Apple F-Box Protein MdMAX2 Regulates Plant Photomorphogenesis and Stress Response

Gene Co-Expression Analysis Reveals the Transcriptome Changes and Hub Genes of Fructan Metabolism in Garlic under Drought Stress

Drought has become a serious environmental factor that affects the growth and yield of plants. Fructan, as an important storage compound in garlic, plays an important role in drought tolerance. Genomic changes in plants under drought stress clarify the molecular mechanism of plants' responses to stress. Therefore, we used RNA-seq to determine the transcriptomic changes in garlic under drought stress and identified the key module related to fructan metabolism by weighted gene co-expression network analysis. We conducted a comprehensive analysis of the garlic transcriptome under drought stress over a time course (0, 3, 6, 9, 12, 15 d). Drought significantly induces changes in gene expression. The number of specifically expressed genes were 1430 (3 d), 399 (6 d), 313 (9 d), 351 (12 d), and 1882 (15 d), and only 114 genes responded at each time point. The number of upregulated DEGs was higher than the number of downregulated DEGs. Gene ontology and a Kyoto Encyclopedia of Genes and Genomes analysis showed that garlic was more likely to cause changes in carbohydrate metabolism pathways under drought stress. Fructan content measurements showed that drought stress significantly induced fructan accumulation in garlic. To determine whether there were modules involved in the transcriptional regulation of fructan content in garlic, we further analyzed the genes related to fructan metabolism using WGCNA. They were enriched in two modules, with F-box protein and GADPH as hub genes, which are involved in garlic fructan metabolism in response to drought stress. These results provide important insights for the future research and cultivation of drought-tolerant garlic varieties.

SUPPRESSOR OF MAX2 LIKE 6, 7, and 8 Interact with DDB1 BINDING WD REPEAT DOMAIN HYPERSENSITIVE TO ABA DEFICIENT 1 to Regulate the Drought Tolerance and Target SUCROSE NONFERMENTING 1 RELATED PROTEIN KINASE 2.3 to Abscisic Acid Response in Arabidopsis

SUPPRESSOR OF MAX2-LIKE 6, 7, and 8 (SMXL6,7,8) function as repressors and transcription factors of the strigolactone (SL) signaling pathway, playing an important role in the development and stress tolerance in Arabidopsis thaliana. However, the molecular mechanism by which SMXL6,7,8 negatively regulate drought tolerance and ABA response remains largely unexplored. In the present study, the interacting protein and downstream target genes of SMXL6,7,8 were investigated. Our results showed that the substrate receptor for the CUL4-based E3 ligase DDB1-BINDING WD-REPEAT DOMAIN (DWD) HYPERSENSITIVE TO ABA DEFICIENT 1 (ABA1) (DWA1) physically interacted with SMXL6,7,8. The degradation of SMXL6,7,8 proteins were partially dependent on DWA1. Disruption of SMXL6,7,8 resulted in increased drought tolerance and could restore the drought-sensitive phenotype of the dwa1 mutant. In addition, SMXL6,7,8 could directly bind to the promoter of SUCROSE NONFERMENTING 1 (SNF1)-RELATED PROTEIN KINASE 2.3 (SnRK2.3) to repress its transcription. The mutations in SnRK2.2/2.3 significantly suppressed the hypersensitivity of smxl6/7/8 to ABA-mediated inhibition of seed germination. Conclusively, SMXL6,7,8 interact with DWA1 to negatively regulate drought tolerance and target ABA-response genes. These data provide insights into drought tolerance and ABA response in Arabidopsis via the SMXL6,7,8-mediated SL signaling pathway.

Strigolactone signaling gene from soybean GmMAX2a enhances the drought and salt-alkaline resistance in Arabidopsis via regulating transcriptional profiles of stress-related genes

