Evaluation of Chinese rice varieties resistant to the root-knot nematode Meloidogyne graminicola

Zhan Li-ping, Ding Zhong, PEng De-liang, PEng huan, Kong Ling-an, Liu Shi-ming, Liu Ying,Li Zhong-cai, huang Wen-kun

1 State Key Laboratory for Biology of Plant Diseases and Insect Pests/Institute of Plant Protection, Chinese Academy of Agricultural Sciences, Beijing 100193, P.R.China

2 College of Plant Protection, Hunan Agricultural University, Changsha 410128, P.R.China

3 Agriculture Bureau of Hanshou County, Hunan Province, Changde 415900, P.R.China

1.introduction

Cultivated rice (Oryza sativa) is the predominant staple food for most countries in Asia and provides 20% of the dietary energy supply worldwide (Schatz et al.2014).Rice varieties in Asia are traditionally classified into two major groups: indica and japonica (Chang 2003).Indica varieties are widely cultivated in lowland tropical areas,whereas japonica varieties are cultivated in both lowland and high-elevation upland areas.Based on differences in isozymes and simple sequence repeats (SSR) markers,the indica group of Asian rice can be further subdivided into two subpopulations (indica and aus) and the japonica group into three subpopulations, temperate japonica, tropical japonica and aromatic (Glaszmann 1987; Garris et al.2005;Zhao et al.2010).These subpopulations exhibit broad differences in geographical distribution, morphological traits, physiological differentiation and genetic divergence(Chang 1976).

The root-knot nematode Meloidogyne graminicola is widely distributed in every rice-producing country in South and Southeast Asia and is considered one of the most important pests affecting Asian rice (Kyndt et al.2012; Ji et al.2013).After second-stage juveniles (J2s) hatch from eggs, they invade at the root tips and inject pharyngeal secretions into vascular cells to induce a specialized nematode feeding site called a giant cell (Kyndt et al.2013).Root-knot nematodes keep these giant cells, which are embedded in galls after J2 root penetration (Gheysen and Mitchum 2011), alive as a food resource throughout their life cycle.These root galls are typically direct reflection of infection level of nematode and always correlate with susceptibility of plants to nematode (Wang et al.2012).M.graminicola infection causes substantial damage to rice root systems in nurseries and significant yield loss in the field (Patil and Gaur 2014; Dimkpa et al.2016).In Bangladesh and Thailand, nematicide application in M.graminicola-infested rice fields resulted in yield increases of 16 to 33% (Arayarungsarit 1987; Sharma-Poudyal et al.2004).In the Philippines, rice yields following two crops of cowpea and treatment with carbofuran resulted in a 34% increase, and nematicide application in upland rice fields in Indonesia resulted in yield increases of 28 to 87%(Dutta et al.2012).Although M.graminicola has been identified in most rice-growing areas in southern China,systematic investigation of rice yield loss after nematode infection is lacking.

Many economical practices have been used alone or in combination to manage M.graminicola population densities below the damage threshold.For example, crop rotation with non-host plants, flooding and fallowing for several months can effectively decrease populations of M.graminicola and reduce yield losses (Ventura et al.1981; Rahman 1990).However, these management practices have a number of drawbacks.The wide host range of M.graminicola limits the use of crop rotation, and flooding cannot be applied in water-limited areas.Furthermore, areas left fallow for several months or a crop season can significantly reduce overall output (De Waele et al.2013).Popular in many riceplanting areas, soil sterilization with chemical nematicides is the most effective management method.Nonetheless,chemical nematicides are expensive, environmentally harmful and pose potential risks to beneficial organisms.Accordingly, searching for resistant or tolerant rice varieties may be an eco-friendly management strategy to manage this nematode.

