Brassinosteroids interact with auxin to promote lateral root development in Arabidopsis

Early transcriptomic changes in cucumber and maize roots in response to FePO4 nanoparticles as a source of P and Fe

The use of nanoparticles as an alternative to traditional fertilizers, aiming at a more efficient use of nutrients, is a recently developed concept that requires a thorough understanding of the processes occurring in the soil-plant system. A crucial aspect in this framework is to decipher the plant responses to the unique characteristics of these materials. In this work, we aim at decoding the transcriptional responses of cucumber and maize roots to FePO4 nanoparticles applied as P and Fe sources, respectively. The results demonstrate that P and Fe supplied as nanoscale salts support plant nutrition with an efficiency comparable to that of ionic forms of the nutrients. This supposition is confirmed by transcriptomic profiles that show no significant upregulation of transcripts typically induced by deficiencies in P and Fe in cucumber and maize plants in which these nutrients were provided by FePO4 nanoparticles. The analysis further revealed that nanoparticles alter the expression of genes involved in root development and stress responses, effect that appeared to be independent on the nutritional status of the plants. Our data further underline the challenge to identify generalizable elements of the impact of nanomaterials on plant species, as responses are intimately linked to the type of nanomaterials and differ among plant species.

HalALMT1 mediates malate efflux in the cortex of mature cluster rootlets of Hakea laurina, occurring naturally in severely phosphorus-impoverished soil

Hakea laurina, a woody Proteaceae, naturally occurs in severely phosphorus (P)-impoverished habitats in southwest Australia. It develops distinctive cluster roots that exhibit a high capacity for carboxylate exudation and acid phosphatase activity, contributing to its P acquisition. However, the molecular mechanisms underlying these physiological functions remain poorly understood. We explored the cluster-root transcriptome using de novo RNA-Seq and identified Hakea laurina Aluminum-activated Malate Transporter 1 (HalALMT1), encoding an aluminum (Al)-activated malate transporter induced in mature cluster roots. We characterized HalALMT1 through electrophysiological assays and overexpression in Arabidopsis thaliana, and localized HalALMT1 expression, acid phosphatase activity, and suberized boundaries in cluster roots. Differentially expressed genes highlighted multiple increased carboxylate-related processes at cluster-root maturity. HalALMT1 released malate, an activity further enhanced by exposure to Al3+. Notably, HalALMT1 was specifically expressed in mature cortex cells of cluster rootlets, which lack a suberized exodermis. Acid phosphatase activity was pronounced throughout the cluster rootlets, unlike in noncluster roots where it was limited to the epidermis and stele. Substantial malate release and acid phosphatase activity in the cortex cells in cluster rootlets, which lack a suberized exodermis, allowed massive exudation. This study sheds light on an exquisite P-acquisition strategy of Proteaceae, enabling survival under extremely low P availability.

Crosstalk between Brassinosteroids and Other Phytohormones during Plant Development and Stress Adaptation

Brassinosteroids (BRs) are a group of polyhydroxylated phytosterols that play essential roles in regulating plant growth and development as well as stress adaptation. It is worth noting that BRs do not function alone, but rather they crosstalk with other endogenous signaling molecules, including the phytohormones auxin, cytokinins, gibberellins, abscisic acid, ethylene, jasmonates, salicylic acid and strigolactones, forming elaborate signaling networks to modulate plant growth and development. BRs interact with other phytohormones mainly by regulating each others' homeostasis, transport or signaling pathway at the transcriptional and posttranslational levels. In this review, we focus our attention on current research progress in BR signal transduction and the crosstalk between BRs and other phytohormones.

RMD and Its Suppressor MAPK6 Control Root Circumnutation and Obstacle Avoidance via BR Signaling

Helical growth of the root tip (circumnutation) that permits surface exploration facilitates root penetration into soil. Here, we reveal that rice actin-binding protein RMD aids in root circumnutation, manifested by wavy roots as well as compromised ability to efficiently explore and avoid obstacles in rmd mutants. We demonstrate that root circumnutation defects in rmd depend on brassinosteroid (BR) signaling, which is elevated in mutant roots. Suppressing BR signaling via pharmacological (BR inhibitor) or genetic (knockout of BR biosynthetic or signaling components) manipulation rescues root defects in rmd. We further reveal that mutations in MAPK6 suppress BR signaling and restore normal root circumnutation in rmd, which may be mediated by the interaction between MAPK6, MAPKK4 and BR signaling factor BIM2. Our study thus demonstrates that RMD and MAPK6 control root circumnutation by modulating BR signaling to facilitate early root growth.

Light at the end of the tunnel: integrating signaling pathways in the coordination of lateral root development

Root system architecture (RSA) encompasses a range of physical root attributes, including the lateral roots (LRs), root hairs and adventitious roots, in addition to the primary or main root. This overall structure is a crucial trait for efficient water and mineral capture alongside providing anchorage to the plant in the soil and is vital for plant productivity and fitness. RSA dynamics are dependent upon various environmental cues such as light, soil pH, water, mineral nutrition and the belowground microbiome. Among these factors, light signaling through HY5 significantly influences the flexibility of RSA by controlling different signaling pathways that converge at photoreceptors-mediated signaling, also present in the 'hidden half'. Furthermore, several phytohormones also drive the formation and emergence of LRs and are critical to harmonize intra and extracellular stimuli in this regard. This review endeavors to elucidate the impact of these interactions on RSA, with particular emphasis on LR development and to enhance our understanding of the fundamental mechanisms governing the light-regulation of LR growth and physiology.

Metabolomics and transcriptomics combined with physiology reveal key metabolic pathway responses in tobacco roots exposed to NaHS

Hydrogen sulfide (H2S) has emerged as a novel endogenous gas signaling molecule, joining the ranks of nitric oxide (NO) and carbon monoxide (CO). Recent research has highlighted its involvement in various physiological processes, such as promoting root organogenesis, regulating stomatal movement and photosynthesis, and enhancing plant growth, development, and stress resistance. Tobacco, a significant cash crop crucial for farmers' economic income, relies heavily on root development to affect leaf growth, disease resistance, chemical composition, and yield. Despite its importance, there remains a scarcity of studies investigating the role of H2S in promoting tobacco growth. This study exposed tobacco seedlings to different concentrations of NaHS (an exogenous H2S donor) - 0, 200, 400, 600, and 800 mg/L. Results indicated a positive correlation between NaHS concentration and root length, wet weight, root activity, and antioxidant enzymatic activities (CAT, SOD, and POD) in tobacco roots. Transcriptomic and metabolomic analyses revealed that treatment with 600 mg/L NaHS significantly effected 162 key genes, 44 key enzymes, and two metabolic pathways (brassinosteroid synthesis and aspartate biosynthesis) in tobacco seedlings. The addition of exogenous NaHS not only promoted tobacco root development but also potentially reduced pesticide usage, contributing to a more sustainable ecological environment. Overall, this study sheds light on the primary metabolic pathways involved in tobacco root response to NaHS, offering new genetic insights for future investigations into plant root development.

Genome-wide association analysis reveal candidate genes and haplotypes related to root weight in cucumber (Cucumis sativus L.)

Background

The plant root system is critical for the absorption of water and nutrients, and have a direct influence on growth and yield. In cucumber, a globally consumed crop, the molecular mechanism of root development remains unclear, and this has implications for developing stress tolerant varieties. This study sought to determine the genetic patterns and related genes of cucumber root weight. A core cucumber germplasms population was used to do the GWAS analysis in three environments.

Results

Here, we investigated four root-weight related traits including root fresh weight (RFW), root dry weight (RDW), ratio of root dry weight to root fresh weight (RDFW) and the comprehensive evaluation index, D-value of root weight (DRW) deduced based on the above three traits for the core germplasm of the cucumber global repository. According to the D-value, we identified 21 and 16 accessions with light and heavy-root, respectively. We also found that the East Asian ecotype accessions had significantly heavier root than other three ecotypes. The genome-wide association study (GWAS) for these four traits reveals that 4 of 10 significant loci (gDRW3.1, gDRW3.2, gDRW4.1 and gDRW5.1) were repeatedly detected for at least two traits. Further haplotype and expression analysis for protein-coding genes positioned within these 4 loci between light and heavy-root accessions predicted five candidate genes (i.e., Csa3G132020 and Csa3G132520 both encoding F-box protein PP2-B1 for gDRW3.1, Csa3G629240 encoding a B-cell receptor-associated protein for gDRW3.2, Csa4G499330 encodes a GTP binding protein for gDRW4.1, and Csa5G286040 encodes a proteinase inhibitor for gDRW5.1).

