hormone signaling have rarely been identified via classical genetics screening, possibly due to genetic redundancy and/or lethal mutation. Chemical genetics that utilize small molecules to perturb and elucidate biological processes could provide a complementary strategy to overcome the limitations in classical genetics. In this review, we summarize current progress in hormonal regulation of the apical hook, and previously reported chemical tools that could assist the understanding of this complex developmental process. We also provide insight into novel strategies for chemical screening and target identification, which could possibly lead to discoveries of new regulatory components in apical hook development, or unidentified signaling crosstalk that is overlooked by classical genetics screening.

ROOT PENETRATION INDEX 3, a major quantitative trait locus associated with root system penetrability in Arabidopsis

Soil mechanical impedance precludes root penetration, confining root system development to shallow soil horizons where mobile nutrients are scarce. Using a two-phase-agar system, we characterized Arabidopsis responses to low and high mechanical impedance at three root penetration stages. We found that seedlings whose roots fail to penetrate agar barriers show a significant reduction in leaf area, root length, and elongation zone and an increment in root diameter, while those capable of penetrating show only minor morphological effects. Analyses using different auxin-responsive reporter lines, exogenous auxins, and inhibitor treatments suggest that auxin responsiveness and PIN-mediated auxin distribution play an important role in regulating root responses to mechanical impedance. The assessment of 21 Arabidopsis accessions revealed that primary root penetrability varies widely among accessions. To search for quantitative trait loci (QTLs) associated to root system penetrability, we evaluated a recombinant inbred population derived from Landsberg erecta (Ler-0, with a high primary root penetrability) and Shahdara (Sha, with a low primary root penetrability) accessions. QTL analysis revealed a major-effect QTL localized in chromosome 3, ROOT PENETRATION INDEX 3 (q-RPI3), which accounted for 29.98% (logarithm of odds=8.82) of the total phenotypic variation. Employing an introgression line (IL-321) with a homozygous q-RPI3 region from Sha in the Ler-0 genetic background, we demonstrated that q-RPI3 plays a crucial role in root penetrability. This multiscale study reveals new insights into root plasticity during the penetration process in hard agar layers, natural variation, and genetic architecture behind primary root penetrability in Arabidopsis.

A feedback regulation between ARF7-mediated auxin signaling and auxin homeostasis involving MES17 affects plant gravitropism

Gravitropism is an essential adaptive response of land plants. Asymmetric auxin gradients across plant organs, interpreted by multiple auxin signaling components including AUXIN RESPONSE FACTOR7 (ARF7), trigger differential growth and bending response. However, how this fundamental process is strictly maintained in nature remains unclear. Here, we report that gravity stimulates the transcription of METHYL ESTERASE17 (MES17) along the lower side of the hypocotyl via ARF7-dependent auxin signaling. The asymmetric distribution of MES17, a methyltransferase that converts auxin from its inactive form methyl indole-3-acetic acid ester (MeIAA) to its biologically active form free-IAA, enhanced the gradient of active auxin across the hypocotyl, which in turn reversely amplified the asymmetric auxin responses and differential growth that shape gravitropic bending. Taken together, our findings reveal the novel role of MES17-mediated auxin homeostasis in gravitropic responses and identify an ARF7-triggered feedback mechanism that reinforces the asymmetric distribution of active auxin and strictly controls gravitropism in plants.

Jasmonic acid coordinates with light, glucose and auxin signalling in regulating branching angle of Arabidopsis lateral roots

The role of jasmonates (JAs) in primary root growth and development and in plant response to external stimuli is already known. However, its role in lateral root (LR) development remains to be explored. Our work identified methyl jasmonate (MeJA) as a key phytohormone in determining the branching angle of Arabidopsis LRs. MeJA inclines the LRs to a more vertical orientation, which was dependent on the canonical JAR1-COI1-MYC2,3,4 signalling. Our work also highlights the dual roles of light in governing LR angle. Light signalling enhances JA biosynthesis, leading to erect root architecture; whereas, glucose (Glc) induces wider branching angles. Combining physiological and molecular assays, we revealed that Glc antagonises the MeJA response via TARGET OF RAPAMYCIN (TOR) signalling. Moreover, physiological assays using auxin mutants, MYC2-mediated transcriptional activation of LAZY2, LAZY4 and auxin biosynthetic gene CYP79B2, and asymmetric distribution of DR5::GFP and PIN2::GFP pinpointed the role of an intact auxin machinery required by MeJA for vertical growth of LRs. We also demonstrated that light perception and signalling are indispensable for inducing vertical angles by MeJA. Thus, our investigation highlights antagonism between light and Glc signalling and how they interact with JA-auxin signals to optimise the branching angle of LRs.