To date, resistance to M.graminicola has been evaluated in a number of Asian rice accessions, the majority of which are susceptible to M.graminicola.Indeed, only a few are truly resistant (Bridge et al.2005).One accession(WL02) of Oryza longistaminata and three accessions of Oryza glaberrima (TOG7235, TOG5674, TOG5675) in the Philippines were found to be resistant to M.graminicola(Soriano et al.1999), and two commonly grown rice cultivars, Masuli and Chaite-6, were moderately resistant to M.graminicola in Nepal (Sharma-Poudyal et al.2004).The recurrent parent Teqing and the donors Type 3,Zihui 100 and Shwe Thwe Yin Hyv were resistant to the nematode in India (Prasad et al.2006).Regardless, in China, limited information is available on the resistance of rice accessions, and very little effort has been devoted to identifying and breeding resistant rice cultivars.Thus, the aim of this study was to evaluate the response of the most commonly grown commercial rice varieties to M.graminicola in China.

2.Materials and methods

2.1.Rice seed collection

In total, 136 rice varieties were used to characterize the response of plants to M.graminicola, as well as one highly resistant (Rongyou 368, a hybrid indica rice variety) and one susceptible (Boyou 998, a hybrid indica rice variety) control.Of the 138 rice varieties, 16 are aus, 33 are hybrid aus, 10 are indica, 45 are hybrid indica, 10 are temperate japonica,and 24 are tropical japonica (Appendix A).Most hybrid varieties were collected from the China National Hybrid Rice R&D Center; others were obtained from seed markets in different provinces.All the information of rice varieties can be find at China Rice Data Center (http://www.ricedata.cn).

2.2.nematode culture and inoculation

M.graminicola was originally collected from an infested field in Hanshou County, Hunan Province, China, and maintained on O.sativa cv.Nipponbare in a greenhouse at 26°C.Eggs of nematodes were collected from root galls and hatched in a 200-μm sieve.J2 nematodes were extracted with a modified flotation-sieving method and collected using a 25-μm sieve, as described in Huang et al.(2015).Nematode number was estimated under a microscope and adjusted to ±150 juveniles per milliliter for subsequent inoculation.

2.3.Pot experiments

Plants were grown in polyvinylchloride (PVC) tubes (3.2 cm diameter and 17.8 cm height) in a greenhouse at 26°C under a 16-h/8-h light/dark regime.Rice seeds were germinated at 30°C for 4 d, and one seed was sown in each pot filled with sand and synthetic absorbent polymer (SAP), as described in Reversat et al.(1999).Each 2-wk-old plant was inoculated with approximately 150 nematode juveniles as Huang et al.(2015).The experiment was performed three times, with six replicates each.The plants were watered and fertilized with 20 mL Hoagland solution twice per week.At 14-days post inoculation (dpi), six plants were uprooted,and the roots were washed free of soil for calculating the root gall index.All roots were further cleared in 0.6% NaClO for 5 min and boiled in 0.8% acetic acid and 0.013% acid fuchsin for 3 min (Nahar et al.2011).After destaining in 4% acidified glycerol for 3 to 4 d, nematodes from different stages inside the roots were assessed under a stereomicroscope.

2.4.Field experiment

The field experiment was performed from July to August in a continuous cropping field in Hanshou County, under irrigated conditions in 2015 and 2016.This field was severely infected with M.graminicola for at least three years,with a density of 157±42 nematodes per gram soil.Soils in this field were mixed well with rotary cultivator before seeding.The management conditions were consistent in different varieties during testing.Seeds were germinated at 30°C for 4 d and directly seeded in each plot at 30 kg ha-1.Each plot measured approximate 20 m2(4 m×5 m),and each treatment was arranged in a randomized complete block design with four replicates.To prevent water loss and the spread of nematodes, an earthen levee with height of 25 cm and width of 30 cm was constructed around each plot (Khanam et al.2016).At 30 d after seeding, 10 plants from each plot were uprooted, and the roots were washed free of soil for calculating the root gall index.

