A Lignin Molecular Brace Controls Precision Processing of Cell Walls Critical for Surface Integrity in Arabidopsis

Exploring the puzzle of reactive oxygen species acting on root hair cells

Reactive oxygen species (ROS) are essential signaling molecules that enable cells to respond rapidly to a range of stimuli. The ability of plants to recognize various stressors, incorporate a variety of environmental inputs, and initiate stress-response networks depends on ROS. Plants develop resilience and defensive systems as a result of these processes. Root hairs are central components of root biology since they increase the surface area of the root, anchor it in the soil, increase its ability to absorb water and nutrients, and foster interactions between microorganisms. In this review, we specifically focused on root hair cells and we highlighted the identification of ROS receptors, important new regulatory hubs that connect ROS production, transport, and signaling in the context of two hormonal pathways (auxin and ethylene) and under low temperature environmental input related to nutrients. As ROS play a crucial role in regulating cell elongation rates, root hairs are rapidly gaining traction as a very valuable single plant cell model for investigating ROS homeostasis and signaling. These promising findings might soon facilitate the development of plants and roots that are more resilient to environmental stressors.

Imaging plant cell walls using fluorescent stains: The beauty is in the details

Plants continuously face various environmental stressors throughout their lifetime. To be able to grow and adapt in different environments, they developed specialized tissues that allowed them to maintain a protected yet interconnected body. These tissues undergo specific primary and secondary cell wall modifications that are essential to ensure normal plant growth, adaptation and successful land colonization. The composition of cell walls can vary among different plant species, organs and tissues. The ability to remodel their cell walls is fundamental for plants to be able to cope with multiple biotic and abiotic stressors. A better understanding of the changes taking place in plant cell walls may help identify and develop new strategies as well as tools to enhance plants' survival under environmental stresses or prevent pathogen attack. Since the invention of microscopy, numerous imaging techniques have been developed to determine the composition and dynamics of plant cell walls during normal growth and in response to environmental stimuli. In this review, we discuss the main advances in imaging plant cell walls, with a particular focus on fluorescent stains for different cell wall components and their compatibility with tissue clearing techniques. Lay Description: Plants are continuously subjected to various environmental stresses during their lifespan. They evolved specialized tissues that thrive in different environments, enabling them to maintain a protected yet interconnected body. Such tissues undergo distinct primary and secondary cell wall alterations essential to normal plant growth, their adaptability and successful land colonization. Cell wall composition may differ among various plant species, organs and even tissues. To deal with various biotic and abiotic stresses, plants must have the capacity to remodel their cell walls. Gaining insight into changes that take place in plant cell walls will help identify and create novel tools and strategies to improve plants' ability to withstand environmental challenges. Multiple imaging techniques have been developed since the introduction of microscopy to analyse the composition and dynamics of plant cell walls during growth and in response to environmental changes. Advancements in plant tissue cleaning procedures and their compatibility with cell wall stains have significantly enhanced our ability to perform high-resolution cell wall imaging. At the same time, several factors influence the effectiveness of cleaning and staining plant specimens, as well as the time necessary for the process, including the specimen's size, thickness, tissue complexity and the presence of autofluorescence. In this review, we will discuss the major advances in imaging plant cell walls, with a particular emphasis on fluorescent stains for diverse cell wall components and their compatibility with tissue clearing techniques. We hope that this review will assist readers in selecting the most appropriate stain or combination of stains to highlight specific cell wall components of interest.

The single RRM domain-containing protein SARP1 is required for establishment of the separation zone in Arabidopsis

Abscission is the shedding of plant organs in response to developmental and environmental cues. Abscission involves cell separation between two neighboring cell types, residuum cells (RECs) and secession cells (SECs) in the floral abscission zone (AZ) in Arabidopsis thaliana. However, the regulatory mechanisms behind the spatial determination that governs cell separation are largely unknown. The class I KNOTTED-like homeobox (KNOX) transcription factor BREVIPEDICELLUS (BP) negatively regulates AZ cell size and number in Arabidopsis. To identify new players participating in abscission, we performed a genetic screen by activation tagging a weak complementation line of bp-3. We identified the mutant ebp1 (enhancer of BP1) displaying delayed floral organ abscission. The ebp1 mutant showed a concaved surface in SECs and abnormally stacked cells on the top of RECs, in contrast to the precisely separated surface in the wild-type. Molecular and histological analyses revealed that the transcriptional programming during cell differentiation in the AZ is compromised in ebp1. The SECs of ebp1 have acquired REC-like properties, including cuticle formation and superoxide production. We show that SEPARATION AFFECTING RNA-BINDING PROTEIN1 (SARP1) is upregulated in ebp1 and plays a role in the establishment of the cell separation layer during floral organ abscission in Arabidopsis.

A dual function of the IDA peptide in regulating cell separation and modulating plant immunity at the molecular level

The abscission of floral organs and emergence of lateral roots in Arabidopsis is regulated by the peptide ligand inflorescence deficient in abscission (IDA) and the receptor protein kinases HAESA (HAE) and HAESA-like 2 (HSL2). During these cell separation processes, the plant induces defense-associated genes to protect against pathogen invasion. However, the molecular coordination between abscission and immunity has not been thoroughly explored. Here, we show that IDA induces a release of cytosolic calcium ions (Ca2+) and apoplastic production of reactive oxygen species, which are signatures of early defense responses. In addition, we find that IDA promotes late defense responses by the transcriptional upregulation of genes known to be involved in immunity. When comparing the IDA induced early immune responses to known immune responses, such as those elicited by flagellin22 treatment, we observe both similarities and differences. We propose a molecular mechanism by which IDA promotes signatures of an immune response in cells destined for separation to guard them from pathogen attack.

Transcriptomic and physiological analysis provide new insight into seed shattering mechanism in Pennisetum alopecuroides 'Liqiu'

KEY MESSAGE

Through the histological, physiological, and transcriptome-level identification of the abscission zone of Pennisetum alopecuroides 'Liqiu', we explored the structure and the genes related to seed shattering, ultimately revealing the regulatory network of seed shattering in P. alopecuroides. Pennisetum alopecuroides is one of the most representative ornamental grass species of Pennisetum genus. It has unique inflorescence, elegant appearance, and strong stress tolerance. However, the shattering of seeds not only reduces the ornamental effect, but also hinders the seed production. In order to understand the potential mechanisms of seed shattering in P. alopecuroides, we conducted morphological, histological, physiological, and transcriptomic analyses on P. alopecuroides cv. 'Liqiu'. According to histological findings, the seed shattering of 'Liqiu' was determined by the abscission zone at the base of the pedicel. Correlation analysis showed that seed shattering was significantly correlated with cellulase, lignin, auxin, gibberellin, cytokinin and jasmonic acid. Through a combination of histological and physiological analyses, we observed the accumulation of cellulase and lignin during 'Liqiu' seed abscission. We used PacBio full-length transcriptome sequencing (SMRT) combined with next-generation sequencing (NGS) transcriptome technology to improve the transcriptome data of 'Liqiu'. Transcriptomics further identified many differential genes involved in cellulase, lignin and plant hormone-related pathways. This study will provide new insights into the research on the shattering mechanism of P. alopecuroides.

Functional Complexity on a Cellular Scale: Why In Situ Analyses Are Indispensable for Our Understanding of Lignified Tissues

Lignins are a key adaptation that enables vascular plants to thrive in terrestrial habitats. Lignin is heterogeneous, containing upward of 30 different monomers, and its function is multifarious: It provides structural support, predetermined breaking points, ultraviolet protection, diffusion barriers, pathogen resistance, and drought resilience. Recent studies, carefully characterizing lignin in situ, have started to identify specific lignin compositions and ultrastructures with distinct cellular functions, but our understanding remains fractional. We summarize recent works and highlight where further in situ lignin analysis could provide valuable insights into plant growth and adaptation. We also summarize strengths and weaknesses of lignin in situ analysis methods.

