Grafting with selected resistant rootstocks for the purpose of controlling diseases, pests and abiotic stresses is an ancient practice conducted on a variety of wood plants. French grape industry that was almost destroyed by phylloxera (Dactylosphaera vitifolii) in nineteenth century was one of the famous examples. The crisis reversed only until grafting susceptible French grapes on a phylloxera resistant rootstock (Mudge et al 2009). Researches on herbaceous vegetable grafting started in 1920th, with watermelon (Citrullus lanatus) grafted on squash rootstocks (Cucurbita moschata) to overcome yield loss caused by Fusarium wilt (Sakata et al 2007). In addition, other soil-borne diseases, even a foliar disease or a plaque caused by insects were reported to be attenuated by grafting. Moreover, increased yield and improved tolerance of abiotic stresses were reported to associate with grafted plants. Due to those advantages, grafting technology has been rapidly extended to other vegetable crops (tomato, pepper, cucumber and melon). To date, virtually all the cucurbits for greenhouse cultivation have been grafted in Asia (Lee and Oda 2003) and 60% to 70% watermelon have been grafted in Israel (Cohen 2007). In North America, as phasing-out of methyl bromide, grafting is regarded as a promising alternative. Thus, much more attention is drawing on disease-alleviated effect of grafting technology.
Intra- and inter-specific grafting
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Grafting with scion and rootstock from the same species is regarded as intra-specific grafting, while with scion and rootstock from close members of the same botanical family is defined as inter-specific grafting (Cohen et al 2007). Generally, intra-specific grafting has less influence on fruit quality and better reunion of grafted site. However, disease resistance is normally limited, or exclusively to specific pathogen races. Commonly used rootstocks in Curcurbitacea family include squash (Cucurbita spp.), bottle gourd (Lagenaria siceraria) and wax gourd (Benincasa hispida). They exhibited broad range of disease resistance thus served as ideal rootstock germplasms for grafting watermelons (Citrullus lanatus), melons (Cucumis melon L.) and cucumbers (Cucumis sativus). However, fruit quality is a critical issue worth special notification when performing inter-specific grafting. It was reported that grafted 'Honey Dew' onto Cucurbita moschata rootstock led to inferior texture and fruit flavor even though some other reports suggested, under best circumstance and specific rootstock/scion combinations, grafting can actually enhance fruit quality (Davis 2008).
Diseases controlled by grafting in Cucurbits
Fusarium wilt The first reported cucurbit diseases controlled by grafting was Fusarium wilt (Fusarium oxysporum Schltdl.). It was reported that chlamydospores, the rest structure of the pathogen, can survive for 5 to 10 years without obtaining anything more than water in the soil (Roberts et al. 1972). Thus, traditional cultural practices like crop rotation and soil solarization are barely effective to this disease. Using resistance cultivars is a common practice for Fusarium wilt management. However, screening and selection resistance cultivars are time consuming. Moreover, occurrences of novel pathogens that overcome existed resistance make breeding work even more difficult (Hirai 2002). Bottle gourd rootstock and squash interspecific hybrids ( Cucurbita maxima - Cucurbita moschata) are the most commonly used rootstocks that have showed high resistance to most species of Fusarium wilt. Using those rootstocks has successfully controlled Fusarium wilt in cucumber, melon, bitter gourd in addition to watermelon.