Conclusions

We conducted a systematic analysis of the root genetic basis and characteristics of cucumber core germplasms population. We detected four novel loci, which regulate the root weight in cucumber. Our study provides valuable candidate genes and haplotypes for the improvement of root system in cucumber breeding.

A mutation in the brassinosteroid biosynthesis gene CpDWF5 disrupts vegetative and reproductive development and the salt stress response in squash (Cucurbita pepo)

A Cucurbita pepo mutant with multiple defects in growth and development has been identified and characterized. The mutant dwfcp displayed a dwarf phenotype with dark green and shrinking leaves, shortened internodes and petioles, shorter but thicker roots and greater root biomass, and reduced fertility. The causal mutation of the phenotype was found to disrupt gene Cp4.1LG17g04540, the squash orthologue of the Arabidopsis brassinosteroid (BR) biosynthesis gene DWF5, encoding for 7-dehydrocholesterol reductase. A single nucleotide transition (G > A) causes a splicing defect in intron 6 that leads to a premature stop codon and a truncated CpDWF5 protein. The mutation co-segregated with the dwarf phenotype in a large BC1S1 segregating population. The reduced expression of CpDWF5 and brassinolide (BL) content in most mutant organs, and partial rescue of the mutant phenotype by exogenous application of BL, showed that the primary cause of the dwarfism in dwfcp is a BR deficiency. The results showed that in C. pepo, CpDWF5 is not only a positive growth regulator of different plant organs but also a negative regulator of salt tolerance. During germination and the early stages of seedling development, the dwarf mutant was less affected by salt stress than the wild type, concomitantly with a greater upregulation of genes associated with salt tolerance, including those involved in abscisic acid (ABA) biosynthesis, ABA and Ca2+ signaling, and those coding for cation exchangers and transporters.

Wheat BREVIS RADIX (BRX) regulates organ size, stomatal density and enhances drought tolerance in Arabidopsis

BREVIS RADIX (BRX) is a small plant-specific and evolutionary conserved gene family with divergent yet partially redundant biological functions including root and shoot growth, stomatal development and tiller angle in plants. We characterized a BRX family gene from wheat (Triticum aestivum) by gain-of-function in Arabidopsis. Overexpression of TaBRXL2A resulted in longer primary roots with increased root meristem size and higher root growth under control and exogenous hormone treatments as compared to wild type (Col-0) plants. Overexpression lines also exhibited significant differences with the wild type such as increased rosette size, higher leaf number and leaf size. At reproductive stage, overexpression lines exhibited wider siliques and higher grain weight per plant. Under drought stress, overexpression lines exhibited enhanced drought tolerance in terms of higher chlorophyll retention and lower oxidative stress, thereby leading to significant recovery from drought stress. The analysis suggests that the inherent lower stomatal density in the leaves of overexpression lines and higher stomatal closure in response to ABA might contribute to lower water loss from the overexpression lines. Furthermore, TaBRXL2A protein showed membrane localization, presence of conserved residues at N-terminal for palmitoylation, and phosphosites in the linker region which are prescribed for its potential role in protophloem differentiation and stomatal lineage. Thus, we identified a TaBRX family gene which is involved in developmental pathways essential for plant growth, and also enhances drought tolerance in Arabidopsis.

Genetic regulation of lateral root development

Lateral roots (LRs) are an important part of plant root systems. In dicots, for example, after plants adapted from aquatic to terrestrial environments, filamentous pseudorhizae evolved to allow nutrient absorption. A typical plant root system comprises a primary root, LRs, root hairs, and a root cap. Classical plant roots exhibit geotropism (the tendency to grow downward into the ground) and can synthesize plant hormones and other essential substances. Root vascular bundles and complex spatial structures enable plants to absorb water and nutrients to meet their nutrient quotas and grow. The primary root carries out most functions during early growth stages but is later overtaken by LRs, underscoring the importance of LR development water and mineral uptake and the soil fixation capacity of the root. LR development is modulated by endogenous plant hormones and external environmental factors, and its underlying mechanisms have been dissected in great detail in Arabidopsis, thanks to its simple root anatomy and the ease of obtaining mutants. This review comprehensively and systematically summarizes past research (largely in Arabidopsis) on LR basic structure, development stages, and molecular mechanisms regulated by different factors, as well as future prospects in LR research, to provide broad background knowledge for root researchers.

Transcriptome analysis reveals the effect of acidic environment on adventitious root differentiation in Camellia sinensis

The generation of adventitious roots (ARs) is the key to the success of cuttings. The appropriate environment for AR differentiation in tea plants is acidic. However, the mechanism is unclear. In this study, pH 4.5 was suitable condition for the differentiation of AR in tea plants. At the base of cuttings, the root primordia differentiated ARs more rapidly at pH 4.5 than pH 7.0, and nine AR differentiation-related genes were found to be differentially expressed in 30 days, the result was also validated by qRT-PCR. The promoter regions of these genes contained auxin and brassinosteroid response elements. The expression levels of several genes which were involved in auxin and brassinosteroid synthesis as well as signaling at pH 4.5 compared to pH 7.0 occurred differential expression. Brassinolide (BL) and indole-3-acetic acid (IAA) could affect the differentiation of ARs under pH 4.5 and pH 7.0. By qRT-PCR analysis of genes during ARs generation, BL and IAA inhibited and promoted the expression of CsIAA14 gene, respectively, to regulate auxin signal transduction. Meanwhile, the expression levels of CsKNAT4, CsNAC2, CsNAC100, CsWRKY30 and CsLBD18 genes were up-regulated upon auxin treatment and were positively correlated with ARs differentiation.This study showed that pH 4.5 was the most suitable environment for the root primordia differentiation of AR in tea plant. Proper acidic pH conditions promoted auxin synthesis and signal transduction. The auxin initiated the expression of AR differentiation-related genes, and promoted its differentiated. BL was involved in ARs formation and elongation by regulating auxin signal transduction.

Fine-tuned nitric oxide and hormone interface in plant root development and regeneration

Plant root growth and developmental capacities reside in a few stem cells of the root apical meristem (RAM). Maintenance of these stem cells requires regenerative divisions of the initial stem cell niche (SCN) cells, self-maintenance, and proliferative divisions of the daughter cells. This ensures sufficient cell diversity to guarantee the development of complex root tissues in the plant. Damage in the root during growth involves the formation of a new post-embryonic root, a process known as regeneration. Post-embryonic root development and organogenesis processes include primary root development and SCN maintenance, plant regeneration, and the development of adventitious and lateral roots. These developmental processes require a fine-tuned balance between cell proliferation and maintenance. An important regulator during root development and regeneration is the gasotransmitter nitric oxide (NO). In this review we have sought to compile how NO regulates cell rate proliferation, cell differentiation, and quiescence of SCNs, usually through interaction with phytohormones, or other molecular mechanisms involved in cellular redox homeostasis. NO exerts a role on molecular components of the auxin and cytokinin signaling pathways in primary roots that affects cell proliferation and maintenance of the RAM. During root regeneration, a peak of auxin and cytokinin triggers specific molecular programs. Moreover, NO participates in adventitious root formation through its interaction with players of the brassinosteroid and cytokinin signaling cascade. Lately, NO has been implicated in root regeneration under hypoxia conditions by regulating stem cell specification through phytoglobins.

