Disease-resistant Mechanism of Pepper against Root-Knot Nematode(Me1oidogyne spp.)

Miao Yue,Liu Ziji,Cao Zhenmu,Zhu Jie*

1.College of Horticulture,Hainan University,Haikou 570228,China;2.Tropical Crops Genetic Resources Institute,Chinese Academy of Tropical Agricultural Sciences,Haikou 571101,China

Abstract The biological characteristics,damage symptoms,pathogenic mechanism,prevention and control methods of Me1oidogyne spp.in pepper,as well as identification and study of pepper resistance against Me1oidogyne spp.are reviewed and summarized.

Keywords Pepper;Me1oidogyne spp.;Disease resistance

Root-knot nematode(Me1oidogyne spp.)belongs to Me1oidogyne,Tylenchida,and root-knot nematode disease caused by Me1oidogyne spp.is a soil-borne disease widely distributed in the world.Me1oidogyne spp.have a wide range of hosts and can parasitize more than 2 000 plants belonging to over 100 families.Me1oidogyne spp.causing great harms mainly include M.incognita,M.arenaria,M.javanica and M.hap1a.

Me1oidogyne spp.are parasitic nematodes in plants,which generally live in the topsoil of 5-30 cm.The life cycle of Me1oidogyne spp.is affected by external environment such as temperature,humidity and host species.Me1oidogyne spp.occur more frequently in southern China,usually 5-6 generations each year,with overlapping generations in the nature.Generally,the disease in sandy loam is more serious than that in clayed soil.Especially in recent years,with the continuous expansion of vegetable planting area in protected areas in China,the damage of Me1oidogyne spp.has become more and more serious,especially in protected areas with long-term continuous cropping of Solanaceae,Cucurbit and Cruciferae plants[1].

1 Damage Symptoms and Pathogenic Mechanism of Me1oidogyne spp.

1.1 Damage symptoms Me1oidogyne spp.harm crop roots,mainly fibrous roots and lateral roots.The apex of injured fibrous roots and lateral roots form globose or conical unequal moniliform nodules,which are called root-knot.Sometimes the whole root is swollen and root quantity is small.Root-knot is white at first,smooth and solid,becoming light brown and rotten.When nodules are cut open,translucent white pinhead-sized particles are seen inside,namely female adults.The symptoms of aboveground plants are inconspicuous in light cases,and plants grow slowly,with yellowing lower leaves,dwarf plants,dysplasia,small and few fruits in severe cases.When the temperature is higher at noon,aboveground part of plants wither,and plant will temporarily return to normal when the temperature is lower in the morning and evening or moisture is sufficient.As the disease progresses,the plant gradually dies[2].

1.2 Pathogenic mechanism The pathogenic mechanism of Me1oidogyne spp.can be theoretically divided into three types:mechanical injury caused by feeding or puncturing;interaction of Me1oidogyne spp.with other pathogens;secreta of Me1oidogyne spp.playing a major role in inducing pathological changes of hosts[3].Me1oidogyne spp.are endoparasitic nematodes of plants.Generally,the 2ndinstar larvae freely activated will infect roots of host,which first invade into cellular space of root tissue from elongation area of roots,then migrate towards root tip along cortex;afterwards,they reach root tip cells through microtubule organization,and establish feeding sites by inducing nuclear division of root tip meristematic cells without cytoplasmic division;the divided cells become multinucleated giant cells,which provide nutrients fornematode growth and development.The plant cells around feeding site constantly differentiate and divide,and eventually form root nodules or root-knot.The size of root nodules varies with different vegetable species.After three times of molting,the invaded nematodes develop into pear-shaped female adults capable of laying eggs.Female adults begin to lay eggs about 6 weeks after nematodes invade the host.Since mechanical injury caused by Me1oidogyne spp.to hosts during parasitic process is insignificant in most cases,the latter two theories are generally supported[4].The wounds left by the 2ndinstar larvae(J2)invading near root tip of hosts are conducive to the occurrence of pepper blight,fusarium wilt and damping off[5].Chen et al.[2]suggested that damaged roots often induced the in-fection of some fungi in the soil,such as Fusarium and Rhizoctonia,which accelerated root rot and caused plant death in advance.Previous studies have confirmed that seven types of fungi commonly present in soil that are not pathogenic to tobacco can cause large-scale rot of roots with Me1oidogyne spp.[6].M.incognita and Rhizoctonia solani cause severe root rot,but individual pathogen will not cause root rot[7].Abad et al.[8]put forward that the pathogenicity of Me1oidogyne spp.was mainly caused by their secreta:the growth and reproduction of Me1oidogyne spp.depended on special breeding sites of roots,so nematodes would not kill host cells,but induced repeated nuclear division of feeder cells in hosts,becoming multi-core feeder cells(also known as giant cells);the root structure was destroyed,which affected normal absorption and hindered growth and development of aboveground part.

