Living with Two Genomes: Grafting and Its Implications for Plant Genome-to-Genome Interactions, Phenotypic Variation, and Evolution

JrGA20ox1-transformed rootstocks deliver drought response signals to wild-type scions in grafted walnut

Targeted regulation using transgrafting technology has become a trend. However, the mechanisms of transgene-derived signal communication between rootstocks and scions remain unclear in woody plants. Here, we grafted wild-type (WT) walnut (Juglans regia L.) on WT (WT/WT), JrGA20ox1 (encodes a gibberellin 20-oxidase)-overexpressing (WT/OE), and JrGA20ox1-RNAi transformation (WT/RNAi) walnut in vitro. We aimed to elucidate the mechanisms of JrGA20ox1-derived signal communication under PEG-simulated drought stress between rootstocks and scions in walnut. We demonstrated that JrGA20ox1-OE and JrGA20ox1-RNAi rootstocks could transport active gibberellins (GAs) and JrGA20ox1-RNAi vector-produced sRNAs to WT scions under PEG-simulated drought stress, respectively. The movement of sRNAs further led to a successive decline in JrGA20ox1 expression and active GA content. Meanwhile, unknown mobile signals may move between rootstocks and scions. These mobile signals reduced the expression of a series of GA-responsive and GA-non-responsive genes, and induced ROS production in guard cells and an increase in ABA content, which may contribute to the drought tolerance of WT/RNAi, while the opposite occurred in WT/OE. The findings suggest that JrGA20ox1-derived rootstock-to-scion movement of signals is involved in drought tolerance of scions. Our research will provide a feasible approach for studying signal communication in woody plants.

Parasitic Plants-Potential Vectors of Phytopathogens

Parasitic plants represent a peculiar group of semi- or fully heterotrophic plants, possessing the ability to extract water, minerals, and organic compounds from other plants. All parasitic plants, either root or stem, hemi- or holoparasitic, establish a vascular connection with their host plants through a highly specialized organ called haustoria. Apart from being the organ responsible for nutrient extraction, the haustorial connection is also a highway for various macromolecules, including DNA, proteins, and, apparently, phytopathogens. At least some parasitic plants are considered significant agricultural pests, contributing to enormous yield losses worldwide. Their negative effect is mainly direct, by the exhaustion of host plant fitness and decreasing growth and seed/fruit formation. However, they may pose an additional threat to agriculture by promoting the trans-species dispersion of various pathogens. The current review aims to summarize the available information and to raise awareness of this less-explored problem. We further explore the suitability of certain phytopathogens to serve as specific and efficient methods of control of parasitic plants, as well as methods for control of the phytopathogens.

Phased genomics reveals hidden somatic mutations and provides insight into fruit development in sweet orange

Although revisiting the discoveries and implications of genetic variations using phased genomics is critical, such efforts are still lacking. Somatic mutations represent a crucial source of genetic diversity for breeding and are especially remarkable in heterozygous perennial and asexual crops. In this study, we focused on a diploid sweet orange (Citrus sinensis) and constructed a haplotype-resolved genome using high fidelity (HiFi) reads, which revealed 10.6% new sequences. Based on the phased genome, we elucidate significant genetic admixtures and haplotype differences. We developed a somatic detection strategy that reveals hidden somatic mutations overlooked in a single reference genome. We generated a phased somatic variation map by combining high-depth whole-genome sequencing (WGS) data from 87 sweet orange somatic varieties. Notably, we found twice as many somatic mutations relative to a single reference genome. Using these hidden somatic mutations, we separated sweet oranges into seven major clades and provide insight into unprecedented genetic mosaicism and strong positive selection. Furthermore, these phased genomics data indicate that genomic heterozygous variations contribute to allele-specific expression during fruit development. By integrating allelic expression differences and somatic mutations, we identified a somatic mutation that induces increases in fruit size. Applications of phased genomics will lead to powerful approaches for discovering genetic variations and uncovering their effects in highly heterozygous plants. Our data provide insight into the hidden somatic mutation landscape in the sweet orange genome, which will facilitate citrus breeding.

DNA barcoding identification of grafted Semen Ziziphi Spinosae and transcriptome study of wild Semen Ziziphi Spinosae

Semen Ziziphi Spinosae (SZS) is the dried and ripe seeds of Ziziphus jujuba var. spinosa. Currently, the yield of naturally grown SZS is unstable owing to environmental factors. Grafting high-quality sour jujube scions onto sour jujube or jujube tree stocks can result in a greater yield. However, the effects of grafting on the quality and gene expression of SZS have rarely been reported. This study used a DNA barcoding technique, high-performance liquid phase-evaporative luminescence detector (HPLC-ELSD), and transcriptomics to investigate the origin and genetic differences between grafted and wild jujube seeds. DNA barcoding identified all samples as Ziziphus jujuba var. spinosa. HPLC-ELSD analysis revealed a higher content of grafted SZS compared to that of the wild SZS. Transcriptome analysis of the metabolic pathways in SZS showed that 22 and 19 differentially expressed gene sequences encoded enzymes related to flavonoids and saponin synthesis, respectively. Weighted correlation network analysis (WGCNA) identified 15 core genes governing the differences in medicinal components between grafted and wild SZS. This study demonstrated the use of DNA barcoding and fingerprint methods to identify jujube seed species and effectively capture ingredient information of medicinal materials. Additionally, transcriptome technology provided data for identifying core differential genes, facilitating studies on quality differences between grafted and wild SZS.

