Knockdown of PagSAP11 Confers Drought Resistance and Promotes Lateral Shoot Growth in Hybrid Poplar (Populus alba × Populus tremula var. glandulosa).

Plants have evolved defense mechanisms to overcome unfavorable climatic conditions. The growth and development of plants are regulated in response to environmental stress. In this study, we investigated the molecular and physiological characteristics of a novel gene PagSAP11 in hybrid poplar (Populus alba × Populus tremula var. glandulosa) under drought stress. PagSAP11, a stress-associated protein (SAP) family gene, encodes a putative protein containing an A20 and AN1 zinc-finger domain at its N- and C-termini, respectively. Knockdown of PagSAP11 transgenic poplars (SAP11-Ri) enhanced their tolerance to drought stress compared with wild type plants. Moreover, the RNAi lines showed increased branching of lateral shoots that led to a gain in fresh weight, even when grown in the living modified organism (LMO) field. In SAP11-Ri transgenic plants, the expression levels of genes involved in axillary bud outgrowth and cell proliferation such as DML10, CYP707A and RAX were increased while the DRM gene which involved in bud dormancy was down-regulated. Taken together, these results indicate that PagSAP11 represents a promising candidate gene for engineering trees with improved stress tolerance and growth during unfavorable conditions.

Efficient knockout of the phytoene desaturase gene in a hybrid poplar (Populus alba × Populus glandulosa) using the CRISPR/Cas9 system with a single gRNA.

The CRISPR/Cas9 system has been used for genome editing in several plant species; however, there are few reports on its use in trees. Here, CRISPR/Cas9 was used to mutate a target gene in Populus alba × Populus glandulosa hybrid poplars. The hybrid poplar is routinely used in molecular biological studies due to the well-established Agrobacterium-mediated transformation method. A single guide RNA (sgRNA) with reported high mutation efficiency in other popular species was designed with a protospacer adjacent motif sequence for the phytoene desaturase 1 (PagPDS1) gene. The pHSE/Cas9-PagPDS1 sgRNA vector was delivered into hybrid poplar cells using Agrobacterium-mediated transformation. The transgenic plants were propagated and classified them into three groups according to their phenotypes. Among a total of 110 lines of transgenic hybrid poplars, 82 lines showed either an albino or a pale green phenotype, indicating around 74.5% phenotypic mutation efficiency of the PagPDS1 gene. The albino phenotypes were observed when the CRISPR/Cas9-mediated mutations in both PagPDS1 alleles in the transgenic plants. There was no off-target modification of the PagPDS2 gene, which has a potential sgRNA target sequence with two mismatches. The results confirmed that the sgRNA can specifically edit PagPDS1 rather than PagPDS2, indicating that CRISPR/Cas9-mediated genome editing can effectively induce target mutations in the hybrid poplar. This technique will be useful to improve tree quality in hybrid poplars (P. alba × P. glandulosa); for example, by enhancing biomass or stress tolerance.

A Raf-like kinase is required for smoke-induced seed dormancy in Arabidopsis thaliana.

Plants sense and integrate diverse stimuli to determine the timing for germination. A smoke compound, 3,4,5-trimethylfuran-2(5H)-one (trimethylbutenolide, TMB), has been identified to inhibit the seed germination of higher plants. To understand the mode of action, we examined various physiological and molecular aspects of the TMB-dependent inhibition of seed germination in Arabidopsis thaliana The results indicated that the effect of TMB is due to the enhanced physiological dormancy, which is modulated by other dormancy regulatory cues such as after-ripening, stratification, and ABA/GA signaling. In addition, gene expression profiling showed that TMB caused genome-wide transcriptional changes, altering the expression of a series of dormancy-related genes. Based on the TMB-responsive physiological contexts in Arabidopsis, we performed mutant screening to isolate genetic components that underpin the TMB-induced seed dormancy. As a result, the TMB-RESISTANT1 (TES1) gene in Arabidopsis, encoding a B2 group Raf-like kinase, was identified. Phenotypic analysis of the tes1 mutant implicated that TES1 has a critical role in the TMB-responsive gene expression and the inhibition of seed germination. Taken together, we propose that plants have been equipped with a TMB sensory pathway through which the TMB induces the seed dormancy in a TES1-dependent way.

Transcriptional network regulation of the brassinosteroid signaling pathway by the BES1-TPL-HDA19 co-repressor complex.

