Involvement of brassinosteroids in the gravitropic response of primary root of maize

Molecular mechanism of brassinosteroids involved in root gravity response based on transcriptome analysis

BACKGROUND

Brassinosteroids (BRs) are a class of phytohormones that regulate a wide range of developmental processes in plants. BR-associated mutants display impaired growth and response to developmental and environmental stimuli.

RESULTS

Here, we found that a BR-deficient mutant det2-1 displayed abnormal root gravitropic growth in Arabidopsis, which was not present in other BR mutants. To further elucidate the role of DET2 in gravity, we performed transcriptome sequencing and analysis of det2-1 and bri1-116, bri1 null mutant allele. Expression levels of auxin, gibberellin, cytokinin, and other related genes in the two mutants of det2-1 and bri1-116 were basically the same. However, we only found that a large number of JAZ (JASMONATE ZIM-domain) genes and jasmonate synthesis-related genes were upregulated in det2-1 mutant, suggesting increased levels of endogenous JA.

CONCLUSIONS

Our results also suggested that DET2 not only plays a role in BR synthesis but may also be involved in JA regulation. Our study provides a new insight into the molecular mechanism of BRs on the root gravitropism.

Transcriptome and hormone metabolome reveal the mechanism of stem bending in water lily (Nymphaea tetragona) cut-flowers

Water lilies are popular ornamental cut-flowers with significant economic and cultural value. However, stem bending affects the preservation of cut-flowers during their vase life. To gain further insights into the molecular mechanisms of stem bending, transcriptome profiling, hormone measurement, and morphological analysis were performed using the stems of the 'Blue Bird' water lily. Transcriptome analysis revealed that 607 differentially expressed genes (DEGs) were associated with the dorsal and ventral stems of the water lily, of which 247 were up-regulated and 360 were down-regulated. Significant differences in genes associated with plant hormones, calcium ions, glucose metabolism, and photosynthesis pathways genes involved in the dorsal and ventral areas of the curved stem. In particular, DEGs were associated with the hormone synthesis, gravity response, starch granules, Ca2+ ions, and photosynthesis. The results of qRT-PCR were consistent with that of the transcriptome sequence analysis. A total of 12 hormones were detected, of which abscisic acid, indole-3-carboxaldehyde, indole-3-carboxaldehyde and jasmonic acid were significantly differentially expressed in the dorsal and ventral stems, and were significantly higher in the dorsal stem than in the ventral stem. The cell morphology in the dorsal and ventral areas of the curved stem clearly changed during vase life. The direction of starch granule settlement was consistent with the bending direction of the water lily stem, as well as the direction of gravity. In conclusion, stem bending in water lily cut-flowers is regulated by multiple factors and genes. This study provides an important theoretical basis for understanding the complex regulatory mechanism of water lily stem bending.

Transcriptomic analysis of Chinese yam (Dioscorea polystachya Turcz.) variants indicates brassinosteroid involvement in tuber development

Dioscorea is an important but underutilized genus of flowering plants that grows predominantly in tropical and subtropical regions. Several species, known as yam, develop large underground tubers and aerial bulbils that are used as food. The Chinese yam (D. polystachya Turcz.) is one of the few Dioscorea species that grows well in temperate regions and has been proposed as a climate-resilient crop to enhance food security in Europe. However, the fragile, club-like tubers are unsuitable for mechanical harvesting, which is facilitated by shorter and thicker storage organs. Brassinosteroids (BRs) play a key role in plant cell division, cell elongation and proliferation, as well as in the gravitropic response. We collected RNA-Seq data from the head, middle and tip of two tuber shape variants: F60 (long, thin) and F2000 (short, thick). Comparative transcriptome analysis of F60 vs. F2000 revealed 30,229 differentially expressed genes (DEGs), 1,393 of which were differentially expressed in the growing tip. Several DEGs are involved in steroid/BR biosynthesis or signaling, or may be regulated by BRs. The quantification of endogenous BRs revealed higher levels of castasterone (CS), 28-norCS, 28-homoCS and brassinolide in F2000 compared to F60 tubers. The highest BR levels were detected in the growing tip, and CS was the most abundant (439.6 ± 196.41 pmol/g in F2000 and 365.6 ± 112.78 pmol/g in F60). Exogenous 24-epi-brassinolide (epi-BL) treatment (20 nM) in an aeroponic system significantly increased the width-to-length ratio (0.045 ± 0.002) compared to the mock-treated plants (0.03 ± 0.002) after 7 weeks, indicating that exogenous epi-BL produces shorter and thicker tubers. In this study we demonstrate the role of BRs in D. polystachya tuber shape, providing insight into the role of plant hormones in yam storage organ development. We found that BRs can influence tuber shape in Chinese yam by regulating the expression of genes involved cell expansion. Our data can help to improve the efficiency of Chinese yam cultivation, which could provide an alternative food source and thus contribute to future food security in Europe.

The nitrification inhibitor 1,9-decanediol from rice roots promotes root growth in Arabidopsis through involvement of ABA and PIN2-mediated auxin signaling

1,9-decanediol (1,9-D) is a biological nitrification inhibitor secreted in roots, which effectively inhibits soil nitrifier activity and reduces nitrogen loss from agricultural fields. However, the effects of 1,9-D on plant root growth and the involvement of signaling pathways in the plant response to 1,9-D have not been investigated. Here, we report that 1,9-D, in the 100-400 μM concentration range, promotes primary root length in Arabidopsis seedlings at 3d and 5d, by 10.1%-33.3% and 6.9%-32.6%, and, in a range of 50-200 μM, leads to an increase in the number of lateral roots. 150 μM 1,9-D was found optimum for the positive regulation of root growth. qRT-PCR analysis reveals that 1,9-D can significantly increase AtABA3 gene expression and that a mutation in ABA3 results in insensitivity of root growth to 1,9-D. Moreover, through pharmacological experiments, we show that exogenous addition of ABA (abscisic acid) with 1,9-D enhances primary root length by 23.5%-63.3%, and an exogenous supply of 1,9-D with the ABA inhibitor Flu reduces primary root length by 1.0%-14.3%. Primary root length of the pin2/eir1-1 is shown to be insensitive to both exogenous addition of 1,9-D and ABA, indicating that the auxin carrier PIN2/EIR1 is involved in promotion of root growth by 1,9-D. These results suggest a novel for 1,9-D in regulating plant root growth through ABA and auxin signaling.

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.

Effects of Exogenous α-Naphthaleneacetic Acid and 24-Epibrassinolide on Fruit Size and Assimilate Metabolism-Related Sugars and Enzyme Activities in Giant Pumpkin

Size is the most important quality attribute of giant pumpkin fruit. Different concentrations and application frequencies of α-naphthaleneacetic acid (NAA) and 24-epibrassinolide (EBR) were sprayed on the leaves and fruits of giant pumpkin at different growth stages to determine their effects and the mechanism responsible for fruit size increase. NAA+EBR application improved source strength, and further analysis indicated that NAA+EBR markedly boosted net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr) and the expression level and activity of galactitol synthetase (GolS), raffinose synthetase (RS), and stachyose synthetase (STS), resulting in an increase in the synthesis of photoassimilate, especially stachyose. Concomitantly, NAA+EBR spray increased stachyose and sucrose contents throughout pumpkin fruit growth and the concentrations of glucose and fructose at 0 and 20 days post-anthesis (DPA) in peduncle phloem sap, implying that such treatment improved the efficiency of assimilate transport from the peduncle to the fruit. Furthermore, it improved the expression and activity of alkaline α-galactosidase (AGA), facilitating assimilate unloading, providing carbon skeletons and energy for fruit growth, and increasing fruit weight by more than 44.1%. Therefore, exogenous NAA and EBR increased source capacity, transportation efficiency, and sink strength, overall promoting the synthesis and distribution of photoassimilate, ultimately increasing fruit size.