Grapevine scion gene expression is driven by rootstock and environment interaction

BACKGROUND

Grafting is a horticultural practice used widely across woody perennial crop species to fuse together the root and shoot system of two distinct genotypes, the rootstock and the scion, combining beneficial traits from both. In grapevine, grafting is used in nearly 80% of all commercial vines to optimize fruit quality, regulate vine vigor, and enhance biotic and abiotic stress-tolerance. Rootstocks have been shown to modulate elemental composition, metabolomic profiles, and the shape of leaves in the scion, among other traits. However, it is currently unclear how rootstock genotypes influence shoot system gene expression as previous work has reported complex and often contradictory findings.

RESULTS

In the present study, we examine the influence of grafting on scion gene expression in leaves and reproductive tissues of grapevines growing under field conditions for three years. We show that the influence from the rootstock genotype is highly tissue and time dependent, manifesting only in leaves, primarily during a single year of our three-year study. Further, the degree of rootstock influence on scion gene expression is driven by interactions with the local environment.

CONCLUSIONS

Our results demonstrate that the role of rootstock genotype in modulating scion gene expression is not a consistent, unchanging effect, but rather an effect that varies over time in relation to local environmental conditions.

The role of DNA methylation in the maintenance of phenotypic variation induced by grafting chimerism in Brassica

Grafting facilitates the interaction between heterologous cells with different genomes, resulting in abundant phenotypic variation, which provides opportunities for crop improvement. However, how grafting-induced variation occurs and is transmitted to progeny remains elusive. A graft chimera, especially a periclinal chimera, which has genetically distinct cell layers throughout the plant, is an excellent model to probe the molecular mechanisms of grafting-induced variation maintenance. Here we regenerated a plant from the T-cell layer of a periclinal chimera, TCC (where the apical meristem was artificially divided into three cell layers - from outside to inside, L1, L2, and L3; T = Tuber mustard, C = red Cabbage), named rTTT0 (r = regenerated). Compared with the control (rsTTT, s = self-grafted), rTTT0 had multiple phenotypic variations, especially leaf shape variation, which could be maintained in sexual progeny. Transcriptomes were analyzed and 58 phenotypic variation-associated genes were identified. Whole-genome bisulfite sequencing analyses revealed that the methylome of rTTT0 was changed, and the CG methylation level was significantly increased by 8.74%. In rTTT0, the coding gene bodies are hypermethylated in the CG context, while their promoter regions are hypomethylated in the non-CG context. DNA methylation changes in the leaf shape variation-associated coding genes, ARF10, IAA20, ROF1, and TPR2, were maintained for five generations of rTTT0. Interestingly, grafting chimerism also affected transcription of the microRNA gene (MIR), among which the DNA methylation levels of the promoters of three MIRs associated with leaf shape variation were changed in rTTT0, and the DNA methylation modification of MIR319 was maintained to the fifth generation of selfed progeny of rTTT0 (rTTT5). These findings demonstrate that DNA methylation of coding and non-coding genes plays an important role in heterologous cell interaction-induced variation formation and its transgenerational inheritance.

Metabolite profiling and transcriptome analyses provide insight into the regulatory network of graft incompatibility in litchi

Litchi is an important commercial fruit crop widely grown in the world. Graft incompatibility between rootstocks and scions is a major constraint for large-scale cultivation of litchi orchards, popularization of new and excellent litchi varieties, and associated industrial development. Further, the genetic mechanism of graft incompatibility is still unclear in litchi. To reduce the incompatibility problems, this study investigated metabolic and transcriptomic differences between graft compatible and incompatible rootstock-scion combinations of litchi. The result of metabolomics analysis showed that incompatible rootstock-scion interaction modified the profiles of several metabolic substances. However, various compounds of flavonoids, phenolic acids, and lignin predominantly exhibited significantly altered abundance in graft incompatible combinations. Transcriptome analysis identified that graft incompatibility induces dynamic gene differences. The majority of these differentially expressed genes were enriched in biosynthetic pathways of phenylpropanoids. The differential expressions of genes in these pathways could be linked to the differential abundance levels of flavonoids, phenolic acids, and lignin compounds. Integrated metabolomic and transcriptomic analyses revealed a strong relationship between differential genes and differential metabolites identified in this study. In addition, identified hub genes and metabolites were closely associated with graft incompatibility of litchi. This study characterized the abundance of metabolites and genes in graft incompatible combinations and further discussed the genetic mechanism of graft incompatibility in litchi. Our results provide a platform to dissect the molecular mechanisms of graft incompatibility in the litchi fruit.

