Light and brassinosteroid signals are integrated via a dark-induced small G protein in etiolated seedling growth

Brassinosteroids regulate cotton fiber elongation by modulating very-long-chain fatty acid biosynthesis

Brassinosteroid (BR), a growth-promoting phytohormone, regulates many plant growth processes including cell development. However, the mechanism by which BR regulates fiber growth is poorly understood. Cotton (Gossypium hirsutum) fibers are an ideal single-cell model in which to study cell elongation due to their length. Here we report that BR controls cotton fiber elongation by modulating very-long-chain fatty acid (VLCFA) biosynthesis. BR deficiency reduces the expression of 3-ketoacyl-CoA synthases (GhKCSs), the rate-limiting enzymes involved in VLCFA biosynthesis, leading to lower saturated VLCFA contents in pagoda1 (pag1) mutant fibers. In vitro ovule culture experiments show that BR acts upstream of VLCFAs. Silencing of BRI1-EMS-SUPPRESOR 1.4 (GhBES1.4), encoding a master transcription factor of the BR signaling pathway, significantly reduces fiber length, whereas GhBES1.4 overexpression produces longer fibers. GhBES1.4 regulates endogenous VLCFA contents and directly binds to BR RESPONSE ELEMENTS (BRREs) in the GhKCS10_At promoter region, which in turn regulates GhKCS10_At expression to increase endogenous VLCFA contents. GhKCS10_At overexpression promotes cotton fiber elongation, whereas GhKCS10_At silencing inhibits cotton fiber growth, supporting a positive regulatory role for GhKCS10_At in fiber elongation. Overall, these results uncover a mechanism of fiber elongation through crosstalk between BR and VLCFAs at the single-cell level.

GhAPC8 regulates leaf blade angle by modulating multiple hormones in cotton (Gossypium hirsutum L.)

Leaf angle, including leaf petiole angle (LPA) and leaf blade angle (LBA), is an important trait affecting plant architecture. Anaphase-promoting complex/cyclosome (APC/C) genes play a vital role in plant growth and development, including regulation of leaf angle. Here, we identified and characterized the APC genes in Upland cotton (G. hirsutum L.) with a focus on GhAPC8, a homolog of soybean GmILPA1 involved in regulation of LPA. We showed that independently silencing the At or Dt sub-genome homoeolog of GhAPC8 using virus-induced gene silencing reduced plant height and LBA, and that reduction of LBA could be caused by uneven growth of cortex parenchyma cells on the adaxial and abaxial sides of the junction between leaf blade and leaf petiole. The junction between leaf blade and leaf petiole of the GhAPC8-silenced plants had an elevated level of brassinosteroid (BR) and a decreased levels of auxin and gibberellin. Consistently, comparative transcriptome analysis found that silencing GhAPC8 activated genes of the BR biosynthesis and signaling pathways as well as genes related to ubiquitin-mediated proteolysis. Weighted gene co-expression network analysis (WGCNA) identified gene modules significantly associated with plant height and LBA, and candidate genes bridging GhAPC8, the pathways of BR biosynthesis and signaling and ubiquitin-mediated proteolysis. These results demonstrated a role of GhAPC8 in regulating LBA, likely achieved by modulating the accumulation and signaling of multiple phytohormones.

Genome-wide identification and expression profiling of durian CYPome related to fruit ripening

Durian (Durio zibethinus L.) is a major economic crop native to Southeast Asian countries, including Thailand. Accordingly, understanding durian fruit ripening is an important factor in its market worldwide, owing to the fact that it is a climacteric fruit with a strikingly limited shelf life. However, knowledge regarding the molecular regulation of durian fruit ripening is still limited. Herein, we focused on cytochrome P450, a large enzyme family that regulates many biosynthetic pathways of plant metabolites and phytohormones. Deep mining of the durian genome and transcriptome libraries led to the identification of all P450s that are potentially involved in durian fruit ripening. Gene expression validation by RT-qPCR showed a high correlation with the transcriptome libraries at five fruit ripening stages. In addition to aril-specific and ripening-associated expression patterns, putative P450s that are potentially involved in phytohormone metabolism were selected for further study. Accordingly, the expression of CYP72, CYP83, CYP88, CYP94, CYP707, and CYP714 was significantly modulated by external treatment with ripening regulators, suggesting possible crosstalk between phytohormones during the regulation of fruit ripening. Interestingly, the expression levels of CYP88, CYP94, and CYP707, which are possibly involved in gibberellin, jasmonic acid, and abscisic acid biosynthesis, respectively, were significantly different between fast- and slow-post-harvest ripening cultivars, strongly implying important roles of these hormones in fruit ripening. Taken together, these phytohormone-associated P450s are potentially considered additional molecular regulators controlling ripening processes, besides ethylene and auxin, and are economically important biological traits.

Integration of Light and Brassinosteroid Signaling during Seedling Establishment

Light and brassinosteroid (BR) are external stimuli and internal cue respectively, that both play critical roles in a wide range of developmental and physiological process. Seedlings grown in the light exhibit photomorphogenesis, while BR promotes seedling etiolation. Light and BR oppositely control the development switch from shotomorphogenesis in the dark to photomorphogenesis in the light. Recent progress report that substantial components have been identified as hubs to integrate light and BR signals. Photomorphogenic repressors including COP1, PIFs, and AGB1 have been reported to elevate BR response, while photomorphogenesis-promoting factors such as HY5, BZS1, and NF-YCs have been proven to repress BR signal. In addition, BR components also modulate light signal. Here, we review the current research on signaling network associated with light and brassinosteroids, with a focus on the integration of light and BR signals enabling plants to thrive in the changeable environment.

Identification and Characterization of Secondary Wall-Associated NAC Genes and Their Involvement in Hormonal Responses in Tobacco (Nicotiana tabacum)

Secondary wall-associated NAC (SWN) genes are a subgroup of NAC (NAM, ATAF, and CUC) transcription factors (TF) that play a key role in regulating secondary cell wall biosynthesis in plants. However, this gene family has not been systematically characterized, and their potential roles in response to hormones are unknown in Nicotiana tabacum. In this study, a total of 40 SWN genes, of which 12 from Nicotiana tomentosiformis, 13 from Nicotiana sylvestris, and 15 from Nicotiana tabacum, were successfully identified. The 15 SWNs from Nicotiana tabacum were further classified into three groups, namely, vascular-related NAC domain genes (NtVNDs), NAC secondary wall thickening promoting factor genes (NtNSTs), and secondary wall-associated NAC domain genes (NtSNDs). The protein characteristic, gene structure, and chromosomal location of 15 NtSWNs (also named Nt1 to Nt15) were also analyzed. The NtVND and NtNST group genes had five conserved subdomains in their N-terminal regions and a motif (LP[Q/x] L[E/x] S[P/A]) in their diverged C- terminal regions. Some hormones, dark and low-temperature related cis-acting elements, were significantly enriched in the promoters of NtSWN genes. A comprehensive expression profile analysis revealed that Nt4 and Nt12 might play a role in vein development. Others might be important for stem development. Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) revealed that in the NtNST group, genes such as Nt7, Nt8, and Nt13 were more sensitive than the genes in NtVND and NtSND groups under abiotic stress conditions. A transactivation assay further suggested that Nt7, Nt8, and Nt13 showed a significant transactivation activity. Overall, SWN genes were finally identified and characterized in diploid and tetraploid tobacco, revealing new insights into their evolution, variation, and homology relationships. Transcriptome, cis-acting element, qRT-PCR, and transactivation assay analysis indicated the roles in hormonal and stress responses, which provided further resources in molecular mechanism and genetic improvement.

