Interactions between the oomycete Pythium arrhenomanes and the rice root-knot nematode Meloidogyne graminicola in aerobic Asian rice varieties

A Large-Scale Hydroponic Evaluation of Rice Mutants for Pythium Resistance

Rice is a major staple crop worldwide. However, the occurrence of rice diseases during cultivation poses a significant challenge to achieving optimal yields. Among the major pathogens, Pythium species, which cause seedling blight, are of particular concern. Pythium infects rice roots through zoospores, mycelia, or oospores, leading to root rot, stunting, yellowing, and ultimately seedling damping-off. While many disease resistance-related genes have been reported in rice, only very limited research has been associated with resistance to Pythium infection. In this study, we aimed to establish a rapid screening system to identify rice lines that are resistant or susceptible to the Pythium pathogen in rice nurseries. We conducted evaluations on important factors, including virulence, inoculation method, seed-soaking period, and the measurement of disease severity. As a result, we successfully developed a screening system that allows for high-throughput and rapid screening of the Taiwan Rice Insertional Mutant (TRIM) library for mutant lines exhibiting resistance to P. arrhenomanes. Furthermore, we identified a slightly resistant TRIM line and explored potential genes encoding endoglucanase-1 precursor and malonyl-CoA decarboxylase that may be involved in conferring resistance to P. arrhenomanes.

Jasmonate-Induced Defense Mechanisms in the Belowground Antagonistic Interaction Between Pythium arrhenomanes and Meloidogyne graminicola in Rice

Next to their essential roles in plant growth and development, phytohormones play a central role in plant immunity against pathogens. In this study we studied the previously reported antagonism between the plant-pathogenic oomycete Pythium arrhenomanes and the root-knot nematode Meloidogyne graminicola, two root pathogens that co-occur in aerobic rice fields. In this manuscript, we investigated if the antagonism is related to imbalances in plant hormone levels, which could be involved in activation of plant defense. Hormone measurements and gene expression analyses showed that the jasmonate (JA) pathway is induced early upon P. arrhenomanes infection. Exogenous application of methyl-jasmonate (MeJA) on the plant confirmed that JA is needed for basal defense against both P. arrhenomanes and M. graminicola in rice. Whereas M. graminicola suppresses root JA levels to increase host susceptibility, Pythium inoculation boosts JA in a manner that prohibits JA repression by the nematode in double-inoculated plants. Exogenous MeJA supply phenocopied the defense-inducing capacity of Pythium against the root-knot nematode, whereas the antagonism was weakened in JA-insensitive mutants. Transcriptome analysis confirmed upregulation of JA biosynthesis and signaling genes upon P. arrhenomanes infection, and additionally revealed induction of genes involved in biosynthesis of diterpenoid phytoalexins, consistent with strong activation of the gene encoding the JA-inducible transcriptional regulator DITERPENOID PHYTOALEXIN FACTOR. Altogether, the here-reported data indicate an important role for JA-induced defense mechanisms in this antagonistic interaction. Next to that, our results provide evidence for induced expression of genes encoding ERF83, and related PR proteins, as well as auxin depletion in P. arrhenomanes infected rice roots, which potentially further contribute to the reduced nematode susceptibility seen in double-infected plants.