Symplastic communication in the root cap directs auxin distribution to modulate root development

Root cap not only protects root meristem, but also detects and transduces the signals of environmental changes to affect root development. The symplastic communication is an important way for plants to transduce signals to coordinate the development and physiology in response to the changing enviroments. However, it is unclear how the symplastic communication between root cap cells affects root growth. Here we exploit an inducible system to specifically block the symplastic communication in the root cap. Transient blockage of plasmodesmata (PD) in differentiated collumella cells severely impairs the root development in Arabidopsis, in particular in the stem cell niche and the proximal meristem. The neighboring stem cell niche is the region that is most sensitive to the disrupted symplastic communication and responds rapidly via the alteration of auxin distribution. In the later stage, the cell division in proximal meristem is inhibited, presumably due to the reduced auxin level in the root cap. Our results reveal the essential role of the differentiated collumella cells in the root cap mediated signaling system that directs root development.

PIN3 positively regulates the late initiation of ovule primordia in Arabidopsis thaliana

Ovule initiation determines the maximum ovule number and has great impact on seed number and yield. However, the regulation of ovule initiation remains largely elusive. We previously reported that most of the ovule primordia initiate asynchronously at floral stage 9 and PINFORMED1 (PIN1) polarization and auxin distribution contributed to this process. Here, we further demonstrate that a small amount of ovule primordia initiate at floral stage 10 when the existing ovules initiated at floral stage 9 start to differentiate. Genetic analysis revealed that the absence of PIN3 function leads to the reduction in pistil size and the lack of late-initiated ovules, suggesting PIN3 promotes the late ovule initiation process and pistil growth. Physiological analysis illustrated that, unlike picloram, exogenous application of NAA can’t restore these defective phenotypes, implying that PIN3-mediated polar auxin transport is required for the late ovule initiation and pistil length. qRT-PCR results indicated that the expression of SEEDSTICK (STK) is up-regulated under auxin analogues treatment while is down-regulated in pin3 mutants. Meanwhile, overexpressing STK rescues pin3 phenotypes, suggesting STK participates in PIN3-mediated late ovule initiation possibly by promoting pistil growth. Furthermore, brassinosteroid influences the late ovule initiation through positively regulating PIN3 expression. Collectively, this study demonstrates that PIN3 promotes the late ovule initiation and contributes to the extra ovule number. Our results give important clues for increasing seed number and yield of cruciferous and leguminous crops.

The volatile cedrene from Trichoderma guizhouense modulates Arabidopsis root development through auxin transport and signalling

Rhizosphere microorganisms interact with plant roots by producing chemical signals that regulate root development. However, the distinct bioactive compounds and signal transduction pathways remain to be identified. Here, we showed that sesquiterpenes are the main volatile compounds produced by plant-beneficial Trichoderma guizhouense NJAU4742. Inhibition of sesquiterpene biosynthesis eliminated the promoting effect of this strain on root growth, indicating its involvement in plant-fungus cross-kingdom signalling. Sesquiterpene component analysis identified cedrene, a highly abundant sesquiterpene in strain NJAU4742, to stimulate plant growth and root development. Genetic analysis and auxin transport inhibition showed that the TIR1 and AFB2 auxin receptors, IAA14 auxin-responsive protein, and ARF7 and ARF19 transcription factors affected the response of lateral roots to cedrene. Moreover, the AUX1 auxin influx carrier and PIN2 efflux carrier were also found to be indispensable for cedrene-induced lateral root formation. Confocal imaging showed that cedrene affected the expression of pPIN2:PIN2:GFP and pPIN3:PIN3:GFP, which might be related to the effect of cedrene on root morphology. These results suggested that a novel sesquiterpene molecule from plant-beneficial T. guizhouense regulates plant root development through the transport and signalling of auxin.