Genome-Wide Identification and Characterization of ABC Transporters in Nine Rosaceae Species Identifying MdABCG28 as a Possible Cytokinin Transporter linked to Dwarfing

Identification and functional characterization of ABC transporters for selenium accumulation and tolerance in soybean

ATP-binding cassette (ABC) transporters were crucial for various physiological processes like nutrition, development, and environmental interactions. Selenium (Se) is an essential micronutrient for humans, and its role in plants depends on applied dosage. ABC transporters are considered to participate in Se translocation in plants, but detailed studies in soybean are still lacking. We identified 196 ABC genes in soybean transcriptome under Se exposure using next-generation sequencing and single-molecule real-time sequencing technology. These proteins fell into eight subfamilies: 8 GmABCA, 51 GmABCB, 39 GmABCC, 5 GmABCD, 1 GmABCE, 10 GmABCF, 74 GmABCG, and 8 GmABCI, with amino acid length 121-3022 aa, molecular weight 13.50-341.04 kDa, and isoelectric point 4.06-9.82. We predicted a total of 15 motifs, some of which were specific to certain subfamilies (especially GmABCB, GmABCC, and GmABCG). We also found predicted alternative splicing in GmABCs: 60 events in selenium nanoparticles (SeNPs)-treated, 37 in sodium selenite (Na2SeO3)-treated samples. The GmABC genes showed differential expression in leaves and roots under different application of Se species and Se levels, most of which are belonged to GmABCB, GmABCC, and GmABCG subfamilies with functions in auxin transport, barrier formation, and detoxification. Protein-protein interaction and weighted gene co-expression network analysis suggested functional gene networks with hub ABC genes, contributing to our understanding of their biological functions. Our results illuminate the contributions of GmABC genes to Se accumulation and tolerance in soybean and provide insight for a better understanding of their roles in soybean as well as in other plants.

Genome-wide analysis of the ABC gene family in almond and functional predictions during flower development, freezing stress, and salt stress

ABC (ATP-binding cassette) transporter proteins are one of the most extensive protein families known to date and are ubiquitously found in animals, plants, and microorganisms. ABCs have a variety of functions, such as plant tissue development regulation, hormone transport, and biotic and abiotic stress resistance. However, the gene characterization and function of the ABC gene family in almond (Prunus dulcis) have not been thoroughly studied. In this study, we identified 117 PdABC genes using the whole genome of 'Wanfeng' almond obtained by sequencing and explored their protein characterization. The PdABC family members were classified into eight subfamilies. The members of the same subfamily had conserved motifs but poorly conserved numbers of exons and introns and were unevenly distributed among the eight subfamilies and on the eight chromosomes. Expression patterns showed that PdABC family members were significantly differentially expressed during almond development, dormant freezing stress, and salt stress. We found that PdABC59 and PdABC77 had extremely high expression levels in pollen. PdABC63 and PdABC64 had high expression levels during almond petal development and multiple stages of flower development. PdABC98 was highly expressed in annual dormant branches after six temperature-freezing stress treatments. PdABC29, PdABC69, and PdABC98 were highly expressed under different concentrations of salt stress. This study preliminarily investigated the expression characteristics of ABC genes in different tissues of almond during flower development, freezing stress and salt stress, and the results will provide a reference for further in-depth research and breeding of almond in the future.

Genome-wide identification and characterization of ABC transporter superfamily in the legume Cajanus cajan

Plant ATP-binding cassette (ABC) protein family is the largest multifunctional highly conserved protein superfamily that transports diverse substrates across biological membranes by the hydrolysis of ATP and is also the part of the several other biological processes like cellular detoxification, growth and development, stress biology, and signaling processes. In the agriculturally important legume crop Cajanus cajan, a genome-wide identification and characterization of the ABC gene family was carried out. A total of 159 ABC genes were identified that belong to eight canonical classes CcABCA to CcABCG and CcABCI based on the phylogenetic analysis. The number of genes was highest in CcABCG followed by CcABCC and CcABCB class. A total of 85 CcABC genes were found on 11 chromosomes and 74 were found on scaffold. Tandem duplication was the major driver of CcABC gene family expansion. The dN/dS ratio revealed the purifying selection. The phylogenetic analysis revealed class-specific eight superclades which reflect their functional importance. The largest clade was found to be CcABCG which reflects their functional significance. CcABC proteins were mainly basic in nature and found to be localized in the plasma membrane. The secondary structure prediction revealed the dominance of α-helix. The canonical transmembrane and nucleotide binding domain, signature motif LSSGQ, Walker A, Walker B region, and Q loop were also identified. A class-specific exon-intron pattern was also observed. In addition to core elements, different cis-acting regulatory elements like stress, hormone, and cellular responsive were also identified. Expression profiling of CcABC genes at various developmental stages of different anatomical tissues was performed and it was noticed that CcABCF3, CcABCF4, CcABCF5, CcABCG66, and CcABCI3 had the highest expression. The results of the current study endow us with the further functional analysis of Cajanus ABC in the future.