2.5.Resistance/susceptibility scoring methods

At 14 dpi in the pot experiment or 30 d after seeding in the field experiment, ten plants in each plot were uprooted randomly to evaluate resistance/susceptibility using the gall index according to Pederson and Windham (1989),with minor modifications.Root galling was rated on a scale of 0 to 5, where level 0=no galls, level 1=1-2, level 2=3-10, level 3=11-20, level 4=21-30, level 5≥30 galls per root system.The gall index (GI) was calculated using the following formula:

Where, Siwas root galling scale of 0, 1, 2, 3, 4, 5.Niwas the number of plants in each root galling scale.N was the total number of evaluated plants in each plot.GI was used to score resistance/susceptibility as follows: immune (I) GI=0;highly resistant (HR) 0.1≤GI≤5.0; resistant (R) 5.1≤GI≤25.0;moderately susceptible (MS) 25.1≤GI≤50.0; susceptible (S)50.1≤GI≤75.0; highly susceptible (HS) GI＞75.0.The highly resistant variety Rongyou 368 and the highly susceptible variety Boyou 998, evaluated by Huang et al.(2011), were used as the controls.

2.6.Relationship between the root gall index and the number of nematodes inside the roots

To assess the relationship between the root gall index and number of nematodes inside the roots, 35 rice varieties exhibiting different susceptibilities to M.graminicola were selected for another pot experiment.The responses of all varieties were evaluated in PVC tubes in a greenhouse, as described in pot experiment.One germinated seed was sown in each tube, and six seedlings for each variety were maintained per replication.After 2 wk, each plant was inoculated with approximately 150 nematode juveniles.The experiment was performed three times, with six replicates each.At 14 dpi, the plants were uprooted, and the root gall number and the number of nematodes inside the roots were calculated.Correlations between the root gall index and the number of nematodes inside the roots were analyzed with SPSS software.

2.7.nematode development in resistant varieties

To investigate nematode development, seeds of a resistant variety and three highly resistant varieties were germinated and transferred to SAP in PVC tubes.The highly susceptible variety Boyou 998 and the resistant variety Rongyou 368 were used as the controls.One germinated seed was sown in each tube, and 12 seedlings for each variety were maintained per replication.After 2 wk, each plant was inoculated with approximately 150 nematode juveniles.At 3 dpi, six plants of each variety were uprooted, the roots were washed clean, and the root galls and number of nematodes inside the roots were calculated as described above.At 14 dpi, the other plants were uprooted, and the different stages of nematodes inside the roots were examined and calculated as described in Kong et al.(2016).The experiment was performed three times, with six replicates each.

2.8.Data analysis

Data were analyzed using SPSS software 22 (IBM SPSS,Inc., Chicago, IL, USA).Correlation between the scoring system and the number of nematodes was determined with the Pearson product-moment correlation coefficient with the significance test (P=0.05).Mean nematode numbers of each rice variety with different susceptibilities were compared using Duncan’s multiple mean comparison test.

3.Results

3.1.Relationship between the root gall index and the number of nematodes inside the roots

Before screening the susceptibility/resistance of rice varieties in field experiments, 35 that showed different susceptibility to M.graminicola in a preliminary experiment were randomly selected to analyze the correlation between the root gall index and the number of nematodes inside the roots.Evaluation at 14 dpi revealed that the number of nematodes observed differed significantly among the 35 selected varieties (P＜0.05, R2=0.76) and correlated strongly(r=0.87, P＜0.05) with the nematode gall index (Fig.1).For the highly resistant control variety Rongyou 368, fewer than three juveniles of M.graminicola penetrated the roots, and only 1 to 2 root galls were observed.In contrast, for the susceptible rice variety Boyou 998, more than 50 juveniles in roots and greater than 30 galls per plant were observed.These results confirm that the number of nematodes in the roots correlates with the number of galls in the roots.The results of repeat analyses with 35 randomly selected rice varieties confirmed the initial assessment (data not shown),indicating that the method is highly repeatable in pot and field experiments.

