Intergeneric Grafts Research Articles

Effects of Intergeneric Grafting of Schisandraceae on Root Morphology, Anatomy and Physiology of Rootstocks

Kadsura coccinea (Lem.) A. C. Smith and Schisandra sphenantha rehd. et wils. are different genus plants of Schisandraceae, distributed in the north and south of China, respectively. These species are non-wood forest trees with important medicinal value. Grafting is one of the technical means by which plants migrate to other regions to adapt to different habitats. However, the current research on the grafting of Schisandraceae only focuses on the effect of the rootstock on the scion, and lacks investigations on the effect of the scion on the rootstock, leading to the mechanism of the rootstock–scion interaction being still unclear. In this study, Kadsura coccinea (K) and Schisandra sphenanthera (S) were used as scion and rootstock for reciprocal grafting, and four grafted combinations, K/S (scion/rootstock), S/S, S/K, and K/K, were obtained, with S/S and K/K being the controls. Comparative analyses of the morphological, anatomical, and physiological characteristics of rootstocks were conducted at 10 d, 30 d, 50 d, and 70 d after grafting. The results showed that compared with the control, the total root length, root volume, root tips, and root biomass of K/S and S/K decreased. However, the root average diameter and root surface area of K/S increased, while those of S/K decreased. All the differences between the above indices and the control were about 10%, and almost all of them changed most significantly at 70 d after grafting. Anatomically, the root cross section, stele diameter, and xylem area of K/S increased, while those of S/K decreased. In addition, the cortex thicknesses of K/S and S/K increased. These anatomical changes were mainly reflected in the second-order and third-order roots. Meanwhile, the changes in root endogenous hormone (IAA, ZR, GA3, ABA) contents in K/S and S/K in relation to days after grafting could explain the differences in root morphology. Moreover, both K/S and S/K had lower root activity and soluble sugar content than the control, and S/K had lower soluble protein and higher malondialdehyde content. This study indicated that the intergeneric grafting of Schisandraceae inhibited the growth of rootstocks, and the degree of inhibition was potentially related to the relative strength of the rootstock and scion, which provides a theoretical basis for further research on the rootstock–scion interaction mechanism.

Grafting compatibility between Okra cultivars and root-knot nematode resistant Kenaf

The use of intergeneric grafting has been reported as an alternative to manage root­knot nematodes in okra, but the compatibility for grafting has only been tested in a few okra (Abelmoschus esculentus L. Moench) cultivars. The kenaf (Hibiscus cannabinus L.) is resistant to root­knot nematode species and is a potential rootstock for okra. The objective was to study the compatibility of kenaf as rootstock with okra cultivars. It was used a completely randomized design, in factorial scheme 3x10, with five repetitions. The compatibility was assessed by measuring several vegetative characteristics. All cultivars are compatible for grafting with kenaf as rootstock. Grafting onto kenaf may be an option to control root­knot nematodes.

Quantitative Analysis of Florigen for the Variability of Floral Induction in Cabbage/Radish Inter-generic Grafting.

Grafting-induced flowering is a key phenomenon to understand systemic floral induction caused by florigen. It can also be used as a breeding technique enabling rapid seed production of crops with long generation times. However, the degree of floral induction in grafted plants is often variable. Moreover, it is difficult in some crop species. Here, we explored the factors promoting variability in the grafting-induced flowering of cabbage (Brassica oleracea L. var. capitata), an important vegetable crop with a long generation time, via the quantitative analysis of florigen accumulation. Significant variability in the flowering response of grafted cabbage was observed when rootstocks of different genotypes were used. As reported previously, B. oleracea rootstocks did not induce the flowering of grafted cabbage plants, but radish (Raphanus sativus L.) rootstocks unstably did, depending on the accessions used. Immunoblotting analysis of the FLOWERING LOCUS T (FT) protein, a main component of florigen, revealed that floral induction was quantitatively correlated with the level of accumulated FT protein in the grafted scion. To identify rootstock factors that cause variability in the floral induction of the grafted scion, we investigated FT protein accumulation and flowering response in grafted scions when the transcription levels of FT and the leaf area of rootstocks were altered by vernalization, daylength and leaf trimming treatments. We concluded that increasing the total amount of FT protein produced in the rootstock is important for the stable floral induction of the grafted cabbage, and this can be accomplished by increasing FT transcription and the leaf area of the rootstock.

