Auxin in Root Development

The whole-genome dissection of root system architecture provides new insights for the genetic improvement of alfalfa (Medicago sativa L.)

Appropriate root system architecture (RSA) can improve alfalfa yield, yet its genetic basis remains largely unexplored. This study evaluated six RSA traits in 171 alfalfa genotypes grown under controlled greenhouse conditions. We also analyzed five yield-related traits in normal and drought stress environments and found a significant correlation (0.50) between root dry weight (RDW) and alfalfa dry weight under normal conditions (N_DW). A genome-wide association study (GWAS) was performed using 1 303 374 single-nucleotide polymorphisms (SNPs) to explore the relationships between RSA traits. Sixty significant SNPs (-log 10 (P) ≥ 5) were identified, with genes within the 50 kb upstream and downstream ranges primarily enriched in GO terms related to root development, hormone synthesis, and signaling, as well as morphological development. Further analysis identified 19 high-confidence candidate genes, including AUXIN RESPONSE FACTORs (ARFs), LATERAL ORGAN BOUNDARIES-DOMAIN (LBD), and WUSCHEL-RELATED HOMEOBOX (WOX). We verified that the forage dry weight under both normal and drought conditions exhibited significant differences among materials with different numbers of favorable haplotypes. Alfalfa containing more favorable haplotypes exhibited higher forage yields, whereas favorable haplotypes were not subjected to human selection during alfalfa breeding. Genomic prediction (GP) utilized SNPs from GWAS and machine learning for each RSA trait, achieving prediction accuracies ranging from 0.70 for secondary root position (SRP) to 0.80 for root length (RL), indicating robust predictive capability across the assessed traits. These findings provide new insights into the genetic underpinnings of root development in alfalfa, potentially informing future breeding strategies aimed at improving yield.

ROS, an Important Plant Growth Regulator in Root Growth and Development: Functional Genes and Mechanism

Roots are fundamental to the growth, development, and survival of plants. Beyond anchoring the plant, roots absorb water and nutrients, supporting the plant's ability to grow and function normally. Root systems, originating from the apical meristem, exhibit significant diversity depending on the plant species. ROS are byproducts of aerobic metabolism, present in both above- and below-ground plant tissues. While ROS were once considered merely harmful byproducts of oxygen metabolism, they are now recognized as critical signaling molecules that regulate plant growth and development. Under stress conditions, plants produce elevated levels of ROS, which can inhibit growth. However, moderate ROS levels act as signals that integrate various regulatory pathways, contributing to normal plant development. However, there is still a lack of comprehensive and systematic research on how ROS precisely regulate root growth and development. This review provides an overview of ROS production pathways and their regulatory mechanisms in plants, with a particular focus on their influence on root development.

Phosphorylation of auxin signaling repressor IAA8 by heat-responsive MPKs causes defective flower development

Heat stress is a substantial and imminent threat to plant growth and development. Understanding its adverse effects on plant development at the molecular level is crucial for sustainable agriculture. However, the molecular mechanism underlying how heat stress causes developmental defects in flowers remains poorly understood. Here, we identified Indole-3-Acetic Acid 8 (IAA8), a repressor of auxin signaling, as a substrate of mitogen-activated protein kinases (MPKs) in Arabidopsis thaliana, and found that MPK-mediated phosphorylation of IAA8 inhibits flower development. MPKs phosphorylated three residues of IAA8: S74, T77, and S135. Interestingly, transgenic plants overexpressing a phospho-mimicking mutant of IAA8 (IAA8DDD OX) exhibited defective flower development due to high IAA8 levels. Furthermore, MPK-mediated phosphorylation inhibited IAA8 polyubiquitination, thereby significantly increasing its stability. Additionally, the expression of key transcription factors involved in flower development, such as bZIP and MYB genes, was significantly perturbed in the IAA8DDD OX plants. Collectively, our study demonstrates that heat stress inhibits flower development by perturbing the expression of flower development genes through the MPK-mediated phosphorylation of IAA8, suggesting that Aux/IAA phosphorylation enables plants to fine-tune their development in response to environmental stress.