Inhibition of the Glycogen Synthase Kinase 3 Family by the Bikinin Alleviates the Long-Term Effects of Salinity in Barley

Crops grown under stress conditions show restricted growth and, eventually, reduced yield. Among others, brassinosteroids (BRs) mitigate the effects of stress and improve plant growth. We used two barley cultivars with differing sensitivities to BRs, as determined by the lamina joint inclination test. Barley plants with the 2nd unfolded leaf were sprayed with a diluted series of bikinin, an inhibitor of the Glycogen Synthase Kinase 3 (GSK3) family, which controls the BR signaling pathway. Barley was grown under salt stress conditions up to the start of the 5th leaf growth stage. The phenotypical, molecular, and physiological changes were determined. Our results indicate that the salt tolerance of barley depends on its sensitivity to BRs. We confirmed that barley treatment with bikinin reduced the level of the phosphorylated form of HvBZR1, the activity of which is regulated by GSK3. The use of two barley varieties with different responses to salinity led to the identification of the role of BR signaling in photosynthesis activity. These results suggest that salinity reduces the expression of the genes controlling the BR signaling pathway. Moreover, the results also suggest that the functional analysis of the GSK3 family in stress responses can be a tool for plant breeding in order to improve crops’ resistance to salinity or to other stresses.

Excavation of Genes Responsive to Brassinosteroids by Transcriptome Sequencing in Adiantum flabellulatum Gametophytes

Brassinosteroids (BRs) are a class of polyhydroxysteroid plant hormones; they play important roles in the development and stress resistance of plants. The research on BRs has mainly been carried out in angiosperms, but in ferns-research is still limited to the physiological level and is not in-depth. In this study, Adiantum flabellulatum gametophytes were used as materials and treated with 10^-6 M brassinolide (BL). The differentially expressed genes (DEGs) responsive to BRs were identified by transcriptome sequencing, GO, KEGG analysis, as well as a quantitative real-time polymerase chain reaction. From this, a total of 8394 DEGs were screened. We found that the expressions of photosynthetic genes were widely inhibited by high concentrations of BL in A. flabellulatum gametophytes. Moreover, we detected many BR synthase genes, except BR6ox2, which may be why castasterone (CS) rather than BL was detected in ferns. Additionally, we identified (for the first time) that the expressions of BR synthase genes (CYP90B1, CYP90C1, CYP90D1, CPD, and BR6ox1) were negatively regulated by BL in fern gametophytes, which indicated that ferns, including gametophytes, also needed the regulatory mechanism for maintaining BR homeostasis. Based on transcriptome sequencing, this study can provide a large number of gene expression data for BRs regulating the development of fern gametophytes.

Low Concentration of Anti-Auxin and Anti-Fungal Agent Accelerates the PLB Regeneration of Dendrobium okinawense under Green LED

Dendrobium okinawense is an endangered epiphytic orchid, and there has been no scientific report so far on its propagation. Protocorm is a mass of cells, and protocorm-like bodies (PLBs) are lookalike protocorms produced by vegetative explants in vitro. Regeneration of PLBs is a widely used technique for orchid micropropagation. We used different light-emitting diodes (LEDs) for the PLB regeneration of D. okinawense. The number of PLBs and fresh weight were increased by 81.1% and 80.8%, respectively, under green LED over the white fluorescent (WF) light. We added different concentrations of PCIB (p-Chlorophenoxyisobutyric acid, an anti-auxin) and HMI (3-Hydroxy-5-methyl isoxazole, an anti-fungal agent) in culture media. The number of PLBs was increased in media having 0.01 mg/L of PCIB (35.9%) compared to control (no PCIB), whereas 19.3% increased in media having 0.01 mL/L of HMI compared to control (no HMI). Green LED in combination with 0.01 mg/L of PCIB significantly increased the number of PLBs (69.0%) compared to the WF–without PCIB combination. In LEDs-PCIB and LED-HMI combinations, HMI did not show better PLBs regeneration compared with PCIB. The results suggested that a combination of low concentrations of PCIB and green LED have the potential to accelerate PLB regeneration of D. okinawense.

Brassinazole Resistant 1 Activity Is Organ-Specific and Genotype-Dependent in Barley Seedlings

Brassinosteroids (BRs) control many plant developmental processes by regulating different groups of transcription factors, and consequently gene expressions. The most known is BZR1, the main member of the BES1 family. However, to date, it is poorly characterized in crop species. The main goal of the presented study was to identify HvBZR1 and determine its activity in 5-day-old barley (the stage is related to one leaf on the main shoot and a few seminal roots) using two cultivars with different sensitivities to BRs. Using the anti-OsBZR1 antibody, we identified the forms of HvBZR1 transcription factor with different molecular weights, which can be related to different phosphorylated forms of serine/threonine residues. Two phosphorylated forms in the shoots and one dephosphorylated form in the roots were determined. A minor amount of the dephosphorylated form of the HvBZR1 in the Haruna Nijo shoots was also found. The phosphorylated forms gave a higher band intensity for Golden Promise than Haruna Nijo. The bands were similar in their intensity, when two different phosphorylated forms were compared in Golden Promise, while a reduced intensity was detected for the phosphorylated form with a lower molecular weight for Haruna Nijo. Degradation of the phosphorylated forms in the shoots (complete degradation in Golden Promise and significant but not complete in Haruna Nijo) and the presence of the dephosphorylated form in the roots were proven for the etiolated barley. In the case of Haruna Nijo, a wider range of the regulators of the BR biosynthesis and signaling pathways induced the expected effects, 24-EBL (0.001 µM) and bikinin (10 and 50 µM) caused low amount of the phosphorylated forms, and at the same time, a tiny band of dephosphorylated form was detected. However, the expression of genes related to the BR biosynthesis and signaling pathways was not a determinant for the protein amount.

Genome-wide identification of Gramineae histone modification genes and their potential roles in regulating wheat and maize growth and stress responses

BACKGROUND

In plants, histone modification (HM) genes participate in various developmental and defense processes. Gramineae plants (e.g., Triticum aestivum, Hordeum vulgare, Sorghum bicolor, Setaria italica, Setaria viridis, and Zea mays) are important crop species worldwide. However, little information on HM genes is in Gramineae species.

RESULTS

Here, we identified 245 TaHMs, 72 HvHMs, 84 SbHMs, 93 SvHMs, 90 SiHMs, and 90 ZmHMs in the above six Gramineae species, respectively. Detailed information on their chromosome locations, conserved domains, phylogenetic trees, synteny, promoter elements, and gene structures were determined. Among the HMs, most motifs were conserved, but several unique motifs were also identified. Our results also suggested that gene and genome duplications potentially impacted the evolution and expansion of HMs in wheat. The number of orthologous gene pairs between rice (Oryza sativa) and each Gramineae species was much greater than that between Arabidopsis and each Gramineae species, indicating that the dicotyledons shared common ancestors. Moreover, all identified HM gene pairs likely underwent purifying selection based on to their non-synonymous (Ka)/synonymous (Ks) nucleotide substitutions. Using published transcriptome data, changes in TaHM gene expression in developing wheat grains treated with brassinosteroid, brassinazole, or activated charcoal were investigated. In addition, the transcription models of ZmHMs in developing maize seeds and after gibberellin treatment were also identified. We also examined plant stress responses and found that heat, drought, salt, insect feeding, nitrogen, and cadmium stress influenced many TaHMs, and drought altered the expression of several ZmHMs. Thus, these findings indicate their important functions in plant growth and stress adaptations.

