Abstract
In this study, we have characterized the role of carotenoids and chlorophyll in the compatible interaction between the sedentary root knot nematode (RKN) Meloidogyne graminicola and the monocot model plant rice (Oryza sativa). Previous transcriptome data showed a differential expression of carotenoid and chlorophyll biosynthesis genes in nematode-induced giant cells and gall tissue. Metabolite measurement showed that galls indeed accumulate chlorophyll a, b and carotenoids, as well as the hormone abscisic acid (ABA). When ABA was externally applied on rice plants, or when ABA-biosynthesis was inhibited, a significant increase in gall formation and nematode development was found, showing the complex role of ABA in this interaction. ABA application suppressed jasmonic acid (JA) levels in the plants, while ABA-biosynthesis inhibition lead to increased JA levels confirming an antagonism between ABA and JA in rice roots. In addition, combined applications of ABA and JA showed that the ABA-effect can overcome JA-induced defense. Based on these observations, we hypothesized that the accumulation of chlorophyll and carotenoid precursors would be beneficial to nematode infection. Indeed, when chemically blocking the carotenoid biosynthesis pathway at different steps, which leads to differential accumulation of carotenoids and chlorophyll in the plants, a positive and clear link between accumulation of carotenoids and chlorophyll and rice susceptibility to RKN was detected.
Highlights
The root knot nematode (RKN) Meloidogyne graminicola is considered to be one of the most damaging biotic causal agents of yield failure in tropical aerobic rice (De Waele and Elsen, 2007; Kreye et al, 2009a,b; Mantelin et al, 2017)
Genes involved in abscisic acid (ABA) catabolism showed a decreasing trend in 3 dai galls and contrasting patterns in 7 dai galls, but they are strongly induced within the giant cells
These data indicate that most chlorophyll, carotenoid and ABA biosynthesis genes are activated in galls
Summary
The root knot nematode (RKN) Meloidogyne graminicola is considered to be one of the most damaging biotic causal agents of yield failure in tropical aerobic rice (De Waele and Elsen, 2007; Kreye et al, 2009a,b; Mantelin et al, 2017). M. graminicola is an obligate, sedentary endoparasite with a wide range of monocot host plants (Mantelin et al, 2017) This RKN transforms some selected root vascular cells into so-called giant cells, which are kept alive as food resource throughout its life cycle (Kyndt et al, 2013). Starting from the observation that chloroplast and chlorophyll-biosynthesis genes were induced in giant cells, the presence of chloroplast-like structures was microscopically confirmed inside giant cells (Ji et al, 2013). These root cells were completely covered from light exposure during these experiments
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