Strigolactone (SL) is a new plant hormone, which not only plays an important role in stimulating seed germination, plant branching, and regulating root development, but also plays an important role in the response of plants to abiotic stresses. In this study, the full-length cDNA of a soybean SL signal transduction gene (GmMAX2a) was isolated, cloned and revealed an important role in abiotic stress responses. Tissue-specific expression analysis by qRT-PCR indicated that GmMAX2a was expressed in all tissues of soybean, but highest expression was detected in seedling stems. Moreover, upregulation of GmMAX2a transcript expression under salt, alkali, and drought conditions were noted at different time points in soybean leaves compared to roots. Additionally, histochemical GUS staining studies revealed the deep staining in P_GmMAX2a: GUS transgenic lines compared to WT indicating active involvement of GmMAX2a promoter region to stress responses. To further investigate the function of GmMAX2a gene in transgenic Arabidopsis, Petri-plate experiments were performed and GmMAX2a OX lines appeared with longer roots and improved fresh biomass compared to WT plants to NaCl, NaHCO₃, and mannitol supplementation. Furthermore, the expression of several stress-related genes such as RD29B, SOS1, NXH1, AtRD22, KIN1, COR15A, RD29A, COR47, H+-APase, NADP-ME, NCED3, and P5CS were significantly high in GmMAX2a OX plants after stress treatment compared to WT plants. In conclusion, GmMAX2a improves soybean tolerance towards abiotic stresses (salt, alkali, and drought). Hence, GmMAX2a can be considered a candidate gene for transgenic breeding against various abiotic stresses in plants.

Role of F-box E3-ubiquitin ligases in plant development and stress responses

KEY MESSAGE

F-box E3-ubiquitin ligases regulate critical biological processes in plant development and stress responses. Future research could elucidate why and how plants have acquired a large number of F-box genes. The ubiquitin-proteasome system (UPS) is a predominant regulatory mechanism employed by plants to maintain the protein turnover in the cells and involves the interplay of three classes of enzymes, E1 (ubiquitin-activating), E2 (ubiquitin-conjugating), and E3 ligases. The diverse and most prominent protein family among eukaryotes, F-box proteins, are a vital component of the multi-subunit SCF (Skp1-Cullin 1-F-box) complex among E3 ligases. Several F-box proteins with multifarious functions in different plant systems have evolved rapidly over time within closely related species, but only a small part has been characterized. We need to advance our understanding of substrate-recognition regulation and the involvement of F-box proteins in biological processes and environmental adaptation. This review presents a background of E3 ligases with particular emphasis on the F-box proteins, their structural assembly, and their mechanism of action during substrate recognition. We discuss how the F-box proteins regulate and participate in the signaling mechanisms of plant development and environmental responses. We highlight an urgent need for research on the molecular basis of the F-box E3-ubiquitin ligases in plant physiology, systems biology, and biotechnology. Further, the developments and outlooks of the potential technologies targeting the E3-ubiquitin ligases for developing crop improvement strategies have been discussed.

Karrikin increases tomato cold tolerance via strigolactone and the abscisic acid signaling network

As a class of biostimulants, karrikins (KARs) were first identified from plant-derived smoke to regulate plant growth, development, and stress tolerance. However, the roles of KARs in plant cold tolerance and their crosstalk with strigolactones (SLs) and abscisic acid (ABA) remain elusive. We studied the interaction among KAR, SLs, and ABA in cold acclimatization with KAI2-, MAX1-, SnRK2.5-silenced, or cosilenced plant materials. KAI2 is involved in smoke-water- (SW-) and KAR-mediated cold tolerance. MAX1 acts downstream of KAR in cold acclimation. ABA biosynthesis and sensitivity are regulated by KAR and SLs, which improve cold acclimation through the SnRK2.5 component. The physiological mechanisms of SW and KAR in improving growth, yield, and tolerance under a long-term sublow temperature environment were also studied. SW and KAR were shown to improve tomato growth and yield under sublow temperature conditions by regulating nutritional uptake, leaf temperature control, photosynthetic defense, ROS scavenging, and CBF transcriptional activation. Together, SW, which functions via the KAR-mediated SL and ABA signaling network, has potential application value for increasing cold tolerance in tomato production.

Unfolding molecular switches for salt stress resilience in soybean: recent advances and prospects for salt-tolerant smart plant production

The increasing sodium salts (NaCl, NaHCO3, NaSO4 etc.) in agricultural soil is a serious global concern for sustainable agricultural production and food security. Soybean is an important food crop, and their cultivation is severely challenged by high salt concentration in soils. Classical transgenic and innovative breeding technologies are immediately needed to engineer salt tolerant soybean plants. Additionally, unfolding the molecular switches and the key components of the soybean salt tolerance network are crucial for soybean salt tolerance improvement. Here we review our understandings of the core salt stress response mechanism in soybean. Recent findings described that salt stress sensing, signalling, ionic homeostasis (Na⁺/K⁺) and osmotic stress adjustment might be important in regulating the soybean salinity stress response. We also evaluated the importance of antiporters and transporters such as Arabidopsis K⁺ Transporter 1 (AKT1) potassium channel and the impact of epigenetic modification on soybean salt tolerance. We also review key phytohormones, and osmo-protectants and their role in salt tolerance in soybean. In addition, we discuss the progress of omics technologies for identifying salt stress responsive molecular switches and their targeted engineering for salt tolerance in soybean. This review summarizes recent progress in soybean salt stress functional genomics and way forward for molecular breeding for developing salt-tolerant soybean plant.

