Grafting in plants: recent discoveries and new applications

Phenotyping xylem connections in grafted plants using X-ray micro-computed tomography

Plants are able to naturally graft or inosculate their trunks, branches and roots together, this mechanism is used by humans to graft together different genotypes for a range of purposes. Grafts are considered successful if functional vascular connections between the two genotypes occur. Various techniques can evaluate xylem connections across the graft interface. However, these methods are generally unable to assess the heterogeneity and three-dimensional (3D) structure of xylem vessel connections. Here we present the use of X-ray micro-computed tomography to characterize the 3D morphology of grafts of grapevine. We show that xylem vessels form between the two plants of natural root and human-made stem grafts. The main novelty of this methodology is that we were able to visualize the 3D network of functional xylem vessels connecting the scion and rootstock in human-made stem grafts thanks to the addition of a contrast agent to the roots and improved image analysis pipelines. In addition, we reveal the presence of extensive diagonal xylem connections between the main axial xylem vessels in 2-year old grapevine stems. In conclusion, we present a method that has the potential to provide new insights into the structure and function of xylem vessels in large tissue samples.

An Integrated Analysis of Anatomical and Sugar Contents Identifies How Night Temperatures Regulate the Healing Process of Oriental Melon Grafted onto Pumpkin

Graft healing is a complex process affected by environmental factors, with temperature being one of the most important influencing factors. Here, oriental melon grafted onto pumpkin was used to study changes in graft union formation and sugar contents at the graft interface under night temperatures of 18 °C and 28 °C. Histological analysis suggested that callus formation occurred 3 days after grafting with a night temperature of 28 °C, which was one day earlier than with a night temperature of 18 °C. Vascular reconnection with a night temperature of 28 °C was established 2 days earlier than with a night temperature of 18 °C. Additionally, nine sugars were significantly enriched in the graft union, with the contents of sucrose, trehalose, raffinose, D-glucose, D-fructose, D-galactose, and inositol initially increasing but then decreasing. Furthermore, we also found that exogenous glucose and fructose application promotes vascular reconnection. However, exogenous sucrose application did not promote vascular reconnection. Taken together, our results reveal that elevated temperatures improve the process of graft union formation through increasing the contents of sugars. This study provides information to develop strategies for improving grafting efficiency under low temperatures.

Transcriptional dynamics reveals the asymmetrical events underlying graft union formation in pecan (Carya illinoinensis)

Grafting is a widely used technique for pecan propagation; however, the background molecular events underlying grafting are still poorly understood. In our study, the graft partners during pecan [Carya illinoinensis (Wangenh.) K. Koch] graft union formation were separately sampled for RNA-seq, and the transcriptional dynamics were described via weighted gene co-expression network analysis. To reveal the main events underlying grafting, the correlations between modules and grafting traits were analyzed. Functional annotation showed that during the entire graft process, signal transduction was activated in the scion, while messenger RNA splicing was induced in the rootstock. At 2 days after grafting, the main processes occurring in the scion were associated with protein synthesis and processing, while the primary processes occurring in the rootstock were energy release-related. During the period of 7-14 days after grafting, defense response was a critical process taking place in the scion; however, the main process functioning in the rootstock was photosynthesis. From 22 to 32 days after grafting, the principal processes taking place in the scion were jasmonic acid biosynthesis and defense response, whereas the highly activated processes associated with the rootstock were auxin biosynthesis and plant-type secondary cell wall biogenesis. To further prove that the graft partners responded asymmetrically to stress, hydrogen peroxide contents as well as peroxidase and β-1,3-glucanase activities were detected, and the results showed that their levels were increased in the scion not the rootstock at certain time points after grafting. Our study reveals that the scion and rootstock might respond asymmetrically to grafting in pecan, and the scion was likely associated with stress response, while the rootstock was probably involved in energy supply and xylem bridge differentiation during graft union formation.

The Effects of a New Citrus Rootstock Citrus junos cv. Shuzhen No. 1 on Performances of Ten Hybrid Citrus Cultivars

The importance of rootstock in citrus production lies in its crucial role in determining tree growth, environmental stress tolerance, and fruit quality. Citrus junos Siebold ex Tanaka cv. Shuzhen No. 1, a recently developed rootstock, demonstrates excellent graft compatibility and abiotic stress tolerance. The objective of this study was to assess ten hybrid citrus cultivars grafted onto two C. junos rootstock selections, with the aim of determining the potential for industrial utilization of the new citrus rootstock. All graft junctions are mature and well established. Vigorous growth characterized all ten citrus cultivars on Shuzhen No. 1, with the largest tree's height reaching 280.33 cm (Wogan scion) and the widest scion's diameter being 67.52 cm (Chunjian scion). However, the scion-to-rootstock diameter ratio was the lowest at 0.62 (Chunxiang scion). C. junos rootstock selections significantly affected fruit weight (five of ten scions) and fruit color (seven of ten scions) but had negligible impact on peel thickness (nine of ten scions). Furthermore, rootstock type had a significant influence on fruit quality. In conclusion, our findings indicate strong graft compatibility between all scions and C. junos rootstocks, which can impact overall size and fruit quality. Based on these results, Shuzhen No. 1 is recommended as a valuable citrus rootstock.

