14-3-3 regulates 1-aminocyclopropane-1-carboxylate synthase protein turnover in Arabidopsis

The Big, Mysterious World of Plant 14-3-3 Proteins

14-3-3 is a family of small regulatory proteins found exclusively in eukaryotic organisms. They selectively bind to phosphorylated molecules of partner proteins and regulate their functions. 14-3-3 proteins were first characterized in the mammalian brain approximately 60 years ago and then found in plants, 30 years later. The multifunctionality of 14-3-3 proteins is exemplified by their involvement in coordination of protein kinase cascades in animal brain and regulation of flowering, growth, metabolism, and immunity in plants. Despite extensive studies of this diverse and complex world of plant 14-3-3 proteins, our understanding of functions of these enigmatic molecules is fragmentary and unsystematic. The results of studies are often contradictory and many questions remain unanswered, including biochemical properties of 14-3-3 isoforms, structure of protein-protein complexes, and direct mechanisms by which 14-3-3 proteins influence the functions of their partners in plants. Although many plant genes coding for 14-3-3 proteins have been identified, the isoforms for in vivo and in vitro studies are often selected at random. This rather limited approach is partly due to an exceptionally large number and variety of 14-3-3 homologs in plants and erroneous a priori assumptions on the equivalence of certain isoforms. The accumulated results provide an extensive but rather fragmentary picture, which poses serious challenges for making global generalizations. This review is aimed to demonstrate the diversity and scope of studies of the functions of plant 14-3-3 proteins, as well as to identify areas that require further systematic investigation and close scientific attention.

Phosphorylation of the transcription factor OsNAC29 by OsMAPK3 activates diterpenoid genes to promote rice immunity

Well-conserved mitogen-activated protein kinase (MAPK) cascades are essential for orchestrating of a wide range of cellular processes in plants, including defense responses against pathogen attack. NAC transcription factors (TFs) play important roles in plant immunity, but their targets and how they are regulated remain largely unknown. Here, we identified the TF OsNAC29 as a key component of a MAPK signaling pathway involved in rice (Oryza sativa) disease resistance. OsNAC29 binds directly to CACGTG motifs in the promoters of OsTPS28 and OsCYP71Z2, which are crucial for the biosynthesis of the phytoalexin 5,10-diketo-casbene and consequently rice blast resistance. OsNAC29 positively regulates rice blast resistance by promoting the expression of of OsTPS28 and OsCYP71Z2, and the function of OsNAC29 is genetically dependent on OsCYP71Z2 and OsTPS28. Furthermore, OsNAC29 interacts with OsRACK1A and OsMAPK3/6 to form an immune complex; OsMAPK3 phosphorylates OsNAC29 at Thr304 to prevent its proteasome-mediated degradation and promote its function against rice blast fungus. Phosphorylation of OsNAC29 at Thr304 is induced upon Magnaporthe oryzae infection and chitin treatment. Our data demonstrate the positive role of the OsMAPK3-OsNAC29-OsTPS28/OsCYP71Z2 module in rice blast resistance, providing insights into the molecular regulatory network and fine-tuning of NAC TFs in rice immunity.

The 'Candidatus Phytoplasma ziziphi' effectors SJP1/2 negatively control leaf size by stabilizing the transcription factor ZjTCP2 in jujube

Phytoplasmas manipulate host plant development to benefit insect vector colonization and their own invasion. However, the virulence factors and mechanisms underlying small-leaf formation caused by jujube witches' broom (JWB) phytoplasmas remain largely unknown. Here, effectors SJP1 and SJP2 from JWB phytoplasmas were identified to induce small-leaf formation in jujube (Ziziphus jujuba). In vivo interaction and expression assays showed that SJP1 and SJP2 interacted with and stabilized the transcription factor ZjTCP2. Overexpression of SJP1 and SJP2 in jujube induced ZjTCP2 accumulation. In addition, the abundance of miRNA319f_1 was significantly reduced in leaves of SJP1 and SJP2 transgenic jujube plants and showed the opposite pattern to the expression of its target, ZjTCP2, which was consistent with the pattern in diseased leaves. Overexpression of ZjTCP2 in Arabidopsis promoted ectopic leaves arising from the adaxial side of cotyledons and reduced leaf size. Constitutive expression of the miRNA319f_1 precursor in the 35S::ZjTCP2 background reduced the abundance of ZjTCP2 mRNA and reversed the cotyledon and leaf defects in Arabidopsis. Therefore, these observations suggest that effectors SJP1 and SJP2 induced small-leaf formation, at least partly, by interacting with and activating ZjTCP2 expression both at the transcriptional and the protein level, providing new insights into small-leaf formation caused by phytoplasmas in woody plants.

The 14-3-3 protein GRF8 modulates salt stress tolerance in apple via the WRKY18-SOS pathway

Salinity is a severe abiotic stress that limits plant survival, growth, and development. 14-3-3 proteins are phosphopeptide-binding proteins that are involved in numerous signaling pathways, such as metabolism, development, and stress responses. However, their roles in salt tolerance are unclear in woody plants. Here, we characterized an apple (Malus domestica) 14-3-3 gene, GENERAL REGULATORY FACTOR 8 (MdGRF8), the product of which promotes salinity tolerance. MdGRF8 overexpression improved salt tolerance in apple plants, whereas MdGRF8-RNA interference (RNAi) weakened it. Yeast 2-hybrid, bimolecular fluorescence complementation, pull-down, and coimmunoprecipitation assays revealed that MdGRF8 interacts with the transcription factor MdWRKY18. As with MdGRF8, overexpressing MdWRKY18 enhanced salt tolerance in apple plants, whereas silencing MdWRKY18 had the opposite effect. We also determined that MdWRKY18 binds to the promoters of the salt-related genes SALT OVERLY SENSITIVE 2 (MdSOS2) and MdSOS3. Moreover, we showed that the 14-3-3 protein MdGRF8 binds to the phosphorylated form of MdWRKY18, enhancing its stability and transcriptional activation activity. Our findings reveal a regulatory mechanism by the MdGRF8-MdWRKY18 module for promoting the salinity stress response in apple.

Molecular evolution and interaction of 14-3-3 proteins with H+-ATPases in plant abiotic stresses

Environmental stresses severely affect plant growth and crop productivity. Regulated by 14-3-3 proteins (14-3-3s), H+-ATPases (AHAs) are important proton pumps that can induce diverse secondary transport via channels and co-transporters for the abiotic stress response of plants. Many studies demonstrated the roles of 14-3-3s and AHAs in coordinating the processes of plant growth, phytohormone signaling, and stress responses. However, the molecular evolution of 14-3-3s and AHAs has not been summarized in parallel with evolutionary insights across multiple plant species. Here, we comprehensively review the roles of 14-3-3s and AHAs in cell signaling to enhance plant responses to diverse environmental stresses. We analyzed the molecular evolution of key proteins and functional domains that are associated with 14-3-3s and AHAs in plant growth and hormone signaling. The results revealed evolution, duplication, contraction, and expansion of 14-3-3s and AHAs in green plants. We also discussed the stress-specific expression of those 14-3-3and AHA genes in a eudicotyledon (Arabidopsis thaliana), a monocotyledon (Hordeum vulgare), and a moss (Physcomitrium patens) under abiotic stresses. We propose that 14-3-3s and AHAs respond to abiotic stresses through many important targets and signaling components of phytohormones, which could be promising to improve plant tolerance to single or multiple environmental stresses.

The Ethylene Biosynthetic Enzymes, 1-Aminocyclopropane-1-Carboxylate (ACC) Synthase (ACS) and ACC Oxidase (ACO): The Less Explored Players in Abiotic Stress Tolerance

Ethylene is an essential plant hormone, critical in various physiological processes. These processes include seed germination, leaf senescence, fruit ripening, and the plant's response to environmental stressors. Ethylene biosynthesis is tightly regulated by two key enzymes, namely 1-aminocyclopropane-1-carboxylate synthase (ACS) and 1-aminocyclopropane-1-carboxylate oxidase (ACO). Initially, the prevailing hypothesis suggested that ACS is the limiting factor in the ethylene biosynthesis pathway. Nevertheless, accumulating evidence from various studies has demonstrated that ACO, under specific circumstances, acts as the rate-limiting enzyme in ethylene production. Under normal developmental processes, ACS and ACO collaborate to maintain balanced ethylene production, ensuring proper plant growth and physiology. However, under abiotic stress conditions, such as drought, salinity, extreme temperatures, or pathogen attack, the regulation of ethylene biosynthesis becomes critical for plants' survival. This review highlights the structural characteristics and examines the transcriptional, post-transcriptional, and post-translational regulation of ACS and ACO and their role under abiotic stress conditions. Reviews on the role of ethylene signaling in abiotic stress adaptation are available. However, a review delineating the role of ACS and ACO in abiotic stress acclimation is unavailable. Exploring how particular ACS and ACO isoforms contribute to a specific plant's response to various abiotic stresses and understanding how they are regulated can guide the development of focused strategies. These strategies aim to enhance a plant's ability to cope with environmental challenges more effectively.

