AT-Hook Transcription Factors Restrict Petiole Growth by Antagonizing PIFs

Promoter of Cassava MeAHL31 Responds to Diverse Abiotic Stresses and Hormone Signals in Transgenic Arabidopsis

The AT-hook motif nuclear-localized (AHL) family is pivotal for the abiotic stress response in plants. However, the function of the cassava AHL genes has not been elucidated. Promoters, as important regulatory elements of gene expression, play a crucial role in stress resistance. In this study, the promoter of the cassava MeAHL31 gene was cloned. The MeAHL31 protein was localized to the cytoplasm and the nucleus. qRT-PCR analysis revealed that the MeAHL31 gene was expressed in almost all tissues tested, and the expression in tuber roots was 321.3 times higher than that in petioles. Promoter analysis showed that the MeAHL31 promoter contains drought, methyl jasmonate (MeJA), abscisic acid (ABA), and gibberellin (GA) cis-acting elements. Expression analysis indicated that the MeAHL31 gene is dramatically affected by treatments with salt, drought, MeJA, ABA, and GA3. Histochemical staining in the proMeAHL31-GUS transgenic Arabidopsis corroborated that the GUS staining was found in most tissues and organs, excluding seeds. Beta-glucuronidase (GUS) activity assays showed that the activities in the proMeAHL31-GUS transgenic Arabidopsis were enhanced by different concentrations of NaCl, mannitol (for simulating drought), and MeJA treatments. The integrated findings suggest that the MeAHL31 promoter responds to the abiotic stresses of salt and drought, and its activity is regulated by the MeJA hormone signal.

ScRNA-seq reveals dark- and light-induced differentially expressed gene atlases of seedling leaves in Arachis hypogaea L

Although the regulatory mechanisms of dark and light-induced plant morphogenesis have been broadly investigated, the biological process in peanuts has not been systematically explored on single-cell resolution. Herein, 10 cell clusters were characterized using scRNA-seq-identified marker genes, based on 13 409 and 11 296 single cells from 1-week-old peanut seedling leaves grown under dark and light conditions. 6104 genes and 50 transcription factors (TFs) displayed significant expression patterns in distinct cell clusters, which provided gene resources for profiling dark/light-induced candidate genes. Further pseudo-time trajectory and cell cycle evidence supported that dark repressed the cell division and perturbed normal cell cycle, especially the PORA abundances correlated with 11 TFs highly enriched in mesophyll to restrict the chlorophyllide synthesis. Additionally, light repressed the epidermis cell developmental trajectory extending by inhibiting the growth hormone pathway, and 21 TFs probably contributed to the different genes transcriptional dynamic. Eventually, peanut AHL17 was identified from the profile of differentially expressed TFs, which encoded protein located in the nucleus promoted leaf epidermal cell enlargement when ectopically overexpressed in Arabidopsis through the regulatory phytohormone pathway. Overall, our study presents the different gene atlases in peanut etiolated and green seedlings, providing novel biological insights to elucidate light-induced leaf cell development at the single-cell level.

Genome-wide analysis of radish AHL gene family and functional verification of RsAHL14 in tomato

The AT-hook motif nuclear localized (AHL) gene family is a highly conserved transcription factors involved in plant growth, development, and stress responses. However, AHLs have not been systematically analyzed in radish (Raphanus sativus). Therefore, we performed genome-wide identification and expression pattern, gene structure, and function verifications of radish AHLs. We identified 52 radish AHLs (RsAHL1-RsAHL52), which were unevenly distributed across nine chromosomes. Phylogenetic analysis showed that the RsAHLs were divided into two clades (A and B) and subdivided into three types (I, II, and III). Collinearity analysis revealed that the 52 RsAHLs produced 49 repeat events. Tissue expression profiles revealed differential expression of RsAHLs across different tissues, with higher expression observed in flower organs, particularly petals and anthers. qRT-PCR results indicated that RsAHLs responded to abscisic acid, methyl jasmonate, and abiotic stress (low and high temperatures and drought). Additionally, RsAHL14 induced a dwarf phenotype in tomato plants, and RsAHL14-overexpression tomato plants presented significantly decreased expression levels of the gibberellin (GA) synthetic genes ent-Copalyl diphosphatase, GA3ox-3/-4/-5, and GA20ox-1/-2/-3, but significantly increased expression of the degradation gene GA2ox-1/-3. Thus, RsAHL14 might affect plant growth by regulating GA content. Collectively, our study comprehensively identified RsAHLs in radish and provided a reference for further research on these genes.

Bridging fungal resistance and plant growth through constitutive overexpression of Thchit42 gene in Pelargonium graveolens

KEY MESSAGE

Thchit42 constitutive expression for fungal resistance showed synchronisation with leaf augmentation and transcriptome analysis revealed the Longifolia and Zinc finger RICESLEEPER gene is responsible for plant growth and development. Pelargonium graveolens essential oil possesses significant attributes, known for perfumery and aromatherapy. However, optimal yield and propagation are predominantly hindered by biotic stress. All biotechnological approaches have yet to prove effective in addressing fungal resistance. The current study developed transgenic geranium bridging molecular mechanism of fungal resistance and plant growth by introducing cassette 35S::Thchit42. Furthermore, 120 independently putative transformed explants were regenerated on kanamycin fortified medium. Primarily transgenic lines were demonstrated peak pathogenicity and antifungal activity against formidable Colletotrichum gloeosporioides and Fusarium oxysporum. Additionally, phenotypic analysis revealed ~ 2fold increase in leaf size and ~ 2.1fold enhanced oil content. To elucidate the molecular mechanisms for genotypic cause, de novo transcriptional profiles were analyzed to indicate that the auxin-regulated longifolia gene is accountable for augmentation in leaf size, and zinc finger (ZF) RICESLEEPER attributes growth upregulation. Collectively, data provides valuable insights into unravelling the mechanism of Thchit42-mediated crosstalk between morphological and chemical alteration in transgenic plants. This knowledge might create novel opportunities to cultivate fungal-resistant geranium throughout all seasons to fulfil demand.