CONCLUSIONS

Based on a comprehensive analysis of Gramineae HMs, we found that TaHMs play potential roles in grain development, brassinosteroid- and brassinazole-mediated root growth, activated charcoal-mediated root and leaf growth, and biotic and abiotic adaptations. Furthermore, ZmHMs likely participate in seed development, gibberellin-mediated leaf growth, and drought adaptation.

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.

Utilizing MIKC-type MADS-box protein SOC1 for yield potential enhancement in maize

Overexpression of Zea mays SOC gene promotes flowering, reduces plant height, and leads to no reduction in grain production per plant, suggesting enhanced yield potential, at least, through increasing planting density. MIKC-type MADS-box gene SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) is an integrator conserved in the plant flowering pathway. In this study, the maize SOC1 (ZmSOC1) gene was cloned and overexpressed in transgenic maize Hi-II genotype. The T₀ plants were backcrossed with nontransgenic inbred B73 to produce first generation backcross (BC₁) seeds. Phenotyping of both transgenic and null segregant (NT) BC₁ plants was conducted in three independent experiments. The BC₁ transgenic plants showed new attributes such as increased vegetative growth, accelerated flowering time, reduced overall plant height, and increased grain weight. Second generation backcross (BC₂) plants were evaluated in the field using two planting densities. Compared to BC₂ NT plants, BC₂ transgenic plants, were 12-18% shorter, flowered 5 days earlier, and showed no reduction in grain production per plant and an increase in fat, starch, and simple sugars in the grain. Transcriptome comparison in young leaves of 56-day-old BC₁ plants revealed that the overexpressed ZmSOC1 resulted in 107 differentially expressed genes. The upregulated transcription factor DNA BINDING WITH ONE FINGER 5.4 (DOF5.4) was among the genes responsible for the reduced plant height. Modulating expression of SOC1 opens a new and effective approach to promote flowering and reduce plant height, which may have potential to enhance crop yield and improve grain quality.

Understanding the Intricate Web of Phytohormone Signalling in Modulating Root System Architecture

Root system architecture (RSA) is an important developmental and agronomic trait that is regulated by various physical factors such as nutrients, water, microbes, gravity, and soil compaction as well as hormone-mediated pathways. Phytohormones act as internal mediators between soil and RSA to influence various events of root development, starting from organogenesis to the formation of higher order lateral roots (LRs) through diverse mechanisms. Apart from interaction with the external cues, root development also relies on the complex web of interaction among phytohormones to exhibit synergistic or antagonistic effects to improve crop performance. However, there are considerable gaps in understanding the interaction of these hormonal networks during various aspects of root development. In this review, we elucidate the role of different hormones to modulate a common phenotypic output, such as RSA in Arabidopsis and crop plants, and discuss future perspectives to channel vast information on root development to modulate RSA components.

State-of-the-Art Technologies for Understanding Brassinosteroid Signaling Networks

Brassinosteroids, the steroid hormones of plants, control physiological and developmental processes through its signaling pathway. The major brassinosteroid signaling network components, from the receptor to transcription factors, have been identified in the past two decades. The development of biotechnologies has driven the identification of novel brassinosteroid signaling components, even revealing several crosstalks between brassinosteroid and other plant signaling pathways. Herein, we would like to summarize the identification and improvement of several representative brassinosteroid signaling components through the development of new technologies, including brassinosteroid-insensitive 1 (BRI1), BRI1-associated kinase 1 (BAK1), BR-insensitive 2 (BIN2), BRI1 kinase inhibitor 1 (BKI1), BRI1-suppressor 1 (BSU1), BR signaling kinases (BSKs), BRI1 ethyl methanesulfonate suppressor 1 (BES1), and brassinazole resistant 1 (BZR1). Furthermore, improvement of BR signaling knowledge, such as the function of BKI1, BES1 and its homologous through clustered regularly interspaced short palindromic repeats (CRISPR), the regulation of BIN2 through single-molecule methods, and the new in vivo interactors of BIN2 identified by proximity labeling are described. Among these technologies, recent advanced methods proximity labeling and single-molecule methods will be reviewed in detail to provide insights to brassinosteroid and other phytohormone signaling pathway studies.

Brassinosteroids Inhibit Autotropic Root Straightening by Modifying Filamentous-Actin Organization and Dynamics

When positioned horizontally, roots grow down toward the direction of gravity. This phenomenon, called gravitropism, is influenced by most of the major plant hormones including brassinosteroids. Epi-brassinolide (eBL) was previously shown to enhance root gravitropism, a phenomenon similar to the response of roots exposed to the actin inhibitor, latrunculin B (LatB). This led us to hypothesize that eBL might enhance root gravitropism through its effects on filamentous-actin (F-actin). This hypothesis was tested by comparing gravitropic responses of maize (Zea mays) roots treated with eBL or LatB. LatB- and eBL-treated roots displayed similar enhanced downward growth compared with controls when vertical roots were oriented horizontally. Moreover, the effects of the two compounds on root growth directionality were more striking on a slowly-rotating two-dimensional clinostat. Both compounds inhibited autotropism, a process in which the root straightened after the initial gravistimulus was withdrawn by clinorotation. Although eBL reduced F-actin density in chemically-fixed Z. mays roots, the impact was not as strong as that of LatB. Modification of F-actin organization after treatment with both compounds was also observed in living roots of barrel medic (Medicago truncatula) seedlings expressing genetically encoded F-actin reporters. Like in fixed Z. mays roots, eBL effects on F-actin in living M. truncatula roots were modest compared with those of LatB. Furthermore, live cell imaging revealed a decrease in global F-actin dynamics in hypocotyls of etiolated M. truncatula seedlings treated with eBL compared to controls. Collectively, our data indicate that eBL-and LatB-induced enhancement of root gravitropism can be explained by inhibited autotropic root straightening, and that eBL affects this process, in part, by modifying F-actin organization and dynamics.

Brassinosteroid signaling delimits root gravitropism via sorting of the Arabidopsis PIN2 auxin transporter

Arabidopsis PIN2 protein directs transport of the phytohormone auxin from the root tip into the root elongation zone. Variation in hormone transport, which depends on a delicate interplay between PIN2 sorting to and from polar plasma membrane domains, determines root growth. By employing a constitutively degraded version of PIN2, we identify brassinolides as antagonists of PIN2 endocytosis. This response does not require de novo protein synthesis, but involves early events in canonical brassinolide signaling. Brassinolide-controlled adjustments in PIN2 sorting and intracellular distribution governs formation of a lateral PIN2 gradient in gravistimulated roots, coinciding with adjustments in auxin signaling and directional root growth. Strikingly, simulations indicate that PIN2 gradient formation is no prerequisite for root bending but rather dampens asymmetric auxin flow and signaling. Crosstalk between brassinolide signaling and endocytic PIN2 sorting, thus, appears essential for determining the rate of gravity-induced root curvature via attenuation of differential cell elongation.