MdGRF11-MdARF19-2 module acts as a positive regulator of drought resistance in apple rootstock

14-3-3 proteins play an important role in the response of plants to drought resistance. In this study, 14-3-3 protein MdGRF11 was cloned from Malus xiaojinensis, and its positive regulation of drought resistance was verified using Orin calli and M. xiaojinensis plants. The transcription factor MdARF19-2 was further screened for interaction with this protein in vitro and in vivo. We also conducted experiments using Orin calli and found that the overexpression of MdARF19-2 decreased the level of reactive oxygen species (ROS) and increased the activity of enzymes that scavenge ROS in plant materials. This indicates that MdARF19-2 is a positive regulator in the drought resistance of plants. The drought tolerance was further improved by the overexpression of both MdGRF11 and MdARF19-2 in the calli. In addition, we examined several genes related to ROS scavenging with auxin response factor binding elements in their promoters and found that their level of expression was regulated by the MdGRF11-MdARF19-2 module. In conclusion, the enhancement of plant drought resistance by MdGRF11 could be owing to its accumulation at the protein level in response to drought, which then combined with MdARF19-2, affecting the expression of MdARF19-2 downstream genes. Thus, it scavenges ROS, which ultimately improves the resistance of plant to drought stress.

Genome-wide identification of WOX family members in nine Rosaceae species and a functional analysis of MdWOX13-1 in drought resistance

WUSCHEL-related homeobox (WOX) transcription factors (TFs) are important in plant development processes and evolutionary novelties. In this study, a genome-wide comprehensive analysis of WOX genes from nine Rosaceae species was carried out, and their potential roles in Malus were subsequently investigated. 125 WOXs in 9 Rosaceae species were identified and classified into three clades, i.e., the ancient, intermediate, and WUS clades. Prunus. domestica contained the most intra-genomic collinearity among the nine Rosaceae species. Additionally, the cis-elements in WOX gene family members were compared and classified into three categories, including phytohormone-responsive, plant growth and development, and abiotic and biotic stresses. Overexpression (OE) of MdWOX13-1 also increased the callus weight and enhanced ROS scavenging against drought stress. Furthermore, via yeast-one hybrid assay and LUC analyses, MdWOX13-1 could directly bind to the MdMnSOD promoter. Therefore, our results will facilitate further study of the WOX genes' function in the Rosaceae family.

Molecular Regulation and Evolution of Cytokinin Signaling in Plant Abiotic Stresses

The sustainable production of crops faces increasing challenges from global climate change and human activities, which leads to increasing instances of many abiotic stressors to plants. Among the abiotic stressors, drought, salinity and excessive levels of toxic metals cause reductions in global agricultural productivity and serious health risks for humans. Cytokinins (CKs) are key phytohormones functioning in both normal development and stress responses in plants. Here, we summarize the molecular mechanisms on the biosynthesis, metabolism, transport and signaling transduction pathways of CKs. CKs act as negative regulators of both root system architecture plasticity and root sodium exclusion in response to salt stress. The functions of CKs in mineral-toxicity tolerance and their detoxification in plants are reviewed. Comparative genomic analyses were performed to trace the origin, evolution and diversification of the critical regulatory networks linking CK signaling and abiotic stress. We found that the production of CKs and their derivatives, pathways of signal transduction and drought-response root growth regulation are evolutionarily conserved in land plants. In addition, the mechanisms of CK-mediated sodium exclusion under salt stress are suggested for further investigations. In summary, we propose that the manipulation of CK levels and their signaling pathways is important for plant abiotic stress and is, therefore, a potential strategy for meeting the increasing demand for global food production under changing climatic conditions.