Genome-Wide Analysis and Characterization of Eggplant F-Box Gene Superfamily: Gene Evolution and Expression Analysis under Stress

F-box genes play an important role in plant growth and resistance to abiotic and biotic stresses. To date, systematic analysis of F-box genes and functional annotation in eggplant (Solanum melongena) is still limited. Here, we identified 389 F-box candidate genes in eggplant. The domain study of F-box candidate genes showed that the F-box domain is conserved, whereas the C-terminal domain is diverse. There are 376 SmFBX candidate genes distributed on 12 chromosomes. A collinearity analysis within the eggplant genome suggested that tandem duplication is the dominant form of F-box gene replication in eggplant. The collinearity analysis between eggplant and the three other species (Arabidopsis thaliana, rice and tomato) provides insight into the evolutionary characteristics of F-box candidate genes. In addition, we analyzed the expression of SmFBX candidate genes in different tissues under high temperature and bacterial wilt stress. The results identified several F-box candidate genes that potentially participate in eggplant heat tolerance and bacterial wilt resistance. Moreover, the yeast two-hybrid assay showed that several representative F-box candidate proteins interacted with representative Skp1 proteins. Overexpression of SmFBX131 and SmFBX230 in tobacco increased resistance to bacterial wilt. Overall, these results provide critical insights into the functional analysis of the F-box gene superfamily in eggplant and provide potentially valuable targets for heat and bacterial resistance.

Genome-wide analysis of the strigolactone biosynthetic and signaling genes in grapevine and their response to salt and drought stresses

Strigolactones (SLs) are a novel class of plant hormones that play critical roles in regulating various developmental processes and stress tolerance. Although the SL biosynthetic and signaling genes were already determined in some plants such as Arabidopsis and rice, the information of SL-related genes in grapevine (Vitis vinifera L.) remains largely unknown. In this study, the SL-related genes were identified from the whole grapevine genome, and their expression patterns under salt and drought stresses were determined. The results indicated that the five genes that involved in the SL biosynthesis included one each of the D27, CCD7, CCD8, MAX1 and LBO genes, as well as the three genes that involved in the SL signaling included one each of the D14, MAX2, D53 genes. Phylogenetic analysis suggested that these SL-related proteins are highly conserved among different plant species. Promoter analysis showed that the prevalence of a variety of cis-acting elements associated with hormones and abiotic stress existed in the promoter regions of these SL-related genes. Furthermore, the transcription expression analysis demonstrated that most SL-related genes are involved in the salt and drought stresses response in grapevine. These findings provided valuable information for further investigation and functional analysis of SL biosynthetic and signaling genes in response to salt and drought stresses in grapevine.

A Bursaphelenchus xylophilus pathogenic protein Bx-FAR-1, as potential control target, mediates the jasmonic acid pathway in pines

BACKGROUND

The pine wilt disease (PWD) caused by Bursaphelenchus xylophilus is a devastating forest disease and its pathogenesis remains unclear. Secreted enzymes and proteins are important pathogenicity determinants and Bx-FAR-1 is an important pathogenic protein involved in the interaction between pine and B. xylophilus. However, the function of the Bx-FAR-1 protein in monitoring and prevention PWD remains unknown.

RESULTS

We found a small peptide of B. xylophilus effector Bx-FAR-1 is sufficient for immunosuppression function in Nicotiana benthamiana. Transient expression of Bx-FAR-1 in N. benthamiana revealed that nuclear localization is required for its function. The results of the ligand binding test showed that Bx-FAR-1 protein had the ability to bind fatty acid and retinol. We demonstrated that Bx-FAR-1 targeted to the nuclei of Pinus thunbergii using the polyclonal antibody by immunologic approach. The content of jasmonic acid (JA) was significantly increased in P. thunbergii infected with B. xylophilus when Bx-FAR-1 was silenced. We identified an F-box protein as the host target of Bx-FAR-1 by yeast two-hybrid and co-immunoprecipitation. Moreover, we found that Pt-F-box-1 was up-regulated during B. xylophilus infection and the expression of Pt-F-box-1 was increased in Bx-FAR-1 double-stranded RNA (dsRNA)-treated host pines.