The t-SNARE protein OsSYP132 is required for vesicle fusion and root morphogenesis in rice

Root morphogenesis is crucial for water and nutrient acquisition, but many aspects of root morphogenesis in crops are not well-understood. Here, we cloned and functionally characterized a key gene for root morphogenesis in rice (Oryza sativa) based on mutant analysis. The stop root morphogenesis 1 (srm1) mutant lacks crown roots (CRs) and lateral roots (LRs) and carries a point mutation in the t-SNARE coding gene SYNTAXIN OF PLANTS 132 (OsSYP132), leading to a premature stop codon and ablating the post-transmembrane (PTM) region of OsSYP132. We identified the functional SNARE complex OsSYP132-OsNPSN13-OsSYP71-OsVAMP721/722 and determined that the integrity of the PTM region of OsSYP132 is essential for OsSYP132-based SNARE complex-mediated fusion of OsVAMP721/722 vesicles with the plasma membrane. The loss of this region in srm1 disrupts the intercellular trafficking and plasma membrane localization of OsPIN1b, preventing proper auxin distribution in the primordia of CRs and LRs and inhibiting their outgrowth.

Decoding the Impact of a Bacterial Strain of Micrococcus luteus on Arabidopsis Growth and Stress Tolerance

Microbes produce various bioactive metabolites that can influence plant growth and stress tolerance. In this study, a plant growth-promoting rhizobacterium (PGPR), strain S14, was identified as Micrococcus luteus (designated as MlS14) using de novo whole-genome assembly. The MlS14 genome revealed major gene clusters for the synthesis of indole-3-acetic acid (IAA), terpenoids, and carotenoids. MlS14 produced significant amounts of IAA, and its volatile organic compounds (VOCs), specifically terpenoids, exhibited antifungal activity, suppressing the growth of pathogenic fungi. The presence of yellow pigment in the bacterial colony indicated carotenoid production. Treatment with MlS14 activated the expression of β-glucuronidase (GUS) driven by a promoter containing auxin-responsive elements. The application of MlS14 reshaped the root architecture of Arabidopsis seedlings, causing shorter primary roots, increased lateral root growth, and longer, denser root hairs; these characteristics are typically controlled by elevated exogenous IAA levels. MlS14 positively regulated seedling growth by enhancing photosynthesis, activating antioxidant enzymes, and promoting the production of secondary metabolites with reactive oxygen species (ROS) scavenging activity. Pretreatment with MlS14 reduced H2O2 and malondialdehyde (MDA) levels in seedlings under drought and heat stress, resulting in greater fresh weight during the post-stress period. Additionally, exposure to MlS14 stabilized chlorophyll content and growth rate in seedlings under salt stress. MlS14 transcriptionally upregulated genes involved in antioxidant defense and photosynthesis. Furthermore, genes linked to various hormone signaling pathways, such as abscisic acid (ABA), auxin, jasmonic acid (JA), and salicylic acid (SA), displayed increased expression levels, with those involved in ABA synthesis, using carotenoids as precursors, being the most highly induced. Furthermore, MlS14 treatment increased the expression of several transcription factors associated with stress responses, with DREB2A showing the highest level of induction. In conclusion, MlS14 played significant roles in promoting plant growth and stress tolerance. Metabolites such as IAA and carotenoids may function as positive regulators of plant metabolism and hormone signaling pathways essential for growth and adaptation to abiotic stress.

