Ascorbate oxidation activates systemic defence against root-knot nematode Meloidogyne graminicola in rice.

Ashley Haeck,Adelahu Mekonene Tegene,Bruno Verstraeten,Kristof Demeestere,Tina Kyndt,Michael Frei,Shahid Siddique,Raimund Tenhaken,Godelieve Gheysen,Somnath Pokhare,Richard Raj Singh,Nick Smirnoff

doi:10.1093/jxb/eraa171

Abstract

Ascorbic acid (AA) is the major antioxidant buffer produced in the shoot tissue of plants. Previous studies on root-knot nematode (RKN; Meloidogyne graminicola)-infected rice (Oryza sativa) plants showed differential expression of AA-recycling genes, although their functional role was unknown. Our results confirmed increased dehydroascorbate (DHA) levels in nematode-induced root galls, while AA mutants were significantly more susceptible to nematode infection. External applications of ascorbate oxidase (AO), DHA, or reduced AA, revealed systemic effects of ascorbate oxidation on rice defence versus RKN, associated with a primed accumulation of H2O2 upon nematode infection. To confirm and further investigate these systemic effects, a transcriptome analysis was done on roots of foliar AO-treated plants, revealing activation of the ethylene (ET) response and jasmonic acid (JA) biosynthesis pathways in roots, which was confirmed by hormone measurements. Activation of these pathways by methyl-JA, or ethephon treatment can complement the susceptibility phenotype of the rice Vitamin C (vtc1) mutant. Experiments on the jasmonate signalling (jar1) mutant or using chemical JA/ET inhibitors confirm that the effects of ascorbate oxidation are dependent on both the JA and ET pathways. Collectively, our data reveal a novel pathway in which ascorbate oxidation induces systemic defence against RKNs.

Highlights

Rice has been cultivated as a staple crop for many years, and due to its fully sequenced genome, availability of functional sustains approximately half of the world’s population (Seck genomic tools, and relatively easy production of transgenic et al, 2012)
Previous transcriptome analyses on rice infected by M. graminicola revealed that genes involved in recycling of Ascorbic acid (AA) are differentially expressed in locally infected tissue (Kyndt et al, 2012; Ji et al, 2013; Supplementary Table S1) as well as in aboveground tissue (Kyndt et al, 2017)
A significantly higher H2O2 level was observed in galls at 3 dai compared with galls at 7 dai (Fig. 1B), showing that oxidative stress signals are triggered as an early response upon nematode infection

Summary

Introduction

Rice has been cultivated as a staple crop for many years, and due to its fully sequenced genome, availability of functional sustains approximately half of the world’s population (Seck genomic tools, and relatively easy production of transgenic et al, 2012). The infective second-stage juveniles (J2s) of M. graminicola enter the root in the elongation zone and move intercellularly until they reach the vascular cylinder where they induce the formation of feeding sites containing 5–8 giant cells, leading to root galls, preferentially at the root tips (Kyndt et al, 2014; Mantelin et al, 2017; EPPO, 2018). It was shown that root application of β-aminobutyric acid (BABA) (Ji et al, 2015; Jisha and Puthur, 2016), thiamine (Huang et al, 2016), or silicon (Zhan et al, 2018) enhances the rice plant defence system, leading to improved tolerance or resistance against root parasitic nematodes. Systemic defence activation has been observed: foliar application of methyl-JA (MeJA), ethephon, the SA analogue BTH, or COS–OGA [Fytosave®, a mixture of chitosan oligomers (COS) and pectin fragments (oligogalacturonides, OGAs)] activates root defence pathways with varying efficacy (Nahar et al, 2011; Singh et al, 2019)

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