Grafting success by cell wall remodeling: Plants that produce great fruit may not always have great roots. Grafting of a productive scion onto a resilient rootstock has provided agriculturalists with solutions to this and other challenges. Notaguchi et al. have now studied why some plant grafts work better than others (see the Perspective by McCann). The tobacco relative Nicotiana benthamiana ( Nb ) turns out to be the superhero of grafting, able to form grafts with plants from a wide range of evolutionary families. A bit of Nb , set as a middleman between a tomato scion and an Arabidopsis rootstock, negotiated a successful junction between these two otherwise nonconversant plant species. The expression of β-1,4-glucanases secreted into the extracellular region turns out to be key in facilitating cell wall reconstruction. Science , this issue p. 698 ; see also p. 618

Plant vascular systems are constructed by specific cell wall modifications through which cells are highly specialized to make conduits for water and nutrients. Xylem vessels are formed by thickened cell walls that remain after programmed cell death, and serve as water conduits from the root to the shoot. In contrast, phloem tissues consist of a complex of living cells, including sieve tube elements and their neighboring companion cells, and translocate photosynthetic assimilates from mature leaves to developing young tissues. Intensive studies on the content of vascular flow fluids have unveiled that plant vascular tissues transport various types of gene product, and the transport of some provides the molecular basis for the long-distance communications. Analysis of xylem sap has demonstrated the presence of proteins in the xylem transpiration stream. Recent studies have revealed that CLE and CEP peptides secreted in the roots are transported to above ground via the xylem in response to plant-microbe interaction and soil nitrogen starvation, respectively. Their leucine-rich repeat transmembrane receptors localized in the shoot phloem are required for relaying the signal from the shoot to the root. These findings well fit to the current scenario of root-to-shoot-to-root feedback signaling, where peptide transport achieves the root-to-shoot signaling, the first half of the signaling process. Meanwhile, it is now well evidenced that proteins and a range of RNAs are transported via the phloem translocation system, and some of those can exert their physiological functions at their destinations, including roots. Thus, plant vascular systems may serve not only as conduits for the translocation of essential substances but also as long-distance communication pathways that allow plants to adapt to changes in internal and external environments at the whole plant level.

Grafting has long been an important technique in agriculture. Nowadays, grafting is a widely used technique also to study systemic long-distance signaling in plants. Plants respond to their surrounding environment, and at that time many aspects of their physiology are regulated systemically; these start from local input signals and are followed by the transmission of information to the rest of the plant. For example, soil nutrient conditions, light/photoperiod, and biotic and abiotic stresses affect plants heterogeneously, and plants perceive such information in specific plant tissues or organs. Such environmental cues are crucial determinants of plant growth and development, and plants drastically change their morphology and physiology to adapt to various events in their life. Hitherto, intensive studies have been conducted to understand systemic signaling in plants, and grafting techniques have permitted advances in this field. The breakthrough technique of micrografting in Arabidopsis thaliana was established in 2002 and led to the development of molecular genetic tools in this field. Thereafter, various phenomena of systemic signaling have been identified at the molecular level, including nutrient fixation, flowering, circadian clock and defense against pathogens. The significance of grafting is that it can clarify the transmission of the stimulus and molecules. At present, many micro- and macromolecules have been identified as mobile signals, which are transported through plant vascular tissues to co-ordinate their physiology and development. In this review, we introduce the various grafting techniques that have been developed, we report on the recent advances in the field of plant systemic signaling where grafting techniques have been applied and provide insights for the future.

Phloem is a conductive tissue that allocates nutrients from mature source leaves to sinks such as young developing tissues. Phloem also delivers proteins and RNA species, such as small RNAs and mRNAs. Intensive studies on plant systemic signaling revealed the essential roles of proteins and RNA species. However, many of their functions are still largely unknown, with the roles of transported mRNAs being particularly poorly understood. A major difficulty is the absence of an accurate and comprehensive list of mobile transcripts. In this study, we used a hetero-graft system with Nicotiana benthamiana as the recipient scion and Arabidopsis as the donor stock, to identify transcripts that moved long distances across the graft union. We identified 138 Arabidopsis transcripts as mobile mRNAs, which we collectively termed the mRNA mobilome. Reverse transcription–PCR, quantitative real-time PCR and droplet digital PCR analyses confirmed the mobility. The transcripts included potential signaling factors and, unexpectedly, more general factors. In our investigations, we found no preferred transcript length, no previously known sequence motifs in promoter or transcript sequences and no similarities between the level of the transcripts and that in the source leaves. Grafting experiments regarding the function of ERECTA, an identified transcript, showed that no function of the transcript mobilized. To our knowledge, this is the first report identifying transcripts that move over long distances using a hetero-graft system between different plant taxa.

Nicotiana benthamiana is widely used as a model plant for dicotyledonous angiosperms. In fact, the strains used in research are highly susceptible to a wide range of viruses. Accordingly, these strains are subject to plant pathology and plant–microbe interactions. In terms of plant–plant interactions, N. benthamiana is one of the plants that exhibit grafting affinity with plants from different families. Thus, N. benthamiana is a good model for plant biology and has been the subject of genome sequencing analyses for many years. However, N. benthamiana has a complex allopolyploid genome, and its previous reference genome is fragmented into 141,000 scaffolds. As a result, molecular genetic analysis is difficult to perform. To improve this effort, de novo whole-genome assembly was performed in N. benthamiana with Hifi reads, and 1,668 contigs were generated with a total length of 3.1 Gb. The 21 longest scaffolds, regarded as pseudomolecules, contained a 2.8-Gb sequence, occupying 95.6% of the assembled genome. A total of 57,583 high-confidence gene sequences were predicted. Based on a comparison of the genome structures between N. benthamiana and N. tabacum, N. benthamiana was found to have more complex chromosomal rearrangements, reflecting the age of interspecific hybridization. To verify the accuracy of the annotations, the cell wall modification genes involved in grafting were analyzed, which revealed not only the previously indeterminate untranslated region, intron and open reading frame sequences but also the genomic locations of their family genes. Owing to improved genome assembly and annotation, N. benthamiana would increasingly be more widely accessible.

Tissue adhesion between plant species occurs both naturally and artificially. Parasitic plants establish intimate relationship with host plants by adhering tissues at roots or stems. Plant grafting, on the other hand, is a widely used technique in agriculture to adhere tissues of two stems. Here we found that the model Orobanchaceae parasitic plant Phtheirospermum japonicum can be grafted on to interfamily species. To understand molecular basis of tissue adhesion between distant plant species, we conducted comparative transcriptome analyses on both infection and grafting by P. japonicum on Arabidopsis. Despite different organs, we identified the shared gene expression profile, where cell proliferation- and cell wall modification-related genes are up-regulated. Among genes commonly induced in tissue adhesion between distant species, we showed a gene encoding a secreted type of β-1,4-glucanase plays an important role for plant parasitism. Our data provide insights into the molecular commonality between parasitism and grafting in plants.