Rhytidome- and cork-type barks of holm oak, cork oak and their hybrids highlight processes leading to cork formation

BACKGROUND

The periderm is basic for land plants due to its protective role during radial growth, which is achieved by the polymers deposited in the cell walls. In most trees, like holm oak, the first periderm is frequently replaced by subsequent internal periderms yielding a heterogeneous outer bark made of a mixture of periderms and phloem tissues, known as rhytidome. Exceptionally, cork oak forms a persistent or long-lived periderm which results in a homogeneous outer bark of thick phellem cell layers known as cork. Cork oak and holm oak distribution ranges overlap to a great extent, and they often share stands, where they can hybridize and produce offspring showing a rhytidome-type bark.

RESULTS

Here we use the outer bark of cork oak, holm oak, and their natural hybrids to analyse the chemical composition, the anatomy and the transcriptome, and further understand the mechanisms underlying periderm development. We also include a unique natural hybrid individual corresponding to a backcross with cork oak that, interestingly, shows a cork-type bark. The inclusion of hybrid samples showing rhytidome-type and cork-type barks is valuable to approach cork and rhytidome development, allowing an accurate identification of candidate genes and processes. The present study underscores that abiotic stress and cell death are enhanced in rhytidome-type barks whereas lipid metabolism and cell cycle are enriched in cork-type barks. Development-related DEGs showing the highest expression, highlight cell division, cell expansion, and cell differentiation as key processes leading to cork or rhytidome-type barks.

CONCLUSION

Transcriptome results, in agreement with anatomical and chemical analyses, show that rhytidome and cork-type barks are active in periderm development, and suberin and lignin deposition. Development and cell wall-related DEGs suggest that cell division and expansion are upregulated in cork-type barks whereas cell differentiation is enhanced in rhytidome-type barks.

Dissection of the IDA promoter identifies WRKY transcription factors as abscission regulators in Arabidopsis

Plants shed organs such as leaves, petals, or fruits through the process of abscission. Monitoring cues such as age, resource availability, and biotic and abiotic stresses allow plants to abscise organs in a timely manner. How these signals are integrated into the molecular pathways that drive abscission is largely unknown. The INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) gene is one of the main drivers of floral organ abscission in Arabidopsis and is known to transcriptionally respond to most abscission-regulating cues. By interrogating the IDA promoter in silico and in vitro, we identified transcription factors that could potentially modulate IDA expression. We probed the importance of ERF- and WRKY-binding sites for IDA expression during floral organ abscission, with WRKYs being of special relevance to mediate IDA up-regulation in response to biotic stress in tissues destined for separation. We further characterized WRKY57 as a positive regulator of IDA and IDA-like gene expression in abscission zones. Our findings highlight the promise of promoter element-targeted approaches to modulate the responsiveness of the IDA signaling pathway to harness controlled abscission timing for improved crop productivity.

Immunocompromisation of wheat host by L-BSO and 2,4-DPA induces susceptibility to the fungal pathogen Fusarium oxysporum

Susceptibility is defined as the disruption of host defence systems that promotes infection or limits pathogenicity. Glutathione (GSH) is a major component of defence signalling pathways that maintain redox status and is synthesised by γ-glutamyl cysteine synthetase (γ-ECS). On the other hand, lignin acts as a barrier in the primary cell wall of vascular bundles (VBs) synthesised by phenylalanine ammonia-lyase (PAL) in the intracellular system of plants. In this study, we used two inhibitors, such as L-Buthionine-sulfoximine (BSO), which irreversibly inhibits γ-ECS, and 2,4-dichlorophenoxyacetic acid (DPA), which reduces PAL activity and leads to the induction of oxidative stress in wheat (Triticum aestivum) seedlings after exposure to Fusarium oxysporum. Seedlings treated with 1 mM L-BSO and 2,4-DPA showed high levels of hydrogen peroxide (H2O2), malondialdehyde (MDA), carbonyl (CO) content, and low activity of antioxidative enzymes [superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR)] as compared to wild-type (WT) seedlings under F. oxysporum infection. Further, the content of reduced glutathione (RGSH), ascorbate (ASC), and lignin was decreased in BSO and DPA treated seedlings as compared to WT seedlings during Fusarium infection. Moreover, treatment with BSO and DPA significantly inhibited the relative activity of γ-ECS and PAL (P ≤ 0.001) in WT seedlings during Fusarium infection, which led to disintegrated VBs and, finally, cell death. Our results demonstrate that inhibition of γ-ECS and PAL by BSO and DPA, respectively, disrupts the defence mechanisms of wheat seedlings and induces susceptibility to F. oxysporum.

Reactive oxygen species: Multidimensional regulators of plant adaptation to abiotic stress and development

Reactive oxygen species (ROS) are produced as undesirable by-products of metabolism in various cellular compartments, especially in response to unfavorable environmental conditions, throughout the life cycle of plants. Stress-induced ROS production disrupts normal cellular function and leads to oxidative damage. To cope with excessive ROS, plants are equipped with a sophisticated antioxidative defense system consisting of enzymatic and non-enzymatic components that scavenge ROS or inhibit their harmful effects on biomolecules. Nonetheless, when maintained at relatively low levels, ROS act as signaling molecules that regulate plant growth, development, and adaptation to adverse conditions. Here, we provide an overview of current approaches for detecting ROS. We also discuss recent advances in understanding ROS signaling, ROS metabolism, and the roles of ROS in plant growth and responses to various abiotic stresses.

Lignin developmental patterns and Casparian strip as apoplastic barriers: A review

Lignin and Casparian strips are two essential components of plant cells that play critical roles in plant development regulate nutrients and water across the plants cell. Recent studies have extensively investigated lignin diversity and Casparian strip formation, providing valuable insights into plant physiology. This review presents the established lignin biosynthesis pathway, as well as the developmental patterns of lignin and Casparian strip and transcriptional network associated with Casparian strip formation. It describes the biochemical and genetic mechanisms that regulate lignin biosynthesis and deposition in different plants cell types and tissues. Additionally, the review highlights recent studies that have uncovered novel lignin biosynthesis genes and enzymatic pathways, expanding our understanding of lignin diversity. This review also discusses the developmental patterns of Casparian strip in roots and their role in regulating nutrient and water transport, focusing on recent genetic and molecular studies that have identified regulators of Casparian strip formation. Previous research has shown that lignin biosynthesis genes also play a role in Casparian strip formation, suggesting that these processes are interconnected. In conclusion, this comprehensive overview provides insights into the developmental patterns of lignin diversity and Casparian strip as apoplastic barriers. It also identifies future research directions, including the functional characterization of novel lignin biosynthesis genes and the identification of additional regulators of Casparian strip formation. Overall, this review enhances our understanding of the complex and interconnected processes that drive plant growth, pathogen defense, regulation and development.

Spraying chitosan on cassava roots reduces postharvest deterioration by promoting wound healing and inducing disease resistance

Postharvest damage makes cassava roots vulnerable to pathogen infections and decay, which significantly hinders the development of the cassava industry. The objective of this study was to assess the antibacterial properties of chitosan in vitro, as well as its effect on wound healing and resistance in cassava roots. The findings demonstrated that the bacteriostatic effect of chitosan became increasingly prominent as the concentration of chitosan enhanced. Chitosan at a concentration of 0.5 mg/mL was revealed to significantly inhibit the germination of P. palmivora spores and damage to their structure. Moreover, chitosan activated the transcription of crucial genes and enzyme activities associated with the phenylpropane metabolism pathway in cassava roots, thus promoting rapid lignin accumulation and resulting in the early formation of a fracture layer. Chitosan was also found to enhance cassava root resistance by promoting the expression of pathogenesis-related proteins and the accumulation of flavonoids and total phenols. After 48 h of inoculation, cassava roots treated with chitosan exhibited a 51.4 % and 53.4 % decrease in lesion area for SC9 and SC6 varieties, respectively. The findings of this study offer a novel approach for managing postharvest deterioration of cassava roots.

