Unmethyl-esterified homogalacturonan and extensins seal Arabidopsis graft union

Damage activates EXG1 and RLP44 to suppress vascular differentiation during regeneration in Arabidopsis

Plants possess remarkable regenerative abilities to form de novo vasculature after damage and in response to pathogens that invade and withdraw nutrients. To identify common factors that affect vascular formation upon stress, we searched for Arabidopsis thaliana genes differentially expressed upon Agrobacterium infection, nematode infection, and plant grafting. One such gene is cell wall-related and highly induced by all three stresses, which we named ENHANCED XYLEM AND GRAFTING1 (EXG1), since its mutations promote ectopic xylem formation in a vascular cell induction system and enhance graft formation. Further observations revealed that exg1 mutants show inhibited cambium development and callus formation but enhanced tissue attachment, syncytium size, phloem reconnection, and xylem formation. Given that brassinosteroids also promote xylem differentiation, we analyzed brassinosteroid-related genes and found that mutations in RLP44 encoding a receptor-like protein cause similar regeneration-related phenotypes as mutations in EXG1. Like EXG1, RLP44 expression is also induced by grafting and wounding. Mutations in EXG1 and RLP44 affect the expression of many genes in common, including those related to cell walls and genes important for vascular regeneration. Our results suggest that EXG1 integrates information from wounding or pathogen stress and functions with RLP44 to suppress vascular differentiation during regeneration and healing.

Autophagy is induced during plant grafting to promote wound healing

Grafting is an agricultural technique that joins tissues from different plants to obtain useful rootstock traits. However, cellular processes involved in joint tissue repair remain poorly understood. We analyzed Nicotiana benthamiana (Nb) and Arabidopsis thaliana (At) interfamily heterografting as a high-stress model and At homografting as a low-stress model. Transmission electron micrographs reveal the formation of autophagic structures in cells near the graft boundary over a long period in Nb/At interfamily grafts and in a short period of a few days in At homografts. Using a GFP-ATG8 marker line, the autophagosomes were observed in the cells near the graft boundary, especially on the scion side, where nutrient depletion occurred. Grafting of At autophagy-defective mutants decreases grafting success rates and post-grafting growth. NbATG5 knockdown suppresses graft establishment in Nb/At interfamily heterografts. Moreover, At autophagy-defective mutants show reduced callus formation directed to wounds under the nutrient-deficient conditions. These results suggest that autophagy is induced during grafting, promoting callus formation and contributing to tissue connectivity.

Cell wall remodeling during plant regeneration

Plant regeneration is the process during which differentiated tissues or cells can reverse or alter their developmental trajectory to repair damaged tissues or form new organs. In the plant regeneration process, the cell wall not only functions as a foundational barrier and scaffold supporting plant cells but also influences cell fates and identities. Cell wall remodeling involves the selective degradation of certain cell wall components or the integration of new components. Recently, accumulating evidence has underscored the importance of cell wall remodeling in plant regeneration. Wounding signals, transmitted by transcription factors, trigger the expressions of genes responsible for cell wall loosening, which is essential for tissue repair. In de novo organ regeneration and somatic embryogenesis, phytohormones orchestrate a transcriptional regulatory network to induce cell wall remodeling, which promotes cell fate reprogramming and organ formation. This review summarizes the effects of cell wall remodeling on various regenerative processes and provides novel insights into the future research of uncharacterized roles of cell wall in plant regeneration.

The effect of silicon supplementation and drought stress on the deposition of callose and chemical components in the cell walls of the Brassica napus roots

BACKGROUND

Silicon has an important role in regulating water management in plants. It is deposited in cell walls and creates a mechanical barrier against external factors. The aim of this study was to determine the role of silicon supplementation in the synthesis and distribution of callose in oilseed rape roots and to characterize the modifications of cell wall structure of these organs after exposure to drought stress. Histological and ultrastructural analyses were performed to determine the changes in the distribution of arabinogalactan proteins, pectins, and extensin in roots of Brassica napus growing under drought and supplemented with silicon. Callose deposition and the accumulation of callose synthase protein were assessed, followed by transcriptional analysis of callose synthase genes.

RESULTS

The results showed that silicon supplementation under drought conditions alter the direction of cortex cell differentiation, promoting fiber formation and proliferation of callose-depositing cells in the roots of the tested plants. This was reflected in an increase in the level of callose synthase and a decrease in the transcriptional activity of the gene encoding this enzyme, indicating regulation based on negative feedback under drought stress. The changes in abundance and distribution of investigated arabinogalactan proteins, pectins and extensin in roots of Si supplemented plants growing under drought stress were observed, indicating cell walls remodeling.