Transcriptome Analysis of the Effects of Grafting Interstocks on Apple Rootstocks and Scions

Apples are a major horticultural crop worldwide. Grafting techniques are widely utilized in apple production to keep the varieties pure. Interstocks are frequently used in Northern China to achieve intensive apple dwarfing cultivation. High-throughput sequencing was used to investigate differentially expressed genes in the phloem tissues of two different xenograft systems, M ('Gala'/'Mac 9'/Malus baccata (L.) Borkh.) and B ('Gala'/Malus baccata (L.) Borkh.). The results showed that dwarfing interstocks could significantly reduce the height and diameters of apple trees while have few effects on the growth of annual branches. The interstocks were found to regulate the expression of genes related to hormone metabolism and tree body control (GH3.9, PIN1, CKI1, ARP1, GA2ox1 and GA20ox1), these effects may attribute the dwarf characters for apple trees with interstocks. Besides, the interstocks reduce photosynthesis-related genes (MADH-ME4 and GAPC), promote carbon (C) metabolism gene expression (AATP1, GDH and PFK3), promote the expression of nitrogen (N)-metabolism-related genes (NRT2.7, NADH and GDH) in rootstocks, and improve the expression of genes related to secondary metabolism in scions (DX5, FPS1, TPS21 and SRG1). We also concluded that the interstocks acquired early blooming traits due to promotion of the expression of flowering genes in the scion (MOF1, FTIP7, AGL12 and AGL24). This study is a valuable resource regarding the molecular mechanisms of dwarf interstocks' influence on various biological processes and transplantation systems in both scions and rootstocks.

Scion-to-Rootstock Mobile Transcription Factor CmHY5 Positively Modulates the Nitrate Uptake Capacity of Melon Scion Grafted on Squash Rootstock

It is generally recognized that the root uptake capacity of grafted plants strongly depends on the rootstocks' well-developed root system. However, we found that grafted plants showed different nitrate uptake capacities when different varieties of oriental melon scion were grafted onto the same squash rootstock, suggesting that the scion regulated the nitrate uptake capacity of the rootstock root. In this study, we estimated the nitrate uptake capacity of grafted plants with the different oriental melon varieties' seedlings grafted onto the same squash rootstocks. The results indicated a significant difference in the nitrate uptake rate and activity of two heterologous grafting plants. We also showed a significant difference in CmoNRT2.1 expression in the roots of two grafting combinations and verified the positive regulation of nitrate uptake by CmoNRT2.1 expression. In addition, the two varieties of oriental melon scion had highly significant differences in CmHY5 expression, which was transported to the rootstock and positively induced CmoHY5-1 and CmoHY5-2 expression in the rootstock roots. Meanwhile, CmHY5 could positively regulate CmoNRT2.1 expression in the rootstock roots. Furthermore, CmoHY5-1 and CmoHY5-2 also positively regulated CmoNRT2.1 expression, respectively, and CmoHY5-1 dominated the positive regulation of CmoNRT2.1, while CmHY5 could interact with CmoHY5-1 and CmoHY5-2, respectively, to jointly regulate CmoNRT2.1 expression. The oriental melon scion regulated the nitrate uptake capacity of the melon/squash grafting plant roots, and the higher expression of CmHY5 in the oriental melon scion leaves, the more substantial the nitrate uptake capacity of squash rootstock roots.

Comparative Transcriptomic Analysis of Inarching Invigorating Rootstock onto Incompatible Grafts in Citrus

Grafting is a technique that is widely used in citrus production. Graft incompatibility often occurs in the orchard. Inarching can effectively improve the vigor of incompatible grafts, but its mechanism remains poorly understood. Our previous studies investigated the scion-rootstock interaction of citrus and highlighted the role of hormonal balance and genes in abscisic acid biosynthesis regulation. To further elucidate the mechanism of inarched grafts rejuvenation, Hm/Pt combination (Citrus maxima (Burm.) Merrill cv. 'Hongmian miyou' grafted onto Poncirus trifoliata) were inarched with 'Pujiang Xiangcheng' (a novel citrus rootstock cultivar recently selected from wild Citrus junos populations), and comprehensive analysis was performed to compare the inarched grafts and controls. Compared with incompatible grafts, the results revealed that inarching could recover the leaf metabolism balance, including reducing starch content, increasing chlorophyll content and restoring the cell structure. Additionally, our results corroborated that hormonal balance and hormone-related genes played a central role in inarching compatibility. Furthermore, the roles of Hsf4, ERF1, NCED3 and PYL were highlighted, and a model for explaining inarched grafts recovery invigoration was proposed. This study shed light on the mechanism of inarching regulation tree vigor and offered deep insights into the scion-rootstock interaction in citrus.

X-ray imaging of 30 year old wine grape wood reveals cumulative impacts of rootstocks on scion secondary growth and Ravaz index

Annual rings from 30 year old vines in a California rootstock trial were measured to determine the effects of 15 different rootstocks on Chardonnay and Cabernet Sauvignon scions. Viticultural traits measuring vegetative growth, yield, berry quality, and nutrient uptake were collected at the beginning (1995 to 1999) and end (2017 to 2020) of the lifetime of a vineyard initially planted in 1991 and removed in 2021. X-ray Computed Tomography (CT) was used to measure ring widths in 103 vines. Ring width was modeled as a function of ring number using a negative exponential model. Early and late wood ring widths, cambium width, and scion trunk radius were correlated with 27 traits. Modeling of annual ring width shows that scions alter the width of the first rings but that rootstocks alter the decay of later rings, consistently shortening ring width throughout the lifetime of the vine. Ravaz index, juice pH, photosynthetic assimilation and transpiration rates, and instantaneous water use efficiency are correlated with scion trunk radius. Ultimately, our research indicates that rootstocks modulate secondary growth over years, altering physiology and agronomic traits. Rootstocks act in similar but distinct ways from climate to modulate ring width, which borrowing techniques from dendrochronology, can be used to monitor both genetic and environmental effects in woody perennial crop species.