Small peptide SiDVL/RTFLs from foxtail millet inhibit root growth through repressing auxin signaling in transgenic Arabidopsis

KEY MESSAGE

SiDVLs inhibit auxin signaling to regulate root growth by enhancing the expression of Aux/IAAs and reducing the protein accumulation of PINs. The DEVIL/ ROTUNDIFOLIA (DVL/RTFL), a small polypeptide family, is conserved in seed plants and important in regulating plant growth and development. However, the molecular mechanisms remain largely unknown. Here, 27 SiDVLs were identified in foxtail millet genome. Overexpression of three SiDVLs in Arabidopsis (Arabidopsis thaliana) strongly repressed the plant growth, especially the root growth. We demonstrate that overexpression of SiDVLs enhances Auxin/Indole-3-Acetic Acids (Aux/IAAs) transcription, thereby weakening auxin signaling in the roots. Furthermore, SiDVLs reduced the protein levels of the auxin transporters PIN-formed 1 (PIN1), PIN2, and PIN7 in the roots. The impaired auxin signaling reduces the cell division and elongation. In conclusion, SiDVLs suppress cell division and elongation in root by inhibiting auxin signaling and transport, which lead to the reduced root growth.

Diverse geotropic responses in the orchid family

In epiphytes, aerial roots are important to combat water-deficient, nutrient-poor, and high-irradiance microhabitats. However, whether aerial roots can respond to gravity and whether auxin plays a role in regulating aerial root development remain open-ended questions. Here, we investigated the gravitropic response of the epiphytic orchid Phalaenopsis aphrodite. Our data showed that aerial roots of P. aphrodite failed to respond to gravity, and this was correlated with a lack of starch granules/statolith sedimentation in the roots and the absence of the auxin efflux carrier PIN2 gene. Using an established auxin reporter, we discovered that auxin maximum was absent in the quiescent center of aerial roots of P. aphrodite. Also, gravity failed to trigger auxin redistribution in the root caps. Hence, loss of gravity sensing and gravity-dependent auxin redistribution may be the genetic factors contributing to aerial root development. Moreover, the architectural and functional innovations that achieve fast gravitropism in the flowering plants appear to be lost in both terrestrial and epiphytic orchids, but are present in the early diverged orchid subfamilies. Taken together, our findings provide physiological and molecular evidence to support the notion that epiphytic orchids lack gravitropism and suggest diverse geotropic responses in the orchid family.

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.

Gallic acid regulates primary root elongation via modulating auxin transport and signal transduction

Gallic acid is an important secondary metabolite in plants, with great value in medicine, food, and chemical industry. However, whether and how this widely existing natural polyphenolic compound affects the growth and development of plants themselves remains elusive. In this study, we revealed that exogenous application of gallic acid has a dual effect on the elongation of primary root in Arabidopsis. While lower concentrations of gallic acid slightly stimulate primary root growth, excessive gallic acid profoundly reduces primary root length and root meristem size in a dose-dependent manner, probably via suppressing cell division in root meristem as indicated by CYCB1;1::GUS. Moreover, as suggested by the DR5::GFP line analysis and confirmed by the LC-MS assay, auxin contents in root tips were dramatically decreased upon excessive gallic acid treatment. Additional application of IAA partially rescued the shortened primary root and root meristem upon excessive gallic acid treatment, suggesting that auxin is required for excessive gallic acid-caused root growth inhibition. Then, we further revealed that excessive gallic acid down-regulated the expression of auxin transporters PIN1, PIN2, PIN3, and PIN7, and triple mutant pin1 pin3 pin7 exhibited a reduced sensitivity to gallic acid treatment. Meanwhile, excessive gallic acid decreased the degradation of AXR3/IAA17 protein as revealed by HS::AXR3NT-GUS reporter line. Auxin signaling mutant tir1 afb2 afb3 and axr3-3 were also less sensitive to excessive gallic acid treatment in terms of primary root length and root meristem size. Taken together, these findings suggested that excessive gallic acid inhibits primary root growth by modulating auxin transport and signaling in Arabidopsis.

The first intron of ARF7 is required for expression in root tips

Auxin regulates plant growth and development through the transcription factors of the AUXIN RESPONSE FACTOR (ARF) gene family. ARF7 is one of five activators that bind DNA and elicit downstream transcriptional responses. In roots, ARF7 regulates growth, gravitropism and redundantly with ARF19, lateral root organogenesis. In this study we analyzed ARF7 cis-regulation, using different non-coding sequences of the ARF7 locus to drive GFP. We show that constructs containing the first intron led to increased signal in the root tip. Although bioinformatics analyses predicted several transcription factor binding sites in the first intron, we were unable to significantly alter expression of GFP in the root by mutating these. We instead observed the intronic sequences needed to be present within the transcribed sequences to drive expression in the root meristem. These data support a mechanism by which intron-mediated enhancement regulates the tissue specific expression of ARF7 in the root meristem.

