A vast number of plant pathogens from viroids of a few hundred nucleotides to higher plants cause diseases in our crops. Their effects range from mild symptoms to catastrophes in which large areas planted to food crops are destroyed. Catastrophic plant disease exacerbates the current deficit of food supply in which at least 800 million people are inadequately fed. Plant pathogens are difficult to control because their populations are variable in time, space, and genotype. Most insidiously, they evolve, often overcoming the resistance that may have been the hard-won achievement of the plant breeder. In order to combat the losses they cause, it is necessary to define the problem and seek remedies. At the biological level, the requirements are for the speedy and accurate identification of the causal organism, accurate estimates of the severity of disease and its effect on yield, and identification of its virulence mechanisms. Disease may then be minimized by the reduction of the pathogen's inoculum, inhibition of its virulence mechanisms, and promotion of genetic diversity in the crop. Conventional plant breeding for resistance has an important role to play that can now be facilitated by marker-assisted selection. There is also a role for transgenic modification with genes that confer resistance. At the political level, there is a need to acknowledge that plant diseases threaten our food supplies and to devote adequate resources to their control.

Plant Disease: A Threat to Global Food Security

Many genera and species of nematodes attack almost all crops affecting their yield potential. Their distribution, diversity and potential damage depend on many factors including their centres of origin and subsequent spread, the agroecological conditions and the tolerance and resistance of the crops used in that agricultural system.

Current Nematode Threats to World Agriculture

Secreted parasitism proteins encoded by parasitism genes expressed in esophageal gland cells mediate infection and parasitism of plants by root-knot nematodes (RKN). Parasitism gene 16D10 encodes a conserved RKN secretory peptide that stimulates root growth and functions as a ligand for a putative plant transcription factor. We used in vitro and in vivo RNA interference approaches to silence this parasitism gene in RKN and validate that the parasitism gene has an essential function in RKN parasitism of plants. Ingestion of 16D10 dsRNA in vitro silenced the target parasitism gene in RKN and resulted in reduced nematode infectivity. In vivo expression of 16D10 dsRNA in Arabidopsis resulted in resistance effective against the four major RKN species. Because no known natural resistance gene has this wide effective range of RKN resistance, bioengineering crops expressing dsRNA that silence target RKN parasitism genes to disrupt the parasitic process represents a viable and flexible means of developing novel durable RKN-resistant crops and could provide crops with unprecedented broad resistance to RKN.

https://www.pnas.org/content/pnas/103/39/14302.full.pdf

Engineering broad root-knot resistance in transgenic plants by RNAi silencing of a conserved and essential root-knot nematode parasitism gene

Most apomictic root-knot nematodes (RKN; Meloidogyne spp.) have host ranges that encompass the majority of flowering plants, and M. incognita is possibly the world's most damaging crop pathogen. The ancestors, age, and origins of the polyphagous RKN are obscure, but there is increasing evidence that M. incognita, M. javanica, and M. arenaria are closely related, heterogeneous species with a recent, hybrid (reticulate) origin. If so, they must owe much of their current worldwide distributions to spread by agriculture. Host resistance appears to be generally durable in the field, but laboratory studies suggest that apomixis does not prevent evolution in response to selection by a parasitic bacterium (Pasteuria penetrans) and host resistance. Maintaining general fitness may be the evolutionary priority for most populations of polyphagous RKN, and a wide host range, important in the field but not in the laboratory, may be conserved by apomixis. Several factors may help confer a wide host range, including suppression of host resistance, perhaps as a consequence of the strength of the induced susceptible response. Resistance genes effective against RKN appear not to have resulted from coevolution. Rates of juvenile invasion and/or development are low in many wild and some crop plants, with the result that they are both poor hosts and sustain less damage. Overall, it is suggested that greater coordination, particularly of fundamental research, is required.

Apomictic, polyphagous root-knot nematodes: exceptionally successful and damaging biotrophic root pathogens.

Complex defense signaling pathways, controlled by different hormones, are involved in the reaction of plants to a wide range of biotic and abiotic stress factors. We studied the ability of salicylic acid, jasmonate (JA), and ethylene (ET) to induce systemic defense in rice (Oryza sativa) against the root knot nematode Meloidogyne graminicola. Exogenous ET (ethephon) and JA (methyl jasmonate) supply on the shoots induced a strong systemic defense response in the roots, exemplified by a major up-regulation of pathogenesis-related genes OsPR1a and OsPR1b, while the salicylic acid analog BTH (benzo-1,2,3-thiadiazole-7-carbothioic acid S-methyl ester) was a less potent systemic defense inducer from shoot to root. Experiments with JA biosynthesis mutants and ET-insensitive transgenics showed that ET-induced defense requires an intact JA pathway, while JA-induced defense was still functional when ET signaling was impaired. Pharmacological inhibition of JA and ET biosynthesis confirmed that JA biosynthesis is needed for ET-induced systemic defense, and quantitative real-time reverse transcription-polymerase chain reaction data revealed that ET application onto the shoots strongly activates JA biosynthesis and signaling genes in the roots. All data provided in this study point to the JA pathway to play a pivotal role in rice defense against root knot nematodes. The expression of defense-related genes was monitored in root galls caused by M. graminicola. Different analyzed defense genes were attenuated in root galls caused by the nematode at early time points after infection. However, when the exogenous defense inducers ethephon and methyl jasmonate were supplied to the plant, the nematode was less effective in counteracting root defense pathways, hence making the plant more resistant to nematode infection.

