Nucleus- and plastid-targeted annexin 5 promotes reproductive development in Arabidopsis and is essential for pollen and embryo formation

Plastid Nucleoids: Insights into Their Shape and Dynamics

Chloroplasts/plastids are unique organelles found in plant cells and some algae and are responsible for performing essential functions such as photosynthesis. The plastid genome, consisting of circular and linear DNA molecules, is packaged and organized into specialized structures called nucleoids. The composition and dynamics of these nucleoids have been the subject of intense research, as they are critical for proper plastid functions and development. In this mini-review, recent advances in understanding the organization and regulation of plastid nucleoids are overviewed, with a focus on the various proteins and factors that regulate the shape and dynamics of nucleoids, including DNA-binding proteins and membrane anchorage proteins. The dynamic nature of nucleoid organization, which is influenced by a variety of developmental cues and the cell cycle, is also examined.

The Evolution, Expression Patterns, and Domestication Selection Analysis of the Annexin Gene Family in the Barley Pan-Genome

Plant annexins constitute a conserved protein family that plays crucial roles in regulating plant growth and development, as well as in responses to both biotic and abiotic stresses. In this study, a total of 144 annexin genes were identified in the barley pan-genome, comprising 12 reference genomes, including cultivated barley, landraces, and wild barley. Their chromosomal locations, physical-chemical characteristics, gene structures, conserved domains, and subcellular localizations were systematically analyzed to reveal the certain differences between wild and cultivated populations. Through a cis-acting element analysis, co-expression network, and large-scale transcriptome analysis, their involvement in growth, development, and responses to various stressors was highlighted. It is worth noting that HvMOREXann5 is only expressed in pistils and anthers, indicating its crucial role in reproductive development. Based on the resequencing data from 282 barley accessions worldwide, genetic variations in thefamily were investigated, and the results showed that 5 out of the 12 identified HvMOREXanns were affected by selection pressure. Genetic diversity and haplotype frequency showed notable reductions between wild and domesticated barley, suggesting that a genetic bottleneck occurred on the annexin family during the barley domestication process. Finally, qRT-PCR analysis confirmed the up-regulation of HvMOREXann7 under drought stress, along with significant differences between wild accessions and varieties. This study provides some insights into the genome organization and genetic characteristics of the annexin gene family in barley at the pan-genome level, which will contribute to better understanding its evolution and function in barley and other crops.

Genome-Wide Identification of the GhANN Gene Family and Functional Validation of GhANN11 and GhANN4 under Abiotic Stress

Annexins (ANNs) are a structurally conserved protein family present in almost all plants. In the present study, 27 GhANNs were identified in cotton and were unevenly distributed across 14 chromosomes. Transcriptome data and RT-qPCR results revealed that multiple GhANNs respond to at least two abiotic stresses. Similarly, the expression levels of GhANN4 and GhANN11 were significantly upregulated under heat, cold, and drought stress. Using virus-induced gene silencing (VIGS), functional characterization of GhANN4 and GhANN11 revealed that, compared with those of the controls, the leaf wilting of GhANN4-silenced plants was more obvious, and the activities of catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD) were lower under NaCl and PEG stress. Moreover, the expression of stress marker genes (GhCBL3, GhDREB2A, GhDREB2C, GhPP2C, GhRD20-2, GhCIPK6, GhNHX1, GhRD20-1, GhSOS1, GhSOS2 and GhSnRK2.6) was significantly downregulated in GhANN4-silenced plants after stress. Under cold stress, the growth of the GHANN11-silenced plants was significantly weaker than that of the control plants, and the activities of POD, SOD, and CAT were also lower. However, compared with those of the control, the elasticity and orthostatic activity of the GhANN11-silenced plants were greater; the POD, SOD, and CAT activities were higher; and the GhDREB2C, GhHSP, and GhSOS2 expression levels were greater under heat stress. These results suggest that different GhANN family members respond differently to different types of abiotic stress.