The YABBY gene SHATTERING1 controls activation rather than patterning of the abscission zone in Setaria viridis

Abscission is predetermined in specialized cell layers called the abscission zone (AZ) and activated by developmental or environmental signals. In the grass family, most identified AZ genes regulate AZ anatomy, which differs among lineages. A YABBY transcription factor, SHATTERING1 (SH1), is a domestication gene regulating abscission in multiple cereals, including rice and Setaria. In rice, SH1 inhibits lignification specifically in the AZ. However, the AZ of Setaria is nonlignified throughout, raising the question of how SH1 functions in species without lignification. Crispr-Cas9 knockout mutants of SH1 were generated in Setaria viridis and characterized with histology, cell wall and auxin immunofluorescence, transmission electron microscopy, hormonal treatment and RNA-Seq analysis. The sh1 mutant lacks shattering, as expected. No differences in cell anatomy or cell wall components including lignin were observed between sh1 and the wild-type (WT) until abscission occurs. Chloroplasts degenerated in the AZ of WT before abscission, but degeneration was suppressed by auxin treatment. Auxin distribution and expression of auxin-related genes differed between WT and sh1, with the signal of an antibody to auxin detected in the sh1 chloroplast. SH1 in Setaria is required for activation of abscission through auxin signaling, which is not reported in other grass species.

Integrated analysis of transcriptome, metabolome, and histochemistry reveals the response mechanisms of different ages Panax notoginseng to root-knot nematode infection

Panax notoginseng (P. notoginseng) is an invaluable perennial medicinal herb. However, the roots of P. notoginseng are frequently subjected to severe damage caused by root-knot nematode (RKN) infestation. Although we have observed that P. notoginseng possessed adult-plant resistance (APR) against RKN disease, the defense response mechanisms against RKN disease in different age groups of P. notoginseng remain unexplored. We aimed to elucidate the response mechanisms of P. notoginseng at different stages of development to RKN infection by employing transcriptome, metabolome, and histochemistry analyses. Our findings indicated that distinct age groups of P. notoginseng may activate the phenylpropanoid and flavonoid biosynthesis pathways in varying ways, leading to the synthesis of phenolics, flavonoids, lignin, and anthocyanin pigments as both the response and defense mechanism against RKN attacks. Specifically, one-year-old P. notoginseng exhibited resistance to RKN through the upregulation of 5-O-p-coumaroylquinic acid and key genes involved in monolignol biosynthesis, such as PAL, CCR, CYP73A, CYP98A, POD, and CAD. Moreover, two-year-old P. notoginseng enhanced the resistance by depleting chlorogenic acid and downregulating most genes associated with monolignol biosynthesis, while concurrently increasing cyanidin and ANR in flavonoid biosynthesis. Three-year-old P. notoginseng reinforced its resistance by significantly increasing five phenolic acids related to monolignol biosynthesis, namely p-coumaric acid, chlorogenic acid, 1-O-sinapoyl-D-glucose, coniferyl alcohol, and ferulic acid. Notably, P. notoginseng can establish a lignin barrier that restricted RKN to the infection site. In summary, P. notoginseng exhibited a potential ability to impede the further propagation of RKN through the accumulation or depletion of the compounds relevant to resistance within the phenylpropanoid and flavonoid pathways, as well as the induction of lignification in tissue cells.

How Extracellular Reactive Oxygen Species Reach Their Intracellular Targets in Plants

Reactive oxygen species (ROS) serve as secondary messengers that regulate various developmental and signal transduction processes, with ROS primarily generated by NADPH OXIDASEs (referred to as RESPIRATORY BURST OXIDASE HOMOLOGs [RBOHs] in plants). However, the types and locations of ROS produced by RBOHs are different from those expected to mediate intracellular signaling. RBOHs produce O2•- rather than H2O2 which is relatively long-lived and able to diffuse through membranes, and this production occurs outside the cell instead of in the cytoplasm, where signaling cascades occur. A widely accepted model explaining this discrepancy proposes that RBOH-produced extracellular O2•- is converted to H2O2 by superoxide dismutase and then imported by aquaporins to reach its cytoplasmic targets. However, this model does not explain how the specificity of ROS targeting is ensured while minimizing unnecessary damage during the bulk translocation of extracellular ROS (eROS). An increasing number of studies have provided clues about eROS action mechanisms, revealing various mechanisms for eROS perception in the apoplast, crosstalk between eROS and reactive nitrogen species, and the contribution of intracellular organelles to cytoplasmic ROS bursts. In this review, we summarize these recent advances, highlight the mechanisms underlying eROS action, and provide an overview of the routes by which eROS-induced changes reach the intracellular space.

Ethylene signals modulate the survival of Arabidopsis leaf explants

BACKGROUND

Leaf explants are major materials in plant tissue cultures. Incubation of detached leaves on phytohormone-containing media, which is an important process for producing calli and regenerating plants, change their cell fate. Although hormone signaling pathways related to cell fate transition have been widely studied, other molecular and physiological events occurring in leaf explants during this process remain largely unexplored.

RESULTS

Here, we identified that ethylene signals modulate expression of pathogen resistance genes and anthocyanin accumulation in leaf explants, affecting their survival during culture. Anthocyanins accumulated in leaf explants, but were not observed near the wound site. Ethylene signaling mutant analysis revealed that ethylene signals are active and block anthocyanin accumulation in the wound site. Moreover, expression of defense-related genes increased, particularly near the wound site, implying that ethylene induces defense responses possibly by blocking pathogenesis via wounding. We also found that anthocyanin accumulation in non-wounded regions is required for drought resistance in leaf explants.

CONCLUSIONS

Our study revealed the key roles of ethylene in the regulation of defense gene expression and anthocyanin biosynthesis in leaf explants. Our results suggest a survival strategy of detached leaves, which can be applied to improve the longevity of explants during tissue culture.

Grain shattering by cell death and fracture in Eragrostis tef

Abscission, known as shattering in crop species, is a highly regulated process by which plants shed parts. Although shattering has been studied extensively in cereals and a number of regulatory genes have been identified, much diversity in the process remains to be discovered. Teff (Eragrostis tef) is a crop native to Ethiopia that is potentially highly valuable worldwide for its nutritious grain and drought tolerance. Previous work has suggested that grain shattering in Eragrostis might have little in common with other cereals. In this study, we characterize the anatomy, cellular structure, and gene regulatory control of the abscission zone (AZ) in E. tef. We show that the AZ of E. tef is a narrow stalk below the caryopsis, which is common in Eragrostis species. X-ray microscopy, scanning electron microscopy, transmission electron microscopy, and immunolocalization of cell wall components showed that the AZ cells are thin walled and break open along with programmed cell death (PCD) at seed maturity, rather than separating between cells as in other studied species. Knockout of YABBY2/SHATTERING1, documented to control abscission in several cereals, had no effect on abscission or AZ structure in E. tef. RNA sequencing analysis showed that genes related to PCD and cell wall modification are enriched in the AZ at the early seed maturity stage. These data show that E. tef drops its seeds using a unique mechanism. Our results provide the groundwork for understanding grain shattering in Eragrostis and further improvement of shattering in E. tef.

Directed growth and fusion of membrane-wall microdomains requires CASP-mediated inhibition and displacement of secretory foci

Casparian strips (CS) are aligned bands of lignin-impregnated cell walls, building an extracellular diffusion barrier in roots. Their structure profoundly differs from tight junctions (TJ), analogous structures in animals. Nonetheless, CS membrane domain (CSD) proteins 1-5 (CASP1-5) are homologues of occludins, TJ components. CASP-marked membranes display cell wall (matrix) adhesion and membrane protein exclusion. A full CASP knock-out now reveals CASPs are not needed for localized lignification, since correctly positioned lignin microdomains still form in the mutant. Ultra-structurally, however, these microdomains are disorganized, showing excessive cell wall growth, lack of exclusion zone and matrix adhesion, and impaired exocyst dynamics. Proximity-labelling identifies a Rab-GTPase subfamily, known exocyst activators, as potential CASP-interactors and demonstrate their localization and function at the CSD. We propose that CASP microdomains displace initial secretory foci by excluding vesicle tethering factors, thereby ensuring rapid fusion of microdomains into a membrane-cell wall band that seals the extracellular space.