Bacillus subtilis promotes plant phosphorus (P) acquisition through P solubilization and stimulation of root and root hair growth

Bacteria can be applied as biofertilizers to improve crop growth in phosphorus (P)-limited conditions. However, their mode of action in a soil environment is still elusive. We used the strain ALC_02 as a case study to elucidate how Bacillus subtilis affects dwarf tomato cultivated in soil-filled rhizoboxes over time. ALC_02 improved plant P acquisition by increasing the size and P content of P-limited plants. We assessed three possible mechanisms, namely root growth stimulation, root hair elongation, and solubilization of soil P. ALC_02 produced auxin, and inoculation with ALC_02 promoted root growth. ALC_02 promoted root hair elongation as the earliest observed response and colonized root hairs specifically. Root and root hair growth stimulation was associated with a subsequent increase in plant P content, indicating that a better soil exploration by the root system improved plant P acquisition. Furthermore, ALC_02 affected the plant-available P content in sterilized soil differently over time and released P from native P pools in the soil. Collectively, ALC_02 exhibited all three mechanisms in a soil environment. To our knowledge, bacterial P biofertilizers have not been reported to colonize and elongate root hairs in the soil so far, and we propose that these traits contribute to the overall effect of ALC_02. The knowledge gained in this research can be applied in the future quest for bacterial P biofertilizers, where we recommend assessing all three parameters, not only root growth and P solubilization, but also root hair elongation. This will ultimately support the development of sustainable agricultural practices.

Hydrogen Peroxide Signaling in the Maintenance of Plant Root Apical Meristem Activity

Hydrogen peroxide (H2O2) is a prevalent reactive oxygen species (ROS) found in cells and takes a central role in plant development and stress adaptation. The root apical meristem (RAM) has evolved strong plasticity to adapt to complex and changing environmental conditions. Recent advances have made great progress in explaining the mechanism of key factors, such as auxin, WUSCHEL-RELATED HOMEOBOX 5 (WOX5), PLETHORA (PLT), SHORTROOT (SHR), and SCARECROW (SCR), in the regulation of RAM activity maintenance. H2O2 functions as an emerging signaling molecule to control the quiescent center (QC) specification and stem cell niche (SCN) activity. Auxin is a key signal for the regulation of RAM maintenance, which largely depends on the formation of auxin regional gradients. H2O2 regulates the auxin gradients by the modulation of intercellular transport. H2O2 also modulates the expression of WOX5, PLTs, SHR, and SCR to maintain RAM activity. The present review is dedicated to summarizing the key factors in the regulation of RAM activity and discussing the signaling transduction of H2O2 in the maintenance of RAM activity. H2O2 is a significant signal for plant development and environmental adaptation.

Impact of ultraviolet-B radiation on early-season morpho-physiological traits of indica and japonica rice genotypes

Ultraviolet (UV)-B radiation is considered one of the major detrimental rays coming from the Sun. UV-B radiation has a harmful impact on plant growth and development. The effect of UV-B radiation was studied on 64 rice (Oryza sativa L.) genotypes during the vegetative season. An equal number of genotypes from the japonica (50%) and indica (50%) subspecies were phenotyped using the Soil-Plant-Atmosphere-Research (SPAR) units. The 10 kJ UV-B was imposed 12 days after planting (DAP) and continued for three weeks (21 d). Based on the combined ultraviolet-B radiation response index (CUVBRI) for each genotype, the 64 rice genotypes were classified into sensitive, moderately sensitive, moderately tolerant, and tolerant. Various shoot traits, such as plant height, tiller, and leaf numbers, were measured. We also studied critical root phenological traits like root volume, diameter, tips, and forks. Out of all the studied shoot traits, leaf area showed maximum reduction for both indica (54%) and japonica (48%). Among the root traits, root length decreased by negligible (1%) for indica as compared to japonica (5%), while root crossing and forks showed a maximum decline for japonica (37 and 42%), respectively. This study is timely, meaningful, and required because it will help breeders select a tolerant or sensitive rice line for better yield and production under abiotic stresses.