3.2.Reaction of rice varieties to M.graminicola in pot experiments

For pot experiments, the root gall index and nematode number in roots were evaluated at 14 dpi (Appendix A).Only 1 to 4 nematodes were observed in highly resistant varieties, with a root gall index of 1.7 to 4.3.Seventeen varieties exhibited resistance, with a root gall index of 7.6 to 23.2, whereas a moderately susceptible response was observed for 41 varieties, with a root gall index of 25.4 to 49.3.Most of the local varieties exhibited high susceptibility,with a root gall index of greater than 50.

Among five tested subpopulations, temperate japonica exhibited the highest gall index, followed by indica, tropical japonica, hybrid indica, aus and hybrid aus (Fig.2).Zhonghua 11, the aus variety from Tianjing, exhibited only 1 to 2 galls in any of six replicates, with a gall index of 1.7 and 2.4 nematodes in the roots of each plant (Fig.3).In addition,another variety from the hybrid aus subpopulation from Hunan, Shenliangyou 1, showed 1 to 2 galls in only one of six replicates, with a root gall index of 3.3 and 5.3 nematodes in the roots of each plant.For another variety from the hybrid indica subpopulation from Hunan, Cliangyou 4418, 1 to 2 galls in one or two of six replicates were found, with a root gall index of 4.4 and 8.6 nematodes in the roots of each plant(Fig.3).The four highest scoring varieties were observed in two hybrid indicas, Wuyou 7 and Jingliangyouhuazhan,and two temperate japonicas, Hajingdao 3 and Liangjing 10.These plants showed a root gall index over 80 and about 57.8 juveniles in the roots of each plant.

Fig.1 Relationship between the root gall index and the actual number of nematodes inside each plant.The responses of 35 randomly selected varieties were evaluated at 14 days post inoculation (dpi) in pot experiments.The experiment was performed three times, with six replicates each.

Fig.2 Box plot of the root gall index for the different subpopulations of rice in pot experiments.The root gall index (GI) was used to score resistance/susceptibility in rice varieties, as follows: immune (I) GI=0; highly resistant (HR)0.1≤GI≤5.0; resistant (R) 5.1≤GI≤25.0; moderately susceptible(MS) 25.1≤GI≤50.0; susceptible (S) 50.1≤GI≤75.0; highly susceptible (HS) GI＞75.0.The bars represent the means±SE of the data from three independent biological replicates, each containing six plants.

3.3.Response of rice varieties to M.graminicola in field experiments

The responses of all varieties to M.graminicola were further evaluated in field experiments, with observations similar to the pot experiments for most resistant varieties.Most varieties showed same resistance level in both 2015 and 2016.However, the root gall index of most varieties was slightly increased in the field compared with the pot experiment.Temperate japonica displayed the highest gall index, followed by tropical japonica, indica, hybrid indica, aus, and hybrid aus (Fig.4).The three highest scorings were among the aus variety Xiangzaoxian 6,the hybrid aus Wangliangyou 1133, and the hybrid indica Fengliangyou 3, with root gall index greater than 90.High gall indices were also observed in the pot experiments for these three varieties, with values exceeding 70.The aus variety Zhonghua 11 from Tianjin, the hybrid aus variety Shenliangyou 1 from Hunan, and the hybrid indica Cliangyou 4418 from Hunan showed only 1 to 2 galls in six replicates, with a root gall index of 2 to 4.These three varieties exhibited high resistance in both pot and field experiments.However, the germination rates of the highly susceptible varieties Xiangzaoxian 6 and Wangliangyou 1133 were only 70%, significantly lower than the other varieties (data not shown).In addition, some plants of these two highly susceptible varieties gradually became yellow and exhibited necrosis one month after seeding.