Brassinosteroid Regulates Root Development with Highly Redundant Genes in Hexaploid Wheat

Brassinosteroid (BR) plays an important role in plant development and biotic and abiotic stress tolerance, but its specific function remains largely unknown in wheat (Triticum aestivum L.), preventing its utilization in this important crop. In this study, the function of BR and its underlying cytological role in wheat root development were comprehensively investigated. Our findings demonstrated that BR has a conserved function in regulating root length in wheat, and novel roles in regulating lateral root emergence and root diameter were uncovered. Analyses of BR homologous gene composition and evolutionary divergence demonstrated that the genetic framework of the wheat BR pathway was close to that of rice, but contained highly redundant homologous copies of genes from the subgenome A, B and D. These homologous copies showed active expression and shared a conserved BR response. The expression of wheat DWF4 and glycogen synthase kinase (GSK) genes in Arabidopsis confirmed that multiple homologous copies maintained their conserved function in regulating root development, highlighting their redundant status and indicating that a special challenge exists in wheat gene modification to deal with this high redundancy. However, our results suggested that the hypermorphic effect of T. aestivum GSK (TaGSK) genes with point mutations may be an effective approach to overcome this redundancy in the manipulation of BR signaling in wheat. Our study provides fundamental data uncovering the function of BR in wheat root development, the underlying genetic basis and a possible strategy to manipulate BR signaling in hexaploid wheat.

BR deficiency causes increased sensitivity to drought and yield penalty in cotton

BACKGROUND

Brassinosteroids (BRs) play crucial roles in drought tolerance, but the underlying molecular mechanisms remain unclear in the important oilseed and fiber crop, cotton (Gossypium hirsutum L.).

RESULTS

To elucidate how BRs mediate drought tolerance in cotton, a cotton brassinosteroid (BR)-deficient mutant, pag1 (pagoda1), was employed for analysis. Importantly, the pag1 mutant showed increased sensitivity to drought stress, with shorter primary roots and fewer lateral roots. The number of stomata was significantly increased in the mutant, and the stomata aperture was much wider than that of the control plants. These mutant plants therefore showed an increased water loss rate. Furthermore, the abscisic acid (ABA) content, photosynthetic efficiency and starch content of the mutant were significantly lower than those of the wild type. The overall performance of the mutant plants was worse than that of the wild-type control under both normal and drought conditions. Moreover, Proteomic analysis revealed reduced levels of stress-related proteins in pag1 plants.

CONCLUSIONS

These results suggest that BRs may modulate the drought tolerance of cotton by regulating much genes that related to drought stress and multiple organ responses to drought, including root growth, stomata development, the stomata aperture and photosynthesis. This study provides an important basis for understanding drought resistance regulated by BRs and cultivating drought-resistant cotton lines.

The maize lilliputian1 (lil1) gene, encoding a brassinosteroid cytochrome P450 C-6 oxidase, is involved in plant growth and drought response

BACKGROUND AND AIMS

Brassinosteroids (BRs) are plant hormones involved in many developmental processes as well as in plant-environment interactions. Their role was investigated in this study through the analysis of lilliputian1-1 (lil1-1), a dwarf mutant impaired in BR biosynthesis in maize (Zea mays).

METHODS

We isolated lil1-1 through transposon tagging in maize. The action of lil1 was investigated through morphological and genetic analysis. Moreover, by comparing lil1-1 mutant and wild-type individuals grown under drought stress, the effect of BR reduction on the response to drought stress was examined.

KEY RESULTS

lil1-1 is a novel allele of the brassinosteroid-deficient dwarf1 (brd1) gene, encoding a brassinosteroid C-6 oxidase. We show in this study that lil1 is epistatic to nana plant1 (na1), a BR gene involved in earlier steps of the pathway. The lill-1 mutation causes alteration in the root gravitropic response, leaf epidermal cell density, epicuticular wax deposition and seedling adaptation to water scarcity conditions.

CONCLUSIONS

Lack of active BR molecules in maize causes a pleiotropic effect on plant development and improves seedling tolerance of drought. BR-deficient maize mutants can thus be instrumental in unravelling novel mechanisms on which plant adaptations to abiotic stress are based.

Striking the Right Chord: Signaling Enigma during Root Gravitropism

Plants being sessile can often be judged as passive acceptors of their environment. However, plants are actually even more active in responding to the factors from their surroundings. Plants do not have eyes, ears or vestibular system like animals, still they "know" which way is up and which way is down? This is facilitated by receptor molecules within plant which perceive changes in internal and external conditions such as light, touch, obstacles; and initiate signaling pathways that enable the plant to react. Plant responses that involve a definite and specific movement are called "tropic" responses. Perhaps the best known and studied tropisms are phototropism, i.e., response to light, and geotropism, i.e., response to gravity. A robust root system is vital for plant growth as it can provide physical anchorage to soil as well as absorb water, nutrients and essential minerals from soil efficiently. Gravitropic responses of both primary as well as lateral root thus become critical for plant growth and development. The molecular mechanisms of root gravitropism has been delved intensively, however, the mechanism behind how the potential energy of gravity stimulus converts into a biochemical signal in vascular plants is still unknown, due to which gravity sensing in plants still remains one of the most fascinating questions in molecular biology. Communications within plants occur through phytohormones and other chemical substances produced in plants which have a developmental or physiological effect on growth. Here, we review current knowledge of various intrinsic signaling mechanisms that modulate root gravitropism in order to point out the questions and emerging developments in plant directional growth responses. We are also discussing the roles of sugar signals and their interaction with phytohormone machinery, specifically in context of root directional responses.

Auxin transport and response requirements for root hydrotropism differ between plant species

The direction of auxin transport changes in gravistimulated roots, causing auxin accumulation in the lower side of horizontally reoriented roots. This study found that auxin was similarly involved in hydrotropism and gravitropism in rice and pea roots, but hydrotropism in Lotus japonicus roots was independent of both auxin transport and response. Application of either auxin transport inhibitors or an auxin response inhibitor decreased both hydrotropism and gravitropism in rice roots, and reduced hydrotropism in pea roots. However, Lotus roots treated with these inhibitors showed reduced gravitropism but an unaltered or an enhanced hydrotropic response. Inhibiting auxin biosynthesis substantially reduced both tropisms in rice and Lotus roots. Removing the final 0.2 mm (including the root cap) from the root tip inhibited gravitropism but not hydrotropism in rice seedling roots. These results suggested that modes of auxin involvement in hydrotropism differed between plant species. In rice roots, although auxin transport and responses were required for both gravitropism and hydrotropism, the root cap was involved in the auxin regulation of gravitropism but not hydrotropism. Hydrotropism in Lotus roots, however, may be regulated by a novel mechanism that is independent of both auxin transport and the TIR1/AFBs auxin response pathway.

ARF7 increases the endogenous contents of castasterone through suppression of BAS1 expression in Arabidopsis thaliana

Homeostasis of brassinosteroids (BRs) maintained by the balance between their biosynthesis and inactivation is important to coordinate the diverse physiological and developmental responses of plants. Although BR signaling regulates the endogenous levels of BRs via negative feedback regulation, it remains largely unknown how the biosynthesis and inactivation of BR are triggered. BAS1 encodes CYP734A1, which inactivates the biologically active BRs via C-26 hydroxylation and is down-regulated by a BR-responsive transcription factor, BZR1. Here it is demonstrated that the expression of the BAS1 gene is regulated by auxin response factors (ARFs) in Arabidopsis thaliana. Two successive E-box motifs on the BAS1 promoter function as BZR1 binding sites and are essential for BR-regulated BAS1 expression. The expression of BAS1 is increased in the arf7 and arf7arf19 mutants. The endogenous level of bioactive BR, castasterone, is greatly decreased in those mutants. ARF7 can bind to the E-box motifs of the BAS1 promoter where BZR1 binds, suggesting that ARF7 and BZR1 mutually compete for the same cis-element of the BAS1 promoter. Additionally, ARF7 directly interacts with BZR1, which inhibits their DNA binding activities and regulation of BAS1 expression. In conclusion, auxin signaling via ARF7 directly modulates the expression of BAS1 by competition with BZR1, thereby increasing the level of castasterone and promoting growth and development in A. thaliana.

