The role of horticulture in plant invasions: how grafting in cultivars of Callery pear (Pyrus calleryana) can facilitate spread into natural areas

Although few plant species escape from cultivation, some horticultural practices used with woody ornamentals may enhance their invasion potential into natural areas. One such procedure is grafting, in which individuals are propagated for commercial sale by joining a clone of a desired cultivar (the scion) with rootstock obtained from a different individual. If the rootstock can sprout and flower, it can potentially cross-pollinate the scion, leading to fruit production in a self-incompatible species. The effect of grafting on invasion success was examined in the Callery pear, Pyrus calleryana, one of the most popular landscaping trees in the United States. Using nine microsatellite markers, scion and rootstock were genotyped for 57 cultivated trees to characterize rootstock genotypes and the rootstock gene pool. Invasive populations were then sampled to determine if rootstock genotypes have contributed to their formation. In no case were scion and rootstock genotypes identical for any given cultivated tree and rootstock genotypes were genetically variable, although some cloned rootstock genotypes were detected. Rootstock genotypes were also observed in invasive populations, with up to 17% of wild individuals having at least one rootstock parent. These results indicate that rootstock can contribute to the formation of invasive populations of the Callery pear through cross-pollination with other available genotypes. Future investigations of woody ornamentals propagated by grafting should consider this horticultural practice as a potential contributor to invasiveness. Furthermore, plant breeders are encouraged to assess the ability of rootstock to sprout as well as its cross-compatibility with the scion or other cultivar genotypes growing nearby.     

Reverse-cleft in vitro micrografting of <Emphasis Type="Italic">Ziziphus jujuba</Emphasis> Mill. Infected with jujube witches’ broom (JWB)

Jujube witches’ broom (JWB) is a destructive disease, caused by a phytoplasma, of Chinese jujube (Ziziphus jujuba Mill.), an important traditional fruit in China and South Korea. Micrografting of plantlets to detect those affected with JWB is difficult due to differences in stem diameter between the scion and rootstock. In this study, an in vitro micrografting technique, reverse-cleft graft, suitable for rootstock thinner than scion was developed and optimized. The JWB phytoplasma was successfully transmitted into the scion from diseased rootstock using this technique as determined by polymerase chain reaction. Micrografted plantlets of Chinese jujube were incubated on Murashige and Skoog medium without plant growth regulators. The survival frequencies of 78–91%, depending on grafting combinations, were obtained.     

Biosafety considerations of RNAi-mediated virus resistance in fruit-tree cultivars and in rootstock

A major application of RNA interference (RNAi) is envisaged for the production of virus-resistant transgenic plants. For fruit trees, this remains the most, if not the only, viable option for the control of plant viral disease outbreaks in cultivated orchards, due to the difficulties associated with the use of traditional and conventional disease-control measures. The use of RNAi might provide an additional benefit for woody crops if silenced rootstock can efficiently transmit the silencing signal to non-transformed scions, as has already been demonstrated in herbaceous plants. This would provide a great opportunity to produce non-transgenic fruit from transgenic rootstock. In this review, we scrutinise some of the concerns that might arise with the use of RNAi for engineering virus-resistant plants, and we speculate that this virus resistance has fewer biosafety concerns. This is mainly because RNAi-eliciting constructs only express small RNA molecules rather than proteins, and because this technology can be applied using plant rootstock that can confer virus resistance to the scion, leaving the scion untransformed. We discuss the main biosafety concerns related to the release of new types of virus-resistant plants and the risk assessment approaches in the application of existing regulatory systems (in particular, those of the European Union, the USA, and Canada) for the evaluation and approval of RNAi-mediated virus-resistant plants, either as transgenic varieties or as plant virus resistance induced by transgenic rootstock.     