Building a foundation for gene family analysis in Rosaceae genomes with a novel workflow: A case study in Pyrus architecture genes

The rapid development of sequencing technologies has led to a deeper understanding of plant genomes. However, direct experimental evidence connecting genes to important agronomic traits is still lacking in most non-model plants. For instance, the genetic mechanisms underlying plant architecture are poorly understood in pome fruit trees, creating a major hurdle in developing new cultivars with desirable architecture, such as dwarfing rootstocks in European pear (Pyrus communis). An efficient way to identify genetic factors for important traits in non-model organisms can be to transfer knowledge across genomes. However, major obstacles exist, including complex evolutionary histories and variable quality and content of publicly available plant genomes. As researchers aim to link genes to traits of interest, these challenges can impede the transfer of experimental evidence across plant species, namely in the curation of high-quality, high-confidence gene models in an evolutionary context. Here we present a workflow using a collection of bioinformatic tools for the curation of deeply conserved gene families of interest across plant genomes. To study gene families involved in tree architecture in European pear and other rosaceous species, we used our workflow, plus a draft genome assembly and high-quality annotation of a second P. communis cultivar, 'd'Anjou.' Our comparative gene family approach revealed significant issues with the most recent 'Bartlett' genome - primarily thousands of missing genes due to methodological bias. After correcting assembly errors on a global scale in the 'Bartlett' genome, we used our workflow for targeted improvement of our genes of interest in both P. communis genomes, thus laying the groundwork for future functional studies in pear tree architecture. Further, our global gene family classification of 15 genomes across 6 genera provides a valuable and previously unavailable resource for the Rosaceae research community. With it, orthologs and other gene family members can be easily identified across any of the classified genomes. Importantly, our workflow can be easily adopted for any other plant genomes and gene families of interest.

Genome-Wide Analysis of the ATP-Binding Cassette (ABC) Transporter Family in Zea mays L. and Its Response to Heavy Metal Stresses

The ATP-binding cassette (ABC) transporter family is one of the largest eukaryotic protein families. Its members play roles in numerous metabolic processes in plants by releasing energy for substrate transport across membranes through hydrolysis of ATP. Maize belongs to the monocotyledonous plant family, Gramineae, and is one of the most important food crops in the world. We constructed a phylogenetic tree with individual ABC genes from maize, rice, sorghum, Arabidopsis, and poplar. This revealed eight families, each containing ABC genes from both monocotyledonous and dicotyledonous plants, indicating that the amplification events of ABC gene families predate the divergence of plant monocotyledons. To further understand the functions of ABC genes in maize growth and development, we analyzed the expression patterns of maize ABC family genes in eight tissues and organs based on the transcriptome database on the Genevestigator website. We identified 133 ABC genes expressed in most of the eight tissues and organs examined, especially during root and leaf development. Furthermore, transcriptome analysis of ZmABC genes showed that exposure to metallic lead induced differential expression of many maize ABC genes, mainly including ZmABC 012, 013, 015, 031, 040, 043, 065, 078, 080, 085, 088, 102, 107, 111, 130 and 131 genes, etc. These results indicated that ZmABC genes play an important role in the response to heavy metal stress. The comprehensive analysis of this study provides a foundation for further studies into the roles of ABC genes in maize.

ATP-binding cassette transporters in nonmodel plants

Knowledge about plant ATP-binding cassette (ABC) proteins is of great value for sustainable agriculture, economic yield, and the generation of high-quality products, especially under unfavorable growth conditions. We have learned much about ABC proteins in model organisms, notably Arabidopsis thaliana; however, the importance of research dedicated to these transporters extends far beyond Arabidopsis biology. Recent progress in genomic and transcriptomic approaches for nonmodel and noncanonical model plants allows us to look at ABC transporters from a wider perspective and consider chemodiversity and functionally driven adaptation as distinctive mechanisms during their evolution. Here, by considering several representatives from agriculturally important families and recent progress in functional characterization of nonArabidopsis ABC proteins, we aim to bring attention to understanding the evolutionary background, distribution among lineages and possible mechanisms underlying the adaptation of this versatile transport system for plant needs. Increasing the knowledge of ABC proteins in nonmodel plants will facilitate breeding and development of new varieties based on, for example, genetic variations of endogenous genes and/or genome editing, representing an alternative to transgenic approaches.

2021 update on ATP-binding cassette (ABC) transporters: how they meet the needs of plants

Recent developments in the field of ABC proteins including newly identified functions and regulatory mechanisms expand the understanding of how they function in the development and physiology of plants.

Identification of ABC transporter G subfamily in white lupin and functional characterization of L.albABGC29 in phosphorus use

BACKGROUND

White lupin (Lupinus albus) is a leguminous crop with elite adaptive ability in phosphorus-deficient soil and used as a model plant for studying phosphorus (P) use. However, the genetic basis of its adaptation to low P (LP) remains unclear. ATPase binding cassette (ABC) transports G subfamily play a crucial role in the transportation of biological molecules across the membrane. To date, identification of this subfamily has been analyzed in some plants, but no systematic analysis of these transporters in phosphorus acquisition is available for white lupin.