Graft Compatibility Classification within Aurantioideae Based on Biometric Traits and the Anatomy of Graft Union

Citrus relatives are a relevant source of valuable traits for use in citrus breeding, including resistance to diseases such as Huanglongbing (HLB). Resistant rootstocks may impact tree responses to HLB. This requires graft compatibility, which has been poorly investigated within the Aurantioideae. In this study, the biometric characteristics and the anatomy of the graft union of 86 scion/rootstock combinations were assessed. This comprised 18 genotypes/species and 8 genera from Citrinae, Balsamocitrinae, and Clauseninae subtribes sensu Swingle and Reece. Most graft combinations were found to be noncompatible. Phylogenetic proximity did not ensure successful grafting as, for example, Orange jasmine autografts failed, whereas some intergeneric grafts were successful (>60% of graft-take). Plant scion height was directly related to graft-take, but the correlation between the scion and rootstock stem diameters was not a reliable indicator of graft compatibility. Rangpur/Tabog, Tabog/Rangpur, Wampee/Rangpur, Wampee/Pomeroy, Wampee/Swingle, Pomeroy/Wampee, and Swingle/Wampee were the most compatible intergeneric graft combinations. Graft-take success for this was at similar levels to those of sweet orange grafted on common citrus rootstocks. The position as a scion or rootstock in the combination affected the performance and was specific to the genotypes tested. The lack of differentiation between xylem-derived calli and the accumulation of phenolic compounds at the graft union were clear anatomical and biochemical markers, respectively, of incompatibility for most Aurantioideae combinations. In the field, within a set of the ten most promising combinations, Hamlin/Rangpur (control) was the only one that became infected by ‘Candidatus Liberibacter asiaticus’. This was first observed 12 months after planting. Overall, the assessment of biometric traits and anatomy of the graft union allowed Aurantioideae genotypes to be divided into four clusters, with respect to their graft compatibility, as follows: fully compatible with high graft-take and plant growth; potentially compatible with high graft-take but lower plant growth; partially incompatible with lower graft-take and poor plant growth; and fully incompatible with a complete absence of graft-take.

A Novel Graft between Pac Choi (Brassica rapa var. chinensis) and Daikon Radish (Raphanus sativus var. longipinnatus)

Vegetable grafting has primarily been used in the commercial production of high-value crops in the Solanaceae and Cucurbitaceae families. In this study, we explored the feasibility of making a novel graft between pac choi (Brassica rapa L. var. chinensis) and daikon radish (Raphanus sativus L. var. longipinnatus) to create a plant with harvestable pac choi leafy vegetable above-ground, and a daikon radish taproot below-ground. ‘Mei Qing Choi’ pac choi (scion) was grafted onto ‘Bora King’ daikon radish (rootstock). Grafted pac choi–daikon radish plants did not show a decrease in SPAD value, canopy size, leaf number, leaf area, or above-ground weight compared with self-grafted pac choi plants. However, taproot formation was reduced in grafted pac choi–daikon radish plants, as shown by decreased taproot length, diameter, fresh weight, and dry weight compared with non- and self-grafted daikon radish plants. Surprisingly, grafting with radish increased the photosynthetic rate of the pac choi. This pilot study demonstrated the potential of creating a new pac choi–daikon radish vegetable product to help save growing space and minimize waste at consumption, as pac choi roots are not eaten and radish leaves are usually discarded. The inter-generic grafting between B. rapa var. chinensis and R. sativus var. longipinnatus could also provide a unique model system to help elucidate scion-rootstock synergy and above- and below-ground sink competition in horticultural crops.