A LcDOF5.6-LcRbohD regulatory module controls the reactive oxygen species-mediated fruitlet abscission in litchi

Reactive oxygen species (ROS) have been emerging as a key regulator in plant organ abscission. However, the mechanism underlying the regulation of ROS homeostasis in the abscission zone (AZ) is not completely established. Here, we report that a DOF (DNA binding with one finger) transcription factor LcDOF5.6 can suppress the litchi fruitlet abscission through repressing the ROS accumulation in fruitlet AZ (FAZ). The expression of LcRbohD, a homolog of the Arabidopsis RBOHs that are critical for ROS production, was significantly increased during the litchi fruitlet abscission, in parallel with an increased accumulation of ROS in FAZ. In contrast, silencing of LcRbohD reduced the ROS accumulation in FAZ and decreased the fruitlet abscission in litchi. Using in vitro and in vivo assays, we revealed that LcDOF5.6 was shown to inhibit the expression of LcRbohD via direct binding to its promoter. Consistently, silencing of LcDOF5.6 increased the expression of LcRbohD, concurrently with higher ROS accumulation in FAZ and increased fruitlet abscission. Furthermore, the expression of key genes (LcIDL1, LcHSL2, LcACO2, LcACS1, and LcEIL3) in INFLORESCENCE DEFICIENT IN ABSCISSION signaling and ethylene pathways were altered in LcRbohD-silenced and LcDOF5.6-silenced FAZ cells. Taken together, our results demonstrate an important role of the LcDOF5.6-LcRbohD module during litchi fruitlet abscission. Our findings provide new insights into the molecular regulatory network of organ abscission.

Lignin Biosynthesis in Postharvest Water Bamboo (Zizania latifolia) Shoots during Cold Storage Is Regulated by RBOH-Mediated Reactive Oxygen Species Signaling

Lignification is a major cause of senescence in fresh shoots of water bamboo (Zizania latifolia), which is a popular vegetable in southeast Asia; however, its physiological and molecular mechanisms is less understood. In the present study, lignin content and transcriptome change in postharvested water bamboo shoots under cold storage were investigated. We found that lignin significantly accumulated in the epidermis of the shoots with the increase of firmness. In the cold storage shoots, the major up-regulated genes were involved in phenylpropanoid biosynthesis, plant-pathogen interactions, and starch and sucrose metabolism. The lignin biosynthesis genes PAL, 4CL, C4H, CCoAOMT, CCR, F5H, CAD, and POD family were up-regulated during cold storage, while HCT and C3H were down-regulated. The MAPK signaling pathway was also up-regulated and respiratory burst oxidase homologue (RBOH) genes were strongly up-regulated. Therefore, we investigated the RBOH gene family and their expression profile in water bamboo shoots. The results indicated that 10 ZlRBOHs were up-regulated in cold storage shoots. Diphenyleneiodonium chloride (DPI), an inhibitor of RBOH oxidase, significantly inhibited the expression of genes involved in lignin deposition and biosynthesis, while H2O2 enhanced these processes. These results suggest that lignification of water bamboo shoots is regulated by RBOH-mediated ROS signaling.

Leaf Senescence of the Seagrass Cymodocea nodosa in Cádiz Bay, Southern Spain

Leaf decay in seagrasses is enhanced in some seasons since large green senescent beach-cast seagrass leaves are frequently recorded during autumn and winter seasons. Here, we explore if senescence is operating in seagrass leaf decay or if hydrodynamic stress is responsible for the seasonal leaf abscission. A seasonal study on the temperate seagrass Cymodocea nodosa was carried out in four locations with contrasting hydrodynamic regimes. The morphological, biomechanical and material properties of C. nodosa were measured. The force required to break the ligule was always lower than that required to break the blade. This could be considered an adaptive strategy to reduce acute drag forces and thus lessen the chance of plant uprooting. The absolute force needed to dislodge the blade at the ligule level varied with season and location, with the lowest forces recorded in autumn. This may indicate that senescence is operating in this species. On the other hand, the minimum estimated failure velocities for leaf abscission were also recorded in autumn. Consequently, this may cause the premature shedding of leaves in this season before the senescence process has finished and can probably explain the occurrence of green beach-cast seagrass leaves usually found during autumn and winter.

RNA-Seq and Genome-Wide Association Studies Reveal Potential Genes for Rice Seed Shattering

The loss of the shattering ability is one of the key events in rice domestication. The strength of the seed shattering ability is closely related to the harvest yield and the adaptability of modern mechanical harvesting methods. In this study, using a population of 587 natural rice cultivars, quantitative trait loci associated with seed shattering were detected by genome-wide association studies (GWASs). We consider the quantitative trait loci (QTLs) qBTS1 and qBTS3 to be the key loci for seed shattering in rice. Additionally, the abscission zone (AZ) and nonabscission zone (NAZ) of materials with a loss of shattering (DZ129) and easy shattering (W517) were subjected to RNA-Seq, and high-quality differential expression profiles were obtained. The AZ-specific differentially expressed genes (DEGs) of W517 were significantly enriched in plant hormone signal transduction, while the AZ-specific DEGs of DZ129 were enriched in phenylpropanoid biosynthesis. We identified candidate genes for the lignin-associated laccase precursor protein (LOC_Os01g63180) and the glycoside hydrolase family (LOC_Os03g14210) in the QTLs qBTS1 (chromosome 1) and qBTS3 (chromosome 3), respectively. In summary, our findings lay the foundation for the further cloning of qBTS1 and qBTS3, which would provide new insights into seed shattering in rice.

Floral organ abscission in Arabidopsis requires the combined activities of three TALE homeodomain transcription factors

Floral organ abscission is a separation process in which sepals, petals, and stamens detach from the plant at abscission zones. Here, we investigated the collective role of three amino-acid-loop-extension (TALE) homeobox genes ARABIDOPSIS THALIANA HOMEOBOX GENE1 (ATH1), KNAT6 (for KNOTTED LIKE from Arabidopsis thaliana) and KNAT2, which form a module that patterns boundaries under the regulation of BLADE-ON-PETIOLE 1 and 2 (BOP1/2) co-activators. These TALE homeodomain transcription factors were shown to maintain boundaries in the flower, functioning as a unit to coordinate the growth, patterning, and activity of abscission zones. Together with BOP1 and BOP2, ATH1 and its partners KNAT6 and KNAT2 collectively contribute to the differentiation of lignified and separation layers of the abscission zone. The genetic interactions of BOP1/2 and ATH1 with INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) were also explored. We showed that BOP1/2 co-activators and ATH1 converge with the IDA signalling pathway to promote KNAT6 and KNAT2 expression in the abscission zone and cell separation. ATH1 acts as a central regulator in floral organ abscission as it controls the expression of other TALE genes in abscission zone cells.

Regulation of PaRBOH1-mediated ROS production in Norway spruce by Ca2+ binding and phosphorylation

Plant respiratory burst oxidase homologs (RBOHs) are plasma membrane-localized NADPH oxidases that generate superoxide anion radicals, which then dismutate to H₂O₂, into the apoplast using cytoplasmic NADPH as an electron donor. PaRBOH1 is the most highly expressed RBOH gene in developing xylem as well as in a lignin-forming cell culture of Norway spruce (Picea abies L. Karst.). Since no previous information about regulation of gymnosperm RBOHs exist, our aim was to resolve how PaRBOH1 is regulated with a focus on phosphorylation. The N-terminal part of PaRBOH1 was found to contain several putative phosphorylation sites and a four-times repeated motif with similarities to the Botrytis-induced kinase 1 target site in Arabidopsis AtRBOHD. Phosphorylation was indicated for six of the sites in in vitro kinase assays using 15 amino-acid-long peptides for each of the predicted phosphotarget site in the presence of protein extracts of developing xylem. Serine and threonine residues showing positive response in the peptide assays were individually mutated to alanine (kinase-inactive) or to aspartate (phosphomimic), and the wild type PaRBOH1 and the mutated constructs transfected to human kidney embryogenic (HEK293T) cells with a low endogenous level of extracellular ROS production. ROS-producing assays with HEK cells showed that Ca²⁺ and phosphorylation synergistically activate the enzyme and identified several serine and threonine residues that are likely to be phosphorylated including a novel phosphorylation site not characterized in other plant species. These were further investigated with a phosphoproteomic study. Results of Norway spruce, the first gymnosperm species studied in relation to RBOH regulation, show that regulation of RBOH activity is conserved among seed plants.