Chromatin attachment to the nuclear matrix represses hypocotyl elongation in Arabidopsis thaliana

The nuclear matrix is a nuclear compartment that has diverse functions in chromatin regulation and transcription. However, how this structure influences epigenetic modifications and gene expression in plants is largely unknown. In this study, we show that a nuclear matrix binding protein, AHL22, together with the two transcriptional repressors FRS7 and FRS12, regulates hypocotyl elongation by suppressing the expression of a group of genes known as SMALL AUXIN UP RNAs (SAURs) in Arabidopsis thaliana. The transcriptional repression of SAURs depends on their attachment to the nuclear matrix. The AHL22 complex not only brings these SAURs, which contain matrix attachment regions (MARs), to the nuclear matrix, but it also recruits the histone deacetylase HDA15 to the SAUR loci. This leads to the removal of H3 acetylation at the SAUR loci and the suppression of hypocotyl elongation. Taken together, our results indicate that MAR-binding proteins act as a hub for chromatin and epigenetic regulators. Moreover, we present a mechanism by which nuclear matrix attachment to chromatin regulates histone modifications, transcription, and hypocotyl elongation.

AT Hook-Like 10 phosphorylation determines ribosomal RNA processing 6-like 1 (RRP6L1) chromatin association and growth suppression during water stress

Phosphorylation of AT Hook-Like 10 (AHL10), one of the AT-hook family of plant-specific DNA binding proteins, is critical for growth suppression during moderate severity drought (low water potential, ψw ) stress. To understand how AHL10 phosphorylation determines drought response, we identified putative AHL10 interacting proteins and further characterized interaction with RRP6L1, a protein involved in epigenetic regulation. RRP6L1 and AHL10 mutants, as well as ahl10-1rrp6l1-2, had similar phenotypes of increased growth maintenance during low ψw . Chromatin precipitation demonstrated that RRP6L1 chromatin association increased during low ψw stress and was dependent upon AHL10 phosphorylation. Transcriptome analyses showed that AHL10 and RRP6L1 have concordant effects on expression of stress- and development-related genes. Together these results indicate that stress signalling can act via AHL10 phosphorylation to control the chromatin association of the key regulatory protein RRP6L1. AHL10 and RRP6L1 interaction in meristem cells is part of a mechanism to downregulate growth during low ψw stress. Interestingly, the loss of AHL13, which is homologous to AHL10 and phosphorylated at a similar C-terminal site, blocked the enhanced growth maintenance of ahl10-1. Thus, AHL10 and AHL13, despite their close homology, are not redundant but rather have distinct roles, likely related to the formation of AHL hetero-complexes.

Spatially resolved gene regulatory networks in Asian rice (Oryza sativa cv. Nipponbare) leaves

Transcriptome profiles in plants are heterogenous at every level of morphological organization. Even within organs, cells of the same type can have different patterns of gene expression depending on where they are positioned within tissues. This heterogeneity is associated with non-uniform distribution of biological processes within organs. The regulatory mechanisms that establish and sustain the spatial heterogeneity are unknown. Here, we identify regulatory modules that support functional specialization of different parts of Oryza sativa cv. Nipponbare leaves by leveraging transcriptome data, transcription factor binding motifs and global gene regulatory network prediction algorithms. We generated a global gene regulatory network in which we identified six regulatory modules that were active in different parts of the leaf. The regulatory modules were enriched for genes involved in spatially relevant biological processes, such as cell wall deposition, environmental sensing and photosynthesis. Strikingly, more than 86.9% of genes in the network were regulated by members of only five transcription factor families. We also generated targeted regulatory networks for the large MYB and bZIP/bHLH families to identify interactions that were masked in the global prediction. This analysis will provide a baseline for future single cell and array-based spatial transcriptome studies and for studying responses to environmental stress and demonstrates the extent to which seven coarse spatial transcriptome analysis can provide insight into the regulatory mechanisms supporting functional specialization within leaves.

Comparative transcriptome profiling to unravel the key molecular signalling pathways and drought adaptive plasticity in shoot borne root system of sugarcane

Sugarcane root system comprises of superficial sett roots as well as deeply-penetrating shoot borne roots (SBR) with latter being the permanent root system. In sugarcane, the healthy SBR contributes to a better crop yield and it also helps to produce multiple ratoon crops after the harvest. There is a dearth of in-depth knowledge on SBR system architecture and its functional role in modern day commercial hybrids. A comprehensive phenotypic, anatomical and whole transcriptome profiling, conducted between the commercial sugarcane hybrids and a wild germplasm Erianthus, found a developmental delay in both initiation and establishment of the SBR in commercial hybrid compared to Erianthus. The SBR system in Erianthus proved to be an extensive drought-adaptive root system architecture that significantly contributes to drought tolerance. On the other hand, SBRs in the commercial hybrids showed an irreversible collapse and damage of the root cells under drought stress. The outcomes from the comparative analysis of the transcriptome data showed a significant upregulation of the genes that regulate important stress signalling pathways viz., sugar, calcium, hormone signalling and phenylpropanoid biosynthesis in the SBRs of Erianthus. It was found that through these key signalling pathways, Erianthus SBRs triggered the downstream signalling cascade to impart physiological responses like osmoprotection, modification of the cell walls, detoxification of reactive oxygen species, expression of drought responsive transcription factors, maintenance of cell stability and lateral root development. The current study forms a basis for further exploration of the Shoot Borne Root system as a valuable breeding target to develop drought tolerant sugarcane genotypes.