Above and belowground phenotypic response to exogenous auxin across Arabidopsis thaliana mutants and natural accessions varies from seedling to reproductive maturity

Background

Plant hormones influence phenology, development, and function of above and belowground plant structures. In seedlings, auxin influences the initiation and development of lateral roots and root systems. How auxin-related genes influence root initiation at early life stages has been investigated from numerous perspectives. There is a gap in our understanding of how these genes influence root size through the life cycle and in mature plants. Across development, the influence of a particular gene on plant phenotypes is partly regulated by the addition of a poly-A tail to mRNA transcripts via alternative polyadenylation (APA). Auxin related genes have documented variation in APA, with auxin itself contributing to APA site switches. Studies of the influence of exogenous auxin on natural plant accessions and mutants of auxin pathway gene families exhibiting variation in APA are required for a more complete understanding of genotype by development by hormone interactions in whole plant and fitness traits.

Methods

We studied Arabidopsis thaliana homozygous mutant lines with inserts in auxin-related genes previously identified to exhibit variation in number of APA sites. Our growth chamber experiment included wildtype Col-0 controls, mutant lines, and natural accession phytometers. We applied exogenous auxin through the life cycle. We quantified belowground and aboveground phenotypes in 14 day old, 21 day old seedlings and plants at reproductive maturity. We contrasted root, rosette and flowering phenotypes across wildtype, auxin mutant, and natural accession lines, APA groups, hormone treatments, and life stages using general linear models.

Results

The root systems and rosettes of mutant lines in auxin related genes varied in response to auxin applications across life stages and varied between genotypes within life stages. In seedlings, exposure to auxin decreased size, but increased lateral root density, whereas at reproductive maturity, plants displayed greater aboveground mass and total root length. These differences may in part be due to a shift which delayed the reproductive stage when plants were treated with auxin. Root traits of auxin related mutants depended on the number of APA sites of mutant genes and the plant's developmental stage. Mutants with inserts in genes with many APA sites exhibited lower early seedling belowground biomass than those with few APA sites but only when exposed to exogenous auxin. As we observed different responses to exogenous auxin across the life cycle, we advocate for further studies of belowground traits and hormones at reproductive maturity. Studying phenotypic variation of genotypes across life stages and hormone environments will uncover additional shared patterns across traits, assisting efforts to potentially reach breeding targets and enhance our understanding of variation of genotypes in natural systems.

RNA Sequencing Analysis and Verification of Paeonia ostii 'Fengdan' CuZn Superoxide Dismutase (PoSOD) Genes in Root Development

Tree peony (Paeonia suffruticosa) is a significant medicinal plant. However, the low rooting number is a bottleneck problem in the micropropagation protocols of P. ostii 'Fengdan'. The activity of superoxide dismutase (SOD) is closely related to root development. But research on the SOD gene's impact on rooting is still lacking. In this study, RNA sequencing (RNA-seq) was used to analyze the four crucial stages of root development in P. ostii 'Fengdan' seedlings, including the early root primordium formation stage (Gmfq), root primordium formation stage (Gmf), root protrusion stage (Gtq), and root outgrowth stage (Gzc). A total of 141.77 GB of data were obtained; 71,718, 29,804, and 24,712 differentially expressed genes (DEGs) were identified in the comparison groups of Gmfq vs. Gmf, Gmf vs. Gtq, and Gtq vs. Gzc, respectively. Among the 20 most highly expressed DEGs in the three comparison groups, only the CuZnSOD gene (SUB13202229, PoSOD) was found to be significantly expressed in Gtq vs. Gzc. The overexpression of PoSOD increased the number of adventitious roots and promoted the activities of peroxidase (POD) and SOD in P. ostii 'Fengdan'. The gene ADVENTITIOUS ROOTING RELATED OXYGENASE1 (PoARRO-1), which is closely associated with the development of adventitious roots, was also significantly upregulated in overexpressing PoSOD plants. Furthermore, PoSOD interacted with PoARRO-1 in yeast two-hybrid (Y2H) and biomolecular luminescence complementation (BiFC) assays. In conclusion, PoSOD could interact with PoARRO-1 and enhance the root development of tube plantlets in P. ostii 'Fengdan'. This study will help us to preliminarily understand the molecular mechanism of adventitious root formation and improve the root quality of tree peony and other medicinal plants.

