ERECTA receptor-kinases play a key role in the appropriate timing of seed germination under changing salinity

Stomatal patterning is differently regulated in adaxial and abaxial epidermis in Arabidopsis

Stomatal pores in leaves mediate CO2 uptake into the plant and water loss via transpiration. Most plants are hypostomatous with stomata present only in the lower leaf surface (abaxial epidermis). Many herbs, including the model plant Arabidopsis, have substantial numbers of stomata also on the upper (adaxial) leaf surface. Studies of stomatal development have mostly focused on abaxial stomata and very little is known of adaxial stomatal formation. We analysed the role of leaf number in determining stomatal density and stomatal ratio, and studied adaxial and abaxial stomatal patterns in Arabidopsis mutants deficient in known abaxial stomatal development regulators. We found that stomatal density in some genetic backgrounds varies between different fully expanded leaves, and thus we recommend using defined leaves for analyses of stomatal patterning. Our results indicate that stomatal development is at least partly independently regulated in adaxial and abaxial epidermis, as (i) plants deficient in ABA biosynthesis and perception have increased stomatal ratios, (ii) the epf1epf2, tmm, and sdd1 mutants have reduced stomatal ratios, (iii) erl2 mutants have increased adaxial but not abaxial stomatal index, and (iv) stomatal precursors preferentially occur in abaxial epidermis. Further studies of adaxial stomata can reveal new insights into stomatal form and function.

Genomic Survey of LRR-RLK Genes in Eriobotrya japonica and Their Expression Patterns Responding to Environmental Stresses

The impact of global warming is increasing and thus exacerbating environmental stresses that affect plant yield and distribution, including the Eriobotrya japonica Lindl (Loquat tree). Eriobotrya japonica, a member of the Rosaceae family, is valued not only for its nutritious fruit but also for its medicinal purposes, landscape uses, and other pharmacological benefits. Nonetheless, the productivity of Eriobotrya japonica has raised a lot of concern in the wake of adverse environmental conditions. Understanding the characteristics of the LRR-RLK gene family in loquat is crucial, as these genes play vital roles in plant stress responses. In this study, 283 LRR-RLK genes were identified in the genome of E. japonica that were randomly positioned on 17 chromosomes and 24 contigs. The 283 EjLRR-RLK proteins clustered into 21 classes and subclasses in the phylogenetic analysis based on domain and protein arrangements. Further explorations in the promoter regions of the EjLRR-RLK genes showed an abundance of cis-regulatory elements that functioned in growth and development, phytohormone, and biotic and abiotic responses. Most cis-elements were present in the biotic and abiotic responses suggesting that the EjLRR-RLK genes are invested in regulating both biotic and abiotic stresses. Additional investigations into the responses of EjLRR-RLK genes to abiotic stress using the RT-qPCR revealed that EjLRR-RLK genes respond to abiotic stress, especially heat and salt stresses. Particularly, EjapXI-1.6 and EjapI-2.5 exhibited constant upregulation in all stresses analyzed, indicating that these may take an active role in regulating abiotic stresses. Our findings suggest the pivotal functions of EjLRR-RLK genes although additional research is still required. This research aims to provide useful information relating to the characterization of EjLRR-RLK genes and their responses to environmental stresses, establishing a concrete base for the following research.

ERECTA Modulates Seed Germination and Fruit Development via Auxin Signaling in Tomato

Tomato (Solanum lycopersicum) breeding for improved fruit quality emphasizes selecting for desirable taste and characteristics, as well as enhancing disease resistance and yield. Seed germination is the initial step in the plant life cycle and directly affects crop productivity and yield. ERECTA (ER) is a receptor-like kinase (RLK) family protein known for its involvement in diverse developmental processes. We characterized a Micro-Tom EMS mutant designated as a knock-out mutant of sler. Our research reveals that SlER plays a central role in controlling critical traits such as inflorescence development, seed number, and seed germination. The elevation in auxin levels and alterations in the expression of ABSCISIC ACID INSENSITIVE 3 (ABI3) and ABI5 in sler seeds compared to the WT indicate that SlER modulates seed germination via auxin and abscisic acid (ABA) signaling. Additionally, we detected an increase in auxin content in the sler ovary and changes in the expression of auxin synthesis genes YUCCA flavin monooxygenases 1 (YUC1), YUC4, YUC5, and YUC6 as well as auxin response genes AUXIN RESPONSE FACTOR 5 (ARF5) and ARF7, suggesting that SlER regulates fruit development via auxin signaling.