Brassinosteroid catabolic enzyme CYP734A129 regulates the morphologies of leaves and floral organs in woodland strawberry

Brassinosteroids (BRs) play critical roles in plant growth and development and regulate many important agronomic traits. However, the functions of BRs in strawberry are unclear. This study identified two mutants, named P6 and R87, in woodland strawberry (Fragaria vesca) from EMS mutagenesis populations that exhibit narrow leaves, petals and sepals. Mapping by sequencing and genetic studies revealed that the F. vesca CYP734A129, encoding a putative BR catabolic enzyme, is the causative gene for both P6 and R87. Overexpression of CYP734A129 in both F. vesca and Arabidopsis causes a severe dwarf phenotype, and the BRI1-EMS-SUPPRESSOR 1 (BES1) protein is less abundant in the CYP734A129-overexpressing Arabidopsis seedlings. This suggests that CYP734A129 is functionally conserved with CYP734A1, as a BR-inactivating enzyme. Transcriptome analysis of young leaves revealed that four BR biosynthetic genes were significantly downregulated in P6 (cyp734a129), and photosynthesis-related genes were highly enriched among the up-regulated genes in P6 compared to the wild type. This further supports that CYP734A129 inactivates BRs in F. vesca. Furthermore, we showed that mutations in CYP734A129 do not affect fruit shape and color during ripening in strawberry. Overall, our results suggest that F. vesca CYP734A129 is a BR catabolic enzyme, and provide insights into the roles of CYP734A129 in strawberry.

BvCPD promotes parenchyma cell and vascular bundle development in sugar beet (Beta vulgaris L.) taproot

Constitutive photomorpogenic dwarf (CPD) is a pivotal enzyme gene for brassinolide (BR) synthesis and plays an important role in plant growth, including increasing plant biomass and plant height, elongating cells, and promoting xylem differentiation. However, little is known about the function of the CPD gene in sugar beet. In the current study, we isolated CPD from Beta vulgaris L. (BvCPD), which encodes protein localized in the nucleus, cell membrane, and cell wall. BvCPD was strongly expressed in parenchyma cells and vascular bundles. The transgenic sugar beet overexpressing BvCPD exhibited larger diameter than that of the wild type (WT), which mainly owing to the increased number and size of parenchyma cells, the enlarged lumen and area of vessel in the xylem. Additionally, overexpression of BvCPD increased the synthesis of endogenous BR, causing changes in the content of endogenous auxin (IAA) and gibberellin (GA) and accumulation of cellulose and lignin in cambium 1-4 rings of the taproot. These results suggest that BvCPD can promote the biosynthesis of endogenous BR, improve cell wall components, promote the development of parenchyma cells and vascular bundle, thereby playing an important role in promoting the growth and development of sugar beet taproot.

24-Epibrassinolide Facilitates Adventitious Root Formation by Coordinating Cell-Wall Polyamine Oxidase- and Plasma Membrane Respiratory Burst Oxidase Homologue-Derived Reactive Oxygen Species in Capsicum annuum L

Adventitious root (AR) formation is a critical process in cutting propagation of horticultural plants. Brassinosteroids (BRs) have been shown to regulate AR formation in several plant species; however, little is known about their exact effects on pepper AR formation, and the downstream signaling of BRs also remains elusive. In this study, we showed that treatment of 24-Epibrassinolide (EBL, an active BR) at the concentrations of 20-100 nM promoted AR formation in pepper (Capsicum annuum). Furthermore, we investigated the roles of apoplastic reactive oxygen species (ROS), including hydrogen peroxide (H₂O₂) and superoxide radical (O₂^•-), in EBL-promoted AR formation, by using physiological, histochemical, bioinformatic, and biochemical approaches. EBL promoted AR formation by modulating cell-wall-located polyamine oxidase (PAO)-dependent H₂O₂ production and respiratory burst oxidase homologue (RBOH)-dependent O₂^•- production, respectively. Screening of CaPAO and CaRBOH gene families combined with gene expression analysis suggested that EBL-promoted AR formation correlated with the upregulation of CaPAO1, CaRBOH2, CaRBOH5, and CaRBOH6 in the AR zone. Transient expression analysis confirmed that CaPAO1 was able to produce H₂O₂, and CaRBOH2, CaRBOH5, and CaRBOH6 were capable of producing O₂^•-. The silencing of CaPAO1, CaRBOH2, CaRBOH5, and CaRBOH6 in pepper decreased the ROS accumulation and abolished the EBL-induced AR formation. Overall, these results uncover one of the regulatory pathways for BR-regulated AR formation, and extend our knowledge of the functions of BRs and of the BRs-ROS crosstalk in plant development.

Specificity in root domain accumulation of Phytoglobin1 and nitric oxide (NO) determines meristematic viability in water-stressed Brassica napus roots

BACKGROUND AND AIMS

Drought reduces plant productivity, especially in the susceptible species Brassica napus. Water stress, mimicked by applications of 10 % polyethylene glycol (PEG), elevates nitric oxide (NO) in root cells after a few hours, contributing to degradation of the root apical meristems (RAMs), the function of which relies on auxin and brassinosteroids (BRs). Phytoglobins (Pgbs) are effective NO scavengers induced by this stress. This study examines the effects of BnPgb1 dysregulation in dehydrating B. napus roots, and the spatiotemporal relationship between Pgb1 and activities of auxin and BRs in the regulation of the RAM.

METHODS

Brassica napus lines over-expressing [BnPgb1(S)] or down-regulating [BnPgb1(RNAi)] BnPgb1 were exposed to PEG-induced water stress. The localization of BnPgb1, NO, auxin and PIN1 were analysed during the first 48 h, while the expression level of biosynthetic auxin and BR genes was measured during the first 24 h. Pharmacological treatments were conducted to assess the requirement of auxin and BR in dehydrating roots.

KEY RESULTS

During PEG stress, BnPgb1 protein accumulated preferentially in the peripheral domains of the root elongation zone, exposing the meristem to NO, which inhibits polar auxin transport (PAT), probably by interfering with PIN1 localization and the synthesis of auxin. Diminished auxin at the root tip depressed the synthesis of BR and caused the degradation of the RAMs. The strength of BnPgb1 signal in the elongation zone was increased in BnPgb1(S) roots, where NO was confined to the most apical cells. Consequently, PAT and auxin synthesis were retained, and the definition of RAMs was maintained. Auxin preservation of the RAM required BRs, although BRs alone was not sufficient to fully rescue drought-damaged RAMs in auxin-depleted environments.

CONCLUSIONS

The tissue-specific localization of BnPgb1 and NO determine B. napus root responses to water stress. A model is proposed in which auxin and BRs act as downstream components of BnPgb1 signalling in the preservation of RAMs in dehydrating roots.

New Paradigms in Brassinosteroids, Strigolactones, Sphingolipids, and Nitric Oxide Interaction in the Control of Lateral and Adventitious Root Formation

The root system is formed by the primary root (PR), which forms lateral roots (LRs) and, in some cases, adventitious roots (ARs), which in turn may produce their own LRs. The formation of ARs is also essential for vegetative propagation in planta and in vitro and for breeding programs. Root formation and branching is coordinated by a complex developmental network, which maximizes the plant's ability to cope with abiotic stress. Rooting is also a response caused in a cutting by wounding and disconnection from the donor plant. Brassinosteroids (BRs) are steroid molecules perceived at the cell surface. They act as plant-growth-regulators (PGRs) and modulate plant development to provide stress tolerance. BRs and auxins control the formation of LRs and ARs. The auxin/BR interaction involves other PGRs and compounds, such as nitric oxide (NO), strigolactones (SLs), and sphingolipids (SPLs). The roles of these interactions in root formation and plasticity are still to be discovered. SLs are carotenoid derived PGRs. SLs enhance/reduce LR/AR formation depending on species and culture conditions. These PGRs possibly crosstalk with BRs. SPLs form domains with sterols within cellular membranes. Both SLs and SPLs participate in plant development and stress responses. SPLs are determinant for auxin cell-trafficking, which is essential for the formation of LRs/ARs in planta and in in vitro systems. Although little is known about the transport, trafficking, and signaling of SPLs, they seem to interact with BRs and SLs in regulating root-system growth. Here, we review the literature on BRs as modulators of LR and AR formation, as well as their crosstalk with SLs and SPLs through NO signaling. Knowledge on the control of rooting by these non-classical PGRs can help in improving crop productivity and enhancing AR-response from cuttings.

Brassinosteroids is involved in methane-induced adventitious root formation via inducing cell wall relaxation in marigold

BACKGROUND

Methane (CH₄) and brassinosteroids (BRs) are important signaling molecules involved in a variety of biological processes in plants.