CONCLUSION

Silicon supplementation in oilseed rape roots induced significant changes in cell wall composition, including increased callose deposition and altered pectins and arabinogalactan proteins distribution. These modifications, along with the formation of fibres in the root cortex, likely contribute to enhanced cell wall strength providing a physical barrier against water loss and mechanical stress, as a probable defence mechanism induced during drought stress.

Deepening the Role of Pectin in the Tissue Assembly Process During Tomato Grafting

Cell walls play essential roles in cell recognition, tissue adhesion, and wound response. In particular, pectins as cell-adhesive agents are expected to play a key role in the early stages of grafting. To test this premise, this study focused on examining the dynamics of the accumulation and degree of methyl-esterification of pectic polysaccharides at the graft junctions using tomato autografts as an experimental model. Monosaccharide analysis showed a marked increase in homogalacturonan from 25% to 32 or 34% at the junction zones early after grafting. In addition, a decrease in the degree of homogalacturonan methyl-esterification up to 38% in the scion and 64% in the rootstock was observed in the first few days after grafting, accompanied by an increase in pectin methyl-esterase activity of up to 20-30% in the tissues surrounding the graft junction. These results shed light on the role of homogalacturonan in grafting and reinforce the key function of pectin as one of the most relevant cell wall components during the grafting process.

H3K4me3 changes occur in cell wall genes during the development of Fagopyrum tataricum morphogenic and non-morphogenic calli

Epigenetic changes accompany the dynamic changes in the cell wall composition during the development of callus cells. H3K4me3 is responsible for active gene expression and reaction to environmental cues. Chromatin immunoprecipitation (ChIP) is a powerful technique for studying the interplay between epigenetic modifications and the DNA regions of interest. In combination with sequencing, it can provide the genome-wide enrichment of the specific epigenetic mark, providing vital information on its involvement in the plethora of cellular processes. Here, we describe the genome-wide distribution of H3K4me3 in morphogenic and non-morphogenic callus of Fagopyrum tataricum. Levels of H3K4me3 were higher around the transcription start site, in agreement with the role of this mark in transcriptional activation. The global levels of methylation were higher in the non-morphogenic callus, which indicated increased gene activation compared to the morphogenic callus. We also employed ChIP to analyse the changes in the enrichment of this epigenetic mark on the cell wall-related genes in both calli types during the course of the passage. Enrichment of H3K4me3 on cell wall genes was specific for callus type, suggesting that the role of this mark in cell-wall remodelling is complex and involved in many processes related to dedifferentiation and redifferentiation. This intricacy of the cell wall composition was supported by the immunohistochemical analysis of the cell wall epitopes' distribution of pectins and extensins. Together, these data give a novel insight into the involvement of H3K4me3 in the regeneration processes in F. tataricum in vitro callus tissue culture.

A conserved graft formation process in Norway spruce and Arabidopsis identifies the PAT gene family as central regulators of wound healing

The widespread use of plant grafting enables eudicots and gymnosperms to join with closely related species and grow as one. Gymnosperms have dominated forests for over 200 million years, and despite their economic and ecological relevance, we know little about how they graft. Here we developed a micrografting method in conifers using young tissues that allowed efficient grafting with closely related species and between distantly related genera. Conifer graft junctions rapidly connected vasculature and differentially expressed thousands of genes including auxin and cell-wall-related genes. By comparing these genes to those induced during Arabidopsis thaliana graft formation, we found a common activation of cambium, cell division, phloem and xylem-related genes. A gene regulatory network analysis in Norway spruce (Picea abies) predicted that PHYTOCHROME A SIGNAL TRANSDUCTION 1 (PAT1) acted as a core regulator of graft healing. This gene was strongly up-regulated during both spruce and Arabidopsis grafting, and Arabidopsis mutants lacking PAT genes failed to attach tissues or successfully graft. Complementing Arabidopsis PAT mutants with the spruce PAT1 homolog rescued tissue attachment and enhanced callus formation. Together, our data show an ability for young tissues to graft with distantly related species and identifies the PAT gene family as conserved regulators of graft healing and tissue regeneration.