Genome-wide identification of the LRR-RLK gene family in peanut and functional characterization of AhLRR-RLK265 in salt and drought stresses

Leucine-rich repeat receptor-like kinases (LRR-RLKs) play important roles in plant developmental regulations and various stress responses. Peanut (Arachis hypogaea L.) is a worldwide important oil crop; however, no systematic identification or analysis of the peanut LRR-RLK gene family has been reported. In present study, 495 LRR-RLK genes in peanut were identified and analyzed. The 495 AhLRR-RLK genes were classed into 14 groups and 10 subgroups together with their Arabidopsis homologs according to phylogenetic analyses, and 491 of 495 AhLRR-RLK genes unequally located on 20 chromosomes. Analyses of gene structure and protein motif organization revealed similarity in exon/intron and motif organization among members of the same subgroup, further supporting the phylogenetic results. Gene duplication events were found in peanut LRR-RLK gene family via syntenic analysis, which were important in LRR-RLK gene family expansion in peanut. We found that the expression of AhLRR-RLK genes was detected in different tissues using RNA-seq data, implying that AhLRR-RLK genes may differ in function. In addition, Arabidopsis plants overexpressing stress-induced AhLRR-RLK265 displayed lower seed germination rates and root lengths compared to wild-type under exogenous ABA treatment. Notably, overexpression of AhLRR-RLK265 enhanced tolerance to salt and drought stresses in transgenic Arabidopsis. Moreover, the AhLRR-RLK265-OE lines were found to have higher activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) under salt and drought stress treatments. We believe these results may provide valuable information about the function of peanut LRR-RLK genes for further analysis.

Transcriptional Landscape of Cotton Roots in Response to Salt Stress at Single-cell Resolution

Increasing soil salinization has led to severe losses of plant yield and quality. Thus, it is urgent to investigate the molecular mechanism of the salt stress response. In this study, we took systematically analyzed cotton root response to salt stress by single-cell transcriptomics technology; 56,281 high-quality cells were totally obtained from 5-days-old lateral root tips of Gossypium arboreum under natural growth and different salt-treatment conditions. Ten cell types with an array of novel marker genes were synthetically identified and confirmed with in situ RNA hybridization, and some specific-type cells of pesudotime analysis also pointed out their potential differentiation trajectory. The prominent changes of cell numbers responding to salt stress were observed on outer epidermal and inner endodermic cells, which were significantly enriched in response to stress, amide biosynthetic process, glutathione metabolism, and glycolysis/gluconeogenesis. Other functional aggregations were concentrated on plant-type primary cell wall biogenesis, defense response, phenylpropanoid biosynthesis and metabolic pathways by analyzing the abundant differentially expressed genes (DEGs) identified from multiple comparisons. Some candidate DEGs related with transcription factors and plant hormones responding to salt stress were also identified, of which the function of Ga03G2153, an annotated auxin-responsive GH3.6, was confirmed by using virus-induced gene silencing (VIGS). The GaGH3.6-silenced plants presented severe stress-susceptive phenotype, and suffered more serious oxidative damages by detecting some physiological and biochemical indexes, indicating that GaGH3.6 might participate in salt tolerance in cotton through regulating oxidation-reduction process. For the first time, a transcriptional atlas of cotton roots under salt stress were characterized at a single-cell resolution, which explored the cellular heterogeneityand differentiation trajectory, providing valuable insights into the molecular mechanism underlying stress tolerance in plants.

CYSTEINE-RICH RECEPTOR-LIKE PROTEIN KINASES: their evolution, structure, and roles in stress response and development

Plant-specific receptor-like protein kinases (RLKs) are central components for sensing the extracellular microenvironment. CYSTEINE-RICH RLKs (CRKs) are members of one of the biggest RLK subgroups. Their physiological and molecular roles have only begun to be elucidated, but recent studies highlight the diverse types of proteins interacting with CRKs, as well as the localization of CRKs and their lateral organization within the plasma membrane. Originally the DOMAIN OF UNKNOWN FUNCTION 26 (DUF26)-containing extracellular region of the CRKs was proposed to act as a redox sensor, but the potential activating post-translational modification or ligands perceived remain elusive. Here, we summarize recent progress in the analysis of CRK evolution, molecular function, and role in plant development, abiotic stress responses, plant immunity, and symbiosis. The currently available information on CRKs and related proteins suggests that the CRKs are central regulators of plant signaling pathways. However, more research using classical methods and interdisciplinary approaches in various plant model species, as well as structural analyses, will not only enhance our understanding of the molecular function of CRKs, but also elucidate the contribution of other cellular components in CRK-mediated signaling pathways.