Transgene mobilization and regulatory uncertainty for non-GE fruit products of transgenic rootstocks

Genetically engineered (GE) rootstocks may offer some advantages for biotechnology applications especially in woody perennial crops such as grape or walnut. Transgrafting combines horticultural grafting practices with modern GE methods for crop improvement. Here, a non-GE conventional scion (upper stem portion) is grafted onto a transgenic GE rootstock. Thus, the scion does not contain the genetic modification present in the rootstock genome. We examined transgene presence in walnut and tomato GE rootstocks and non-GE fruit-bearing scions. Mobilization of transgene DNA, protein, and mRNA across the graft was not detected. Though transgenic siRNA mobilization was not observed in grafted tomatoes or walnut scions, transgenic siRNA signal was detected in walnut kernels. Prospective benefits from transgrafted plants include minimized risk of GE pollen flow (Lev-Yadun and Sederoff, 2001), possible use of more than one scion per approved GE rootstock which could help curb the estimated US$136 million (CropLife International, 2011) cost to bring a GE crop to international markets, as well as potential for improved consumer and market acceptance since the consumable product is not itself GE. Thus, transgrafting provides an alternative option for agricultural industries wishing to expand their biotechnology portfolio.     

植物嫁接愈合分子机制研究进展

嫁接能显著改良单一品种的产量、品质和抗逆等性状, 已广泛应用于农业生产。促进砧木和接穗在嫁接面的快速愈合有利于提高嫁接效率。目前对嫁接愈合调控机制尚了解不足, 因此短时间内难以进行有效的技术改良。嫁接愈合过程包括先后发生的创伤应激响应、愈伤组织形成、砧穗细胞通讯以及砧穗再生重连等生理事件, 均涉及复杂而交联的激素应答及基因调控模式。近年来, 相关领域的研究成果为综合解析嫁接愈合的调控机制奠定了基础。该文综述了在嫁接愈合过程中发挥核心作用的植物激素及其应答方式, 以及激素依赖或非依赖的基因表达调控模式, 以期为深入揭示嫁接愈合分子机制提供参考。     

In vitro shoot-tip grafting improves recovery of cotton plants from culture

A rapid in vitro shoot-tip grafting (STG) technique was adapted to increase recovery of intact cotton plants from shoots developed in culture. Induction of root organogenesis in cotton shoots is genotype dependent and unreliable. The resulting loss of regeneration potential due to failure to form roots can vary from 30 to 80% according to genotype and represents a significant bottleneck in the overall recovery of plants from culture. If the non-rooting shoots are transgenic, the loss in regenerated plant material can be substantial. In vitro grafting of cotton shoots to seedling rootstock proved to be a simple and reliable method allowing 90–100% recovery of non-rooting shoots from culture. Success of any given graft was directly related to scion size (0.8–1.0 cm) and age (14–35 days) of the seedling rootstock. The method appeared to be genotype independent, and varietal differences between rootstock and scion did not effect the rate of plant recovery from culture. This revised version was published online in June 2006 with corrections to the Cover Date.     

Stimulation of scion bud release by rol gene transformed rootstocks of Rosa hybrida L

For improvement of the root characteristics of roses, the rolA, B and C genes from Agrobacterium rhizogenes were introduced in the cultivar Moneyway, which is used as a rootstock. Four independent rolA+B+C transformants were produced. One rolC expressing transformant, showing good growth and clearly decreased apical dominance, was selected for a grafting experiment with the cut rose cultivar Madelon as a scion. Grafting on this rolA+B+C transformed rootstock resulted in a stimulation of both root development of the rootstock and axillary bud release of the untransformed scion. Axillary bud release increased from 0.1 to 0.6 and from 0.3 to 1.3 basal shoots per plant at 15C and 20C, respectively. Since basal shoots from flowers, this altered plant architecture will presumably lead to a more efficient flower production.Keywords: Agrobacterium transformation, apical dominance, rol genes, rootstock, rose.      

