The Impact of Plant Parasitic Nematodes on Rise Crops Cultivation

Cereals comprise of the world’s major source of food. Amongst cereals, rice, maize and wheat are the major crops in terms of production, acreage and source of nutrition, especially in developing countries. Around 70% of cultivated land is committed towards the cereal crops. The global population is predicted to expand around 9 billion in 2050 and henceforth, demand will increase for staple cereals such as rice, maize and wheat, etc (Dixon et al. 2009).

Rice (Oryza sativa L) is a member of the Graminae family and as a cereal grain, major food source for a larger part of the world’s population. Along with the maize, rice is cultivated in most tropical and subtropical regions. Rice is cultivating around 114 countries around the world in Asia, Africa, Central and South America and Northern Australia. Asia comprises 90% of world rice production in China, India, Indonesia, Bangladesh and Vietnam. There are various systems for cultivating rice that have evolved to acclimatize specific environments, such as irrigated, rainfed lowland, deepwater, tidal wetlands and upland. Irrigated rice is the major and dominant growing system in the world. Among the total food grain production, rice contributes 43% while 46% for total cereal production. It plays most imperative role in national food grain reservoir. Rice crop ranks third after wheat and maize in regards of worldwide production. In India, rice is in the first position among the cereals with respect to area and production. Rice is grown most of the states of India. West Bengal is the highest rice producing state and Tamil Nadu having highest productivity (Simon and Anamika 2011). In India, rice cultivated around 44 million ha area and annual production of about 110. 15 million tonnes (Directorate of Economics and Statistics, Department of Agriculture, Cooperation and Farmers Welfare 2017-2018). Rice crop infected by various pests and diseases among them several plant nematodes infect the rice host. Plant parasitic nematodes (PPNs) adapted with each and every rice cultivation system with foliar and root parasites (Nicol et al. 2011). The foliar parasites consist of Aphelenchoides besseyi and Ditylenchus angustus. A. besseyi is a seed borne nematode causing disease of rice. The symptom produced by A. besseyi is a whitening of leaf tip portion (White tip disease) which turns into necrosis, distortion of the flag leaf where panicle is enclosed. A. besseyi infected plants are stunted, decline vigour and their panicles become distorted leads to small grain (Ou 1985). Another foliar parasite, D. angustus (Ufra disease) distributed in Southeast Asia particularly in lowland and deepwater rice cultivation systems. Nematode parasites of root (rice) consist of migratory endoparasites (Hirschmanniella spp. ), sedentary endoparasites (cyst and root knot nematodes) and several ectoparasites. Cyst nematode species distributed in lowland, upland and flooded rice cultivation systems are Heterodera oryzicola, H. oryzae, H. sacchari, etc. The root-knot nematodes (RKN) are the important pests of vegetables, fruits, ornamentals, other dicotyledonous and few monocotyledonous plants. Major species of RKN are Meloidogyne incognita, M. javanica, M. arenaria, M. graminicola, etc. widely distributed in tropics/subtropics whereas M. hapla in sub-temperate climate. (Sasser 1980; Sasser et al. 1984; Dasgupta and Gaur 1986; Soriano and Reversat 2003; Somasekhar and Prasad 2009). Being a staple food crop, rice has diverting more attention from nematologists for studying physiological and molecular interaction between rice and PPNs to aid for improving rice yield production worldwide.

There are several RKN species infected in rice and the major species is Meloidogyne graminicola (RRKN) (Golden and Birchfield, 1965) widely distributed in South and Southeast Asia such as Burma, Bangladesh, Laos, Thailand, Vietnam, India, China and Philippines (Pankaj et al. 2010) in upland, irrigated, lowland rainfed, and deepwater rice (Arayarungsarit 1987; Bridge 1990; Bridge and Page 1982; Cuc and Prot 1992; Gaur et al. 1993, 1996; Sharma et al. 2001). A socioeconomic factors and climatic changes can leads to increasing water shortage, increasing cost of production and also severely limiting yield of rice, which make threatening to food security. Lowland rice production has raised international issue since traditional paddy production system consumes a huge amount of water in Southeast Asian region; water requirement is very high to sustain this type of rice production. In Asia, out of total 79 million ha of irrigated rice, presently 17 million ha suffering with water scarcity and by 2025 water scarcity will be expected to 22 million ha area. Therefore it is obligatory to confide on water saving rice production system such as direct wet seeding, intermittent irrigation, raised bed, aerobic rice and many others. But the large scale introduction of these methods leads to favoring the development of a huge population of M. graminicola (Waele and Elsen 2007). M. graminicola is very well adopted under flooded condition enabling to continue multiplying inside the host tissue even roots are under the deep water. Second stage juveniles of M. graminicola penetrate rice root in upland condition behind the root tip. Juveniles cannot penetrate when rice roots under flooded condition but immediately invade when the soil is drained. Populations of RRKN decline rapidly after 4 months while juveniles and numerous egg masses remain viable at least 5 to 14 months under waterlogged soil condition. M. graminicola has a very short life cycle, completes within 15-20 days at 22-29⁰C.