CONCLUSION

This study illustrated that Bx-FAR-1 might mediate the JA pathway to destroy the immune system of P. thunbergii, indicating that PWN likely secretes effectors to facilitate parasitism and promote infection, which could better reveal the pathogenesis mechanisms of B. xylophilus and would be beneficial for developing disease control strategies.

Silencing of MsD14 Resulted in Enhanced Forage Biomass through Increasing Shoot Branching in Alfalfa (Medicago sativa L.)

Branching is one of the key determinants of plant architecture that dramatically affects crop yield. As alfalfa is the most important forage crop, understanding the genetic basis of branching in this plant can facilitate breeding for a high biomass yield. In this study, we characterized the strigolactone receptor gene MsD14 in alfalfa and demonstrated that MsD14 was predominantly expressed in flowers, roots, and seedpods. Furthermore, we found that MsD14 expression could significantly respond to strigolactone in alfalfa seedlings, and its protein was located in the nucleus, cytoplasm, and cytomembrane. Most importantly, transformation assays demonstrated that silencing of MsD14 in alfalfa resulted in increased shoot branching and forage biomass. Significantly, MsD14 could physically interact with AtMAX2 and MsMAX2 in the presence of strigolactone, suggesting a similarity between MsD14 and AtD14. Together, our results revealed the conserved D14-MAX2 module in alfalfa branching regulation and provided candidate genes for alfalfa high-yield molecular breeding.

The F-box E3 ubiquitin ligase AtSDR is involved in salt and drought stress responses in Arabidopsis

F-box protein genes have been shown to play vital roles in plant development and stress respones. In Arabidopsis, there are more than 600 F-box proteins, and most of their functions are unclear. The present study shows that the F-box (SKP1-Cullin/CDC53-F-box) gene At5g15710 (Salt and Drought Responsiveness, SDR) is involved in abiotic stress responses in Arabidopsis. SDR is expressed in all tissues of Arabidopsis and is upregulated by salt and heat stresses and ABA treatment but downregulated by drought stress. Subcellular localization analysis shows that the SDR protein colocalizes with the nucleus. 35S:AntiSDR plants are hypersensitive to salt stress, but 35S:SDR plants display a salt-tolerant phenotype. Furthermore, 35S:SDR plants are hypersensitive to drought stress, while 35S:AntiSDR plants are significantly more drought tolerant. Overall, our results suggest that SDR is involved in salt and drought stress responses in Arabidopsis.

Proteomic analysis reveals responsive mechanisms for saline-alkali stress in alfalfa

Saline-alkali stress is a major abiotic stress that limits plant growth, yield, and geographical distribution. Alfalfa is a perennial legume with the largest planting area in the world because of its high protein content, good palatability, and long utilization life. However, saline-alkali stress seriously affects alfalfa yield and quality. To better understand the saline-alkali stress response mechanisms of alfalfa, an isobaric tags proteomics method was used to compare and analyse alfalfa under saline-alkali stress for 0, 1, and 7 days, and 126 (1 vs. 0 days) and 1869 (7 vs. 0 days) differentially abundant proteins (DAPs) were found. Through integrative analysis with differentially expressed genes (DEGs), we found correlated DEGs-DAPs of RNA and protein with similar expression trends at the mRNA and protein levels; these were mainly involved in ABA and Ca²⁺ signal pathways, regulation of photosynthesis, ROS scavenging, secondary metabolism, and transcription factors (TFs) related to saline-alkali stress. Some genes not exhibiting such trends may have been regulated post-transcriptionally. Furthermore, through transgenic experiments, MsFTL was found to significantly improve the saline-alkali tolerance of plants. Overall, our findings provide important clues for understanding the molecular mechanisms underlying the response of alfalfa to saline-alkali stress.

Exogenous Strigolactones alleviate KCl stress by regulating photosynthesis, ROS migration and ion transport in Malus hupehensis Rehd

AIMS

In recent years, the application of large amounts of potash fertilizer in apple orchards leads to worsening KCl stress. Strigolactone (SL), as a novel phytohormone, reportedly participates in plant tolerance to NaCl and drought stresses. However, the underlying mechanism and the effects of exogenous SL on the KCl stress of apple seedlings remain unclear.

