Abstract
Anticipating an increased ecological awareness, scientists have been exploring new strategies to reduce the use of chemical pesticides to control pests and diseases. Triggering the intrinsic plant defense system is one of the promising strategies to reduce yield loss by pathogenic organisms, such as nematodes. Ascorbate oxidase (AO) enzyme plays an important role in plant defense by regulating the apoplastic ascorbate/dehydroascorbate (DHA) ratio via the ascorbate oxidation process. Ascorbate oxidation is known to induce systemic resistance in rice against parasitic root-knot nematodes (RKN). Here, we sought to evaluate if AO- or DHA-induced resistance (IR) against RKN M. graminicola involves activation of the phenylpropanoid pathway and whether this IR phenotype has potential effects on growth of rice seedlings under stressed and unstressed conditions. Our results show that AO/DHA-IR against these parasitic nematodes is dependent on activation of phenylalanine ammonia lyase (PAL). However, application of reduced ascorbic acid (AA) did not induce this response. Gene expression analysis via qRT-PCR showed that OsPAL2 and OsPAL4 are highly expressed in AO/DHA-sprayed nematode-infected roots and PAL-activity measurements confirmed that AO/DHA spraying triggers the plants for primed activation of this enzyme upon nematode infection. AO/DHA-IR is not effective in plants sprayed with a chemical PAL inhibitor confirming that AO/DHA-induced resistance is dependent on PAL activity. Improved plant growth and low nematode infection in AO/DHA-sprayed plants was found to be correlated with an increase in shoot chlorophyll fluorescence (Fv/Fm), chlorophyll index (ChlIdx), and modified anthocyanin reflection index which were proven to be good above-ground parameters for nematode infestation. A detailed growth analysis confirmed the improved growth of AO/DHA-treated plants under nematode-infected conditions. Taken together, our results indicate that ascorbate oxidation enhances the phenylpropanoid-based response to nematode infection and leads to a tolerance phenotype in treated rice plants.
Highlights
Rice (Oryza sativa L.) is one of the world’s most valuable agricultural commodities, a staple food for half of the global population, and an important model plant for the study of the interaction between a monocotyledonous plant and plant parasitic nematodes at molecular and physiological level (De Waele and Elsen, 2007; Kyndt et al, 2014)
We evaluated the effect of the compounds (AA, ascorbate oxidase (AO), and DHA) on the number of galls and number of nematodes and a significantly lower number of galls and nematodes was observed in rice plants sprayed with AO or with DHA, but not with ascorbic acid (AA) (Singh et al, 2020b)
The effect of foliar AA, AO, or DHA treatment on rice plants was independently confirmed by counting the number of galls and extended by counting the total number of egg-laying females (ELFs)
Summary
Rice (Oryza sativa L.) is one of the world’s most valuable agricultural commodities, a staple food for half of the global population, and an important model plant for the study of the interaction between a monocotyledonous plant and plant parasitic nematodes at molecular and physiological level (De Waele and Elsen, 2007; Kyndt et al, 2014). Hyperplasia and hypertrophy of the surrounding cells lead to the development of root knots (galls) and hooked root tips, typical root symptoms induced by M. graminicola infection (Bridge et al, 2005; Karssen et al, 2006; Karssen and Moens, 2006; Kyndt et al, 2014) This gall formation disturbs normal root physiological functions, such as water and nutrient transport, causing chlorosis and stunted growth and observable patchy growth in rice fields (Bridge and Page, 1982)
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