Genome-Wide Identification and Expression Analysis of 1-Aminocyclopropane-1-Carboxylate Synthase (ACS) Gene Family in Chenopodium quinoa

Ethylene plays an important role in plant development and stress resistance. The rate-limiting enzyme in ethylene biosynthesis is 1-aminocyclopropane-1-carboxylic acid synthase (ACS). C. quinoa (Chenopodium quinoa) is an important food crop known for its strong tolerance to abiotic stresses. However, knowledge regarding the ACS gene family in C. quinoa remains restricted. In this study, we successfully identified 12 ACS genes (CqACSs) from the C. quinoa genome. Through thorough analysis of their sequences and phylogenetic relationships, it was verified that 8 out of these 12 CqACS isozymes exhibited substantial resemblance to ACS isozymes possessing ACS activity. Furthermore, these eight isozymes could be categorized into three distinct groups. The four remaining CqACS genes grouped under category IV displayed notable similarities with AtACS10 and AtACS12, known as amido transferases lacking ACS activity. The CqACS proteins bore resemblance to the AtACS proteins and had the characteristic structural features typically observed in plant ACS enzymes. Twelve CqACS genes were distributed across 8 out of the 18 chromosomes of C. quinoa. The CqACS genes were expanded from segment duplication. Many cis-regulatory elements related with various abiotic stresses, phytohormones, and light were found. The expression patterns of ACS genes varied across different tissues of C. quinoa. Furthermore, the analysis of gene expression patterns under abiotic stress showed that CqACS genes can be responsive to various stresses, implying their potential functions in adapting to various abiotic stresses. The findings from this research serve as a foundation for delving deeper into the functional roles of CqACS genes.

Time-Dependent Proteomic Signatures Associated with Embryogenic Callus Induction in Carica papaya L

Sex segregation increases the cost of Carica papaya production through seed-based propagation. Therefore, in vitro techniques are an attractive option for clonal propagation, especially of hermaphroditic plants. Here, we performed a temporal analysis of the proteome of C. papaya calli aiming to identify the key players involved in embryogenic callus formation. Mature zygotic embryos used as explants were treated with 20 μM 2,4-dichlorophenoxyacetic acid to induce embryogenic callus. Total proteins were extracted from explants at 0 (zygotic embryo) and after 7, 14, and 21 days of induction. A total of 1407 proteins were identified using a bottom-up proteomic approach. The clustering analysis revealed four distinct patterns of protein accumulation throughout callus induction. Proteins related to seed maturation and storage are abundant in the explant before induction, decreasing as callus formation progresses. Carbohydrate and amino acid metabolisms, aerobic respiration, and protein catabolic processes were enriched throughout days of callus induction. Protein kinases associated with auxin responses, such as SKP1-like proteins 1B, accumulated in response to callus induction. Additionally, regulatory proteins, including histone deacetylase (HD2C) and argonaute 1 (AGO1), were more abundant at 7 days, suggesting their role in the acquisition of embryogenic competence. Predicted protein-protein networks revealed the regulatory role of proteins 14-3-3 accumulated during callus induction and the association of proteins involved in oxidative phosphorylation and hormone response. Our findings emphasize the modulation of the proteome during embryogenic callus initiation and identify regulatory proteins that might be involved in the activation of this process.

Ethylene, ACC, and the Plant Growth-Promoting Enzyme ACC Deaminase

Here, a brief summary of the biosynthesis of 1-aminocyclopropane-1-carboxylate (ACC) and ethylene in plants, as well as overviews of how ACC and ethylene act as signaling molecules in plants, is presented. Next, how the bacterial enzyme ACC deaminase cleaves plant-produced ACC and thereby decreases or prevents the ethylene or ACC modulation of plant gene expression is considered. A detailed model of ACC deaminase functioning, including the role of indoleacetic acid (IAA), is presented. Given that ACC is a signaling molecule under some circumstances, this suggests that ACC, which appears to have evolved prior to ethylene, may have been a major signaling molecule in primitive plants prior to the evolution of ethylene and ethylene signaling. Due to their involvement in stimulating ethylene production, the role of D-amino acids in plants is then considered. The enzyme D-cysteine desulfhydrase, which is structurally very similar to ACC deaminase, is briefly discussed and the possibility that ACC deaminase arose as a variant of D-cysteine desulfhydrase is suggested.

Transcriptional Profiling Reveals the Wheat Defences against Fusarium Head Blight Disease Regulated by a NAC Transcription Factor

The wheat NAC transcription factor TaNACL-D1 enhances resistance to the economically devastating Fusarium head blight (FHB) disease. The objective of this study was to decipher the alterations in gene expression, pathways and biological processes that led to enhanced resistance as a result of the constitutive expression of TaNACL-D1 in wheat. Transcriptomic analysis was used to determine the genes and processes enhanced in wheat due to TaNACL-D1 overexpression, both in the presence and absence of the causal agent of FHB, Fusarium graminearum (0- and 1-day post-treatment). The overexpression of TaNACL-D1 resulted in more pronounced transcriptional reprogramming as a response to fungal infection, leading to the enhanced expression of genes involved in detoxification, immune responses, secondary metabolism, hormone biosynthesis, and signalling. The regulation and response to JA and ABA were differentially regulated between the OE and the WT. Furthermore, the results suggest that the OE may more efficiently: (i) regulate the oxidative burst; (ii) modulate cell death; and (iii) induce both the phenylpropanoid pathway and lignin synthesis. Thus, this study provides insights into the mode of action and downstream target pathways for this novel NAC transcription factor, further validating its potential as a gene to enhance FHB resistance in wheat.

A 14-3-3 protein positively regulates rice salt tolerance by stabilizing phospholipase C1

The phosphatidylinositol-specific phospholipase Cs (PI-PLCs) catalyze the hydrolysis of phosphatidylinositols, which play crucial roles in signaling transduction during plant development and stress response. However, the regulation of PI-PLC is still poorly understood. A previous study showed that a rice PI-PLC, OsPLC1, was essential to rice salt tolerance. Here, we identified a 14-3-3 protein, OsGF14b, as an interaction partner of OsPLC1. Similar to OsPLC1, OsGF14b also positively regulates rice salt tolerance, and their interaction can be promoted by NaCl stress. OsGF14b also positively regulated the hydrolysis activity of OsPLC1, and is essential to NaCl-induced activation of rice PI-PLCs. We further discovered that OsPLC1 was degraded via ubiquitin-proteasome pathway, and OsGF14b could inhibit the ubiquitination of OsPLC1 to protect OsPLC1 from degradation. Under salt stress, the OsPLC1 protein level in osgf14b was lower than the corresponding value of WT, whereas overexpression of OsGF14b results in a significant increase of OsPLC1 stability. Taken together, we propose that OsGF14b can interact with OsPLC1 and promote its activity and stability, thereby improving rice salt tolerance. This study provides novel insights into the important roles of 14-3-3 proteins in regulating protein stability and function in response to salt stress.

Genome-Wide Identification and Expression Analysis of the 14-3-3 (TFT) Gene Family in Tomato, and the Role of SlTFT4 in Salt Stress

The 14-3-3 proteins, which are ubiquitous and highly conserved in eukaryotic cells, play an essential role in various areas of plant growth, development, and physiological processes. The tomato is one of the most valuable vegetable crops on the planet. The main objective of the present study was to perform genome-wide identification and analysis of the tomato 14-3-3 (SlTFT) family to investigate its response to different abiotic stresses and phytohormone treatments in order to provide valuable information for variety improvement. Here, 13 SlTFTs were identified using bioinformatics methods. Characterization showed that they were categorized into ε and non-ε groups with five and eight members, accounting for 38.5% and 61.5%, respectively. All the SlTFTs were hydrophilic, and most of them did not contain transmembrane structural domains. Meanwhile, the phylogeny of the SlTFTs had a strong correlation with the gene structure, conserved domains, and motifs. The SlTFTs showed non-random chromosomal distribution, and the promoter region contained more cis-acting elements related to abiotic stress tolerance and phytohormone responses. The results of the evolutionary analysis showed that the SlTFTs underwent negative purifying selection during evolution. Transcriptional profiling and gene expression pattern analysis showed that the expression levels of the SlTFTs varied considerably in different tissues and periods, and they played a specific role under various abiotic stresses and phytohormone treatments. Meanwhile, the constructed protein-based interaction network systematically broadens our understanding of SlTFTs. Finally, the virus-induced gene silencing of SlTFT4 affected the antioxidant and reactive oxygen species defense systems, increased the degree of cellular damage, and reduced salt resistance in tomatoes.

Genome-wide identification and gene expression analysis of the 14-3-3 gene family in potato (Solanum tuberosum L.)

BACKGROUND

14-3-3 proteins are essential in regulating various biological processes and abiotic stress responses in plants. Although 14-3-3 proteins have been studied in model plants such as Arabidopsis thaliana and Oryza sativa, there is a lack of research on the 14-3-3 gene family in potatoes (Solanum tuberosum L.).