Comprehensive analysis of annexin gene family and its expression in response to branching architecture and salt stress in crape myrtle

BACKGROUND

Annexin (ANN) is calcium (Ca2+)-dependent and phospholipid binding protein family, which is involved in plant growth and development and response to various stresses. However, little known about ANN genes were identified from crape myrtle, an ornamental horticultural plant widely cultivated in the world.

RESULTS

Here, 9 LiANN genes were identified from Lagerstroemia indica, and their characterizations and functions were investigated in L. indica for the first time. The LiANN genes were divided into 2 subfamilies. The gene structure, chromosomal location, and collinearity relationship were also explored. In addition, the GO annotation analysis of these LiANNs indicated that they are enriched in molecular functions, cellular components, and biological processes. Moreover, transcription factors (TFs) prediction analysis revealed that bHLH, MYB, NAC, and other TFs can interact with the LiANN promoters. Interestingly, the LiANN2/4/6-9 were demonstrated to play critical roles in the branching architecture of crape myrtle. Furthermore, the LiANN2/6/8/9 were differentially expressed under salt treatment, and a series of TFs regulating LiANN2/6/8/9 expression were predicted to play essential roles in salt resistance.

CONCLUSIONS

These results shed light on profile and function of the LiANN gene family, and lay a foundation for further studies of the LiANN genes.

Ion Channels in Electrical Signaling in Higher Plants

Electrical signals (ESs) appearing in plants under the action of various external factors play an important role in adaptation to changing environmental conditions. Generation of ES in higher plant cells is associated with activation of Ca2+, K+, and anion fluxes, as well as with changes in the activity of plasma membrane H+-ATPase. In the present review, molecular nature of the ion channels contributing to ESs transmission in higher plants is analyzed based on comparison of the data from molecular-genetic and electrophysiological studies. Based on such characteristics of ion channels as selectivity, activation mechanism, and intracellular and tissue localization, those ion channels that meet the requirements for potential participation in ES generation were selected from a wide variety of ion channels in higher plants. Analysis of the data of experimental studies performed on mutants with suppressed or enhanced expression of a certain channel gene revealed those channels whose activation contributes to ESs formation. The channels responsible for Ca2+ flux during generation of ESs include channels of the GLR family, for K+ flux - GORK, for anions - MSL. Consideration of the prospects of further studies suggests the need to combine electrophysiological and genetic approaches along with analysis of ion concentrations in intact plants within a single study.

High temporal-resolution transcriptome landscapes of maize embryo sac and ovule during early seed development

Here we provided a high temporal-resolution transcriptome atlas of maize embryo sac and ovule to reveal the gene activity dynamic during early seed development. The early maize (Zea mays) seed development is initiated from double fertilization in the embryo sac and needs to undergo a highly dynamic and complex development process to form the differentiated embryo and endosperm. Despite the importance of maize seed for food, feed, and biofuel, many regulators responsible for controlling its early development are not known yet. Here, we reported a high temporal-resolution transcriptome atlas of embryo sac and ovule based on 44 time point samples collected within the first four days of seed development. A total of 25,187 genes including 1598 transcription factors (TFs) involved in early seed development were detected. Global comparisons of the expressions of these genes revealed five distinct development stages of early seed, which are mainly related to double fertilization, asymmetric cell division of the zygote, as well as coenocyte formation, cellularization and differentiation in endosperm. We identified 3327 seed-specific genes, which more than one thousand seed-specific genes with main expressions during early seed development were newly identified here, including 859 and 186 genes predominantly expressed in the embryo sac and ovule, respectively. Combined with the published transcriptome data of seed, we uncovered the dominant auxin biosynthesis, transport and signaling related genes at different development stages and subregions of seed. These results are helpful for understanding the genetic control of early seed development.