Recent Advances in Studying the Regulation of Fruit Ripening in Tomato Using Genetic Engineering Approaches

Tomato (Solanum lycopersicum L.) is one of the most commercially essential vegetable crops cultivated worldwide. In addition to the nutritional value, tomato is an excellent model for studying climacteric fruits' ripening processes. Despite this, the available natural pool of genes that allows expanding phenotypic diversity is limited, and the difficulties of crossing using classical selection methods when stacking traits increase proportionally with each additional feature. Modern methods of the genetic engineering of tomatoes have extensive potential applications, such as enhancing the expression of existing gene(s), integrating artificial and heterologous gene(s), pointing changes in target gene sequences while keeping allelic combinations characteristic of successful commercial varieties, and many others. However, it is necessary to understand the fundamental principles of the gene molecular regulation involved in tomato fruit ripening for its successful use in creating new varieties. Although the candidate genes mediate ripening have been identified, a complete picture of their relationship has yet to be formed. This review summarizes the latest (2017-2023) achievements related to studying the ripening processes of tomato fruits. This work attempts to systematize the results of various research articles and display the interaction pattern of genes regulating the process of tomato fruit ripening.

Maintenance of stem cell activity in plant development and stress responses

Stem cells residing in plant apical meristems play an important role during postembryonic development. These stem cells are the wellspring from which tissues and organs of the plant emerge. The shoot apical meristem (SAM) governs the aboveground portions of a plant, while the root apical meristem (RAM) orchestrates the subterranean root system. In their sessile existence, plants are inextricably bound to their environment and must adapt to various abiotic stresses, including osmotic stress, drought, temperature fluctuations, salinity, ultraviolet radiation, and exposure to heavy metal ions. These environmental challenges exert profound effects on stem cells, potentially causing severe DNA damage and disrupting the equilibrium of reactive oxygen species (ROS) and Ca2+ signaling in these vital cells, jeopardizing their integrity and survival. In response to these challenges, plants have evolved mechanisms to ensure the preservation, restoration, and adaptation of the meristematic stem cell niche. This enduring response allows plants to thrive in their habitats over extended periods. Here, we presented a comprehensive overview of the cellular and molecular intricacies surrounding the initiation and maintenance of the meristematic stem cell niche. We also delved into the mechanisms employed by stem cells to withstand and respond to abiotic stressors.

Comparative transcriptome analysis reveals the regulatory effects of exogenous auxin on lateral root development and tanshinone accumulation in Salvia miltiorrhiza

MAIN CONCLUSION

The physiological and transcriptome analysis revealed that auxin was a positive regulator of lateral root development and tanshinone accumulation in Salvia miltiorrhiza. Roots of S. miltiorrhiza are widely used as medicinal materials in China, and the root morphology and content of bioactive compounds [such as phenolic acids and diterpenoid quinones (tanshinones)] are the main factors to determine the quality of this herb. Auxin regulates root development and secondary metabolism in many plant species, but little is known about its function in S. miltiorrhiza. In this study, S. miltiorrhiza seedlings were treated (exogenous application) with the auxin indole-3-acetic acid (IAA) and the polar auxin transport inhibitor N-1-naphthylphthalamic acid (NPA) to investigate the regulatory roles of auxin in S. miltiorrhiza. The results indicated that exogenous IAA promoted both lateral root development and tanshinones biosynthesis in S. miltiorrhiza. The NPA application suppressed the lateral root development but showed no obvious effects on tanshinones accumulation. Based on the RNA-seq analysis, expressions of genes related to auxin biosynthesis and signaling transduction were altered in both treated groups. Coincidental with the enhanced content of tanshinones, transcripts of several key enzyme genes in the tanshinones biosynthetic pathway were stimulated after the exogenous IAA application. The expression profiles of seven common transcription factor domain-containing gene families were analyzed, and the results implied that some AP2/ERF genes were probably responsible for the auxin-induced lateral root development in S. miltiorrhiza. These findings shed new light on the regulatory roles of auxin on root development and bioactive compounds biosynthesis in S. miltiorrhiza, and lay the groundwork for future research into the detailed molecular mechanism underlying these biological functions.