RESULTS

A total of 18 14-3-3 genes encoding proteins containing a typical conserved PF00244 domain were identified by genome-wide analysis in potatoes. The St14-3-3 gene family members were unevenly distributed across the chromosomes, and gene structure analysis showed that gene length and intron number varied greatly among the members. Phylogenetic analysis of 14-3-3 proteins in potatoes and other plant species showed that they could be divided into two distinct groups (ε and non-ε). Members in the ε group tended to have similar exon-intron structures and conserved motif patterns. Promoter sequence analysis showed that the St14-3-3 gene promoters contained multiple hormone-, stress-, and light-responsive cis-regulatory elements. Synteny analysis suggested that segmental duplication events contributed to the expansion of the St14-3-3 gene family in potatoes. The observed syntenic relationships between some 14-3-3 genes from potato, Arabidopsis, and tomato suggest that they evolved from a common ancestor. RNA-seq data showed that St14-3-3 genes were expressed in all tissues of potatoes but that their expression patterns were different. qRT-PCR assays revealed that the expression levels of nearly all tested St14-3-3 genes were affected by drought, salt, and low-temperature stresses and that different St14-3-3 genes had different responses to these stresses.

CONCLUSIONS

In summary, genome-wide identification, evolutionary, and expression analyses of the 14-3-3 gene family in potato were conducted. These results provide important information for further studies on the function and regulation of St14-3-3 gene family members in potatoes.

Natural variations of ZmSRO1d modulate the trade-off between drought resistance and yield by affecting ZmRBOHC-mediated stomatal ROS production in maize

While crop yields have historically increased, drought resistance has become a major concern in the context of global climate change. The trade-off between crop yield and drought resistance is a common phenomenon; however, the underlying molecular modulators remain undetermined. Through genome-wide association study, we revealed that three non-synonymous variants in a drought-resistant allele of ZmSRO1d-R resulted in plasma membrane localization and enhanced mono-ADP-ribosyltransferase activity of ZmSRO1d toward ZmRBOHC, which increased reactive oxygen species (ROS) levels in guard cells and promoted stomatal closure. ZmSRO1d-R enhanced plant drought resilience and protected grain yields under drought conditions, but it led to yield drag under favorable conditions. In contrast, loss-of-function mutants of ZmRBOHC showed remarkably increased yields under well-watered conditions, whereas they showed compromised drought resistance. Interestingly, by analyzing 189 teosinte accessions, we found that the ZmSRO1d-R allele was present in teosinte but was selected against during maize domestication and modern breeding. Collectively, our work suggests that the allele frequency reduction of ZmSRO1d-R in breeding programs may have compromised maize drought resistance while increased yields. Therefore, introduction of the ZmSRO1d-R allele into modern maize cultivars would contribute to food security under drought stress caused by global climate change.

Genome-wide analysis of the G-box regulating factors protein family reveals its roles in response to Sclerotinia sclerotiorum infection in rapeseed (Brassica napus L.)

The G-box regulating factors (GRFs) are involved in a wide array of signal transduction pathway and play important roles in plant physiological and developmental processes and stress responses. The GRF proteins have previously been described in several plant species, but not in rapeseed (Brassica napus L.). In this study, we carried out genome-wide analysis of GRFs in B. napus based on the available genome sequence information, and analyzed their expression in different tissues under different hormone treatments and after inoculation with Sclerotinia sclerotiorum. We identified 46 putative BnaGRF genes in rapeseed, unevenly distributed on 18 chromosomes. Like the 14-3-3 proteins in other plant species, the 46 putative BnaGRFs could be classified into two major evolutionary branches: epsilon (ε) group and non-epsilon (non-ε) group. Evolutionary analysis indicated that the BnaGRF gene family expanded in both groups much before speciation. We discovered an expansion of the 14-3-3 gene family that likely occurred during a recent gene duplication event. Collinearity analysis revealed that most of the BnaGRF genes shared syntenic relationships. Global gene expression profiling of BnaGRFs by RNA-seq analysis showed 41.3% (19/46) response to S. sclerotiorum infection, and this response was probably mediated through jasmonic acid (JA) and salicylic acid (SA) signaling pathways. These results provide key insights into the role of 14-3-3s in the biotic stress response and enhance our understanding of their multiple functions in B. napus.

Potential Roles of 1-Aminocyclopropane-1-carboxylic Acid Synthase Genes in the Response of Gossypium Species to Abiotic Stress by Genome-Wide Identification and Expression Analysis

Ethylene plays a pivotal role in plant stress resistance and 1-aminocyclopropane-1-carboxylic acid synthase (ACS) is the rate-limiting enzyme in ethylene biosynthesis. Upland cotton (Gossypium hirsutum L.) is the most important natural fiber crop, but the function of ACS in response to abiotic stress has rarely been reported in this plant. We identified 18 GaACS, 18 GrACS, and 35 GhACS genes in Gossypiumarboreum, Gossypium raimondii and Gossypiumhirsutum, respectively, that were classified as types I, II, III, or IV. Collinearity analysis showed that the GhACS genes were expanded from diploid cotton by the whole-genome-duplication. Multiple alignments showed that the C-terminals of the GhACS proteins were conserved, whereas the N-terminals of GhACS10 and GhACS12 were different from the N-terminals of AtACS10 and AtACS12, probably diverging during evolution. Most type II ACS genes were hardly expressed, whereas GhACS10/GhACS12 were expressed in many tissues and in response to abiotic stress; for example, they were highly and hardly expressed at the early stages of cold and heat exposure, respectively. The GhACS genes showed different expression profiles in response to cold, heat, drought, and salt stress by quantitative PCR analysis, which indicate the potential roles of them when encountering the various adverse conditions, and provide insights into GhACS functions in cotton’s adaptation to abiotic stress.

Crosstalk between the Arabidopsis Glutathione Peroxidase-Like 5 Isoenzyme (AtGPXL5) and Ethylene

Glutathione peroxidases (GPXs) are important antioxidant enzymes in animals. Plants contain GPX-like (GPXL) enzymes, which-in contrast to GPXs-contain cysteine in their active site instead of selenocysteine. Although several studies proved their importance in development and stress responses, their interaction with ethylene (ET) signalling is not known. Our aim was to investigate the involvement of AtGPXL5 in ET biosynthesis and/or signalling using Atgpxl5 mutant and AtGPXL5 cDNA-overexpressing (OX-AtGPXL5) lines. Four-day-old dark-grown Atgpxl5 seedlings had shorter hypocotyls and primary roots, while OX-AtGPXL5 seedlings exhibited a similar phenotype as wild type under normal conditions. Six-week-old OX-AtGPXL5 plants contained less H₂O₂ and malondialdehyde, but higher polyamine and similar ascorbate- and glutathione contents and redox potential (E_GSH) than the Col-0. One-day treatment with the ET-precursor 1-aminocyclopropane-1-carboxylic acid (ACC) induced the activity of glutathione- and thioredoxin peroxidases and some other ROS-processing enzymes. In the Atgpxl5 mutants, the E_GSH became more oxidised; parallelly, it produced more ethylene after the ACC treatment than other genotypes. Although the enhanced ET evolution measured in the Atgpxl5 mutant can be the result of the increased ROS level, the altered expression pattern of ET-related genes both in the Atgpxl5 and OX-AtGPXL5 plants suggests the interplay between AtGPXL5 and ethylene signalling.

EPIP as an abscission promoting agent in the phytohormonal pathway

Understanding the mechanisms underlying the activation of the abscission zone (AZ) responsible for organ separation from plant body in crop species will help improve their yielding and economic importance. Special attention has been given recently to the role of the INFLORESCENCE DEFICIENT IN ABSCISSION protein, particularly its functional fragment, EPIP peptide. Its stimulatory effect on abscission in different crops has been demonstrated. Recently we described the role of EPIP in the redox, lipid, and pectin-related events taking place in AZ of Lupinus luteus flowers, which undergo massive abscission in natural conditions. To further examine EPIP contribution in AZ functioning, here, we analyze its impact on the ultrastructural changes, synthesis of two hormonal abscission stimulators - abscisic acid (ABA) and ethylene (ET), and the appearance of phosphoproteins. As our results show, the response of flower AZ to exogenous EPIP involves the induction of distinct modifications related to the one hand with upregulation of cell activity but on the other hand degradation processes and possible autophagy. Furthermore, the EPIP stimulated biosynthesis pathways of ABA and ET precisely in AZ cells. In addition, progressive phosphorylation of proteins has been observed under EPIP influence. The highly accumulated ones were identified as those, related to primary metabolism and reactive oxygen species homeostasis, and their role in abscission has been discussed. To summarizing, the presented detailed description of EPIP action in AZ cells in combination with our previous data offers new insights into its regulatory function and provides opportunities to counteract excessive flower abscission in lupine.