Transcriptome profiling provides insights into molecular mechanism in Peanut semi-dwarf mutant

BACKGROUND

Plant height, mainly decided by main stem height, is the major agronomic trait and closely correlated to crop yield. A number of studies had been conducted on model plants and crops to understand the molecular and genetic basis of plant height. However, little is known on the molecular mechanisms of peanut main stem height.

RESULTS

In this study, a semi-dwarf peanut mutant was identified from ⁶⁰Co γ-ray induced mutant population and designated as semi-dwarf mutant 2 (sdm2). The height of sdm2 was only 59.3% of its wild line Fenghua 1 (FH1) at the mature stage. The sdm2 has less internode number and short internode length to compare with FH1. Gene expression profiles of stem and leaf from both sdm2 and FH1 were analyzed using high throughput RNA sequencing. The differentially expressed genes (DEGs) were involved in hormone biosynthesis and signaling pathways, cell wall synthetic and metabolic pathways. BR, GA and IAA biosynthesis and signal transduction pathways were significantly enriched. The expression of several genes in BR biosynthesis and signaling were found to be significantly down-regulated in sdm2 as compared to FH1. Many transcription factors encoding genes were identified as DEGs.

CONCLUSIONS

A large number of genes were found differentially expressed between sdm2 and FH1. These results provide useful information for uncovering the molecular mechanism regulating peanut stem height. It could facilitate identification of causal genes for breeding peanut varieties with semi-dwarf phenotype.

Regulation of Brassinosteroid Homeostasis in Higher Plants

Brassinosteroids (BRs) are known as one of the major classes of phytohormones essential for various processes during normal plant growth, development, and adaptations to biotic and abiotic stresses. Significant progress has been achieved on revealing mechanisms regulating BR biosynthesis, catabolism, and signaling in many crops and in model plant Arabidopsis. It is known that BRs control plant growth and development in a dosage-dependent manner. Maintenance of BR homeostasis is therefore critical for optimal functions of BRs. In this review, updated discoveries on mechanisms controlling BR homeostasis in higher plants in response to internal and external cues are discussed.

Inhibitors of Brassinosteroid Biosynthesis and Signal Transduction

Chemical inhibitors are invaluable tools for investigating protein function in reverse genetic approaches. Their application bears many advantages over mutant generation and characterization. Inhibitors can overcome functional redundancy, their application is not limited to species for which tools of molecular genetics are available and they can be applied to specific tissues or developmental stages, making them highly convenient for addressing biological questions. The use of inhibitors has helped to elucidate hormone biosynthesis and signaling pathways and here we review compounds that were developed for the plant hormones brassinosteroids (BRs). BRs are steroids that have strong growth-promoting capacities, are crucial for all stages of plant development and participate in adaptive growth processes and stress response reactions. In the last two decades, impressive progress has been made in BR inhibitor development and application, which has been instrumental for studying BR modes of activity and identifying and characterizing key players. Both, inhibitors that target biosynthesis, such as brassinazole, and inhibitors that target signaling, such as bikinin, exist and in a comprehensive overview we summarize knowledge and methodology that enabled their design and key findings of their use. In addition, the potential of BR inhibitors for commercial application in plant production is discussed.

DWARF4 accumulation in root tips is enhanced via blue light perception by cryptochromes

Brassinosteroid (BR) signalling is known to be coordinated with light signalling in above ground tissue. Many studies focusing on the shade avoidance response in above ground tissue or hypocotyl elongation in darkness have revealed the contribution of the BR signalling pathway to these processes. We previously analysed the expression of DWARF 4 (DWF4), a key BR biosynthesis enzyme, and revealed that light perception in above ground tissues triggered DWF4 accumulation in root tips. To determine the required wavelength of light and photoreceptors responsible for this regulation, we studied DWF4-GUS marker plants grown in several monochromatic light conditions. We revealed that monochromatic blue LED light could induce DWF4 accumulation in primary root tips and root growth as much as white light, whereas monochromatic red LED could not. Consistent with this, a cryptochrome1/2 double mutant showed retarded root growth under white light whereas a phytochromeA/B double mutant did not. Taken together, our data strongly indicated that blue light signalling was important for DWF4 accumulation in root tips and root growth. Furthermore, DWF4 accumulation patterns in primary root tips were not altered by auxin or sugar treatment. Therefore, we hypothesize that blue light signalling from the shoot tissue is different from auxin and sugar signalling.

Identification of differentially expressed proteins in Ostrinia furnacalis adults after exposure to ultraviolet A

Ultraviolet A (UVA), the major component of solar UV irradiation, is an important environmental factor inducing damage to insects including cell death, photoreceptor damage, and oxidative stress. In order to improve understanding of the adaptation mechanisms of insect after UVA exposure, a comparative proteomic analysis was carried out to reveal differential protein expression in Ostrinia furnacalis. Three-day-old adults were treated with UVA for 1 h. Total proteins of control and UVA-treated insects were examined using two-dimensional electrophoresis (2-DE). 2-DE analysis demonstrated that 19 proteins were increased and 18 proteins were decreased significantly in O. furnacalis after UVA exposure, respectively. Thirty differentially expressed proteins were successfully identified by mass spectrometry. The identified proteins were involved in diverse biological processes, such as signal transduction, transport processing, cellular stress, metabolisms, and cytoskeleton organization. Our results reveal that the response patterns of O. furnacalis to UVA irradiation are complex and provide novel insights into the adaptation response to UVA irradiation stress.

Global identification, structural analysis and expression characterization of cytochrome P450 monooxygenase superfamily in rice

BACKGROUND

The cytochrome P450 monooxygenases (CYP450, CYP, P450) catalyze numerous monooxygenation/hydroxylation reactions in biochemical pathways. Although CYP superfamily has been systematically studied in a few species, the genome-scale research about it in rice has not been done.

RESULTS

In this study, a total of 355 CYPs encoded by 326 genes were identified in japonica genome. The OsCYP genes are classified into 10 clans including 45 families according to phylogenetic analysis. More than half of the genes are distributed in 53 tandem duplicated gene clusters. Intron-exon structure of OsCYPs exhibits highly conserved and specificity within a family, and divergences of duplicate genes in gene structure result in non-functionalization, neo-functionalization or sub-functionalization. Selection pressure analysis showed that rice CYPs are under purifying selection. The microarray data analysis shows that some genes are tissue-specific expression, such as OsCYP710A5 and OsCYP71X14 in endosperm, OsCYP99A3 and OsCYP78A16 in root and OsCYP93G2 and OsCYP97D7 in leaf. Analysis of RNA-seq data derived from rice leaf developmental gradient indicates that some OsCYPs exhibit zone-specific expression patterns. OsCYP87C2, OsCYP96B5, OsCYP96B8 and OsCYP84A5 were specifically expressed in leaf base and transitional zone. The transcripts of lineages II and IV-1 members were highly abundant in maturing zone. Eighty three OsCYPs are differentially expressed in response to drought stress, of which OsCYP51G3, OsCYP709C9, OsCYP709C5, OsCYP81A6, OsCYP72A18 and OsCYP704A5 are strongly induced and OsCYP78A16, OsCYP89C9 and OsCYP704A5 are down-regulated significantly, and some of the results were validated by qPCR. And 23 up-regulated and 17 down-regulated genes are specific to Osbhlh148 mutation under drought stress. Compared to those in wild type, the changes in transcript levels of several genes are slight in the mutant, such as OsCYP51G3, OsCYP94C2, OsCYP709C9 and OsCYP709C5.