Relationships between Rootstock-Scion Combinations and Growing Regions on Watermelon Fruit Quality

Grafting of vegetable plants is done primarily to reduce the potential for damage caused by soil-borne diseases. Most of the watermelons (Citrullus) grown in the Mediterranean Basin, including in Israel, are grafted, mainly on interspecific hybrid pumpkin (Cucurbita) rootstocks. Biblical law (Leviticus 19:19) does not allow intergeneric grafting, so in recent years, great efforts have been made in Israel to find or breed watermelon rootstocks. Both interspecific and intergeneric grafting can have negative or positive effects on fruit yield and quality after harvest. The inconsistencies in fruit quality and shelf-life parameters can be attributed to differences in production environments. However, many farmers are grafting and planting the same rootstock-scion combination all over the country, regardless of local soil, water, and climactic conditions. We studied the effect of similar rootstock-scion combinations on watermelon yield and fruit quality in three regions of Israel differing in soil type and altitude. Fruit-quality parameters were evaluated after 4 days at 21 °C (local marketing simulation). Fruit quality was significantly affected, mainly by the growing region, based on factorial analysis, but also by rootstock-scion combination, regardless of rootstock vigor. Therefore, the best rootstock-scion combination needs to be found and adopted for each growing region. Grafting was essential for watermelon crop survival in contaminated soils and improved both plant performance and postharvest fruit quality, but was not a factor in non-contaminated soils.

Methods for grafting Arabidopsis thaliana and Eutrema salsugineum

BackgroundGrafting, an ancient agronomic technique, is an artificial mode of asexual reproduction in plants. Recently, grafting research has gradually shifted from modifying agronomic traits to the study of molecular mechanism. Grafting is an excellent tool to study long-range signaling processes in plants. And the grafting between species will help elucidate the molecular mechanisms underlying contrasting differences between different species. Arabidopsis thaliana is a salt-sensitive model glycophyte and Eutrema salsugineum (previously Thellungiella salsuginea, salt cress) is a salt-tolerant model halophyte. Successful grafting of these two model plants will help further study the physiological and molecular mechanisms underlying salt tolerance in plants. The aim of this study was to demonstrate two sterile micro-grafting methods for Arabidopsis and salt cress.ResultsWe developed the methods for sterile grafting between A. thaliana and E. salsugineum; this is the first report on inter-generic grafting between Arabidopsis and Eutrema. The method involves cut-in grafting under sterile conditions. The grafted plant part was placed in half strength Murashige and Skoog medium with 1% agar and 1% sugar, and then cultured vertically with 22 °C/18 °C short-day/night cycles. The plants were then transferred to half strength Hoagland nutrient solution for hydroponics. The reported method is simple and easy to operate. Self-grafted Arabidopsis–Arabidopsis and Eutrema–Eutrema plants were used as controls, which were obtained with an improved hypocotyl-cutting grafting method. Ion contents in grafted plants were detected by inductively coupled plasma optical emission spectroscopy. The results showed that the ion content in salt cress and Arabidopsis changed to different degrees after grafting.ConclusionsThe inter-species grafting technique described here makes it possible to study hybrid plants between Arabidopsis and Eutrema and will contribute to further understanding of long-distance communications in plants. This technique also provides a reference for improving plant varieties using grafting, such as gardening plants, as well as fruit and vegetable crops.