SnRK2 subfamily I protein kinases regulate ethylene biosynthesis by phosphorylating HB transcription factors to induce ACO1 expression in apple

Ethylene (ETH) controls climacteric fruit ripening and can be triggered by osmotic stress. However, the mechanism regulating ETH biosynthesis during fruit ripening and under osmotic stress is largely unknown in apple (Malus domestica). Here, we explored the roles of SnRK2 protein kinases in ETH biosynthesis related to fruit ripening and osmoregulation. We identified the substrates of MdSnRK2-I using phosphorylation analysis techniques. Finally, we identified the MdSnRK2-I-mediated signaling pathway for ETH biosynthesis related to fruit ripening and osmoregulation. The activity of two MdSnRK2-I members, MdSnRK2.4 and MdSnRK2.9, was significantly upregulated during ripening or following mannitol treatment. Overexpression of MdSnRK2-I increased ETH biosynthesis under normal and osmotic conditions in apple fruit. MdSnRK2-I phosphorylated the transcription factors MdHB1 and MdHB2 to enhance their protein stability and transcriptional activity on MdACO1. MdSnRK2-I also interacted with MdACS1 and increased its protein stability through two phosphorylation sites. The increased MdACO1 expression and MdACS1 protein stability resulted in higher ETH production in apple fruit. In addition, heterologous expression of MdSnRK2-I or manipulation of SlSnRK2-I expression in tomato (Solanum lycopersicum) fruit altered fruit ripening and ETH biosynthesis. We established that MdSnRK2-I functions in fruit ripening and osmoregulation, and identified the MdSnRK2-I-mediated signaling pathway controlling ETH biosynthesis.

Stigma/Style Cell-Cycle Inhibitor 1, a Regulator of Cell Proliferation, Interacts With a Specific 14-3-3 Protein and Is Degraded During Cell Division

The final shape and size of plant organs are determined by a network of genes that modulate cell proliferation and expansion. Among those, SCI1 (Stigma/style Cell-cycle Inhibitor 1) functions by inhibiting cell proliferation during pistil development. Alterations in SCI1 expression levels can lead to remarkable stigma/style size changes. Recently, we demonstrated that SCI1 starts to be expressed at the specification of the Nicotiana tabacum floral meristem and is expressed at all floral meristematic cells. To elucidate how SCI1 regulates cell proliferation, we screened a stigma/style cDNA library through the yeast two-hybrid (Y2H) system, using SCI1 as bait. Among the interaction partners, we identified the 14-3-3D protein of the Non-Epsilon group. The interaction between SCI1 and 14-3-3D was confirmed by pulldown and co-immunoprecipitation experiments. 14-3-3D forms homo- and heterodimers in the cytoplasm of plant cells and interacts with SCI1 in the nucleus, as demonstrated by Bimolecular Fluorescence Complementation (BiFC). Analyses of SCI1-GFP fluorescence through the cell-cycle progression revealed its presence in the nucleoli during interphase and prophase. At metaphase, SCI1-GFP fluorescence faded and was no longer detected at anaphase, reappearing at telophase. Upon treatment with the 26S proteasome inhibitor MG132, SCI1-GFP was stabilized during cell division. Site-directed mutagenesis of seven serines into alanines in the predicted 14-3-3 binding sites on the SCI1 sequence prevented its degradation during mitosis. Our results demonstrate that SCI1 degradation at the beginning of metaphase is dependent on the phosphorylation of serine residues and on the action of the 26S proteasome. We concluded that SCI1 stability/degradation is cell-cycle regulated, consistent with its role in fine-tuning cell proliferation.

The role of 14-3-3 proteins in plant growth and response to abiotic stress

The 14-3-3 proteins widely exist in almost all plant species. They specifically recognize and interact with phosphorylated target proteins, including protein kinases, phosphatases, transcription factors and functional proteins, offering an array of opportunities for 14-3-3s to participate in the signal transduction processes. 14-3-3s are multigene families and can form homo- and heterodimers, which confer functional specificity of 14-3-3 proteins. They are widely involved in regulating biochemical and cellular processes and plant growth and development, including cell elongation and division, seed germination, vegetative and reproductive growth, and seed dormancy. They mediate plant response to environmental stresses such as salt, alkaline, osmotic, drought, cold and other abiotic stresses, partially via hormone-related signalling pathways. Although many studies have reviewed the function of 14-3-3 proteins, recent research on plant 14-3-3s has achieved significant advances. Here, we provide a comprehensive overview of the fundamental properties of 14-3-3 proteins and systematically summarize and dissect the emerging advances in understanding the roles of 14-3-3s in plant growth and development and abiotic stress responses. Some ambiguous questions about the roles of 14-3-3s under environmental stresses are reviewed. Interesting questions related to plant 14-3-3 functions that remain to be elucidated are also discussed.

The 14-3-3 protein GF14c positively regulates immunity by modulating the protein homoeostasis of the GRAS protein OsSCL7 in rice

Various types of transcription factors have been reported to be involved in plant-pathogen interactions by regulating defence-related genes. GRAS proteins, plant- specific transcription factors, have been shown to play essential roles in plant growth, development and stress responses. By performing a transcriptome study on rice early defence responses to Magnaporthe oryzae, we identified a GRAS protein, OsSCL7, which was induced by M. oryzae infection. We characterized the function of OsSCL7 in rice disease resistance. OsSCL7 was upregulated upon exposure to M. oryzae and pathogen-associated molecular pattern treatments, and knocking out OsSCL7 resulted in decreased disease resistance of rice to M. oryzae. In contrast, overexpression of OsSCL7 could improve rice disease resistance to M. oryzae. OsSCL7 was mainly localized in the nucleus and showed transcriptional activity. OsSCL7 can interact with GF14c, a 14-3-3 protein, and loss-of-function GF14c leads to enhanced susceptibility to M. oryzae. Additionally, OsSCL7 protein levels were reduced in the gf14c mutant and knocking out OsSCL7 affected the expression of a series of defence-related genes. Taken together, these findings uncover the important roles of OsSCL7 and GF14c in plant immunity and a potential mechanism by which plants fine-tune immunity by regulating the protein stability of a GRAS protein via a 14-3-3 protein.

Elevated tyrosine results in the cytosolic retention of 3-deoxy-d-arabino-heptulosonate 7-phosphate synthase in Arabidopsis thaliana

The shikimate pathway plays a central role in the biosynthesis of aromatic amino acids and specialized metabolites in plants. The first enzyme, 3-deoxy-d-arabino-heptulosonate 7-phosphate synthase (DAHPS) serves as a key regulatory point for the pathway in various organisms. These enzymes are important in regulating the shikimate pathway in multiple microbial systems. The mechanism of regulation of DAHPS is poorly understood in plants, and the role of tyrosine (Tyr) with respect to the three DAHPS isozymes from Arabidopsis thaliana was investigated. In vitro enzymatic analyses established that Tyr does not function as an allosteric regulator for the A. thaliana DAHPS isozymes. In contrast, Arabidopsis T-DNA insertional mutants for the DAHPS1 locus, dahps1, are hypersensitive to elevated Tyr. Tyr hypersensitivity can be reversed with tryptophan and phenylalanine supplementation, indicating that Tyr is affecting the shikimate pathway flux in the dahps1 mutant. Tyr treatment of Arabidopsis seedlings showed reduced accumulation of overexpressed DAHPS2 in the chloroplast. Further, bimolecular fluorescence complementation studies revealed that DAHPS2 interacts with a 14-3-3 protein in the cytosol, and this interaction is enhanced with Tyr treatment. This interaction with 14-3-3 may retain DAHPS2 in the cytosol, which prevents its ability to function in the chloroplast with elevated Tyr.

Genome-Wide Identification and Expression Analysis of the 14-3-3 Gene Family in Mango (Mangifera indica L.)

Members of the Mi14-3-3 gene family interact with target proteins that are widely involved in plant hormone signal transduction and physiology-related metabolism and play important roles in plant growth, development and stress responses. In this study, 14-3-3s family members are identified by the bioinformatic analysis of the mango (Mangifera indica L.) genome. The gene structures, chromosomal distributions, genetic evolution, and expression patterns of these genes and the physical and chemical properties and conserved motifs of their proteins are analysed systematically. The results identified 16 members of the 14-3-3 genes family in the mango genome. The members were not evenly distributed across the chromosomes, and the gene structure analysis showed that the gene sequence length and intron number varied greatly among the different members. Protein sequence analysis showed that the Mi14-3-3 proteins had similar physical and chemical properties and secondary and tertiary structures, and protein subcellular localization showed that the Mi14-3-3 family proteins were localized to the nucleus. The sequence analysis of the Mi14-3-3s showed that all Mi14-3-3 proteins contain a typical conserved PFAM00244 domain, and promoter sequence analysis showed that the Mi14-3-3 promoters contain multiple hormone-, stress-, and light-responsive cis-regulatory elements. Expression analysis showed that the 14-3-3 genes were expressed in all tissues of mango, but that their expression patterns were different. Drought, salt and low temperature stresses affected the expression levels of 14-3-3 genes, and different 14-3-3 genes had different responses to these stresses. This study provides a reference for further studies on the function and regulation of Mi14-3-3 family members.

The regulation of ACC synthase protein turnover: a rapid route for modulating plant development and stress responses

The phytohormone ethylene regulates plant growth, development, and stress responses. The strict fine-tuning of the regulation of ethylene biosynthesis contributes to the diverse roles of ethylene in plants. Pyridoxal 5'-phosphate-dependent 1-aminocyclopropane-1-carboxylic acid synthase, a rate-limiting enzyme in ethylene biosynthesis, is central and often rate-limiting to regulate ethylene concentration in plants. The post-translational regulation of ACS is a major pathway controlling ethylene biosynthesis in response to various stimuli. We conclude that the regulation of ACS turnover may serve as a central hub for the rapid integration of developmental, environmental, and hormonal signals, all of which influence plant growth and stress responses.