Genome-wide identification and analyses of the AHL gene family in rice (Oryza sativa)

AHL (AT-HOOK MOTIF CONTAINING NUCLEAR LOCALIZED) family members play a critical role in stress resistance regulation by DNA-protein and protein-protein interactions in a number of plant biological processes. Using genomic data, an attempt was made to evaluate AHL genes in rice. Using a genome database, we performed in silico detection and characterization of AHL family genes in rice. The data of the gene were obtained from the Rice Genome Annotation Project (RGAP) database. The rice genome data were analyzed using bioinformatics software. The main objectives of the research are genome-wide recognition, expression, structural examination, phylogenetic analysis of AHL gene family, classification of AHL proteins into different classes based on motif and domain composition, analysis of promoter regions to identify stress and phytohormone-associated cis-elements, expression analysis of OsAHL genes in diverse tissues and stressful situations and understanding the roles of AHLs in controlling rice plant development. The genome-wide recognition, expression, and structural examination of the AHL gene family were undertaken in this research to evaluate the structural activities of AHLs in rice. From the Oryza sativa genome, 26 AHL genes have been identified. WoLF PSORT analysis predicted different subcellular localizations for these proteins, including nuclear, cytoplasmic, chloroplast, and endoplasmic reticulum. According to a phylogenetic study, rice AHLs resulted in two clades: Clade-A with no introns (excluding OsAHL15 and OsAHL21) and Clade-B with four introns. Depending on the AT-hook motif (s) (AHM) and PPC/DUF 296 domain composition, the AHL proteins are categorized into the following three classes: Type-I, Type-II, and Type-III, among Type-I AHLs constituting Clade-A, Type-II, and Type-III creating Clade-B. Type-I was the largest gene family, representing 57.69% of OsAHL genes. The exon-intron organization within clades of OsAHL genes was similar. Multiple sequence alignment identified 15 conserved motifs, including AT-hook motifs and the PPC domain, suggesting DNA-binding functionality. OsAHL genes were distributed across 12 chromosomes, with chromosome 2 and 8 harboring the highest number of genes. Gene duplication analysis revealed eight paralogous pairs, indicating evolutionary divergence between 13.32 and 35.59 million years ago. The emergence of OsAHL paralogous pairs was favored by purifying selection. Synteny analysis between rice and Arabidopsis demonstrated collinearity among AHL gene pairs, implying comparable structure and function in the two species. The role of stress- and phytohormone-associated cis-elements in the OsAHL genes was discovered by promoter analysis. OsAHL genes participated in various biological processes, with a prominent involvement in cellular and metabolic processes. They exhibited a significant enrichment in binding functions, including a substantial proportion of transcription regulators. OsAHL genes displayed diverse expression patterns in different tissues and under abiotic stress conditions. According to their expression patterns, the majority of OsAHLs of Clade-B were expressed mainly in the pistil indicating their roles in flower formation, while Clade-A OsAHLs had the minimal expression in pistil and highly expressed in embryos, indicating that the AHLs within each clade had the same expression patterns. Some OsAHL genes were also expressed in stressful situations, such as cold, salt, and drought. Protein interaction analysis revealed networks involving AHL proteins and other proteins, suggesting their participation in phytohormone responses, abiotic stress, and plant development. In this work, 26 OsAHL genes were found in the genome of rice. Rice OsAHLs were grouped into two phylogenetic groups. It is further divided into three types on the basis of the motif and domain composition. At various phases of development, the expression analysis of OsAHLs showed numerous variations in expression levels in diverse tissues and stress situations. Our findings shed light on the significant roles of AHLs in controlling rice plant development.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03666-0.

Functions of Phytochrome-Interacting Factors (PIFs) in the regulation of plant growth and development: A comprehensive review

Transcription factors play important roles in governing plant responses upon changes in their ambient conditions. Any fluctuation in the supply of critical requirements for plants, such as optimum light, temperature, and water leads to the reprogramming of gene-signaling pathways. At the same time, plants also evaluate and shift their metabolism according to the various stages of development. Phytochrome-Interacting Factors are one of the most important classes of transcription factors that regulate both developmental and external stimuli-based growth of plants. This review focuses on the identification of PIFs in various organisms, regulation of PIFs by various proteins, functions of PIFs of Arabidopsis in diverse developmental pathways such as seed germination, photomorphogenesis, flowering, senescence, seed and fruit development, and external stimuli-induced plant responses such as shade avoidance response, thermomorphogenesis, and various abiotic stress responses. Recent advances related to the functional characterization of PIFs of crops such as rice, maize, and tomato have also been incorporated in this review, to ascertain the potential of PIFs as key regulators to enhance the agronomic traits of these crops. Thus, an attempt has been made to provide a holistic view of the function of PIFs in various processes in plants.

