Globally, plant parasitic nematodes constitute a major constrain for crop production. Although significant contributions and achievements have been made in recent years, the emerging nematode problems limiting the production of major food, feed and industrial crops and their management strategies currently are big challenges. Advanced Plant Nematology is taught as a core course in PG degree programmes at various universities, with a common syllabus. Not a single textbook covering the prescribed syllabus is available. Thus, a dire need of such a textbook has been felt for quite some time for the benefit of students specializing in the subject. This book, therefore, has been designed to cover the course outline of “Advanced Plant Nematology”, prescribed by the Indian Council of Agricultural Research, New Delhi, India

Nematodes are invertebrate roundworms that inhabit marine, freshwater and terrestrial environments. They comprise the phylum Nematoda or Nemata, which includes parasites of plants and of animals, including humans, as well as species that feed on bacteria, fungi, algae, and on other nematodes. Four out of every five multicellular animals on the planet are nematodes. All nematodes may look alike, the smallest being scaled down versions of the largest. The phrase tube-within-a-tube is a convenient way to think of nematode body structure and also a term used to refer to a major trend in the evolution of triploblastic metazoa. It refers to the development of a fluid-filled cavity between the outer body wall and the digestive tube. The nature of this body cavity has led to the grouping of metazoa into three grades, acoelomate, pseudocoelomate and eucoelomate. Nematodes together with Rotifera, Gastrotricha, Kinorhyncha, Nematomorpha, Acanthocephala and Entoprocta are traditionally grouped together as pseudocoelomates on the basis of possessing a body cavity that is not formed from the mesoderm or fully lined by peritoneum.

Keeping the medical, ecological and economical importance of nematode phylum in mind, it is remarkable to see that nematode systematics is far from established. It has a long history of constant revision and there may be as many classifications as there are nematode taxonomists. Ferris and Ferris (1987) anticipated about the growing sense of excitement pervading systematics as new techniques make it possible a depth of understanding of phylogenetic relationships and affinities never before thought possible. They further stated that Darwin’s ‘genealogical taxonomy’ based on the concepts of descent with modification, is linked directly with two approaches to phylogentic inference, viz., phonetics and cladistics. In both these, patterns of descent take precedence over processes and in classifications based on these procedures, ‘grades’ and ‘gaps’ beloved by the evolutionary systematics are ignored and categories are usually of lesser importance. The phonetic approach deals with ‘natural classification’ based on overall similarity and the belief that the more characters a classification is based on, the more reliable it will be.

Future areas of emphasis for research and scholarship in nematode ecology are indicated by pressing agricultural and environmental issues, by new directions in applied nematology, and by current technological advances. Studies in nematode ecology must extend beyond observation, counting, and simple statistical analysis. Experimentation and the testing of hypotheses are needed for understanding the biological mechanisms of ecological systems. Opportunities for fruitful experimentation in nematode ecology are emerging at the ecosystem, community, population, and individual levels. Nematode ecologists will best promote their field of study by closely monitoring and participating in the advances, initiatives, developments, and directions in the larger f ield of ecology. Some major concepts in ecology have been explored including the dynamics of species interaction, the relationship between complexity and stability in ecosystems and the characterization of organisms as “r” or “K” selected (Howard Ferris, 1993). There has been some application of concepts of plant-herbivore coevolution and density-dependent regulation of populations.

Phytonematodes spend much of their lives inside or in contact with host tissue, and molecular interactions constantly occur and shape the outcome of parasitism.Eggs of these parasites generally hatch in the soil, and the juveniles must locate and infect an appropriate host before their stored energy is exhausted Components of host exudate are evaluated by the nematode and direct its migration to its infection site.Host plants recognize approaching nematodes before physical contact through molecules released by the nematodes and launch a defense response In turn, nematodes deploy numerous mechanisms to counteract plant defenses. A review by Shahid Siddique et al., (2022) focused on these early stages of the interaction between plants and nematodes and discussed how nematodes perceive and find suitable hosts, how plants perceive and mount a defense response against the approaching parasites, and how nematodes fight back against host defenses.

Recent years have seen a rapid increase in the application of the various techniques of molecular, cytogenetical and serological approaches to problems in plant Nematology. The application of these approaches has been considered to problems of genetic variation and classical taxonomy. Two principal approaches are available. Firstly the use of “random” techniques which aim to sample the entire genome of the animaIs or populations of interest and which are mostly used in studies of genetic variation. Secondly “non-random “ techniques which target defined regions of the genome such as particular genes or gene products and are most frequently used in identification to species or sub-species level. The defined region ofthe genome may be the ribosomal genes, the mitochondrial genome or a conserved nuc1ear gene. Whilst analysis of these regions may allow information about taxonomic relationships to be gathered, focusing on one region of the genome imposes certain limitations on the experimenter.