Reciprocal antagonistic regulation of E3 ligases controls ACC synthase stability and responses to stress

Ethylene influences plant growth, development, and stress responses via crosstalk with other phytohormones; however, the underlying molecular mechanisms are still unclear. Here, we describe a mechanistic link between the brassinosteroid (BR) and ethylene biosynthesis, which regulates cellular protein homeostasis and stress responses. We demonstrate that as a scaffold, 1-aminocyclopropane-1-carboxylic acid (ACC) synthases (ACS), a rate-limiting enzyme in ethylene biosynthesis, promote the interaction between Seven-in-Absentia of Arabidopsis (SINAT), a RING-domain containing E3 ligase involved in stress response, and ETHYLENE OVERPRODUCER 1 (ETO1) and ETO1-like (EOL) proteins, the E3 ligase adaptors that target a subset of ACS isoforms. Each E3 ligase promotes the degradation of the other, and this reciprocally antagonistic interaction affects the protein stability of ACS. Furthermore, 14-3-3, a phosphoprotein-binding protein, interacts with SINAT in a BR-dependent manner, thus activating reciprocal degradation. Disrupted reciprocal degradation between the E3 ligases compromises the survival of plants in carbon-deficient conditions. Our study reveals a mechanism by which plants respond to stress by modulating the homeostasis of ACS and its cognate E3 ligases.

Physiological and interactomic analysis reveals versatile functions of Arabidopsis 14-3-3 quadruple mutants in response to Fe deficiency

To date, few phenotypes have been described for Arabidopsis 14-3-3 mutants or the phenotypes showing the role of 14-3-3 in plant responding to abiotic stress. Although one member of the 14-3-3 protein family (14-3-3 omicron) was shown to be involved in the proper operation of Fe acquisition mechanisms at physiological and gene expression levels in Arabidopsis thaliana, it remains to be explored whether other members play a role in regulating iron acquisition. To more directly and effectively observe whether members of 14-3-3 non-epsilon group have a function in Fe-deficiency adaptation, three higher order quadruple KOs, kappa/lambda/phi/chi (klpc), kappa/lambda/upsilon/nu(klun), and upsilon/nu/phi/chi (unpc) were generated and studied for physiological analysis in this study. The analysis of iron-utilization efficiency, root phenotyping, and transcriptional level of Fe-responsive genes suggested that the mutant with kl background showed different phenotypes from Wt when plants suffered Fe starved, while these phenotypes were absent in the unpc mutant. Moreover, the absence of the four 14-3-3 isoforms in the klun mutant has a clear impact on the 14-3-3 interactome upon Fe deficiency. Dynamics of 14-3-3-client interactions analysis showed that 27 and 17 proteins differentially interacted with 14-3-3 in Wt and klun roots caused by Fe deficiency, respectively. Many of these Fe responsive proteins have a role in glycolysis, oxidative phosphorylation and TCA cycle, the FoF1-synthase and in the cysteine/methionine synthesis. A clear explanation for the observed phenotypes awaits a more detailed analysis of the functional aspects of 14-3-3 binding to the target proteins identified in this study.

Symbiotic nitrogen fixation in the reproductive structures of a basidiomycete fungus

Nitrogen (N) fixation is a driving force for the formation of symbiotic associations between N₂-fixing bacteria and eukaryotes.¹ Limited examples of these associations are known in fungi, and none with sexual structures of non-lichenized species.^2-6 The basidiomycete Guyanagaster necrorhizus is a sequestrate fungus endemic to the Guiana Shield.⁷ Like the root rot-causing species in its sister genera Armillaria and Desarmillaria, G. necrorhizus sporocarps fruit from roots of decaying trees (Figures 1A-1C),⁸ and genome sequencing is consistent with observations that G. necrorhizus is a white-rotting decomposer. This species also represents the first documentation of an arthropod-dispersed sequestrate fungus. Numerous species of distantly related wood-feeding termites, which scavenge for N-rich food, feed on the mature spore-bearing tissue, or gleba, of G. necrorhizus. During feeding, mature spores adhere to termites for subsequent dispersal.⁹ Using chemical assays, isotope analysis, and high-throughput sequencing, we show that the sporocarps harbor actively N₂-fixing Enterobacteriaceae species and that the N content within fungal tissue increases with maturation. Untargeted proteomic profiling suggests that ATP generation in the gleba is accomplished via fermentation. The use of fermentation-an anaerobic process-indicates that the sporocarp environment is anoxic, likely an adaptation to protect the oxygen-sensitive nitrogenase enzyme. Sporocarps also have a thick outer covering, possibly to limit oxygen diffusion. The enriched N content within mature sporocarps may offer a dietary inducement for termites in exchange for spore dispersal. These results show that the flexible metabolic capacity of fungi may facilitate N₂-fixing associations, as well as higher-level organismal associations.

Essential Roles of the Linker Sequence Between Tetratricopeptide Repeat Motifs of Ethylene Overproduction 1 in Ethylene Biosynthesis

Ethylene Overproduction 1 (ETO1) is a negative regulator of ethylene biosynthesis. However, the regulation mechanism of ETO1 remains largely unclear. Here, a novel eto1 allele (eto1-16) was isolated with typical triple phenotypes due to an amino acid substitution of G480C in the uncharacterized linker sequence between the TPR1 and TPR2 motifs. Further genetic and biochemical experiments confirmed the eto1-16 mutation site. Sequence analysis revealed that G480 is conserved not only in two paralogs, EOL1 and EOL2, in Arabidopsis, but also in the homologous protein in other species. The glycine mutations (eto1-11, eto1-12, and eto1-16) do not influence the mRNA abundance of ETO1, which is reflected by the mRNA secondary structure similar to that of WT. According to the protein-protein interaction analysis, the abnormal root phenotype of eto1-16 might be caused by the disruption of the interaction with type 2 1-aminocyclopropane-1-carboxylic acid (ACC) synthases (ACSs) proteins. Overall, these data suggest that the linker sequence between tetratricopeptide repeat (TPR) motifs and the glycine in TPR motifs or the linker region are essential for ETO1 to bind with downstream mediators, which strengthens our knowledge of ETO1 regulation in balancing ACSs.

The apple 14-3-3 protein MdGRF11 interacts with the BTB protein MdBT2 to regulate nitrate deficiency-induced anthocyanin accumulation

Nitrogen is an important factor that affects plant anthocyanin accumulation. In apple, the nitrate-responsive BTB/TAZ protein MdBT2 negatively regulates anthocyanin biosynthesis. In this study, we found that MdBT2 undergoes posttranslational modifications in response to nitrate deficiency. Yeast two-hybrid, protein pull-down, and bimolecular fluorescence complementation (BiFC) assays showed that MdBT2 interacts with MdGRF11, a 14-3-3 protein; 14-3-3 proteins compose a family of highly conserved phosphopeptide-binding proteins involved in multiple physiological and biological processes. The interaction of MdGRF11 negatively regulated the stability of the MdBT2 protein via a 26S proteasome-dependent pathway, which increased the abundance of MdMYB1 proteins to activate the expression of anthocyanin biosynthesis-related genes. Taken together, the results demonstrate the critical role of 14-3-3 proteins in the regulation of nitrate deficiency-induced anthocyanin accumulation. Our results provide a novel avenue to elucidate the mechanism underlying the induction of anthocyanin biosynthesis in response to nitrate deficiency.

The regulation of ethylene biosynthesis: a complex multilevel control circuitry

The gaseous plant hormone ethylene is produced by a fairly simple two-step biosynthesis route. Despite this pathway's simplicity, recent molecular and genetic studies have revealed that the regulation of ethylene biosynthesis is far more complex and occurs at different layers. Ethylene production is intimately linked with the homeostasis of its general precursor S-adenosyl-l-methionine (SAM), which experiences transcriptional and posttranslational control of its synthesising enzymes (SAM synthetase), as well as the metabolic flux through the adjacent Yang cycle. Ethylene biosynthesis continues from SAM by two dedicated enzymes: 1-aminocyclopropane-1-carboxylic (ACC) synthase (ACS) and ACC oxidase (ACO). Although the transcriptional dynamics of ACS and ACO have been well documented, the first transcription factors that control ACS and ACO expression have only recently been discovered. Both ACS and ACO display a type-specific posttranslational regulation that controls protein stability and activity. The nonproteinogenic amino acid ACC also shows a tight level of control through conjugation and translocation. Different players in ACC conjugation and transport have been identified over the years, however their molecular regulation and biological significance is unclear, yet relevant, as ACC can also signal independently of ethylene. In this review, we bring together historical reports and the latest findings on the complex regulation of the ethylene biosynthesis pathway in plants.