Root-knot nematode Root-knot nematodes (Meloidogyne spp) attack nearly all the species of cucurbit plants. Galling on roots of susceptible plants is a typical symptom of the disease, which results in poor absorption of water and nutrients. In addition, wounds caused by nematodes on plant root give rise to secondary diseases caused by fungus and bacteria, resulting in more severe damage to plant root systems (Robert 1972). Therefore, breeding work of M. incognita resistant cucurbits is extremely difficult. At present, all the available bottle gourd and Cucurbita rootstocks are susceptible to root-knot nematode (Kousik et al. 2010). Resistant to M. incognita was found in Cucumis.metuliferus, C.licifolius, C. heptadactylus and C. longipes. But attempts to incorporate this resistance into C. melo have not been successful (Fassuliotis 1970). Grafting research indicated that susceptible melon grafted onto C. metuliferus reduced the levels of root galling as well as the nematode number at harvest (Siguenza et al. 2005). Progresses were also made on selecting resistant wild watermelons (Citrullus lanatus var. citroides) (Thies et al. 2010a). Recent study suggested that susceptible watermelon grafted on wild watermelon germplasm lines had significantly less root galling than non-grafted watermelon and plants with squash hybrid and bottle gourd rootstocks (Thies et al. 2010b). Recently, two Cucurbita cultivars that did not produce any galls when inoculated with root-knot nematode have been selected out from 23 tested cultivars. Interestingly, even galls on the roots of susceptible Cucurbita were smaller than those on susceptible muskmelons, watermelons, and cucumbers (Cohen et al. 2007). Altogether, these progresses provide promising future of using grafting to control root-knot nematode.
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Monosporascus sudden wilt Monosporascus sudden wilt, caused by Monosporascus cannonballus, is a common disease in hot and semiarid area, at where melon and watermelon cultivation are under severe threaten of this disease (Martyn and Miller 1996). Researches indicated that watermelon and melon grafted on Cucurbita rootstocks obtained improved disease tolerance against sudden wilt (Edelstein et al. 1999) although Cucurbita was normally regarded as host of Monosporascus cannonballus (Mertely et al. 1993, Kousik et al. 2010). By comparing disease development of non-grafted muskmelons, watermelons, with watermelons grafted onto Cucurbita moshata - C. maxima rootstocks, it was found out that the amount of pathogen isolated from Cucurbita roots was considerably lower, no perithecia observed, and soil population of ascospores remained stable as the development of vine decline symptoms (Beltrán 2008). However, the result that grafted plants with improved sudden wilt tolerance and better yield was inconsistent among different rootstock-scion combinations and growing conditions (Mertely et al. 1993, Cohen et al. 2005, Cohen et al. 2000a).
Verticillim wilt Verticillim wilt, caused by Verticillium dahlia, is another vascular wilt disease commonly affects cucurbit plants. Currently, genetic resistance to Verticillium wilt in cucurbits is still unavailable, thus crop rotation, soil sterilization and soil solarization are the alternative strategies for disease management. Studies have been conducted on watermelon, melon and cucumber that grafted on commercial rootstocks under infection of V.dahlia. Two out of thirteen combinations were identified as tolerance since foliar symptoms were significantly delayed although grafted plants finally got disease (Paplomatas and Elena 2002). Moreover, grafted plants were reported to have lower disease incidence, more vigorous growth and higher yield compared with non-grafted plants (Paroussi et al. 2007). Interestingly, it was observed that same rootstocks reacted differently when different species of scion was used, indicating both scion and rootstock contribute to performance of grafted combination (Paplomatas and Elena 2002).
Other diseases crown rot, caused by Phytophthora capsici, was regarded as the most destructive disease of cucurbits, because it may result in total crop loss in some situations. Genetic resistance was not available thus pathogen exclusion and water management are the primary controlling practice for the disease (McGrath 2001). There were reports documented that cucumber grafted on L. siceraria, C. moschata and Benincasa hispida had higher yields and more vigorous growth under infection of P.capsici (Wang 2004). Furthermore, plant introductions of bottle gourd for resistance against crown rot have been evaluated and grafted watermelons with selected bottle gourd rootstocks have shown tolerance to crown rot (Kousik 2010).
The effects of grafting on viral disease are controversial. Wang reported an increased antivirus performance of grafted seedless melon (Wang 2002). In Israel, controlling Melon necrotic spot virus (MNSV) was regarded as a significant advantage of grafting compared with soil fumigation, which has no effect on viral diseases (Cohen et al. 2007). However, some reports indicated that grafted plants were more vulnerable to virus disease. This is probable because lacking grafting compatibility weakened the scions thus plants are more vulnerable to virus infection (Davisa et al. 2008).