Structural variation and parallel evolution of apomixis in citrus during domestication and diversification

Apomixis, or asexual seed formation, is prevalent in Citrinae via a mechanism termed nucellar or adventitious embryony. Here, multiple embryos of a maternal genotype form directly from nucellar cells in the ovule and can outcompete the developing zygotic embryo as they utilize the sexually derived endosperm for growth. Whilst nucellar embryony enables the propagation of clonal plants of maternal genetic constitution, it is also a barrier to effective breeding through hybridization. To address the genetics and evolution of apomixis in Citrinae, a chromosome-level genome of the Hongkong kumquat (Fortunella hindsii) was assembled following a genome-wide variation map including structural variants (SVs) based on 234 Citrinae accessions. This map revealed that hybrid citrus cultivars shelter genome-wide deleterious mutations and SVs into heterozygous states free from recessive selection, which may explain the capability of nucellar embryony in most cultivars during Citrinae diversification. Analyses revealed that parallel evolution may explain the repeated origin of apomixis in different genera of Citrinae. Within Fortunella, we found that apomixis of some varieties originated via introgression. In apomictic Fortunella, the locus associated with apomixis contains the FhRWP gene, encoding an RWP-RK domain-containing protein previously shown to be required for nucellar embryogenesis in Citrus. We found the heterozygous SV in the FhRWP and CitRWP promoters from apomictic Citrus and Fortunella, due to either two or three miniature inverted transposon element (MITE) insertions. A transcription factor, FhARID, encoding an AT-rich interaction domain-containing protein binds to the MITEs in the promoter of apomictic varieties, which facilitates induction of nucellar embryogenesis. This study provides evolutionary genomic and molecular insights into apomixis in Citrinae and has potential ramifications for citrus breeding.

Lateral transfers lead to the birth of momilactone biosynthetic gene clusters in grass

Momilactone A, an important plant labdane-related diterpenoid, functions as a phytoalexin against pathogens and an allelochemical against neighboring plants. The genes involved in the biosynthesis of momilactone A are found in clusters, i.e., momilactone A biosynthetic gene clusters (MABGCs), in the rice and barnyardgrass genomes. In addition, we know little about the origin and evolution of MABGCs. Here, we integrated results from comprehensive phylogeny and comparative genomic analyses of the core genes of MABGC-like clusters and MABGCs in 40 monocot plant genomes, providing convincing evidence for the birth and evolution of MABGCs in grass species. The MABGCs found in the PACMAD clade of the core grass lineage (including Panicoideae and Chloridoideae) originated from a MABGC-like cluster in Triticeae (BOP clade) via lateral gene transfer (LGT) and followed by recruitment of MAS1/2 and CYP76L1 genes. The MABGCs in Oryzoideae originated from PACMAD through another LGT event and lost CYP76L1 afterwards. The Oryza MABGC and another Oryza diterpenoid cluster c2BGC are two distinct clusters, with the latter originating from gene duplication and relocation within Oryzoideae. Further comparison of the expression patterns of the MABGC genes between rice and barnyardgrass in response to pathogen infection and allelopathy provides novel insights into the functional innovation of MABGCs in plants. Our results demonstrate LGT-mediated origination of MABGCs in grass and shed lights into the evolutionary innovation and optimization of plant biosynthetic pathways.

Temporal and environmental factors interact with rootstock genotype to shape leaf elemental composition in grafted grapevines

Plants take up elements through their roots and transport them to their shoot systems for use in numerous biochemical, physiological, and structural functions. Elemental composition of above-ground plant tissues, such as leaves, reflects both above- and below-ground activities of the plant, as well the local environment. Perennial, grafted plants, where the root system of one individual is fused to the shoot system of a genetically distinct individual, offer a powerful experimental system in which to study how genetically distinct root systems influence the elemental composition of a common shoot system. We measured elemental composition of over 7,000 leaves in the grapevine cultivar "Chambourcin" growing ungrafted and grafted to three rootstock genotypes. Leaves were collected over multiple years and phenological stages (across the season) and along a developmental time series. Temporal components of this study had the largest effect on leaf elemental composition, and rootstock genotype interacted with year, phenological stage, and leaf age to differentially modulate leaf elemental composition. Further, the local, above-ground environment affected leaf elemental composition, an effect influenced by rootstock genotype. This work highlights the dynamic nature by which root systems interact with shoot systems to respond to temporal and environmental variation.

Metabolomic and transcriptomic analyses reveal the effects of self- and hetero-grafting on anthocyanin biosynthesis in grapevine