ATANN3 Is Involved in Extracellular ATP-Regulated Auxin Distribution in Arabidopsis thaliana Seedlings

Extracellular ATP (eATP) plays multiple roles in plant growth and development, and stress responses. It has been revealed that eATP suppresses growth and alters the growth orientation of the root and hypocotyl of Arabidopsis thaliana by affecting auxin transport and localization in these organs. However, the mechanism of the eATP-stimulated auxin distribution remains elusive. Annexins are involved in multiple aspects of plant cellular metabolism, while their role in response to apoplastic signals remains unclear. Here, by using the loss-of-function mutations, we investigated the role of AtANN3 in the eATP-regulated root and hypocotyl growth. Firstly, the inhibitory effects of eATP on root and hypocotyl elongation were weakened or impaired in the AtANN3 null mutants (atann3-1 and atann3-2). Meanwhile, the distribution of DR5-GUS and DR5-GFP indicated that the eATP-induced asymmetric distribution of auxin in the root tips or hypocotyl cells occurred in wild-type control plants, while in atann3-1 mutant seedlings, it was not observed. Further, the eATP-induced asymmetric distribution of PIN2-GFP in root-tip cells or that of PIN3-GFP in hypocotyl cells was reduced in atann3-1 seedlings. Finally, the eATP-induced asymmetric distribution of cytoplasmic vesicles in root-tip cells was impaired in atann3-1 seedlings. Based on these results, we suggest that AtANN3 may be involved in eATP-regulated seedling growth by regulating the distribution of auxin and auxin transporters in vegetative organs.

A critical review on plant annexin: Structure, function, and mechanism

Plant annexins are evolutionary conserved protein family widely exist in almost all plant species, characterized by a shorter N-terminal region and four conservative annexin repeats. Plant annexins have Ca²⁺ channel-regulating activity and peroxidase as well as ATPase/GTPase activities, which give annexins functional specificity. They are widely involved in regulating diverse aspects of biochemical and cellular processes, plant growth and development, and responses to biotic and abiotic environmental stresses. Though many studies have reviewed the function of annexins, great progress have been made in the study of plant annexins recently. In this review, we outline the current understanding of basic properties of plant annexins and summarize the emerging advances in understanding the functional roles of annexins in plants and highlight the regulation mechanisms of annexin protein in response to stress especially to salt and cold stress. The interesting questions related to plant annexin that remain to be further elucidated are also discussed.

Arabidopsis annexin 5 is involved in maintenance of pollen membrane integrity and permeability

In Arabidopsis, a dry stigma surface enables a gradual hydration of pollen grains by a controlled release of water. Occasionally the grains may be exposed to extreme precipitations that cause rapid water influx and swelling, eventually leading to pollen membrane rupture. In metazoans, calcium- and phospholipid-binding proteins, referred to as annexins, participate in the repair of plasma membrane damages. It remains unclear, however, how this process is conducted in plants. Here, we examined whether plant annexin 5 (ANN5), the most abundant member of the annexin family in pollen, is involved in the restoration of pollen membrane integrity. We analyzed the cellular dynamics of ANN5 in pollen grains undergoing hydration in favorable or stress conditions. We observed a transient association of ANN5 with the pollen membrane during in vitro hydration that did not occur in the pollen grains being hydrated on the stigma. To simulate a rainfall, we performed spraying of the pollinated stigma with deionized water that induced ANN5 accumulation at the pollen membrane. Interestingly, calcium or magnesium application affected pollen membrane properties differently, causing rupture or shrinkage of pollen membrane, respectively. Both treatments, however, induced ANN5 recruitment to the pollen membrane. Our data suggest a model in which ANN5 is involved in the maintenance of membrane integrity in pollen grains exposed to osmotic or ionic imbalances.