M. graminicola was first described in 1965 from grasses and oats in Louisiana. This nematode causes severe damage to upland, lowland, deepwater and irrigated rice. The most notable symptom on the rice root includes swollen and hooked root tips. The above ground symptoms consist of stunting and chlorosis leading to decline tillers and yield. This nematode has lacking specific above ground symptoms which underestimate the below ground damage by growers (Mantelin et al. 2017). Grain yield loss due to M. graminicola in upland rice is predicted around 2. 6% for every 1000 nematodes present around the rhizosphere of young seedlings (Rao and Biswas 1973). The tolerance level of rice seedlings has been determined as less than one second stage juvenile/cm3 of soil in flooded system (Plowright and Bridge 1990). Second stage juveniles (J2) penetrate the rice root behind the root tip zone, inside vascular tissue and produce a typical feeding cell, known as the giant cell (GC) act as nematode feeding site. Cells surrounding the GC become hyperplasia and hypertrophied to form the macroscopic hook-like galls on the root system (Kumari et al. 2016). M. graminicola is a problematic nematode pest of rice wheat cropping system in Indo-Gangetic plains and causing significant yield loss (17-30%). Also it occurs in all rice growing states of India with heavy loss in rice production (MacGowan 1989; Jain et al. 2007).

To combat M. graminicola, there are various management strategies such as cultural, biological, physical, mechanical, and chemical methods which are accessible but every method having certain restrictions. Among all, chemical method is a most effective, but because of chemical toxicity, environmental issues and scarcity of availability in the market, it has a limited use. Soil solarization is only possible in small scale and not recommended in temperate regions.

Crop rotation is an efficacious option and able to manage nematode effectively, may not be realistic in Southeast Asia because of limitation of land, choice of crop, seasonal flooding and priority of growers to take rice crop. Therefore the development of nematode resistant cultivars is a most economical and sustainable strategy for nematode management. It is prime need to search resistant source to manage M. graminicola. Resistance sources has been discovered in African rice, Oryza glaberrima and O. longistaminata against M. graminicola (Soriano et al. 1999) and variability up to a certain level reported in the Indian context as well (Kumari et al. 2016). Wild relatives of African rice (O. glaberrima, O. longistaminata, and O. rufipogon) which are partially or completely resistant M. graminicola can act as resistant donors for interspecific crosses with Asian rice cultivars, O. sativa (Plowright et al. 1999; Soriano et al. 1999). Minimal breeding efforts have been made for developing nematode resistant rice cultivars (Bridge et al. 2005). Various approaches and methodologies have been used for searching the sources of resistance against M. graminicola in rice. The proper screening protocol used for identifying nematode resistant breeding lines will allow the evaluating thousands of genotypes for breeding programme (Boerma and Hussey 1992). Several protocols has been published for searching resistance source against other root-knot nematode species such as M. arenaria, M. incognita, M. javanica and M. hapla in soybean, tomato, potato, lettuce, pepper and few other crops (Hussey and Janssen 2002) but very limited for M. graminicola in rice and wheat (Kumari et al. 2016).

Advances have been made in developing powerful molecular genetics tools for use in life sciences. These techniques can be employed for improving the yield, abiotic and biotic stress resistance, and quality traits of crop. Development of various biotechnological tools correlate with the recognition of the utility of landraces, wild relatives and cultivated varieties of different crop species as a source of valuable genes to develop a resistance against nematodes/other pathogens and countless traits of agronomic/horticultural value (Yencho et al. 2000). Activation tagging/insertional mutagenesis has been reported to powerful genomics strategy to find novel candidate genes and demonstrate variability for a particular trait (Weigel et al. 2000; Moin et al. 2016).