CONCLUSION

The whole-genomic analysis of rice P450 superfamily provides a clue to understanding biological function of OsCYPs in development regulation and drought stress response, and is helpful to rice molecular breeding.

Oxidative rearrangement of (+)-sesamin by CYP92B14 co-generates twin dietary lignans in sesame

(+)-Sesamin, (+)-sesamolin, and (+)-sesaminol glucosides are phenylpropanoid-derived specialized metabolites called lignans, and are rich in sesame (Sesamum indicum) seed. Despite their renowned anti-oxidative and health-promoting properties, the biosynthesis of (+)-sesamolin and (+)-sesaminol remained largely elusive. Here we show that (+)-sesamolin deficiency in sesame is genetically associated with the deletion of four C-terminal amino acids (Del4C) in a P450 enzyme CYP92B14 that constitutes a novel clade separate from sesamin synthase CYP81Q1. Recombinant CYP92B14 converts (+)-sesamin to (+)-sesamolin and, unexpectedly, (+)-sesaminol through an oxygenation scheme designated as oxidative rearrangement of α-oxy-substituted aryl groups (ORA). Intriguingly, CYP92B14 also generates (+)-sesaminol through direct oxygenation of the aromatic ring. The activity of CYP92B14 is enhanced when co-expressed with CYP81Q1, implying functional coordination of CYP81Q1 with CYP92B14. The discovery of CYP92B14 not only uncovers the last steps in sesame lignan biosynthesis but highlights the remarkable catalytic plasticity of P450s that contributes to metabolic diversity in nature.

A Small G Protein as a Novel Component of the Rice Brassinosteroid Signal Transduction

Brassinosteroids (BRs) are a class of steroid hormones that are essential for plant growth and development. The BR signal transduction pathway in the dicot model plant Arabidopsis is well established, but the components connecting the BR signaling steps in rice have not been fully explored. For example, how the BR signaling is fine-tuned in rice, especially at the BR receptor level, is largely unknown. Here we show that OsPRA2, a rice small G protein, plays a repressive role in the BR signaling pathway. Lamina inclination, coleoptile elongation, and root inhibition assays indicated that rice plants with suppressed expression of OsPRA2 were more sensitive to exogenously applied brassinolide than the wild-type plants. Conversely, rice overexpressing OsPRA2 was less sensitive to exogenous brassinolide. Further study uncovered that OsPRA2 inhibited the dephosphorylation of, and thus inactivated the transcription factor BRASSINAZOLE-RESISTANT 1 (OsBZR1). More importantly, OsPRA2 was found to co-localize with and directly bind to rice BR receptor BRASSINOSTEROID-INSENSITIVE 1 (OsBRI1) at the plasma membrane. Additionally, the in vitro assays showed that OsPRA2 inhibits its autophosphorylation. This OsPRA2-OsBRI1 interaction led to the dissociation of OsBRI1 from its co-receptor OsBAK1, and abolished OsBRI1-mediated phosphorylation of OsBAK1. Together, these results reveal a possible working mechanism of OsPRA2 as a novel negative regulator on OsBRI1 and OsBZR1 and extend the knowledge about the regulatory mechanism of rice BR signaling.

The MYB182 protein down-regulates proanthocyanidin and anthocyanin biosynthesis in poplar by repressing both structural and regulatory flavonoid genes

Trees in the genus Populus (poplar) contain phenolic secondary metabolites including the proanthocyanidins (PAs), which help to adapt these widespread trees to diverse environments. The transcriptional activation of PA biosynthesis in response to herbivory and ultraviolet light stress has been documented in poplar leaves, and a regulator of this process, the R2R3-MYB transcription factor MYB134, has been identified. MYB134-overexpressing transgenic plants show a strong high-PA phenotype. Analysis of these transgenic plants suggested the involvement of additional MYB transcription factors, including repressor-like MYB factors. Here, MYB182, a subgroup 4 MYB factor, was found to act as a negative regulator of the flavonoid pathway. Overexpression of MYB182 in hairy root culture and whole poplar plants led to reduced PA and anthocyanin levels as well as a reduction in the expression of key flavonoid genes. Similarly, a reduced accumulation of transcripts of a MYB PA activator and a basic helix-loop-helix cofactor was observed in MYB182-overexpressing hairy roots. Transient promoter activation assays in poplar cell culture demonstrated that MYB182 can disrupt transcriptional activation by MYB134 and that the basic helix-loop-helix-binding motif of MYB182 was essential for repression. Microarray analysis of transgenic plants demonstrated that down-regulated targets of MYB182 also include shikimate pathway genes. This work shows that MYB182 plays an important role in the fine-tuning of MYB134-mediated flavonoid metabolism.

Genome-wide identification and expression analyses of cytochrome P450 genes in mulberry (Morus notabilis)

Cytochrome P450s play critical roles in the biosynthesis of physiologically important compounds in plants. These compounds often act as defense toxins to prevent herbivory. In the present study, a total of 174 P450 genes of mulberry (Morus notabilis C.K.Schn) were identified based on bioinformatics analyses. These mulberry P450 genes were divided into nine clans and 47 families and were found to be expressed in a tissue-preferential manner. These genes were compared to the P450 genes in Arabidopsis thaliana. Families CYP80, CYP92, CYP728, CYP733, CYP736, and CYP749 were found to exist in mulberry, and they may play important roles in the biosynthesis of mulberry secondary metabolites. Analyses of the functional and metabolic pathways of these genes indicated that mulberry P450 genes may participate in the metabolism of lipids, other secondary metabolites, xenobiotics, amino acids, cofactors, vitamins, terpenoids, and polyketides. These results provide a foundation for understanding of the structures and biological functions of mulberry P450 genes.

Brassinosteroid-mediated regulation of agronomic traits in rice

Brassinosteroids (BRs) are a group of steroid phytohormones with wide-ranging biological activity. Genetic, genomic and proteomic studies have greatly advanced our understanding of BR signaling in Arabidopsis and revealed a connected signal transduction pathway from the cell surface receptor kinase BRASSINOSTEROID-INSENSITIVE1 (BRI1) and BRI1-ASSOCIATED RECEPTOR KINASE 1 (BAK1) to the BRASSINAZOLE-RESISTANT1 (BZR1) family of transcription factors and their targets mediating physiological functions. However, compared with the dicot model plant Arabidopsis, much less is known about BR signaling in rice, which is a monocot. In this review, we provide an update on the progress made by BR studies in rice and discuss how BR regulates various important agronomic traits to determine rice grain yield. Specifically, we discuss the function of novel components including LEAF AND TILLER ANGLE INCREASED CONTROLLER (LIC), DWARF and LOW-TILLERING (DLT), DWARF1 (D1) and TAIHU DWARF1 (TUD1) in rice BR signaling, and provide a rice BR-signaling pathway model that involves a BRI1-dependent pathway as well as a G-protein α subunit-mediated signaling pathway. The recent significant advances in our understanding of BR-mediated molecular mechanisms underlying agronomic traits will be of great help for rice molecular breeding.