RESULTS

This study identified 66 ABCG gene family members in the white lupin genome using comprehensive approaches. Phylogenetic analysis of white lupin ABCG transporters revealed six subclades based on their counterparts in Arabidopsis, displaying distinct gene structure and motif distribution in each cluster. Influences of the whole genome duplication on the evolution of L.albABCGs were investigated in detail. Segmental duplications appear to be the major driving force for the expansion of ABCGs in white lupin. Analysis of the Ka/Ks ratios indicated that the paralogs of the L.albABCG subfamily members principally underwent purifying selection. However, it was found that L.albABCG29 was a result of both tandem and segmental duplications. Overexpression of L.albABCG29 in white lupin hairy root enhanced P accumulation in cluster root under LP and improved plant growth. Histochemical GUS staining indicated that L.albABCG29 expression increased under LP in white lupin roots. Further, overexpression of L.albABCG29 in rice significantly improved P use under combined soil drying and LP by improving root growth associated with increased rhizosheath formation.

CONCLUSION

Through systematic and comprehensive genome-wide bioinformatics analysis, including conserved domain, gene structures, chromosomal distribution, phylogenetic relationships, and gene duplication analysis, the L.albABCG subfamily was identified in white lupin, and L.albABCG29 characterized in detail. In summary, our results provide deep insight into the characterization of the L.albABCG subfamily and the role of L.albABCG29 in improving P use.

Group-C/S1 bZIP heterodimers regulate MdIPT5b to negatively modulate drought tolerance in apple species

Cytokinins play a central role in delaying senescence, reducing oxidative damage and maintaining plant growth during drought. This study showed that the ectopic expression of ProRE-deleted MdIPT5b, a key enzyme involved in cytokinin metabolism, increased the drought tolerance of transgenic Malus domestica (apple) callus and Solanum lycopersicum (tomato) seedlings by maintaining cytokinin homeostasis, and thus maintaining redox balance. Under restricted watering regimes, the yields of transgenic tomato plants were enhanced. Heterodimers of C/S1 bZIP are involved in the cytokinin-mediated drought response. The heterodimers bind the ProRE of MdIPT5b promoter, thus directly suppressing gene transcription. Single C/S1 bZIP members could not independently function as suppressors. However, specific paired members (heterodimers of MdbZIP80 with MdbZIP2 or with MdbZIP39) effectively suppressed transcription. The α-helical structure is essential for the heterodimerization of C/S1 bZIP members and for synergistic transcriptional suppression. As negative regulators of drought tolerance, suppressing either MdbZIP2 or MdbZIP39 alone does not improve the expression of MdIPT5b and did not increase the drought tolerance of transgenic apple callus. However, this could be achieved when they were co-suppressed. The suppression of MdbZIP80 alone could improve MdIPT5b expression and increase the drought tolerance of transgenic apple callus. However, these effects were reversed in response to the cosuppression of MdbZIP80 and MdIPT5b. Similar results were also observed during delayed dark-induced senescence in apple leaves. In conclusion, the apple C/S1 bZIP network (involving MdbZIP2, MdbZIP39 and MdbZIP80) directly suppressed the expression of MdIPT5b, thus negatively modulating drought tolerance and dark-induced senescence in a functionally redundant manner.

Genome-Wide Identification of Soybean ABC Transporters Relate to Aluminum Toxicity

ATP-binding cassette (ABC) transporter proteins are a gene super-family in plants and play vital roles in growth, development, and response to abiotic and biotic stresses. The ABC transporters have been identified in crop plants such as rice and buckwheat, but little is known about them in soybean. Soybean is an important oil crop and is one of the five major crops in the world. In this study, 255 ABC genes that putatively encode ABC transporters were identified from soybean through bioinformatics and then categorized into eight subfamilies, including 7 ABCAs, 52 ABCBs, 48 ABCCs, 5 ABCDs, 1 ABCEs, 10 ABCFs, 111 ABCGs, and 21 ABCIs. Their phylogenetic relationships, gene structure, and gene expression profiles were characterized. Segmental duplication was the main reason for the expansion of the GmABC genes. Ka/Ks analysis suggested that intense purifying selection was accompanied by the evolution of GmABC genes. The genome-wide collinearity of soybean with other species showed that GmABCs were relatively conserved and that collinear ABCs between species may have originated from the same ancestor. Gene expression analysis of GmABCs revealed the distinct expression pattern in different tissues and diverse developmental stages. The candidate genes GmABCB23, GmABCB25, GmABCB48, GmABCB52, GmABCI1, GmABCI5, and GmABCI13 were responsive to Al toxicity. This work on the GmABC gene family provides useful information for future studies on ABC transporters in soybean and potential targets for the cultivation of new germplasm resources of aluminum-tolerant soybean.

