Carbonic Anhydrases Function in Anther Cell Differentiation Downstream of the Receptor-Like Kinase EMS1

Single-cell RNA sequencing reveals dynamics of gene expression for 2D elongation and 3D growth in Physcomitrium patens

The transition from two-dimensional (2D) to 3D growth likely facilitated plants to colonize land, but its heterogeneity is not well understood. In this study, we utilized single-cell RNA sequencing to analyze the moss Physcomitrium patens, whose morphogenesis involves a transition from 2D to 3D growth. We profiled over 17,000 single cells covering all major vegetative tissues, including 2D filaments (chloronema and caulonema) and 3D structures (bud and gametophore). Pseudotime analyses revealed larger numbers of candidate genes that determine cell fates for 2D tip elongation or 3D bud differentiation. Using weighted gene co-expression network analysis, we identified a module that connects β-type carbonic anhydrases (βCAs) with auxin. We further validated the cellular expression patterns of βCAs and demonstrated their roles in 3D gametophore development. Overall, our study provides insights into cellular heterogeneity in a moss and identifies molecular signatures that underpin the 2D-to-3D growth transition at single-cell resolution.

Integrative regulatory mechanisms of stomatal movements under changing climate

Global climate change-caused drought stress, high temperatures and other extreme weather profoundly impact plant growth and development, restricting sustainable crop production. To cope with various environmental stimuli, plants can optimize the opening and closing of stomata to balance CO2 uptake for photosynthesis and water loss from leaves. Guard cells perceive and integrate various signals to adjust stomatal pores through turgor pressure regulation. Molecular mechanisms and signaling networks underlying the stomatal movements in response to environmental stresses have been extensively studied and elucidated. This review focuses on the molecular mechanisms of stomatal movements mediated by abscisic acid, light, CO2 , reactive oxygen species, pathogens, temperature, and other phytohormones. We discussed the significance of elucidating the integrative mechanisms that regulate stomatal movements in helping design smart crops with enhanced water use efficiency and resilience in a climate-changing world.

Receptor-like kinases and their signaling cascades for plant male fertility: loyal messengers

Receptor-like kinases (RLKs) are evolved for plant cell-cell communications. The typical RLK protein contains an extracellular and hypervariable N-terminus to perceive various signals, a transmembrane domain to anchor into plasma membrane, and a cytoplasmic, highly conserved kinase domain to phosphorylate target proteins. To date, RLKs have manifested their significance in a myriad of biological processes during plant reproductive growth, especially in male fertility. This review first summarizes a recent update on RLKs and their interacting protein partners controlling anther and pollen development, pollen release from dehisced anther, and pollen function during pollination and fertilization. Then, regulatory networks of RLK signaling pathways are proposed. In addition, we predict RLKs in maize and rice genome, obtain homologs of well-studied RLKs from phylogeny of three subfamilies and then analyze their expression patterns in developing anthers of maize and rice to excavate potential RLKs regulating male fertility in crops. Finally, current challenges and future prospects regarding RLKs are discussed. This review will contribute to a better understanding of plant male fertility control by RLKs, creating potential male sterile lines, and inspiring innovative crop breeding methods.

Decoding the guardians of cotton resilience: A comprehensive exploration of the βCA genes and its role in Verticillium dahliae resistance

Plant Carbonic anhydrases (Cas) have been shown to be stress-responsive enzymes that may play a role in adapting to adverse conditions. Cotton is a significant economic crop in China, with upland cotton (Gossypium hirsutum) being the most widely cultivated species. We conducted genome-wide identification of the βCA gene in six cotton species and preliminary analysis of the βCA gene in upland cotton. In total, 73 βCA genes from six cotton species were identified, with phylogenetic analysis dividing them into five subgroups. GHβCA proteins were predominantly localized in the chloroplast and cytoplasm. The genes exhibited conserved motifs, with motifs 1, 2, and 3 being prominent. GHβCA genes were unevenly distributed across chromosomes and were associated with stress-responsive cis-regulatory elements, including those responding to light, MeJA, salicylic acid, abscisic acid, cell cycle regulation, and defence/stress. Expression analysis indicated that GHβCA6, GHβCA7, GHβCA10, GHβCA15, and GHβCA16 were highly expressed under various abiotic stress conditions, whereas GHβCA3, GHβCA9, GHβCA10, and GHβCA18 had higher expression patterns under Verticillium dahliae infection at different time intervals. In Gossypium thurberi, GthβCA1, GthβCA2, and GthβCA4 showed elevated expression across stress conditions and tissues. Silencing GHβCA10 through VIGS increased Verticillium wilt severity and reduced lignin deposition compared to non-silenced plants. GHβCA10 is crucial for cotton's defense against Verticillium dahliae. Further research is needed to understand the underlying mechanisms and develop strategies to enhance resistance against Verticillium wilt.

Ascorbic acid-mediated reactive oxygen species homeostasis modulates the switch from tapetal cell division to cell differentiation in Arabidopsis

The major antioxidant L-ascorbic acid (AsA) plays important roles in plant growth, development, and stress responses. However, the importance of AsA concentration and the regulation of AsA metabolism in plant reproduction remain unclear. In Arabidopsis (Arabidopsis thaliana) anthers, the tapetum monolayer undergoes cell differentiation to support pollen development. Here, we report that a transcription factor, DEFECTIVE IN TAPETAL DEVELOPMENT AND FUNCTION 1 (TDF1), inhibits tapetal cell division leading to cell differentiation. We identified SKEWED5-SIMILAR 18 (SKS18) as a downstream target of TDF1. Enzymatic assays showed that SKS18, annotated as a multicopper oxidase-like protein, has ascorbate oxidase activity, leading to AsA oxidation. We also show that VITAMIN C DEFECTIVE1 (VTC1), an AsA biosynthetic enzyme, is negatively controlled by TDF1 to maintain proper AsA contents. Consistently, either knockout of SKS18 or VTC1 overexpression raised AsA concentrations, resulting in extra tapetal cells, while SKS18 overexpression in tdf1 or the vtc1-3 tdf1 double mutant mitigated their defective tapetum. We observed that high AsA concentrations caused lower accumulation of reactive oxygen species (ROS) in tapetal cells. Overexpression of ROS scavenging genes in tapetum restored excess cell divisions. Thus, our findings demonstrate that TDF1-regulated AsA balances cell division and cell differentiation in the tapetum through governing ROS homeostasis.