Grafting, which joins a scion from a cultivar with the stem of a rootstock from a grapevine wild relative, is commonly used in viticulture. Grafting has crucial effects on various phenotypes of the cultivar, including its phenology, biotic and abiotic resistance, berry metabolome, and coloration, but the underlying genetics and regulatory mechanisms are largely unexplored. In this study, we investigated the phenotypic, metabolomic, and transcriptomic profiles at three developmental stages (45, 75, and 105 days after flowering) of the Crimson Seedless cultivar (Vitis vinifera) grafted onto four rootstocks (three heterografts, CS/101-14, CS/SO4, and CS/110R and one self-graft, CS/CS) with own-rooted graft-free Crimson Seedless (CS) as the control. All the heterografts had a significant effect on berry reddening as early as ~45 days after flowering. The grafting of rootstocks promoted anthocyanin biosynthesis and accumulation in grape berries. The metabolomic features showed that cyanidin 3-O-glucoside, delphinidin 3-O-glucoside, malvidin 3-O-glucoside, peonidin 3-O-glucoside, and petunidin 3-O-glucoside were the pigments responsible for the purplish-red peel color. Transcriptomic analyses revealed that anthocyanin biosynthesis-related genes, from upstream (phenylalanine ammonia-lyase) to downstream (anthocyanidin 3-O-glucosyltransferase and anthocyanidin synthase), were upregulated with the accumulation of anthocyanins in the heterografted plants. At the same time, all these genes were also highly expressed and more anthocyanin was accumulated in self-grafted CS/CS samples compared with own-rooted graft-free CS samples, suggesting that self-grafting may also have promoted berry reddening in grapevine. Our results reveal global transcriptomic and metabolomic features in berry color regulation under different grafting conditions that may be useful for improving berry quality in viticulture.

Genomic insights into the evolution of Echinochloa species as weed and orphan crop

As one of the great survivors of the plant kingdom, barnyard grasses (Echinochloa spp.) are the most noxious and common weeds in paddy ecosystems. Meanwhile, at least two Echinochloa species have been domesticated and cultivated as millets. In order to better understand the genomic forces driving the evolution of Echinochloa species toward weed and crop characteristics, we assemble genomes of three Echinochloa species (allohexaploid E. crus-galli and E. colona, and allotetraploid E. oryzicola) and re-sequence 737 accessions of barnyard grasses and millets from 16 rice-producing countries. Phylogenomic and comparative genomic analyses reveal the complex and reticulate evolution in the speciation of Echinochloa polyploids and provide evidence of constrained disease-related gene copy numbers in Echinochloa. A population-level investigation uncovers deep population differentiation for local adaptation, multiple target-site herbicide resistance mutations of barnyard grasses, and limited domestication of barnyard millets. Our results provide genomic insights into the dual roles of Echinochloa species as weeds and crops as well as essential resources for studying plant polyploidization, adaptation, precision weed control and millet improvements.

Gene regulatory networks for compatible versus incompatible grafts identify a role for SlWOX4 during junction formation

Grafting has been adopted for a wide range of crops to enhance productivity and resilience; for example, grafting of Solanaceous crops couples disease-resistant rootstocks with scions that produce high-quality fruit. However, incompatibility severely limits the application of grafting and graft incompatibility remains poorly understood. In grafts, immediate incompatibility results in rapid death, but delayed incompatibility can take months or even years to manifest, creating a significant economic burden for perennial crop production. To gain insight into the genetic mechanisms underlying this phenomenon, we developed a model system using heterografting of tomato (Solanum lycopersicum) and pepper (Capsicum annuum). These grafted plants express signs of anatomical junction failure within the first week of grafting. By generating a detailed timeline for junction formation, we were able to pinpoint the cellular basis for this delayed incompatibility. Furthermore, we inferred gene regulatory networks for compatible self-grafts and incompatible heterografts based on these key anatomical events, which predict core regulators for grafting. Finally, we examined the role of vascular development in graft formation and uncovered SlWOX4 as a potential regulator of graft compatibility. Following this predicted regulator up with functional analysis, we show that Slwox4 homografts fail to form xylem bridges across the junction, demonstrating that indeed, SlWOX4 is essential for vascular reconnection during grafting, and may function as an early indicator of graft failure.

Gene regulatory networks for compatible versus incompatible grafts identify a role for SlWOX4 during junction formation

Grafting has been adopted for a wide range of crops to enhance productivity and resilience; for example, grafting of Solanaceous crops couples disease-resistant rootstocks with scions that produce high-quality fruit. However, incompatibility severely limits the application of grafting and graft incompatibility remains poorly understood. In grafts, immediate incompatibility results in rapid death, but delayed incompatibility can take months or even years to manifest, creating a significant economic burden for perennial crop production. To gain insight into the genetic mechanisms underlying this phenomenon, we developed a model system using heterografting of tomato (Solanum lycopersicum) and pepper (Capsicum annuum). These grafted plants express signs of anatomical junction failure within the first week of grafting. By generating a detailed timeline for junction formation, we were able to pinpoint the cellular basis for this delayed incompatibility. Furthermore, we inferred gene regulatory networks for compatible self-grafts and incompatible heterografts based on these key anatomical events, which predict core regulators for grafting. Finally, we examined the role of vascular development in graft formation and uncovered SlWOX4 as a potential regulator of graft compatibility. Following this predicted regulator up with functional analysis, we show that Slwox4 homografts fail to form xylem bridges across the junction, demonstrating that indeed, SlWOX4 is essential for vascular reconnection during grafting, and may function as an early indicator of graft failure.