Senescence-related receptor kinase 1 functions downstream of WRKY53 in regulating leaf senescence in Arabidopsis

Receptor-like kinases (RLKs) are the most important class of cell surface receptors, and play crucial roles in plant development and stress responses. However, few studies have been reported about the biofunctions of RLKs in leaf senescence. Here, we characterized a novel Arabidopsis RLK-encoding gene, SENESCENCE-RELATED RECEPTOR KINASE 1 (SENRK1), which was significantly down-regulated during leaf senescence. Notably, the loss-of-function senrk1 mutants displayed an early leaf senescence phenotype, while overexpression of SENRK1 significantly delayed leaf senescence, indicating that SENRK1 negatively regulates age-dependent leaf senescence in Arabidopsis. Furthermore, the senescence-promoting transcription factor WRKY53 repressed the expression of SENRK1. While the wrky53 mutant showed a delayed senescence phenotype as previously reported, the wrky53 senrk1-1 double mutant exhibited precocious leaf senescence, suggesting that SENRK1 functions downstream of WRKY53 in regulating age-dependent leaf senescence in Arabidopsis.

Multifaceted roles of the ERECTA family in plant organ morphogenesis

Receptor-like kinases (RLKs) can participate in multiple signalling pathways and are considered one of the most critical components of the early events of intercellular signalling. As an RLK, the ERECTA family (ERf), which comprises ERECTA (ER), ERECTA-Like1 (ERL1), and ERECTA-Like2 (ERL2) in Arabidopsis, regulates multiple signalling pathways in plant growth and development. Despite its indispensability, detailed information on ERf-manipulated signalling pathways remains elusive. In this review, we attempt to summarize the essential roles of the ERf in plant organ morphogenesis, including shoot apical meristem, stem, and reproductive organ development.

Receptor-like Kinases (LRR-RLKs) in Response of Plants to Biotic and Abiotic Stresses

Plants live under different biotic and abiotic stress conditions, and, to cope with the adversity and severity, plants have well-developed resistance mechanisms. The mechanism starts with perception of the stimuli followed by molecular, biochemical, and physiological adaptive measures. The family of LRR-RLKs (leucine-rich repeat receptor-like kinases) is one such group that perceives biotic and abiotic stimuli and also plays important roles in different biological processes of development. This has been mostly studied in the model plant, Arabidopsis thaliana, and to some extent in other plants, such as Solanum lycopersicum, Nicotiana benthamiana, Brassica napus, Oryza sativa, Triticum aestivum, Hordeum vulgare, Brachypodium distachyon, Medicago truncatula, Gossypium barbadense, Phaseolus vulgaris, Solanum tuberosum, and Malus robusta. Most LRR-RLKs tend to form different combinations of LRR-RLKs-complexes (dimer, trimer, and tetramers), and some of them were observed as important receptors in immune responses, cell death, and plant development processes. However, less is known about the function(s) of LRR-RLKs in response to abiotic and biotic stresses. Here, we give recent updates about LRR-RLK receptors, specifically focusing on their involvement in biotic and abiotic stresses in the model plant, A. thaliana. Furthermore, the recent studies on LRR-RLKs that are homologous in other plants is also reviewed in relation to their role in triggering stress response processes against biotic and abiotic stimuli and/or in exploring their additional function(s). Furthermore, we present the interactions and combinations among LRR-RLK receptors that have been confirmed through experiments. Moreover, based on GENEINVESTIGATOR microarray database analysis, we predict some potential LRR-RLK genes involved in certain biotic and abiotic stresses whose function and mechanism may be explored.