Comprehensive Insight into Tapetum-Mediated Pollen Development in Arabidopsis thaliana

In flowering plants, pollen development is a key process that is essential for sexual reproduction and seed set. Molecular and genetic studies indicate that pollen development is coordinatedly regulated by both gametophytic and sporophytic factors. Tapetum, the somatic cell layer adjacent to the developing male meiocytes, plays an essential role during pollen development. In the early anther development stage, the tapetal cells secrete nutrients, proteins, lipids, and enzymes for microsporocytes and microspore development, while initiating programmed cell death to provide critical materials for pollen wall formation in the late stage. Therefore, disrupting tapetum specification, development, or function usually leads to serious defects in pollen development. In this review, we aim to summarize the current understanding of tapetum-mediated pollen development and illuminate the underlying molecular mechanism in Arabidopsis thaliana.

Role of C4 photosynthetic enzyme isoforms in C3 plants and their potential applications in improving agronomic traits in crops

As compared to C3, C4 plants have higher photosynthetic rates and better tolerance to high temperature and drought. These traits are highly beneficial in the current scenario of global warming. Interestingly, all the genes of the C4 photosynthetic pathway are present in C3 plants, although they are involved in diverse non-photosynthetic functions. Non-photosynthetic isoforms of carbonic anhydrase (CA), phosphoenolpyruvate carboxylase (PEPC), malate dehydrogenase (MDH), the decarboxylating enzymes NAD/NADP-malic enzyme (NAD/NADP-ME), and phosphoenolpyruvate carboxykinase (PEPCK), and finally pyruvate orthophosphate dikinase (PPDK) catalyze reactions that are essential for major plant metabolism pathways, such as the tricarboxylic acid (TCA) cycle, maintenance of cellular pH, uptake of nutrients and their assimilation. Consistent with this view differential expression pattern of these non-photosynthetic C3 isoforms has been observed in different tissues across the plant developmental stages, such as germination, grain filling, and leaf senescence. Also abundance of these C3 isoforms is increased considerably in response to environmental fluctuations particularly during abiotic stress. Here we review the vital roles played by C3 isoforms of C4 enzymes and the probable mechanisms by which they help plants in acclimation to adverse growth conditions. Further, their potential applications to increase the agronomic trait value of C3 crops is discussed.

Specification of female germline by microRNA orchestrated auxin signaling in Arabidopsis

Germline determination is essential for species survival and evolution in multicellular organisms. In most flowering plants, formation of the female germline is initiated with specification of one megaspore mother cell (MMC) in each ovule; however, the molecular mechanism underlying this key event remains unclear. Here we report that spatially restricted auxin signaling promotes MMC fate in Arabidopsis. Our results show that the microRNA160 (miR160) targeted gene ARF17 (AUXIN RESPONSE FACTOR17) is required for promoting MMC specification by genetically interacting with the SPL/NZZ (SPOROCYTELESS/NOZZLE) gene. Alterations of auxin signaling cause formation of supernumerary MMCs in an ARF17- and SPL/NZZ-dependent manner. Furthermore, miR160 and ARF17 are indispensable for attaining a normal auxin maximum at the ovule apex via modulating the expression domain of PIN1 (PIN-FORMED1) auxin transporter. Our findings elucidate the mechanism by which auxin signaling promotes the acquisition of female germline cell fate in plants.

bHLH010/089 Transcription Factors Control Pollen Wall Development via Specific Transcriptional and Metabolic Networks in Arabidopsis thaliana

The pollen wall is a specialized extracellular cell wall that protects male gametophytes from various environmental stresses and facilitates pollination. Here, we reported that bHLH010 and bHLH089 together are required for the development of the pollen wall by regulating their specific downstream transcriptional and metabolic networks. Both the exine and intine structures of bhlh010 bhlh089 pollen grains were severely defective. Further untargeted metabolomic and transcriptomic analyses revealed that the accumulation of pollen wall morphogenesis-related metabolites, including polysaccharides, glyceryl derivatives, and flavonols, were significantly changed, and the expression of such metabolic enzyme-encoding genes and transporter-encoding genes related to pollen wall morphogenesis was downregulated in bhlh010 bhlh089 mutants. Among these downstream target genes, CSLB03 is a novel target with no biological function being reported yet. We found that bHLH010 interacted with the two E-box sequences at the promoter of CSLB03 and directly activated the expression of CSLB03. The cslb03 mutant alleles showed bhlh010 bhlh089–like pollen developmental defects, with most of the pollen grains exhibiting defective pollen wall structures.

Pan-brassinosteroid signaling revealed by functional analysis of NILR1 in land plants

Brassinosteroid (BR) signaling has been identified from the ligand BRs sensed by the receptor Brassinosteroid Insensitive 1 (BRI1) to the final activation of Brassinozole Resistant 1/bri1 EMS-Suppressor 1 through a series of transduction events. Extensive studies have been conducted to characterize the role of BR signaling in various biological processes. Our previous study has shown that Excess Microsporocytes 1 (EMS1) and BRI1 control different aspects of plant growth and development via conserved intracellular signaling. Here, we reveal that another receptor, NILR1, can complement the bri1 mutant in the absence of BRs, indicating a pathway that resembles BR signaling activated by NILR1. Genetic analysis confirms the intracellular domains of NILR1, BRI1 and EMS1 have a common signal output. Furthermore, we demonstrate that NILR1 and BRI1 share the coreceptor BRI1 Associated Kinase 1 and substrate BSKs. Notably, the NILR1-mediated downstream pathway is conserved across land plants. In summary, we provide evidence for the signaling cascade of NILR1, suggesting pan-brassinosteroid signaling initiated by a group of distant receptor-ligand pairs in land plants.

Nitric oxide, salicylic acid and oxidative stress: Is it a perfect equilateral triangle?