RESULTS

Here, marigold (Tagetes erecta L. 'Marvel') was used to investigate the role and relationship between CH₄ and BRs during adventitious root (AR) formation. The results showed a dose-dependent effect of CH₄ and BRs on rooting, with the greatest biological effects of methane-rich water (MRW, CH₄ donor) and 2,4-epibrassinolide (EBL) at 20% and 1 μmol L^- 1, respectively. The positive effect of MRW on AR formation was blocked by brassinoazole (Brz, a synthetic inhibitor of EBL), indicating that BRs might be involved in MRW-regulated AR formation. MRW promoted EBL accumulation during rooting by up-regulating the content of campestanol (CN), cathasterone (CT), and castasterone (CS) and the activity of Steroid 5α-reductase (DET2), 22α-hydroxylase (DWF4), and BR-6-oxidase (BR6ox), indicating that CH₄ could induce endogenous brassinolide (BR) production during rooting. Further results showed that MRW and EBL significantly down-regulated the content of cellulose, hemicellulose and lignin during rooting and significantly up-regulated the hydrolase activity, i.e. cmcase, xylanase and laccase. In addition, MRW and EBL also significantly promoted the activity of two major cell wall relaxing factors, xyloglucan endotransglucosylase/hydrolase (XTH) and peroxidase, which in turn promoted AR formation. While, Brz inhibited the role of MRW on these substances.

CONCLUSIONS

BR might be involved in CH₄-promoted AR formation by increasing cell wall relaxation.

24-epibrassinolide improves cadmium tolerance and lateral root growth associated with regulating endogenous auxin and ethylene in Kentucky bluegrass

The application of phytohormones is a viable technique to increase the efficiency of phytoremediation in heavy metal-contaminated soils. The objective of this study was to determine how the application of 24-epibrassinolide (EBR), a brassinosteroid analog, could regulate root growth and tolerance to cadmium (Cd) stress in Kentucky bluegrass. As a result, the number of lateral root primordia and total root length in the Cd-treated seedlings decreased by 33.1 % and 56.5 %, respectively. After the application of EBR, Cd accumulation in roots and leaves, and the negative effect of Cd on root growth were reduced under Cd stress. Additionally, the expression of the brassinosteroid signaling gene PpBRI1 was significantly upregulated by exogenous EBR. Moreover, exogenous EBR upregulated the expression of genes encoding antioxidant enzymes and improved the activity of antioxidant enzymes, thereby reduced oxidative stress in roots. Finally, targeted hormonomics analysis highlighted the utility of the application of EBR to alleviate the effect of Cd on the reduction in auxin (IAA) content and the increase in ethylene (ACC) content. These were known to be associated with the upregulation in the expression of auxin biosynthesis gene PpYUCCA1 and downregulation in the expression of ethylene biosynthesis gene PpACO1 in the roots treated with Cd stress. Overall, the application of EBR alleviated Cd-induced oxidative stress in addition to improving root elongation and lateral root growth crosstalk with auxin and ethylene in Kentucky bluegrass subjected to Cd stress. This study further highlights the potential role of brassinosteroids in improving the efficiency of phytoremediation for Cd-contaminated soils.

Signaling events for photomorphogenic root development

A germinating seedling incorporates environmental signals such as light into developmental outputs. Light is not only a source of energy, but also a central coordinative signal in plants. Traditionally, most research focuses on aboveground organs' response to light; therefore, our understanding of photomorphogenesis in roots is relatively scarce. However, root development underground is highly responsive to light signals from the shoot and understanding these signaling mechanisms will give a better insight into early seedling development. Here, we review the central light signaling hubs and their role in root growth promotion of Arabidopsis thaliana seedlings.

The Arabidopsis TETRATRICOPEPTIDE-REPEAT THIOREDOXIN-LIKE (TTL) family members are involved in root system formation via their interaction with cytoskeleton and cell wall remodeling

Lateral roots (LR) are essential components of the plant edaphic interface; contributing to water and nutrient uptake, biotic and abiotic interactions, stress survival, and plant anchorage. We have identified the TETRATRICOPEPTIDE-REPEAT THIOREDOXIN-LIKE 3 (TTL3) gene as being related to LR emergence and later development. Loss of function of TTL3 leads to a reduced number of emerged LR due to delayed development of lateral root primordia (LRP). This trait is further enhanced in the triple mutant ttl1ttl3ttl4. TTL3 interacts with microtubules and endomembranes, and is known to participate in the brassinosteroid (BR) signaling pathway. Both ttl3 and ttl1ttl3ttl4 mutants are less sensitive to BR treatment in terms of LR formation and primary root growth. The ability of TTL3 to modulate biophysical properties of the cell wall was established under restrictive conditions of hyperosmotic stress and loss of root growth recovery, which was enhanced in ttl1ttl3ttl4. Timing and spatial distribution of TTL3 expression is consistent with its role in development of LRP before their emergence and subsequent growth of LR. TTL3 emerged as a component of the root system morphogenesis regulatory network.

Root hair-specific transcriptome reveals response to low phosphorus in Cicer arietinum

Root hairs (RH) are a single-cell extension of root epidermal cells. In low phosphorus (LP) availability, RH length and density increase thus expanding the total root surface area for phosphate (Pi) acquisition. However, details on genes involved in RH development and response to LP are missing in an agronomically important leguminous crop, chickpea. To elucidate this response in chickpea, we performed tissue-specific RNA-sequencing and analyzed the transcriptome modulation for RH and root without RH (Root-RH) under LP. Root hair initiation and cellular differentiation genes like RSL TFs and ROPGEFs are upregulated in Root-RH, explaining denser, and ectopic RH in LP. In RH, genes involved in tip growth processes and phytohormonal biosynthesis like cell wall synthesis and loosening (cellulose synthase A catalytic subunit, CaEXPA2, CaGRP2, and CaXTH2), cytoskeleton/vesicle transport, and ethylene biosynthesis are upregulated. Besides RH development, genes involved in LP responses like lipid and/or pectin P remobilization and acid phosphatases are induced in these tissues summarizing a complete molecular response to LP. Further, RH displayed preferential enrichment of processes involved in symbiotic interactions, which provide an additional benefit during LP. In conclusion, RH shows a multi-faceted response that starts with molecular changes for epidermal cell differentiation and RH initiation in Root-RH and later induction of tip growth and various LP responses in elongated RH.

Brassinosteroid signaling restricts root lignification by antagonizing SHORT-ROOT function in Arabidopsis

Cell wall lignification is a key step in forming functional endodermis and protoxylem (PX) in plant roots. Lignified casparian strips (CS) in endodermis and tracheary elements of PX are essential for selective absorption and transport of water and nutrients. Although multiple key regulators of CS and PX have been identified, the spatial information that drives the developmental shift to root lignification remains unknown. Here, we found that brassinosteroid (BR) signaling plays a key role in inhibiting root lignification in the root elongation zone. The inhibitory activity of BR signaling occurs partially through the direct binding of BRASSINAZOLE-RESISTANT 1 (BZR1) to SHORT-ROOT (SHR), repressing the SHR-mediated activation of downstream genes that are involved in root lignification. Upon entering the mature root zone, BR signaling declines rapidly, which releases SHR activity and initiates root lignification. Our results provide a mechanistic view of the developmental transition to cell wall lignification in Arabidopsis thaliana roots.

Root system architecture in rice: impacts of genes, phytohormones and root microbiota

To feed the continuously expanding world's population, new crop varieties have been generated, which significantly contribute to the world's food security. However, the growth of these improved plant varieties relies primarily on synthetic fertilizers, which negatively affect the environment and human health; therefore, continuous improvement is needed for sustainable agriculture. Several plants, including cereal crops, have the adaptive capability to combat adverse environmental changes by altering physiological and molecular mechanisms and modifying their root system to improve nutrient uptake efficiency. These plants operate distinct pathways at various developmental stages to optimally establish their root system. These processes include changes in the expression profile of genes, changes in phytohormone level, and microbiome-induced root system architecture (RSA) modification. Several studies have been performed to understand microbial colonization and their involvement in RSA improvement through changes in phytohormone and transcriptomic levels. This review highlights the impact of genes, phytohormones, and particularly root microbiota in influencing RSA and provides new insights resulting from recent studies on rice root as a model system and summarizes the current knowledge about biochemical and central molecular mechanisms.