Role of reactive oxygen species in the modulation of auxin flux and root development in Arabidopsis thaliana

Reactive oxygen species (ROS) play a dual role in plant biology, acting as important signal transduction molecules and as toxic byproducts of aerobic metabolism that accumulate in cells upon exposure to different stressors and lead to cell death. In plants, root architecture is regulated by the distribution and intercellular flow of the phytohormone auxin. In this study, we identified ROS as an important modulator of auxin distribution and response in the root. ROS production is necessary for root growth, proper tissue patterning, cell growth, and lateral root (LR) induction. Alterations in ROS balance led to altered auxin distribution and response in SOD and RHD2 loss-of-function mutants. Treatment of Arabidopsis seedlings with additional sources of ROS (hydrogen peroxide) or an ROS production inhibitor (diphenylene iodonium) induced phenocopies of the mutants studied. Simultaneous application of auxin and ROS increased LR primordia induction, and PIN-FORMED protein immunolocalization further demonstrated the existing link between auxin and ROS in orchestrating cell division and auxin flux during root development. In Arabidopsis roots, genetic alterations in ROS balance led to defective auxin distribution and growth-related responses in roots. Exogenous hydrogen peroxide alters the establishment of the endogenous auxin gradient in the root meristem through regulation of PIN-FORMED polarity, while the simultaneous application of hydrogen peroxide and auxin enhanced LR induction in a dose- and position-dependent manner through activation of cell division.

Asymmetric cell division in plant development

Asymmetric cell division (ACD) is a fundamental process that generates new cell types during development in eukaryotic species. In plant development, post-embryonic organogenesis driven by ACD is universal and more important than in animals, in which organ pattern is preset during embryogenesis. Thus, plant development provides a powerful system to study molecular mechanisms underlying ACD. During the past decade, tremendous progress has been made in our understanding of the key components and mechanisms involved in this important process in plants. Here, we present an overview of how ACD is determined and regulated in multiple biological processes in plant development and compare their conservation and specificity among different model cell systems. We also summarize the molecular roles and mechanisms of the phytohormones in the regulation of plant ACD. Finally, we conclude with the overarching paradigms and principles that govern plant ACD and consider how new technologies can be exploited to fill the knowledge gaps and make new advances in the field.

Chemical genetic screening identifies nalacin as an inhibitor of GH3 amido synthetase for auxin conjugation

Auxin inactivation is critical for plant growth and development. To develop plant growth regulators functioning in auxin inactivation pathway, we performed a phenotype-based chemical screen in Arabidopsis and identified a chemical, nalacin, that partially mimicked the effects of auxin. Genetic, pharmacological, and biochemical approaches demonstrated that nalacin exerts its auxin-like activities by inhibiting indole-3-acetic acid (IAA) conjugation that is mediated by Gretchen Hagen 3 (GH3) acyl acid amido synthetases. The crystal structure of Arabidopsis GH3.6 in complex with D4 (a derivative of nalacin) together with docking simulation analysis revealed the molecular basis of the inhibition of group II GH3 by nalacin. Sequence alignment analysis indicated broad bioactivities of nalacin and D4 as inhibitors of GH3s in vascular plants, which were confirmed, at least, in tomato and rice. In summary, our work identifies nalacin as a potent inhibitor of IAA conjugation mediated by group II GH3 that plays versatile roles in hormone-regulated plant development and has potential applications in both basic research and agriculture.