MAIN CONCLUSION: The brassinosteroid-related BES1 and BZR1 transcription factors dynamically modulate downstream gene networks via the TPL-HDA19 co-repressor complex in BR-signaling pathways in Arabidopsis thaliana. Brassinosteroids (BRs) are plant steroid hormones that are essential for diverse growth and developmental processes across the whole life cycle of plants. In Arabidopsis thaliana, the BR-related transcription factors BRI1-EMS-SUPPRESSOR 1 (BES1) and BRASSINAZOLE-RESISTANT 1 (BZR1) regulate a range of global gene expression in response to BR and several external signaling cues; however, the molecular mechanisms by which they mediate the reprogramming of downstream transcription remain unclear. We here report that formation of a protein complex between BES1 and BZR1 and Histone Deacetylase 19 (HDA19) via the conserved ERF-associated amphiphilic repression (EAR) motif proved essential for regulation of BR-signaling-related gene expression. Defects in BR-related functions of BES1 and BZR1 proteins containing a mutated EAR motif were completely rescued by artificial fusion with EAR-repression domain (SRDX), TOPLESS (TPL), or HDA19 proteins. RNA-sequencing analysis of Arabidopsis plants over-expressing bes1-DmEAR or bes1-DmEAR-HDA19 revealed an essential role for HDA19 activity in regulation of BES1/BZR1-mediated BR signaling. In addition to BR-related gene expression, the BES1-HDA19 transcription factor complex was important for abiotic stress-related drought stress tolerance and organ boundary formation. These results suggested that integrating activation of BR-signaling pathways with the formation of the protein complex containing BES1/BZR1 and TPL-HDA19 via the EAR motif was important in fine-tuning BR-related gene networks in plants.

A missense allele of KARRIKIN-INSENSITIVE2 impairs ligand-binding and downstream signaling in Arabidopsis thaliana.

A smoke-derived compound, karrikin (KAR), and an endogenous but as yet unidentified KARRIKIN INSENSITIVE2 (KAI2) ligand (KL) have been identified as chemical cues in higher plants that impact on multiple aspects of growth and development. Genetic screening of light-signaling mutants in Arabidopsis thaliana has identified a mutant designated as ply2 (pleiotropic long hypocotyl2) that has pleiotropic light-response defects. In this study, we used positional cloning to identify the molecular lesion of ply2 as a missense mutation of KAI2/HYPOSENSITIVE TO LIGHT, which causes a single amino acid substitution, Ala219Val. Physiological analysis and genetic epistasis analysis with the KL-signaling components MORE AXILLARY GROWTH2 (MAX2) and SUPPRESSOR OF MAX2 1 suggested that the pleiotropic phenotypes of the ply2 mutant can be ascribed to a defect in KL-signaling. Molecular and biochemical analyses revealed that the mutant KAI2ply2 protein is impaired in its ligand-binding activity. In support of this conclusion, X-ray crystallography studies suggested that the KAI2ply2 mutation not only results in a narrowed entrance gate for the ligand but also alters the structural flexibility of the helical lid domains. We discuss the structural implications of the Ala219 residue with regard to ligand-specific binding and signaling of KAI2, together with potential functions of KL-signaling in the context of the light-regulatory network in Arabidopsis thaliana.

PIF4 Integrates Multiple Environmental and Hormonal Signals for Plant Growth Regulation in Arabidopsis.

As sessile organisms, plants must be able to adapt to the environment. Plants respond to the environment by adjusting their growth and development, which is mediated by sophisticated signaling networks that integrate multiple environmental and endogenous signals. Recently, increasing evidence has shown that a bHLH transcription factor PIF4 plays a major role in the multiple signal integration for plant growth regulation. PIF4 is a positive regulator in cell elongation and its activity is regulated by various environmental signals, including light and temperature, and hormonal signals, including auxin, gibberellic acid and brassinosteroid, both transcriptionally and post-translationally. Moreover, recent studies have shown that the circadian clock and metabolic status regulate endogenous PIF4 level. The PIF4 transcription factor cooperatively regulates the target genes involved in cell elongation with hormone-regulated transcription factors. Therefore, PIF4 is a key integrator of multiple signaling pathways, which optimizes growth in the environment. This review will discuss our current understanding of the PIF4-mediated signaling networks that control plant growth.

Bakuchiol augments MyoD activation leading to enhanced myoblast differentiation.

Myoblast differentiation is fundamental to skeletal muscle development and regeneration after injury and defects in this process are implicated in muscle atrophy associated with aging or pathological conditions. MyoD family transcription factors function as mater regulators in induction of muscle-specific genes during myoblast differentiation. We have identified bakuchiol, a prenylated phenolic monoterpene, as an inducer of MyoD-mediated transcription and myogenic differentiation. C2C12 myoblasts treated with bakuchiol exhibit enhanced muscle-specific gene expression and myotube formation. A key promyogenic kinase p38MAPK is activated dramatically by bakuchiol which in turn induced the formation of MyoD/E protein active transcription complexes. Consistently, the recruitment of MyoD and Baf60c to the Myogenin promoter is enhanced in bakuchiol-treated myoblasts. Furthermore, bakuchiol rescues defective p38MAPK activation and myogenic differentiation caused by Cdo-depletion or in RD rhabdomyosarcoma cells. Taken together, these results indicate that bakuchiol enhances myogenic differentiation through p38MAPK and MyoD activation. Thus bakuchiol can be developed into a potential agent to improve muscular regeneration and repair to treat muscular atrophy.