Genome-Wide Characterization and Abiotic Stresses Expression Analysis of Annexin Family Genes in Poplar

Poplar is an illustrious industrial woody plant with rapid growth, providing a range of materials, and having simple post-treatment. Various kinds of environmental stresses limit its output. Plant annexin (ANN) is a calcium-dependent phospholipid-binding protein involved in plant metabolism, growth and development, and cooperatively regulating drought resistance, salt tolerance, and various stress responses. However, the features of the PtANN gene family and different stress responses remain unknown in poplar. This study identified 12 PtANN genes in the P. trichocarpa whole-genome and PtANNs divided into three subfamilies based on the phylogenetic tree. The PtANNs clustered into the same clade shared similar gene structures and conserved motifs. The 12 PtANN genes were located in ten chromosomes, and segmental duplication events were illustrated as the main duplication method. Additionally, the PtANN4 homogenous with AtANN1 was detected localized in the cytoplasm and plasma membrane. In addition, expression levels of PtANNs were induced by multiple abiotic stresses, which indicated that PtANNs could widely participate in response to abiotic stress. These results revealed the molecular evolution of PtANNs and their profiles in response to abiotic stress.

The durum wheat annexin, TdAnn6, improves salt and osmotic stress tolerance in Arabidopsis via modulation of antioxidant machinery

TdAnn6 is a gene encoding an annexin protein in durum wheat (Triticum durum). The function of TdAnn6 in plant response to stress is not yet clearly understood. Here, we isolated TdAnn6 and characterized it in genetically modified Arabidopsis thaliana. Expressing TdAnn6 in Arabidopsis coincided with an improvement in stress tolerance at germination and seedling stages. In addition, TdAnn6-expressing seedling antioxidant activities were improved with lower level of malondialdehyde, and enhanced transcript levels of six stress-related genes during salt/osmotic stresses. Under greenhouse conditions, the TdAnn6 plants exhibited increased tolerance to salt or drought stress. To deepen our understanding of TdAnn6 function, we isolated a 1515-bp genomic fragment upstream of its coding sequence, designated as PrTdAnn6. The PrTdAnn6 promoter was fused to the β-glucuronidase reporter gene and transferred to Arabidopsis. By histochemical GUS staining, GUS activity was detected in the roots, leaves, and floral organs, but no activity was detected in the seeds. Furthermore, we noticed a high stimulation of promoter activity when A. thaliana seedlings were exposed to NaCl, mannitol, ABA, GA, and cold conditions. This cross-talk between tissue-specific expression and exogenous stress stimulation may provide additional layers of regulation for salt and osmotic stress responses in crops.

Characterization of Annexin gene family and functional analysis of RsANN1a involved in heat tolerance in radish (Raphanus sativus L.)

Plant annexins are a kind of conserved Ca²⁺-dependent phospholipid-binding proteins which are involved in plant growth, development and stress tolerance. Radish is an economically important annual or biennial root vegetable crop worldwide. However, the genome-wide characterization of annexin (RsANN) gene family remain largely unexplored in radish. In this study, a comprehensive identification of annexin gene family was performed at the whole genome level in radish. In total, ten RsANN genes were identified, and these putative RsANN proteins shared typical characteristics of the annexin family proteins. Phylogenetic analysis showed that the RsANNs together with annexin from Arabidopsis and rice were clustered into five groups with shared similar motif patterns. Chromosomal localization showed that these ten RsANN genes were distributed on six chromosomes (R3-R8) of radish. Several cis-elements involved in abiotic stress response were identified in the promoter regions of RsANN genes. Expression profile analysis indicated that the RsANN genes exhibited tissue-specific patterns at different growth stages and tissues. The Real-time quantitative PCR (RT-qPCR) revealed that the expression of most RsANN genes was induced under various abiotic stresses including heat, drought, salinity, oxidization and ABA stress. In addition, stress assays showed that overexpression of RsANN1a improved plant's growth and heat tolerance, while artificial microRNAs (amiRNA)-mediated knockdown of RsANN1a caused dramatically decreased survival ratio of Arabidopsis plants. These findings not only demonstrate that RsANN1a might play a critical role in the heat stress response of radish, but also facilitate clarifying the molecular mechanism of RsANN genes in regulating the biological process governing plant growth and development.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01056-5.