Brassinosteroids Regulate Root Growth, Development, and Symbiosis

Brassinosteroids (BRs) are natural plant hormones critical for growth and development. BR deficient or signaling mutants show significantly shortened root phenotypes. However, for a long time, it was thought that these phenotypes were solely caused by reduced cell elongation in the mutant roots. Functions of BRs in regulating root development have been largely neglected. Nonetheless, recent detailed analyses, revealed that BRs are not only involved in root cell elongation but are also involved in many aspects of root development, such as maintenance of meristem size, root hair formation, lateral root initiation, gravitropic response, mycorrhiza formation, and nodulation in legume species. In this review, current findings on the functions of BRs in mediating root growth, development, and symbiosis are discussed.

Glucose control of root growth direction in Arabidopsis thaliana

Directional growth of roots is a complex process that is modulated by various environmental signals. This work shows that presence of glucose (Glc) in the medium also extensively modulated seedling root growth direction. Glc modulation of root growth direction was dramatically enhanced by simultaneous brassinosteroid (BR) application. Glc enhanced BR receptor BRASSINOSTEROID INSENSITIVE1 (BRI1) endocytosis from plasma membrane to early endosomes. Glc-induced root deviation was highly enhanced in a PP2A-defective mutant, roots curl in naphthyl phthalamic acid 1-1 (rcn1-1) suggesting that there is a role of phosphatase in Glc-induced root-growth deviation. RCN1, therefore, acted as a link between Glc and the BR-signalling pathway. Polar auxin transport worked further downstream to BR in controlling Glc-induced root deviation response. Glc also affected other root directional responses such as root waving and coiling leading to altered root architecture. High light intensity mimicked the Glc-induced changes in root architecture that were highly reduced in Glc-signalling mutants. Thus, under natural environmental conditions, changing light flux in the environment may lead to enhanced Glc production/response and is a way to manipulate root architecture for optimized development via integrating several extrinsic and intrinsic signalling cues.

Cereal grass pulvini: agronomically significant models for studying gravitropism signaling and tissue polarity

Cereal grass pulvini have emerged as model systems that are not only valuable for the study of gravitropism, but are also of agricultural and economic significance. The pulvini are regions of tissue that are apical to each node and collectively return a reoriented stem to a more vertical position. They have proven to be useful for the study of gravisensing and response and are also providing clues about the establishment of polarity across tissues. This review will first highlight the agronomic significance of these stem regions and their benefits for use as model systems and provide a brief historical overview. A detailed discussion of the literature focusing on cell signaling and early changes in gene expression will follow, culminating in a temporal framework outlining events in the signaling and early growth phases of gravitropism in this tissue. Changes in cell wall composition and gene expression that occur well into the growth phase will be touched upon briefly. Finally, some ongoing research involving both maize and wheat pulvini will be introduced along with prospects for future investigations.

Regulation of plant gravity sensing and signaling by the actin cytoskeleton

Gravitropism is a process by which plant organs readjust their growth toward or away from the gravity vector when the plant is reoriented. The actin cytoskeleton has often been a significant component of models explaining gravitropism, but its role in this process has become somewhat controversial in light of reports showing that actin inhibitors enhance the gravitropic response. The work with inhibitors implies that actin might function as a negative regulator of gravitropism. In this article, possibilities for how such a role might be accomplished are presented. First, the organization of actin in statocytes is revisited in an attempt to rationalize how compressive forces exerted by statoliths on membranes can lead to enhanced gravity sensing. Second, recent genetic work in the model plant Arabidopsis thaliana is discussed, focusing on the potential involvement of the protein degradation machinery in actin-mediated control of statolith dynamics and on the intriguing possibility that an actin-regulated, ligand-receptor mechanism for gravity signal transduction might operate in higher plants. Third, modifications in the trafficking of auxin efflux transporters are considered as possible mechanisms for the enhanced gravity responses observed in plant organs when the actin cytoskeleton is disrupted by chemical inhibitors. The various possibilities presented in this review emphasize the large amount of research that remains to be done before we can fully understand how the actin cytoskeleton modulates tropisms in higher plants.

Role of actin cytoskeleton in brassinosteroid signaling and in its integration with the auxin response in plants

In plants, developmental programs and tropisms are modulated by the phytohormone auxin. Auxin reconfigures the actin cytoskeleton, which controls polar localization of auxin transporters such as PIN2 and thus determines cell-type-specific responses. In conjunction with a second growth-promoting phytohormone, brassinosteroid (BR), auxin synergistically enhances growth and gene transcription. We show that BR alters actin configuration and PIN2 localization in a manner similar to that of auxin. We describe a BR constitutive-response mutant that bears an allele of the ACTIN2 gene and shows altered actin configuration, PIN2 delocalization, and a broad array of phenotypes that recapitulate BR-treated plants. Moreover, we show that actin filament reconfiguration is sufficient to activate BR signaling, which leads to an enhanced auxin response. Our results demonstrate that the actin cytoskeleton functions as an integration node for the BR signaling pathway and auxin responsiveness.

Characterization of gravitropic inflorescence bending in brassinosteroid biosynthesis and signaling Arabidopsis mutants

The interaction between the plant hormones, brassinosteroids and auxins has been documented in various processes using a variety of plants and plant parts. In this study, detached inflorescences from brassinosteroid biosynthesis and signaling Arabidopsis mutants were evaluated for their gravitropic bending in response to epibrassinolide (EBR) and indole-3-acetic acid (IAA). EBR supplied to the base of detached inflorescences stimulated gravitropic bending in all BR biosynthetic mutants but there was no effect on the BR signaling mutant or wild type plants. When IAA was supplied to the base of BR mutant inflorescences both natural and EBR-induced gravitropic bending was inhibited. Treatment with the auxin inhibitors also decreased both natural and EBR-induced gravitropic bending. No gravitropic bending was observed when the apical tips of BR mutant inflorescences were removed. IAA treatment to the tips of decapitated BR mutant inflorescences restored gravitropic bending to values observed in the inflorescences with an apical tip, however, EBR applied to the tip had no effect. When decapitated inflorescences from BR mutants were treated with IAA to the base and either gel, EBR or IAA was applied to the tip; there was no gravitropic bending. These results show that brassinosteroids have a role in the gravitropic bending response in Arabidopsis and mutants serve to uncover this hidden contributor.

The mechanisms of brassinosteroids' action: from signal transduction to plant development

Brassinosteroids play diverse roles in plant growth and development. Plants deficient in brassinosteroid (BR) biosynthesis or defective in signal transduction show many abnormal developmental phenotypes, indicating the importance of both BR biosynthesis and the signaling pathway in regulating these biological processes. Recently, using genetics, proteomics, genomics, cell biology, and many other approaches, more components involved in the BR signaling pathway were identified. Furthermore, the physiological, cellular, and molecular mechanisms by which BRs regulate various aspects of plant development, are being discovered. These include root development, anther and pollen development and formation, stem elongation, vasculature differentiation, and cellulose biosynthesis, suggesting that the biological functions of BRs are far beyond promoting cell elongation. This review will focus on the up-to-date progresses about regulatory mechanisms of the BR signaling pathway and the physiological and molecular mechanisms whereby BRs regulate plant growth and development.