Shotgun proteomic analysis of the Mexican lime tree infected with "CandidatusPhytoplasma aurantifolia"

Infection of Mexican lime trees (Citrus aurantifolia L.) with the specialized bacterium "CandidatusPhytoplasma aurantifolia" causes witches' broom disease. Witches' broom disease has the potential to cause significant economic losses throughout western Asia and North Africa. We used label-free quantitative shotgun proteomics to study changes in the proteome of Mexican lime trees in response to infection by "Ca. Phytoplasma aurantifolia". Of 990 proteins present in five replicates of healthy and infected plants, the abundances of 448 proteins changed significantly in response to phytoplasma infection. Of these, 274 proteins were less abundant in infected plants than in healthy plants, and 174 proteins were more abundant in infected plants than in healthy plants. These 448 proteins were involved in stress response, metabolism, growth and development, signal transduction, photosynthesis, cell cycle, and cell wall organization. Our results suggest that proteomic changes in response to infection by phytoplasmas might support phytoplasma nutrition by promoting alterations in the host's sugar metabolism, cell wall biosynthesis, and expression of defense-related proteins. Regulation of defense-related pathways suggests that defense compounds are induced in interactions with susceptible as well as resistant hosts, with the main differences between the two interactions being the speed and intensity of the response.

Regulation of brassinosteroid biosynthesis and inactivation

Brassinosteroids (BRs) are a group of naturally-occurring steroidal phytohormones playing fundamental roles during normal plant growth and development. Using a combination of experimental approaches, including analytical chemistry, genetics, and biochemistry, the major BR biosynthetic pathway has been largely elucidated. The least-understood knowledge in the BR research field is probably the molecular mechanisms controlling the bioactive levels of BRs in response to various developmental and environmental cues. In this review, we focus our discussion on a recently-proposed, 8-step predominant BR biosynthetic pathway, several newly-identified transcription factors regulating the expression of key enzymes that catalyze BR biosynthesis, and up-to-date information about the mechanisms that plants use to inactivate unnecessary BRs.

meta-Tyrosine in Festuca rubra ssp. commutata (Chewings fescue) is synthesized by hydroxylation of phenylalanine

m-Tyrosine is a non-protein amino acid that is structurally similar to the common protein amino acids p-tyrosine and phenylalanine. Copious amounts of m-tyrosine can be found in root exudates of the fine fescue cultivar, Festuca rubra L. ssp. commutata (Chewings fescue). The phytotoxicity of m-tyrosine may contribute to the allelopathic potential of F. rubra. m-Tyrosine in Euphorbia myrsinites (donkey-tail spurge), was previously shown to be synthesized via transamination of m-hydroxyphenylpyruvate. Here we show that m-tyrosine biosynthesis in F. rubra occurs through direct hydroxylation of phenylalanine in the root tips, perhaps through the activity of a cytochrome P450 enzyme. Hence, E. myrsinites and F. rubra, the only two plant species known to produce m-tyrosine, use distinct biosynthetic pathways that likely arose independently in evolutionary history.

Recent advances in the regulation of brassinosteroid signaling and biosynthesis pathways

Brassinosteroids (BRs) play important roles in plant growth, development and responses to environmental cues. BRs signal through plasma membrane receptor BRI1 and co-receptor BAK1, and several positive (BSK1, BSU1, PP2A) and negative (BKI1, BIN2 and 14-3-3) regulators to control the activities of BES1 and BZR1 family transcription factors, which regulate the expression of hundreds to thousands of genes for various BR responses. Recent studies identified novel signaling components in the BR pathways and started to establish the detailed mechanisms on the regulation of BR signaling. In addition, the molecular mechanism and transcriptional network through which BES1 and BZR1 control gene expression and various BR responses are beginning to be revealed. BES1 recruits histone demethylases ELF6 and REF6 as well as a transcription elongation factor IWS1 to regulate target gene expression. Identification of BES1 and BZR1 target genes established a transcriptional network for BR response and crosstalk with other signaling pathways. Recent studies also revealed regulatory mechanisms of BRs in many developmental processes and regulation of BR biosynthesis. Here we provide an overview and discuss some of the most recent progress in the regulation of BR signaling and biosynthesis pathways.

Auxin acts independently of DELLA proteins in regulating gibberellin levels

Shoot elongation is a vital process for plant development and productivity, in both ecological and economic contexts. Auxin and bioactive gibberellins (GAs), such as GA1, play critical roles in the control of elongation, along with environmental and endogenous factors, including other hormones such as the brassinosteroids. The effect of auxins, such as indole-3-acetic acid (IAA), is at least in part mediated by its effect on GA metabolism, since auxin up-regulates biosynthesis genes such as GA 3-oxidase and GA 20-oxidase and down regulates GA catabolism genes such as GA 2-oxidases, leading to elevated levels of bioactive GA 1. In our recent paper, we have provided evidence that this action of IAA is largely independent of DELLA proteins, the negative regulators of GA action, since the auxin effects are still present in the DELLA-deficient la cry-s genotype of pea. This was a crucial issue to resolve, since like auxin, the DELLAs also promote GA 1 synthesis and inhibit its deactivation. DELLAs are deactivated by GA, and thereby mediate a feedback system by which bioactive GA regulates its own level. However, our recent results, in themselves, do not show the generality of the auxin-GA relationship across species and phylogenetic groups or across different tissue types and responses. Further, they do not touch on the ecological benefits of the auxin-GA interaction. These issues are discussed below as well as the need for the development of suitable experimental systems to allow this process to be examined.

CESTA, a positive regulator of brassinosteroid biosynthesis

Brassinosteroids (BRs) are steroid hormones that are essential for the development of plants. A tight control of BR homeostasis is vital for modulating their impact on growth responses. Although it is recognized that the rapid adaptation of de novo synthesis has a key role in adjusting required BR levels, our knowledge of the mechanisms governing feedback control is limited. In this study, we identify the transcription factor CESTA as a regulator of BR biosynthesis. ces-D was isolated in a screen of Arabidopsis mutants by BR over-accumulation phenotypes. Loss-of-function analysis and the use of a dominant repressor version revealed functional overlap among CESTA and its homologues and confirmed the role of CESTA in the positive control of BR-biosynthetic gene expression. We provide evidence that CESTA interacts with its homologue BEE1 and can directly bind to a G-box motif in the promoter of the BR biosynthesis gene CPD. Moreover, we show that CESTA subnuclear localization is BR regulated and discuss a model, in which CESTA interplays with BEE1 to control BR biosynthesis and other BR responses.