3.4.nematode development in resistant varieties

Nematode infection and development in three highly resistant varieties, Zhonghua 11, Shenliangyou 1, and Cliangyou 4418, were assessed in another pot experiment.In total, the number of M.graminicola individuals in the roots of the highly resistant varieties was considerably reduced compared with the highly susceptible varieties.At 3 dpi, only 3 to 6 J2s and 1 to 2 galls were detected in the roots of the highly resistant varieties Zhonghua 11 and Shenliangyou 1,and 6.9 J2s and 2.3 galls were present in the highly resistant variety Rongyou 368.At the same time point,10.9 J2s and 4.8 galls were observed in the resistant variety Cliangyou 4418.Conversely, 44.2 J2s and 20.8 galls were found in the susceptible variety Boyou 998 (Table 1).The resistant variety that exhibited the fewest root galls at 3 dpi was further investigated to evaluate nematode development.At 14 dpi, examination of nematode-infected rice roots revealed that all M.graminicola J2s failed to develop into fourth-stage juveniles (J4s) within the roots of Zhonghua 11 and remained at third-stage juveniles (J3s) (Table 1,Fig.5).Approximately 38.7% of juveniles in Shengliangyou 1 exhibited delayed development into females and remained at the J3s+J4s, and approximately 25.2% juveniles in the highly resistant variety Cliangyou 4418 showed delayed development into females and remained at the J3s+J4s.In the resistant control variety Rongyou 368, only 18.9%juveniles exhibited delayed development into females and remained at the J3s+J4s.In contrast, most nematodes that infected the susceptible variety Boyou 998 gradually developed into the female stage at 14 dpi, with only 6.3%juveniles remaining at the J3s+J4s.The lower number of infected nematodes and those that failed to develop into the J3s in Zhonghua 11 or the female stage in Shengliangyou 1 and Cliangyou 4418 revealed that these highly resistant varieties are able to inhibit nematode development inside their roots.

Fig.3 Responses of resistant rice varieties to the root-knot nematode Meloidogyne graminicola at 14 days post inoculation (dpi) in pot experiments.The response of each variety was evaluated in terms of the root gall index of each variety (A) and the number of nematodes per plant (B).The resistant variety Rongyou 368 and the susceptible variety Boyou 998 were used as control treatments to analyze the susceptibility of rice varieties Zhonghua 11, Shenliangyou 1 and Cliangyou 4418.Each plant was inoculated with±150 second-stage juveniles (J2s).Each bar with standard error represents the average number of gall index or nematodes.Varieties with different letters on the error bars are statistically significant (P≤0.05).The experiment was performed three times,with six replicates each.C, root symptom of high resistance and high susceptible of rice varieties.The highly resistant variety Rongyou 368 and the highly susceptible variety Boyou 998 were used as the controls.Zhonghua 11 and Cliangyou 4418 were used as representatives of tested highly resistant varieties.

Fig.4 Box plot of the root gall index for the different subpopulations of rice in field experiments.The root gall index(GI) was used to score resistance/susceptibility in rice varieties,as follows: immune (I) GI=0; highly resistant (HR) 0.1≤GI≤5.0;resistant (R) 5.1≤GI≤25.0; moderately susceptible (MS)25.1≤GI≤50.0; susceptible (S) 50.1≤GI≤75.0; highly susceptible(HS) GI＞75.0.The experiment was performed twice, with six replicates each.The bars represent the means±SE of the data from two independent biological replicates, each containing six plants.

Fig.5 Development of the root-knot nematode Meloidogyne graminicola inside the roots of tested plants at 14 days post inoculation (dpi).The different developmental stages of M.graminicola inside three resistant varieties, Zhonghua 11,Shenliangyou 1 and Cliangyou 4418, were examined by staining the roots with acid fuchsin.The resistant variety Rongyou 368 and the susceptible variety Boyou 998 were used as control treatments.Each plant was inoculated with ±150 second-stage juveniles (J2s).The experiment was performed three times,with six replicates each.