Other diseases reported to be controlled by grafting include gummy stem blight, a foliar disease caused by Didymella bryoniae (Crino 2007, Letícia Akemi Ito 2009), corky root (Phrenochaeta lycopersici) of eggplant and tomato bacteria wilt (Ralstonia solanacearum). Moreover, one report indicated that the ability of controlling carmine spider mite (Tetranychus cinnabarinus Boisduval), a foliar pest of cucurbits, can be transmitted from resistant rootstock L. siceraria to susceptible C. maxima scion and L. siceraria maintained its resistance to T. cinnabarinus even when susceptible C. maxima served as rootstock. However, this phenomenon did not apply when susceptible melon was grafted on L. siceraria (Edelstein et al. 2000).
Mechanisms of grafting afforded disease control
Mechanisms associated with grafting afforded disease control have not been thoroughly investigated. It is presumed that rootstocks' inherent resistances inhibit pathogen infection and propagation, and it serves as the primary reason for grafting related disease control. This presumption has been fully elaborated in the case of controlling Fusarium wilt since 1920th (Sakata et al. 2007). Because most commonly used Cucurbit rootstocks are highly resistant to F. oxysporum, this disease was successfully controlled by grafting technology. However, commonly used rootstocks are not generally considered as resistant hosts to other above-mentioned diseases, therefore, alternative mechanisms should be involved in the process.
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One of the main purposes for grafting is the utilization of vigorous root systems of rootstocks. So, grafted plants usually show increased uptake of water and minerals that result in more vigorous growth and higher yield (Yetisir and Sari 2003, Salehi and Kashi 2010). It is presumed that larger root area and increased vigor help plants develop in the presence of disease stresses. This proposition may help to explain why incidences of some foliar diseases and virus diseases were also reduced by using grafting techtology.
Plants defend themselves on two levels. The first level is the physical or chemical barriers, such as trichome and epicuticular wax, preventing them from pathogen approaching or invading. The second level is the induced plant defense mechanisms. Diverse sets of defense responses are activated by plant-pathogen recognition. Among them, accumulation of reactive oxygen species (ROS), production of antimicrobial compounds, expression of pathogen-relative genes (PR genes), synthesis of nitric oxide (NO), and hypersensitive responses (HR reaction) are the major indicators of plant defense responses (Buchanan et al 2000). Defense responses are vital for plants, but they are also costly decisions for plants. Therefore, conferring plant tissue the immunity for the subsequent infection is a cost-effective strategy. These pre-conditionings could be pathogen attack. Systemic acquired resistance (SAR) is a typical example, which refers to one pathogen infection inducing a long-lasting and broad -spectrum disease resistance to subsequent infections (Durrant and Dong, 2004). In addition, pre-conditionings can be plant growth-promoting rhizobacteria (PGPR). It was realized that PGPR can reduce diseases in above-ground plant parts through induction of systemic resistance (van Loon 2007). Moreover, localized tissue damage, is also reported to elicit an array of systemic defense responses (Schilmiller and Howe 2005).
Accumulated evidence suggested that grafting could also act as a pre-conditioning and lead to induced systemic defense. Li (2009) conducted proteomics study on leaves of both grafted and non-grafted cucumbers in seedling stage, flowering stage and fruiting stage by two-dimensional electrophoresis (2-DE) and mass spectrum (MS). Results showed that two types of proteins in grafted plants were significantly higher than those from non-grafted plants. The first type was R-protein (RGC 693), a stress tolerance related squalene synthase. The second type was photosynthesis related proteins. Referring to gene expression, cDNA-amplified fragment length polymorphism was used to address the potential mechanism. Zheng et al. (2009) examined gene expression in grafted hickory at four time points (0, 3, 6, and 14 days) during grafting process. 49 genes that uniquely involved in the grafting process were identified and some of them were closely related to plant defense response. Plant secondary product is another source for plant defense responses. Phenylalanine ammonia lyase (PAL) is a key enzyme in phenolic synthesis, its metabolic products such as phytoauxion and phenolic compounds, play an essential role in plant defense response. Zhou (1998, 2000) compared PAL activity of grafted eggplants with that of non-grafted plants. In both shoot and root, PAL activity was higher in grafted plants than non-grafted ones.