Morphology based nematode taxonomy and biodiversity studies have historically challenged most biologists. In the past few decades, there have been efforts to integrate molecular methods and digital 3D image-capturing technology in nematode taxonomy, the former to enhance the accuracy of identification of such a taxonomically challenging group and the latter to communicate morphological data. While the employment of these two methods is growing in recent taxonomic, biodiversity and biogeographic studies, a movement to abandon traditional phenotypic identification methods altogether has emerged (Eyualem Abebe et al?, 2011). Proponents try to justify this trend by citing the challenging gap between the high estimated number of undescribed species and the limited ability of traditional taxonomy to accomplish the task of documenting such diversity.

7.1. Culturing of Nematodes The need to use host plant tissue makes the culture of phytonematodes more difficult and expensive than the culture of fungi and bacteria. Relatively few species of phytonematodes have been cultured, compared to fungi and bacteria. Populations derived out of extraction procedures are often not sterile, an additional hindrance to many types of biological studies. Axenic culture of free living, insect/animal parasitic nematodes in chemically defined or nondefined media has been achieved with some success, but development of similar culture system for plant parasitic nematodes has been less compared to former ones. However, these techniques form useful research tools in Nematology, which help procure clean nematodes, free of contamination for wide range of studies. Dual culture of nematode and plant tissue is aimed to establish plant parasitic nematodes in sterile cultures. Since phytophagous nematodes are obligate parasites they need a plant tissue as a food source. The term ’Gnotobiology’ is normally used to culture/study a single species of a nematode in the absence of other nematodes /organisms or in the presence of other known species. These techniques are generally considered as tedious and time consuming, require expertise and specialized equipments, nematodes are rarely found associated with only one or few other organisms and there is a need to get non-sterile soil and/or plant tissues, which does not go with gnotobiotic techniques.

Sex determination is one of the first aspects of nematode biology to be systematically characterized with genetic analysis and the depth of this analysis has made it a major topic in developmental biology. It is both fair and interesting to ask how general the Caenorhabditis elegans model is likely to be, both for other nematodes as well as for animals in general. As nematodes employ many different reproductive strategies, often related to parasitic life histories, one might expect similarly variable sex determination mechanisms. However, comparative studies of sex determination based on the C? elgans model have revealed both rapidly evolving and surprisingly well conserved features (Haag, 2005). This mixture of old and new (or slow and fast), along with its obvious relevance to evolution, ecology, and applied fields like agricultural and medical parasitology, has made the evolution of nematode sex determination an increasingly active research area.

Pheromones are neuronal signaling molecules synthesized by various organisms and then excreted into the environment, where they typically stimulate individuals of the same species to react to environmental changes (e.g., temperature shifts, biological stimuli, or nutritional changes) (Jun Young Park et al., 2019). It is thought that most organisms, from prokaryotes to higher animals such as humans, can produce and use pheromones for communication between nonspecific individuals. In most cases, pheromones trigger neuronal events that are linked to various behavioral responses. The outcome of such neuronal stimulation includes the modulation of developmental and/or physiological programs that can support adaptation to new environments. 

Nematoda, a diverse animal phylum that comprises an estimated million species, inhabit very broad ranges of ecological niches throughout earth. These animals, ranging from microscopic to a meter in size, are extremely successful in adapting different environments and have different lifestyles as free-living or parasitic to plants, animals and humans. As a result, nematodes have evolved to communicate with a wide variety of organisms that they live and interact with, including microbes, plants, insects, other animals, and nematodes of the same and different species. These communications play a key role in the mutualism, parasitism, predatory and prey, host and pathogen relationships between nematodes and other organisms and are critical to the ecological fitness of nematodes (Hsueh et al., 2014)

A current focus of the research community is to advance our understanding of phytonematode biology in sufficient detail to develop novel management methods. Progress in this aim is held back by a lack of functional genetic tools: forward genetics in the sedentary endoparasites is restricted to the root-knot nematode Meloidogyne hapla, and relies on natural variants as the source of mappable polymorphisms (Olaf Kranse et al, 2021) and reverse genetics is entirely reliant on RNA interference and is limited by the variable penetrance and stability of the effect. Despite these restrictions, meaningful progress recently has been made. There is nevertheless an expectation that the development of functional genetic tools would accelerate progress in understanding the biology of plant-parasitic nematodes, and thereby also indirectly the development of novel control solutions.