CUL3 E3 ligases in plant development and environmental response

Thirty years of research have revealed the fundamental role of the ubiquitin-proteasome system in diverse aspects of cellular regulation in eukaryotes. The ubiquitin-protein ligases or E3s are central to the ubiquitin-proteasome system since they determine the specificity of ubiquitylation. The cullin-RING ligases (CRLs) constitute one large class of E3s that can be subdivided based on the cullin isoform and the substrate adapter. SCF complexes, composed of CUL1 and the SKP1/F-box protein substrate adapter, are perhaps the best characterized in plants. More recently, accumulating evidence has demonstrated the essential roles of CRL3 E3s, consisting of a CUL3 protein and a BTB/POZ substrate adaptor. In this Review, we describe the variety of CRL3s functioning in plants and the wide range of processes that they regulate. Furthermore, we illustrate how different classes of E3s may cooperate to regulate specific pathways or processes.

Insights into the genes involved in the ethylene biosynthesis pathway in Arabidopsis thaliana and Oryza sativa

Background

Ethylene is a gaseous plant hormone that acts as a requisite role in many aspects of the plant life cycle, and it is also a regulator of plant responses to abiotic and biotic stresses. In this study, we attempt to provide comprehensive information through analyses of existing data using bioinformatics tools to compare the identified ethylene biosynthesis genes between Arabidopsis (as dicotyledonous) and rice (as monocotyledonous).

Results

The results exposed that the Arabidopsis proteins of the ethylene biosynthesis pathway had more potential glycosylation sites than rice, and 1-aminocyclopropane-1-carboxylate oxidase proteins were less phosphorylated than 1-aminocyclopropane-1-carboxylate synthase and S-adenosylmethionine proteins. According to the gene expression patterns, S-adenosylmethionine genes were more involved in the rice-ripening stage while in Arabidopsis, ACS2, and 1-aminocyclopropane-1-carboxylate oxidase genes were contributed to seed maturity. Furthermore, the result of miRNA targeting the transcript sequences showed that ath-miR843 and osa-miR1858 play a key role to regulate the post-transcription modification of S-adenosylmethionine genes in Arabidopsis and rice, respectively. The discovered cis- motifs in the promoter site of all the ethylene biosynthesis genes of A. thaliana genes were engaged to light-induced response in the cotyledon and root genes, sulfur-responsive element, dehydration, cell cycle phase-independent activation, and salicylic acid. The ACS4 protein prediction demonstrated strong protein-protein interaction in Arabidopsis, as well as, SAM2, Os04T0578000, Os01T0192900, and Os03T0727600 predicted strong protein-protein interactions in rice.

Conclusion

In the current study, the complex between miRNAs with transcript sequences of ethylene biosynthesis genes in A. thaliana and O. sativa were identified, which could be helpful to understand the gene expression regulation after the transcription process. The binding sites of common transcription factors such as MYB, WRKY, and ABRE that control target genes in abiotic and biotic stresses were generally distributed in promoter sites of ethylene biosynthesis genes of A. thaliana. This was the first time to wide explore the ethylene biosynthesis pathway using bioinformatics tools that markedly showed the capability of the in silico study to integrate existing data and knowledge and furnish novel insights into the understanding of underlying ethylene biosynthesis pathway genes that will be helpful for more dissection.

The Arabidopsis Proteins AtNHR2A and AtNHR2B Are Multi-Functional Proteins Integrating Plant Immunity With Other Biological Processes

AtNHR2A (Arabidopsis thaliana nonhost resistance 2A) and AtNHR2B (Arabidopsis thaliana nonhost resistance 2B) are two proteins that participate in nonhost resistance, a broad-spectrum mechanism of plant immunity that protects plants against the majority of potential pathogens. AtNHR2A and AtNHR2B are localized to the cytoplasm, chloroplasts, and other subcellular compartments of unknown identity. The multiple localizations of AtNHR2A and AtNHR2B suggest that these two proteins are highly dynamic and versatile, likely participating in multiple biological processes. In spite of their importance, the specific functions of AtNHR2A and AtNHR2B have not been elucidated. Thus, to aid in the functional characterization of these two proteins and identify the biological processes in which these proteins operate, we used immunoprecipitation coupled with mass spectrometry (IP-MS) to identify proteins interacting with AtNHR2A and AtNHR2B and to generate their interactome network. Further validation of three of the identified proteins provided new insights into specific pathways and processes related to plant immunity where AtNHR2A and AtNHR2B participate. Moreover, the comprehensive analysis of the AtNHR2A- and AtNHR2B-interacting proteins using published empirical information revealed that the functions of AtNHR2A and AtNHR2B are not limited to plant immunity but encompass other biological processes.

BES1 directly binds to the promoter of the ACC oxidase 1 gene to regulate gravitropic response in the roots of Arabidopsis thaliana

Brassinosteroids (BRs) are known to be endogenous regulators of ethylene production, suggesting that some BR activity in plant growth and development is associated with ethylene. Here, we demonstrated that ethylene production in Arabidopsis thaliana roots is increased by BR signaling via the ethylene biosynthetic gene for ACC oxidase 1 (ACO1). Electrophoretic mobility shift and chromatin immune-precipitation assays showed that the BR transcription factor BES1 directly binds to two E-box sequences located in the intergenic region of ACO1. GUS expression using site mutations of the E-box sequences verified that ACO1 is normally expressed only when BES1 binds to the E-boxes in the putative promoter of ACO1, indicating that this binding is essential for ACO1 expression and the subsequent production of ethylene in A. thaliana roots. BR exogenously applied to A. thaliana roots enhanced the gravitropic response. Additionally, bes1-D exhibited a greater gravitropic response than did the wild-type specimens, proving that BR is a positive regulator of the gravitropic response in A. thaliana roots. The knock-down mutant aco1-1 showed a slightly lower gravitropic response than did the wild-type specimens, while bes1-D X aco1-1 exhibited a lower gravitropic response than did bes1-D. Therefore, ACO1 is a direct downstream target for BR transcription factor BES1, which controls ethylene production for gravitropism in A. thaliana roots.

Phosphoproteomic Profiling Reveals the Importance of CK2, MAPKs and CDPKs in Response to Phosphate Starvation in Rice

Phosphorus is one of the most important macronutrients required for plant growth and development. The importance of phosphorylation modification in regulating phosphate (Pi) homeostasis in plants is emerging. We performed phosphoproteomic profiling to characterize proteins whose degree of phosphorylation is altered in response to Pi starvation in rice root. A subset of 554 proteins, including 546 down-phosphorylated and eight up-phosphorylated proteins, exhibited differential phosphorylation in response to Pi starvation. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis with the differentially phosphorylated proteins indicated that RNA processing, transport, splicing and translation and carbon metabolism played critical roles in response to Pi starvation in rice. Levels of phosphorylation of four mitogen-activated protein kinases (MAPKs), including OsMAPK6, five calcium-dependent protein kinases (CDPKs) and OsCK2β3 decreased in response to Pi starvation. The decreased phosphorylation level of OsMAPK6 was confirmed by Western blotting. Mutation of OsMAPK6 led to Pi accumulation under Pi-sufficient conditions. Motif analysis indicated that the putative MAPK, casein kinase 2 (CK2) and CDPK substrates represented about 54.4%, 21.5% and 4.7%, respectively, of the proteins exhibiting differential phosphorylation. Based on the motif analysis, 191, 151 and 46 candidate substrates for MAPK, CK2 and CDPK were identified. These results indicate that modification of phosphorylation profiles provides complementary information on Pi-starvation-induced processes, with CK2, MAPK and CDPK protein kinase families playing key roles in these processes in rice.

MdGRF11, an apple 14-3-3 protein, acts as a positive regulator of drought and salt tolerance

The 14-3-3 proteins are a family of highly conserved phosphoserine-binding proteins that participate in the regulation of diverse physiological and developmental processes. In this research, twenty 14-3-3 genes in apples, which contained a highly conserved 14-3-3 domain, were identified and divided into two subgroups. Among them, MdGRF11 was further cloned and investigated. qRT-PCR analyses and GUS staining show that MdGRF11 is expressed in various organs and tissues with the highest expression levels found in the fruit. MdGRF11 was upregulated by polyethylene glycol 6000 (PEG 6000), NaCl, abscisic acid (ABA) and low temperature (4 °C) treatments. MdGRF11-overexpressing transgenic Arabidopsis and apple calli exhibited reduced sensitivity to salt and PEG 6000 treatments. Moreover, the ectopic expression of MdGRF11 improved the tolerance of transgenic tobacco to salt and drought stresses, which grew longer roots, underwent more growth, and presented higher chlorophyll levels than the wild-type control under salt and drought stress conditions. Furthermore, MdGRF11 expression remarkably reduced electrolyte leakage, malondialdehyde content levels, H₂O₂ and O2- accumulation under salt and drought stress conditions, which relied on the regulation of ROS-scavenging signaling to reduce oxidative damage of cells after salt and drought stress treatment. MdGRF11 also enhanced tolerance to stress by upregulating expression levels of ROS-scavenging and stress-related genes, especially improving responses to drought stress by modifying the water loss rates and stomatal aperture. Moreover, MdGRF11 could interact with MdAREB/ABF transcription factors through yeast two hybrid analyses. In conclusion, our results indicate that MdGRF11 acts as a positive regulator of salt and drought stress responses through regulating ROS scavenging and other signaling systems.