METHODS

We sprayed different concentrations of exogenous SL on Malus hupehensis Rehd. under KCl stress and measured the physiological indexes like, photosynthetic parameter, content of ROS, osmolytes and mineral element. In addition, the expressions of KCl-responding genes and SL-signaling genes were also detected and analyzed.

RESULTS

Application of exogenous SL protected the chlorophyll and maintained the photosynthetic rate of apple seedlings under KCl stress. Exogenous SL strengthened the enzyme activities of peroxidase and catalase, thereby eliminating reactive oxygen species production induced by KCl stress, promoting the accumulation of proline, and maintaining osmotic balance. Exogenous SL expelled K⁺ outside of the cytoplasm and compartmentalized K⁺ into the vacuole, increased the contents of Na⁺, Mg²⁺, Fe²⁺, and Mn²⁺ in the cytoplasm to maintain the ion homeostasis under KCl stress.

CONCLUSIONS

Exogenous SL can regulate photosynthesis, ROS migration and ion transport in apple seedlings to alleviate KCl stress.

Apple MdSAT1 encodes a bHLHm1 transcription factor involved in salinity and drought responses

This study identified a new bHLHm1 transcription factor MdSAT1 which functioned in mediating tolerance to salt and drought resistance. Changes in the expression of stress-related genes play crucial roles in response to environmental stress. Basic helix-loop-helix (bHLH) proteins are the largest superfamily of transcription factors and a large number of bHLH proteins function in plant responses to abiotic stresses. We identified a new bHLHm1 transcription factor from apple and named it MdSAT1. β-Glucuronidase (GUS) staining showed that MdSAT1 expressed in various tissues with highly expressed in leaves. Promoter analysis revealed that MdSAT1 contained multiple response elements and its transcription was induced by several environmental cues, particularly salt and drought stresses. Overexpression of MdSAT1 in apple calli and Arabidopsis resulted in a phenotype of increased tolerance to salt and drought. Altering abscisic acid (ABA) treatment increased the sensitivity of MdSAT1-OE Arabidopsis to ABA, and heavy metal stress, osmotic stress, and ethylene did not participate in MdSAT1 mediated plant development. These findings reveal the abiotic stress functions of MdSAT1 and pave the way for further functional investigation.

How Plant Hormones Mediate Salt Stress Responses

Salt stress is one of the major environmental stresses limiting plant growth and productivity. To adapt to salt stress, plants have developed various strategies to integrate exogenous salinity stress signals with endogenous developmental cues to optimize the balance of growth and stress responses. Accumulating evidence indicates that phytohormones, besides controlling plant growth and development under normal conditions, also mediate various environmental stresses, including salt stress, and thus regulate plant growth adaptation. In this review, we mainly discuss and summarize how plant hormones mediate salinity signals to regulate plant growth adaptation. We also highlight how, in response to salt stress, plants build a defense system by orchestrating the synthesis, signaling, and metabolism of various hormones via multiple crosstalks.

Identification of Phytochrome-Interacting Factor Family Members and Functional Analysis of MdPIF4 in Malus domestica

Phytochrome-interacting factors (PIFs), members of the basic helix-loop-helix transcription factor family that have been extensively investigated in Arabidopsis thaliana, play essential roles in plant growth and development. However, PIF members have not been systematically investigated in apples, a worldwide perennial woody crop of economic importance. Here, seven PIF genes were identified from the Malus × domestica reference genome. Chromosomal locations, gene structures, and phylogenetic relationships of these members were analyzed. Analysis of cis-acting elements in promoter regions of MdPIF genes indicated that various elements were related to light, abiotic stress, and plant hormone responsiveness. Subsequently, subcellular localization and transcriptional activity analysis revealed that MdPIFs were typical nuclear transcription factors with transcriptional activation ability. Expression analysis demonstrated that MdPIF genes had different gene expression patterns for various abiotic factors. Moreover, overexpressed MdPIF4 reduced the sensitivity of apple calluses to abscisic acid (ABA). Our work lays foundations for further investigation of PIF functions in plant growth and development in apples.