Plasma membrane-to-organelle communication in plant stress signaling

Intracellular compartments engage in extensive communication with one another, an essential ability for cells to respond and adapt to changing environmental and developmental conditions. The plasma membrane (PM), as the interface between the cellular and the outside media, plays a central role in the perception and relay of information about external stimuli, which needs to be ultimately addressed to the relevant subcellular organelles. Interest in PM-organelle communication has increased dramatically in recent years, as examples arise that illustrate different strategies through which information from the PM can be transmitted. In this review, we will discuss mechanisms enabling PM-to-organelle communication in plants, specifically in biotic and abiotic stress signaling.

BREVIPEDICELLUS and ERECTA control the expression of AtPRX17 to prevent Arabidopsis callus browning

Efficient in vitro callus generation is required for tissue culture propagation, a process that allows for plant regeneration and transgenic breeding for desired phenotypes. Identifying genes and regulatory elements that prevent impaired callus growth and callus browning is essential for the development of in vitro callus systems. Here, we show that the BREVIPEDICELLUS and ERECTA pathways in Arabidopsis calli converge to prevent callus browning, and positively regulate the expression of the isoperoxidase gene AtPRX17 in rapidly growing calli. Loss-of-function mutations in both BREVIPEDICELLUS and ERECTA resulted in markedly increased callus browning. Transgenic lines expressing 35S pro::AtPRX17 in the bp-5 er105 double mutant background fully rescued this phenotypic abnormality. Using in vivo (chromatin immunoprecipitation-PCR and transient expression) and in vitro (electrophoretic mobility shift assays) assays, we observed that the BREVIPEDICELLUS protein binds directly to the upstream sequence of AtPRX17 to promote its transcription during callus growth. ERECTA is a ubiquitous factor required for cell proliferation and growth. We show that ERECTA positively regulates the expression of the transcription factor WRKY6, which directly binds to a separate site on the AtPRX17 promoter, further increasing its expression. Our data reveal an important molecular mechanism involved in the regulation of peroxidase isozyme expression to reduce Arabidopsis callus browning.

Tomato fruit quality traits and metabolite content are affected by reciprocal crosses and heterosis

Heterosis occurs when the F1s outperform their parental lines for a trait. Reciprocal hybrids are obtained by changing the cross direction of parental genotypes. Both biological phenomena could affect the external and internal attributes of fleshy fruits. This work aimed to detect reciprocal effects and heterosis in tomato (Solanum lycopersicum) fruit quality traits and metabolite content. Twelve agronomic traits and 28 metabolites identified and estimated by 1H-NMR were evaluated in five cultivars grown in two environments. Given that the genotype component was more important than the phenotype, the traits were evaluated following a full diallel mating design among those cultivars, in a greenhouse. Hybrids showed a higher phenotypic diversity than parental lines. Interestingly, the metabolites, mainly amino acids, displayed more reciprocal effects and heterosis. Agronomic traits were more influenced by general combining ability (GCA) and metabolites by specific combining ability (SCA). Furthermore, the genetic distance between parental lines was not causally related to the occurrence of reciprocal effects or heterosis. Hybrids with heterosis and a high content of metabolites linked to tomato flavour and nutritious components were obtained. Our results highlight the impact of selecting a cultivar as male or female in a cross to enhance the variability of fruit attributes through hybrids as well as the possibility to exploit heterosis for fruit composition.

One-Pot Extractive Transesterification of Fatty Acids Followed by DMOX Derivatization for Location of Double Bonds Using GC-EI-MS

Fatty acids are an essential structural and energy storage component of cells and hence there is much interest in their metabolism, requiring identification and quantification with readily available instrumentation, such as GC-MS. Fatty acid methyl esters (FAMEs) can be generated and extracted directly from biological tissue, in a one-pot process, and following high resolution GC, their respective chain length, degrees of unsaturation, and other functionalities can be readily identified using EI-MS. Defining the positions of the double bonds in the alkyl chain requires conversion of the FAMEs into their respective dimethyloxazoline (DMOX) derivatives. Following EI, this derivative allows charge retention on the heterocycle, and concomitant charge remote fragmentation of the alkyl chain to yield key double bond position identifying ions. The protocols described herein have been applied to the identification and quantification of fatty acids harvested from microalgae grown to produce biofuels and to the screening of salt tolerant Arabidopsis mutants.