Genome-Wide Identification and Transcriptional Expression Analysis of Annexin Genes in Capsicum annuum and Characterization of CaAnn9 in Salt Tolerance

Annexin (Ann) is a polygenic, evolutionarily conserved, calcium-dependent and phospholipid-binding protein family, which plays key roles in plant growth, development, and stress response. However, a comprehensive understanding of CaAnn genes of pepper (Capsicum annuum) at the genome-wide level is limited. Based on the available pepper genomic information, we identified 15 members of the CaAnn gene family. Phylogenetic analysis showed that CaAnn proteins could be categorized into four different orthologous groups. Real time quantitative RT-PCR analysis showed that the CaAnn genes were tissue-specific and were widely expressed in pepper leaves after treatments with cold, salt, and drought, as well as exogenously applied MeJA and ABA. In addition, the function of CaAnn9 was further explored using the virus-induced gene silencing (VIGS) technique. CaAnn9-silenced pepper seedlings were more sensitive to salt stress, reflected by the degradation of chlorophyll, the accumulation of reactive oxygen species (ROS), and the decrease of antioxidant defense capacity. This study provides important information for further study of the role of pepper CaAnn genes and their coding proteins in growth, development, and environmental responses.

Overexpression of Cassava MeAnn2 Enhances the Salt and IAA Tolerance of Transgenic Arabidopsis

Annexins are a superfamily of soluble calcium-dependent phospholipid-binding proteins that have considerable regulatory effects in plants, especially in response to adversity and stress. The Arabidopsis thaliana AtAnn1 gene has been reported to play a significant role in various abiotic stress responses. In our study, the cDNA of an annexin gene highly similar to AtAnn1 was isolated from the cassava genome and named MeAnn2. It contains domains specific to annexins, including four annexin repeat sequences (I–IV), a Ca²⁺-binding sequence, Ca²⁺-independent membrane-binding-related tryptophan residues, and a salt bridge-related domain. MeAnn2 is localized in the cell membrane and cytoplasm, and it was found to be preferentially expressed in the storage roots of cassava. The overexpression of MeAnn2 reduced the sensitivity of transgenic Arabidopsis to various Ca²⁺, NaCl, and indole-3-acetic acid (IAA) concentrations. The expression of the stress resistance-related gene AtRD29B and auxin signaling pathway-related genes AtIAA4 and AtLBD18 in transgenic Arabidopsis was significantly increased under salt stress, while the Malondialdehyde (MDA) content was significantly lower than that of the control. These results indicate that the MeAnn2 gene may increase the salt tolerance of transgenic Arabidopsis via the IAA signaling pathway.

RAB GTPASE HOMOLOG 8D is required for the maintenance of both the root stem cell niche and the meristem

Previous studies have suggested that the plastid translation elongation factor, elongation factor thermo unstable (EF-Tu), encoded by RAB GTPASE HOMOLOG 8D (RAB8D) is essential for plant growth. Here, through analyzing the root phenotypes of two knock-down alleles of RAB8D (rab8d-1 and rab8d-2), we further revealed a vital role for RAB8D in primary root development through the maintenance of both the stem cell niche (SCN) and the meristem. Our results showed that RAB8D deficiency affects the root auxin response and SCN maintenance signaling. RAB8D interacts with GENOMES UNCOUPLED 1 (GUN1) in vivo. Further analysis revealed that GUN1 is over-accumulated and is required for both stem cell death and maintenance of root architecture in rab8d Arabidopsis mutants. The ATAXIA-TELANGIECTASIA-MUTATED (ATM)-SUPPRESSOR OF GAMMA RESPONSE 1 pathway is involved in the regulation of root meristem size through upregulating SIAMESE-RELATED 5 expression in the rab8d-2 allele. Moreover, ETHYLENE RESPONSE FACTOR 115 is highly expressed in rab8d-2, which plays a role in further quiescent center division. Our observations not only characterized the role of RAB8D in root development, but also uncovered functions of GUN1 and ATM in response to plastid EF-Tu deficiency.