Root-knot nematode modulates plant CLE3-CLV1 signaling as a long-distance signal for successful infection

Plants use many long-distance and systemic signals to modulate growth and development, as well as respond to biotic and abiotic stresses. Parasitic nematodes infect host plant roots and cause severe damage to crop plants. However, the molecular mechanisms that regulate parasitic nematode infections are still unknown. Here, we show that plant parasitic root-knot nematodes (RKNs), Meloidogyne incognita, modulate the host CLAVATA3 (CLV3)/EMBRYO SURROUNDING REGION (CLE)-CLV1 signaling module to promote the infection progression. Plants deficient in the CLE signaling pathway show enhanced RKN resistance, whereas CLE overexpression leads to increased susceptibility toward RKN. Grafting analysis shows that CLV1 expression in the shoot alone is sufficient to positively regulate RKN infection. Together with results from the split-root culture system, infection assays, and CLE3-CLV1 binding assays, we conclude that mobile root-derived CLE signals are perceived by CLV1 in the shoot, which subsequently produce systemic signals to promote gall formation and RKN reproduction.

Systematical Characterization of the AT-Hook Gene Family in Juglans regia L. and the Functional Analysis of the JrAHL2 in Flower Induction and Hypocotyl Elongation

AT-hook motif nuclear localization (AHL) proteins play essential roles in various plant biological processes. Yet, a comprehensive understanding of AHL transcription factors in walnut (Juglans regia L.) is missing. In this study, 37 AHL gene family members were first identified in the walnut genome. Based on the evolutionary analysis, JrAHL genes were grouped into two clades, and their expansion may occur due to segmental duplication. The stress-responsive nature and driving of developmental activities of JrAHL genes were revealed by cis-acting elements and transcriptomic data, respectively. Tissue-specific expression analysis showed that JrAHLs had a profound transcription in flower and shoot tip, JrAHL2 in particular. Subcellular localization showed that JrAHL2 is anchored to the nucleus. Overexpression of JrAHL2 in Arabidopsis adversely affected hypocotyl elongation and delayed flowering. Our study, for the first time, presented a detailed analysis of JrAHL genes in walnut and provided theoretical knowledge for future genetic breeding programs.

Identification of GA20ox2 as a target of ATHB2 and TCP13 during shade response

The shade avoidance syndrome (SAS) is a collective adaptive response of plants under shade highlighted by characteristic phenotypes such as hypocotyl elongation, which is largely mediated by concerted actions of auxin and GA. We identified ATHB2, a homeodomain-leucine zipper (HD-Zip) domain transcription factor known to be rapidly induced under shade condition, as a positive regulator of GA biosynthesis necessary for the SAS by transactivating the expression of GA20ox2, a key gene in the GA biosynthesis pathway. Based on promoter deletion analysis, EMSA and ChIP assay, ATHB2 appears to regulate the GA20ox2 expression as a direct binding target. We also found that the GA20ox2 expression is under negative control by TCP13, the effect of which can be suppressed by presence of ATHB2. Considering a rapid induction kinetics of ATHB2, this relationship between ATHB2 and TCP13 may allow ATHB2 to play a shade-specific activator for GA20ox by derepressing a pre-existing activity of TCP13.

High temperature restricts cell division and leaf size by coordination of PIF4 and TCP4 transcription factors

High ambient temperature suppresses Arabidopsis (Arabidopsis thaliana) rosette leaf area and elongates the stem and petiole. While the mechanism underlying the temperature-induced elongation response has been extensively studied, the genetic basis of temperature regulation of leaf size is largely unknown. Here, we show that warm temperature inhibits cell proliferation in Arabidopsis leaves, resulting in fewer cells compared to the control condition. Cellular phenotyping and genetic and biochemical analyses established the key roles of PHYTOCHROME-INTERACTING FACTOR4 (PIF4) and TEOSINTE BRANCHED1/CYCLOIDEA/PCF4 (TCP4) transcription factors in the suppression of Arabidopsis leaf area under high temperature by a reduction in cell number. We show that temperature-mediated suppression of cell proliferation requires PIF4, which interacts with TCP4 and regulates the expression of the cell cycle inhibitor KIP-RELATED PROTEIN1 (KRP1) to control leaf size under high temperature. Warm temperature induces binding of both PIF4 and TCP4 to the KRP1 promoter. PIF4 binding to KRP1 under high temperature is TCP4 dependent as TCP4 regulates PIF4 transcript levels under high temperature. We propose a model where a warm temperature-mediated accumulation of PIF4 in leaf cells promotes its binding to the KRP1 promoter in a TCP4-dependent way to regulate cell production and leaf size. Our finding of high temperature-mediated transcriptional upregulation of KRP1 integrates a developmental signal with an environmental signal that converges on a basal cell regulatory process.

Transcriptional activation of auxin biosynthesis drives developmental reprogramming of differentiated cells

Plant cells exhibit remarkable plasticity of their differentiation states, enabling regeneration of whole plants from differentiated somatic cells. How they revert cell fate and express pluripotency, however, remains unclear. In this study, we demonstrate that transcriptional activation of auxin biosynthesis is crucial for reprogramming differentiated Arabidopsis (Arabidopsis thaliana) leaf cells. Our data show that interfering with the activity of histone acetyltransferases dramatically reduces callus formation from leaf mesophyll protoplasts. Histone acetylation permits transcriptional activation of PLETHORAs, leading to the induction of their downstream YUCCA1 gene encoding an enzyme for auxin biosynthesis. Auxin biosynthesis is in turn required to accomplish initial cell division through the activation of G2/M phase genes mediated by MYB DOMAIN PROTEIN 3-RELATED (MYB3Rs). We further show that the AUXIN RESPONSE FACTOR 7 (ARF7)/ARF19 and INDOLE-3-ACETIC ACID INDUCIBLE 3 (IAA3)/IAA18-mediated auxin signaling pathway is responsible for cell cycle reactivation by transcriptionally upregulating MYB3R4. These findings provide a mechanistic model of how differentiated plant cells revert their fate and reinitiate the cell cycle to become pluripotent.