Changes in intracellular NAD status affect stomatal development in an abscisic acid-dependent manner

Nicotinamide adenine dinucleotide (NAD) plays a central role in redox metabolism in all domains of life. Additional roles in regulating posttranslational protein modifications and cell signaling implicate NAD as a potential integrator of central metabolism and programs regulating stress responses and development. Here we found that NAD negatively impacts stomatal development in cotyledons of Arabidopsis thaliana. Plants with reduced capacity for NAD⁺ transport from the cytosol into the mitochondria or the peroxisomes exhibited reduced numbers of stomatal lineage cells and reduced stomatal density. Cotyledons of plants with reduced NAD⁺ breakdown capacity and NAD⁺ -treated cotyledons also presented reduced stomatal number. Expression of stomatal lineage-related genes was repressed in plants with reduced expression of NAD⁺ transporters as well as in plants treated with NAD⁺ . Impaired NAD⁺ transport was further associated with an induction of abscisic acid (ABA)-responsive genes. Inhibition of ABA synthesis rescued the stomatal phenotype in mutants deficient in intracellular NAD⁺ transport, whereas exogenous NAD⁺ feeding of aba-2 and ost1 seedlings, impaired in ABA synthesis and ABA signaling, respectively, did not impact stomatal number, placing NAD upstream of ABA. Additionally, in vivo measurement of ABA dynamics in seedlings of an ABA-specific optogenetic reporter - ABAleon2.1 - treated with NAD⁺ showed increases in ABA content suggesting that NAD⁺ impacts on stomatal development through ABA synthesis and signaling. Our results demonstrate that intracellular NAD⁺ homeostasis as set by synthesis, breakdown and transport is essential for normal stomatal development, and provide a link between central metabolism, hormone signaling and developmental plasticity.

Melon short internode (CmSi) encodes an ERECTA-like receptor kinase regulating stem elongation through auxin signaling

Plant height is one of the most important agronomic traits that directly determines plant architecture, and compact or dwarf plants can allow for increased planting density and land utilization as well as increased lodging resistance and economic yield. At least four dwarf/semidwarf genes have been identified in different melon varieties, but none of them have been cloned, and little is known about the molecular mechanisms underlying internode elongation in melon. Here, we report map-based cloning and functional characterization of the first semidwarf gene short internode (Cmsi) in melon, which encodes an ERECTA-like receptor kinase regulating internode elongation. Spatial-temporal expression analyses revealed that CmSI exhibited high expression in the vascular bundle of the main stem during internode elongation. The expression level of CmSI was positively correlated with stem length in the different melon varieties examined. Ectopic expression of CmSI in Arabidopsis and cucumber suggested CmSI as a positive regulator of internode elongation in both species. Phytohormone quantitation and transcriptome analysis showed that the auxin content and the expression levels of a number of genes involved in the auxin signaling pathway were altered in the semidwarf mutant, including several well-known auxin transporters, such as members of the ABCB family and PIN-FORMED genes. A melon polar auxin transport protein CmPIN2 was identified by protein-protein interaction assay as physically interacting with CmSI to modulate auxin signaling. Thus, CmSI functions in an auxin-dependent regulatory pathway to control internode elongation in melon. Our findings revealed that the ERECTA family gene CmSI regulates stem elongation in melon through auxin signaling, which can directly affect polar auxin transport.

Enhancing Conservation of a Globally Imperiled Rockland Herb (Linum arenicola) through Assessments of Seed Functional Traits and Multi-Dimensional Germination Niche Breadths

Humans currently face an extraordinary period of plant biodiversity loss. One strategy to stem further losses involves the development of species-level recovery plans that guide conservation actions. Seeds represent an important component in the life history of plants and are crucial for conservation activities. Yet, most recovery plans contain meager seed biology information. We set out to examine seed functional traits and germination niche breadth of Linum arenicola seeds exposed to a range of thermal, photoperiodic, and salinity gradients to gain perspectives on the seed biology of this endangered species that may inform conservation decision making and assist recovery plan development. We found that fresh seeds possess non-deep physiological dormancy, which may be alleviated via a four-week dry after-ripening treatment. The germination response of non-dormant seeds is subsequently promoted by constant rather than alternating temperatures. The optimum germination temperature range is 20–22 °C. Non-dormant seeds do not possess an absolute light requirement for germination, but are sensitive to low levels of salinity (EC₅₀ = 6.34 ppth NaCl). The narrow thermal and salinity germination niche breadths reported here suggest a specialized reproductive strategy that may require careful consideration when planning ex and in situ conservation activities.