Unveiling the mechanism of source-sink rebalancing in cucumber-pumpkin heterografts: the buffering roles of rootstock cotyledon

Grafting onto pumpkin rootstock is widely applied in cucumber production to improve growth and yield, as well as to overcome soil-borne diseases and enhance resistance to abiotic stresses. In this study, we constructed the cucumber-pumpkin heterografts with the one-cotyledon grafting method, and examined the effects of heterografting on biomass allocation and sugar partitioning, with cucumber and pumpkin self-grafts used as control. Compared with cucumber self-grafts, heterografting onto pumpkin rootstock promoted photosynthesis in cucumber scion, and led to higher sucrose contents in the 1st true leaf (source) and newly emerged leaf (sink). Thereby, the scion part of heterografts accumulated more biomass than cucumber self-grafts. In contrast, when compared to pumpkin self-grafts, grafting with cucumber scion reduced root vigor and biomass but promoted cotyledon growth in pumpkin rootstock. The roots (sink) of heterografts contained less sucrose and hexoses, and showed reduced sucrose synthase (SuSy) and hexokinase (HXK) activities. However, the rootstock cotyledon (source) contained more sucrose and starch, and showed higher activities of HXK, cell-wall invertase (CWIN), and enzymes for starch synthesis and degradation. Furthermore, removal or shade of rootstock cotyledon led to reduced growth of root and scion. Silencing of CmoMEX1a gene in rootstock cotyledon inhibited maltose export and reduced root growth of heterografts. These results indicated that rootstock cotyledon, especially its starch content, played a buffering role in the growth regulation of cucumber-pumpkin heterografts. Taken together, our results provided a major contribution to our understanding of source-sink sugar partitioning and scion-rootstock growth balancing in cucumber-pumpkin heterografts.

Comprehensive review: Effects of climate change and greenhouse gases emission relevance to environmental stress on horticultural crops and management

Global climate change exerts a significant impact on sustainable horticultural crop production and quality. Rising Global temperatures have compelled the agricultural community to adjust planting and harvesting schedules, often necessitating earlier crop cultivation. Notably, climate change introduces a suite of ominous factors, such as greenhouse gas emissions (CGHs), including elevated temperature, increased carbon dioxide (CO2) concentrations, nitrous oxide (N2O) and methane (CH4) ozone depletion (O3), and deforestation, all of which intensify environmental stresses on crops. Consequently, climate change stands poised to adversely affect crop yields and livestock production. Therefore, the primary objective of the review article is to furnish a comprehensive overview of the multifaceted factors influencing horticulture production, encompassing fruits, vegetables, and plantation crops with a particular emphasis on greenhouse gas emissions and environmental stressors such as high temperature, drought, salinity, and emission of CO2. Additionally, this review will explore the implementation of novel horticultural crop varieties and greenhouse technology that can contribute to mitigating the adverse impact of climate change on agricultural crops.

Genome-wide identification and expression analysis of xyloglucan endotransglucosylase/hydrolase genes family in Salicaceae during grafting

BACKGROUND

Poplar (Populus cathayana)and willow (Salix rehderiana) are important fast-growing trees in China. Grafting plays an important role in improving plant stress resistance and construction of ornamental plants. It is found that willow scions grafted onto poplar rootstocks can form ornamental plants. However, this grafted combination has a low survival rate. Many studies have reported that the xyloglucan endotransglucosylase/hydrolase (XTH) family plays an important role in the healing process of grafts.

RESULTS

A total of 38 PtrXTHs and 32 SpuXTHs were identified in poplar and willow respectively, and were classified into three subfamilies. Tandem duplication was the main reason for the expansion of the PtrXTHs. Grafting treatment and Quantitative real time PCR (RT-qPCR) analysis revealed that five XTH genes differentially expressed between self-grafted and reciprocal grafted combinations. Specifically, the high expression levels of SrXTH16, SrXTH17, SrXTH25, PcXTH22 and PcXTH17 may contribute to the high survival rate of the grafted combination with willow scion and poplar rootstock. Subcellular localization identified that the SrXTH16, SrXTH17, SrXTH25, PcXTH17 and PcXTH22 proteins were located on the cell walls. Transcription factors (NAC, MYB and DOF) may regulate the five XTH genes.

CONCLUSIONS

This study provides a new understanding of the roles of PcXTH and SrXTH genes and their roles in grafting. Our results will give some hints to explore the molecular mechanisms of PcXTH and SrXTH genes involved in grafting in the future.