The chemical cross talk between rice and barnyardgrass

The chemical cross talk between rice and barnyardgrass which is one of the most noxious weeds in rice cultivation was investigated. Allelopathic activity of rice was increased by the presence of barnyardgrass seedlings or barnyardgrass root exudates. Rice allelochemical, momilactone B, concentration in rice seedlings and momilactone B secretion level from rice were also increased by the presence of barnyardgrass seedlings or barnyardgrass root exudates. As momilactone B possesses strong growth inhibitory activity and acts as an allelochemical, barnyardgrass-induced rice allelopathy may be due to the increased momilactone B secretion. These results suggest that rice may respond to the presence of neighboring barnyardgrass by sensing the chemical components in barnyardgrass root exudates and increase allelopathic activity by elevated production and secretion levels of momilactone B. Thus, rice allelopathy may be one of the inducible defense mechanisms by chemical-mediated plant interaction between rice and barnyardgrass and the induced-allelopathy may provide a competitive advantage for rice through suppression of the growth of barnyardgrass.Key words: allelopathy, Echinochloa, chemical interaction, induced-allelopathy, momilactone, Oryza sativaThe chemical cross talk between host and symbiotic or parasitic plants is an essential process for the development of physical connections in symbiosis and parasitism.^1–3 Barnyardgrass is one of the most common and noxious weeds in rice paddy fields.⁴ Although barnyardgrass is adapted rice production system due to its similarity in growth habit, the reason why barnyardgrass so often invades into the rice paddy fields is unknown. There might be some special interactions between both plant species.Plants are able to accumulate phytoalexins around infection sites of pathogens soon after sensing elicitors of pathogen origin. This accumulation of phytoalexins can protect the plants from further pathogen infection.^5,6 Plants are also able to activate defense mechanisms against attacking herbivores by sensing volatile compounds, such as methacrolein and methyl jasmonate, released by herbivore-attacked plant cells. The volatile-sensed plants increase the production of phenolics, alkaloids, terpenes and defense proteins, which reduce herbivory attacks.^7,8 Therefore, plants are able to elevate the defense mechanisms against several biotic stress conditions by detection of various compounds.Allelopathy is the direct influence of organic chemicals released from plants on the growth and development of other plants.^9–11 Allelochemicals are such organic chemicals involved in the allelopathy.^12,13 Allelochemicals can provide a competitive advantage for host-plants through suppression of soil microorganism and inhibition of the growth of competing plant species because of their antibacterial, antifungal and growth inhibitory activities.^3,14,15Rice has been extensively studied with respect to its allelopathy as part of a strategy for sustainable weed management, such as breeding allelopathic rice strains. A large number of rice varieties were found to inhibit the growth of several plant species when these rice varieties were grown together with these plants under the field or/and laboratory conditions.^16–20 These findings suggest that rice may produce and release allelochemicals into the neighboring environments and may inhibit the growth of the neighboring plants by the allelochemicals.Potent allelochemical, momilactone B, was isolated from rice root exudates.²¹ Momilactone B inhibits the growth of typical rice weeds like barnyardgrass and Echinochloa colonum at concentrations greater than 1 µM and the toxicity of momilactone B to rice itself was very low.²² In addition, rice plants secrete momilactone B from the roots into the rhizosphere over their entire life cycle.²² The observations suggest rice allelopathy may be primarily dependant on the secretion levels of momilactone B from the rice seedlings.^22,23Allelopathic activity of rice exhibited 5.3- to 6.3-fold increases when rice and barnyardgrass seedlings were grown together. Root exudates of barnyardgrass seedlings also increased allelopathic activity and momilactone B concentration in rice seedlings. The increasing the exudate concentration increased the allelopathic activity and momilactone B concentration in rice.²⁴ Thus, the chemical components in barnyardgrass root exudates may affect gene expressions involved in momilactone B biosynthesis. However, effects of the barnyardgrass root exudates on the secretion level of mimilactone B from rice has not yet reported.Rice seedlings were incubated in the medium containing barnyardgrass root exudates for 10 d, and secretion level of momilactone B by rice was determined (Fig. 1). The root exudates increased the secretion level significantly at concentrations greater than 30 mg/L of barnyardgrass root exudates, and increasing the concentration increased the secretion level. At concentrations of 300 mg/L of the root exudates, the secretion level was 10-fold greater than that in control (0 mg of root exudate). There was no significant difference in the osmotic potential between the medium contained barnyardgrass root exudates and control medium (all about 10 mmol/kg), and pH value of the medium was maintained at 6.0 throughout the experiments.²⁵ These results suggest that unknown chemical components in the barnyardgrass root exudates may induce the secretion of momilactone B from rice. As momilactone B possesses strong phytotoxic and allelopathic activities,^21–23,25 the elevated production and secretion of momilactone B in rice may provide a competitive advantage for root establishment through local suppression of pathogens and inhibition of the growth of competing plant species including barnyardgrass. Thus, barnyardgrass-induced rice allelopathy may be caused by the chemical components in the barnyardgrass root exudates.Open in a separate windowFigure 1Effects of barnyardgrass root exudates on momilactone B secretion level in rice. Rice seedlings were incubated in the medium containing barnyardgrass root exudates for 10 d, and secretion level of momilactone B was determined as described by Kato-Noguchi.²⁴ The experiment was repeated six times with three assays for each determination. Different letters show significant difference (p < 0.01) according to Tukey''s HSD test.Although mechanisms of the exudation are not well understood, it is suggested that plants are able to secrete a wide variety of compounds from root cells by plasmalemma-derived exudation, endoplasmic-derived exudation and proton-pumping mechanisms.^3,15 Through the root exudation of compounds, plants are able to regulate the soil microbial community in their immediate vicinity, change the chemical and physical properties of the soil, and inhibit the growth of competing plant species.^3,14,15 The present research suggests that rice may be aware of the presence of neighboring barnyardgrass by detection of certain key in barnyardgrass root exudates, and this sensorial function may trigger a signal cascade resulting in increasing rice allelopathy through increasing production of momilactone B and secretion of momilactone B into the rhizosphere. Therefore, rice allelopathy may potentially be an inducible defense mechanism by chemical-mediated plant interactions between rice and barnyardgrass.