Plant grafting: Molecular mechanisms and applications

People have grafted plants since antiquity for propagation, to increase yields, and to improve stress tolerance. This cutting and joining of tissues activates an incredible regenerative ability as different plants fuse and grow as one. For over a hundred years, people have studied the scientific basis for how plants graft. Today, new techniques and a deepening knowledge of the molecular basis for graft formation have allowed a range of previously ungraftable combinations to emerge. Here, we review recent developments in our understanding of graft formation, including the attachment and vascular formation steps. We analyze why plants graft and how biotic and abiotic factors influence successful grafting. We also discuss the ability and inability of plants to graft, and how grafting has transformed both horticulture and fundamental plant science. As our knowledge about plant grafting improves, new combinations and techniques will emerge to allow an expanded use of grafting for horticultural applications and to address fundamental research questions.

Immunohistochemical dynamics of cell wall matrix polymers during tomato autograft healing

A large part of the production of tomato plants is grafted. Although it has recently been described that cell walls play an important role in tomato graft healing, the spatiotemporal dynamics of cell wall changes in this critical process remains largely unknown. The aim of this work was to immunolocalize changes in the major cell wall matrix components of autograft union tissues throughout the course of healing, from 1 to 20 days after grafting (DAG). Homogalacturonan was de novo synthetized and deposited in the cut edges, displaying the low methyl-esterified homogalacturonan a stronger labelling. Labelling of galactan side chains of rhamnogalacturonan increased until 8 DAG, although remarkably a set of cells at the graft union did not show labelling for this epitope. Changes in xylan immunolocalization were associated to the xylem vasculature development throughout, while those of xyloglucan revealed early synthesis at the cut edges. Arabinogalactan proteins increased up to 8 DAG and showed scion-rootstock asymmetry, with a higher extent in the scion. The combination of these changes appears to be related with the success of the autograft, specifically facilitating the adhesion phase between scion-rootstock tissues. This knowledge paves the way for improved grafting using methods that facilitate appropriate changes in the time and space dynamics of these cell wall compounds.

Grafting in plants: recent discoveries and new applications

Grafting is a traditional horticultural technique that makes use of plant wound healing mechanisms to join two different genotypes together to form one plant. In many agricultural systems, grafting with rootstocks controls the vigour of the scion and/or provides tolerance to deleterious soil conditions such as the presence of soil pests or pathogens or limited or excessive water or mineral nutrient supply. Much of our knowledge about the limits to grafting different genotypes together comes from empirical knowledge of horticulturalists. Until recently, researchers believed that grafting monocotyledonous plants was impossible, because they lack a vascular cambium, and that graft compatibility between different scion/rootstock combinations was restricted to closely related genotypes. Recent studies have overturned these ideas and open up the possibility of new research directions and applications for grafting in agriculture. The objective of this review is to describe and assess these recent advances in the field of grafting and, in particular, the molecular mechanisms underlining graft union formation and graft compatibility between different genotypes. The challenges of characterizing the different stages of graft union formation and phenotyping graft compatibility are examined.

A correlative light electron microscopy approach reveals plasmodesmata ultrastructure at the graft interface

Despite recent progress in our understanding of graft union formation, we still know little about the cellular events underlying the grafting process. This is partially due to the difficulty of reliably targeting the graft interface in electron microscopy to study its ultrastructure and three-dimensional architecture. To overcome this technological bottleneck, we developed a correlative light electron microscopy (CLEM) approach to study the graft interface with high ultrastructural resolution. Grafting hypocotyls of Arabidopsis thaliana lines expressing yellow FP or monomeric red FP in the endoplasmic reticulum (ER) allowed efficient targeting of the grafting interface for examination under light and electron microscopy. To explore the potential of our method to study sub-cellular events at the graft interface, we focused on the formation of secondary plasmodesmata (PD) between the grafted partners. We showed that four classes of PD were formed at the interface and that PD introgression into the cell wall was initiated equally by both partners. Moreover, the success of PD formation appeared not systematic with a third of PD not spanning the cell wall entirely. Characterizing the ultrastructural characteristics of these incomplete PD gives us insights into the process of secondary PD biogenesis. We found that the establishment of successful symplastic connections between the scion and rootstock occurred predominantly in the presence of thin cell walls and ER-plasma membrane tethering. The resolution reached in this work shows that our CLEM method advances the study of biological processes requiring the combination of light and electron microscopy.