Intergeneric grafting for enhanced growth, yield and nutrient acquisition in greenhouse cucumber during winter

Intergeneric grafting for enhanced growth, yield and nutrient acquisition in greenhouse cucumber during winter

Effects of tomato and potato heterografting on photosynthesis, quality and yield of grafted parents

Grafting has been widely used to improve quality, yield, and tolerance to biotic and abiotic stresses in plants. Tomato/potato heterografting combines two different species to create a new variety that can harvest tomato fruits and tubers from scions and rootstocks, respectively. This could be an optimal agricultural technique for improving yield and utilizing nature resources effectively, but the growth and development of rootstocks and scions after grafting are rarely described. The present study aimed to determine the effects of tomato/potato heterografting on the physiological characters, quality and yield of fruits and tubers and study the changes in photosynthetic capacity of tomato scion grafted to potato rootstock. In this study, the Zhongyan988 (ZY988) tomato cultivar four potato cultivars, Lishu6 (LS6), Qingshu9 (QS9), Hezuo88 (HZ88) and Diantongshu1 (DTS1), served as grafting parents and ungrafted tomato and potato plants were used as controls. Intergeneric grafting between tomato and potato was an effective agricultural approach that improved the yield per unit area on existing cultivated land. Under normal growth conditions, grafting had no significant effect on the photosynthetic capacity of the tomato leaves. Some characteristics related to the tomato fruit quality were improved by the potato rootstocks, including vitamin C (Vc), total soluble solids and soluble sugars; however, the titratable acidity content decreased in these grafted plants. The tomato fruit size and fruit number were affected by grafting. Grafting produced more fruit with lower average weight per fruit. Grafting had a significantly promoted tuber sprouting during the harvest period and decreased the tuber number per plant. Furthermore, the starch content and Vc level in the potato tubers were significantly decreased after grafting, but the reducing sugar content significantly increased. Scion and rootstock changes caused by grafting may be related to the accumulation and distribution of photosynthetic products and scion-rootstock interactions.

Improved drought resistance by intergeneric graftingin Salicaceae plants under water deficits

In this study, intergeneric grafting was employed between Populus cathayana and Salix rehderiana to investigate the grafting compatibility of the two Salicaceae plants and to reveal whether grafting can improve their drought resistance. Under different grafting combinations (P. cathayana scion with P. cathayana rootstock, P/P; P. cathayana scion with S. rehderiana rootstock, P/S; S. rehderiana scion with S. rehderiana rootstock, S/S; and S. rehderiana scion with P. cathayana rootstock, S/P), the survival and growth rate, biomass accumulation and allocation, photosynthetic traits, carbon isotope composition (δ13C), relative water content (RWC) and non-structural carbohydrates (NSCs) were measured. The results showed that the grafting compatibility between P. cathayana and S. rehderiana was very high, as the survival rates ranged from 76% to 100% under different grafting combinations. Drought significantly decreased growth, biomass accumulation, photosynthetic pigment contents, net photosynthesis rates (Pn) and RWC, and increased δ13C in all grafting combinations. Under drought stress, biomass accumulation, total chlorophyll, transpiration rate (E) and Pn were higher in P/P and P/S than in S/S and S/P. Compared with P/P, the growth rate, biomass accumulation, root/aboveground ratio (R/A ratio), carotenoid, RWC, starch and total soluble sugar (TSS) of P/S were less affected by drought. The height growth rate (GRH), R/A ratio, carotenoid, chlorophyll a, total chlorophyll, WUEi and TSS of S/P were lower than those of S/S under water-limited conditions. Moreover, a principal component analysis indicated that P/S and S/S had higher drought resistance than P/P and S/P under water deficits. The used method allows combining specific advantageous traits from P. cathayana and S. rehderiana, which may be a highly useful tool to enhance drought resistance in the cultivation of Salicaceae plants.

Common and divergent shoot-root signalling in legume symbioses.