Two AT-Hook proteins regulate A/NINV7 expression to modulate sucrose catabolism for cold tolerance in Poncirus trifoliata

Invertase (INV)-mediated sucrose (Suc) hydrolysis, leading to the irreversible production of glucose (Glc) and fructose (Frc), plays an essential role in abiotic stress tolerance of plants. However, the regulatory network associated with the Suc catabolism in response to cold environment remains largely elusive. Herein, the cold-induced alkaline/neutral INV gene PtrA/NINV7 of trifoliate orange (Poncirus trifoliata (L.) Raf.) was shown to function in cold tolerance via mediating the Suc hydrolysis. Meanwhile, a nuclear matrix-associated region containing A/T-rich sequences within its promoter was indispensable for the cold induction of PtrA/NINV7. Two AT-Hook Motif Containing Nuclear Localized (AHL) proteins, PtrAHL14 and PtrAHL17, were identified as upstream transcriptional activators of PtrA/NINV7 by interacting with the A/T-rich motifs. PtrAHL14 and PtrAHL17 function positively in the cold tolerance by modulating PtrA/NINV7-mediated Suc catabolism. Furthermore, both PtrAHL14 and PtrAHL17 could form homo- and heterodimers between each other, and interacted with two histone acetyltransferases (HATs), GCN5 and TAF1, leading to elevated histone3 acetylation level under the cold stress. Taken together, our findings unraveled a new cold-responsive signaling module (AHL14/17-HATs-A/NINV7) for orchestration of Suc catabolism and cold tolerance, which shed light on the molecular mechanisms underlying Suc catabolism catalyzed by A/NINVs under cold stress.

SUPPRESSOR OF PHYTOCHROME B-4 #3 reduces the expression of PIF-activated genes and increases expression of growth repressors to regulate hypocotyl elongation in short days

SUPPRESSOR OF PHYTOCHROME B-4 #3 (SOB3) is a member of the AT-HOOK MOTIF CONTAINING NUCLEAR LOCALIZED (AHL) family of transcription factors that are involved in light-mediated growth in Arabidopsis thaliana, affecting processes such as hypocotyl elongation. The majority of the research on the AHLs has been conducted in continuous light. However, there are unique molecular events that promote growth in short days (SD) compared to constant light conditions. Therefore, we investigated how AHLs affect hypocotyl elongation in SD. Firstly, we observed that AHLs inhibit hypocotyl growth in SD, similar to their effect in constant light. Next, we identified AHL-regulated genes in SD-grown seedlings by performing RNA-seq in two sob3 mutants at different time points. Our transcriptomic data indicate that PHYTOCHROME INTERACTING FACTORS (PIFs) 4, 5, 7, and 8 along with PIF-target genes are repressed by SOB3 and/or other AHLs. We also identified PIF target genes that are repressed and have not been previously described as AHL-regulated, including PRE1, PIL1, HFR1, CDF5, and XTR7. Interestingly, our RNA-seq data also suggest that AHLs activate the expression of growth repressors to control hypocotyl elongation, such as HY5 and IAA17. Notably, many growth-regulating and other genes identified from the RNA-seq experiment were differentially regulated between these two sob3 mutants at the time points tested. Surprisingly, our ChIP-seq data suggest that SOB3 mostly binds to similar genes throughout the day. Collectively, these data suggest that AHLs affect gene expression in a time point-specific manner irrespective of changes in binding to DNA throughout SD.

AT-HOOK MOTIF NUCLEAR LOCALIZED (AHL) proteins of ancient origin radiate new functions

AHL (AT-HOOK MOTIF NUCLEAR LOCALIZED) protein is an important transcription factor in plants that regulates a wide range of biological process. It is considered to have evolved from an independent PPC domain in prokaryotes to a complete protein in modern plants. AT-hook motif and PPC conserved domains are the main functional domains of AHL. Since the discovery of AHL, their evolution and function have been continuously studied. The AHL gene family has been identified in multiple species and the functions of several members of the gene family have been studied. Here, we summarize the evolution and structural characteristics of AHL genes, and emphasize their biological functions. This review will provide a basis for further functional study and crop breeding.

Overexpression of AHL9 accelerates leaf senescence in Arabidopsis thaliana

BACKGROUND

Leaf senescence, the final stage of leaf growth and development, is regulated by numerous internal factors and environmental cues. Ethylene is one of the key senescence related hormones, but the underlying molecular mechanism of ethylene-induced leaf senescence remains poorly understood.

RESULTS

In this study, we identified one AT-hook like (AHL) protein, AHL9, as a positive regulator of leaf senescence in Arabidopsis thaliana. Overexpression of AHL9 significantly accelerates age-related leaf senescence and promotes dark-induced leaf chlorosis. The early senescence phenotype observed in AHL9 overexpressing lines is inhibited by the ethylene biosynthesis inhibitor aminooxyacetic acid suggesting the involvement of ethylene in the AHL9-associated senescence. RNA-seq and quantitative reverse transcription PCR (qRT-PCR) data identified numerous senescence-associated genes differentially expressed in leaves of AHL9 overexpressing transgenic plants.

CONCLUSIONS

Our investigation demonstrates that AHL9 functions in accelerating the leaf senescence process via ethylene synthesis or signalling.