Genome-Wide Identification of the DOF Gene Family Involved in Fruitlet Abscission in Areca catechu L

Fruitlet abscission frequently occurs in Areca catechu L. and causes considerable production loss. However, the inducement mechanism of fruitlet abscission remains mysterious. In this study, we observed that the cell architecture in the abscission zone (AZ) was distinct with surrounding tissues, and varied obviously before and after abscission. Transcriptome analysis of the “about-to-abscise” and “non-abscised” AZs were performed in A. catechu, and the genes encoding the plant-specific DOF (DNA-binding with one finger) transcription factors showed a uniform up-regulation in AZ, suggesting a role of the DOF transcription in A. catechu fruitlet abscission. In total, 36 members of the DOF gene family distributed in 13 chromosomes were identified from the A. catechu genome. The 36 AcDOF genes were classified into nine subgroups based on phylogenic analysis. Six of them showed an AZ-specific expression pattern, and their expression levels varied according to the abscission process. In total, nine types of phytohormone response cis-elements and five types of abiotic stress related cis-elements were identified in the promoter regions of the AcDOF genes. In addition, histochemical staining showed that lignin accumulation of vascular bundles in AZ was significantly lower than that in pedicel and mesocarp, indicating the specific characteristics of the cell architecture in AZ. Our data suggests that the DOF transcription factors might play a role in fruitlet abscission regulation in A. catechu.

Cell-wall microdomain remodeling controls crucial developmental processes

Plant cell walls display cellular and subcellular specificities. At the subcellular level, wall regional territories with specific compositions are necessary for macroscopic developmental processes. These regional specificities were named differently throughout the years, and are unified here under the term 'cell-wall microdomains' that define the local composition and organization of wall polymers underlying territories of wall loosening and/or softening or stiffening. We review the occurrence and developmental role of wall microdomains in different cell types. We primarily focus on the contribution of two categories of wall-remodeling molecular actors: fine-tuning of homogalacturonan (HG; pectin) demethylesterification patterns and two classes of oxidoreductases [class III peroxidases (CIII PRXs) and laccases (LACs)], but we also highlight two different molecular scaffolds recently identified for positioning specific CIII PRXs.

Cytochrome b 5: A versatile electron carrier and regulator for plant metabolism

Cytochrome b ₅ (CB5) is a small heme-binding protein, known as an electron donor delivering reducing power to the terminal enzymes involved in oxidative reactions. In plants, the CB5 protein family is substantially expanded both in its isoform numbers and cellular functions, compared to its yeast and mammalian counterparts. As an electron carrier, plant CB5 proteins function not only in fatty acid desaturation, hydroxylation and elongation, but also in the formation of specialized metabolites such as flavonoids, phenolic esters, and heteropolymer lignin. Furthermore, plant CB5s are found to interact with different non-catalytic proteins such as ethylene signaling regulator, cell death inhibitor, and sugar transporters, implicating their versatile regulatory roles in coordinating different metabolic and cellular processes, presumably in respect to the cellular redox status and/or carbon availability. Compared to the plentiful studies on biochemistry and cellular functions of mammalian CB5 proteins, the cellular and metabolic roles of plant CB5 proteins have received far less attention. This article summarizes the fragmentary information pertaining to the discovery of plant CB5 proteins, and discusses the conventional and peculiar functions that plant CB5s might play in different metabolic and cellular processes. Gaining comprehensive insight into the biological functions of CB5 proteins could offer effective biotechnological solutions to tailor plant chemodiversity and cellular responses to environment stimuli.

MSD2-mediated ROS metabolism fine-tunes the timing of floral organ abscission in Arabidopsis

The timely removal of end-of-purpose flowering organs is as essential for reproduction and plant survival as timely flowering. Despite much progress in understanding the molecular mechanisms of floral organ abscission, little is known about how various environmental factors are integrated into developmental programmes that determine the timing of abscission. Here, we investigated whether reactive oxygen species (ROS), mediators of various stress-related signalling pathways, are involved in determining the timing of abscission and, if so, how they are integrated with the developmental pathway in Arabidopsis thaliana. MSD2, encoding a secretory manganese superoxide dismutase, was preferentially expressed in the abscission zone of flowers, and floral organ abscission was accelerated by the accumulation of ROS in msd2 mutants. The expression of the genes encoding the receptor-like kinase HAESA (HAE) and its cognate peptide ligand INFLORESCENCE DEFICIENT IN ABSCISSION (IDA), the key signalling components of abscission, was accelerated in msd2 mutants, suggesting that MSD2 acts upstream of IDA-HAE. Further transcriptome and pharmacological analyses revealed that abscisic acid and nitric oxide facilitate abscission by regulating the expression of IDA and HAE during MSD2-mediated signalling. These results suggest that MSD2-dependent ROS metabolism is an important regulatory point integrating environmental stimuli into the developmental programme leading to abscission.

Development and diversity of lignin patterns

Different patterns of lignified cell walls are associated with diverse functions in a variety of plant tissues. These functions rely on the stiffness and hydrophobicity that lignin polymers impart to the cell wall. The precise pattern of subcellular lignin deposition is critical for the structure-function relationship in each lignified cell type. Here, we describe the role of xylem vessels as water pipes, Casparian strips as apoplastic barriers, and the role of asymmetrically lignified endocarp b cells in exploding seed pods. We highlight similarities and differences in the genetic mechanisms underpinning local lignin deposition in these diverse cell types. By bringing together examples from different developmental contexts and different plant species, we propose that comparative approaches can benefit our understanding of lignin patterning mechanisms.

Explosive seed dispersal depends on SPL7 to ensure sufficient copper for localized lignin deposition via laccases

The sudden explosion of seed pods in popping cress (Cardamine hirsuta) takes less than 3 ms to accelerate seeds away from the plant. This explosive mechanism relies on polar deposition of the cell-wall polymer lignin. To investigate the genetic basis for polar lignin deposition, we conducted a mutant screen and identified SQUAMOSA PROMOTER-BINDING PROTEIN-LIKE 7 (SPL7)—a transcriptional regulator of copper homeostasis. We discovered three multicopper laccases, LAC4, 11, and 17, that precisely colocalize with, and are required for, the polar deposition of lignin in explosive seed pods. Activity of these three laccases depends on SPL7 to acclimate to copper deficiency. Our findings demonstrate how mineral nutrition is integrated with polar lignin deposition to facilitate dispersal.

Exploding seed pods evolved in the Arabidopsis relative Cardamine hirsuta via morphomechanical innovations that allow the storage and rapid release of elastic energy. Asymmetric lignin deposition within endocarpb cell walls is one such innovation that is required for explosive seed dispersal and evolved in association with the trait. However, the genetic control of this novel lignin pattern is unknown. Here, we identify three lignin-polymerizing laccases, LAC4, 11, and 17, that precisely colocalize with, and are redundantly required for, asymmetric lignification of endocarpb cells. By screening for C. hirsuta mutants with less lignified fruit valves, we found that loss of function of the transcription factor gene SQUAMOSA PROMOTER-BINDING PROTEIN-LIKE 7 (SPL7) caused a reduction in endocarpb cell-wall lignification and a consequent reduction in seed dispersal range. SPL7 is a conserved regulator of copper homeostasis and is both necessary and sufficient for copper to accumulate in the fruit. Laccases are copper-requiring enzymes. We discovered that laccase activity in endocarpb cell walls depends on the SPL7 pathway to acclimate to copper deficiency and provide sufficient copper for lignin polymerization. Hence, SPL7 links mineral nutrition to efficient dispersal of the next generation.

Identification of Golovinomyces artemisiae Causing Powdery Mildew, Changes in Chlorophyll Fluorescence Parameters, and Antioxidant Levels in Artemisia selengensis

Artemisia selengensis Turcz. is a valuable edible and medicinal vegetable crop widely cultivated in Northeast China. Powdery mildew (PM) disease occurs during field and greenhouse cultivation, resulting in production losses and quality deterioration. The pathogen in A. selengensis was Golovinomyces artemisiae identified using optical microscopic and scanning electron microscopic observations, morphological identification, and molecular biological analyses. Parameters of chlorophyll fluorescence (ChlF) and antioxidant system responses as well as callose and lignin contents in A. selengensis were analyzed with inoculating G. artemisiae. Obvious of PM-infected leaves were confirmed with significantly lower values in electron transport rate (ETR), non-photochemical quenching (NPQ), photochemical quenching (qP), and actual photochemical efficiency [Y(II)], but higher values in non-adjusting energy dissipation yield [Y(NO)], supposed that maximal photosystem II quantum yield (Fv/Fm) value and images could be used to monitor PM degree on infectedA. selengensis. In addition, malondialdehyde (MDA), superoxide anion (O2 -), callose, lignin contents, and peroxidase (POD) activity increased, while superoxide dismutase (SOD) activity, catalase (CAT) activity, and ascorbic acid (AsA) content decreased significantly in infected leaves compared to mock-inoculated leaves, indicated that lignin and protective enzymes are the key indicators for detecting PM resistant in A. selengensis. These results suggest that PM caused by G. artemisiae disrupted the photosynthetic capacity and induced imbalance of antioxidant system inA. selengensis. The findings were of great significance for designing a feasible approach to effectively prevent and control the PM disease in A. selengensis as well as in other vegetable crops.