Secretory Vesicles Targeted to Plasma Membrane During Pollen Germination and Tube Growth

Pollen germination and pollen tube growth are important biological events in the sexual reproduction of higher plants, during which a large number of vesicle trafficking and membrane fusion events occur. When secretory vesicles are transported via the F-actin network in proximity to the apex of the pollen tube, the secretory vesicles are tethered and fused to the plasma membrane by tethering factors and SNARE proteins, respectively. The coupling and uncoupling between the vesicle membrane and plasma membrane are also regulated by dynamic cytoskeleton, proteins, and signaling molecules, including small G proteins, calcium, and PIP2. In this review, we focus on the current knowledge regarding secretory vesicle delivery, tethering, and fusion during pollen germination and tube growth and summarize the progress in research on how regulators and signaling molecules participate in the above processes.

QTL-Seq identifies quantitative trait loci of relative electrical conductivity associated with heat tolerance in bottle gourd (Lagenaria siceraria)

Heat is a major abiotic stress that seriously affects watermelon (Citrullus lanatus) production. However, its effects may be mitigated through grafting watermelon to heat tolerant bottle gourd (Lagenaria siceraria) rootstocks. Understanding the genetic basis of heat tolerance and development of reliable DNA markers to indirectly select for the trait are necessary in breeding for new varieties with heat tolerance. The objectives of this study were to investigate the inheritance of heat tolerance and identify molecular markers associated with heat tolerance in bottle gourd. A segregating F₂ population was developed from a cross between two heat tolerant and sensitive inbred lines. The population was phenotyped for relative electrical conductivity (REC) upon high temperature treatment which was used as an indicator for heat tolerance. QTL-seq was performed to identify regions associated with heat tolerance. We found that REC-based heat tolerance in this population exhibited recessive inheritance. Seven heat-tolerant quantitative trait loci (qHT1.1, qHT2.1, qHT2.2, qHT5.1, qHT6.1, qHT7.1, and qHT8.1) were identified with qHT2.1 being a promising major-effect QTL. In the qHT2.1 region, we identified three non-synonymous SNPs that were potentially associated with heat tolerance. These SNPs were located in the genes that may play roles in pollen sterility, intracellular transport, and signal recognition. Association of the three SNPs with heat tolerance was verified in segregating F₂ populations, which could be candidate markers for marker assisted selection for heat tolerance in bottle gourd. The qHT2.1 region is an important finding that may be used for fine mapping and discovery of novel genes associated with heat tolerance in bottle gourd.

Comprehensive analyses of the annexin (ANN) gene family in Brassica rapa, Brassica oleracea and Brassica napus reveals their roles in stress response

Annexins (ANN) are a multigene, evolutionarily conserved family of calcium-dependent and phospholipid-binding proteins that play important roles in plant development and stress resistance. However, a systematic comprehensive analysis of ANN genes of Brassicaceae species (Brassica rapa, Brassica oleracea, and Brassica napus) has not yet been reported. In this study, we identified 13, 12, and 26 ANN genes in B. rapa, B. oleracea, and B. napus, respectively. About half of these genes were clustered on various chromosomes. Molecular evolutionary analysis showed that the ANN genes were highly conserved in Brassicaceae species. Transcriptome analysis showed that different group ANN members exhibited varied expression patterns in different tissues and under different (abiotic stress and hormones) treatments. Meanwhile, same group members from Arabidopsis thaliana, B. rapa, B. oleracea, and B. napus demonstrated conserved expression patterns in different tissues. The weighted gene coexpression network analysis (WGCNA) showed that BnaANN genes were induced by methyl jasmonate (MeJA) treatment and played important roles in jasmonate (JA) signaling and multiple stress response in B. napus.