http://www.plantphysiol.org/content/plantphysiol/157/1/305.full.pdf

The Jasmonate Pathway Is a Key Player in Systemically Induced Defense against Root Knot Nematodes in Rice

The first edition of this title was published in 1990. This fully updated second edition covers all aspects of plant nematology in subtropical and tropical agriculture. It covers nematode pests of all the major food and cash crops including; rice, cereals, root and tubers, potato, food legumes, vegetables, peanut, citrus, tree and fruit crops, coconut and other palms, coffee, cocoa, tea, bananas, sugarcane, tobacco, pineapple, cotton, other tropical fibers, spices, condiments, and medicinal plants. It also provides practical guidance on methods of extraction, processing and diagnosing of different plant and soil nematodes. The information in this book is supported by abundant illustrations, including 24 color plates, making this an invaluable combination of theory and practice for those studying and working in the area of plant nematology and crop protection.

Plant parasitic nematodes in subtropical and tropical agriculture

Root-knot nematodes: Meloidogyne species.

Nematode parasites of rice.

Starr, J. L., Bridge, J., Cook, R. (2002). Plant Resistance to Parasitic Nematodes. CABI Publishing, Wallingford, ISBN: 0851994660, 288 pp. Table of Contents Contributors Preface 1. Resistance to Plant-parasitic Nematodes: History, Current Use and Future Potential , J.L. Starr, J. Bridge and R. Cook 2. Concepts and Consequences of Resistance , P.A. Roberts 3. Root-knot Nematodes: Meloidogyne Species , R.S. Hussey and G.J.W. Janssen 4. Cyst Nematodes: Globodera and Heterodera Species , R. Cook and G.R. Noel 5. Ditylenchus Species , R.A. Plowright, G. Caubel and K.A. Mizen 6. Foliar Nematodes: Aphelenchoides Species , D. De Waele 7. Reniform Nematodes: Rotylenchulus Species , A.F. Robinson 8. Migratory Endoparasites: Pratylenchus and Radopholus Species , D. De Waele and A. Elsen 9. The Citrus Nematode: Tylenchulus semipenetrans , S. Verdejo-Lucas and D.T. Kaplan 10. The Yam Nematode: Scutellonema bradys , C. Kwoseh, R.A. Plowright and J. Bridge 11. Ectoparasitic Nematodes , J.L. Starr and I.F. Bendezu 12. Marker-assisted Selection for Soybean Cyst Nematode Resistance , N.D. Young and J. Mudge Index Contributors Preface 1. Resistance to Plant-parasitic Nematodes: History, Current Use and Future Potential , J.L. Starr, J. Bridge and R. Cook 2. Concepts and Consequences of Resistance , P.A. Roberts 3. Root-knot Nematodes: Meloidogyne Species , R.S. Hussey and G.J.W. Janssen 4. Cyst Nematodes: Globodera and Heterodera Species , R. Cook and G.R. Noel 5. Ditylenchus Species , R.A. Plowright, G. Caubel and K.A. Mizen 6. Foliar Nematodes: Aphelenchoides Species , D. De Waele 7. Reniform Nematodes: Rotylenchulus Species , A.F. Robinson 8. Migratory Endoparasites: Pratylenchus and Radopholus Species , D. De Waele and A. Elsen 9. The Citrus Nematode: Tylenchulus semipenetrans , S. Verdejo-Lucas and D.T. Kaplan 10. The Yam Nematode: Scutellonema bradys , C. Kwoseh, R.A. Plowright and J. Bridge 11. Ectoparasitic Nematodes , J.L. Starr and I.F. Bendezu 12. Marker-assisted Selection for Soybean Cyst Nematode Resistance , N.D. Young and J. Mudge Index

J. Bridge

Papers

Plant parasitic nematodes in subtropical and tropical agriculture

Root-knot nematodes: Meloidogyne species.

Nematode parasites of rice.

Plant Resistance to Parasitic Nematodes

Concepts and consequences of resistance.