The role of shoot-root signals in the control of nodulation and arbuscular mycorrhizal (AM) development were examined in the divergent legume species pea and blue lupin. These species were chosen as pea can host both symbionts, whereas lupin can nodulate but has lost the ability to form AM. Intergeneric grafts between lupin and pea enabled examination of key long-distance signals in these symbioses. The role of strigolactones, auxin and elements of the autoregulation of nodulation (AON) pathway were investigated. Grafting studies were combined with loss-of-function mutants to monitor symbioses (nodulation, AM) and hormone effects (levels, gene expression and application studies). Lupin shoots suppress AM colonization in pea roots, in part by downregulating strigolactone exudation involving reduced expression of the strigolactone biosynthesis gene PsCCD8. By contrast, lupin shoots enhance pea nodulation, independently of strigolactones, possibly due to a partial incompatibility in AON shoot-root signalling between pea and lupin. This study highlights that nodulation and AM symbioses can be regulated independently and this may be due to long-distance signals, a phenomenon we were able to uncover by working with divergent legumes. We also identify a role for strigolactone exudation in determining the status of non-AM hosts.

Characterization of the growth and fruit quality of tomato grafted on a woody medicinal plant, Lycium chinense

Grafting has been widely used in vegetables, particularly in the plants of Solanaceae and Cucurbitaceae families because of its many significant advantages. Substantial studies have revealed that grafting can affect the yield and fruit quality of tomato through the vigorous rootstocks as reported previously. The selection of rootstocks plays an important role in improving the fruit yield and quality of scions. Lycium species (popularly known as goji or goji berry), also belonging to the Solanaceae family, is widely used as the functional fruits and vegetables, as well as traditional Chinese herbs. Goji plants also exhibit the excellent salt-tolerant ability. The intergeneric grafting across different genera, importantly with tomato and plant from different genus is little described. In this study, two commercial cultivars of tomato, Zhongshu 5 (ZS5) and TA209 as scions were grafted on goji plants (L. chinense Mill.), and the growth and fruit quality of tomato were investigated. The ungrafted self-rooted tomato plants were used as the control plants. The results showed that the growth of the grafted tomato plants was slightly depressed by grafting at the early development, but was restored like the self-rooted plants during the late development, and the net photosynthetic rate and chlorophyll content were higher in the grafted plants’ leaves at the fruit ripening stage. The size, shape, fresh weight and pigmentation pattern of tomato fruits were remarkably affected by grafting. Grafting produced smaller tomato fruit with lower fresh weight. An interesting observation was noticed that the pigmentation pattern of tomato fruits was close to that of goji berry. Some traits associated with fruit quality were improved by grafting approach, including total soluble solids (TSS), titratable acidity (TA), ratio of TSS/TA and vitamin C (Vc). The natural amino acids and nine mineral elements in tomato fruits were also affected by grafting. It was suggested that the increase of fruit quality may be attributed to the reduction of fruit water content and the effect of rootstock. This is the first report on intergeneric grafting between tomato and goji plants. The grafting procedure had a remarkable influence on the growth and fruit quality of tomato plants. With the perspective of high salt-tolerance of goji rootstocks, it is well worthy to study the performance of grafted tomato under saline condition in future.

Improving survival of inter-generic grafts of nematode-susceptible watermelon cultivars and nematode-resistant Cucumis species

Inter-generic grafting of highly nematode susceptible watermelon (Citrullus lanatus) cultivars onto nematode-resistant wild watermelon (Cucumis africanus) and wild cucumber (Cucumis myricoarpus) had on average 36% graft survival ratios, which was attributed to unequal stem diameters at the graft union. Cucumis species had smaller stem diameters at the graft union, whereas Citrullus had bigger ones. The objective of the study was to improve inter-generic graft compatibility of Citrullus and Cucumis through optimizing the sizes of stem diameters during grafting. Cucumis species were raised in a 160-hole seedling tray and primed seeds of Citrullus were raised in a 200-hole seedling tray seven days after emergence of Cucumis in order to reduce the stem diameters. At grafting, stem diameter ratios in various treatments were equal to one, whereas at 66 days after grafting the ratios were grater than one, with the exception of those of intact plants. Survival of grafts from grafting to 66 days after grafting was 100%, translating into relative improvement of 186%. Improved survival of grafts would invariably promote the potential uses of inter-generic grafting among wild and cultivated watermelon in management of soil-borne pathogens.