The graft framework: Quantitative changes in cell wall matrix polysaccharides throughout the tomato graft union formation

Plant cell walls provide essential functions in cell recognition, differentiation, adhesion and wound responses. Therefore, it is tempting to hypothesize that cell walls play a key role in grafting, but to date there are no quantitative studies targeting on cell wall changes during grafting. The aim of this work was to investigate the dynamics of pectic and hemicellulosic polysaccharides at the graft junctions in tomato homografts throughout the first 12 days after grafting. Cell wall fractionation, combined with ATR-FTIR spectroscopy and gas-chromatography, evidenced a marked increase in pectin content and a decrease in the degree of methyl-esterification of homogalacturonan in scion and rootstock throughout grafting. Also, recovery of tightly-bound hemicelluloses decreased at late times after grafting suggesting an increase of cross-linked hemicelluloses along grafting. In addition, immuno-dot assays revealed an increase in xyloglucan and arabinogalactan proteins in the first days after grafting, pointing to a presumed role in tissue adhesion-cohesion.

Inhibition of Carotenoid Biosynthesis by CRISPR/Cas9 Triggers Cell Wall Remodelling in Carrot

Recent data indicate that modifications to carotenoid biosynthesis pathway in plants alter the expression of genes affecting chemical composition of the cell wall. Phytoene synthase (PSY) is a rate limiting factor of carotenoid biosynthesis and it may exhibit species-specific and organ-specific roles determined by the presence of psy paralogous genes, the importance of which often remains unrevealed. Thus, the aim of this work was to elaborate the roles of two psy paralogs in a model system and to reveal biochemical changes in the cell wall of psy knockout mutants. For this purpose, Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) and CRISPR associated (Cas9) proteins (CRISPR/Cas9) vectors were introduced to carotenoid-rich carrot (Daucus carota) callus cells in order to induce mutations in the psy1 and psy2 genes. Gene sequencing, expression analysis, and carotenoid content analysis revealed that the psy2 gene is critical for carotenoid biosynthesis in this model and its knockout blocks carotenogenesis. The psy2 knockout also decreased the expression of the psy1 paralog. Immunohistochemical staining of the psy2 mutant cells showed altered composition of arabinogalactan proteins, pectins, and extensins in the mutant cell walls. In particular, low-methylesterified pectins were abundantly present in the cell walls of carotenoid-rich callus in contrast to the carotenoid-free psy2 mutant. Transmission electron microscopy revealed altered plastid transition to amyloplasts instead of chromoplasts. The results demonstrate for the first time that the inhibited biosynthesis of carotenoids triggers the cell wall remodelling.

Similarities and Differences in the GFP Movement in the Zygotic and Somatic Embryos of Arabidopsis

Intercellular signaling during embryo patterning is not well understood and the role of symplasmic communication has been poorly considered. The correlation between the symplasmic domains and the development of the embryo organs/tissues during zygotic embryogenesis has only been described for a few examples, including Arabidopsis. How this process occurs during the development of somatic embryos (SEs) is still unknown. The aim of these studies was to answer the question: do SEs have a restriction in symplasmic transport depending on the developmental stage that is similar to their zygotic counterparts? The studies included an analysis of the GFP distribution pattern as expressed under diverse promoters in zygotic embryos (ZEs) and SEs. The results of the GFP distribution in the ZEs and SEs showed that 1/the symplasmic domains between the embryo organs and tissues in the SEs was similar to those in the ZEs and 2/the restriction in symplasmic transport in the SEs was correlated with the developmental stage and was similar to the one in their zygotic counterparts, however, with the spatio-temporal differences and different PDs SEL value between these two types of embryos.

Histological Changes Associated with the Graft Union Development in Tomato

Despite the importance of grafting in horticultural crops such as tomato (Solanum lycopersicum L.), the structural changes that occur during the graft establishment are little understood. Using histological techniques, the present work examines the time course of changes on the anatomical structure of the graft junction in functional tomato homografts and compares it to that of heterografts and non-functional grafts. No apparent differences were detected between homo- and heterografts, showing similar tissue development. At 10 days after grafting, the cell walls of the scion and rootstock in the area of the graft junction were thicker than usual. Undifferentiated cells and new vascular tissue emerged from the pre-existing vasculature. Adventitious roots appeared mainly on the scion, arising from the pre-existing vasculature. At 20 days, more pronounced vascular tissue was visible, along with large areas showing vascular connection. At 210 days, vestiges of the changes undergone in graft development were still visible. Generally, non-functional grafts presented layers of necrotic remains and deposition of cell wall material in the cut edges, impeding the suitable scion-rootstock connection. Our results show that accurate changes in pre-existing vasculature and the cell walls of the adhesion line are crucial to the development of functional grafts.