Rootin, a compound that inhibits root development through modulating PIN-mediated auxin distribution.

Plant roots anchor the plant to the soil and absorb water and nutrients for growth. Understanding the molecular mechanisms regulating root development is essential for improving plant survival and agricultural productivity. Extensive molecular genetic studies have provided important information on crucial components for the root development control over the last few decades. However, it is becoming difficult to identify new regulatory components in root development due to the functional redundancy and lethality of genes involved in root development. In this study, we performed a chemical genetic screen to identify novel synthetic compounds that regulate root development in Arabidopsis seedlings. The screen yielded a root growth inhibitor designated as 'rootin', which inhibited Arabidopsis root development by modulating cell division and elongation, but did not significantly affect shoot development. Transcript analysis of phytohormone marker genes revealed that rootin preferentially altered the expression of auxin-regulated genes. Furthermore, rootin reduced the accumulation of PIN1, PIN3, and PIN7 proteins, and affected the auxin distribution in roots, which consequently may lead to the observed defects in root development. Our results suggest that rootin could be utilized to unravel the mechanisms underlying root development and to investigate dynamic changes in PIN-mediated auxin distribution.

The homeodomain-leucine zipper ATHB23, a phytochrome B-interacting protein, is important for phytochrome B-mediated red light signaling.

Phytochromes are red (R)/far-red (FR) photoreceptors that are central to the regulation of plant growth and development. Although it is well known that photoactivated phytochromes are translocated into the nucleus where they interact with a variety of nuclear proteins and ultimately regulate genome-wide transcription, the mechanisms by which these photoreceptors function are not completely understood. In an effort to enhance our understanding of phytochrome-mediated light signaling networks, we attempted to identify novel proteins interacting with phytochrome B (phyB). Using affinity purification in Arabidopsis phyB overexpressor, coupled with mass spectrometry analysis, 16 proteins that interact with phyB in vivo were identified. Interactions between phyB and six putative phyB-interacting proteins were confirmed by bimolecular fluorescence complementation (BiFC) analysis. Involvement of these proteins in phyB-mediated signaling pathways was also revealed by physiological analysis of the mutants defective in each phyB-interacting protein. We further characterized the athb23 mutant impaired in the homeobox protein 23 (ATHB23) gene. The athb23 mutant displayed altered hypocotyl growth under R light, as well as defects in phyB-dependent seed germination and phyB-mediated cotyledon expansion. Taken together, these results suggest that the ATHB23 transcription factor is a novel component of the phyB-mediated R light signaling pathway.

Forward chemical genetic screening.

Chemical genetics utilizes small molecules to perturb biological processes. Unlike conventional genetics methods, which involve the alteration of genetic information mostly with lasting effects, chemical genetics allows temporary and reversible alterations of biological processes. Furthermore, it enables the alteration of biological processes in a dose-dependent manner, providing an advantage over conventional genetics. In the present chapter, the general procedures of forward chemical genetic screening are described. Forward chemical genetic screening can be performed in three steps. The first step involves the identification of small molecules that induce phenotypic or physiological changes in a biological system from a chemical library. In the second step, cellular targets that interact with the isolated chemical, which are mostly proteins, are identified. Although several methods can be applied in the second step, the most common one is affinity pull-down assay using a target protein that binds to the isolated compound. However, affinity pull-down of a target protein is a formidable barrier in forward chemical genetics. We introduced a tagged chemical library approach that significantly facilitates the identification of target proteins. The third step consists of the validation of the target protein, which should include the assessment of target specificity. This step is critical because small molecules often show pleiotropic effects due to low specificity. The specificity test may include a competition assay using cold competitors and a genetic study using mutants or transgenic lines modified for the cellular target.

CRY1 inhibits COP1-mediated degradation of BIT1, a MYB transcription factor, to activate blue light-dependent gene expression in Arabidopsis.

Cryptochromes (CRY) are one of the two major classes of photoreceptors that perceive light stimuli in the UV-A to blue light region and they are involved in multiple aspects of plant growth and development. However, knowledge regarding their signaling transduction components and mechanisms remains limited. Here, we report that a MYB transcription factor Blue Insensitive Trait 1 (BIT1), plays an important role in controlling blue light responses. Hypocotyl growth responses indicate that BIT1 functions as a positive element in blue light signaling, since BIT1 antisense and knock-out lines show a reduced light response in blue light. BIT1 controls blue light-dependent expression of various genes such as PsbS, a member of the light-harvesting complex gene family. A transactivation assay showed that BIT1 regulates promoter activity of PsbS in a blue light-dependent manner and that it requires CRY1 for activation of the PsbS promoter. BIT1 undergoes degradation in darkness and CRY1 functions to stabilize BIT1 in a blue light-dependent manner. In contrast, COP1 binds to BIT1 and mediates its degradation. We propose that the PsbS promoter is activated in blue light via the blue light-dependent stabilization of BIT1 by CRY1, while in darkness BIT1 is degraded by COP1-mediated proteolysis.