Use of indigenous Cucumis technologies as alternative to synthetic nematicides in management of root-knot nematodes in low-input agricultural farming systems: A review

The ”Indigenous Cucumis Technologies” (ICTs) were researched and developed for the management of plant-parasitic nematodes, particularly Meloidogyne species, in an attempt to alleviate the void created by the withdrawal of synthetic nematicides from the agro-chemical markets and the drawbacks associated with the use of conventional organic matter as a nematode management practice. Currently, ICTs comprises of four technology types, namely (1) ground leaching, (2) nematode resistance, (3) inter-generic grafting and (4) fermented crude extracts. ICTs, in their various forms, consistently suppressed the nematode numbers and improved crop yields in experimental trials carried out in Limpopo Province, Republic of South Africa. The present paper reviews a decade of successful research and development in ICTs for the management of root-knot nematodes in low-input agricultural farming systems. Key words: Cucumis species, fermented crude extract, ground leaching technology, inter-generic grafting, nematode resistance.

In vivo grafting of wild Lens species to Vicia faba rootstocks

Faba bean (Vicia faba L.) was used as the rootstock for lentil scions to test the feasibility of using in vivo inter-generic grafting techniques as a substitute for root induction and as a tool in lentil genetic improvement. An accession of each of the six wild Lens species was used as the scion in grafts to faba bean breeding line FB50-9 rootstock. Successful grafts were obtained for all species with survival of grafts to seed maturity between 70.7 and 87.7% except for Lens orientalis PI 72735 with 55.3% survival. Days to flower remained the same after grafting, except for scions of L. nigricans PI 72560 and L. orientalis PI 72735 which had a lag phase of 9 and 7 d, respectively. For all six wild species, seed diameter and seed weight were not significantly different between non-grafted controls and scions grafted onto faba bean rootstocks. This simple approach opens the possibility of using in vivo grafting techniques to rescue inter-specific hybrids of lentil. The technique has potential as a useful tool in lentil breeding, as a means of improving seed multiplication rate of rare genetic resources of wild lentil and as a way to reduce the costs of germplasm multiplication of wild lentil species.

Genetic Modification of Soybean with a Novel Grafting Technique: Downregulating the FAD2-1 Gene Increases Oleic Acid Content

A hairpin RNA-encoding construct targeting gmFAD2-1 was transformed into soybean, and an optimised Agrobacterium-mediated embryonic tip system was employed. A novel intergeneric grafting method using transgenic plantlets as scions was used instead of the conventional rooting method. Compared with traditional acclimatisation, the survival ratio of cleft grafts increased by 70%, and the culture period was shortened by about 40 days. The regeneration frequency of the grafted shoots in this embryonic tip system was approximately 74.6%. Soybean transformants were confirmed by Southern and Northern blot hybridisation analyses. The fatty acid composition of the T1 and T2 seeds from the transformed plants was determined by gas chromatography. The resulting downregulation of the gmFAD2-1 gene substantially increased the level of oleic acid from 16% to 55% as indicated by the oleic desaturation proportion (ODP). The ratio of plants with high ODP, moderate ODP and low ODP was about 1:2:1, which was consistent with a single-gene segregation pattern.