The Glutamate Receptor-Like Protein GLR3.7 Interacts With 14-3-3ω and Participates in Salt Stress Response in Arabidopsis thaliana

Ionotropic glutamate receptors (iGluRs) are ligand-gated cation channels that mediate fast excitatory neurotransmission in the mammalian central nervous system. In the model plant Arabidopsis thaliana, a family of 20 glutamate receptor-like proteins (GLRs) shares similarities to animal iGluRs in sequence and predicted secondary structure. However, the function of GLRs in plants is little known. In the present study, a serine site (Ser-860) of AtGLR3.7 phosphorylated by a calcium-dependent protein kinase (CDPK) was identified and confirmed by an in vitro kinase assay. Using a bimolecular fluorescence complementation and quartz crystal microbalance analyses, the physical interaction between AtGLR3.7 and the 14-3-3ω protein was confirmed. The mutation of Ser-860 to alanine abolished this interaction, indicating that Ser-860 is the 14-3-3ω binding site of AtGLR3.7. Compared with wild type, seed germination of the glr3.7-2 mutant was more sensitive to salt stress. However, the primary root growth of GLR3.7-S860A overexpression lines was less sensitive to salt stress than that of the wild-type line. In addition, the increase of cytosolic calcium ion concentration by salt stress was significantly lower in the glr3.7-2 mutant line than in the wild-type line. Moreover, association of 14-3-3 proteins to microsomal fractions was less in GLR3.7-S860A overexpression lines than in GLR3.7 overexpression line under 150 mM NaCl salt stress condition. Overall, our results indicated that GLR3.7 is involved in salt stress response in A. thaliana by affecting calcium signaling.

Biotechnological Potential of LSD1, EDS1, and PAD4 in the Improvement of Crops and Industrial Plants

Lesion Simulating Disease 1 (LSD1), Enhanced Disease Susceptibility (EDS1) and Phytoalexin Deficient 4 (PAD4) were discovered a quarter century ago as regulators of programmed cell death and biotic stress responses in Arabidopsis thaliana. Recent studies have demonstrated that these proteins are also required for acclimation responses to various abiotic stresses, such as high light, UV radiation, drought and cold, and that their function is mediated through secondary messengers, such as salicylic acid (SA), reactive oxygen species (ROS), ethylene (ET) and other signaling molecules. Furthermore, LSD1, EDS1 and PAD4 were recently shown to be involved in the modification of cell walls, and the regulation of seed yield, biomass production and water use efficiency. The function of these proteins was not only demonstrated in model plants, such as Arabidopsis thaliana or Nicotiana benthamiana, but also in the woody plant Populus tremula x tremuloides. In addition, orthologs of LSD1, EDS1, and PAD4 were found in other plant species, including different crop species. In this review, we focus on specific LSD1, EDS1 and PAD4 features that make them potentially important for agricultural and industrial use.

Recent advances in enhancement of oil content in oilseed crops

Plant oils are very valuable agricultural commodity. The manipulation of seed oil composition to deliver enhanced fatty acid compositions, which are appropriate for feed or fuel, has always been a main objective of metabolic engineers. The last two decennary have been noticeable by numerous significant events in genetic engineering for identification of different gene targets to improve oil yield in oilseed crops. Particularly, genetic engineering approaches have presented major breakthrough in elevating oil content in oilseed crops such as Brassica napus and soybean. Additionally, current research efforts to explore the possibilities to modify the genetic expression of key regulators of oil accumulation along with biochemical studies to elucidate lipid biosynthesis will establish protocols to develop transgenic oilseed crops along much improved oil content. In this review, we describe current distinct genetic engineering approaches investigated by researchers for ameliorating oil content and its nutritional quality. Moreover, we will also discuss some auspicious and innovative approaches and challenges for engineering oil content to yield oil at much higher rate in oilseed crops.

Light-induced stabilization of ACS contributes to hypocotyl elongation during the dark-to-light transition in Arabidopsis seedlings

Hypocotyl growth during seedling emergence is a crucial developmental transition influenced by light and phytohormones such as ethylene. Ethylene and light antagonistically control hypocotyl growth in either continuous light or darkness. However, how ethylene and light regulate hypocotyl growth, including seedling emergence, during the dark-to-light transition remains elusive. Here, we show that ethylene and light cooperatively stimulate a transient increase in hypocotyl growth during the dark-to-light transition via the light-mediated stabilization of 1-aminocyclopropane-1-carboxylic acid (ACC) synthases (ACSs), the rate-limiting enzymes in ethylene biosynthesis. We found that, in contrast to the known inhibitory role of light in hypocotyl growth, light treatment transiently increases hypocotyl growth in wild-type etiolated seedlings. Moreover, ACC, the direct precursor of ethylene, accentuates the effects of light on hypocotyl elongation during the dark-to-light transition. We determined that light leads to the transient elongation of hypocotyls by stabilizing the ACS5 protein during the dark-to-light transition. Furthermore, biochemical analysis of an ACS5 mutant protein bearing an alteration in the C-terminus indicated that light stabilizes ACS5 by inhibiting the degradation mechanism that acts through the C-terminus of ACS5. Our study reveals that plants regulate hypocotyl elongation during seedling establishment by coordinating light-induced ethylene biosynthesis at the post-translational level. Moreover, the stimulatory role of light on hypocotyl growth during the dark-to-light transition provides additional insights into the known inhibitory role of light in hypocotyl development.

1-Aminocyclopropane 1-Carboxylic Acid and Its Emerging Role as an Ethylene-Independent Growth Regulator

1-Aminocyclopropane 1-carboxylic acid (ACC) is the direct precursor of the plant hormone ethylene. ACC is synthesized from S-adenosyl-L-methionine (SAM) by ACC synthases (ACSs) and subsequently oxidized to ethylene by ACC oxidases (ACOs). Exogenous ACC application has been used as a proxy for ethylene in numerous studies as it is readily converted by nearly all plant tissues to ethylene. However, in recent years, a growing body of evidence suggests that ACC plays a signaling role independent of the biosynthesis. In this review, we briefly summarize our current knowledge of ACC as an ethylene precursor, and present new findings with regards to the post-translational modifications of ACS proteins and to ACC transport. We also summarize the role of ACC in regulating plant development, and its involvement in cell wall signaling, guard mother cell division, and pathogen virulence.

Light Modulates Ethylene Synthesis, Signaling, and Downstream Transcriptional Networks to Control Plant Development

The inhibition of hypocotyl elongation by ethylene in dark-grown seedlings was the basis of elegant screens that identified ethylene-insensitive Arabidopsis mutants, which remained tall even when treated with high concentrations of ethylene. This simple approach proved invaluable for identification and molecular characterization of major players in the ethylene signaling and response pathway, including receptors and downstream signaling proteins, as well as transcription factors that mediate the extensive transcriptional remodeling observed in response to elevated ethylene. However, the dark-adapted early developmental stage used in these experiments represents only a small segment of a plant's life cycle. After a seedling's emergence from the soil, light signaling pathways elicit a switch in developmental programming and the hormonal circuitry that controls it. Accordingly, ethylene levels and responses diverge under these different environmental conditions. In this review, we compare and contrast ethylene synthesis, perception, and response in light and dark contexts, including the molecular mechanisms linking light responses to ethylene biology. One powerful method to identify similarities and differences in these important regulatory processes is through comparison of transcriptomic datasets resulting from manipulation of ethylene levels or signaling under varying light conditions. We performed a meta-analysis of multiple transcriptomic datasets to uncover transcriptional responses to ethylene that are both light-dependent and light-independent. We identified a core set of 139 transcripts with robust and consistent responses to elevated ethylene across three root-specific datasets. This "gold standard" group of ethylene-regulated transcripts includes mRNAs encoding numerous proteins that function in ethylene signaling and synthesis, but also reveals a number of previously uncharacterized gene products that may contribute to ethylene response phenotypes. Understanding these light-dependent differences in ethylene signaling and synthesis will provide greater insight into the roles of ethylene in growth and development across the entire plant life cycle.

The Glutamate Receptor-Like Protein GLR3.7 Interacts With 14-3-3ω and Participates in Salt Stress Response in Arabidopsis thaliana

Ionotropic glutamate receptors (iGluRs) are ligand-gated cation channels that mediate fast excitatory neurotransmission in the mammalian central nervous system. In the model plant Arabidopsis thaliana, a family of 20 glutamate receptor-like proteins (GLRs) shares similarities to animal iGluRs in sequence and predicted secondary structure. However, the function of GLRs in plants is little known. In the present study, a serine site (Ser-860) of AtGLR3.7 phosphorylated by a calcium-dependent protein kinase (CDPK) was identified and confirmed by an in vitro kinase assay. Using a bimolecular fluorescence complementation and quartz crystal microbalance analyses, the physical interaction between AtGLR3.7 and the 14-3-3ω protein was confirmed. The mutation of Ser-860 to alanine abolished this interaction, indicating that Ser-860 is the 14-3-3ω binding site of AtGLR3.7. Compared with wild type, seed germination of the glr3.7-2 mutant was more sensitive to salt stress. However, the primary root growth of GLR3.7-S860A overexpression lines was less sensitive to salt stress than that of the wild-type line. In addition, the increase of cytosolic calcium ion concentration by salt stress was significantly lower in the glr3.7-2 mutant line than in the wild-type line. Moreover, association of 14-3-3 proteins to microsomal fractions was less in GLR3.7-S860A overexpression lines than in GLR3.7 overexpression line under 150 mM NaCl salt stress condition. Overall, our results indicated that GLR3.7 is involved in salt stress response in A. thaliana by affecting calcium signaling.