Transcriptional auxin-brassinosteroid crosstalk: who's talking?

The plant hormones auxin and brassinosteroid are both essential regulators of plant growth and known to influence both cell division and cell elongation in various developmental contexts. These physiological effects of auxin and brassinosteroid have been known for many years. Based on observations from external simultaneous application of both hormones to plant tissues, it has been suggested that they act in an interdependent and possibly synergistic manner. Recent work in the model plant Arabidopsis thaliana suggests that, at the molecular level, auxin-brassinosteroid synergism manifests itself in the regulation of the expression of common target genes. However, whether this reflects genuine hormone pathway-dependent crosstalk modulation of the transcription machinery or rather indirect effects of hormone action on other cellular activities, such as hormone biosynthesis or the polar transport of auxin, is not entirely clear. This article reviews the evidence for transcriptional crosstalk between auxin and brassinosteroid and its molecular basis.

Quantitative analysis of auxin-regulated proteins from basal part of leaf sheaths in rice by two-dimensional difference gel electrophoresis

To identify the effects of auxin on rice root formation, proteins induced by exogenous addition of auxin to rice seedlings were analyzed by a proteomic approach. Root formation by rice seedlings was promoted by 0.45microM 2,4-dichlorophenoxyacetic acid (2,4-D) and repressed by 60microM p-chlorophenoxyisobutyric acid (PCIB). Proteins extracted from the basal part of leaf sheaths of rice seedlings treated with 2,4-D or PCIB for 48h were labeled with Cy3 and Cy5, and separated by two-dimensional polyacrylamide gel electrophoresis. Out of nine proteins up-regulated by 2,4-D and down-regulated by PCIB, five proteins showing significant difference in abundance were used for expression analysis at the transcript abundance level. Transcript abundance of the mitochondrial complex I subunit slightly increased with 2,4-D treatment and were repressed by PCIB. The transcript abundance of EF-1beta', myosin heavy chain and mitochondrial [Mn]SOD increased with 2,4-D treatment but did not decrease with PCIB. The transcript abundance of aldehyde dehydrogenase was not effected by 2,4-D or PCIB. These results indicate that mitochondrial complex I subunit is part of the downstream signal cascade of PCIB, whereas myosin heavy chain, mitochondrial [Mn]SOD and EF-1beta' are involved in the 2,4-D signal cascade but are probably upstream of PCIB.

Phytohormone collaboration: zooming in on auxin-brassinosteroid interactions

Similar to animal hormones, classic plant hormones are small organic molecules that regulate physiological and developmental processes. In development, this often involves the regulation of growth through the control of cell size or division. The plant hormones auxin and brassinosteroid modulate both cell expansion and proliferation and are known for their overlapping activities in physiological assays. Recent molecular genetic analyses in the model plant Arabidopsis suggest that this reflects interdependent and often synergistic action of the two hormone pathways. Such pathway interactions probably occur through the combinatorial regulation of common target genes by auxin- and brassinosteroid-controlled transcription factors. Moreover, auxin and brassinosteroid signaling and biosynthesis and auxin transport might be linked by an emerging upstream connection involving calcium-calmodulin and phosphoinositide signaling.

Elongation and gravitropic responses of Arabidopsis roots are regulated by brassinolide and IAA

Exogenously applied brassinolide (BL) increased both gravitropic curvature and length of primary roots of Arabidopsis at low concentration (10(-10) M), whereas at higher concentration, BL further increased gravitropic curvature while it inhibited primary root growth. BRI1-GFP plants possessing a high steady-state expression level of a brassinosteroid (BR) receptor kinase rendered the plant's responses to gravity and root growth more sensitive, while BR-insensitive mutants, bri1-301 and bak1, delayed root growth and reduced their response to the gravitropic stimulus. The stimulatory effect of BL on the root gravitropic curvature was also enhanced in auxin transport mutants, aux1-7 and pin2, relative to wild-type plants, and increasing concentration of auxin attenuated BL-induced root sensitivity to gravity. Interestingly, IAA treatment to the roots of bri1-301 and bak1 plants or of plants pretreated with a BL biosynthetic inhibitor, brassinazole, increased their sensitivity to gravity, while these treatments for the BL-hypersensitive transgenic plants, BRI1-GFP and 35S-BAK1, were less effective. Expression of a CYP79B2 gene, encoding an IAA biosynthetic enzyme, was suppressed in BL-hypersensitive plant types and enhanced in BL-insensitive or -deficient plants. In conclusion, our results indicate that BL interacts negatively with IAA in the regulation of plant gravitropic response and root growth, and its regulation is achieved partly by modulating biosynthetic pathways of the counterpart hormone.

LAZY1 controls rice shoot gravitropism through regulating polar auxin transport

Tiller angle of rice (Oryza sativa L.) is an important agronomic trait that contributes to grain production, and has long attracted attentions of breeders for achieving ideal plant architecture to improve grain yield. Although enormous efforts have been made over the past decades to study mutants with extremely spreading or compact tillers, the molecular mechanism underlying the control of tiller angle of cereal crops remains unknown. Here we report the cloning of the LAZY1 (LA1) gene that regulates shoot gravitropism by which the rice tiller angle is controlled. We show that LA1, a novel grass-specific gene, is temporally and spatially expressed, and plays a negative role in polar auxin transport (PAT). Loss-of-function of LA1 enhances PAT greatly and thus alters the endogenous IAA distribution in shoots, leading to the reduced gravitropism, and therefore the tiller-spreading phenotype of rice plants.

Inhibition of brassinosteroid biosynthesis by either a dwarf4 mutation or a brassinosteroid biosynthesis inhibitor rescues defects in tropic responses of hypocotyls in the arabidopsis mutant nonphototropic hypocotyl 4

The nonphototropic hypocotyl 4 (nph4)/auxin response factor 7 (arf7) mutant of Arabidopsis (Arabidopsis thaliana) is insensitive to auxin and has defects in hypocotyl tropism, hook formation, differential leaf growth, and lateral root formation. To understand an auxin-signaling pathway through NPH4, we carried out screening of suppressor mutants of nph4-103 and obtained a dwarf suppressor mutant, suppressor of nph4 (snp2). snp2 had short hypocotyls in the dark condition and dark green and round leaves, short petioles, and more lateral shoots than the wild type in the light condition. The snp2 phenotypes were rescued by adding brassinolide to the growth medium in both light and dark conditions. Genetic mapping, sequence analysis, and a complementation test indicated that snp2 was a weak allele of DWARF4 (DWF4), which functions in brassinosteroid (BR) biosynthesis. snp2, which was renamed dwf4-101, exhibited photo- and gravitropisms of hypocotyls similar to those of the wild type with a slightly faster response in gravitropism. dwf4-101 almost completely suppressed defects in both tropisms of nph4-103 hypocotyls and completely suppressed hyponastic growth of nph4-103 leaves. Treatment with brassinazole, an inhibitor of BR biosynthesis, also partially rescued the tropic defects in nph4-103. Hypocotyls of nph4-103 were auxin insensitive, whereas hypocotyls of dwf4-101 were more sensitive than those of the wild type. dwf4-101 nph4-103 hypocotyls were as sensitive as those of dwf4-101. Auxin inducibility of massugu 2 (MSG2)/IAA19 gene expression was reduced in nph4-103. mRNA level of MSG2 was reduced in dwf4-101 and dwf4-101 nph4-103, but both mutants exhibited greater auxin inducibility of MSG2 than the wild type. Taken together, dwf4-101 was epistatic to nph4-103. These results strongly suggest that BR deficiency suppresses nph4-103 defects in tropic responses of hypocotyls and differential growth of leaves and that BR negatively regulates tropic responses.