Effects of mutual grafting on the cadmium accumulation characteristics of two ecotypes of Solanum photeinocarpum

Abstract

The effects of mutual grafting on the cadmium (Cd) accumulation characteristics of two ecotypes (farmland and mining) of the potential Cd-hyperaccumulator Solanum photeinocarpum were studied through a pot experiment for one month. Four treatments were used in the experiment: ungrafted farmland ecotype (F-CK), ungrafted mining ecotype (M-CK), the farmland ecotype as the scion grafted onto rootstocks of the mining ecotype (F-Scion), and the mining ecotype as the scion grafted onto rootstocks of the farmland ecotype (M-Scion). Mutual grafting increased the rootstock biomass of both S. photeinocarpum ecotypes. However, mutual grafting decreased the scion biomass of F-Scion compared with F-CK and M-CK, and the scion biomass of M-Scion was higher than that of M-CK and lower than that of F-CK. The Cd content in the rootstock of M-Scion increased compared with F-CK, and the Cd content in the rootstock of F-Scion increased compared with M-CK, but mutual grafting decreased the Cd content in scions of both S. photeinocarpum ecotypes. Mutual grafting increased Cd extraction by rootstocks of both S. photeinocarpum ecotypes, but decreased extraction by scions. Therefore, mutual grafting can increase Cd accumulation in S. photeinocarpum rootstocks but not increase Cd accumulation in S. photeinocarpum scions in a short period.     

A Rootstock Provides Water Conservation for a Grafted Commercial Tomato (Solanum lycopersicum L.) Line in Response to Mild-Drought Conditions: A Focus on Vegetative Growth and Photosynthetic Parameters

The development of water stress resistant lines of commercial tomato by breeding or genetic engineering is possible, but will take considerable time before commercial varieties are available for production. However, grafting commercial tomato lines on drought resistant rootstock may produce drought tolerant commercial tomato lines much more rapidly. Due to changing climates and the need for commercial production of vegetables in low quality fields there is an urgent need for stress tolerant commercial lines of vegetables such as tomato. In previous observations we identified a scion root stock combination (‘BHN 602’ scion grafted onto ‘Jjak Kkung’ rootstock hereafter identified as 602/Jjak) that had a qualitative drought-tolerance phenotype when compared to the non-grafted line. Based on this initial observation, we studied photosynthesis and vegetative above-ground growth during mild-drought for the 602/Jjak compared with another scion-rootstock combination (‘BHN 602’ scion grafted onto ‘Cheong Gang’ rootstock hereafter identified as 602/Cheong) and a non-grafted control. Overall above ground vegetative growth was significantly lower for 602/Jjak in comparison to the other plant lines. Moreover, water potential reduction in response to mild drought was significantly less for 602/Jjak, yet stomatal conductance of all plant-lines were equally inhibited by mild-drought. Light saturated photosynthesis of 602/Jjak was less affected by low water potential than the other two lines as was the % reduction in mesophyll conductance. Therefore, the Jjak Kkung rootstock caused aboveground growth reduction, water conservation and increased photosynthetic tolerance of mild drought. These data show that different rootstocks can change the photosynthetic responses to drought of a high yielding, commercial tomato line. Also, this rapid discovery of one scion-rootstock combination that provided mild-drought tolerance suggests that screening more scion-rootstock combination for stress tolerance may rapidly yield commercially viable, stress tolerant lines of tomato.     

植物离体茎段嫁接

植物体茎段嫁接系统是在无菌条件下将茎切段嫁接后放入培养基中、使接穗和砧木分别与含不同成分的培养基接触,再署光下培养的一个模拟植物正常生理过程、环境条件可控的实验系统,离体茎段嫁接体的发育与整体类拟,包括接穗与砧木粘连、愈伤组织产生、次生甩间连丝形成和维管束分化等几个步骤,发育进程受植物激素如生长素和细胞分裂素调节。该系统的建立为阐明嫁接体发机理及嫁接亲和性机制提供了重要的依据。     

A Study of the graft union in in vitro micrografted apple

Light microscopy was used to study graft union formation in in vitro micrografts of tissue cultured apple (Malus domestica. Borkh). Micrografts were constructed using horizontal incisions to form the grafting surface, and placing the cut ends of rootstock and scion into sterile silicon tubing to permit graft formation to occur.The outer morphological and histological development was similar for different stock-scion micrograft combinations but graft union formation was slower in heterografts than in autografts. Initial leaf expansion at the scion shoot apex occurred in all micrografted plantlets within 1–4 days and was not indicative of graft success. Progressive scion growth and development could be used as an indication of graft success by ten to fourteen days after grafting and probably was related to establisment of cell to cell contact at the graft interface. Microscopy showed initiation of callus proliferation in the vascular cumbium and the pith ten days after grafting. Differentiation was observed subsequently and this was reflected in scion development. Longitudinally orientated cambial cells began to differentiate between twenty and forty days after grafting, and formed a bridge between the vascular cylinders of scion and rootstock. The scions at this stage had as many as eight newly expanded leaves and micrografts were strong enough to permit silicon sleeve removal without damage. Continuity of new vascular elements in rootstock and scion was established around forty days. New vascular elements curved slightly in towards the pith to form a c shaped bridge across the graft union. Vascular development continued until it reached completion after six months.     