Table 1 Time course of root-knot nematode Meloidogyne graminicola development within the roots of tested plants1)

4.Discussion

In the present research, 136 rice varieties were evaluated after nematode infection in both pot and field experiments to identify effective resources of resistance against M.graminicola in China.Significant variation in the level of susceptibility of O.sativa varieties to M.graminicola infection was identified in different subpopulations, supporting the findings of others that varieties differ in their reaction to M.graminicola (Shrestha et al.2007; Dimkpa et al.2016).The remarkable finding in this research is the high resistance to M.graminicola of the aus variety Zhonghua 11, hybrid aus Shenliangyou 1 and hybrid indica Cliangyou 4418.Our results indicate a mechanism of pre-infection resistance to M.graminicola in the resistant varieties examined.The significant positive relationship between the number of nematodes in roots and the root gall index in pot experiments and the consistent susceptibility in both pot and field experiments for most varieties confirm the repeatability of the method.

Analysis of infected resistant rice varieties at 3 dpi revealed reduced nematode penetration for Zhonghua 11 and Shenliangyou 1, suggesting that these rice varieties may prevent or delay penetration at a very early stage(Dimkpa et al.2016).Further research on M.graminicola development in highly resistant varieties revealed that very few nematodes developed into later stages.Previous research suggested that susceptible plants would allow root-knot nematodes to feed and develop successfully but that resistant plants may allow nematodes to penetrate but prevent maturation (Trudgill 1991).De Waele et al.(2013)found delayed nematode reproduction in three resistant genotypes of O.glaberrima: CG14, TOG5674 and TOG5675.Indeed, compared with susceptible genotypes, the fecundity and size of M.graminicola females were reduced (Cabasan et al.2012; De Waele et al.2013).In addition, Dimkpa et al.(2016) reported no nematode penetration at 2 dpi and no nematode galls during the 5-wk experiment for two immune accessions, LD24 (an aus from Thailand) and Khao Pahk Maw (an indica from Sri Lanka).Kumari et al.(2016) observed that M.graminicola penetrated, developed and reproduced more rapidly in the susceptible rice variety Pusa 1121 than in the resistant rice variety Vandana.The data from these similar studies suggest that the response of rice accessions resistant to M.graminicola is associated with reduced penetration (Cabasan et al.2012).However,this resistance mechanism differs from that of the Mi gene,whereby the root-knot nematode M.incognita penetrates into tomato roots but induces rapid and localized host cell necrosis when attempting to establish giant cells(Williamson and Hussey 1996).Resistant rice plants might express pre-infection resistance factors at the root surface, enhancing their ability to limit root penetration by nematode juveniles.Such a mechanism may involve root epidermal barriers or biochemical secretions (Huang 1985).Moreover, differences in root anatomy may also contribute to the observed differences in penetration.Such physical root barriers and biochemical defenses were previously suggested in resistant cotton infected with M.incognita(Anwar et al.1994) and resistant grape rootstock infected with M.arenaria (Aanwa and Mckenry 2000).