Nitric oxide (NO) is an endogenous free radical involved in the regulation of a wide array of physio-biochemical phenomena in plants. The biological activity of NO directly depend on its cellular concentration which usually changes under stress conditions, it participates in maintaining cellular redox equilibrium and regulating target checkpoints which control switches among development and stress. It is one of the key players in plant signalling and a plethora of evidence supports its crosstalk with other phytohormones. NO and salicylic acid (SA) cooperation is also of great physiological relevance, where NO modulates the immune response by regulating SA linked target proteins i.e., non-expressor of pathogenesis-related genes (NPR-1 and NPR-2) and Group D bZIP (basic leucine zipper domain transcription factor). Many experimental data suggest a functional cooperative role between NO and SA in mitigating the plant oxidative stress which suggests that these relationships could constitute a metabolic "equilateral triangle".

SlRBP1 promotes translational efficiency via SleIF4A2 to maintain chloroplast function in tomato

Many glycine-rich RNA-binding proteins (GR-RBPs) have critical functions in RNA processing and metabolism. Here, we describe a role for the tomato (Solanum lycopersicum) GR-RBP SlRBP1 in regulating mRNA translation. We found that SlRBP1 knockdown mutants (slrbp1) displayed reduced accumulation of total chlorophyll and impaired chloroplast ultrastructure. These phenotypes were accompanied by deregulation of the levels of numerous key transcripts associated with chloroplast functions in slrbp1. Furthermore, native RNA immunoprecipitation-sequencing (nRIP-seq) recovered 61 SlRBP1-associated RNAs, most of which are involved in photosynthesis. SlRBP1 binding to selected target RNAs was validated by nRIP-qPCR. Intriguingly, the accumulation of proteins encoded by SlRBP1-bound transcripts, but not the mRNAs themselves, was reduced in slrbp1 mutants. Polysome profiling followed by RT-qPCR assays indicated that the polysome occupancy of target RNAs was lower in slrbp1 plants than in wild-type. Furthermore, SlRBP1 interacted with the eukaryotic translation initiation factor SleIF4A2. Silencing of SlRBP1 significantly reduced SleIF4A2 binding to SlRBP1-target RNAs. Taking these observations together, we propose that SlRBP1 binds to and channels RNAs onto the SleIF4A2 translation initiation complex and promotes the translation of its target RNAs to regulate chloroplast functions.

Engineering Chimeras by Fusing Plant Receptor-like Kinase EMS1 and BRI1 Reveals the Two Receptors' Structural Specificity and Molecular Mechanisms

Brassinosteriods (BRs) are plant hormones essential for plant growth and development. The receptor-like kinase (RLK) BRI1 perceives BRs to initiate a well-known transduction pathway which finally activate the transcription factors BZR1/BES1 specifically regulating BR-mediated gene expression. The RLK EMS1 governs tapetum formation via the same signaling pathway shared with BRI1. BRI1 and EMS1 have a common signal output, but the gene structural specificity and the molecular response remain unclear. In this study, we identified that the transmembrane (TM), intracellular juxtamembrane (iJM), kinase, and leucin-rich repeats 1-13 (LRR1-13) domains of EMS1 could replace the corresponding BRI1 domain to maintain the BR receptor function, whereas the extracellular juxtamembrane (eJM) and LRR1-14 domains could not, indicating that the LRR14-EJM domain conferred functional specificity to BRI1. We compared the kinase domains of EMS1 and BRI1, and found that EMS1’s kinase activity was weaker than BRI1’s. Further investigation of the specific phosphorylation sites in BRI1 and EMS1 revealed that the Y1052 site in the kinase domain was essential for the BRI1 biological function, but the corresponding site in EMS1 showed no effect on the biological function of EMS1, suggesting a site regulation difference in the two receptors. Furthermore, we showed that EMS1 shared the substrate BSKs with BRI1. Our study provides insight into the structural specificity and molecular mechanism of BRI1 and EMS1, as well as the origin and divergence of BR receptors.

Epigenetic Regulation of Heat Stress in Plant Male Reproduction

In flowering plants, male reproductive development is highly susceptible to heat stress. In this mini-review, we summarized different anomalies in tapetum, microspores, and pollen grains during anther development under heat stress. We then discussed how epigenetic control, particularly DNA methylation, is employed to cope with heat stress in male reproduction. Further understanding of epigenetic mechanisms by which plants manage heat stress during male reproduction will provide new genetic engineering and molecular breeding tools for generating heat-resistant crops.

Identification and molecular characterization of the alternative spliced variants of beta carbonic anhydrase 1 (βCA1) from Arabidopsis thaliana

Carbonic anhydrases (CAs) are ubiquitous zinc metalloenzymes that catalyze the interconversion of carbon dioxide and bicarbonate. Higher plants mainly contain the three evolutionarily distinct CA families αCA, βCA, and γCA, with each represented by multiple isoforms. Alternative splicing (AS) of the CA transcripts is common. However, there is little information on the spliced variants of individual CA isoforms. In this study, we focused on the characterization of spliced variants of βCA1 from Arabidopsis. The expression patterns and subcellular localization of the individual spliced variants of βCA1 were examined. The results showed that the spliced variants of βCA1 possessed different subcellular and tissue distributions and responded differently to environmental stimuli. Additionally, we addressed the physiological role of βCA1 in heat stress response and its protein-protein interaction (PPI) network. Our results showed that βCA1 was regulated by heat stresses, and βca1 mutant was hypersensitive to heat stress, indicating a role for βCA1 in heat stress response. Furthermore, PPI network analysis revealed that βCA1 interacts with multiple proteins involved in several processes, including photosynthesis, metabolism, and the stress response, and these will provide new avenues for future investigations of βCA1.

Immunoproteomic identification of allergenic proteins in pecan (Carya illinoinensis) pollen

Pecan (C. illinoinensis) pollen is an important cause of allergic respiratory disease. Pecan is distributed worldwide as shade, ornamental or cultivation tree. To date three well known pecan food allergens have been reported, however, pollen allergens have not been identified. Here, we describe the first identification of IgE recognized pecan pollen proteins, for which proteins were analyzed by 2-DE and immunoblotting using a pool of 8 sera from pecan sensitive patients as primary antibody. IgE recognized protein spots were analyzed by LC-MS/MS and identified using a database of translated protein sequences obtained by the assembly of C. illinoinensis public transcriptomic information. This study has identified 17 IgE binding proteins from pecan pollen including proteins widely recognized as allergens and panallergens. These findings will contribute to develop specific diagnosis and treatment of pecan pollen allergy. SIGNIFICANCE: Pecan is a tree highly valued for its fruits that have a great commercial value. To date three pecan seed storage proteins have been officially recognized by the WHO/IUIS allergen nomenclature subcommittee as food allergens (Car i 1, Car i 2 and Car i 4). Pecan tree pollen is highly allergenic and a clinically relevant cause of allergies in North America (USA and Mexico) and regions where the tree is extensively cultivated (Israel, South Africa, Australia, Egypt, Peru, Argentina, and Brazil). Here, we describe the first identification of IgE recognized pollen proteins using an immunoproteomics approach and a protein database created by the assembly of pecan public transcriptomic information. The findings described here will allow the development of new diagnostic and therapeutic modalities for pecan pollen allergy.

