Rice-Meloidogyne graminicola Interactions: Molecular Mechanisms and Genetic Basis of Host Resistance

Meloidogyne graminicola, also known as the rice root-knot nematode (RRKN), is one of the major plant-parasitic nematodes threatening global rice production. With the expansion of water-saving and labor-saving cultivation systems such as direct-seeded rice, paddy soils are often maintained under moist rather than deeply flooded conditions, which provides a favorable environment for RRKN egg hatching, second-stage juvenile migration, and root invasion. Consequently, both its distribution range and damage severity have shown an increasing trend. This review summarizes recent progress in RRKN distribution and damage, effector-mediated pathogenic mechanisms, multilayered rice defense responses, genetic basis of resistance, and resistant germplasm resources. RRKN secretes diverse effector proteins into host cells through its stylet, thereby compromising rice cell-wall integrity, suppressing reactive oxygen species bursts, interfering with pattern-triggered immunity/effecter-triggered immunity (PTI/ETI) signaling, disrupting hormonal balance and nutrient metabolism, and inducing the reprogramming of vascular parenchyma cells to form giant-cell feeding sites, ultimately enabling parasitism and reproduction. In response, rice restricts nematode invasion and development through multiple defense layers, including physical barriers such as the root epidermis, exodermis, endodermal Casparian strips, and suberin deposition; chemical defenses involving phenolic compounds, callose, lignin, and defense-related enzymes; and immune networks jointly regulated by jasmonic acid, salicylic acid, ethylene, abscisic acid, and other hormonal signals. In terms of genetic resources, resistance is mainly enriched in wild rice, while useful resistant materials have also been identified in Asian cultivated rice, including 'Huaidao 5', 'Zhonghua 11', 'Huahang 31', and several local varieties, providing a foundation for resistance gene mapping, mechanistic analysis, and the development of breeding parents. Genetic studies have mapped multiple RRKN resistance-associated QTLs and cloned the first major resistance gene,  MG1, which encodes a canonical CC-NB-LRR protein and represents an important genetic resource for marker-assisted selection breeding. However, efficient and stable resistance genes that can be directly used in breeding remain limited, and most resistance loci are strongly affected by genetic background and environmental conditions. Future research should integrate multi-omics approaches, effector-target analysis, genome editing, and marker-assisted selection to accelerate the discovery, pyramiding, and utilization of resistance genes, thereby developing RRKN-resistant rice varieties with broad-spectrum and durable resistance as well as desirable agronomic traits, and providing theoretical support and technical strategies for the sustainable management of nematode diseases.