A novel chemical inhibitor of polar auxin transport promotes shoot regeneration by local enhancement of HD-ZIP III transcription

De novo shoot organogenesis is a prerequisite for numerous applications in plant research and breeding but is often a limiting factor, for example, in genome editing approaches. Class III homeodomain-leucine zipper (HD-ZIP III) transcription factors have been characterized as crucial regulators of shoot specification, however up-stream components controlling their activity during shoot regeneration are only partially identified. In a chemical genetic screen, we isolated ZIC2, a novel activator of HD-ZIP III activity. Using molecular, physiological and hormone transport analyses in Arabidopsis and sunflower (Helianthus annuus), we examined the molecular mechanism by which the drug promotes HD-ZIP III expression. ZIC2-dependent upregulation of HD-ZIP III transcription promotes shoot regeneration in Arabidopsis and is accompanied by the induction of shoot specifying factors WUS and RAP2.6L and a subset of cytokinin biosynthesis enzymes. ZIC2's effect on HD-ZIP III expression and regeneration is based on its ability to limit polar auxin transport. We further provide evidence that chemical modulation of auxin efflux can enhance de novo shoot formation in the regeneration recalcitrant species sunflower. Activation of HD-ZIP III transcription during shoot regeneration depends on the local distribution of auxin and chemical modulation of auxin transport can be used to overcome poor shoot organogenesis in tissue culture.

The brassinosteroid biosynthesis gene TaD11-2A controls grain size and its elite haplotype improves wheat grain yields

TaD11-2A affects grain size and root length and its natural variations are associated with significant differences in yield-related traits in wheat. Brassinosteroids (BRs) control many important agronomic traits and therefore the manipulation of BR components could improve crop productivity and performance. However, the potential effects of BR-related genes on yield-related traits and stress tolerance in wheat (Triticum aestivum L.) remain poorly understood. Here, we identified TaD11 genes in wheat (rice D11 orthologs) that encoded enzymes involved in BR biosynthesis. TaD11 genes were highly expressed in roots (Zadoks scale: Z11) and grains (Z75), while expression was significantly suppressed by exogenous BR (24-epiBL). Ectopic expression of TaD11-2A rescued the abnormal panicle structure and plant height (PH) of the clustered primary branch 1 (cpb1) mutant, and also increased endogenous BR levels, resulting in improved grain yields and grain quality in rice. The tad11-2a-1 mutant displayed dwarfism, smaller grains, sensitivity to 24-epiBL, and reduced endogenous BR contents. Natural variations in TaD11-2A were associated with significant differences in yield-related traits, including PH, grain width, 1000-grain weight, and grain yield per plant, and its favorable haplotype, TaD11-2A-HapI was subjected to positive selection during wheat breeding. Additionally, TaD11-2A influenced root length and salt tolerance in rice and wheat at seedling stages. These results indicated the important role of BR TaD11 biosynthetic genes in controlling grain size and root length, and also highlighted their potential in the molecular biological analysis of wheat.

Integrated transcriptome and hormonal analysis of naphthalene acetic acid-induced adventitious root formation of tea cuttings (Camellia sinensis)

BACKGROUND

Tea plant breeding or cultivation mainly involves propagation via cuttings, which not only ensures the inheritance of the excellent characteristics of the mother plant but also facilitates mechanized management. The formation of adventitious root (AR) determines the success of cutting-based propagation, and auxin is an essential factor involved in this process. To understand the molecular mechanism underlying AR formation in nodal tea cuttings, transcriptome and endogenous hormone analysis was performed on the stem bases of red (mature)- and green (immature)-stem cuttings of 'Echa 1 hao' tea plant as affected by a pulse treatment with naphthalene acetic acid (NAA).

RESULTS

In this study, NAA significantly promoted AR formation in both red- and green-stem cuttings but slightly reduced callus formation. External application of NAA reduced the levels of endogenous indole-3-acetic acid (IAA) and cytokinin (TZR, trans-zeatin riboside). The number of DEGs (NAA vs. CK) identified in the green-stem cuttings was significantly higher than that in the red-stem cuttings, which corresponded to a higher rooting rate of green-stem cuttings under the NAA treatment. A total of 82 common DEGs were identified as being hormone-related and involved in the auxin, cytokinin, abscisic acid, ethylene, salicylic acid, brassinosteroid, and jasmonic acid pathways. The negative regulation of NAA-induced IAA and GH3 genes may explain the decrease of endogenous IAA. NAA reduced endogenous cytokinin levels and further downregulated the expression of cytokinin signalling-related genes. By the use of weighted gene co-expression network analysis (WGCNA), several hub genes, including three [cellulose synthase (CSLD2), SHAVEN3-like 1 (SVL1), SMALL AUXIN UP RNA (SAUR21)] that are highly related to root development in other crops, were identified that might play important roles in AR formation in tea cuttings.

CONCLUSIONS

NAA promotes the formation of AR of tea cuttings in coordination with endogenous hormones. The most important endogenous AR inductor, IAA, was reduced in response to NAA. DEGs potentially involved in NAA-mediated AR formation of tea plant stem cuttings were identified via comparative transcriptome analysis. Several hub genes, such as CSLD2, SVL1 and SAUR21, were identified that might play important roles in AR formation in tea cuttings.

The interplay of auxin and brassinosteroid signaling tunes root growth under low and different nitrogen forms

The coordinated signaling activity of auxin and brassinosteroids (BRs) is critical for optimal plant growth and development. Nutrient-derived signals regulate root growth by modulating the levels and spatial distribution of growth hormones to optimize nutrient uptake and assimilation. However, the effect of the interaction of these two hormones and their signaling on root plasticity during low and differential availability of nitrogen (N) forms (NH4+/NO3-) remains elusive. We demonstrate that root elongation under low N (LN) is an outcome of the interdependent activity of auxin and BR signaling pathways in Arabidopsis (Arabidopsis thaliana). LN promotes root elongation by increasing BR-induced auxin transport activity in the roots. Increased nuclear auxin signaling and its transport efficiency have a distinct impact on root elongation under LN conditions. High auxin levels reversibly inhibit BR signaling via BRI1 KINASE INHIBITOR1. Using the tissue-specific approach, we show that BR signaling from root vasculature (stele) tissues is sufficient to promote cell elongation and, hence, root growth under LN condition. Further, we show that N form-defined root growth attenuation or enhancement depends on the fine balance of BR and auxin signaling activity. NH4+ as a sole N source represses BR signaling and response, which in turn inhibits auxin response and transport, whereas NO3- promotes root elongation in a BR signaling-dependent manner. In this study, we demonstrate the interplay of auxin and BR-derived signals, which are critical for root growth in a heterogeneous N environment and appear essential for root N foraging response and adaptation.

Unearthing the Alleviatory Mechanisms of Brassinolide in Cold Stress in Rice

Cold stress inhibits rice germination and seedling growth. Brassinolide (BR) plays key roles in plant growth, development, and stress responses. In this study, we explored the underlying mechanisms whereby BR helps alleviate cold stress in rice seedlings. BR application to the growth medium significantly increased seed germination and seedling growth of the early rice cultivar “Zhongzao 39” after three days of cold treatment. Specifically, BR significantly increased soluble protein and soluble sugar contents after three days of cold treatment. Moreover, BR stimulated the activity of superoxide dismutase, catalase, peroxidase, and ascorbate peroxidase; thereby alleviating cold-induced damage and increasing glutathione content and the GSH/GSSG ratio while concomitantly reducing H2O2 content. BR upregulated the expression levels of cold-response-related genes, including OsICE1, OsFer1, OsCOLD1, OsLti6a, OsSODB, OsMyb, and OsTERF2, and downregulated that of OsWRKY45, overall alleviating cold stress symptoms. Thus, BR not only upregulated cellular osmotic content and the antioxidant enzyme system to maintain the physiological balance of reactive oxygen species under cold but, additionally, it regulated the expression of cold-response-related genes to alleviate cold stress symptoms. These results provide a theoretical basis for rice breeding for cold resistance using young seedlings.