Integration of light- and brassinosteroid-signaling pathways by a GATA transcription factor in Arabidopsis

Light and brassinosteroid (BR) antagonistically regulate the developmental switch from etiolation in the dark to photomorphogenesis in the light in plants. Here, we identify GATA2 as a key transcriptional regulator that mediates the crosstalk between BR- and light-signaling pathways. Overexpression of GATA2 causes constitutive photomorphogenesis in the dark, whereas suppression of GATA2 reduces photomorphogenesis caused by light, BR deficiency, or the constitutive photomorphogenesis mutant cop1. Genome profiling and chromatin immunoprecipitation experiments show that GATA2 directly regulates genes that respond to both light and BR. BR represses GATA2 transcription through the BR-activated transcription factor BZR1, whereas light causes accumulation of GATA2 protein and feedback inhibition of GATA2 transcription. Dark-induced proteasomal degradation of GATA2 is dependent on the COP1 E3 ubiquitin ligase, and COP1 can ubiquitinate GATA2 in vitro. This study illustrates a molecular framework for antagonistic regulation of gene expression and seedling photomorphogenesis by BR and light.

Whole genome co-expression analysis of soybean cytochrome P450 genes identifies nodulation-specific P450 monooxygenases

BACKGROUND

Cytochrome P450 monooxygenases (P450s) catalyze oxidation of various substrates using oxygen and NAD(P)H. Plant P450s are involved in the biosynthesis of primary and secondary metabolites performing diverse biological functions. The recent availability of the soybean genome sequence allows us to identify and analyze soybean putative P450s at a genome scale. Co-expression analysis using an available soybean microarray and Illumina sequencing data provides clues for functional annotation of these enzymes. This approach is based on the assumption that genes that have similar expression patterns across a set of conditions may have a functional relationship.

RESULTS

We have identified a total number of 332 full-length P450 genes and 378 pseudogenes from the soybean genome. From the full-length sequences, 195 genes belong to A-type, which could be further divided into 20 families. The remaining 137 genes belong to non-A type P450s and are classified into 28 families. A total of 178 probe sets were found to correspond to P450 genes on the Affymetrix soybean array. Out of these probe sets, 108 represented single genes. Using the 28 publicly available microarray libraries that contain organ-specific information, some tissue-specific P450s were identified. Similarly, stress responsive soybean P450s were retrieved from 99 microarray soybean libraries. We also utilized Illumina transcriptome sequencing technology to analyze the expressions of all 332 soybean P450 genes. This dataset contains total RNAs isolated from nodules, roots, root tips, leaves, flowers, green pods, apical meristem, mock-inoculated and Bradyrhizobium japonicum-infected root hair cells. The tissue-specific expression patterns of these P450 genes were analyzed and the expression of a representative set of genes were confirmed by qRT-PCR. We performed the co-expression analysis on many of the 108 P450 genes on the Affymetrix arrays. First we confirmed that CYP93C5 (an isoflavone synthase gene) is co-expressed with several genes encoding isoflavonoid-related metabolic enzymes. We then focused on nodulation-induced P450s and found that CYP728H1 was co-expressed with the genes involved in phenylpropanoid metabolism. Similarly, CYP736A34 was highly co-expressed with lipoxygenase, lectin and CYP83D1, all of which are involved in root and nodule development.

CONCLUSIONS

The genome scale analysis of P450s in soybean reveals many unique features of these important enzymes in this crop although the functions of most of them are largely unknown. Gene co-expression analysis proves to be a useful tool to infer the function of uncharacterized genes. Our work presented here could provide important leads toward functional genomics studies of soybean P450s and their regulatory network through the integration of reverse genetics, biochemistry, and metabolic profiling tools. The identification of nodule-specific P450s and their further exploitation may help us to better understand the intriguing process of soybean and rhizobium interaction.

A small GTPase activator protein interacts with cytoplasmic phytochromes in regulating root development

Phytochromes enable plants to sense light information and regulate developmental responses. Phytochromes interact with partner proteins to transmit light signals to downstream components for plant development. PIRF1 (phytochrome-interacting ROP guanine-nucleotide exchange factor (RopGEF 1)) functions as a light-signaling switch regulating root development through the activation of ROPs (Rho-like GTPase of plant) in the cytoplasm. In vitro pulldown and yeast two-hybrid assays confirmed the interaction between PIRF1 and phytochromes. PIRF1 interacted with the N-terminal domain of phytochromes through its conserved PRONE (plant-specific ROP nucleotide exchanger) region. PIRF1 also interacted with ROPs and activated them in a phytochrome-dependent manner. The Pr form of phytochrome A enhanced the RopGEF activity of PIRF1, whereas the Pfr form inhibited it. A bimolecular fluorescence complementation analysis demonstrated that PIRF1 was localized in the cytoplasm and bound to the phytochromes in darkness but not in light. PIRF1 loss of function mutants (pirf1) of Arabidopsis thaliana showed a longer root phenotype in the dark. In addition, both PIRF1 overexpression mutants (PIRF1-OX) and phytochrome-null mutants (phyA-211 and phyB-9) showed retarded root elongation and irregular root hair formation, suggesting that PIRF1 is a negative regulator of phytochrome-mediated primary root development. We propose that phytochrome and ROP signaling are interconnected through PIRF1 in regulating the root growth and development in Arabidopsis.

Physiological regulation and functional significance of shade avoidance responses to neighbors

Plants growing in dense vegetations compete with their neighbors for resources such as water, nutrients and light. The competition for light has been particularly well studied, both for its fitness consequences as well as the adaptive behaviors that plants display to win the battle for light interception. Aboveground, plants detect their competitors through photosensory cues, notably the red:far-red light ratio (R:FR). The R:FR is a very reliable indicator of future competition as it decreases in a plant-specific manner though red light absorption for photosynthesis and is sensed with the phytochrome photoreceptors. In addition, also blue light depletion is perceived for neighbor detection. As a response to these light signals plants display a suite of phenotypic traits defined as the shade avoidance syndrome (SAS). The SAS helps to position the photosynthesizing leaves in the higher zones of a canopy where light conditions are more favorable. In this review we will discuss the physiological control mechanisms through which the photosensory signals are transduced into the adaptive phenotypic responses that make up the SAS. Using this mechanistic knowledge as a starting point, we will discuss how the SAS functions in the context of the complex multi-facetted environments that plants usually grow in.

Arabidopsis RAB geranylgeranyl transferase beta-subunit mutant is constitutively photomorphogenic, and has shoot growth and gravitropic defects

RAB GTPases are important directional regulators of intracellular vesicle transport. Membrane localization of RAB GTPases is mediated by C-terminal double geranylgeranylation. This post-translational modification is catalyzed by the alpha-beta-heterodimer catalytic core of RAB geranylgeranyl transferase (RAB-GGT), which cooperates with the RAB escort protein (REP) that presents a nascent RAB. Here, we show that RAB-geranylgeranylation activity is significantly reduced in two homozygous mutants of the major Arabidopsis beta-subunit of RAB-GGT (AtRGTB1), resulting in unprenylated RAB GTPases accumulation in the cytoplasm. Both endocytosis and exocytosis are downregulated in rgtb1 homozygotes defective in shoot growth and morphogenesis. Root gravitropism is normal in rgtb1 roots, but is significantly compromised in shoots. Mutants are defective in etiolation and show constitutive photomorphogenic phenotypes that cannot be rescued by brassinosteroid treatment, similarly to the det3 mutant that is also defective in the secretory pathway. Transcriptomic analysis revealed an upregulation of specific RAB GTPases in etiolated wild-type plants. Taken together, these data suggest that the downregulation of the secretory pathway is interpreted as a photomorphogenic signal in Arabidopsis.

