Farnesylation mediates brassinosteroid biosynthesis to regulate abscisic acid responses

Crosstalk of the Brassinosteroid Signalosome with Phytohormonal and Stress Signaling Components Maintains a Balance between the Processes of Growth and Stress Tolerance

Brassinosteroids (BRs) are a class of phytohormones, which regulate various processes during plant life cycle. Intensive studies conducted with genetic, physiological and molecular approaches allowed identification of various components participating in the BR signaling—from the ligand perception, through cytoplasmic signal transduction, up to the BR-dependent gene expression, which is regulated by transcription factors and chromatin modifying enzymes. The identification of new components of the BR signaling is an ongoing process, however an emerging view of the BR signalosome indicates that this process is interconnected at various stages with other metabolic pathways. The signaling crosstalk is mediated by the BR signaling proteins, which function as components of the transmembrane BR receptor, by a cytoplasmic kinase playing a role of the major negative regulator of the BR signaling, and by the transcription factors, which regulate the BR-dependent gene expression and form a complicated regulatory system. This molecular network of interdependencies allows a balance in homeostasis of various phytohormones to be maintained. Moreover, the components of the BR signalosome interact with factors regulating plant reactions to environmental cues and stress conditions. This intricate network of interactions enables a rapid adaptation of plant metabolism to constantly changing environmental conditions.

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.

BRexit: Possible Brassinosteroid Export and Transport Routes

The movement and differential distribution of endogenous plant hormones are the determining factors for many developmental processes. Brassinosteroids (BRs) are a group of plant steroidal hormones that promote growth and development. Although synthesis and signalling of BRs are well described and characterized, the exit mechanism of these compounds from the cell remains uncharacterizd. Whether BRs are able to move within the apoplast and whether the BR synthesis in one cell can trigger the signalling in its neighbours is also unknown. Here, we draw the attention to these aspects of the BR biology, propose several BR cell export routes and discuss possible consequences of local BR hormonal gradients, resulting from localized biosynthesis and a short-distance transport, for plant development.

Rice SUB1A constrains remodelling of the transcriptome and metabolome during submergence to facilitate post-submergence recovery

The rice (Oryza sativa L.) ethylene-responsive transcription factor gene SUB1A-1 confers tolerance to prolonged, complete submergence by limiting underwater elongation growth. Upon desubmergence, SUB1A-1 genotypes rapidly recover photosynthetic function and recommence development towards flowering. The underpinnings of the transition from stress amelioration to the return to homeostasis are not well known. Here, transcriptomic and metabolomic analyses were conducted to identify mechanisms by which SUB1A improves physiological function over the 24 hr following a sublethal submergence event. Evaluation of near-isogenic genotypes after submergence and over a day of reaeration demonstrated that SUB1A transiently constrains the remodelling of cellular activities associated with growth. SUB1A influenced the abundance of ca. 1,400 transcripts and had a continued impact on metabolite content, particularly free amino acids, glucose, and sucrose, throughout the recovery period. SUB1A promoted recovery of metabolic homeostasis but had limited influence on mRNAs associated with growth processes and photosynthesis. The involvement of low energy sensing during submergence and recovery was supported by dynamics in trehalose-6-phosphate and mRNAs encoding key enzymes and signalling proteins, which were modulated by SUB1A. This study provides new evidence of convergent signalling pathways critical to the rapidly reversible management of carbon and nitrogen metabolism in submergence resilient rice.

Regulation of Arabidopsis brassinosteroid receptor BRI1 endocytosis and degradation by plant U-box PUB12/PUB13-mediated ubiquitination

Plants largely rely on plasma membrane (PM)-resident receptor-like kinases (RLKs) to sense extracellular and intracellular stimuli and coordinate cell differentiation, growth, and immunity. Several RLKs have been shown to undergo internalization through the endocytic pathway with a poorly understood mechanism. Here, we show that endocytosis and protein abundance of the Arabidopsis brassinosteroid (BR) receptor, BR INSENSITIVE1 (BRI1), are regulated by plant U-box (PUB) E3 ubiquitin ligase PUB12- and PUB13-mediated ubiquitination. BR perception promotes BRI1 ubiquitination and association with PUB12 and PUB13 through phosphorylation at serine 344 residue. Loss of PUB12 and PUB13 results in reduced BRI1 ubiquitination and internalization accompanied with a prolonged BRI1 PM-residence time, indicating that ubiquitination of BRI1 by PUB12 and PUB13 is a key step in BRI1 endocytosis. Our studies provide a molecular link between BRI1 ubiquitination and internalization and reveal a unique mechanism of E3 ligase-substrate association regulated by phosphorylation.