Hydrogen sulfide inhibits the abscission of tomato pedicel through reconstruction of a basipetal auxin gradient

Abscission is an important developmental process and an essential agricultural trait. Auxin and ethylene are two phytohormones with important roles in the complex, but still elusive signaling network of abscission. Here, we found that hydrogen sulfide (H₂S), a newly identified gasotransmitter, inhibits the initiation of tomato pedicel abscission. The underlying mechanism was explored through transcriptome profile analysis in various pedicel tissues with or without H₂S treatment in the early abscission stage. The data suggested that H₂S strongly influences the global transcription of pedicel tissues, exerts differential expression regulation along the pedicel, and markedly influences both the auxin and ethylene signaling pathways. Computational analysis revealed that H₂S reconstructs a basipetal auxin gradient along the pedicel at 4 h after treatment; this finding was further substantiated by the GUS-staining results of DR5::GUS pedicels. The inhibitory effect of H₂S to the ethylene signaling pathway might be an indirect action. Moreover, the subtilisin-like proteinase family members involved in the release of peptide signal molecules are critical components of the abscission signaling network downstream of auxin and ethylene.

MSD2, an apoplastic Mn-SOD, contributes to root skotomorphogenic growth by modulating ROS distribution in Arabidopsis

Reactive oxygen species (ROS) play essential roles as a second messenger in various physiological processes in plants. Due to their oxidative nature, ROS can also be harmful. Thus, the generation and homeostasis of ROS are tightly controlled by multiple enzymes. Membrane-localized NADPH oxidases are well known to generate ROS during developmental and stress responses, but the metabolic pathways of the superoxide (O2^-) generated by them in the apoplast are poorly understood, and the identity of the apoplastic superoxide dismutase (SOD) is unknown in Arabidopsis. Here, we show that a putative manganese SOD, MSD2 is secreted and possesses a SOD activity that can be inhibited by nitration at tyrosine 68. The expression of MSD2 in roots is light condition-dependent, suggesting that MSD2 may act on ROS metabolism in roots during the light-to-dark transition. Root architecture is governed by ROS distribution that exhibits opposite gradient of H₂O₂ and O2^-, which is indeed altered in etiolated msd2 mutants and accompanied by changes in the onset of differentiation. These results provide a missing link in our understanding of ROS metabolism and suggest that MSD2 plays a role in root skotomorphogenesis by regulating ROS distribution, thereby playing a pivotal role in plant growth and development.

Dynamics of pectic homogalacturonan in cellular morphogenesis and adhesion, wall integrity sensing and plant development

Homogalacturonan (HG) is the most abundant pectin subtype in plant cell walls. Although it is a linear homopolymer, its modification states allow for complex molecular encoding. HG metabolism affects its structure, chemical properties, mobility and binding capacity, allowing it to interact dynamically with other polymers during wall assembly and remodelling and to facilitate anisotropic cell growth, cell adhesion and separation, and organ morphogenesis. HGs have also recently been found to function as signalling molecules that transmit information about wall integrity to the cell. Here we highlight recent advances in our understanding of the dual functions of HG as a dynamic structural component of the cell wall and an initiator of intrinsic and environmental signalling. We also predict how HG might interconnect the cell wall, plasma membrane and intracellular components with transcriptional networks to regulate plant growth and development.

Apoplastic class III peroxidases PRX62 and PRX69 promote Arabidopsis root hair growth at low temperature

Root Hairs (RHs) growth is influenced by endogenous and by external environmental signals that coordinately regulate its final cell size. We have recently determined that RH growth was unexpectedly boosted when Arabidopsis thaliana seedlings are cultivated at low temperatures. It was proposed that RH growth plasticity in response to low temperature was linked to a reduced nutrient availability in the media. Here, we explore the molecular basis of this RH growth response by using a Genome Wide Association Study (GWAS) approach using Arabidopsis thaliana natural accessions. We identify the poorly characterized PEROXIDASE 62 (PRX62) and a related protein PRX69 as key proteins under moderate low temperature stress. Strikingly, a cell wall protein extensin (EXT) reporter reveals the effect of peroxidase activity on EXT cell wall association at 10 °C in the RH apical zone. Collectively, our results indicate that PRX62, and to a lesser extent PRX69, are key apoplastic PRXs that modulate ROS-homeostasis and cell wall EXT-insolubilization linked to RH elongation at low temperature.

Regulatory Modules Involved in the Degradation and Modification of Host Cell Walls During Cuscuta campestris Invasion

Haustoria of parasitic plants have evolved sophisticated traits to successfully infect host plants. The degradation and modification of host cell walls enable the haustorium to effectively invade host tissues. This study focused on two APETALA2/ETHYLENE RESPONSE FACTOR (ERF) genes and a set of the cell wall enzyme genes principally expressed during the haustorial invasion of Cuscuta campestris Yuncker. The orthogroups of the TF and cell wall enzyme genes have been implicated in the cell wall degradation and modification activities in the abscission of tomatoes, which are currently the phylogenetically closest non-parasitic model species of Cuscuta species. Although haustoria are generally thought to originate from root tissues, our results suggest that haustoria have further optimized invasion potential by recruiting regulatory modules from other biological processes.

Production of reactive oxygen species by PuRBOHF is critical for stone cell development in pear fruit

The production of reactive oxygen species (ROS) by NADPH oxidase, which is also referred to as respiratory burst oxidase homolog (RBOH), affects several processes in plants. However, the role of RBOHs in cell wall lignification is not well understood. In this study, we show that PuRBOHF, an RBOH isoform, plays an important role in secondary wall formation in pear stone cells. ROS were closely associated with lignin deposition and stone cell formation according to microscopy data. In addition, according to the results of an in situ hybridization analysis, the stage-specific expression of PuRBOHF was higher in stone cells than in cells of other flesh tissues. Inhibitors of RBOH activity suppressed ROS accumulation and stone cell lignification in pear fruit. Moreover, transient overexpression of PuRBOHF caused significant changes in the amount of ROS and lignin that accumulated in pear fruit and flesh calli. We further showed that PuMYB169 regulates PuRBOHF expression, while PuRBOHF-derived ROS induces the transcription of PuPOD2 and PuLAC2. The findings of this study indicate that PuRBOHF-mediated ROS production, which is regulated by a lignin-related transcriptional network, is essential for monolignol polymerization and stone cell formation in pear fruit.

A Non-Shedding Fruit Elaeis oleifera Palm Reveals Perturbations to Hormone Signaling, ROS Homeostasis, and Hemicellulose Metabolism

The developmentally programmed loss of a plant organ is called abscission. This process is characterized by the ultimate separation of adjacent cells in the abscission zone (AZ). The discovery of an American oil palm (Elaeis oleifera) variant that does not shed its has allowed for the study of the mechanisms of ripe fruit abscission in this species. A comparative transcriptome analysis was performed to compare the fruit AZs of the non-shedding E. oleifera variant to an individual of the same progeny that sheds its ripe fruit normally. The study provides evidence for widespread perturbation to gene expression in the AZ of the non-shedding variant, compared to the normal fruit-shedding control, and offers insight into abscission-related functions. Beyond the genes with known or suspected roles during organ abscission or indehiscence that were identified, a list of genes with hormone-related functions, including ethylene, jasmonic acid, abscisic acid, cytokinin and salicylic acid, in addition to reactive oxygen species (ROS) metabolism, transcriptional responses and signaling pathways, was compiled. The results also allowed a comparison between the ripe fruit abscission processes of the African and American oil palm species at the molecular level and revealed commonalities with environmental stress pathways.