Molecular Characterization of Carbonic Anhydrase Genes in Lotus japonicus and Their Potential Roles in Symbiotic Nitrogen Fixation

Carbonic anhydrase (CA) plays a vital role in photosynthetic tissues of higher plants, whereas its non-photosynthetic role in the symbiotic root nodule was rarely characterized. In this study, 13 CA genes were identified in the model legume Lotus japonicus by comparison with Arabidopsis CA genes. Using qPCR and promoter-reporter fusion methods, three previously identified nodule-enhanced CA genes (LjαCA2, LjαCA6, and LjβCA1) have been further characterized, which exhibit different spatiotemporal expression patterns during nodule development. LjαCA2 was expressed in the central infection zone of the mature nodule, including both infected and uninfected cells. LjαCA6 was restricted to the vascular bundle of the root and nodule. As for LjβCA1, it was expressed in most cell types of nodule primordia but only in peripheral cortical cells and uninfected cells of the mature nodule. Using CRISPR/Cas9 technology, the knockout of LjβCA1 or both LjαCA2 and its homolog, LjαCA1, did not result in abnormal symbiotic phenotype compared with the wild-type plants, suggesting that LjβCA1 or LjαCA1/2 are not essential for the nitrogen fixation under normal symbiotic conditions. Nevertheless, the nodule-enhanced expression patterns and the diverse distributions in different types of cells imply their potential functions during root nodule symbiosis, such as CO₂ fixation, N assimilation, and pH regulation, which await further investigations.

Dynamic expression of miRNAs and functional analysis of target genes involved in the response to male sterility of the wheat line YS3038

Thermosensitive cytoplasmic male sterile (TCMS) lines play an important role in wheat breeding, heterosis utilization, and germplasm innovation. MicroRNAs (miRNAs) can regulate the expression level of target genes by inhibiting the translation of these genes. YS3038 is a wheat TCMS line. In this study, the fertility conversion mechanism of YS3038 was studied by examining the abortion characteristics of YS3038, the regulation pattern of miRNAs and the target genes of miRNAs in YS3038. MiRNA-seq was performed on three important stages of YS3038 under sterile and fertile conditions. Then, the clean reads were aligned with some databases to filter other ncRNAs and repeats. The known miRNAs and novel miRNAs were predicted by sequence comparison with known miRNAs from miRbase. Differential expression of miRNAs between different stages and between different fertile conditions was analyzed, and functional analysis of target genes with opposite expression patterns as those of the miRNAs was conducted. The Ubisch bodies and microspores of sterile anthers were covered with filamentous materials. The degradation of the tapetum cells, the chloroplast structure of endothecium cells, and the microspore structure were abnormal. Microspore development was hindered from the late uninucleate stage to the binucleate stage. Twenty, 52, and 68 differentially expressed miRNAs (DEmiRs) were identified at the early uninucleate, late uninucleate, and binucleate stages, respectively, and there were 0, 7, and 72 differentially expressed target genes (DETGs), respectively, at these three stages. At the binucleate stage, 29 DEmiRs had 41 target mRNAs in total, and the expression patterns of the 41 target mRNAs were opposite to those of the 29 miRNAs. Fifteen significantly enriched KEGG pathways were associated with the 41 target mRNAs. Leucine-rich repeat receptor-like kinases (LRR-RLKs) play important roles in plant developmental and physiological processes. Some studies have shown that the expression of LRR-RLKs is related to the differentiation of microsporocytes and tapetum cells and to male sterility. An LRR-RLK (TaeRPK) gene was silenced by the barley stripe mosaic virus-induced gene silencing (BSMV-VIGS) method, and the seed setting rates of the TaeRPK-silenced plants (3.51%) were significantly lower than those of the negative control plants (88.78%) (P < 0.01). Thus, the TaeRPK gene is likely to be involved in the fertility conversion of YS3038.

High CO2 - and pathogen-driven expression of the carbonic anhydrase βCA3 confers basal immunity in tomato

Atmospheric CO₂ concentrations exert a strong influence on the susceptibility of plants to pathogens. However, the mechanisms involved in the CO₂ -dependent regulation of pathogen resistance are largely unknown. Here we show that the expression of tomato (Solanum lycopersicum) β-CARBONIC ANHYDRASE 3 (βCA3) is induced by the virulent pathogen Pseudomonas syringae pv. tomato DC3000. The role of βCA3 in the high CO₂ -mediated response in tomato and two other Solanaceae crops is distinct from that in Arabidopsis thaliana. Using βCA3 knock-out and over-expression plants, we demonstrate that βCA3 plays a positive role in the activation of basal immunity, particularly under high CO₂ . βCA3 is transcriptionally activated by the transcription factor NAC43 and is also post-translationally regulated by the receptor-like kinase GRACE1. The βCA3 pathway of basal immunity is independent on stomatal- and salicylic-acid-dependent regulation. Global transcriptome analysis and cell wall metabolite measurement implicate cell wall metabolism/integrity in βCA3-mediated basal immunity under both CO₂ conditions. These data not only highlight the importance of βCA3 in plant basal immunity under high CO₂ in a well-studied susceptible crop-pathogen system, but they also point to new targets for disease management strategies in a changing climate.