From dwarves to giants? Plant height manipulation for biomass yield

The increasing demand for lignocellulosic biomass for the production of biofuels provides value to vegetative plant tissue and leads to a paradigm shift for optimizing plant architecture in bioenergy crops. Plant height (PHT) is among the most important biomass yield components and is the focus of this review, with emphasis on the energy grasses maize (Zea mays) and sorghum (Sorghum bicolor). We discuss the scientific advances in the identification of PHT quantitative trait loci (QTLs) and the understanding of pathways and genes controlling PHT, especially gibberellins and brassinosteroids. We consider pleiotropic effects of QTLs or genes affecting PHT on other agronomically important traits and, finally, we discuss strategies for applying this knowledge to the improvement of dual-purpose or dedicated bioenergy crops.

Genome-wide analysis revealed the complex regulatory network of brassinosteroid effects in photomorphogenesis

Light and brassinosteroids (BRs) have been proved to be crucial in regulating plant growth and development; however, the mechanism of how they synergistically function is still largely unknown. To explore the underlying mechanisms in photomorphogenesis, genome-wide analyses were carried out through examining the gene expressions of the dark-grown WT or BR biosynthesis-defective mutant det2 seedlings in the presence of light stimuli or exogenous Brassinolide (BL). Results showed that BR deficiency stimulates, while BL treatment suppresses, the expressions of light-responsive genes and photomorphogenesis, confirming the negative effects of BR in photomorphogenesis. This is consistent with the specific effects of BR on the expression of genes involved in cell wall modification, cellular metabolism and energy utilization during dark-light transition. Further analysis revealed that hormone biosynthesis and signaling-related genes, especially those of auxin, were altered under BL treatment or light stimuli, indicating that BR may modulate photomorphogenesis through synergetic regulation with other hormones. Additionally, suppressed ubiquitin-cycle pathway during light-dark transition hinted the presence of a complicated network among light, hormone, and protein degradation. The study provides the direct evidence of BR effects in photomorphogenesis and identified the genes involved in BR and light signaling pathway, which will help to elucidate the molecular mechanism of plant photomorphogenesis.

Arabidopsis nucleoside diphosphate kinase-2 as a plant GTPase activating protein

Nucleoside diphosphate kinase (NDPK) is involved in multiple signaling pathways in mammalian systems, including G-protein signaling. Arabidopsis NDPK2, like its mammalian counterparts, is multifunctional despite its initial discovery phytochrome-interacting protein. This similarity raises the possibility that NDPK2 may play a role in G-protein signaling in plants. In the present study, we explore the potential relationship between NDPK2 and the small G proteins, Pra2 and Pra3, as well as the heterotrimeric G protein, GPA1. We report a physical interaction between NDPK2 and these small G proteins, and demonstrate that NDPK2 can stimulate their GTPase activities. Our results suggest that NDPK2 acts as a GTPase-activating protein for small G proteins in plants. We propose that NDPK2 might be a missing link between the phytochromemediated light signaling and G protein-mediated signaling.

Brassinosteroids, de-etiolation and the re-emerging art of plant hormone quantification

An increase in the use of molecular techniques has provided a significant insight into the function of genes, and how they are regulated and interact. However, in the field of plant hormone physiology, the increased use of these techniques has been accompanied by a reduction in the direct measurement of plant hormone levels by physiochemical methods. Instead, the transcript (mRNA) levels of genes involved in hormone metabolism are often used to predict endogenous hormone levels. The validity of this approach was recently tested by comparing the expression of a range of genes involved in BR synthesis, catabolism and perception, with the actual endogenous BR levels in pea seedlings grown under different light conditions.1,2 Based on this comparison, we now argue that gene expression analysis alone is not always a reliable indicator of endogenous hormone levels.

Tomato Rab11a characterization evidenced a difference between SYP121-dependent and SYP122-dependent exocytosis

The regulatory functions of Rab proteins in membrane trafficking lie in their ability to perform as molecular switches that oscillate between a GTP- and a GDP-bound conformation. The role of tomato LeRab11a in secretion was analyzed in tobacco protoplasts. Green fluorescent protein (GFP)/red fluorescent protein (RFP)-tagged LeRab11a was localized at the trans-Golgi network (TGN) in vivo. Two serines in the GTP-binding site of the protein were mutagenized, giving rise to the three mutants Rab11S22N, Rab11S27N and Rab11S22/27N. The double mutation reduced secretion of a marker protein, secRGUS (secreted rat beta-glucuronidase), by half, whereas each of the single mutations alone had a much smaller effect, showing that both serines have to be mutated to obtain a dominant negative effect on LeRab11a function. The dominant negative mutant was used to determine whether Rab11 is involved in the pathway(s) regulated by the plasma membrane syntaxins SYP121 and SYP122. Co-expression of either of these GFP-tagged syntaxins with the dominant negative Rab11S22/27N mutant led to the appearance of endosomes, but co-expression of GFP-tagged SYP122 also labeled the endoplasmic reticulum and dotted structures. However, co-expression of Rab11S22/27N with SYP121 dominant negative mutants decreased secretion of secRGUS further compared with the expression of Rab11S22/27N alone, whereas co-expression of Rab11S22/27N with SYP122 had no synergistic effect. With the same essay, the difference between SYP121- and SYP122-dependent secretion was then evidenced. The results suggest that Rab11 regulates anterograde transport from the TGN to the plasma membrane and strongly implicate SYP122, rather than SYP121. The differential effect of LeRab11a supports the possibility that SYP121 and SYP122 drive independent secretory events.

Is kinase activity essential for biological functions of BRI1?

Brassinosteroids (BRs) are a major group of plant hormones that regulate plant growth and development. BRI1, a protein localized to the plasma membrane, functions as a BR receptor and it has been proposed that its kinase activity has an essential role in BR-regulated plant growth and development. Here we report the isolation and molecular characterization of a new allele of bri1, bri1-301, which shows moderate morphological phenotypes and a reduced response to BRs under normal growth conditions. Sequence analysis identified a two-base alteration from GG to AT, resulting in a conversion of 989G to 989I in the BRI1 kinase domain. An in vitro assay of kinase activity showed that bri1-301 has no detectable autophosphorylation activity or phosphorylation activity towards the BRI1 substrates TTL and BAK1. Furthermore, our results suggest that bri1-301, even with extremely impaired kinase activity, still retains partial function in regulating plant growth and development, which raises the question of whether BRI1 kinase activity is essential for BR-mediated growth and development in higher plants.

Spatial redistribution of key transcriptional regulators in brassinosteroid signaling

Brassinosteroids (BRs) are steroid phytohormones required for plant growth and development. The perception of BRs at the plasma membrane initiates intracellular signaling and induces dephosphorylation of two key transcription factors, BZR1 and BZR2/BES1. Phosphorylation of these factors is modulated by the GSK3 kinase BIN2 and phosphatase BSU1 and, in turn, controls DNA binding, protein stabilization, or/and nuclear translocation of BZR1 and BZR2/BES1. However, cytosolic signaling events and the biological roles of phosphorylation in BR signaling are still largely unknown. Recently, we demonstrated that BZR1 itself acts as a cytosolic signaling mediator and regulates expression of BR-responsive genes via phosphorylation-mediated nucleocytoplasmic shuttling. BIN2-mediated phosphorylation mediates nuclear export of BZR1 via interaction with a 14-3-3 protein, while BR activated phosphatases induce nuclear import of BZR1. The temporal and spatial expression of BIN2 appears to be essential in BR signaling. In this addendum, we summarize new findings in BR signaling and discuss the possibility that light and brassinosteroid signals intersect at BIN2 expression.

