Rice Allelopathy Induced by Methyl Jasmonate and Methyl Salicylate

Abstract

Methyl jasmonate (MeJA) and methyl salicylate (MeSA) are important signaling molecules that induce plant defense against insect herbivores and microbial pathogens. We tested the hypothesis that allelopathy is an inducible defense mechanism, and that the JA and SA signaling pathways may activate allelochemicals release. Exogenous application of MeJA and MeSA to rice (Oryza sativa L.) enhanced rice allelopathic potential and led to accumulation of phenolics, an increase in enzymatic activities, and gene transcription of phenylalanine ammonia-lyase (PAL) and cinnamate 4-hydroxylase (C4H), two key enzymes in the phenylpropanoid pathway. Aqueous extracts of the leaves of rice IAC165, a putative allelopathic variety, treated with MeSA (5 mM) or MeJA (0.05 mM), showed increased inhibitory effects (25 and 21%, respectively) on root growth of barnyardgrass (Echinochloa crus-galli L.), and increased inhibitory effects (18 and 23%, respectively) on shoot growth. Aqueous extracts from leaves of Huajingxian 1 rice, a putative nonallelopathic variety treated with MeJA and MeSA, caused 63 and 24% inhibition of root growth in barnyardgrass seedlings. The root exudates of both IAC165 and Huajingxian 1 plants treated with MeJA and MeSA for 48 hr also showed significant increases in their inhibitory effects on root growth of barnyardgrass seedlings. At the four-leaf stage, levels of 3,4-hydroxybenzoic acid, vanillic acid, coumaric acid, and ferulic acid that accumulated in the leaves were 5.3-, 31.3-, 2.2-, and 1.7-fold higher in response to MeJA exposure, and 3.3-, 13.1-, 2.0-, and 2.2-fold higher in response to MeSA. Treatments of MeSA and MeJA enhanced the PAL activity in the rice leaves up to 52.3 and 80.1%, respectively, whereas C4H activity was increased by 40.2 and 67%. Gene transcription of PAL and C4H in rice leaves significantly increased after the plants were subjected to treatment with MeJA and MeSA. These results suggest that allelopathy may be an active defense mechanism, and that plant signaling compounds are potentially valuable in its regulation.

Analysis of gene expressions associated with increased allelopathy in rice (Oryza sativa L.) induced by exogenous salicylic acid

“…The present result suggested that the SA-mediated cascade reaction in rice to defend against barnyardgrass attack might be similar to the responses to nutrient deficiency through the regulation of carbon-nutrient balance (Bryant et al 1987). This in turn results in the accumulation of phenolics and more phenolic allelochemicals being released into external environment (culture solution) from the roots to depress the target weed growth including the inhibition of protective enzyme activity and root uptake ability, which was consistent with the previous studies (Lin et al 2000;Bi et al 2007;Xiong et al 2007b;Shen et al 2008;Song et al 2008). This raises the question as to why the autotoxicity does not occur during this process.…”

“…The allelopathic potential of rice was closely related to the types and concentrations of phenolic compounds especially under stress conditions, in which the activities of related enzymes involved in the phenylpropanoid metabolic pathway in allelopathic rice was significantly increased (Xiong et al 2007b). This has been considered to be related to salicylic acid (SA)-mediated signal transduction pathway (Shen et al 2008;Xiong et al 2007b;Bi et al 2007). Previous studies have indicated that SA is an immanent compound of phenol in plants and is involved in many physiological reactions as well as signal transduction that trigger off cascade reactions functioning in the induction of systemic acquired resistance (SAR) (Rasbin 1992;Ling and Hu 1997).…”

“…Previous studies have indicated that SA is an immanent compound of phenol in plants and is involved in many physiological reactions as well as signal transduction that trigger off cascade reactions functioning in the induction of systemic acquired resistance (SAR) (Rasbin 1992;Ling and Hu 1997). It was also shown that exogenous SA could induce the accumulation of phenylalanine ammonia-lyase (PAL) mRNA and the synthesis of new PAL protein, resulting in increased enzyme PAL activity in crop plants against pest and weed infection (Wen et al 2005;Bi et al 2007). Hence, application of plant activators to induce resistance or increase allelopathic potential has become a new strategy for weed control and is expected to be an alternative for effective and sustainable management.…”

Chemical interaction between undamaged plants – Effects on herbivores and natural enemies

“…A) Herbivory influences production of allelopathic substances (Thelen et al, 2005;Kong et al, 2002). B) Chemical inducers enhance production of allelopathic substances (Bi et al, 2007). C) Chemical compounds have both allelopathic and insect behavioural activity (Bouda et al, 2001;Kong et al, 2004;Kim et al, 2006;Cippollini et al, 2008;Wang et al, 2009).…”

“…Methyl salicylate, the volatile ester of the plant hormone salicylic acid, is often found among the volatiles released by plants in response to herbivory (Dicke and Hilker, 2003), and has been shown to enhance the allelopathic activity of exposed rice plants (Bi et al, 2007). Methyl salicylate is a known plant defence inducer and has been used successfully in the field to protect cereal crops against pest aphids Pettersson et al, 1994;Prinsloo et al, 2007), but it is not known whether stimulation of the host plants' allelopathic potential was involved.…”