Transcriptome Analysis Reveals Potential Mechanisms for Ethylene-Inducible Pedicel–Fruit Abscission Zone Activation in Non-Climacteric Sweet Cherry (Prunus avium L.)

The harvesting of sweet cherry (Prunus avium L.) fruit is a labor-intensive process. The mechanical harvesting of sweet cherry fruit is feasible; however, it is dependent on the formation of an abscission zone at the fruit–pedicel junction. The natural propensity for pedicel—fruit abscission zone (PFAZ) activation varies by cultivar, and the general molecular basis for PFAZ activation is not well characterized. In this study, ethylene-inducible change in pedicel fruit retention force (PFRF) was recorded in a developmental time-course with a concomitant analysis of the PFAZ transcriptome from three sweet cherry cultivars. In ‘Skeena’, mean PFRF for both control and treatment fruit dropped below the 0.40 kg-force (3.92 N) threshold for mechanical harvesting, indicating the activation of a discrete PFAZ. In ‘Bing’, mean PFRF for both control and treatment groups decreased over time. However, a mean PFRF conducive to mechanical harvesting was achieved only in the ethylene-treated fruit. While in ‘Chelan’ the mean PFRF of the control and treatment groups did not meet the threshold required for efficient mechanical harvesting. Transcriptome analysis of the PFAZ region followed by the functional annotation, differential expression analysis, and gene ontology (GO) enrichment analyses of the data facilitated the identification of phytohormone-responsive and abscission-related transcripts, as well as processes that exhibited differential expression and enrichment in a cultivar-dependent manner over the developmental time-course. Additionally, read alignment-based variant calling revealed several short variants in differentially expressed genes, associated with enriched gene ontologies and associated metabolic processes, lending potential insight into the genetic basis for different abscission responses between the cultivars. These results provide genetic targets for the induction or inhibition of PFAZ activation, depending on the desire to harvest the fruit with or without the stem attached. Understanding the genetic mechanisms underlying the development of the PFAZ will inform future cultivar development while laying a foundation for mechanized sweet cherry harvest.

Seed Shattering: A Trait of Evolutionary Importance in Plants

Seed shattering refers to the natural shedding of seeds when they ripe, a phenomenon typically observed in wild and weedy plant species. The timing and extent of this phenomenon varies considerably among plant species. Seed shattering is primarily a genetically controlled trait; however, it is significantly influenced by environmental conditions, management practices and their interactions, especially in agro-ecosystems. This trait is undesirable in domesticated crops where consistent efforts have been made to minimize it through conventional and molecular breeding approaches. However, this evolutionary trait serves as an important fitness and survival mechanism for most weeds that utilize it to ensure efficient dispersal of their seeds, paving the way for persistent soil seedbank development and sustained future populations. Weeds have continuously evolved variations in seed shattering as an adaptation under changing management regimes. High seed retention is common in many cropping weeds where weed maturity coincides with crop harvest, facilitating seed dispersal through harvesting operations, though some weeds have notoriously high seed shattering before crop harvest. However, high seed retention in some of the most problematic agricultural weed species such as annual ryegrass (Lolium rigidum), wild radish (Raphanus raphanistrum), and weedy amaranths (Amaranthus spp.) provides an opportunity to implement innovative weed management approaches such as harvest weed seed control, which aims at capturing and destroying weed seeds retained at crop harvest. The integration of such management options with other practices is important to avoid the rapid evolution of high seed shattering in target weed species. Advances in genetics and molecular biology have shown promise for reducing seed shattering in important crops, which could be exploited for manipulating seed shattering in weed species. Future research should focus on developing a better understanding of various seed shattering mechanisms in plants in relation to changing climatic and management regimes.

Localised laccase activity modulates distribution of lignin polymers in gymnosperm compression wood

The woody stems of coniferous gymnosperms produce specialised compression wood to adjust the stem growth orientation in response to gravitropic stimulation. During this process, tracheids develop a compression-wood-specific S₂ L cell wall layer with lignins highly enriched with p-hydroxyphenyl (H)-type units derived from H-type monolignol, whereas lignins produced in the cell walls of normal wood tracheids are exclusively composed of guaiacyl (G)-type units from G-type monolignol with a trace amount of H-type units. We show that laccases, a class of lignin polymerisation enzymes, play a crucial role in the spatially organised polymerisation of H-type and G-type monolignols during compression wood formation in Japanese cypress (Chamaecyparis obtusa). We performed a series of chemical-probe-aided imaging analysis on C. obtusa compression wood cell walls, together with gene expression, protein localisation and enzymatic assays of C. obtusa laccases. Our data indicated that CoLac1 and CoLac3 with differential oxidation activities towards H-type and G-type monolignols were precisely localised to distinct cell wall layers in which H-type and G-type lignin units were preferentially produced during the development of compression wood tracheids. We propose that, not only the spatial localisation of laccases, but also their biochemical characteristics dictate the spatial patterning of lignin polymerisation in gymnosperm compression wood.

IDA (INFLORESCENCE DEFICIENT IN ABSCISSION)-like peptides and HAE (HAESA)-like receptors regulate corolla abscission in Nicotiana benthamiana flowers

BACKGROUND

Abscission is an active, organized, and highly coordinated cell separation process enabling the detachment of aerial organs through the modification of cell-to-cell adhesion and breakdown of cell walls at specific sites on the plant body known as abscission zones. In Arabidopsis thaliana, abscission of floral organs and cauline leaves is regulated by the interaction of the hormonal peptide INFLORESCENCE DEFICIENT IN ABSCISSION (IDA), a pair of redundant receptor-like protein kinases, HAESA (HAE) and HAESA-LIKE2 (HSL2), and SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) co-receptors. However, the functionality of this abscission signaling module has not yet been demonstrated in other plant species.

RESULTS

The expression of the pair of NbenIDA1 homeologs and the receptor NbenHAE.1 was supressed at the base of the corolla tube by the inoculation of two virus-induced gene silencing (VIGS) constructs in Nicotiana benthamiana. These gene suppression events arrested corolla abscission but did not produce any obvious effect on plant growth. VIGS plants retained a higher number of corollas attached to the flowers than control plants, an observation related to a greater corolla breakstrength. The arrest of corolla abscission was associated with the preservation of the parenchyma tissue at the base of the corolla tube that, in contrast, was virtually collapsed in normal corollas. In contrast, the inoculation of a viral vector construct that increased the expression of NbenIDA1A at the base of the corolla tube negatively affected the growth of the inoculated plants accelerating the timing of both corolla senescence and abscission. However, the heterologous ectopic overexpression of citrus CitIDA3 and Arabidopsis AtIDA in N. benthamiana did not alter the standard plant phenotype suggesting that the proteolytic processing machinery was unable to yield active peptides.

CONCLUSION

Here, we demonstrate that the pair of NbenIDA1 homeologs encoding small peptides of the IDA-like family and the receptor NbenHAE.1 control cellular breakdown at the base of the corolla tube awhere an adventitious AZ should be formed and, therefore, corolla abscission in N. benthamiana flowers. Altogether, our results provide the first evidence supporting the notion that the IDA-HAE/HSL2 signaling module is conserved in angiosperms.

Cold stress triggers premature fruit abscission through ABA-dependent signal transduction in early developing apple

Fruit abscission is a complex physiological process that is regulated by internal and environmental factors. During early development, apple fruit are exposed to extreme temperature fluctuations that are associated with premature fruit drop; however, their effect on fruit abscission is largely unknown. We hypothesized that fruit abscission is triggered by cold stress and investigated the molecular basis of premature fruit drop using RNA-Seq and metabolomics data from apple fruit undergoing abscission following cold stress in the field. Genes responsive to abscisic acid signaling and cell wall degradation were upregulated during abscission, consistent with the increased abscisic acid concentrations detected by liquid chromatography-mass spectrometry. We performed ex vivo cold shock experiments with excised tree subunits consisting of a branch, pedicel, and fruit. Abscission induction occurred in the cold-stressed subunits with concurrent upregulation of abscisic acid biosynthesis (MdNCED1) and metabolism (MdCYP707A) genes, and ethylene biosynthesis (MdACS1) and receptor (MdETR2) genes in the pedicel. Another key finding was the activation of cytoplasmic streaming in abscission-zone cells detected by electron microscopy. Our results provide a novel insight into the molecular basis of fruit abscission physiology in response to cold stress in apple.