Use of a Mini-Dome Bioassay and Grafting to Study Resistance of Chickpea to Ascochyta Blight

A mini‐dome bioassay was developed to study pathogenicity of Ascochyta rabiei and relative resistance of chickpea (Cicer arietanium). It was determined that the best condition for assaying pathogenicity of A. rabiei was to use 2 × 10⁵ spores/ml as inoculum and to maintain a leaf wetness period of 24 h under mini‐domes at a temperature between 16 and 22°C. This mini‐dome pathogenicity assay was used to determine relative resistance of six chickpea cultivars (cvs) to isolates of two pathotypes of A. rabiei. Grafting was employed to detect any translocated factors produced in the chickpea plant that mediate disease response, which could help elucidate possible resistance mechanisms to Ascochyta blight. The six chickpea cv. were grafted in all possible scion–rootstock combinations, and then inoculated with isolates of two pathotypes of A. rabiei using the mini‐dome technique. Results showed that self‐grafted‐resistant plants remained resistant and self‐grafted‐susceptible plants stayed susceptible, indicating the grafting procedure did not alter host response to infection by A. rabiei. Susceptible scions always exhibited high and similar levels of disease severity regardless of rootstock genotypes, and resistant scions always showed low and similar levels of disease severity when they were grafted onto any of the six rootstock genotypes. Orthogonal contrasts showed that scion genotypes determined disease phenotype, and that rootstock genotypes had no contribution to disease phenotype of the scions. The pathogenicity assay did not detect any translocated disease‐mediating agents responsible for susceptibility or resistance in chickpea. Disease phenotypes of Ascochyta blight of chickpea were conditioned locally by scion genotypes.     

Relative Importance of Rootstock and Scion in Determining Growth and Fruiting in Young Apple Trees

Principal component analysis has been used on annual measuresof vegetative and reproductive activity of young apple trees.The trial consisted of four clones (M.2, M.4, M.9, and M.16)used as both rootstock and scion in all possible combinationsto give 16 tree types. The results have suggested that theseannual records can best be summarized by their sum and lineartrend with time. Further analyses using these new measures ofsum and trend have enabled the effects of rootstock and scionto be partitioned. Increase in stem dry weight is largely rootstock-controlledbut trend with time is mainly scion-controlled. The reverseoccurs with fruit number where total fruit is largely scion-controlledbut the trend is dominated by the rootstock. Measures of leadergrowth are intermediate between stem growth and fruiting intheir responses. Statistical interactions are greatest withfruiting records which suggests that this activity is less harmoniouslycontrolled by rootstock and scion than other activities. Use of sum and linear trend is recommended for future analysesof apple tree trials without recourse to further multivariateanalyses.     

The effect of shoot and root genotype on phosphorus concentrations of shoots and roots

Genotypes of two morphologically different populations of white clover (Trifolium repens L.) were reciprocally and self-grafted. Ungrafted stolon tips were also grown as controls. Grafting per se had no significant effect on shoot size, root size, leaflet width or shoot and root % P. The scion genotype had a significant effect on shoot and root size, and leaflet width. Neither scion nor rootstock genotype had a significant effect on either shoot or root % P. However, there was a significant scion × rootstock × P level interaction for shoot % P. This along with other evidence suggests that conflicting results regarding effects of scion and rootstock on % P content of plants within species is probaby due to the interaction of scion and rootstock with environment.This work was undertaken at the Department of Agricultural Botany, University of Reading, Reading, England.This work was undertaken at the Department of Agricultural Botany, University of Reading, Reading, England.