Comparative transcriptomic analysis on compatible/incompatible grafts in citrus

Grafting is a useful cultivation technology to resist abiotic and biotic stresses and is an integral part of citrus production. However, some widely utilized rootstocks may still exhibit graft incompatibility in the orchard. "Hongmian miyou" (Citrus maxima (Burm.) Merrill) is mutated from "Guanxi miyou", but these two scions showed different compatibility with available Poncirus trifoliata rootstock. Foliage etiolation is an observed symptom of graft incompatibility, but its mechanism remains poorly understood. This study is the first to investigate the morphological, physiological, and anatomical differences between the compatible/incompatible grafts, and perform transcriptome profiling at crucial stages of the foliage etiolation process. Based on the comprehensive analyses, hormonal balance was disordered, and two rate-limiting genes, NCED3 (9-cis-epoxycarotenoid dioxygenases 3) and NCED5, being responsible for ABA (abscisic acid) accumulation, were highlighted. Further correlation analysis indicated that IAA (indole-3-acetic acid) and ABA were the most likely inducers of the expression of stresses-related genes. In addition, excessive starch accumulation was observed in lamina and midribs of incompatible grafts leaves. These results provided a new insight into the role of the hormonal balance and abscisic acid biosynthesis genes in regulation and contribution to the graft incompatibility, and will further define and deploy candidate genes to explore the mechanisms underlying citrus rootstock- scion interactions.

Mobile Flowering Locus T RNA - Biological Relevance and Biotechnological Potential

Many systemically mobile mRNAs have been revealed in phloem. However, very few of them have been found to be of clear signaling functions. One of such rare examples is the mobile Flowering locus T (FT) mRNA despite the continuous debate about its mobility and biological relevance to the control of flowering time in plants. Nevertheless, accumulating evidence supports the notion of the long-distance movement of FT mRNA from leaf to shoot apex meristem and its role in flowering. In this review, we discuss the discovery of florigenic FT, the initial debate on long-distance movement of FT mRNA, emerging evidence to prove its mobility, and the use of mobile FT mRNA to generate heritable transgenerational gene editing in plants. We elaborate on evidence from virus-based RNA mobility assay, plant grafting, RNA with fluorescent protein labeling, and CRISPR/Cas9 gene-editing technology, to demonstrate that the FT mRNA besides the FT protein can move systemically and function as an integral component of the florigenic signal in flowering. We also propose a model to prompt further research on the molecular mechanism underlying the long-distance movement of this important mobile signaling RNA in plants.

Comparative Transcriptome Analysis in Homo- and Hetero-Grafted Cucurbit Seedlings

Watermelon (Citrullus lanatus) is a valuable horticultural crop with nutritional benefits grown worldwide. It is almost exclusively cultivated as grafted scions onto interspecific squash rootstock (Cucurbita maxima × Cucurbita moschata) to improve the growth and yield and to address the problems of soilborne diseases and abiotic stress factors. This study aimed to examine the effect of grafting (homo- and hetero-grafting) on the transcriptome level of the seedlings. Therefore, we compared homo-grafted watermelon (WW) with non-grafted watermelon control (W), homo-grafted squash (SS) with non-grafted squash control (S), hetero-grafted watermelon onto squash (WS) with SS, and WS with WW. Different numbers of differentially expressed genes (DEGs) were identified in each comparison. In total, 318 significant DEGs were detected between the transcriptomes of hetero-grafts and homo-grafts at 16 h after grafting. Overall, a significantly higher number of downregulated transcripts was detected among the DEGs. Only one gene showing increased expression related to the cytokinin synthesis was common in three out of four comparisons involving WS, SS, and S. The highest number of differentially expressed (DE) transcripts (433) was detected in the comparison between SS and S, followed by the 127 transcripts between WW and W. The study provides a description of the transcriptomic nature of homo- and hetero-grafted early responses, while the results provide a start point for the elucidation of the molecular mechanisms and candidate genes for the functional analyses of hetero-graft and homo-graft systems in Cucurbitaceae and generally in the plants.

Herbivory-induced systemic signals are likely to be evolutionarily conserved in euphyllophytes

Herbivory-induced systemic signaling has been demonstrated in monocots and dicots, and is essential for plant defense against insects. However, the nature and evolution of herbivory-induced systemic signals remain unclear. Grafting is widely used for studying systemic signaling; however, grafting between dicot plants from different families is difficult, and grafting is impossible for monocots. In this study, we took advantage of dodder's extraordinary capability of parasitizing various plant species. Field dodder (Cuscuta campestris) was employed to connect pairs of species that are phylogenetically very distant, ranging from fern to monocot and dicot plants, and so determine whether interplant signaling occurs after simulated herbivory. It was found that simulated herbivory-induced systemic signals can be transferred by dodder between a monocot and a dicot plant and even between a fern and a dicot plant, and the plants that received the systemic signals all exhibited elevated defenses. Thus, we inferred that the herbivory-induced systemic signals are likely to be evolutionarily well conserved among vascular plants. Importantly, we also demonstrate that the jasmonate pathway is probably an ancient regulator of the biosynthesis and/or transport of systemic signals in vascular plants. These findings provide new insight into the nature and evolution of systemic signaling.