Stress-Related Changes in the Expression and Activity of Plant Carbonic Anhydrases

The data on stress-related changes in the expression and activity of plant carbonic anhydrases (CAs) suggest that they are generally upregulated at moderate stress severity. This indicates probable involvement of CAs in adaptation to drought, high salinity, heat, high light, C_i deficit, and excess bicarbonate. The changes in CA levels under cold stress are less studied and generally represented by the downregulation of CAs excepting βCA2. Excess Cd²⁺ and deficit of Zn²⁺ specifically reduce CA activity and reduce its synthesis. Probable roles of βCAs in stress adaptation include stomatal closure, ROS scavenging and partial compensation for decreased mesophyll CO₂ conductance. βCAs play contrasting roles in pathogen responses, interacting with phytohormone signaling networks. Their role can be either negative or positive, probably depending on the host-pathogen system, pathogen initial titer, and levels of ·NO and ROS. It is still not clear why CAs are suppressed under severe stress levels. It should be noted, that the role of βCAs in the facilitation of CO₂ diffusion and their involvement in redox signaling or ROS detoxication are potentially antagonistic, as they are inactivated by oxidation or nitrosylation. Interestingly, some chloroplastic βCAs may be relocated to the cytoplasm under stress conditions, but the physiological meaning of this effect remains to be studied.

Arabidopsis Transmembrane Receptor-Like Kinases (RLKs): A Bridge between Extracellular Signal and Intracellular Regulatory Machinery

Receptors form the crux for any biochemical signaling. Receptor-like kinases (RLKs) are conserved protein kinases in eukaryotes that establish signaling circuits to transduce information from outer plant cell membrane to the nucleus of plant cells, eventually activating processes directing growth, development, stress responses, and disease resistance. Plant RLKs share considerable homology with the receptor tyrosine kinases (RTKs) of the animal system, differing at the site of phosphorylation. Typically, RLKs have a membrane-localization signal in the amino-terminal, followed by an extracellular ligand-binding domain, a solitary membrane-spanning domain, and a cytoplasmic kinase domain. The functional characterization of ligand-binding domains of the various RLKs has demonstrated their essential role in the perception of extracellular stimuli, while its cytosolic kinase domain is usually confined to the phosphorylation of their substrates to control downstream regulatory machinery. Identification of the several ligands of RLKs, as well as a few of its immediate substrates have predominantly contributed to a better understanding of the fundamental signaling mechanisms. In the model plant Arabidopsis, several studies have indicated that multiple RLKs are involved in modulating various types of physiological roles via diverse signaling routes. Here, we summarize recent advances and provide an updated overview of transmembrane RLKs in Arabidopsis.

Carbonic anhydrases CA1 and CA4 function in atmospheric CO2-modulated disease resistance

Carbonic anhydrases CA1 and CA4 attenuate plant immunity and can contribute to altered disease resistance levels in response to changing atmospheric CO₂ conditions. β-Carbonic anhydrases (CAs) play an important role in CO₂ metabolism and plant development, but have also been implicated in plant immunity. Here we show that the bacterial pathogen Pseudomonas syringae and application of the microbe-associated molecular pattern (MAMP) flg22 repress CA1 and CA4 gene expression in Arabidopsis thaliana. Using the CA double-mutant ca1ca4, we provide evidence that CA1 and CA4 play an attenuating role in pathogen- and flg22-triggered immune responses. In line with this, ca1ca4 plants exhibited enhanced resistance against P. syringae, which was accompanied by an increased expression of the defense-related genes FRK1 and ICS1. Under low atmospheric CO₂ conditions (150 ppm), when CA activity is typically low, the levels of CA1 transcription and resistance to P. syringae in wild-type Col-0 were similar to those observed in ca1ca4. However, under ambient (400 ppm) and elevated (800 ppm) atmospheric CO₂ conditions, CA1 transcription was enhanced and resistance to P. syringae reduced. Together, these results suggest that CA1 and CA4 attenuate plant immunity and that differential CA gene expression in response to changing atmospheric CO₂ conditions contribute to altered disease resistance levels.

Translocation of Drought-Responsive Proteins from the Chloroplasts

Some chloroplast proteins are known to serve as messengers to transmit retrograde signals from chloroplasts to the nuclei in response to environmental stresses. However, whether particular chloroplast proteins respond to drought stress and serve as messengers for retrograde signal transduction are unclear. Here, we used isobaric tags for relative and absolute quantitation (iTRAQ) to monitor the proteomic changes in tobacco (Nicotiana benthamiana) treated with drought stress/re-watering. We identified 3936 and 1087 differentially accumulated total leaf and chloroplast proteins, respectively, which were grouped into 16 categories. Among these, one particular category of proteins, that includes carbonic anhydrase 1 (CA1), exhibited a great decline in chloroplasts, but a remarkable increase in leaves under drought stress. The subcellular localizations of CA1 proteins from moss (Physcomitrella patens), Arabidopsis thaliana and rice (Oryza sativa) in P. patens protoplasts consistently showed that CA1 proteins gradually diminished within chloroplasts but increasingly accumulated in the cytosol under osmotic stress treatment, suggesting that they could be translocated from chloroplasts to the cytosol and act as a signal messenger from the chloroplast. Our results thus highlight the potential importance of chloroplast proteins in retrograde signaling pathways and provide a set of candidate proteins for further research.

Tapetum-Dependent Male Meiosis Progression in Plants: Increasing Evidence Emerges

In higher plants, male meiosis is a key process during microsporogenesis and is crucial for male fertility and seed set. Meiosis involves a highly dynamic organization of chromosomes and cytoskeleton and specifically takes place within sexual cells. However, studies in multiple plant species have suggested that the normal development of tapetum, the somatic cell layer surrounding the developing male meiocytes, is indispensable for the completion of the male meiotic cell cycle. Disrupted tapetum development causes alterations in the expression of a large range of genes involved in male reproduction. Moreover, recent experiments suggest that small RNAs (sRNAs) present in the anthers, including microRNAs (miRNAs) and phased, secondary, small interfering RNAs (phasiRNAs), play a potential but important role in controlling male meiosis, either by influencing the expression of meiotic genes in the meiocytes or through other unclear mechanisms, supporting the hypothesis that male meiosis is non-cell autonomously regulated. In this mini review, we summarize the recorded meiotic defects that occur in plants with defective tapetum development in both Arabidopsis and crops. Thereafter, we outline the latest understanding on the molecular mechanisms that potentially underpin the tapetum-dependent regulation of male meiosis, and we especially discuss the regulatory role of sRNAs. At the end, we propose several outstanding questions that should be addressed in future studies.