AHLs' life in plants: Especially their potential roles in responding to Fusarium wilt and repressing the seed oil accumulation

Members of the AT-hook motif nuclear localized (AHL) family contain diverse but poorly understood biological functions. We identified 371 AHLs in 20 land plants, varying from the early diverging lycophyte Selagineila moellendorfi to a variety of higher plants. The AHLs were divided into two clades (Clade-A and Clade-B) with three different types (Type-I, Type-II, and Type-III AHLs). The divergence between Clade-A and Clade-B likely occurred before the separation of S. moellendorfi from the vascular plant lineages. Members of the AHLs family expanded with the specific whole-genome duplication (WGD)/segmental duplication in some genomes, such as Hevea brasiliensis. The ortholog (Vf00G1914/Amo018442) exhibited opposite expression patterns between two Vernicia species (V. fordii and V. montana), indicating that it was implicated in resistance to Fusarium wilt disease. The expression of Vf09G2138 exhibited a negative correlation with lipid biosynthesis in V. fordii seeds during different stages of development, suggesting that this gene might repress the seed oil accumulation. The core AT-hook motif and PPC domain were responsible for guiding the localization of AHL in the nucleus. This study helps us to understand the evolution of AHLs in multiple plants, further highlight their functions during V. fordii seed development and response to Fusarium wilt disease.

Combining Fine Mapping, Whole-Genome Re-Sequencing, and RNA-Seq Unravels Candidate Genes for a Soybean Mutant with Short Petioles and Weakened Pulvini

A short petiole is an important agronomic trait for the development of plant ideotypes with high yields. However, the genetic basis underlying this trait remains unclear. Here, we identified and characterized a novel soybean mutant with short petioles and weakened pulvini, designated as short petioles and weakened pulvini (spwp). Compared with the wild type (WT), the spwp mutant displayed shortened petioles, owing to the longitudinally decreased cell length, and exhibited a smaller pulvinus structure due to a reduction in motor cell proliferation and expansion. Genetic analysis showed that the phenotype of the spwp mutant was controlled by two recessive nuclear genes, named as spwp1 and spwp2. Using a map-based cloning strategy, the spwp1 locus was mapped in a 183 kb genomic region on chromosome 14 between markers S1413 and S1418, containing 15 annotated genes, whereas the spwp2 locus was mapped in a 195 kb genomic region on chromosome 11 between markers S1373 and S1385, containing 18 annotated genes. Based on the whole-genome re-sequencing and RNA-seq data, we identified two homologous genes, Glyma.11g230300 and Glyma.11g230600, as the most promising candidate genes for the spwp2 locus. In addition, the RNA-seq analysis revealed that the expression levels of genes involved in the cytokinin and auxin signaling transduction networks were altered in the spwp mutant compared with the WT. Our findings provide new gene resources for insights into the genetic mechanisms of petiole development and pulvinus establishment, as well as soybean ideotype breeding.

Wound-inducible WUSCHEL-RELATED HOMEOBOX 13 is required for callus growth and organ reconnection

Highly efficient tissue repair is pivotal for surviving damage-associated stress. Plants generate callus upon injury to heal wound sites, yet regulatory mechanisms of tissue repair remain elusive. Here, we identified WUSCHEL-RELATED HOMEOBOX 13 (WOX13) as a key regulator of callus formation and organ adhesion in Arabidopsis (Arabidopsis thaliana). WOX13 belongs to an ancient subclade of the WOX family, and a previous study shows that WOX13 orthologs in the moss Physcomitrium patens (PpWOX13L) are involved in cellular reprogramming at wound sites. We found that the Arabidopsis wox13 mutant is totally defective in establishing organ reconnection upon grafting, suggesting that WOX13 is crucial for tissue repair in seed plants. WOX13 expression rapidly induced upon wounding, which was partly dependent on the activity of an AP2/ERF transcription factor, WOUND-INDUCED DEDIFFERENTIATION 1 (WIND1). WOX13 in turn directly upregulated WIND2 and WIND3 to further promote cellular reprogramming and organ regeneration. We also found that WOX13 orchestrates the transcriptional induction of cell wall-modifying enzyme genes, such as GLYCOSYL HYDROLASE 9Bs, PECTATE LYASE LIKEs and EXPANSINs. Furthermore, the chemical composition of cell wall monosaccharides was markedly different in the wox13 mutant. These data together suggest that WOX13 modifies cell wall properties, which may facilitate efficient callus formation and organ reconnection. Furthermore, we found that PpWOX13L complements the Arabidopsis wox13 mutant, suggesting that the molecular function of WOX13 is partly conserved between mosses and seed plants. This study provides key insights into the conservation and functional diversification of the WOX gene family during land plant evolution.

Comprehensive In Silico Analysis and Transcriptional Profiles Highlight the Importance of Mitochondrial Dicarboxylate Carriers (DICs) on Hypoxia Response in Both Arabidopsis thaliana and Eucalyptus grandis