2 Prevention and Control Methods of Me1oidogyne spp.

At present,physical control,agricultural control and chemical control are most commonly used methods for the prevention and control of Me1oidogyne spp.Physical control measures mainly include high temperature tightly greenhouse,waterflooding treatment,lime nitrogen,steam disinfection,soil electrical disinfection,ozone water disinfection,X-ray or ultrasonic treatment,etc.[9].Agricultural control measures mainly include cultivation of disease-free seedlings,removal of diseased root residues,deep plowing of soil,crop rotation,and selection of resistant and tolerant varieties.Selection of disease-resistant or disease-tolerant varieties is the most environmentally friendly and lowcarbon measure,but due to the scarcity of disease-resistant varieties,or high input and low output of disease-resistant varieties,chemical control is still the major means for the prevention and control of Me1oidogyne spp.in production.Soil fumigation with methyl bromide or chemical nematocide with high toxicity and residue are major chemical measures against Me1oidogyne spp.[10].The improper use of such control measures not only affects the edible safety of vegetable products,but also causes serious pollution to the environment,leading to the degradation of soil quality.European Union countries have banned the use of chemical agents in soil[11].Therefore,breeding crop varieties with both resistance to Me1oidogyne spp.and high agronomic traits is a task that breeders have to complete.In recent years,with the continuous development and maturity of molecular biology technology,comprehensive disease resistance of plants is improved by genetic engineering technology and methods,which provides a new idea for the prevention and control of Me1oidogyne spp.

3 Identification Indexes of Pepper Resistance against Me1oidogyne spp.

The disease resistance of pepper against Me1oidogyne spp.was identified by egg mass index(EI),gall index(GI)and reproductive factor(RF).GI usually refers to the number of nodules in root caused by infection of Me1oidogyne spp.EI usually refers to the number of egg masses in root due to colonization by Me1oidogyne spp.RF refers to the ratio of egg mass number in root system to the number of invaded nematodes,and RF is generally between 0 and 1.At present,GI[12]and EI[13]are mostly used in the research.However,researchers have different understandings of GI and EI,which lead to differences in the definition of resistance and susceptibility.Therefore,it is of great significance to select pepper varieties with obvious resistance for comparison in practical work.For example,Djian-Capora1Ino et al.[11]used EI for identification of disease resistance,and defined that plants were resistant when EI≤5,and susceptible when EI＞5.Other researchers mainly used RI for disease resistance evaluation and analysis,and RI was different from RF.RI=(the number of egg masses per gram root in identified plant/the number of egg masses per gram root in susceptible control)×100,and the criteria were defined as immunity(RI=0),high resistance(RI＜1),extreme resistance (1≤RI＜10),moderate resistance (10≤RI＜25),weak resistance(25≤RI＜50),and susceptibility(RI≥50).

4 Research Progress of Pepper Resistance against Me1oidogyne spp.

The research of pepper resistance against Me1oidogyne spp.started late in China.In 1984,Martin first reported the resistance of pepper varieties against Me1oidogyne spp.at abroad,marking the beginning of pepper resistance breeding against Me1oidogyne spp.In 1957,Hare first reported that the resistance of pepper against M.incognita was controlled by a single dominant gene,and the first gene resistant to Me1oidogyne spp.was named N by Hare.In 1958,Martin[14]selected Carolinahot,a variety of dry pepper resistant to Me1oidogyne spp.Subsequently,several genes against Me1oidogyne spp.were found in wild pepper resources.So far,at least 10 dominant single-genes,such as N gene and Me family gene,have been found to control the resistance of pepper to Me1oidogyne spp.However,N gene shows unstable resistance under different temperature conditions.The resistance would be partially lost at temperatures above 28℃,and it would be significantly reduced or even disappeared in higher humidity environment.There are a large number of family members of ME gene.At present,seven dominant genes against Me1oidogyne spp.have been found in pepper resources such as PM217 and PM687,including Me1,Me2,Me3,Me4,Me5,Me6,Me7,Mech1 and Mech2;Me1 and Me3 have a wide resistance spectrum and are resistant to M.renaria,M.incogita and M.javanica[15].Me1 and Me3 have similar resistance spectrum,but quite dif-ferent mode of action and thermosensitive reactions;the resistance mediated by ME1 gene starts only after nematodes have established their feeding sites,and some resistance are lost in high temperature environment;the resistance mediated by Me3 gene occurs in early stage of contact between nematodes and host rhizosphere,and shows anaphylactic necrosis reaction,thereby preventing nematodes from further infection.Moreover,ME3 gene can also maintain complete activity at 42℃,so it is of great utilization value.Me2 is a dominant gene sensitive to temperature,it is mainly resistant to M.javanica;Me4 is a dominant gene linked to Me3,and has relatively stable response to temperature,and is mainly resistant to M.arrenaria;Me5 has not been determined to be dominant or recessive,it is mainly resistant to M.javanica,but with weak resistance,and it is difficult to use the gene.Me7 gene shows thermal stability,it is mainly resistant to M.arenaria,M.incognita and M.javanica.Mech1 and Mech2 mainly show resistance to M.chitwoodi[16].

5 Conclusions

Root-knot nematode disease is a common disease worldwide,and it seriously affects the production of pepper.There are relatively many researches on the prevention and control methods against root-knot nematode disease in China,but few efforts have been dedicated to genetic rules and resistance breeding of pepper root knot nematode disease.More efforts should be made in breeding varieties resistant to Me1oidogyne spp.,so as to speed up the breeding pace of pepper resistance against Me1oidogyne spp.in China.Molecular marker-assisted breeding will greatly improve the breeding speed.Although at least 10 dominant single genes have been found to control the resistance of pepper to Me1oidogyne spp.,some of the resistances are not stable enough to perform molecular marker-assisted selection.More research should be done to understand the mechanism of host resistance to Me1oidogyne spp.,and accelerate the development and utilization of molecular markers closely linked to Me1oidogyne spp.genes.In order to accelerate the breeding process,we should establish the breeding target of pepper resistant to Me1oidogyne spp.as soon as possible,and actively carry out identification,screening and innovation of resistant materials against Me1oidogyne spp.,so as to lay a foundation for breeding new varieties.