The Orphan Crop Crassocephalum crepidioides Accumulates the Pyrrolizidine Alkaloid Jacobine in Response to Nitrogen Starvation

Crassocephalum crepidioides is an African orphan crop that is used as a leafy vegetable and medicinal plant. Although it is of high regional importance in Sub-Saharan Africa, the plant is still mainly collected from the wild and therefore efforts are made to promote its domestication. However, in addition to beneficial properties, there was first evidence that C. crepidioides can accumulate the highly toxic pyrrolizidine alkaloid (PA) jacobine and here it was investigated, how jacobine production is controlled. Using ecotypes from Africa and Asia that were characterized in terms of their PA profiles, it is shown that the tetraploid C. crepidioides forms jacobine, an ability that its diploid close relative Crassocephalum rubens appears to lack. Evidence is provided that nitrogen (N) deficiency strongly increases jacobine in the leaves of C. crepidioides, that this capacity depends more strongly on the shoot than the root system, and that homospermidine synthase (HSS) activity is not rate-limiting for this reaction. A characterization of HSS gene representation and transcription showed that C. crepidioides and C. rubens possess two functional versions, one of which is conserved, that the HSS transcript is mainly present in roots and that its abundance is not controlled by N deficiency. In summary, this work improves our understanding of how environmental cues impact PA biosynthesis in plants and provides a basis for the development of PA-free C. crepidioides cultivars, which will aid its domestication and safe use.

Time Will Tell: Intercellular Communication in the Plant Clock

Multicellular organisms have evolved local and long-distance signaling mechanisms to synchronize development and response to stimuli among a complex network of cells, tissues, and organs. Biological timekeeping is one such activity that is suggested to be coordinated within an organism to anticipate and respond to daily and seasonal patterns in the environment. New research into the plant clock suggests circadian rhythms are communicated between cells and across long distances. However, further clarity is required on the nature of the signaling molecules and the mechanisms underlying signal translocation. Here we summarize the roles and properties of tissue-specific circadian rhythms, discuss the evidence for local and long-distance clock communication, and evaluate the potential signaling molecules and transport mechanisms involved in this system.

Effect of Transgenic Rootstock Grafting on the Omics Profiles in Tomato

Grafting of non-transgenic scion onto genetically modified (GM) rootstocks provides superior agronomic traits in the GM rootstock, and excellent fruits can be produced for consumption. In such grafted plants, the scion does not contain any foreign genes, but the fruit itself is likely to be influenced directly or indirectly by the foreign genes in the rootstock. Before market release of such fruit products, the effects of grafting onto GM rootstocks should be determined from the perspective of safety use. Here, we evaluated the effects of a transgene encoding β-glucuronidase (GUS) on the grafted tomato fruits as a model case. An edible tomato cultivar, Stella Mini Tomato, was grafted onto GM Micro-Tom tomato plants that had been transformed with the GUS gene. The grafted plants showed no difference in their fruit development rate and fresh weight regardless of the presence or absence of the GUS gene in the rootstock. The fruit samples were subjected to transcriptome (NGS-illumina), proteome (shotgun LC-MS/MS), metabolome (LC-ESI-MS and GC-EI-MS), and general food ingredient analyses. In addition, differentially detected items were identified between the grafted plants onto rootstocks with or without transgenes (more than two-fold). The transcriptome analysis detected approximately 18,500 expressed genes on average, and only 6 genes were identified as differentially expressed. Principal component analysis of 2,442 peaks for peptides in proteome profiles showed no significant differences. In the LC-ESI-MS and GC-EI-MS analyses, a total of 93 peak groups and 114 peak groups were identified, respectively, and only 2 peak groups showed more than two-fold differences. The general food ingredient analysis showed no significant differences in the fruits of Stella scions between GM and non-GM Micro-Tom rootstocks. These multiple omics data showed that grafting on the rootstock harboring the GUS transgene did not induce any genetic or metabolic variation in the scion.

Epigenetic Changes and Transcriptional Reprogramming Upon Woody Plant Grafting for Crop Sustainability in a Changing Environment

Plant grafting is an ancient agricultural practice widely employed in crops such as woody fruit trees, grapes, and vegetables, in order to improve plant performance. Successful grafting requires the interaction of compatible scion and rootstock genotypes. This involves an intricate network of molecular mechanisms operating at the graft junction and associated with the development and the physiology of the scion, ultimately leading to improved agricultural characteristics such as fruit quality and increased tolerance/resistance to abiotic and biotic factors. Bidirectional transfer of molecular signals such as hormones, nutrients, proteins, and nucleic acids from the rootstock to the scion and vice versa have been well documented. In recent years, studies on rootstock-scion interactions have proposed the existence of an epigenetic component in grafting reactions. Epigenetic changes such as DNA methylation, histone modification, and the action of small RNA molecules are known to modulate chromatin architecture, leading to gene expression changes and impacting cellular function. Mobile small RNAs (siRNAs) migrating across the graft union from the rootstock to the scion and vice versa mediate modifications in the DNA methylation pattern of the recipient partner, leading to altered chromatin structure and transcriptional reprogramming. Moreover, graft-induced DNA methylation changes and gene expression shifts in the scion have been associated with variations in graft performance. If these changes are heritable they can lead to stably altered phenotypes and affect important agricultural traits, making grafting an alternative to breeding for the production of superior plants with improved traits. However, most reviews on the molecular mechanisms underlying this process comprise studies related to vegetable grafting. In this review we will provide a comprehensive presentation of the current knowledge on the epigenetic changes and transcriptional reprogramming associated with the rootstock-scion interaction focusing on woody plant species, including the recent findings arising from the employment of advanced-omics technologies as well as transgrafting methodologies and their potential exploitation for generating superior quality grafts in woody species. Furthermore, will discuss graft-induced heritable epigenetic changes leading to novel plant phenotypes and their implication to woody crop improvement for yield, quality, and stress resilience, within the context of climate change.