Organization and function of the actin cytoskeleton in developing root cells

The actin cytoskeleton is a highly dynamic structure, which mediates various cellular functions in large part through accessory proteins that tilt the balance between monomeric G-actin and filamentous actin (F-actin) or by facilitating interactions between actin and the plasma membrane, microtubules, and other organelles. Roots have become an attractive model to study actin in plant development because of their simple anatomy and accessibility of some root cell types such as root hairs for microscopic analyses. Roots also exhibit a remarkable developmental plasticity and possess a delicate sensory system that is easily manipulated, so that one can design experiments addressing a range of important biological questions. Many facets of root development can be regulated by the diverse actin network found in the various root developmental regions. Various molecules impinge on this actin scaffold to define how a particular root cell type grows or responds to a specific environmental signal. Although advances in genomics are leading the way toward elucidating actin function in roots, more significant strides will be realized when such tools are combined with improved methodologies for accurately depicting how actin is organized in plant cells.

Brassinosteroids stimulate plant tropisms through modulation of polar auxin transport in Brassica and Arabidopsis

Brassinosteroids (BRs) are important plant growth regulators in multiple developmental processes. Previous studies have indicated that BR treatment enhanced auxin-related responses, but the underlying mechanisms remain unknown. Using (14)C-labeled indole-3-acetic acid and Arabidopsis thaliana plants harboring an auxin-responsive reporter construct, we show that the BR brassinolide (BL) stimulates polar auxin transport capacities and modifies the distribution of endogenous auxin. In plants treated with BL or defective in BR biosynthesis or signaling, the transcription of PIN genes, which facilitate functional auxin transport in plants, was differentially regulated. In addition, BL enhanced plant tropistic responses by promoting the accumulation of the PIN2 protein from the root tip to the elongation zone and stimulating the expression and dispersed localization of ROP2 during tropistic responses. Constitutive overexpression of ROP2 results in enhanced polar accumulation of PIN2 protein in the root elongation region and increased gravitropism, which is significantly affected by latrunculin B, an inhibitor of F-actin assembly. The ROP2 dominant negative mutants (35S-ROP2-DA/DN) show delayed tropistic responses, and this delay cannot be reversed by BL addition, strongly supporting the idea that ROP2 modulates the functional localization of PIN2 through regulation of the assembly/reassembly of F-actins, thereby mediating the BR effects on polar auxin transport and tropistic responses.

A brassinosteroid-hypersensitive mutant of BAK1 indicates that a convergence of photomorphogenic and hormonal signaling modulates phototropism

The phototropic response of Arabidopsis (Arabidopsis thaliana) is induced by the phototropin photoreceptors and modulated by the cryptochrome and phytochrome photoreceptors. Downstream of these photoreceptors, asymmetric lateral redistribution of auxin underlies the differential growth, which results in phototropism. Historical physiological evidence and recent analysis of hormone-induced gene expression demonstrate that auxin and brassinosteroid signaling function interdependently. Similarly, in this study we report evidence that interactions between brassinosteroids and auxin signaling modulate phototropic responsiveness. We found that elongated, a previously identified photomorphogenesis mutant, enhances high-light phototropism and represents a unique allele of BAK1/SERK3, a receptor kinase implicated in brassinosteroid perception. Altogether, our results support the hypothesis that phototropic responsiveness is modulated by inputs that influence control of auxin response factor-mediated transcription.

Brassinosteroids are inherently biosynthesized in the primary roots of maize, Zea mays L

GC-MS analysis revealed that primary roots of maize contain 6-deoxocathasterone, 6-deoxoteasterone and 6-deoxotyphasterol. These brassinosteroids, and the previously identified campesterol, campestanol, 6-deoxocastasterone and castasterone, in the roots are members of a biosynthetic pathway to castasterone, namely the late C-6 oxidation pathway, suggesting that its biosynthetic pathway is operative in the roots. To verify this, a cell-free enzyme extract was prepared from maize roots, and enzymatic conversions from campesterol to castasterone through the aforementioned sterols and brassinosteroids were examined. The presence for the biosynthetic sequences, campesterol-->24-methylcholest-4-en-3beta-ol-->24-methylcholest-4-en-3-one-->24-methylcholest-5 alpha-cholestan-3-one-->campestanol and 6-deoxoteasterone-->6-deoxo-3-dehydroteasterone-->6-deoxotyphasterol-->6-deoxocastasterone-->castasterone were demonstrated. These results indicate that maize roots contain a complete set of enzymes involved in the late C-6 oxidation pathway, thereby demonstrating that endogenous brassinosteroids are biosynthesized in the roots.

Gibberellin-regulated XET is differentially induced by auxin in rice leaf sheath bases during gravitropic bending

The asymmetric distribution of auxin plays a fundamental role in plant gravitropism, yet little is understood about how its lateral distribution stimulates growth. In the present work, the asymmetric distribution not only of auxin, but also that of gibberellins (GAs), was observed in rice leaf sheath bases following gravistimulation. Gravistimulation induced the transient accumulation of greater amounts of both IAA and GA in the lower halves of the leaf sheath bases of rice seedlings. OsGA3ox1, a gene of active GA synthesis, was differentially induced by gravistimulation. Furthermore, 2,3,5-tri-iodobenzoic acid (TIBA), an inhibitor of auxin transport, substantially decreased the asymmetric distribution of IAA and the gradient of OsGA3ox1 expression. Externally applied GA(3) restored the gravitropic curvature of rice leaf sheaths inhibited by either TIBA or by ancymidol, a GA synthesis inhibitor. The expression of XET (encoding xyloglucan endotransglycosylase) was differentially induced in the lower halves of gravistimulated leaf sheath bases and was also up-regulated by exogenous IAA and GA(3). Both ancymidol and TIBA decreased the gradient of XET expression. These data suggest that the asymmetric distribution of auxin effected by gravistimulation induced a gradient of GAs via asymmetric expression of OsGA3ox1 in rice leaf sheath bases, and hence caused the asymmetric expression of XET. Cell wall loosening in the curvature site of the leaf sheath triggered by the expression of XET would contribute to gravitropic growth.

Auxin: regulation, action, and interaction

BACKGROUND

The phytohormone auxin is critical for plant growth and orchestrates many developmental processes.

SCOPE

This review considers the complex array of mechanisms plants use to control auxin levels, the movement of auxin through the plant, the emerging view of auxin-signalling mechanisms, and several interactions between auxin and other phytohormones. Though many natural and synthetic compounds exhibit auxin-like activity in bioassays, indole-3-acetic acid (IAA) is recognized as the key auxin in most plants. IAA is synthesized both from tryptophan (Trp) using Trp-dependent pathways and from an indolic Trp precursor via Trp-independent pathways; none of these pathways is fully elucidated. Plants can also obtain IAA by beta-oxidation of indole-3-butyric acid (IBA), a second endogenous auxin, or by hydrolysing IAA conjugates, in which IAA is linked to amino acids, sugars or peptides. To permanently inactivate IAA, plants can employ conjugation and direct oxidation. Consistent with its definition as a hormone, IAA can be transported the length of the plant from the shoot to the root; this transport is necessary for normal development, and more localized transport is needed for tropic responses. Auxin signalling is mediated, at least in large part, by an SCFTIR1 E3 ubiquitin ligase complex that accelerates Aux/IAA repressor degradation in response to IAA, thereby altering gene expression. Two classes of auxin-induced genes encode negatively acting products (the Aux/IAA transcriptional repressors and GH3 family of IAA conjugating enzymes), suggesting that timely termination of the auxin signal is crucial. Auxin interaction with other hormone signals adds further challenges to understanding auxin response.