A Model of Hormonal Regulation of Stamen Abortion during Pre-Meiosis of Litsea cubeba

Litsea cubeba (Lour.) Pers., a popular essential oil plant, is a dioecious species with degenerative sexual organs in both male and female individuals. Yet, the mechanism of degenerative organs development in male and female flowers is poorly understood. Here, we analyzed the morphological characters of degenerative organ development by morphological and histological observations, and determined the critical stage of abortion that occurs at pre-meiosis in male and female flowers. We also conducted RNA sequencing (RNA-seq) to understand the genetic basis of stamen abortion in female flowers. The differentially expressed genes (DEGs) were identified during the staminode development in female flowers; functional enrichment analysis revealed some important biological pathways involved the regulation of stamen abortion, including plant hormone signal transduction, phenylpropanoid biosynthesis, flavonoid biosynthesis and monoterpenoid biosynthesis. Furthermore, 15 DEGs involved in the hormone pathways were found to regulate stamen development. By HPLC-MS/MS analysis, there were a salicylic acid (SA) content peak and the gibberellin (GA) content lowest point in the abortion processes in female flowers, suggesting a vital function of hormonal processes. Co-expression network analysis further identified several hub genes that potentially played significant roles in the stamen abortion of L. cubeba. Taken together, we proposed a model involved in plant hormones pathways underlying stamen abortion during pre-meiosis in female flowers of L. cubeba.

BES1 is activated by EMS1-TPD1-SERK1/2-mediated signaling to control tapetum development in Arabidopsis thaliana

BES1 and BZR1 were originally identified as two key transcription factors specifically regulating brassinosteroid (BR)-mediated gene expression. They belong to a family consisting of six members, BES1, BZR1, BEH1, BEH2, BEH3, and BEH4. bes1 and bzr1 single mutants do not exhibit any characteristic BR phenotypes, suggesting functional redundancy of these proteins. Here, by generating higher order mutants, we show that a quintuple mutant is male sterile due to defects in tapetum and microsporocyte development in anthers. Our genetic and biochemical analyses demonstrate that BES1 family members also act as downstream transcription factors in the EMS1-TPD1-SERK1/2 pathway. Ectopic expression of both TPD1 and EMS1 in bri1-116, a BR receptor null mutant, leads to the accumulation of non-phosphorylated, active BES1, similar to activation of BES1 by BRI1-BR-BAK1 signaling. These data suggest that two distinctive receptor-like kinase-mediated signaling pathways share BES1 family members as downstream transcription factors to regulate different aspects of plant development.

BES1 and BZR1 transcription factors are activated by the BRI1-BAK1 receptor complex during brassinosteroid signaling. Here the authors show that BES1-family members also act in anthers, downstream of another receptor-like kinase-mediated signaling pathway, EMS1-TPD1-SERK1/2, to promote tapetum development.

EMS1 and BRI1 control separate biological processes via extracellular domain diversity and intracellular domain conservation

In flowering plants, EMS1 (Excess Microsporocytes 1) perceives TPD1 (Tapetum Determinant 1) to specify tapeta, the last somatic cell layer nurturing pollen development. However, the signaling components downstream of EMS1 are relatively unknown. Here, we use a molecular complementation approach to investigate the downstream components in EMS1 signaling. We show that the EMS1 intracellular domain is functionally interchangeable with that of the brassinosteroid receptor BRI1 (Brassinosteroid Insensitive 1). Furthermore, expressing EMS1 together with TPD1 in the BRI1 expression domain could partially rescue bri1 phenotypes, and led to the dephosphorylation of BES1, a hallmark of active BRI1 signaling. Conversely, expressing BRI1 in the EMS1 expression domain could partially rescue ems1 phenotypes. We further show that PpEMS1 and PpTPD1 from the early land plant Physcomitrella patens could completely rescue ems1 and tpd1 phenotypes, respectively. We propose that EMS1 and BRI1 have evolved distinct extracellular domains to control different biological processes but can act via a common intracellular signaling pathway.

EMS1 is a receptor-like kinase that recognizes the peptide ligand TPD1 to specify tapeta in Arabidopsis. Here, via a reciprocal complementation approach, the authors provide evidence that intracellular signaling by EMS1 is interchangeable with that of the brassinosteroid receptor BRI1.

Small RNA profiling from meiotic and post-meiotic anthers reveals prospective miRNA-target modules for engineering male fertility in sorghum

Understanding male gametophyte development is essential to augment hybrid production in sorghum. Although small RNAs are known to critically influence anther/pollen development, their roles in sorghum reproduction have not been deciphered yet. Here, we report small RNA profiling and high-confidence annotation of microRNAs (miRNAs) from meiotic and post-meiotic anthers in sorghum. We identified 262 miRNAs (82 known and 180 novel), out of which 58 (35 known and 23 novel) exhibited differential expression between two stages. Out of 35 differentially expressed known miRNAs, 13 are known to regulate anther/pollen development in other plant species. We also demonstrated conserved spatiotemporal patterns of 21- and 24-nt phasiRNAs and their respective triggers, miR2118 and miR2275, in sorghum anthers as evidenced in other monocots. miRNA target identification yielded 5622 modules, of which 46 modules comprising 16 known and 8 novel miRNA families with 38 target genes are prospective candidates for engineering male fertility in grasses.

Molecular Regulation of Host Defense Responses Mediated by Biological Anti-TMV Agent Ningnanmycin

Ningnanmycin (NNM) belongs to microbial pesticides that display comprehensive antiviral activity against plant viruses. NNM treatment has been shown to efficiently delay or suppress the disease symptoms caused by tobacco mosaic virus (TMV) infection in local-inoculated or systemic-uninoculated tobacco leaves, respectively. However, the underlying molecular mechanism of NNM-mediated antiviral activity remains to be further elucidated. In this study, 414 differentially expressed genes (DEGs), including 383 which were up-regulated and 31 down-regulated, caused by NNM treatment in TMV-infected BY-2 protoplasts, were discovered by RNA-seq. In addition, KEGG analysis indicated significant enrichment of DEGs in the plant-pathogen interaction and MAPK signaling pathway. The up-regulated expression of crucial DEGs, including defense-responsive genes, such as the receptor-like kinase FLS2, RLK1, and the mitogen-activated protein kinase kinase kinase MAPKKK, calcium signaling genes, such as the calcium-binding protein CML19, as well as phytohormone responsive genes, such as the WRKY transcription factors WRKY40 and WRKY70, were confirmed by RT-qPCR. These findings provided valuable insights into the antiviral mechanisms of NNM, which indicated that the agent induces tobacco systemic resistance against TMV via activating multiple plant defense signaling pathways.