Grapevine Microbiota Reflect Diversity among Compartments and Complex Interactions within and among Root and Shoot Systems

Grafting connects root and shoot systems of distinct individuals, bringing microbial communities of different genotypes together in a single plant. How do root system and shoot system genotypes influence plant microbiota in grafted grapevines? To address this, we utilized clonal replicates of the grapevine ‘Chambourcin’, growing ungrafted and grafted to three different rootstocks in three irrigation treatments. Our objectives were to (1) characterize the microbiota (bacteria and fungi) of below-ground compartments (roots, adjacent soil) and above-ground compartments (leaves, berries), (2) determine how rootstock genotype, irrigation, and their interaction influences grapevine microbiota in different compartments, and (3) investigate abundance of microorganisms implicated in the late-season grapevine disease sour rot (Acetobacterales and Saccharomycetes). We found that plant compartment had the largest influence on microbial diversity. Neither rootstock genotype nor irrigation significantly influenced microbial diversity or composition. However, differential abundance of bacterial and fungal taxa varied as a function of rootstock and irrigation treatment; in particular, Acetobacterales and Saccharomycetes displayed higher relative abundance in berries of grapevines grafted to ‘1103P’ and ‘SO4’ rootstocks and varied across irrigation treatments. This study demonstrates that grapevine compartments retain distinct microbiota and identifies associations between rootstock genotypes, irrigation treatment, and the relative abundance of agriculturally relevant microorganisms in the berries.

Epigenetic Mechanisms of Plant Adaptation to Biotic and Abiotic Stresses

Unlike animals, plants are immobile and could not actively escape the effects of aggressive environmental factors, such as pathogenic microorganisms, insect pests, parasitic plants, extreme temperatures, drought, and many others. To counteract these unfavorable encounters, plants have evolved very high phenotypic plasticity. In a rapidly changing environment, adaptive phenotypic changes often occur in time frames that are too short for the natural selection of adaptive mutations. Probably, some kind of epigenetic variability underlines environmental adaptation in these cases. Indeed, isogenic plants often have quite variable phenotypes in different habitats. There are examples of successful “invasions” of relatively small and genetically homogenous plant populations into entirely new habitats. The unique capability of quick environmental adaptation appears to be due to a high tendency to transmit epigenetic changes between plant generations. Multiple studies show that epigenetic memory serves as a mechanism of plant adaptation to a rapidly changing environment and, in particular, to aggressive biotic and abiotic stresses. In wild nature, this mechanism underlies, to a very significant extent, plant capability to live in different habitats and endure drastic environmental changes. In agriculture, a deep understanding of this mechanism could serve to elaborate more effective and safe approaches to plant protection.

Cell-cell adhesion in plant grafting is facilitated by β-1,4-glucanases

Plant grafting is conducted for fruit and vegetable propagation, whereby a piece of living tissue is attached to another through cell-cell adhesion. However, graft compatibility limits combinations to closely related species, and the mechanism is poorly understood. We found that Nicotiana is capable of graft adhesion with a diverse range of angiosperms. Comparative transcriptomic analyses on graft combinations indicated that a subclade of β-1,4-glucanases secreted into the extracellular region facilitates cell wall reconstruction near the graft interface. Grafting was promoted by overexpression of the β-1,4-glucanase. Using Nicotiana stem as an interscion, we produced tomato fruits on rootstocks from other plant families. These findings demonstrate that the process of cell-cell adhesion is a potential target to enhance plant grafting techniques.

Mechanisms Underlying Graft Union Formation and Rootstock Scion Interaction in Horticultural Plants

Grafting is a common practice for vegetative propagation and trait improvement in horticultural plants. A general prerequisite for successful grafting and long term survival of grafted plants is taxonomic proximity between the root stock and scion. For the success of a grafting operation, rootstock and scion should essentially be closely related. Interaction between the rootstock and scion involves complex physiological-biochemical and molecular mechanisms. Successful graft union formation involves a series of steps viz., lining up of vascular cambium, generation of a wound healing response, callus bridge formation, followed by vascular cambium formation and subsequent formation of the secondary xylem and phloem. For grafted trees compatibility between the rootstock/scion is the most essential factor for their better performance and longevity. Graft incompatibility occurs on account of a number of factors including of unfavorable physiological responses across the graft union, transmission of virus or phytoplasma and anatomical deformities of vascular tissue at the graft junction. In order to avoid the incompatibility problems, it is important to predict the same at an early stage. Phytohormones, especially auxins regulate key events in graft union formation between the rootstock and scion, while others function to facilitate the signaling pathways. Transport of macro as well as micro molecules across long distances results in phenotypic variation shown by grafted plants, therefore grafting can be used to determine the pattern and rate of recurrence of this transport. A better understanding of rootstock scion interactions, endogenous growth substances, soil or climatic factors needs to be studied, which would facilitate efficient selection and use of rootstocks in the future. Protein, hormones, mRNA and small RNA transport across the junction is currently emerging as an important mechanism which controls the stock/scion communication and simultaneously may play a crucial role in understanding the physiology of grafting more precisely. This review provides an understanding of the physiological, biochemical and molecular basis underlying grafting with special reference to horticultural plants.