Ascorbic acid (AsA) is an important antioxidant in plants and regulates various physiological processes. In this study, we show that exogenous treatments with the oxidized form of AsA, that is, dehydroascorbate (DHA), activates induced systemic resistance in rice against the root‐knot nematode Meloidogyne graminicola , and investigate the molecular and biochemical mechanisms underlying this phenotype. Detailed transcriptome analysis on roots of rice plants showed an early and robust transcriptional response on foliar DHA treatment, with induction of several genes related to plant stress responses, immunity, antioxidant activity, and secondary metabolism already at 1 day after treatment. Quantitative and qualitative evaluation of H 2 O 2 levels confirmed the appearance of a reactive oxygen species (ROS) burst on DHA treatment, both at the site of treatment and systemically. Experiments using chemical ROS inhibitors or scavengers confirmed that H 2 O 2 accumulation contributes to DHA‐based induced resistance. Furthermore, hormone measurements in DHA‐treated plants showed a significant systemic accumulation of the defence hormone salicylic acid (SA). The role of the SA pathway in DHA‐based induced resistance was confirmed by nematode infection experiments using an SA‐signalling deficient WRKY45 ‐RNAi line and reverse transcription‐quantitative PCR on SA marker genes. Our results collectively reveal that DHA activates induced systemic resistance in rice against the root‐knot nematode M. graminicola , mediated through the production of ROS and activation of the SA pathway.

Rice diterpenoid phytoalexins (DPs) are secondary metabolites with a well known role in resistance to foliar pathogens. As DPs are also known to be produced and exuded by rice roots, we hypothesised that they might play an important role in plant-nematode interactions, and particularly in defence against phytoparasitic nematodes.We used transcriptome analysis on rice roots to analyse the effect of infection by the rootknot nematode Meloidogyne graminicola or treatment with resistance-inducing chemical stimuli on DP biosynthesis genes, and assessed the susceptibility of mutant rice lines impaired in DP biosynthesis to M. graminicola. Moreover, we grew these mutants and their wild-type in field soil and used metabarcoding to assess the effect of impairment in DP biosynthesis on rhizosphere and root nematode communities.We show that M. graminicola suppresses DP biosynthesis genes early in its invasion process and, conversely, that resistance-inducing stimuli transiently induce the biosynthesis of DPs. Moreover, we show that loss of DPs increases susceptibility to M. graminicola. Metabarcoding on wild-type and DP-deficient plants grown in field soil reveals that DPs significantly alter the composition of rhizosphere and root nematode communities.Diterpenoid phytoalexins are important players in basal and inducible defence against nematode pathogens of rice and help shape rice-associated nematode communities.

Induced resistance (IR), a phenotypic state induced by an exogenous stimulus and characterized by enhanced resistance to future (a)biotic challenge, is an important component of plant immunity. Numerous IR-inducing stimuli have been described in various plant species, but relatively little is known about ‘core’ systemic responses shared by these distinct IR stimuli and the effects of IR on plant-associated microbiota. In this study, we foliarly applied four distinct IR stimuli (β-aminobutyric acid, acibenzolar-S-methyl, dehydroascorbic acid and piperonylic acid) capable of inducing systemic IR in rice (Oryza sativa) against the root-knot nematode Meloidogyne graminicola and evaluated their effect on the root transcriptome, exudome and root-associated nematode communities. Our results reveal shared transcriptional responses –notably induction of jasmonic acid and phenylpropanoid metabolism – and shared alterations to the exudome that include increased amino acid, benzoate and fatty acid exudation. In rice plants grown in soil from a rice field, IR stimuli significantly affected the composition of rhizosphere nematode communities three days after treatment, but by 14 days after treatment these changes had largely reverted . Notably, IR stimuli did not reduce nematode diversity, which suggests that IR might offer a sustainable option for managing plant-parasitic nematodes.