Heat-shock protein 40 is the key farnesylation target in meristem size control, abscisic acid signaling, and drought resistance

In this study, Barghetti et al. investigate the functions of protein farnesylation in plants. They show that defective farnesylation of a single factor—heat-shock protein 40 (HSP40), encoded by the J2 and J3 genes—is sufficient to confer ABA hypersensitivity, drought resistance, late flowering, and enlarged meristems, indicating that altered function of chaperone client proteins underlies most farnesyl transferase mutant phenotypes.

Protein farnesylation is central to molecular cell biology. In plants, protein farnesyl transferase mutants are pleiotropic and exhibit defective meristem organization, hypersensitivity to the hormone abscisic acid, and increased drought resistance. The precise functions of protein farnesylation in plants remain incompletely understood because few relevant farnesylated targets have been identified. Here, we show that defective farnesylation of a single factor—heat-shock protein 40 (HSP40), encoded by the J2 and J3 genes—is sufficient to confer ABA hypersensitivity, drought resistance, late flowering, and enlarged meristems, indicating that altered function of chaperone client proteins underlies most farnesyl transferase mutant phenotypes. We also show that expression of an abiotic stress-related microRNA (miRNA) regulon controlled by the transcription factor SPL7 requires HSP40 farnesylation. Expression of a truncated SPL7 form mimicking its activated proteolysis fragment of the membrane-bound SPL7 precursor partially restores accumulation of SPL7-dependent miRNAs in farnesyl transferase mutants. These results implicate the pathway directing SPL7 activation from its membrane-bound precursor as an important target of farnesylated HSP40, consistent with our demonstration that HSP40 farnesylation facilitates its membrane association. The results also suggest that altered gene regulation via select miRNAs contributes to abiotic stress-related phenotypes of farnesyl transferase mutants.

WRKY transcription factors are involved in brassinosteroid signaling and mediate the crosstalk between plant growth and drought tolerance

Brassinosteroids (BRs) are critical for the plant growth and development. BRs signal through the plasma membrane localized receptor-like kinases to downstream transcription factors BES1/BZR1 to regulate the expression of thousands of genes for various BR responses. In addition to the role in plant growth and development, BRs have been implicated in responses to environmental stresses such as drought. However, the mechanism through which BRs regulate drought have just begun emerging. We have recently found that a group of WRKY transcription factors, WRKY46, WRKY54, WRKY70, which are well known for the function in abiotic and biotic stress, cooperates with BES1 to mediate BR-regulated drought response. The wrky46 wrky54 wrky70 triple mutants showed growth defect, likely due to impaired BR signaling as well as some reduction of endogenous BR level. WRKY46/54/70 cooperates with BES1 to regulate the expression of BR target genes to promote growth. We also found that WRKY46/54/70 negatively modulates drought tolerance by globally repressing drought-inducible gene expression. Thus, our result uncovers a new role for WRKY transcription factors in BR signaling and provides the molecular mechanism for BR-regulated plant growth and drought stress through WRKY46/54/70 and BES1 transcription factors.

Overexpression of Arabidopsis thaliana brassinosteroid-related acyltransferase 1 gene induces brassinosteroid-deficient phenotypes in creeping bentgrass

Brassinosteroids (BRs) are naturally occurring steroidal hormones that play diverse roles in various processes during plant growth and development. Thus, genetic manipulation of endogenous BR levels might offer a way of improving the agronomic traits of crops, including plant architecture and stress tolerance. In this study, we produced transgenic creeping bentgrass (Agrostis stolonifera L.) overexpressing a BR-inactivating enzyme, Arabidopsis thaliana BR-related acyltransferase 1 (AtBAT1), which is known to catalyze the conversion of BR intermediates to inactive acylated conjugates. After putative transgenic plants were selected using herbicide resistance assay, genomic integration of the AtBAT1 gene was confirmed by genomic PCR and Southern blot analysis, and transgene expression was validated by northern blot analysis. The transgenic creeping bentgrass plants exhibited BR-deficient phenotypes, including reduced plant height with shortened internodes (i.e., semi-dwarf), reduced leaf growth rates with short, wide, and thick architecture, high chlorophyll contents, decreased numbers of vascular bundles, and large lamina joint bending angles (i.e., erect leaves). Subsequent analyses showed that the transgenic plants had significantly reduced amounts of endogenous BR intermediates, including typhasterol, 6-deoxocastasterone, and castasterone. Moreover, the AtBAT1 transgenic plants displayed drought tolerance as well as delayed senescence. Therefore, the results of the present study demonstrate that overexpression of an Arabidopsis BR-inactivating enzyme can reduce the endogenous levels of BRs in creeping bentgrass resulting in BR-deficient phenotypes, indicating that the AtBAT1 gene from a dicot plant is also functional in the monocot crop.