Our understanding of plant–nematode interactions has increased significantly. The f irst genome sequences of two root-knot nematodes species, M? incognita and M? hapla, have been described, which were significantly different from genome of the free-living nematode, Caenorhabditis elegans. Both M? incognita and M? hapla have definite set of proteins that determine the virulence in plant species. The secretomes (set of secreted proteins through the stylets) of different phytonematodes have demonstrated a number of effector proteins that are involved in compatible plant nematode interactions (Ali et?al., 2017). In response to infection of various nematodes, plant’s transcriptome resulted in increased metabolic activity in the feeding cells and suppression of defense mechanisms of the plants in most of the cases. Most of these studies revealed considerable progress toward an understanding of plant–nematode interactions under natural conditions.

In the soil environment plants are constantly exposed to a range of microorganisms which are likely to influence one another, as they occupy the same habitat. Phytonematodes are often considered as pathogens in their own right and are capable of producing a single, recognizable disease. Apart from this, they also get associated with other soil pathogens that result in the complex diseases, which are more devastating and cause huge crop losses. Phytonematodes are major predisposing factors for other potential soil pathogens, which deserve more attention. In the case of soil borne pathogens, further opportunities exist for interactions with other microorganisms occupying the same ecological niche. The significant role of nematodes in the development of diseases caused by soil borne pathogens has been demonstrated in many crops throughout the world. In many cases, such nematode–fungus disease complexes involve root-knot nematodes (Meloidogyne spp.), although several other endoparasitic (Globodera spp., Heterodera spp., Rotylenchulus spp., Pratylenchus \spp.) and ectoparasitic (Xiphinema spp., Longidorus spp.) nematodes have been associated with diseases caused by soil borne fungal pathogens (Back et al., 2002).

14.1 Model Organism: A model organism is a species that has been widely studied, usually because it is easy to maintain and breed in a laboratory setting and has particular experimental advantages. Model organisms are non-human species that are used in the laboratory to help scientists understand biological processes. They are usually organisms that are easy to maintain and breed in a laboratory setting. For example, they may have particularly robust embryos that are easily studied and manipulated in the lab, this is useful for scientists studying development. They may occupy a pivotal position in the evolutionary tree, this is useful for scientists studying evolution.

Genetic resistance is the more efficient and the more environmentally friendly way to protect crops against phytonematodes. Nematode management is complex, hence, the prevention of an initial phytonematode infestation in pathogen-free areas is of fundamental importance. However, after the entry of the nematode into an area, it is virtually impossible to eradicate because it is a soil-dwelling organism. In this regard, the genetic resistance of plants to nematodes is considered one of the most efficient and economically feasible methods to prevent production losses (Matsuo et?al?, 2012). Thus, breeding for nematode resistance is essential in crop management to obtain a high yield and stable production.

Modeling phytonematodes is helpful in the investigation of several host plant interactions and may yield useful information to be exploited in the integration of management strategies and biological control. Modeling the phytonematodes density changes in a soil/root microcosm may allow the identification of main factors active in such systems (Ciancio et?al?, 2022). The multiple tri-trophic interactions linking roots, phytonematodes and antagonists, however, originate a very complex system. It requires knowledge about the many parameters describing the nematode life stages, the interactions with the antagonistic organisms present in the microcosm, as well as their capabilities for effective and durable host regulation. Descriptive variables should also account for the effects of roots and of other environmental factors at work in the tri-trophic interaction system. Models may provide benefits, including the possibility of identifying the best moment for the inoculation of a biocontrol agent, the optimal amount of its inoculum applied to maximize its biocontrol efficacy, as well as the possibility to simulate and thus investigate its behavior once introduced in soil.

Omics technologies based on genes and proteins involved in the nematode activities and the plant resistance responses are key factors to novel nematode management. Techniques such as next-generation sequencing of genomes and transcriptomes, RNAi, PIs, MAS, chemo-disruptive peptides and genetic transformation systems with reproducible results must be available within phytonematodes management strategies to enhance crop yields (Abd-Elgawad, 2022). Linking long-read sequencing to the use of high density genetic mapping can also support the detection and characterization of nematode-virulent genes. Progress in computational biology, bioinformatics, and analyzing the omics large-scale data can efficiently boost these techniques to offer accurate recognition of components and pathways engaged in nematode parasitism and plant response. Updated genome-editing devices will serve classical plant breeding and precisely translate how gene actions are linked to phenotypic performances. Yet, these methods must be cautiously used to avoid unwanted effects such as nematode virulence and pleiotropic impacts on qualitative and quantitative crop yields. Definite issues such as those related to the transfer of multiple disease resistance traits, gene original construct, and specificity of the virulent nematodes to the R-gene may arise in particular cases.