Differential regulation of triterpene biosynthesis induced by an early failure in cuticle formation in apple

Waxy apple cuticles predominantly accumulate ursane-type triterpenes, but the profile shifts with the induction of skin russeting towards lupane-type triterpenes. We previously characterised several key enzymes in the ursane-type and lupane-type triterpene pathways, but this switch in triterpene metabolism associated with loss of cuticle integrity is not fully understood. To analyse the relationship between triterpene biosynthesis and russeting, we used microscopy, RNA-sequencing and metabolite profiling during apple fruit development. We compared the skin of three genetically-close clones of 'Golden Delicious' (with waxy, partially russeted and fully russeted skin). We identified a unique molecular profile for the russet clone, including low transcript abundance of multiple cuticle-specific metabolic pathways in the early stages of fruit development. Using correlation analyses between gene transcription and metabolite concentration we found MYB transcription factors strongly associated with lupane-type triterpene biosynthesis. We showed how their transcription changed with the onset of cuticle cracking followed by russeting and that one factor, MYB66, was able to bind the promoter of the oxidosqualene cyclase OSC5, to drive the production of lupeol derivatives. These results provide insights into the breakdown of cuticle integrity leading to russet and how this drives MYB-regulated changes to triterpene biosynthesis.

Genome-Wide Identification of Brassinosteroid Signaling Downstream Genes in Nine Rosaceae Species and Analyses of Their Roles in Stem Growth and Stress Response in Apple

Brassinosteroid signaling downstream genes regulate many important agronomic traits in rice. However, information on such genes is limited in Arabidopsis and Rosaceae species. We identified these genes in Arabidopsis and nine Rosaceae species. They were, respectively, named based on chromosomal locations. Segmental duplication and whole-genome duplication under purifying selection, as determined by Ka/Ks analysis, likely contributed to Rosaceae gene expansion. Apple (Malus domestica), Arabidopsis, and rice genes were generally similar, while several Rosaceae genes differed from their rice homologs in various characteristics, such as gene length, subcellular localization, transmembrane topology, conserved domains, secondary structures, and responses to external signals. The brassinosteroid downstream genes in apple were, respectively, induced or repressed by five phytohormones. Furthermore, these apple downstream genes were differentially expressed in different apple grafting combinations (“Nagafu No. 2”/“Malling 9” and “Nagafu No. 2”/“Nagafu No. 2”) and long–short shoot varieties (“Yanfu No. 6” and “Nagafu No. 2”). Responses of the MdBZR genes to diverse stress signals were examined and candidate hub genes were identified. These findings indicated that several brassinosteroid signaling downstream genes in Rosaceae functionally differed from their rice homologs, and certain apple genes may play roles in plant height and stress responses. This study provided valuable information and presented enriched biological theories on brassinosteroid signaling downstream genes in apple. Identification of such genes serve to help expand apple breeding and growth. This study provides useful information for brassinosteroid signaling downstream genes.

The Hulks and the Deadpools of the Cytokinin Universe: A Dual Strategy for Cytokinin Production, Translocation, and Signal Transduction

Cytokinins are plant hormones, derivatives of adenine with a side chain at the N⁶-position. They are involved in many physiological processes. While the metabolism of trans-zeatin and isopentenyladenine, which are considered to be highly active cytokinins, has been extensively studied, there are others with less obvious functions, such as cis-zeatin, dihydrozeatin, and aromatic cytokinins, which have been comparatively neglected. To help explain this duality, we present a novel hypothesis metaphorically comparing various cytokinin forms, enzymes of CK metabolism, and their signalling and transporter functions to the comics superheroes Hulk and Deadpool. Hulk is a powerful but short-lived creation, whilst Deadpool presents a more subtle and enduring force. With this dual framework in mind, this review compares different cytokinin metabolites, and their biosynthesis, translocation, and sensing to illustrate the different mechanisms behind the two CK strategies. This is put together and applied to a plant developmental scale and, beyond plants, to interactions with organisms of other kingdoms, to highlight where future study can benefit the understanding of plant fitness and productivity.