INTERGENERIC GRAFTING OF APPLE CULTIVARS IN THE 'JAPONÊS' QUINCE TREE

Researches developed by the Agronomic Institute (IAC) and EPAMIG, in Brazil, determinated the 'Japones' quince (Chaenomeles sinensis Koehne) as a rootstock for quince plants, in reason of its high seeds number per fruits (above of 180), high germination and emergence, and high grafting ability. New researches have been carried out, in an attempt to make the use of 'Japanese' quince possible for other pomoideae plants, as the loquat and the pear tree, concerning the nanism promoted by this intergeneric graft, that culminate in the plant size reduction and cultural management facilities. So that, the present experiment was conducted with the objective to evaluate the 'Japanese' quince as a rootstock for apple trees. The nursery trees used had 90 centimeters length and a diameter of eight millimeters (in the graft point), they where produced in plastic bags with a capacity of 3 L of substrate. In the first experiment the apple trees 'Anna', 'Imperial', 'Centenaria', 'Einshemer' and 'Michal' were used, and they were grafted by the cleft method in the hibernal period (July). In the second experiment the apple trees used were 'Eva', 'Ohio', 'Soberana', 'Rainha' and 'Dulcina' and they were grafted by bud method, in the vegetation phase (summer - January). After 60 days of the grafting, the bud-sprouting percentage was evaluated, and the length and graft average diameter were evaluated after 90 and 150 days of the plants' development. The cleft graft provided better results, with sprouting indices higher than 86%. The best results using the cleft grafting method were obtained with 'Einshemer' and 'Michal' apple trees, and with 'Eva' and 'Soberana' apple trees grafted by the bud method.

Histological observation of graft union in intergeneric grafting of mulberry

Histological observation of graft union in intergeneric grafting of mulberry

Identification of translocatable RNA‐binding phloem proteins from melon, potential components of the long‐distance RNA transport system

Phloem proteins (P-proteins) are an enigmatic group of proteins present in most angiosperm species. The best characterized P-proteins (PP1 and PP2) are synthesized in companion cells, transported into sieve elements via pore plasmodesmata and translocated through the plant. Characteristics such as long-distance translocation, RNA-binding activity and capacity of increasing plasmodesmata exclusion size suggest that certain phloem proteins could be involved in RNA transport within the plant, forming translocatable ribonucleoprotein complexes with endogenous or pathogenic RNAs. Long-distance movement of RNA through the phloem is a process known to occur, but both the mechanisms involved and the components constituting this potential information network remain unclear. Here, we demonstrate that several melon phloem proteins have a wide RNA-binding activity. Serological assays strongly suggest that one of these proteins is the melon phloem protein 2 (CmmPP2). Mass spectrometry analysis undoubtedly identifies another one as the recently characterized melon phloem lectin (CmmLec17). Grafting experiments demonstrate that the CmmLec17 is a translocatable phloem protein, able to move through intergeneric grafts from melon to pumpkin. Translocatability and RNA-binding activity was also demonstrated for an uncharacterized protein of approximately 14 kDa. In light of these results the possible involvement of these phloem proteins in the long-distance transport of melon RNAs is discussed.

A long-distance translocatable phloem protein from cucumber forms a ribonucleoprotein complex in vivo with Hop stunt viroid RNA.

Viroids are highly structured plant pathogenic RNAs that do not code for any protein, and thus, their long-distance movement within the plant must be mediated by direct interaction with cellular factors, the nature of which is presently unknown. In addition to this type of RNAs, recent evidence indicates that endogenous RNAs move through the phloem acting as macromolecular signals involved in plant defense and development. The form in which these RNA molecules are transported to distal parts of the plant is unclear. Viroids can be a good model system to try to identify translocatable proteins that could assist the vascular movement of RNA molecules. Here, we demonstrate by use of immunoprecipitation experiments, that the phloem protein 2 from cucumber (CsPP2) is able to interact in vivo with a viroid RNA. Intergeneric graft assays revealed that both the CsPP2 and the Hop stunt viroid RNA were translocated to the scion. The translocated viroid is symptomatic in the nonhost scion, indicating that the translocated RNA is functional. The CsPP2 gene was cloned and sequenced. The analysis of its primary structure revealed the existence of a potential double-spaced-RNA-binding motif, previously identified in a set of proteins that bind to highly structured RNAs, which could explain its RNA-binding properties. The possible involvement of this phloem protein in assisting the long-distance movement of the viroid RNA within the plant is discussed.