Nitric oxide molecular targets: reprogramming plant development upon stress

Plants are sessile organisms that need to complete their life cycle by the integration of different abiotic and biotic environmental signals, tailoring developmental cues and defense concomitantly. Commonly, stress responses are detrimental to plant growth and, despite the fact that intensive efforts have been made to understand both plant development and defense separately, most of the molecular basis of this trade-off remains elusive. To cope with such a diverse range of processes, plants have developed several strategies including the precise balance of key plant growth and stress regulators [i.e. phytohormones, reactive nitrogen species (RNS), and reactive oxygen species (ROS)]. Among RNS, nitric oxide (NO) is a ubiquitous gasotransmitter involved in redox homeostasis that regulates specific checkpoints to control the switch between development and stress, mainly by post-translational protein modifications comprising S-nitrosation of cysteine residues and metals, and nitration of tyrosine residues. In this review, we have sought to compile those known NO molecular targets able to balance the crossroads between plant development and stress, with special emphasis on the metabolism, perception, and signaling of the phytohormones abscisic acid and salicylic acid during abiotic and biotic stress responses.

Roles of malic enzymes in plant development and stress responses

Malic enzyme (ME) comprises a family of proteins with multiple isoforms located in different compartments of eukaryotic cells. It is a key enzyme regulating malic acid metabolism and can catalyze the reversible reaction of oxidative decarboxylation of malic acid. And it is also one of the important enzymes in plant metabolism and is involved in multiple metabolic processes. ME is widely present in plants and mainly discovered in cytoplasmic stroma, mitochondria, chloroplasts. It is involved in plant growth, development, and stress response. Plants are stressed by various environmental factors such as drought, high salt, and high temperature during plant growth, and the mechanisms of plant response to various environmental stresses are synergistic. Numerous studies have shown that ME participates in the process of coping with the above environmental factors by increasing water use efficiency, improving photosynthesis of plants, providing reducing power, and so on. In this review, we discuss the important role of ME in plant development and plant stress response, and prospects for its application. It provides a theoretical basis for the future use of ME gene for molecular resistance breeding.

Therapeutic targeting of cellular stress responses in cancer

Similar to bacteria, yeast, and other organisms that have evolved pathways to respond to environmental stresses, cancer cells develop mechanisms that increase genetic diversity to facilitate adaptation to a variety of stressful conditions, including hypoxia, nutrient deprivation, exposure to DNA-damaging agents, and immune responses. To survive, cancer cells trigger mechanisms that drive genomic instability and mutation, alter gene expression programs, and reprogram the metabolic pathways to evade growth inhibition signaling and immune surveillance. A deeper understanding of the molecular mechanisms that underlie the pathways used by cancer cells to overcome stresses will allow us to develop more efficacious strategies for cancer therapy. Herein, we overview several key stresses imposed on cancer cells, including oxidative, metabolic, mechanical, and genotoxic, and discuss the mechanisms that drive cancer cell responses. The therapeutic implications of these responses are also considered, as these factors pave the way for the targeting of stress adaption pathways in order to slow cancer progression and block resistance to therapy.

Identification of Salt Stress Responding Genes Using Transcriptome Analysis in Green Alga Chlamydomonas reinhardtii

Salinity is one of the most important abiotic stresses threatening plant growth and agricultural productivity worldwide. In green alga Chlamydomonas reinhardtii, physiological evidence indicates that saline stress increases intracellular peroxide levels and inhibits photosynthetic-electron flow. However, understanding the genetic underpinnings of salt-responding traits in plantae remains a daunting challenge. In this study, the transcriptome analysis of short-term acclimation to salt stress (200 mM NaCl for 24 h) was performed in C. reinhardtii. A total of 10,635 unigenes were identified as being differently expressed by RNA-seq, including 5920 up- and 4715 down-regulated unigenes. A series of molecular cues were screened for salt stress response, including maintaining the lipid homeostasis by regulating phosphatidic acid, acetate being used as an alternative source of energy for solving impairment of photosynthesis, and enhancement of glycolysis metabolism to decrease the carbohydrate accumulation in cells. Our results may help understand the molecular and genetic underpinnings of salt stress responses in green alga C. reinhardtii.

CIK Receptor Kinases Determine Cell Fate Specification during Early Anther Development in Arabidopsis

Appropriate cell division and differentiation ensure normal anther development in angiosperms. BARELY ANY MERISTEM 1/2 (BAM1/2) and RECEPTOR-LIKE PROTEIN KINASE2 (RPK2), two groups of leucine-rich repeat receptor-like protein kinases, are required for early anther cell specification. However, little is known about the molecular mechanisms underlying these two RLK-mediated signaling pathways. Here, we show that CLAVATA3 INSENSITIVE RECEPTOR KINASEs (CIKs), a group of novel coreceptor protein kinase-controlling stem cell homeostasis, play essential roles in BAM1/2- and RPK2-regulated early anther development in Arabidopsis thaliana The archesporial cells of cik1/2/3 triple and cik1/2/3/4 quadruple mutant anthers perform anticlinal division instead of periclinal division. Defective cell division and specification of the primary and inner secondary parietal cells occur in these mutant anthers. The disordered divisions and specifications of anther wall cells finally result in excess microsporocytes and a lack of one to three parietal cell layers in mutant anthers, resembling rpk2 or bam1/2 mutant anthers. Genetic and biochemical analyses indicate that CIKs function as coreceptors of BAM1/2 and RPK2 to regulate archesporial cell division and determine the specification of anther parietal cells.