Cell Wall Composition as a Marker of the Reprogramming of the Cell Fate on the Example of a Daucus carota (L.) Hypocotyl in Which Somatic Embryogenesis Was Induced

Changes in the composition of the cell walls are postulated to accompany changes in the cell's fate. We check whether there is a relationship between the presence of selected pectic, arabinogalactan proteins (AGPs), and extensins epitopes and changes in cell reprogramming in order to answer the question of whether they can be markers accompanying changes of cell fate. Selected antibodies were used for spatio-temporal immunolocalization of wall components during the induction of somatic embryogenesis. Based on the obtained results, it can be concluded that (1) the LM6 (pectic), LM2 (AGPs) epitopes are positive markers, but the LM5, LM19 (pectic), JIM8, JIM13 (AGPs) epitopes are negative markers of cells reprogramming to the meristematic/pluripotent state; (2) the LM8 (pectic), JIM8, JIM13, LM2 (AGPs) and JIM11 (extensin) epitopes are positive markers, but LM6 (pectic) epitope is negative marker of cells undergoing detachment; (3) JIM4 (AGPs) is a positive marker, but LM5 (pectic), JIM8, JIM13, LM2 (AGPs) are negative markers for pericycle cells on the xylem pole; (4) LM19, LM20 (pectic), JIM13, LM2 (AGPs) are constitutive wall components, but LM6, LM8 (pectic), JIM4, JIM8, JIM16 (AGPs), JIM11, JIM12 and JIM20 (extensins) are not constitutive wall components; (5) the extensins do not contribute to the cell reprogramming.

Extracellular matrix and wall composition are diverse in the organogenic and non-organogenic calli of Actinidia arguta

Differences in the composition and the structural organisation of the extracellular matrix correlate with the morphogenic competence of the callus tissue that originated from the isolated endosperm of kiwifruit. The chemical composition and structural organisation of the extracellular matrix, including the cell wall and the layer on its surface, may correspond with the morphogenic competence of a tissue. In the presented study, this relationship was found in the callus tissue that had been differentiated from the isolated endosperm of the kiwiberry, Actinidia arguta. The experimental system was based on callus samples of exactly the same age that had originated from an isolated endosperm but were cultured under controlled conditions promoting either an organogenic or a non-organogenic pathway. The analyses which were performed using bright field, fluorescence and scanning electron microscopy techniques showed significant differences between the two types of calli. The organogenic tissue was compact and the outer walls of the peripheral cells were covered with granular structures. The non-organogenic tissue was composed of loosely attached cells, which were connected via a net-like structure. The extracellular matrices from both the non- and organogenic tissues were abundant in pectic homogalacturonan and extensins (LM19, LM20, JIM11, JIM12 and JIM20 epitopes), but the epitopes that are characteristic for rhamnogalacturonan I (LM5 and LM6), hemicellulose (LM25) and the arabinogalactan protein (LM2) were detected only in the non-organogenic callus. Moreover, we report the epitopes, which presence is characteristic for the Actinidia endosperm (LM21 and LM25, heteromannan and xyloglucan) and for the endosperm-derived cells that undergo dedifferentiation (loss of LM21 and LM25; appearance or increase in the content of LM5, LM6, LM19, JIM11, JIM12, JIM20, JIM8 and JIM16 epitopes).

Development of Parasitic Organs of a Stem Holoparasitic Plant in Genus Cuscuta

Parasitic plants infect a broad range of plant species including economically important crops. They survive by absorbing water, minerals, and photosynthates from their hosts. To support their way of life, parasitic plants generally establish parasitic organs that allow them to attach to their hosts and to efficiently absorb substances from the vascular system of the host. Here, we summarize the recent progress in understanding the mechanisms underlying the formation of these parasitic organs, focusing on the process depicted in the stem holoparasitic genus, Cuscuta. An attachment structure called "holdfast" on the stem surface is induced by the light and contact stimuli. Concomitantly with holdfast formation, development of an intrusive structure called haustorium initiates in the inner cortex of the Cuscuta stem, and it elongates through apoplastic space of the host tissue. When haustoria reaches to host vascular tissues, they begin to form vascular conductive elements to connect vascular tissue of Cuscuta stem to those of host. Recent studies have shown parasite-host interaction in the interfacial cell wall, and regulation of development of these parasitic structures in molecular level. We also briefly summarize the role of host receptor in the control of compatibility between Cuscuta and hosts, on which occurrence of attachment structure depends, and the role of plant-to-plant transfer of long-distance signals after the establishment of conductive structure.