Constitutive overexpression of a stress-inducible small GTP-binding protein PgRab7 from Pennisetum glaucum enhances abiotic stress tolerance in transgenic tobacco

The Rab GTPases are important components of endocytic network in plant cells. Endocytosis participates in the cell's reaction to extracellular stimuli by desensitizing, down-regulating or recycling receptors and membrane proteins. Rab7 is a small GTP-binding protein involved in intracellular vesicle trafficking from late endosome to the vacuole. We have isolated Rab7 cDNA from Pennisetum glaucum, a relatively drought-stress tolerant food grain crop grown commonly in India, during cDNA-subtractive hybridization of dehydration-stress treated plants. The PgRab7 ORF, encoding 207 aminoacids, was over-expressed in E. coli. The recombinant PgRab7 protein showed GTP-binding and GTPase activity. Transcript expression of PgRab7 gene was differentially up-regulated by different environmental stimuli such as cold, dehydration and NaCl and also by a plant hormone IAA. Overexpression of PgRab7 gene enhanced tolerance to NaCl and mannitol in transgenic tobacco. Transgenic plants also had increased alkaline phosphatase (ALP) activity. These results show that PgRab7 is a potential candidate gene for developing both salinity and dehydration tolerance in planta.

Circadian rhythms: rho-related signals in time-specific light perception

A recent study shows that a small GTPase, LIF1, helps to coordinate the plant circadian clock with the daily light-dark cycle.

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.

Arabidopsis thaliana Circadian Clock Is Regulated by the Small GTPase LIP1

BACKGROUND

At the core of the eukaryotic circadian network, clock genes/proteins form multiple transcriptional/translational negative-feedback loops and generate a basic approximately 24 hr oscillation, which provides daily regulation for a wide range of processes. This temporal organization enhances the fitness of the organism only if it corresponds to the natural day/night cycles. Light is the most effective signal in synchronizing the oscillator to environmental cycles.

RESULTS

The lip1-1 (light insensitive period 1) mutant isolated from the model plant Arabidopsis thaliana displays novel circadian phenotypes arising from specific defects in the light input pathway to the oscillator. In wild-type plants, period length shortens with increasing light fluence rates and the phase of rhythms can be shifted by light pulses administered to dark-adapted plants. In contrast, in lip1-1, period length is nearly insensitive to light intensity and significantly larger phase shifts (delays) can be induced during the subjective night. The mutant also displays elevated photomorphogenic responses to red and blue light, which cannot be explained by the circadian defect, suggesting distinct functions for LIP1 in the circadian light input and photomorphogenesis. The LIP1 gene encodes a functional, plant-specific atypical small GTPase, and therefore we postulate that it acts similarly to ZEITLUPE at postranscriptional level.

CONCLUSIONS

LIP1 represents the first small GTPase implicated in the circadian system of plants. LIP1 plays a unique negative role in controlling circadian light input and is required for precise entrainment of the plant clock.

Regulation of brassinosteroid responses by phytochrome B in rice

Plant growth and development are coordinately controlled by environmental signals and internal factors. Light signals, mediated by phytochromes, regulate photomorphogenesis by interacting with endogenous programmes that involve multiple phytohormones. Brassinosteroids (BRs) are a group of growth-promoting phytohormones with a crucial role in the light-dependent development of plants. However, the interaction between light-signalling pathways and BR signalling is not well understood. Here, we examined the responses of lamina joint inclination and coleoptile elongation to exogenous brassinolide (BL) under light or dark conditions. Both responses were more pronounced under darkness, implying that BR signalling is inhibited by light. To elucidate which phytochrome is involved in this interaction, we isolated rice phytochrome-deficient mutants (osphyA, osphyB and osphyC) from a T-DNA insertional population. Whereas the osphyA and osphyC knockout mutants did not differ from the wild-type plants in their BL responses, osphyB mutants were more sensitive. In addition, RT-PCR analysis revealed enhanced expression of BR-inducible genes and decreased transcript levels of BR-biosynthetic genes in osphyB plants. These results suggest that Phytochrome B acts as a negative regulator of BL-regulated growth and development processes in rice.

Roles of brassinosteroids and related mRNAs in pea seed growth and germination

The levels of endogenous brassinosteroids (BRs) and the expression of the biosynthesis/metabolism/perception genes involved have been investigated during the development and germination of pea (Pisum sativum) seeds. When seeds were rapidly growing, the level of biologically active BRs (brassinolide [BL] and castasterone [CS]) and the transcript levels of two BR C-6 oxidases (CYP85A1 and CYP85A6) reached a maximum, suggesting the significance of BL and CS in seed development. In the early stages of germination, CS, but not BL, appeared and its level increased in the growing tissues in which the transcript level of CYP85A1 and CYP85A6 was high, suggesting the significance of CS in seed germination and early seedling growth of pea. 6-Deoxocathasterone (6-deoxoCT) was the quantitatively major BR in mature seeds. At the early stage of germination, the level of 6-deoxoCT was specifically decreased, whereas the levels of downstream intermediates were increased. It seems that 6-deoxoCT is the major storage BR and is utilized during germination and early growth stages. The level of the mRNAs of BR biosynthesis and perception genes fluctuated during seed development. In mature seeds, most of mRNAs were present, but the level was generally lower compared with immature seeds. However, CYP90A9 mRNA rapidly increased during seed development and reached the maximum in mature seeds. The mRNAs stored in mature pea seeds seem to be utilized when seeds germinate. However, it was found that de novo transcription of mRNAs also starts as early as during seed imbibition.

Brassinosteroid-independent function of BRI1/CLV1 chimeric receptors

CLAVATA1 (CLV1) and BRASSINOSTEROID INSENSITIVE 1 (BRI1) belong to the leucine-rich repeat receptor-like kinase (LRR-RLK) family, comprising more than 200 members in Arabidopsis thaliana (L.) Heynh. and playing important roles in development and defence responses in many plant species (Diévart and Clark 2003, 2004; Shiu and Bleecker 2001a, b). To dissect the mechanisms of receptor function, we assessed the ability of chimeric proteins containing regions from two different receptors to function in vivo. Using domains from the receptor-kinases CLAVATA1 and BRASSINOSTEROID INSENSITIVE1, we tested the ability of the resulting chimeric receptors to replace CLV1 function. Receptors with the BRI1 extracellular domain and CLV1 kinase domain were able to partially replace CLV1 function. Both loss-of-function and gain-of-function mutations within the BRI1 leucine-rich repeats (LRRs) altered the extent of rescue. Chimeric receptor function was unaffected by addition of either exogenous brassinosteroids (BR) or BR biosynthesis inhibitors, suggesting that the chimeric receptors function in a ligand-independent fashion. We propose that the BRI1 LRR domain drives chimeric receptor homodimerisation, and that the BRI1 LRR domain mutations influence homodimerisation efficiency independent of ligand binding.