CONCLUSIONS

Nearly six decades after the structural elucidation of IAA, many aspects of auxin metabolism, transport and signalling are well established; however, more than a few fundamental questions and innumerable details remain unresolved.

Brassinosteroid-promoted growth

Brassinosteroids (BRs) are highly potent growth-promoting sterol derivatives. BR-deficient or BR-insensitive mutants display dwarfism. Whole plants and excised tissues have been used to analyse the mechanisms involved in BR-promoted growth. BR stimulates cell elongation and cell division, and BR has specific effects on differentiation. Underlying physiological pathways include modification of cell wall properties, effects on carbohydrate assimilation and allocation, and control of aquaporin activities. BR apparently coordinates and integrates diverse processes required for growth, partly via interactions with other phytohormones setting the frame for BR responses. Ultimately, BR-promoted growth is mediated through genomic pathways. Positive regulators of the BR response (such as BZR1 and BES1) and putative downstream components (such as EXO) are involved in the regulation of BR-responsive genes and growth promotion. BR-responsive genes have been identified in several plant species. However, causal links between physiological effects and changes of transcript patterns, for the most part, are still unresolved. This review focuses on physiology and molecular mechanisms underlying BR-promoted growth in the different plant organs. Interactions with other phytohormones are discussed.

Gravitropic bending and plant hormones

Gravitropism is a complex multistep process that redirects the growth of roots and various above-ground organs in response to changes in the direction of the gravity vector. The anatomy and morphology of these graviresponding organs indicates a certain spatial separation between the sensing region and the responding one, a situation that strongly suggests the requirement of phytohormones as mediators to coordinate the process. The Cholodny-Went hypothesis suggested auxin as the main mediator of gravitropism. So far, ample evidence has been gathered with regard to auxin asymmetrical detection, polar and lateral transport involving influx and efflux carriers, response signaling pathway, and possible modes of action in differential cell elongation, supports its major role in gravitropism at least in roots. However, it is becoming clear that the participation of other hormones, acting in concert with auxin, is necessary as well. Of particular importance is the role of ethylene in shoot gravitropism, possibly associated with the modulation of auxin transport or sensitivity, and the key role implicated for cytokinin as the putative root cap inhibitor that controls early root gravitropism. Therefore, the major advances in the understanding of transport and signaling of auxin, ethylene, and cytokinin may shed light on the possibly tight and complicated interactions between them in gravitropism. Not much convincing evidence has been accumulated regarding the participation of other phytohormones, such as gibberellins, abscisic acid, brassinosteroids, jasmonates, and salicylic acid, in gravitropism. However, the emerging concept of cooperative hormone action opens new possibilities for a better understanding of the complex interactions of all phytohormones and their possible synergistic effects and involvement in the gravitropic bending process.

The fast and transient transcriptional network of gravity and mechanical stimulation in the Arabidopsis root apex

Plant root growth is affected by both gravity and mechanical stimulation (Massa GD, Gilroy S [2003] Plant J 33: 435-445). A coordinated response to both stimuli requires specific and common elements. To delineate the transcriptional response mechanisms, we carried out whole-genome microarray analysis of Arabidopsis root apices after gravity stimulation (reorientation) and mechanical stimulation and monitored transcript levels of 22,744 genes in a time course during the first hour after either stimulus. Rapid, transient changes in the relative abundance of specific transcripts occurred in response to gravity or mechanical stimulation, and these transcript level changes reveal clusters of coordinated events. Transcriptional regulation occurs in the root apices within less than 2 min after either stimulus. We identified genes responding specifically to each stimulus as well as transcripts regulated in both signal transduction pathways. Several unknown genes were specifically induced only during gravitropic stimulation (gravity induced genes). We also analyzed the network of transcriptional regulation during the early stages of gravitropism and mechanical stimulation.

Overexpression of OsRAA1 causes pleiotropic phenotypes in transgenic rice plants, including altered leaf, flower, and root development and root response to gravity

There are very few root genes that have been described in rice as a monocotyledonous model plant so far. Here, the OsRAA1 (Oryza sativa Root Architecture Associated 1) gene has been characterized molecularly. OsRAA1 encodes a 12.0-kD protein that has 58% homology to the AtFPF1 (Flowering Promoting Factor 1) in Arabidopsis, which has not been reported as modulating root development yet. Data of in situ hybridization and OsRAA1::GUS transgenic plant showed that OsRAA1 expressed specifically in the apical meristem, the elongation zone of root tip, steles of the branch zone, and the young lateral root. Constitutive expression of OsRAA1 under the control of maize (Zea mays) ubiquitin promoter resulted in phenotypes of reduced growth of primary root, increased number of adventitious roots and helix primary root, and delayed gravitropic response of roots in seedlings of rice (Oryza sativa), which are similar to the phenotypes of the wild-type plant treated with auxin. With overexpression of OsRAA1, initiation and growth of adventitious root were more sensitive to treatment of auxin than those of the control plants, while their responses to 9-hydroxyfluorene-9-carboxylic acid in both transgenic line and wild type showed similar results. OsRAA1 constitutive expression also caused longer leaves and sterile florets at the last stage of plant development. Analysis of northern blot and GUS activity staining of OsRAA1::GUS transgenic plants demonstrated that the OsRAA1 expression was induced by auxin. At the same time, overexpression of OsRAA1 also caused endogenous indole-3-acetic acid to increase. These data suggested that OsRAA1 as a new gene functions in the development of rice root systems, which are mediated by auxin. A positive feedback regulation mechanism of OsRAA1 to indole-3-acetic acid metabolism may be involved in rice root development in nature.

Cytochrome P450-catalyzed brassinosteroid pathway activation through synthesis of castasterone and brassinolide in Phaseolus vulgaris

The last reaction in the biosynthesis of brassinolide has been examined enzymatically. A microsomal enzyme preparation from cultured cells of Phaseolus vulgaris catalyzed a conversion from castasterone to brassinolide, indicating that castasterone 6-oxidase (brassinolide synthase) is membrane associated. This enzyme preparation also catalyzed the conversions of 6-deoxocastasterone and typhasterol to castasterone which have been reported to be catalyzed by cytochrome P450s, CYP85A1 of tomato and CYP92A6 of pea, respectively. The activities of these enzymes require molecular oxygen as well as NADPH as a cofactor. The enzyme activities were strongly inhibited by carbon monoxide, an inhibitor of cytochrome P450, and this inhibition was recovered by blue light irradiation in the presence of oxygen. Commercial cytochrome P450 inhibitors including cytochrome c, SKF 525A, 1-aminobenzotriazole and ketoconazole also inhibited the enzyme activities. The present work presents unanimous enzymological evidence that cytochrome P450s are responsible for the synthesis of brassinolide from castasterone as well as of castasterone from typhasterol and 6-deoxocastasterone, which have been deemed activation steps of BRs.