MdCER2 conferred to wax accumulation and increased drought tolerance in plants

Drought can activate many stress responses in plant growth and development, including the synthesis of epidermal wax and the induction of abscisic acid (ABA), and increased wax accumulation will improve plant drought resistance. Therefore, an examination of wax biosynthesis genes could help to better understand the molecular mechanism of environmental factors regulating wax biosynthesis and the wax associated stress response. Here, we identified the MdCER2 gene from the 'Gala' (Malus× domestica Borkh.) variety of domestic apple, which is a homolog of Arabidopsis AtCER2. It possesses a transferase domain and the protein localizes on the cell membrane. The MdCER2 gene was constitutively expressed in apple tissues and was induced by drought treatment. Finally, we transformed the MdCER2 gene into Arabidopsis to identify its function, and found ectopic expression of MdCER2 promoted accumulation of cuticular wax in both leaves and stems, decreased water loss and permeability in leaves, increased lateral root number, changed plant ABA sensitivity, and increased drought resistance.

Karrikin Improves Osmotic and Salt Stress Tolerance via the Regulation of the Redox Homeostasis in the Oil Plant Sapium sebiferum

Karrikins are reported to stimulate seed germination, regulate seedling growth, and increase the seedling vigor in abiotic stress conditions in plants. Nevertheless, how karrikins alleviate abiotic stress remains largely elusive. In this study, we found that karrikin (KAR1) could significantly alleviate both drought and salt stress in the important oil plant Sapium sebiferum. KAR1 supplementation in growth medium at a nanomolar (nM) concentration was enough to recover seed germination under salt and osmotic stress conditions. One nanomolar of KAR1 improved seedling biomass, increased the taproot length, and increased the number of lateral roots under abiotic stresses, suggesting that KAR1 is a potent alleviator of abiotic stresses in plants. Under abiotic stresses, KAR1-treated seedlings had a higher activity of the key antioxidative enzymes, such as superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase, in comparison with the control, which leads to a lower level of hydrogen peroxide, malondialdehyde, and electrolyte leakage. Moreover, the metabolome analysis showed that KAR1 treatment significantly increased the level of organic acids and amino acids, which played important roles in redox homeostasis under stresses, suggesting that karrikins might alleviate abiotic stresses via the regulation of redox homeostasis. Under abiotic stresses, applications of karrikins did not increase the endogenous abscisic acid level but altered the expression of several ABA signaling genes, such as SNF1-RELATED PROTEIN KINASE2.3, SNF1-RELATED PROTEIN KINASE2.6, ABI3, and ABI5, suggesting potential interactions between karrikins and ABA signaling in the stress responses. Conclusively, we not only provided the physiological and molecular evidence to clarify the mechanism of karrikins in the regulation of stress adaptation in S. sebiferum but also showed the potential value of karrikins in agricultural practices, which will lay a foundation for further studies about the role of karrikins in abiotic stress alleviation in plants.

Genome-wide analysis and characterization of F-box gene family in Gossypium hirsutum L

Background

F-box proteins are substrate-recognition components of the Skp1-Rbx1-Cul1-F-box protein (SCF) ubiquitin ligases. By selectively targeting the key regulatory proteins or enzymes for ubiquitination and 26S proteasome mediated degradation, F-box proteins play diverse roles in plant growth/development and in the responses of plants to both environmental and endogenous signals. Studies of F-box proteins from the model plant Arabidopsis and from many additional plant species have demonstrated that they belong to a super gene family, and function across almost all aspects of the plant life cycle. However, systematic exploration of F-box family genes in the important fiber crop cotton (Gossypium hirsutum) has not been previously performed. The genome-wide analysis of the cotton F-box gene family is now possible thanks to the completion of several cotton genome sequencing projects.

Results

In current study, we first conducted a genome-wide investigation of cotton F-box family genes by reference to the published F-box protein sequences from other plant species. 592 F-box protein encoding genes were identified in the Gossypium hirsutume acc.TM-1 genome and, subsequently, we were able to present their gene structures, chromosomal locations, syntenic relationships with their parent species. In addition, duplication modes analysis showed that cotton F-box genes were distributed to 26 chromosomes, with the maximum number of genes being detected on chromosome 5. Although the WGD (whole-genome duplication) mode seems play a dominant role during cotton F-box gene expansion process, other duplication modes including TD (tandem duplication), PD (proximal duplication), and TRD (transposed duplication) also contribute significantly to the evolutionary expansion of cotton F-box genes. Collectively, these bioinformatic analysis suggest possible evolutionary forces underlying F-box gene diversification. Additionally, we also conducted analyses of gene ontology, and expression profiles in silico, allowing identification of F-box gene members potentially involved in hormone signal transduction.