The Arabidopsis ARGOS-LIKE gene regulates cell expansion during organ growth

Cell expansion, and its coordination with cell division, plays a critical role in the growth and development of plant organs. However, the genes controlling cell expansion during organogenesis are largely unknown. Here, we demonstrate that a novel Arabidopsis gene, ARGOS-LIKE (ARL), which has some sequence homology to the ARGOS gene, is involved in this process. Reduced expression or overexpression of ARL in Arabidopsis results in smaller or larger cotyledons and leaves as well as other lateral organs, respectively. Anatomical examination of cotyledons and leaves in ARL transgenic plants demonstrates that the alteration in size can be attributed to changes in cell size rather than cell number, indicating that ARL plays a role in cell expansion-dependent organ growth. ARL is upregulated by brassinosteroid (BR) and this induction is impaired in the BR-insensitive mutant bri1, but not in the BR-deficient mutant det2. Ectopic expression of ARL in bri1-119 partially restores cell growth in cotyledons and leaves. Our results suggest that ARL acts downstream of BRI1 and partially mediates BR-related cell expansion signals during organ growth.

Diurnal regulation of plant growth

Life occurs in an ever-changing environment. Some of the most striking and predictable changes are the daily rhythms of light and temperature. To cope with these rhythmic changes, plants use an endogenous circadian clock to adjust their growth and physiology to anticipate daily environmental changes. Most studies of circadian functions in plants have been performed under continuous conditions. However, in the natural environment, diurnal outputs result from complex interactions of endogenous circadian rhythms and external cues. Accumulated studies using the hypocotyl as a model for plant growth have shown that both light signalling and circadian clock mutants have growth defects, suggesting strong interactions between hypocotyl elongation, light signalling and the circadian clock. Here, we review evidence suggesting that light, plant hormones and the circadian clock all interact to control diurnal patterns of plant growth.

The regulation of DWARF4 expression is likely a critical mechanism in maintaining the homeostasis of bioactive brassinosteroids in Arabidopsis

Mutants that are defective in brassinosteroid (BR) biosynthesis or signaling display severely retarded growth patterns due to absence of growth-promoting effects by BRs. Arabidopsis (Arabidopsis thaliana) DWARF4 (DWF4) catalyzes a flux-determining step in the BR biosynthetic pathways. Thus, it is hypothesized that the tissues of DWF4 expression may represent the sites of BR biosynthesis in Arabidopsis. Here we show that DWF4 transcripts accumulate in the actively growing tissues, such as root, shoot apices with floral clusters, joint tissues of root and shoot, and dark-grown seedlings. Conforming to the RNA gel-blot analysis, DWF4:beta-glucuronidase (GUS) histochemical analyses more precisely define the tissues that express the DWF4 gene. Examination of the endogenous levels of BRs in six and seven different tissues of wild type and brassinosteroid insensitive1-5 mutant, respectively, revealed that BRs are significantly enriched in roots, shoot tips, and joint tissues of roots and shoots. In addition, DWF4:GUS expression was negatively regulated by BRs. DWF4:GUS activity was increased by treatment with brassinazole, a BR biosynthetic inhibitor, and decreased by exogenous application of bioactive BRs. When DWF4:GUS was expressed in a different genetic background, its level was down-regulated in brassinazole resistant1-D, confirming that BRASSINAZOLE RESISTANT1 acts as a negative regulator of DWF4. Interestingly, in the brassinosteroid insensitive2/dwf12-1D background, DWF4:GUS expression was intensified and delocalized to elongating zones of root, suggesting that BRASSINOSTEROID INSENSITIVE2 is an important factor that limits DWF4 expression. Thus, it is likely that the DWF4 promoter serves as a focal point in maintaining homeostasis of endogenous bioactive BR pools in specific tissues of Arabidopsis.

Molecular mechanisms of steroid hormone signaling in plants

Brassinosteroids (BRs), the polyhydroxylated steroid hormones of plants, regulate the growth and differentiation of plants throughout their life cycle. Over the past several years, genetic and biochemical approaches have yielded great progress in understanding BR signaling. Unlike their animal counterparts, BRs are perceived at the plasma membrane by direct binding to the extracellular domain of the BRI1 receptor S/T kinase. BR perception initiates a signaling cascade, acting through a GSK3 kinase, BIN2, and the BSU1 phosphatase, which in turn modulates the phosphorylation state and stability of the nuclear transcription factors BES1 and BZR1. Microarray technology has been used extensively to provide a global view of BR genomic effects, as well as a specific set of target genes for BES1 and BZR1. These gene products thus provide a framework for how BRs regulate the growth of plants.

OsGLU1, a putative membrane-bound endo-1,4-beta-D-glucanase from rice, affects plant internode elongation

A dwarf mutant glu was identified from screening of T-DNA tagged rice population. Genetic analysis of the T1 generation of glu revealed that a segregation ratio of wild-type:dwarf phenotype was 3:1, suggesting that the mutated phenotype was controlled by a single recessive nuclear locus. The mutated gene OsGLU1, identified by Tail-PCR, encodes a putative membrane-bound endo-1,4-beta-D-glucanase, which is highly conserved between mono- and dicotyledonous plants. Mutation of OsGLU1 resulted in a reduction in cell elongation, and a decrease in cellulose content but an increase in pectin content, suggesting that OsGLU1 affects the internode elongation and cell wall components of rice plants. Transgenic glu mutants harboring the OsGLU1 gene complemented the mutation and displayed the wild-type phenotype. In addition, OsGLU1 RNAi plants showed similar phenotype as the glu mutant has. These results indicate that OsGLU1 plays important roles in plant cell growth. Gibberellins and brassinosteroids induced OsGLU1 expression. In rice genome, endo-1,4-beta-D-glucanases form a multiple gene family with 15 members, and each may have a distinct expression pattern in different organs. These results indicate that endo-1,4-beta-D-glucanases may play diverse roles in growth and developmental process of rice plants.

Ca2+/calmodulin is critical for brassinosteroid biosynthesis and plant growth

Brassinosteroids are plant-specific steroid hormones that have an important role in coupling environmental factors, especially light, with plant growth and development. How the endogenous brassinosteroids change in response to environmental stimuli is largely unknown. Ca2+/calmodulin has an essential role in sensing and transducing environmental stimuli. Arabidopsis DWARF1 (DWF1) is responsible for an early step in brassinosteroid biosynthesis that converts 24-methylenecholesterol to campesterol. Here we show that DWF1 is a Ca2+/calmodulin-binding protein and this binding is critical for its function. Molecular genetic analysis using site-directed and deletion mutants revealed that loss of calmodulin binding completely abolished the function of DWF1 in planta, whereas partial loss of calmodulin binding resulted in a partial dwarf phenotype in complementation studies. These results provide direct proof that Ca2+/calmodulin-mediated signalling has a critical role in controlling the function of DWF1. Furthermore, we observed that DWF1 orthologues from other plants have a similar Ca2+/calmodulin-binding domain, implying that Ca2+/calmodulin regulation of DWF1 and its homologues is common in plants. These results raise the possibility of producing size-engineered crops by altering the Ca2+/calmodulin-binding property of their DWF1 orthologues.