EPIP-Evoked Modifications of Redox, Lipid, and Pectin Homeostasis in the Abscission Zone of Lupine Flowers

Yellow lupine is a great model for abscission-related research given that excessive flower abortion reduces its yield. It has been previously shown that the EPIP peptide, a fragment of LlIDL (INFLORESCENCE DEFICIENT IN ABSCISSION) amino-acid sequence, is a sufficient molecule to induce flower abortion, however, the question remains: What are the exact changes evoked by this peptide locally in abscission zone (AZ) cells? Therefore, we used EPIP peptide to monitor specific modifications accompanied by early steps of flower abscission directly in the AZ. EPIP stimulates the downstream elements of the pathway-HAESA and MITOGEN-ACTIVATED PROTEIN KINASE6 and induces cellular symptoms indicating AZ activation. The EPIP treatment disrupts redox homeostasis, involving the accumulation of H2O2 and upregulation of the enzymatic antioxidant system including superoxide dismutase, catalase, and ascorbate peroxidase. A weakening of the cell wall structure in response to EPIP is reflected by pectin demethylation, while a changing pattern of fatty acids and acyl lipids composition suggests a modification of lipid metabolism. Notably, the formation of a signaling molecule-phosphatidic acid is induced locally in EPIP-treated AZ. Collectively, all these changes indicate the switching of several metabolic and signaling pathways directly in the AZ in response to EPIP, which inevitably leads to flower abscission.

Biosynthetic labeling with 3-O-propargylcaffeyl alcohol reveals in vivo cell-specific patterned lignification in loquat fruits during development and postharvest storage

Lignification is a major cell wall modification that often results in the formation of sophisticated subcellular patterns during plant development or in response to environmental stresses. Precise localization of the spatiotemporal deposition of lignin is of great importance for revealing the lignification regulatory mechanism of individual cells. In loquat fruits, lignification typically increases the flesh lignin content and firmness, reducing their edibility and processing quality. However, the precise localization of the spatiotemporal active zones of lignification inside loquat fruit flesh remains poorly understood, and little is known about the contribution of patterned lignification to cell wall structure dynamics and the subsequent fruit-quality deterioration. Here, we performed an emerging bioorthogonal chemistry imaging technique to trace the in vivo patterned lignification dynamics in cells of loquat fruit flesh during development and storage. In developing fruits, lignified cells (LCs) and vascular bundles (VBs) were the zones of active lignification, and ring-like LCs deposited lignin at both the inner wall layer and the outer periphery sides. The domino effect of the generation of LCs was preliminarily visualized. In mature fruits, the newly formed lignin in the flesh of fruits during storage was specifically deposited in the corners and middle lamellae of parenchyma cells surrounding the VBs, resulting in the development of a reticular structure. Based on the findings, distinct spatiotemporal patterned lignification modes for different flesh cells in loquat fruits were proposed. These findings provide loquat lignification dynamics together with spatiotemporal data that can improve our understanding of the lignification process in planta.

Multi-scale comparative transcriptome analysis reveals key genes and metabolic reprogramming processes associated with oil palm fruit abscission

BACKGROUND

Fruit abscission depends on cell separation that occurs within specialized cell layers that constitute an abscission zone (AZ). To determine the mechanisms of fleshy fruit abscission of the monocot oil palm (Elaeis guineensis Jacq.) compared with other abscission systems, we performed multi-scale comparative transcriptome analyses on fruit targeting the developing primary AZ and adjacent tissues.

RESULTS

Combining between-tissue developmental comparisons with exogenous ethylene treatments, and naturally occurring abscission in the field, RNAseq analysis revealed a robust core set of 168 genes with differentially regulated expression, spatially associated with the ripe fruit AZ, and temporally restricted to the abscission timing. The expression of a set of candidate genes was validated by qRT-PCR in the fruit AZ of a natural oil palm variant with blocked fruit abscission, which provides evidence for their functions during abscission. Our results substantiate the conservation of gene function between dicot dry fruit dehiscence and monocot fleshy fruit abscission. The study also revealed major metabolic transitions occur in the AZ during abscission, including key senescence marker genes and transcriptional regulators, in addition to genes involved in nutrient recycling and reallocation, alternative routes for energy supply and adaptation to oxidative stress.

CONCLUSIONS

The study provides the first reference transcriptome of a monocot fleshy fruit abscission zone and provides insight into the mechanisms underlying abscission by identifying key genes with functional roles and processes, including metabolic transitions, cell wall modifications, signalling, stress adaptations and transcriptional regulation, that occur during ripe fruit abscission of the monocot oil palm. The transcriptome data comprises an original reference and resource useful towards understanding the evolutionary basis of this fundamental plant process.

Highlighting reactive oxygen species as multitaskers in root development

Reactive oxygen species (ROS) are naturally produced by several redox reactions during plant regular metabolism such as photosynthesis and respiration. Due to their chemical properties and high reactivity, ROS were initially described as detrimental for cells during oxidative stress. However, they have been further recognized as key players in numerous developmental and physiological processes throughout the plant life cycle. Recent studies report the important role of ROS as growth regulators during plant root developmental processes such as in meristem maintenance, in root elongation, and in lateral root, root hair, endodermis, and vascular tissue differentiation. All involve multifaceted interplays between steady-state levels of ROS with transcriptional regulators, phytohormones, and nutrients. In this review, we attempt to summarize recent findings about how ROS are involved in multiple stages of plant root development during cell proliferation, elongation, and differentiation.

Comparative genomic analysis of superoxide dismutase (SOD) genes in three Rosaceae species and expression analysis in Pyrus bretschneideri

Superoxide dismutases (SODs) are antioxidant enzymes that play a critical role in the polymerization of lignin monomers. Although current research has indicated that SODs are involved in plant growth and development, information on SODs in pear (Pyrus bretschneideri) and their function in lignin formation is scarce. In this study, 25 SODs, containing three kinds of plant SODs (Cu/Zn-SODs, Mn-SODs, and Fe-SODs), were identified from three Rosaceae species, and 11 of these genes were found in pear. According to the evolutionary analysis, the genes were divided into four subgroups, the division of which is consistent with the intron-exon and conserved motif analyses. These PbSODs were randomly scattered across 7 chromosomes. We have analysed the conserved domains and gene family evolution and predicted the cis-elements of the promoter. Ka/Ks analysis pointed that SOD genes mainly underwent purifying selection. Subsequently, the expression patterns of 11 PbSODs were examined in different tissues, at different developmental periods, in different pear varieties and under different hormone treatments. Gene expression analysis showed that PbCSD3 exhibited transcript levels consistent with the typical changes in lignin content. The changes in SOD activity and hydrogen peroxide (H₂O₂) content combined with the results of a spatio-temporal expression analysis showed that PbCSD3 was a candidate gene in reactive oxygen species (ROS) metabolism during the lignification of pear stone cells. Thus, our research reveals the evolutionary features of the SOD family in Rosaceae species and provide useful information for analysis of functional genome of the SOD family in pear.

SUPPLEMENTARY INFORMATION

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The plant cell wall: Biosynthesis, construction, and functions

The plant cell wall is composed of multiple biopolymers, representing one of the most complex structural networks in nature. Hundreds of genes are involved in building such a natural masterpiece. However, the plant cell wall is the least understood cellular structure in plants. Due to great progress in plant functional genomics, many achievements have been made in uncovering cell wall biosynthesis, assembly, and architecture, as well as cell wall regulation and signaling. Such information has significantly advanced our understanding of the roles of the cell wall in many biological and physiological processes and has enhanced our utilization of cell wall materials. The use of cutting-edge technologies such as single-molecule imaging, nuclear magnetic resonance spectroscopy, and atomic force microscopy has provided much insight into the plant cell wall as an intricate nanoscale network, opening up unprecedented possibilities for cell wall research. In this review, we summarize the major advances made in understanding the cell wall in this era of functional genomics, including the latest findings on the biosynthesis, construction, and functions of the cell wall.