The histone deacetylase 1/GSK3/SHAGGY-like kinase 2/BRASSINAZOLE-RESISTANT 1 module controls lateral root formation in rice

Lateral roots (LRs) are a main component of the root system of rice (Oryza sativa) that increases root surface area, enabling efficient absorption of water and nutrients. However, the molecular mechanism regulating LR formation in rice remains largely unknown. Here, we report that histone deacetylase 1 (OsHDAC1) positively regulates LR formation in rice. Rice OsHDAC1 RNAi plants produced fewer LRs than wild-type plants, whereas plants overexpressing OsHDAC1 exhibited increased LR proliferation by promoting LR primordia formation. Brassinosteroid treatment increased the LR number, as did mutation of GSK3/SHAGGY-like kinase 2 (OsGSK2), whereas overexpression of OsGSK2 decreased the LR number. Importantly, OsHDAC1 could directly interact with and deacetylate OsGSK2, inhibiting its activity. OsGSK2 deacetylation attenuated the interaction between OsGSK2 and BRASSINAZOLE-RESISTANT 1 (OsBZR1), leading to accumulation of OsBZR1. The overexpression of OsBZR1 increased LR formation by regulating Auxin/IAA signaling genes. Taken together, the results indicate that OsHDAC1 regulates LR formation in rice by deactivating OsGSK2, thereby preventing degradation of OsBZR1, a positive regulator of LR primordia formation. Our findings suggest that OsHDAC1 is a breeding target in rice that can improve resource capture.

Transcriptome analysis reveals that cytokinins inhibit adventitious root formation through the MdRR12-MdCRF8 module in apple rootstock

Adventitious root (AR) formation is great significance for apple rootstock breeding. Transcriptome analyses were performed with cytokinins (CTKs) signal treatments to analyze the mechanism of AR formation. The results showed that 6-benzyadenine (6-BA) treatment inhibited AR formation. Histological analysis also observed that AR primordium cell formation was significantly suppressed by 6-BA treatment; the ratio of auxin/cytokinins exhibited the lowest values at 1 and 3 day (d) in the 6-BA treatment group. Furthermore, the differentially expressed genes were divided into five categories, including auxin, cytokinins, other hormones, cell cycle, and carbohydrate metabolism pathways. Due to the study of cytokinins signal treatment, it is important to understand the particular module mediated by the cytokinins pathway. The expression level of MdRR12 (a family member of B-type cytokinins-responsive factors) was significantly upregulated at 3 d by 6-BA treatment. Compared to the wild type, the 35S::MdRR12 transgenic tobaccos suppressed AR formation. The promoter sequence of MdCRF8 contains AGATT motif elements that respond to MdRR12. RNA-seq and RT-qPCR assays predicted cytokinins response factor (MdCRF8) to be a downstream gene regulated by MdRR12. The activity of the pro-MdCRF8-GUS promoter was obviously induced by 6-BA treatment and inhibited by lovastatin (Lov) treatment. Yeast one-hybrid, dual-luciferase reporter, and GUS coexpression assays revealed that MdRR12 could directly bind to the MdCRF8 promoter. Additionally, 35S::MdCRF8 transgenic tobaccos also blocked AR growth. Compared to the wild type, 35S::MdRR12 and 35S::MdCRF8 transgenic tobaccos enhanced sensitivity to cytokinins. Thus, we describe that MdRR12 and MdCRF8 function as integrators of cytokinins signals that affect cell cycle- and carbohydrate metabolism-related genes to regulate cell fate transition during AR formation. On the basis of these results, we concluded that the MdRR12-MdCRF8 module is involved in the negative regulation of AR formation in apple rootstock and can potentially be applied in agriculture using genetic approaches.

Gibberellins regulate lateral root development that is associated with auxin and cell wall metabolisms in cucumber

Cucumber is an economically important crop cultivated worldwide. Gibberellins (GAs) play important roles in the development of lateral roots (LRs), which are critical for plant stress tolerance and productivity. Therefore, it is of great importance for cucumber production to study the role of GAs in LR development. Here, the results showed that GAs regulated cucumber LR development in a concentration-dependent manner. Treatment with 1, 10, 50 and 100 μM GA₃ significantly increased secondary root length, tertiary root number and length. Of these, 50 μM GA₃ treatment had strong effects on increasing root dry weight and the root/shoot dry weight ratio. Pairwise comparisons identified 417 down-regulated genes enriched for GA metabolism-related processes and 447 up-regulated genes enriched for cell wall metabolism-related processes in GA₃-treated roots. A total of 3523 non-redundant DEGs were identified in our RNA-Seq data through pairwise comparisons and linear factorial modeling. Of these, most of the genes involved in auxin and cell wall metabolisms were up-regulated in GA₃-treated roots. Our findings not only shed light on LR regulation mediated by GA but also offer an important resource for functional studies of candidate genes putatively involved in the regulation of LR development in cucumber and other crops.

Molecular components associated with the regulation of flavonoid biosynthesis

Flavonoids exhibit amazing structural diversity and play different roles in plants. Besides, these compounds have been associated with several health benefits in humans. Several exogenous and endogenous cues, for example, light, temperature, nutrient status, and phytohormones have been reported as modulators of biosynthesis and accumulation of flavonoids. Thus, multiple hormones and stress-related signaling pathways are involved in the regulation of gene expression associated with this pathway. The transcriptional regulators belonging to the MYB and bHLH family transcription factors are well documented as the direct regulators of the structural genes associated with flavonoid biosynthesis. Recent studies also suggest that some of these factors are regulated by molecular components involved in stress and hormone signaling pathways. Adapter proteins for transcriptional activation or repression via recruitment of co-activators and co-repressors, respectively, E2 ubiquitin ligases, miRNA processing complex, and DNA methylation/demethylation factors have been recently discovered in various plants to play key roles in fine-tuning flavonoids synthesis. In the present review, we aim to provide comprehensive information about the role of different factors in the regulation of flavonoid biosynthesis. Besides, we describe the potential upstream regulators involved in the regulation of flavonoid biosynthesis within the context of available information. To sum up, the present review furnishes an updated account of signal transduction pathways modulating the biosynthesis of flavonoids.

Brassinosteroids are required for efficient root tip regeneration in Arabidopsis

Compared with other organisms, plants have an extraordinary capacity for self-repair. Even if the entire tissues, including the stem cells, are resected, most plant species are able to completely regenerate whole tissues. However, the mechanism by which plants efficiently regenerate the stem cell niche during tissue reorganization is still largely unknown. Here, we found that the signaling mediated by plant steroid hormones brassinosteroids is activated during stem cell formation after root tip excision in Arabidopsis. Treatment with brassinazole, an inhibitor of brassinosteroid biosynthesis, delayed the recovery of stem cell niche after root tip excision. Regeneration of root tip after resection was also delayed in a brassinosteroid receptor mutant. Therefore, we propose that brassinosteroids participate in efficient root tip regeneration, thereby enabling efficient tissue regeneration to ensure continuous root growth after resection.

Endophytic Bacillus subtilis TR21 Improves Banana Plant Resistance to Fusarium oxysporum f. sp. cubense and Promotes Root Growth by Upregulating the Jasmonate and Brassinosteroid Biosynthesis Pathways

The banana (Musa spp.) industry experiences dramatic annual losses from Fusarium wilt of banana disease, which is caused by the fungus Fusarium oxysporum f. sp. cubense (FOC). Pisang Awak banana 'Fenza No. 1' (Musa spp. cultivar Fenza No. 1), a major banana cultivar with high resistance to F. oxysporum f. sp. cubense race 4, is considered to be ideal for growth in problematic areas. However, 'Fenza No. 1' is still affected by F. oxysporum f. sp. cubense race 1 in the field. TR21 is an endophytic Bacillus subtilis strain isolated from orchids (Dendrobium sp.). Axillary spraying of banana plants with TR21 controls Fusarium wilt of banana, decreasing the growth period and increasing yields in the field. In this study, we established that TR21 increases root growth in different monocotyledonous plant species. By axillary inoculation, TR21 induced a similar transcriptomic change as that induced by F. oxysporum f. sp. cubense race 1 but also upregulated the biosynthetic pathways for the phytohormones brassinosteroid and jasmonic acid in 'Fenza No. 1' root tissues, indicating that TR21 increases Fusarium wilt of banana resistance, shortens growth period, and increases yield of banana by inducing specific transcriptional reprogramming and modulating phytohormone levels. These findings will contribute to the identification of candidate genes related to plant resistance against fungi in a nonmodel system and facilitate further study and exploitation of endophytic biocontrol agents.