Plant dicarboxylate carriers (DICs) transport a wide range of dicarboxylates across the mitochondrial inner membrane. The Arabidopsis thalianaDIC family is composed of three genes (AtDIC1, 2 and 3), whereas two genes (EgDIC1 and EgDIC2) have been retrieved in Eucalyptus grandis. Here, by combining in silico and in planta analyses, we provide evidence that DICs are partially redundant, important in plant adaptation to environmental stresses and part of a low-oxygen response in both species. AtDIC1 and AtDIC2 are present in most plant species and have very similar gene structure, developmental expression patterns and absolute expression across natural Arabidopsis accessions. In contrast, AtDIC3 seems to be an early genome acquisition found in Brassicaceae and shows relatively low (or no) expression across these accessions. In silico analysis revealed that both AtDICs and EgDICs are highly responsive to stresses, especially to cold and submergence, while their promoters are enriched for stress-responsive transcription factors binding sites. The expression of AtDIC1 and AtDIC2 is highly correlated across natural accessions and in response to stresses, while no correlation was found for AtDIC3. Gene ontology enrichment analysis suggests a role for AtDIC1 and AtDIC2 in response to hypoxia, and for AtDIC3 in phosphate starvation. Accordingly, the investigated genes are induced by submergence stress in A. thaliana and E. grandis while AtDIC2 overexpression improved seedling survival to submergence. Interestingly, the induction of AtDIC1 and AtDIC2 is abrogated in the erfVII mutant that is devoid of plant oxygen sensing, suggesting that these genes are part of a conserved hypoxia response in Arabidopsis.

Chromatin phosphoproteomics unravels a function for AT-hook motif nuclear localized protein AHL13 in PAMP-triggered immunity

In many eukaryotic systems during immune responses, mitogen-activated protein kinases (MAPKs) link cytoplasmic signaling to chromatin events by targeting transcription factors, chromatin remodeling complexes, and the RNA polymerase machinery. So far, knowledge on these events is scarce in plants and no attempts have been made to focus on phosphorylation events of chromatin-associated proteins. Here we carried out chromatin phosphoproteomics upon elicitor-induced activation of Arabidopsis The events in WT were compared with those in mpk3, mpk4, and mpk6 mutant plants to decipher specific MAPK targets. Our study highlights distinct signaling networks involving MPK3, MPK4, and MPK6 in chromatin organization and modification, as well as in RNA transcription and processing. Among the chromatin targets, we characterized the AT-hook motif containing nuclear localized (AHL) DNA-binding protein AHL13 as a substrate of immune MAPKs. AHL13 knockout mutant plants are compromised in pathogen-associated molecular pattern (PAMP)-induced reactive oxygen species production, expression of defense genes, and PAMP-triggered immunity. Transcriptome analysis revealed that AHL13 regulates key factors of jasmonic acid biosynthesis and signaling and affects immunity toward Pseudomonas syringae and Botrytis cinerea pathogens. Mutational analysis of the phosphorylation sites of AHL13 demonstrated that phosphorylation regulates AHL13 protein stability and thereby its immune functions.

Two bHLH transcription factors, bHLH48 and bHLH60, associate with phytochrome interacting factor 7 to regulate hypocotyl elongation in Arabidopsis

The basic helix-loop-helix (bHLH) transcription factor PHYTOCHROME-INTERACTING FACTOR 7 (PIF7) is a central regulator that promotes stem growth by activating growth-related gene expression during shade-avoidance responses. Studying the co-factors of PIF7 can facilitate understanding of the mechanism of PIFs and light signal transduction. Here, we describe the identification of two bHLH transcription factors, bHLH48 and bHLH60 (bHLH48/bHLH60), as essential partners for PIF7-dependent modulation of hypocotyl elongation and function downstream of phytochrome B. These two bHLH factors display DNA binding activity and interact with PIF7. Genetic analysis indicated that bHLH48/bHLH60 and PIF7 are interdependent in promoting hypocotyl elongation. Chromatin immunoprecipitation sequencing (ChIP-seq) analysis identified the substantially overlapping downstream targets of bHLH60 and PIF7. Biochemical analysis revealed that bHLH48/bHLH60 enhance the DNA binding ability of PIF7. These results provide evidence that bHLH48/bHLH60 act as positive partners of PIF7 for mutual benefit in the regulation of hypocotyl elongation.

Comprehensive analysis of AHL gene family and their expression under drought stress and ABA treatment in Populus trichocarpa

The AT-hook motif nuclear-localized (AHL) family is a plant transcription factor family, which plays an important role in growth and development and stress responses. We identified and analyzed 37 AHL genes in poplar (Populus trichocarpa). Phylogenetic analysis classified the PtrAHL members into three subfamilies based on their conserved domain. All PtrAHL paralogous pairs evolved under purifying selection. The promoter analysis revealed the presence of stress-related and phytohormone-related cis-elements of the PtrAHL genes. Our analysis of the tissue-specific expression pattern of PtrAHL genes indicated their significance in tissue and organ development. Network-based prediction suggested that PtrAHL genes may interact with histone deacetylases (HDAC) and participate in the development of organs, such as roots. Drought negatively impacts plant growth and development. ABA is produced under osmotic stress condition, and it takes an important part in the stress response and tolerance of plants. Real-time quantitative PCR (qRT-PCR) showed that PtrAHL genes were induced by drought stress and ABA treatment. These insights into the expression of PtrAHL genes under stress provide a basis for PtrAHL gene functional analysis. Our study will help develop new breeding strategies to improve drought tolerance in poplar.