N. Hickerson N, Samuel MA. Farnesylation-mediated subcellular localization is required for CYP85A2 function

Protein farnesylation refers to the addition of a 15-carbon farnesyl isoprenoid to the cysteine residue of the CaaX motif at the carboxy terminus of target proteins. In spite of its known roles in plant development and abiotic stress tolerance, how these processes are precisely regulated by farnesylation had remained elusive. We recently showed that CYP85A2, the cytochrome P450, which converts castasterone to brassinolide in the last step of brassinosteroid synthesis must be farnesylated in order to function in this pathway. Lack of either CYP85A2 or the farnesylation motif of CYP85A2 resulted in reduced brassinolide accumulation, hypersensitivity to ABA, and increased plant drought tolerance. In this study, we have assessed the influence of the N-terminal secretory signal and the C-terminal CaaX motif of CYP85A2 in mediating CYP85A2 function and targeting to endomembrane compartments. We show that CaaX motif could still target CYPA85A2 in the absence of an intact N-terminal secretory signal to the respective membrane compartments and partially rescue cyp85a2-2 phenotypes. However, in the absence of both the CaaX motif and the secretory signal, CYP85A2 is not targeted to the membranes and becomes unstable.

Genome-wide transcriptome profiling provides overwintering mechanism of Agropyron mongolicum

BACKGROUND

The mechanism of winter survival for perennials involves multiple levels of gene regulation, especially cold resistance. Agropyron mongolicum is one important perennial grass species, but there is little information regarding its overwintering mechanism. We performed a comprehensive transcriptomics study to evaluate global gene expression profiles regarding the winter survival of Agropyron mongolicum. A genome-wide gene expression analysis involving four different periods was identified. Twenty-eight coexpression modules with distinct patterns were performed for transcriptome profiling. Furthermore, differentially expressed genes (DEGs) and their functional characterization were defined using a genome database such as NT, NR, COG, and KEGG.

RESULT

A total of 79.6% of the unigenes were characterized to be involved in 136 metabolic pathways. In addition, the expression level of ABA receptor genes, regulation of transcription factors, and a coexpression network analysis were conducted using transcriptome data. We found that ABA receptors regulated downstream gene expression by activating bZIP and NAC transcription factors to improve cold resistance and winter survival.

CONCLUSION

This study provides comprehensive transcriptome data for the characterization of overwintering-related gene expression profiles in A. mongolicum. Genomics resources can help provide a better understanding of the overwintering mechanism for perennial gramineae species. By analyzing genome-wide expression patterns for the four key stages of tiller bud development, the functional characteristics of the DEGs were identified that participated in various metabolic pathways and have been shown to be strongly associated with cold tolerance. These results can be further exploited to determine the mechanism of overwintering in perennial gramineae species.

Cross-talk of Brassinosteroid signaling in controlling growth and stress responses

Plants are faced with a barrage of stresses in their environment and must constantly balance their growth and survival. As such, plants have evolved complex control systems that perceive and respond to external and internal stimuli in order to optimize these responses, many of which are mediated by signaling molecules such as phytohormones. One such class of molecules called Brassinosteroids (BRs) are an important group of plant steroid hormones involved in numerous aspects of plant life including growth, development and response to various stresses. The molecular determinants of the BR signaling pathway have been extensively defined, starting with the membrane-localized receptor BRI1 and co-receptor BAK1 and ultimately culminating in the activation of BES1/BZR1 family transcription factors, which direct a transcriptional network controlling the expression of thousands of genes enabling BRs to influence growth and stress programs. Here, we highlight recent progress in understanding the relationship between the BR pathway and plant stress responses and provide an integrated view of the mechanisms mediating cross-talk between BR and stress signaling.