Conquering compacted soils: uncovering the molecular components of root soil penetration

Global agriculture and food security face paramount challenges due to climate change and land degradation. Human-induced soil compaction severely affects soil fertility, impairing root system development and crop yield. There is a need to design compaction-resilient crops that can thrive in degraded soils and maintain high yields. To address plausible solutions to this challenging scenario, we discuss current knowledge on plant root penetration ability and delineate potential approaches based on root-targeted genetic engineering (RGE) and genomics-assisted breeding (GAB) for developing crops with enhanced root system penetrability (RSP) into compacted soils. Such approaches could lead to crops with improved resilience to climate change and marginal soils, which can help to boost CO₂ sequestration and storage in deeper soil strata.

Fourteen Stations of Auxin

Auxin has always been at the forefront of research in plant physiology and development. Since the earliest contemplations by Julius von Sachs and Charles Darwin, more than a century-long struggle has been waged to understand its function. This largely reflects the failures, successes, and inevitable progress in the entire field of plant signaling and development. Here I present 14 stations on our long and sometimes mystical journey to understand auxin. These highlights were selected to give a flavor of the field and to show the scope and limits of our current knowledge. A special focus is put on features that make auxin unique among phytohormones, such as its dynamic, directional transport network, which integrates external and internal signals, including self-organizing feedback. Accented are persistent mysteries and controversies. The unexpected discoveries related to rapid auxin responses and growth regulation recently disturbed our contentment regarding understanding of the auxin signaling mechanism. These new revelations, along with advances in technology, usher us into a new, exciting era in auxin research.

Brassinosteroids Mitigate Cadmium Effects in Arabidopsis Root System without Any Cooperation with Nitric Oxide

The heavy metal cadmium (Cd) affects root system development and quiescent center (QC)-definition in Arabidopsis root-apices. The brassinosteroids-(BRs)-mediated tolerance to heavy metals has been reported to occur by a modulation of nitric oxide (NO) and root auxin-localization. However, how BRs counteract Cd-action in different root types is unknown. This research aimed to find correlations between BRs and NO in response to Cd in Arabidopsis’s root system, monitoring their effects on QC-definition and auxin localization in root-apices. To this aim, root system developmental changes induced by low levels of 24-epibrassinolide (eBL) or by the BR-biosynthesis inhibitor brassinazole (Brz), combined or not with CdSO₄, and/or with the NO-donor nitroprusside (SNP), were investigated using morpho-anatomical and NO-epifluorescence analyses, and monitoring auxin-localization by the DR5::GUS system. Results show that eBL, alone or combined with Cd, enhances lateral (LR) and adventitious (AR) root formation and counteracts QC-disruption and auxin-delocalization caused by Cd in primary root/LR/AR apices. Exogenous NO enhances LR and AR formation in Cd-presence, without synergism with eBL. The NO-signal is positively affected by eBL, but not in Cd-presence, and BR-biosynthesis inhibition does not change the low NO-signal caused by Cd. Collectively, results show that BRs ameliorate Cd-effects on all root types acting independently from NO.

Lessons Learned from the Studies of Roots Shaded from Direct Root Illumination

The root is the below-ground organ of a plant, and it has evolved multiple signaling pathways that allow adaptation of architecture, growth rate, and direction to an ever-changing environment. Roots grow along the gravitropic vector towards beneficial areas in the soil to provide the plant with proper nutrients to ensure its survival and productivity. In addition, roots have developed escape mechanisms to avoid adverse environments, which include direct illumination. Standard laboratory growth conditions for basic research of plant development and stress adaptation include growing seedlings in Petri dishes on medium with roots exposed to light. Several studies have shown that direct illumination of roots alters their morphology, cellular and biochemical responses, which results in reduced nutrient uptake and adaptability upon additive stress stimuli. In this review, we summarize recent methods that allow the study of shaded roots under controlled laboratory conditions and discuss the observed changes in the results depending on the root illumination status.