BYPASS1-LIKE, A DUF793 Family Protein, Participates in Freezing Tolerance via the CBF Pathway in Arabidopsis

The C-REPEAT BINDING FACTOR signaling pathway is strictly modulated by numerous factors and is essential in the cold response of plants. Here, we show that the DUF793 family gene BYPASS1-LIKE modulates freezing tolerance through the CBFs in Arabidopsis. The expression of B1L was rapidly induced under cold treatment. Comparing to wild type, B1L knockout mutants were more sensitive to freezing treatment, whereas B1L-overexpressing lines were more tolerant. The expression of CBFs and CBF target genes was significantly decreased in b1l mutant. Using yeast two-hybrid screening system, 14-3-3λ was identified as one of proteins interacting with B1L. The interaction was confirmed with bimolecular fluorescence complementation assay and co-immunoprecipitation assay. Biochemical assays revealed that b1l mutation promoted the degradation of CBF3 compared to wild type, whereas 14-3-3κλ mutant and b1l 14-3-3κλ mutant suppressed the degradation of CBF3. Consistently, 14-3-3κλ and b1l 14-3-3κλ mutants showed enhanced freezing tolerance compared to wild type. These results indicate that B1L enhances the freezing tolerance of plants, at least partly through stabilizing CBF. Our findings improve our understanding of the regulation of CBF in response to cold stress.

Affinity chromatography revealed 14-3-3 interactome of tomato (Solanum lycopersicum L.) during blue light-induced de-etiolation

De-etiolation is the first developmental process under light control allowing the heterotrophic seedling to become autotrophic. The phytohormones cytokinins (CKs) largely contribute to this process. Reversible phosphorylation is a key event of cell signaling, allowing proteins to become active or generating a binding site for specific protein interaction. 14-3-3 proteins regulate a variety of plant responses. The expression, hormonal regulation, and proteomic network under the control of 14-3-3s were addressed in tomato (Solanum lycopersicum L.) during blue light-induced photomorphogenesis. Two isoforms were specifically investigated due to their high expression during tomato de-etiolation. The multidisciplinary approach demonstrated that TFT9 expression, but not TFT6, was regulated by CKs and identified cis-regulating elements required for this response. Our study revealed >130 potential TFT6/9 interactors. Their functional annotation predicted that TFTs might regulate the activity of proteins involved notably in cell wall strengthening or primary metabolism. Several potential interactors were also predicted to be CK-responsive. For the first time, the 14-3-3 interactome linked to de-etiolation was investigated and evidenced that 14-3-3s might be involved in CK signaling pathway, cell expansion inhibition and steady-state growth rate establishment, and reprograming from heterotrophy to autotrophy. BIOLOGICAL SIGNIFICANCE: Tomato (Solanum lycopersicum L.) is one of the most important vegetables consumed all around the world and represents probably the most preferred garden crop. Regulation of hypocotyl growth by light plays an important role in the early development of a seedling, and consequently the homogeneity of the culture. The present study focuses on the importance of tomato 14-3-3/TFT proteins in this process. We provide here the first report of 14-3-3 interactome in the regulation of light-induced de-etiolation and subsequent photomorphogenesis. Our data provide new insights into light-induced de-etiolation and open new horizons for dissecting the post-transcriptional regulations.

Tomato 14-3-3 Proteins Are Required for Xv3 Disease Resistance and Interact with a Subset of Xanthomonas euvesicatoria Effectors

The 14-3-3 phospho-binding proteins with scaffolding activity play central roles in the regulation of enzymes and signaling complexes in eukaryotes. In plants, 14-3-3 isoforms are required for disease resistance and key targets of pathogen effectors. Here, we examined the requirement of the tomato (Solanum lycopersicum) 14-3-3 isoform (TFT) protein family for Xv3 disease resistance in response to the bacterial pathogen Xanthomonas euvesicatoria. In addition, we determined whether TFT proteins interact with the repertoire of X. euvesicatoria type III secretion effector proteins, including AvrXv3, the elicitor of Xv3 resistance. We show that multiple TFT contribute to Xv3 resistance. We also show that one or more TFT proteins physically interact with multiple effectors (AvrXv3, XopE1, XopE2, XopN, XopO, XopQ, and XopAU). Genetic analyses indicate that none of the identified effectors interfere with AvrXv3-elicited resistance into Xv3 tomato leaves; however, XopE1, XopE2, and XopO are required to suppress symptom development in susceptible tomato leaves. Phospho-peptide mapping revealed that XopE2 is phosphorylated at multiple residues in planta and residues T66, T131, and S334 are required for maximal binding to TFT10. Together, our data support the hypothesis that multiple TFT proteins are involved in immune signaling during X. euvesicatoria infection.

Arabidopsis ACC Oxidase 1 Coordinated by Multiple Signals Mediates Ethylene Biosynthesis and Is Involved in Root Development

Ethylene regulates numerous aspects of plant growth and development. Multiple external and internal factors coordinate ethylene production in plant tissues. Transcriptional and post-translational regulations of ACC synthases (ACSs), which are key enzymes mediating a rate-limiting step in ethylene biosynthesis have been well characterized. However, the regulation and physiological roles of ACC oxidases (ACOs) that catalyze the final step of ethylene biosynthesis are largely unknown in Arabidopsis. Here, we show that Arabidopsis ACO1 exhibits a tissue-specific expression pattern that is regulated by multiple signals, and plays roles in the lateral root development in Arabidopsis. Histochemical analysis of the ACO1 promoter indicated that ACO1 expression was largely modulated by light and plant hormones in a tissue-specific manner. We demonstrated that point mutations in two E-box motifs on the ACO1 promoter reduce the light-regulated expression patterns of ACO1. The aco1-1 mutant showed reduced ethylene production in root tips compared to wild-type. In addition, aco1-1 displayed altered lateral root formation. Our results suggest that Arabidopsis ACO1 integrates various signals into the ethylene biosynthesis that is required for ACO1's intrinsic roles in root physiology.

Shade-induced nuclear localization of PIF7 is regulated by phosphorylation and 14-3-3 proteins in Arabidopsis

Shade avoidance syndrome enables shaded plants to grow and compete effectively against their neighbors. In Arabidopsis, the shade-induced de-phosphorylation of the transcription factor PIF7 (PHYTOCHROME-INTERACTING FACTOR 7) is the key event linking light perception to stem elongation. However, the mechanism through which phosphorylation regulates the activity of PIF7 is unclear. Here, we show that shade light induces the de-phosphorylation and nuclear accumulation of PIF7. Phosphorylation-resistant site mutations in PIF7 result in increased nuclear localization and shade-induced gene expression, and consequently augment hypocotyl elongation. PIF7 interacts with 14-3-3 proteins. Blocking the interaction between PIF7 and 14-3-3 proteins or reducing the expression of 14-3-3 proteins accelerates shade-induced nuclear localization and de-phosphorylation of PIF7, and enhances the shade phenotype. By contrast, the 14-3-3 overexpressing line displays an attenuated shade phenotype. These studies demonstrate a phosphorylation-dependent translocation of PIF7 when plants are in shade and a novel mechanism involving 14-3-3 proteins, mediated by the retention of PIF7 in the cytoplasm that suppresses the shade response.

Shade-induced nuclear localization of PIF7 is regulated by phosphorylation and 14-3-3 proteins in Arabidopsis

Shade avoidance syndrome enables shaded plants to grow and compete effectively against their neighbors. In Arabidopsis, the shade-induced de-phosphorylation of the transcription factor PIF7 (PHYTOCHROME-INTERACTING FACTOR 7) is the key event linking light perception to stem elongation. However, the mechanism through which phosphorylation regulates the activity of PIF7 is unclear. Here, we show that shade light induces the de-phosphorylation and nuclear accumulation of PIF7. Phosphorylation-resistant site mutations in PIF7 result in increased nuclear localization and shade-induced gene expression, and consequently augment hypocotyl elongation. PIF7 interacts with 14-3-3 proteins. Blocking the interaction between PIF7 and 14-3-3 proteins or reducing the expression of 14-3-3 proteins accelerates shade-induced nuclear localization and de-phosphorylation of PIF7, and enhances the shade phenotype. By contrast, the 14-3-3 overexpressing line displays an attenuated shade phenotype. These studies demonstrate a phosphorylation-dependent translocation of PIF7 when plants are in shade and a novel mechanism involving 14-3-3 proteins, mediated by the retention of PIF7 in the cytoplasm that suppresses the shade response.