Virus-induced down-regulation of GmERA1A and GmERA1B genes enhances the stomatal response to abscisic acid and drought resistance in soybean

Drought is a major threat to global soybean production. The limited transformation potential and polyploid nature of soybean have hindered functional analysis of soybean genes. Previous research has implicated farnesylation in the plant's response to abscisic acid (ABA) and drought tolerance. We therefore used virus-induced gene silencing (VIGS) to evaluate farnesyltransferase genes, GmERA1A and GmERA1B (Glycine max Enhanced Response to ABA1-A and -B), as potential targets for increasing drought resistance in soybean. Apple latent spherical virus (ALSV)-mediated GmERA1-down-regulated soybean leaves displayed an enhanced stomatal response to ABA and reduced water loss and wilting under dehydration conditions, suggesting that GmERA1A and GmERA1B negatively regulate ABA signaling in soybean guard cells. The findings provide evidence that the ALSV-VIGS system, which bypasses the need to generate transgenic plants, is a useful tool for analyzing gene function using only a single down-regulated leaf. Thus, the ALSV-VIGS system could constitute part of a next-generation molecular breeding pipeline to accelerate drought resistance breeding in soybean.

Overexpression of SoCYP85A1, a Spinach Cytochrome p450 Gene in Transgenic Tobacco Enhances Root Development and Drought Stress Tolerance

Brassinosteroids (BRs) play an essential role in plant growth, development, and responses to diverse abiotic stresses. However, previous studies mainly analyzed how exogenous BRs influenced plant physiological reactions to drought stress, therefore, genetic evidences for the endogenous BRs-mediated regulation of plant responses still remain elusive. In this study, a key BRs biosynthetic gene, SoCYP85A1 was cloned from Spinacia oleracea, which has a complete open reading frame of 1,392 bp encoding a 464 amino acid peptide and shares high sequence similarities with CYP85A1 from other plants. The expression of SoCYP85A1 which was higher in leaf compared with root and stem, was induced by treatments of PEG6000, abscisic acid (ABA), low temperature and high salt. Increases in both SoCYP85A1 transcripts and endogenous BRs in transgenic tobacco which resulted in longer primary root and more lateral roots enhanced drought tolerance compared with wild types. The transgenic tobacco accumulated much lower levels of reactive oxygen species and malondialdehyde (MDA) than wild types did, accompanied by significantly higher content of proline and notably enhanced activities of antioxidant enzymes. Besides, transcriptional expressions of six stress-responsive genes were regulated to higher levels in transgenic lines under drought stress. Taken together, our results demonstrated that SoCYP85A1 involves in response to drought stress by promoting root development, scavenging ROS, and regulating expressions of stress-responsive genes.

Barley Brassinosteroid Mutants Provide an Insight into Phytohormonal Homeostasis in Plant Reaction to Drought Stress

Brassinosteroids (BRs) are a class of steroid phytohormones, which regulate various processes of morphogenesis and physiology-from seed development to regulation of flowering and senescence. An accumulating body of evidence indicates that BRs take part in regulation of physiological reactions to various stress conditions, including drought. Many of the physiological functions of BRs are regulated by a complicated, and not fully elucidated network of interactions with metabolic pathways of other phytohormones. Therefore, the aim of this study was to characterize phytohormonal homeostasis in barley (Hordeum vulgare) in reaction to drought and validate role of BRs in regulation of this process. Material of this study included the barley cultivar "Bowman" and five Near-Isogenic Lines (NILs) representing characterized semi-dwarf mutants of several genes encoding enzymes participating in BR biosynthesis and signaling. Analysis of endogenous BRs concentrations in these NILs confirmed that their phenotypes result from abnormalities in BR metabolism. In general, concentrations of 18 compounds, representing various classes of phytohormones, including brassinosteroids, auxins, cytokinins, gibberellins, abscisic acid, salicylic acid and jasmonic acid were analyzed under control and drought conditions in the "Bowman" cultivar and the BR-deficient NILs. Drought induced a significant increase in accumulation of the biologically active form of BRs-castasterone in all analyzed genotypes. Another biologically active form of BRs-24-epi-brassinolide-was identified in one, BR-insensitive NIL under normal condition, but its accumulation was drought-induced in all analyzed genotypes. Analysis of concentration profiles of several compounds representing gibberellins allowed an insight into the BR-dependent regulation of gibberellin biosynthesis. The concentration of the gibberellic acid GA₇ was significantly lower in all NILs when compared with the "Bowman" cultivar, indicating that GA₇ biosynthesis represents an enzymatic step at which the stimulating effect of BRs on gibberellin biosynthesis occurs. Moreover, the accumulation of GA₇ is significantly induced by drought in all the genotypes. Biosynthesis of jasmonic acid is also a BR-dependent process, as all the NILs accumulated much lower concentrations of this hormone when compared with the "Bowman" cultivar under normal condition, however the accumulation of jasmonic acid, abscisic acid and salicylic acid were significantly stimulated by drought.