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.

Transcriptomic and physiological analysis identifies a gene network module highly associated with brassinosteroid regulation in hybrid sweetgum tissues differing in the capability of somatic embryogenesis

Somatic embryogenesis is a preferred method for large-scale production of forest trees due to its high propagation efficiency. In this study, hybrid sweetgum leaves with phase changes from mature to embryogenic state were selected as experimental material to study somatic embryo initiation. Embryogenicity ranged from high to low, i.e. from 45%, 25%, and 12.5% to 0, with the samples of embryogenic callus (EC), whiten leaf edge (WLI), whiten leaf (WLII), and green leaf (GL) respectively. High correlations existed between embryogenicity and endogenous brassinosteroids (BRs) (r = 0.95, p < 0.05). Similarly, concentrations of endogenous BRs of the sample set correlated positively (r = 0.93, 0.99, 0.87, 0.99, 0.96 respectively, P < 0.05) to expression of somatic embryo (SE)-related genes, i.e. BBM, LEC2, ABI3, PLT2, and WOX2. Hierarchical cluster and weighted gene coexpression network analysis identified modules of coexpressed genes and network in 4820 differentially expressed genes (DEGs) from All-BR-Regulated Genes (ABRG). Moreover, exogenously-supplemented epiBR, together with 2,4-D and 6-BA, increased embryogenicity of GL-sourced callus, and expression of SE- and auxin-related genes, while brassinazole (BRZ), a BR biosynthesis inhibitor, reduced embryogenicity. Evidences obtained in this study revealed that BRs involved in phase change of leaf explants and may function in regulating gene expression and enhancing auxin effects. This study successfully established protocols for inducing somatic embryogenesis from leaf explants in hybrid sweetgum, which could facilitate the propagation process greatly, and provide theoretical basis for manipulating SE competence of explants in ornamental woody plants.

Integrated regulation of periclinal cell division by transcriptional module of BZR1-SHR in Arabidopsis roots

The timing and extent of cell division are crucial for the correct patterning of multicellular organism. In Arabidopsis, root ground tissue maturation involves the periclinal cell division of the endodermis to generate two cell layers: endodermis and middle cortex. However, the molecular mechanism underlying this pattern formation remains unclear. Here, we report that phytohormone brassinosteroid (BR) and redox signal hydrogen peroxide (H₂ O₂ ) interdependently promote periclinal division during root ground tissue maturation by regulating the activity of SHORT-ROOT (SHR), a master regulator of root growth and development. BR-activated transcription factor BRASSINAZOLE RESISTANT1 (BZR1) directly binds to the promoter of SHR to induce its expression, and physically interacts with SHR to increase the transcripts of RESPIRATORY BURST OXIDASE HOMOLOGs (RBOHs) and elevate the levels of H₂ O₂ , which feedback enhances the interaction between BZR1 and SHR. Additionally, genetic analysis shows that SHR is required for BZR1-promoted periclinal division, and BZR1 enhances the promoting effects of SHR on periclinal division. Together, our finding reveals that the transcriptional module of BZR1-SHR fine-tunes periclinal division during root ground tissue maturation in response to hormone and redox signals.

Coupled Development of Salt Glands, Stomata, and Pavement Cells in Limonium bicolor

Salt-resistant plants have different mechanisms to limit the deleterious effects of high salt in soil; for example, recretohalophytes secrete salt from unique structures called salt glands. Salt glands are the first differentiated epidermal structure of the recretohalophyte sea lavender (Limonium bicolor), followed by stomata and pavement cells. While salt glands and stomata develop prior to leaf expansion, it is not clear whether these steps are connected. Here, we explored the effects of the five phytohormones salicylic acid, brassinolide, methyl jasmonate, gibberellic acid, and abscisic acid on the development of the first expanded leaf of L. bicolor and its potential connection to salt gland, stomata, and pavement cell differentiation. We calculated the total number of salt glands, stomata, and pavement cells, as well as leaf area and pavement cell area, and assessed the correlations between these parameters. We detected strong and positive correlations between salt gland number and pavement cell area, between stomatal number and pavement cell area, and between salt gland number and stomatal number. We observed evidence of coupling between the development of salt glands, stomata, and pavement cells in L. bicolor, which lays the foundation for further investigation of the mechanism behind salt gland development.

Formation and Development of Taproots in Deciduous Tree Species

Trees are generally long-lived and are therefore exposed to numerous episodes of external stimuli and adverse environmental conditions. In certain trees e.g., oaks, taproots evolved to increase the tree's ability to acquire water from deeper soil layers. Despite the significant role of taproots, little is known about the growth regulation through internal factors (genes, phytohormones, and micro-RNAs), regulating taproot formation and growth, or the effect of external factors, e.g., drought. The interaction of internal and external stimuli, involving complex signaling pathways, regulates taproot growth during tip formation and the regulation of cell division in the root apical meristem (RAM). Assuming that the RAM is the primary regulatory center responsible for taproot growth, factors affecting the RAM function provide fundamental information on the mechanisms affecting taproot development.

Transcriptome analysis reveals the inhibitory nature of high nitrate during adventitious roots formation in the apple rootstock

In the process of vegetative propagation of apple rootstocks, the development of adventitious roots (ARs) has crucial importance. Nitrate is an essential nutrient necessary for plant growth; however, the inhibitory effect of high nitrate on ARs formation has not been explored. The physiological and molecular mechanisms underlying ARs inhibition were examined in this study. Stem cuttings of B9 apple rootstock were cultured on two nitrate treatments (T1 = 18.7 mM L^-1 and T2 = 37.5 mM L^-1 ), where T2 was identified as ARs inhibiting treatment. Morphological and anatomical observations advocating that high availability of nitrate inhibited AR formation by delaying the ARs initiation and emergence stages, where the root number was 287%, and the length was 604.6% lower than the T1 cuttings. Moreover, the contents of endogenous hormones were also elevated in response to T2 at most of the time points, which may cause a hormonal imbalance within the plant body and drive toward ARs inhibition. Furthermore, 3686 genes were differentially expressed by high-throughput sequencing. Out of these, 1797 genes were upregulated, and 1889 genes were downregulated. Approximately 238 genes related to nitrate, hormones, root development, and cell-cycle induction pathways were selected according to their potential to be involved in ARs regulation. This is the first study providing information regarding the inhibitory effect of high nitrate on ARs formation in apple rootstock.

Knockdown of Succinate Dehydrogenase Assembly Factor 2 Induces Reactive Oxygen Species-Mediated Auxin Hypersensitivity Causing pH-Dependent Root Elongation

Metabolism, auxin signaling and reactive oxygen species (ROS) all contribute to plant growth, and each is linked to plant mitochondria and the process of respiration. Knockdown of mitochondrial succinate dehydrogenase assembly factor 2 (SDHAF2) in Arabidopsis thaliana lowered succinate dehydrogenase activity and led to pH-inducible root inhibition when the growth medium pH was poised at different points between 7.0 and 5.0, but this phenomenon was not observed in wildtype (WT). Roots of sdhaf2 mutants showed high accumulation of succinate, depletion of citrate and malate and up-regulation of ROS-related and stress-inducible genes at pH 5.5. A change of oxidative status in sdhaf2 roots at low pH was also evidenced by low ROS staining in root tips and altered root sensitivity to H2O2. sdhaf2 had low auxin activity in root tips via DR5-GUS staining but displayed increased indole-3-acetic acid (IAA, auxin) abundance and IAA hypersensitivity, which is most likely caused by the change in ROS levels. On this basis, we conclude that knockdown of SDHAF2 induces pH-related root elongation and auxin hyperaccumulation and hypersensitivity, mediated by altered ROS homeostasis. This observation extends the existing evidence of associations between mitochondrial function and auxin by establishing a cascade of cellular events that link them through ROS formation, metabolism and root growth at different pH values.