In the present research, juvenile development was delayed in resistant varieties, with most nematodes failing to develop into females at 14 dpi, which is consistent with observations in other resistant plants (Cabasan et al.2012).For example, the number of M.graminicola females developing in resistant rice genotypes TOG564 and Khao Pahk Maw was significantly reduced compared with susceptible rice genotypes (Dimkpa et al.2016).Additionally, compared with the susceptible variety CO47, the life cycle of M.graminicola was completed 7 d later in the moderately resistant rice ADT45 (Priya and Subramanian 2013).In soybean cultivars resistant to M.arenaria, fewer nematodes developed fully into females,and most nematodes remained in the juvenile stage (Noe 1991; Pedrosa et al.1996).Similarly, in a cotton genotype resistant to M.incognita, the majority of nematodes did not develop past the juvenile stage, and those that developed into females were 2 d later compared with susceptible plants (Moura et al.1993).The delayed development of root-knot nematodes in resistant varieties indicates that the interaction between nematode and host differs for resistant and susceptible plants (Cabasan et al.2012).In resistant O.sativa varieties, necrosis was observed in the cortex and delayed establishment of the feeding site and M.graminicola juvenile development (Jena and Rao 1977; Cabasan et al.2012).Similar results were observed in O.glaberrima resistant to M.graminicola, whereby J2 penetration was significantly reduced and giant cell development arrested (Kyndt et al.2014).Moreover, giant cells were gradually degraded to cells with a low cytoplasmic content and high vacuolation, which appears to be the most common resistance response to M.graminicola.In Vitis spp., cortical necrosis not only halted the migration of juveniles but also reduced female size and fecundity in M.arenaria (Aanwa and Mckenry 2000).As giant cells serve as obligate nutrient sources after infection, the formation of giant cells is correlated with nematode development (Ji et al.2013), and phytohormones play a role in giant cell formation and host-nematode interaction (Bird and Koltai 2000).For instance, gibberellins are important stimulators of cell division and elongation and have a critical function in the development, maintenance, and maturation of giant cells (Richards et al.2001).Transient transcriptional activation of DR5, an auxin reporter gene, in early stages of feeding cell induction by root-knot nematodes demonstrated that auxin is involved in the formation of giant cells in the roots of Arabidopsis (Karczmarek et al.2004).However,transcription of genes necessary for jasmonate biosynthesis were suppressed in early-stage giant cells, and activation of the jasmonic acid pathway via external application of methyl jasmonate is an effective method for protecting rice from root-knot nematode infection (Nahar et al.2011; Kyndt et al.2012).In addition, a series of defense-related pathways were strongly activated in the infected root system at the initial time point (Kyndt et al.2014; Kumari et al.2016).Furthermore, Nahar et al.(2011) demonstrated that ethylene(ET) plays a role in defense against M.graminicola in rice plants, and pharmacological inhibition of ET biosynthesis in rice led to a significantly increased susceptibility to the nematode.Brassinosteroids are involved in plant innate immunity, and an exogenous supply of high concentrations of epibrassinolide invoked systemic defense against M.graminicola (Nahar et al.2013).Therefore, differences in M.incognita infection and development in resistant and susceptible rice varieties can be attributed to changes in molecular and histological responses to nematode infection among these plants (Cabasan et al.2012).

In the early stage of infection, gall formation and female nematode growth in the host plant can be useful for evaluating the response of rice.Understanding the response of different varieties to M.graminicola and the mechanism of resistance could help to establish criteria for traditional breeding selection in rice (Cabasan et al.2012).In China, hybrid rice has a yield advantage of 10 to 20%over conventional varieties and is commercially grown in more than 50% of rice-growing areas (Cheng et al.2007).Compared with hybrid indica, japonica/indica hybrid rice exhibits strong heterosis and increased yield potential(Yuan 1994; Wei et al.2016).In the Chinese super-rice breeding program, new male sterile germplasm and intersubspecies (indica×japonica) crosses have been employed to broaden the genetic base of parental lines and improve yield and quality (Cheng et al.2007; Zhu et al.2017).The promising resistant varieties included in our study can be used for identification of resistance gene(s) that will provide valuable information to develop root-knot nematode resistant varieties (Soriano and Reversat 2003; Gheysen and Jones 2006; Cabasan et al.2012).Based on molecular analyses of plant-nematode interactions, hybrid rice varieties with valuable agronomic traits and resistance to M.graminicola can provide a highly practical measure for nematode management in most rice-growing areas.

5.Conclusion

In the present study, resistance/susceptibility to M.graminicola in different subpopulations of O.sativa was evaluated in pot and field experiments.Three varieties, Zhonghua 11 (aus),Shenliangyou 1 (hybrid aus) and Cliangyou 4418 (hybrid indica) were observed highly resistant to M.graminicola under both pot and field conditions.M.graminicola penetrated less often into highly resistant varieties and failed to develop into females more often than with susceptible varieties.The promising varieties found in the present research might be useful for nematode management.

Acknowledgements

This work was financially supported by the grants from the National Natural Science Foundation of China (31571986)and the National Basic Research Programme of China(2013CB127502).

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