To better understand the reduced disease incidence on grafted plants, defense responses after pathogen infection have also been examined. Increased electrolytic leakage and proline content in plants are normally associated with pathogen infection. These two factors were evaluated between grafted and non-grafted eggplants after V. dahlia infection. Results showed that electrolytic leakage and proline content were increased after pathogen infection in both grafted and non-grafted plants. However, the increasing rates in grafted plants were significantly lower than that in self-rooted ones (Zhou, 1998b), indicating damage caused by V. dahlia was less severe on grafted plant compared with self-rooted plants. Peroxidase (POD) is a family of enzymes function as antioxidant. POD isozyme patterns were also compared between grafted and non-grafted plants after V. dahlia infection. Significant alternations of POD isozyme pattern were observed in root and above-ground tissues. Interestly, the largest change occurred in egg plants with Solanum torvum rootstock, which showed the highest disease tolerance (Zhou 1998; Zhou 2000), indicating the potential relationship between POD isozyme pattern and defense response. Moreover, since alternation of POD isozyme pattern was not only observed at root area but also in shoot and leaves, which suggested that the POD isozyme difference was a systemic response induced by grafting. PAL activity was also compared after pathogen infection. Results showed that PAL activities in both shoot and root of grafted plants were significantly higher than that of non-grafted plants. In gene expression level, Jensen et al. (2003) studied the gene expression of apple tress grafted on Erwinia amylovora resistant and susceptible rootstocks, respectively. Under infection of E. amylovora, differentially expressed genes were identified and their functions include stress tolerance, tree stature, photosynthetic activity, and fire blight resistance.
Regarding to induced systemic resistance, long-distance signal transduction is the central point to understand the underline mechanisms of plant defense responses. It is well recognized that accumulation of salicylic acid (SA) is associated with System Acquired Response (SAR) decades ago. Till recently, however, it is realized that the signal molecular is not SA, but SA-derivative methyl salicylate (MeSA), which acts as a long-distance mobile signal for SAR (Park et al. 2007). Different from SA, jasmonic acid (JA) are usually associated with wound responses, inducing plant defense against necrotrophic pathogens and herbivorous insects. It was documented that JA accumulated rapidly in response to tissue damage, and exogenous application of JA induced PR gene expression. The question of whether JA could act as a long-distance signal is controversial. Though many evidence supported the assumption, some researcher suggested that it was jasmonates, but not JA itself functioned as mobile signals. In addition to SA and JA signaling, several other metabolites are likely function as signal molecular, for example, Ethylene (ET)-signaling, which is believed to operate synergistically with JA-signaling. Furthermore, volatile organic compounds released by plants have been reported to induce resistance in neighboring plants or attracting parasitic and predatory insects.
In addition to plant metabolites, mRNA, small RNA, and proteins are all involved in vascular long-distance trafficking (Lough and Lucas 2006). Direct evidence of protein transmission came from Cucurbits research. It was reported that additional proteins in scion after grafting has exactly the same size as proteins in rootstocks, indicating proteins from rootstock can be transmitted to scion during grafting union (Golecki and Schulz 1998, Tiedemann and Carstens-behrens 1994). Functions of these proteins, however, are largely unknown. It would be very interesting to investigate whether they can act as signals to active grafting related systemic defense responses. Small RNAs (including miRNA and siRNA) that playing significant roles in regulation of gene expression, are found out to be involved in different layers of plant defense response (Padmanabhan 2009). Therefore, exploring their roles in grafted plants will be exciting field for further study.
More recently, Stegemann and Bock (2009) provided a new insight on grafting induced variation. They found out that genetic material can transfer between cells at the grafted site. In their research, cells at graft site underwent gene exchange. The newly produced cells combined active genetic materials from different cell compounds, and conferred new cells with heritable characteristics from both rootstock and scion. Even though based on their results, directly DNA switch was restricted to the contact zone between scion and rootstock. It did not exclude the possibility that this mechanism could occur at remote areas of grafted plants. If this turn out to be true, we would expect that disease resistant dominated gene may directly fuse into scion genome, conferring scion capability to withstand disease infection.