Shaping the root system: the interplay between miRNA regulatory hubs and phytohormones

The root system commonly lies underground, where it provides anchorage for the aerial organs, as well as nutrients and water. Both endogenous and environmental cues contribute to the establishment of the root system. Among the endogenous cues, microRNAs (miRNAs), transcription factors, and phytohormones modulate root architecture. miRNAs belong to a subset of endogenous hairpin-derived small RNAs that post-transcriptionally control target gene expression, mostly transcription factors, comprising the miRNA regulatory hubs. Phytohormones are signaling molecules involved in most developmental processes. Some miRNAs and targets participate in more than one hormonal pathway, thereby providing new bridges in plant hormonal crosstalk. Unraveling the intricate network of molecular mechanisms underlying the establishment of root systems is a central aspect in the development of novel strategies for plant breeding to increase yield and optimize agricultural land use. In this review, we summarize recent findings describing the molecular mechanisms associated with the interplay between miRNA regulatory hubs and phytohormones to ensure the establishment of a proper root system. We focus on post-embryonic growth and development of primary, lateral, and adventitious roots. In addition, we discuss novel insights for future research on the interaction between miRNAs and phytohormones in root architecture.

Auxin Interactions with Other Hormones in Plant Development

Auxin is a crucial growth regulator that governs plant development and responses to environmental perturbations. It functions at the heart of many developmental processes, from embryogenesis to organ senescence, and is key to plant interactions with the environment, including responses to biotic and abiotic stimuli. As remarkable as auxin is, it does not act alone, but rather solicits the help of, or is solicited by, other endogenous signals, including the plant hormones abscisic acid, brassinosteroids, cytokinins, ethylene, gibberellic acid, jasmonates, salicylic acid, and strigolactones. The interactions between auxin and other hormones occur at multiple levels: hormones regulate one another's synthesis, transport, and/or response; hormone-specific transcriptional regulators for different pathways physically interact and/or converge on common target genes; etc. However, our understanding of this crosstalk is still fragmentary, with only a few pieces of the gigantic puzzle firmly established. In this review, we provide a glimpse into the complexity of hormone interactions that involve auxin, underscoring how patchy our current understanding is.

De novo transcriptome analysis provides insights into formation of in vitro adventitious root from leaf explants of Arnebia euchroma

BACKGROUND

Adventitious root formation is considered a major developmental step during the propagation of difficult to root plants, especially in horticultural crops. Recently, adventitious roots induced through plant tissue culture methods have also been used for production of phytochemicals such as flavonoids, anthocyanins and anthraquinones. It is rather well understood which horticultural species will easily form adventitious roots, but the factors affecting this process at molecular level or regulating the induction process in in vitro conditions are far less known. The present study was conducted to identify transcripts involved in in vitro induction and formation of adventitious roots using Arnebia euchroma leaves at different time points (intact leaf (control), 3 h, 12 h, 24 h, 3 d, 7 d, 10 d and 15 d). A. euchroma is an endangered medicinal Himalayan herb whose root contains red naphthoquinone pigments. These phytoconstituents are widely used as an herbal ingredient in Asian traditional medicine as well as natural colouring agent in food and cosmetics.

RESULTS

A total of 137.93 to 293.76 million raw reads were generated and assembled to 54,587 transcripts with average length of 1512.27 bps and N50 of 2193 bps, respectively. In addition, 50,107 differentially expressed genes were identified and found to be involved in plant hormone signal transduction, cell wall modification and wound induced mitogen activated protein kinase signalling. The data exhibited dominance of auxin responsive (AUXIN RESPONSE FACTOR8, IAA13, GRETCHEN HAGEN3.1) and sucrose translocation (BETA-31 FRUCTOFURANOSIDASE and MONOSACCHARIDE-SENSING protein1) genes during induction phase. In the initiation phase, the expression of LATERAL ORGAN BOUNDARIES DOMAIN16, EXPANSIN-B15, ENDOGLUCANASE25 and LEUCINE-rich repeat EXTENSION-like proteins was increased. During the expression phase, the same transcripts, with exception of LATERAL ORGAN BOUNDARIES DOMAIN16 were identified. Overall, the transcriptomic analysis revealed a similar patterns of genes, however, their expression level varied in subsequent phases of in vitro adventitious root formation in A. euchroma.

CONCLUSION

The results presented here will be helpful in understanding key regulators of in vitro adventitious root development in Arnebia species, which may be deployed in the future for phytochemical production at a commercial scale.

Transcriptome analysis reveals the promotive effect of potassium by hormones and sugar signaling pathways during adventitious roots formation in the apple rootstock

Apples are economically valuable and widely consumed fruits. The adventitious roots (ARs) formation is gridlock for apple trees mass propagation. The possible function of multiple hormones and sugar signaling pathways regulating ARs formation has not been completely understood in apple. In this study, B9 stem cuttings were treated with KCl treatment, where the highest root numbers (220) and maximum root length of 731.2 cm were noticed in KCl-treated cuttings, which were 98.2% and 215% higher than control cuttings. The content of endogenous hormones: IAA, ZR, JA, GA, and ABA were detected higher in response to KCl at most time-points. To figure out the molecular mechanisms underlying this effect, we investigated transcriptome analysis. In total, 4631 DEGs were determined, from which about 202 DEGs were considerably enriched in pathways associated with hormone signaling, sugar metabolism, root development, and cell cycle-related and were thereupon picked out on their potential involvements in ARs formation. Though, IAA accumulation and up-regulation of various genes contribute to induce AR formation. These results suggest that AR formation is a complex biological process in apple rootstocks, influenced mainly by the auxin signaling pathway and sugar metabolism.

Consequences of LED Lights on Root Morphological Traits and Compounds Accumulation in Sarcandra glabra Seedlings

This study evaluated the effects of different light spectra (white light; WL, blue light; BL and red light; RL) on the root morphological traits and metabolites accumulation and biosynthesis in Sarcandra glabra. We performed transcriptomic and metabolomic profiling by RNA-seq and ultra-performance liquid chromatography-electrospray ionization-tandem mass spectrometry (UPLC-ESI-MS/MS), respectively. When morphological features were compared to WL, BL substantially increased under-ground fresh weight, root length, root surface area, and root volume, while RL inhibited these indices. A total of 433 metabolites were identified, of which 40, 18, and 68 compounds differentially accumulated in roots under WL (WG) vs. roots under BL (BG), WG vs. roots under RL (RG), and RG vs. BG, respectively. In addition, the contents of sinapyl alcohol, sinapic acid, fraxetin, and 6-methylcoumarin decreased significantly in BG and RG. In contrast, chlorogenic acid, rosmarinyl glucoside, quercitrin and quercetin were increased considerably in BG. Furthermore, the contents of eight terpenoids compounds significantly reduced in BG. Following transcriptomic profiling, several key genes related to biosynthesis of phenylpropanoid-derived and terpenoids metabolites were differentially expressed, such as caffeic acid 3-O-methyltransferase) (COMT), hydroxycinnamoyl-CoA shikimate hydroxycinnamoyl transferase (HCT), O-methyltransferase (OMT), and 1-deoxy-D-xylulose-5-phosphate synthetase (DXS). In summary, our findings showed that BL was suitable for growth and accumulation of bioactive metabolites in root tissue of S. glabra. Exposure to a higher ratio of BL might have the potential to improve the production and quality of S. glabra seedlings, but this needs to be confirmed further.

Root-System Architectures of Two Cuban Rice Cultivars with Salt Stress at Early Development Stages

Soil salinity is a critical problem for rice production and is also often associated with phosphors (P) deficiency. Plant hormones, like brassinosteroids, were shown to play a role in plant responses to different stresses and are also expected to mitigate salt stress. The aim of this study was to compare shoot growth and root architecture traits of two rice cultivars (INCA LP-5 and Perla de Cuba) during early plant development in response to salt, P limitation and a brassinosteroid. Seeds were placed in (I) paper rolls for 7 days and (II) mini-rhizotrons for 21 days without or with salt (50 mM NaCl), without or with 24-epibrassinolide (10⁻⁶ M) pre-treatment, and with two levels of P (10 or 1 ppm). The root system of LP-5 was larger in size and extent, while the roots of Perla were growing denser. Salt affected mainly the size- and extent-related root characteristics and explained about 70% of the variance. The effect of P was more pronounced without salt treatment. In Perla, P supply reduced the salt effect on root growth. The brassinosteroid had hardly any effect on the development of the plants in both experiments. Due to the high dependence on experimental factors, root length and related traits can be recommended for selecting young rice cultivars regarding salt stress and P deprivation.