Conclusion

The results of this study provide first insights into the Gossypium hirsutum F-box gene family, which lays the foundation for future studies of functionality, particularly those involving F-box protein family members that play a role in hormone signal transduction.

Overexpression of the Stress-Inducible SsMAX2 Promotes Drought and Salt Resistance via the Regulation of Redox Homeostasis in Arabidopsis

Recent studies have demonstrated that strigolactones (SLs) also participate in the regulation of stress adaptation; however, the regulatory mechanism remains elusive. In this study, the homolog of More Axillary Branches 2, which encodes a key component in SL signaling, in the perennial oil plant Sapium sebiferum was identified and functionally characterized in Arabidopsis. The results showed that the expression of SsMAX2 in S. sebiferum seedlings was stress-responsive, and SsMAX2 overexpression (OE) in Arabidopsis significantly promoted resistance to drought, osmotic, and salt stresses. Moreover, SsMAX2 OE lines exhibited decreased chlorophyll degradation, increased soluble sugar and proline accumulation, and lower water loss ratio in response to the stresses. Importantly, anthocyanin biosynthesis and the activities of several antioxidant enzymes, such as superoxide dismutase (SOD), peroxidase (POD), and ascorbate peroxidase (APX), were enhanced in the SsMAX2 OE lines, which further led to a significant reduction in hydrogen peroxide levels. Additionally, the SsMAX2 OE lines exhibited higher expression level of several abscisic acid (ABA) biosynthesis genes, suggesting potential interactions between SL and ABA in the regulation of stress adaptation. Overall, we provide physiological and biochemical evidence demonstrating the pivotal role of SsMAX2 in the regulation of osmotic, drought, and salt stress resistance and show that MAX2 can be a genetic target to improve stress tolerance.

Filling the Gap: Functional Clustering of ABC Proteins for the Investigation of Hormonal Transport in planta

Plant hormones regulate a myriad of plant processes, from seed germination to reproduction, from complex organ development to microelement uptake. Much has been discovered on the factors regulating the activity of phytohormones, yet there are gaps in knowledge about their metabolism, signaling as well as transport. In this review we analyze the potential of the characterized phytohormonal transporters belonging to the ATP-Binding Cassette family (ABC proteins), thus to identify new candidate orthologs in model plants and species important for human health and food production. Previous attempts with phylogenetic analyses on transporters belonging to the ABC family suggested that sequence homology per se is not a powerful tool for functional characterization. However, we show here that sequence homology might indeed support functional conservation of characterized members of different classes of ABC proteins in several plant species, e.g., in the case of ABC class G transporters of strigolactones and ABC class B transporters of auxinic compounds. Also for the low-affinity, vacuolar abscisic acid (ABA) transporters belonging to the ABCC class we show that localization-, rather than functional-clustering occurs, possibly because of sequence conservation for targeting the tonoplast. The ABC proteins involved in pathogen defense are phylogenetically neighboring despite the different substrate identities, suggesting that sequence conservation might play a role in their activation/induction after pathogen attack. Last but not least, in case of the multiple lipid transporters belong to different ABC classes, we focused on ABC class D proteins, reported to transport/affect the synthesis of hormonal precursors. Based on these results, we propose that phylogenetic approaches followed by transport bioassays and in vivo investigations might accelerate the discovery of new hormonal transport routes and allow the designing of transgenic and genome editing approaches, aimed to improve our knowledge on plant development, plant-microbe symbioses, plant nutrient uptake and plant stress resistance.

An apple NAC transcription factor enhances salt stress tolerance by modulating the ethylene response

It is known that ethylene signaling is involved in the regulation of the salt stress response. However, the molecular mechanism of ethylene-regulated salt stress tolerance remains largely unclear. In this study, an apple NAM ATAF CUC transcription factor, MdNAC047, was isolated and functionally characterized to be involved in ethylene-modulated salt tolerance. MdNAC047 gene was significantly induced by salt treatment and its overexpression conferred increased tolerance to salt stress and facilitated the release of ethylene. Quantitative real-time-PCR analysis demonstrated that overexpression of MdNAC047 increased the expression of ethylene-responsive genes. Electrophoretic mobility shift assay, yeast one-hybrid and dual-luciferase assays suggested that MdNAC047 directly binds to the MdERF3 (ETHYLENE RESPONSE FACTOR) promoter and activates its transcription. In addition, genetic analysis assays indicated that MdNAC047 regulates ethylene production at least partially in an MdERF3-dependent pathway. Overall, we found a novel 'MdNAC047-MdERF3-ethylene-salt tolerance' regulatory pathway, which provide new insight into the link between ethylene and salt stress.