Molecular pathways regulating elongation of aerial plant organs: a focus on light, the circadian clock, and temperature

Organs such as hypocotyls and petioles rapidly elongate in response to shade and temperature cues, contributing to adaptive responses that improve plant fitness. Growth plasticity in these organs is achieved through a complex network of molecular signals. Besides conveying information from the environment, this signaling network also transduces internal signals, such as those associated with the circadian clock. A number of studies performed in Arabidopsis hypocotyls, and to a lesser degree in petioles, have been informative for understanding the signaling networks that regulate elongation of aerial plant organs. In particular, substantial progress has been made towards understanding the molecular mechanisms that regulate responses to light, the circadian clock, and temperature. Signals derived from these three stimuli converge on the BAP module, a set of three different types of transcription factors that interdependently promote gene transcription and growth. Additional key positive regulators of growth that are also affected by environmental cues include the CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) and SUPPRESSOR OF PHYA-105 (SPA) E3 ubiquitin ligase proteins. In this review we summarize the key signaling pathways that regulate the growth of hypocotyls and petioles, focusing specifically on molecular mechanisms important for transducing signals derived from light, the circadian clock, and temperature. While it is clear that similarities abound between the signaling networks at play in these two organs, there are also important differences between the mechanisms regulating growth in hypocotyls and petioles.

Dissection of Functional Modules of AT-HOOK MOTIF NUCLEAR LOCALIZED PROTEIN 4 in the Development of the Root Xylem

Xylem development in the Arabidopsis root apical meristem requires a complex cross talk between plant hormone signaling and transcriptional factors (TFs). The key processes involve fine-tuning between neighboring cells, mediated via the intercellular movement of signaling molecules. As an example, we previously reported that AT-HOOK MOTIF NUCLEAR LOCALIZED PROTEIN (AHL) 4 (AHL4), a member of the 29 AT-hook family TFs in Arabidopsis, moves into xylem precursors from their neighbors to determine xylem differentiation. As part of the effort to understand the molecular functions of AHL4, we performed domain swapping analyses using AHL1 as a counterpart, finding that AHL4 has three functionally distinctive protein modules. The plant and prokaryotes conserved (PPC) domain of AHL4 acts as a mediator of protein-protein interactions with AHL members. The N-terminus of AHL4 is required for the regulation of xylem development likely via its unique DNA-binding activity. The C-terminus of AHL4 confers intercellular mobility. Our characterization of modules in the AHL4 protein will augment our understanding of the complexity of regulation and the evolution of intercellular mobility in AHL4 and its relatives.

Insights Into the Molecular Evolution of AT-Hook Motif Nuclear Localization Genes in Brassica napus

AT-hook motif nuclear localization (AHL) proteins belong to a family of transcription factors, and play important roles in plant growth and development and response to various stresses through protein-DNA and protein-protein interactions. To better understand the Brassica napus AHL gene family, AHL genes in B. napus and related species were analyzed. Using Arabidopsis as a reference, 122 AHL gene family members were first identified in B. napus. According to the phylogenetic tree and gene organization, the BnaAHLs were classified into two clades (Clade-A and Clade-B) and three types (Type-I, Type-II, and Type-III). Gene organization and motif distribution analysis suggested that the AHL gene family is relatively conserved during evolution. These BnaAHLs are unevenly distributed on 38 chromosomes and expanded by whole-genome duplication (WGD) or segmental duplication. And large-scale loss events have also occurred in evolution. All types of BnaAHLs are subject to purification or neutral selection, while some positive selection sites are also identified in Type-II and Type-III groups. At the same time, the purification effect of Type-I members are stronger than that of the others. In addition, RNA-seq data and cis-acting element analysis also suggested that the BnaAHLs play important roles in B. napus growth and development, as well as in response to some abiotic and biotic stresses. Protein-protein interaction analysis identified some important BnaAHL-binding proteins, which also play key roles in plant growth and development. This study is helpful to fully understand the origin and evolution of the AHL gene in B. napus, and lays the foundation for their functional studies.

Overexpression of AtAHL20 causes delayed flowering in Arabidopsis via repression of FT expression

BACKGROUND

The 29-member Arabidopsis AHL gene family is classified into three main classes based on nucleotide and protein sequence evolutionary differences. These differences include the presence or absence of introns, type and/or number of conserved AT-hook and PPC domains. AHL gene family members are divided into two phylogenetic clades, Clade-A and Clade-B. A majority of the 29 members remain functionally uncharacterized. Furthermore, the biological significance of the DNA and peptide sequence diversity, observed in the conserved motifs and domains found in the different AHL types, is a subject area that remains largely unexplored.

RESULTS

Transgenic plants overexpressing AtAHL20 flowered later than the wild type under both short and long days. Transcript accumulation analyses showed that 35S:AtAHL20 plants contained reduced FT, TSF, AGL8 and SPL3 mRNA levels. Similarly, overexpression of AtAHL20's orthologue in Camelina sativa, Arabidopsis' closely related Brassicaceae family member species, conferred a late-flowering phenotype via suppression of CsFT expression. However, overexpression of an aberrant AtAHL20 gene harboring a missense mutation in the AT-hook domain's highly conserved R-G-R core motif abolished the late-flowering phenotype. Data from targeted yeast-two-hybrid assays showed that AtAHL20 interacted with itself and several other Clade-A Type-I AHLs which have been previously implicated in flowering-time regulation: AtAHL19, AtAHL22 and AtAHL29.

CONCLUSION

We showed via gain-of-function analysis that AtAHL20 is a negative regulator of FT expression, as well as other downstream flowering time regulating genes. A similar outcome in Camelina sativa transgenic plants overexpressing CsAHL20 suggest that this is a conserved function. Our results demonstrate that AtAHL20 acts as a photoperiod-independent negative regulator of transition to flowering.

Plant Biology: AHL Transcription Factors Inhibit Growth-Promoting PIFs

How do plants respond to abiotic stresses such as drought, salt or cold? A new study in Arabidopsis reveals that the stress-responsive AHLs antagonize the function of the PIF transcription factors to restrict rosette growth and allow resource reallocation for stress-adaptive responses.