This technology can be one of the auspicious tools to discover resistant source against M. graminicola in rice. Development of T-DNA activated rice mutants is a potential approach to generate variants with different phenotypic characters. Screening of mutants for desired phenotypic trait and molecular characterization of the insertion sequences provide a clue about genes responsible for bringing variation in the phenotype. Furthermore, this potential tool is competent of producing vast number of independent transformed lines with the probability of gain-of-function mutagenesis. High throughput profiling of these activation tagging lines provides useful resources to identify genes which are involved in regulatory/biosynthesis pathways. T-DNA tagged insertional mutants have been extensively used to generate knowledge and identify genes responsible for various traits in rice for biotic and abiotic stress (Jeong et al. 2002). Among biotic stress, the mutant lines have been a great resource in the area of bacterial and fungal diseases (Lin et al. 2004). So far there has been no report demonstrated for the utility of variability of activation tagged mutants to investigate plant-nematode interaction. An activation tagging vector pMN20 with four copies of CaMV 35S enhancers and plant selectable marker glyphosate tolerant gene modified EPSPS was cloned into pMN20. The resultant binary vector pMN20 EPSPS with 4X 35S enhancers was used to develop transformants which upon integration in the recipient plant genome can function in either orientation, thereby making transcriptional activation of nearby genes leading to a particular phenotype.

Development of large number of activation tagged mutants adequate to assess for variability to different traits has been limited in the Indian sub-continent. The main concern is the deficiency in high throughput, amenable and genotype independent transformation strategies. In this direction, it has been developed an apical meristem-targeted non-tissue culture-based Agrobacterium tumefaciens- mediated in planta transformation strategy to transform an array of crops including rice for different traits (Nagaveni et al. 2011). The advantage of the strategy is the ability to develop a large number of transformants where tissue culture step is totally avoided. However a stringent selection agent based screening is needed for identification of putative transformants (Shivakumara et al. 2017). Activation tagged mutants in the background of a superior rice genotype JBT 36/14 has been developed by in planta transformation technique (Udaya Kumar personal commun). This pool of transformants is a potential source to screen for any desired trait. Initial screening of few of such transformed rice events has shown some resistance against M. graminicola (Udaya Kumar personal commun). Henceforth these activation tagged lines are utilized for screening against M. graminicola.

Nematodes suspended in PF-127 (pluronic gel), can move freely in three dimensions in response to stable chemical gradients emanating from the host roots. Pluronic gel most suitable for screening of rice plants against M. graminicola under in vitro conditions. PF-127 is a copolymer of propylene oxide and ethylene oxide has rarely toxicity to nematodes or plant tissues (Wang et al. 2009; Dutta et al. 2011; Kumari et al. 2016).

Host plants are ordinarily exposed to biotic and abiotic stress, components of biotic stress obtained from fungi, bacteria, viruses, insects in addition to nematodes. Hosts suffer from any disease when there is an interaction between host plant and pathogen. The successful pathogenesis requires attachment to the host plant, puncture the cell wall act as a physical barrier and finally conquer the defence mechanism of plant. There are two types of pathogens, one is necrotrophic which feed by killing host cells and second one feeding on host cell without killing known as biotrophic pathogen which live inside the host tissues. In resistant host, having a potential to respond the pathogen and activate the defense responsive mechanism against pathogen. The ‘zigzag’ model explains the interaction between host and pathogen. Upon infection, pathogen associated molecular patterns initiate the pattern recognition receptors for activating pathogen analogous molecular pattern triggered immunity in host plant. The effectors secreted by pathogen make plants vulnerable by augmenting effector triggered susceptibility.

Thereafter R genes from host initiate the effector triggered immunity in contrast to pathogen (Jones and Dangl 2006). It has been reported that nematode outsmart the rice plant in an insightful way for successful interaction (Kyndt et al. 2014). To maintain the compatible host parasite association, M. graminicola manipulates the defence mechanism of the plant (Mantelin et al. 2017). Upon entry of pathogen, host plant generally activates the expression of several defence responsive genes which resist the pathogen infection. During the process of infection, nematodes incite a variation in the pattern of gene expression, both locally and systemically in the host plant (Gheysen and Mitchum 2011). Expression of plant defence proteins and other related changes in any host plant is mainly regulated by phytohormones such as salicylic acid (SA), jasmonic acid (JA) and ethylene (ET). In dicotyledons, biotrophic pathogens that nurture on living host cells generally induce the SA pathway whereas, pathways of JA and ET are induced by necrotrophic pathogens (Pieterse et al. 2009) however, with exceptions (Glazebrook 2005). Participation of differential expression of plant defence responsive genes has been studied in some dicotyledons such as Arabidopsis thaliana, tomato, and soybean upon invasion of sedentary plant nematodes (Kyndt et al. 2012b). However, it has been studied that the nematode represses JA pathway and few phenylpropanoid pathway genes during the establishment of infection in the rice plants (Ji et al. 2013; Ji et al. 2015a; Kumari et al. 2016; Nahar et al. 2011). Scanty information available on the precise role of plant hormones in triggering local and systemic defense responses in monocotyledonous plant especially in rice upon invasion of nematodes.