Identification and functional characterization of a MAX2 ortholog from switchgrass (Panicum virgatum L.)

Switchgrass (Panicum virgatum L.) is a sustainable cellulosic energy crop with high biomass yield on marginal soils. Tillering, an important agronomic characteristic related to biomass production in gramineous plants, is regulated by complex interacting factors, such as plant hormones. Strigolactones (SLs) comprise a novel class of plant hormones that inhibit shoot branching. The MORE AXILLARY GROWTH2 (MAX2)/DWARF 3 (D3)/RAMOSUS (RMS4) genes encode proteins involved in the SL signaling pathway in various plants. The switchgrass tetraploid genome likely contains two high-similarity MAX2 homologs, one of which is 6 bp longer than the other. The longest is named PvMAX2 and is the ortholog of MAX2 in Arabidopsis, D3 in rice, and RMS4 in petunia. PvMAX2 is expressed ubiquitously in switchgrass tissues, with higher expression levels observed in the stem and shoot. PvMAX2 gene expression is upregulated by GR24, a synthetic SL analog. Ectopic expression of PvMAX2 in the Arabidopsis max2 mutant rescued the dwarf and bushy phenotypes and small leaf size in the mutant, suggesting that functions of AtMAX2 in Arabidopsis are conserved in PvMAX2. Ectopic PvMAX2 expression also restored the wild-type primary root and hypocotyl length phenotypes and restored the response to GR24. These results indicate that PvMAX2 may play an important role in switchgrass tillering through the SL pathway.

A tomato proline-, lysine-, and glutamic-rich type gene SpPKE1 positively regulates drought stress tolerance

Plant abiotic resistance in cultivated species features limited variability. Using genes of wild species serves as a valid approach for improving abiotic resistance of cultivated plants. In this study, we uncovered a previously uncharacterized proline-, lysine-, and glutamic-rich protein gene (SpPKE1), which was isolated from drought-resistant wild tomato species Solanum pennellii (LA0716). When M82, which is a drought-sensitive tomato cultivar, was engineered to overexpress SpPKE1, its tolerance under drought stress was significantly improved by the accumulation of more chlorophyll, proline, and limited malondialdehyde compared with that in RNA interference (RNAi)-suppression lines, which were more sensitive than the wild-type plants. Several ion transporter genes, abiotic-related transcriptional factors, and reactive oxygen species-scavenging genes were upregulated in PKE1 overexpression (OE) lines but downregulated in RNAi plants. OE of SpPKE1 enhanced drought tolerance in tobacco. Screening results of yeast two-hybrid protein-protein interaction revealed that SpPKE1 can bind to an F-box protein that plays an important role in plant drought resistance. We posited that PKE1 enhanced drought tolerance by modulating the expressions of stress-responsive genes and interacting with the F-box protein.

The bZIP transcription factor MdHY5 regulates anthocyanin accumulation and nitrate assimilation in apple

The basic leucine zipper (bZIP) transcription factor HY5 plays a multifaceted role in plant growth and development. Here the apple MdHY5 gene was cloned based on its homology with Arabidopsis HY5. Expression analysis demonstrated that MdHY5 transcription was induced by light and abscisic acid treatments. Electrophoretic mobility shift assays and transient expression assays subsequently showed that MdHY5 positively regulated both its own transcription and that of MdMYB10 by binding to E-box and G-box motifs, respectively. Furthermore, we obtained transgenic apple calli that overexpressed the MdHY5 gene, and apple calli coloration assays showed that MdHY5 promoted anthocyanin accumulation by regulating expression of the MdMYB10 gene and downstream anthocyanin biosynthesis genes. In addition, the transcript levels of a series of nitrate reductase genes and nitrate uptake genes in both wild-type and transgenic apple calli were detected. In association with increased nitrate reductase activities and nitrate contents, the results indicated that MdHY5 might be an important regulator in nutrient assimilation. Taken together, these results indicate that MdHY5 plays a vital role in anthocyanin accumulation and nitrate assimilation in apple.