Analysis of Biomolecules Based on the Surface Enhanced Raman Spectroscopy

Analyzing biomolecules is essential for disease diagnostics, food safety inspection, environmental monitoring and pharmaceutical development. Surface-enhanced Raman spectroscopy (SERS) is a powerful tool for detecting biomolecules due to its high sensitivity, rapidness and specificity in identifying molecular structures. This review focuses on the SERS analysis of biomolecules originated from humans, animals, plants and microorganisms, combined with nanomaterials as SERS substrates and nanotags. Recent advances in SERS detection of target molecules were summarized with different detection strategies including label-free and label-mediated types. This comprehensive and critical summary of SERS analysis of biomolecules might help researchers from different scientific backgrounds spark new ideas and proposals.

Non-canonical functions of the mitotic kinesin Eg5

Kinesins are widely expressed, microtubule-dependent motors that play vital roles in microtubule-associated cellular activities, such as cell division and intracellular transport. Eg5, also known as kinesin-5 or kinesin spindle protein, is a member of the kinesin family that contributes to the formation and maintenance of the bipolar mitotic spindle during cell division. Small-molecule compounds that inhibit Eg5 activity have been shown to impair spindle assembly, block mitotic progression, and possess anti-cancer activity. Recent studies focusing on the localization and functions of Eg5 in plants have demonstrated that in addition to spindle organization, this motor protein has non-canonical functions, such as chromosome segregation and cytokinesis, that have not been observed in animals. In this review, we discuss the structure, function, and localization of Eg5 in various organisms, highlighting the specific role of this protein in plants. We also propose directions for the future studies of novel Eg5 functions based on the lessons learned from plants.

The role of receptor-like kinases in regulating plant male reproduction

RLKs in anther development. The cell-to-cell communication is essential for specifying different cell types during plant growth, development and adaption to the ever-changing environment. Plant male reproduction, in particular, requires the exquisitely synchronized development of different cell layers within the male tissue, the anther. Receptor-like kinases (RLKs) belong to a large group of kinases localized on the cell surfaces, perceiving extracellular signals and thereafter regulating intracellular processes. Here we update the role of RLKs in early anther development by defining the cell fate and anther patterning, responding to the changing environment and controlling anther carbohydrate metabolism. We provide speculation of the poorly characterized ligands and substrates of these RLKs. The conserved and diversified aspects underlying the function of RLKs in anther development are discussed.

The many types of carbonic anhydrases in photosynthetic organisms

Carbonic anhydrases (CAs) are enzymes that catalyze the interconversion of CO₂ and HCO₃^-. In nature, there are multiple families of CA, designated with the Greek letters α through θ. CAs are ubiquitous in plants, algae and photosynthetic bacteria, often playing essential roles in the CO₂ concentrating mechanisms (CCMs) which enhance the delivery of CO₂ to Rubisco. As algal CCMs become better characterized, it is clear that different types of CAs are playing the same role in different algae. For example, an α-CA catalyzes the conversion of accumulated HCO₃^- to CO₂ in the green alga Chlamydomonas reinhardtii, while a θ-CA performs the same function in the diatom Phaeodactylum tricornutum. In this review we argue that, in addition to its role of delivering CO₂ for photosynthesis, other metabolic roles of CA have likely changed as the Earth's atmospheric CO₂ level decreased. Since the algal and plant lineages diverged well before the decrease in atmospheric CO₂, it is likely that plant, algae and photosynthetic bacteria all adapted independently to the drop in atmospheric CO₂. In light of this, we will discuss how the roles of CAs may have changed over time, focusing on the role of CA in pH regulation, how CAs affect CO₂ supply for photosynthesis and how CAs may help in the delivery of HCO₃^- for other metabolic reactions.

Molecular mechanisms underlying stress response and adaptation

Environmental stresses are ubiquitous and unavoidable to all living things. Organisms respond and adapt to stresses through defined regulatory mechanisms that drive changes in gene expression, organismal morphology, or physiology. Immune responses illustrate adaptation to bacterial and viral biotic stresses in animals. Dysregulation of the genotoxic stress response system is frequently associated with various types of human cancer. With respect to plants, especially halophytes, complicated systems have been developed to allow for plant growth in high salt environments. In addition, drought, waterlogging, and low temperatures represent other common plant stresses. In this review, we summarize representative examples of organismal response and adaptation to various stresses. We also discuss the molecular mechanisms underlying the above phenomena with a focus on the improvement of organismal tolerance to unfavorable environments.

Rhizobium sp. IRBG74 Alters Arabidopsis Root Development by Affecting Auxin Signaling

Rhizobium sp. IRBG74 not only nodulates Sesbania cannabina but also can enhance rice growth; however, the underlying molecular mechanisms are not clear. Here, we show that Rhizobium sp. IRBG74 colonizes the roots of Arabidopsis thaliana, which leads to inhibition in the growth of main root but enhancement in the formation of lateral roots. The promotion of lateral root formation by Rhizobium sp. IRBG74 in the fls2-1 mutant, which is insensitive to flagellin, is similar to the wild-type plant, while the auxin response deficient mutant tir1-1 is significantly less sensitive to Rhizobium sp. IRBG74 than the wild type in terms of the inhibition of main root elongation and the promotion of lateral root formation. Further transcriptome analysis of Arabidopsis roots inoculated with Rhizobium sp. IRBG74 revealed differential expression of 50 and 211 genes at 24 and 48 h, respectively, and a majority of these genes are involved in auxin signaling. Consistent with the transcriptome analysis results, Rhizobium sp. IRBG74 treatment induces expression of the auxin responsive reporter DR5:GUS in roots. Our results suggest that in Arabidopsis Rhizobium sp. IRBG74 colonizes roots and promotes the lateral root formation likely through modulating auxin signaling. Our work provides insight into the molecular mechanisms of interactions between legume-nodulating rhizobia and non-legume plants.

Environmental sensing and morphological plasticity in plants

All creatures on earth are affected by their surrounding environment. Animals can move and escape unfavorable environmental changes, whereas plants must respond to environmental stimuli. Plants adapt to changes with cellular-level responses to short-term environmental changes, but may adapt to changes in the environment by regulating their development and growth. In this review, we considered changes in atmospheric CO₂ concentrations, dry/wet moisture conditions, flooding, and temperature as examples of environmental stimuli. We mainly focused on leaf morphology and stomatal density as examples of developmental and growth patterns of plants in response to the environment.