The role of rice phenolics efflux transporter in solubilizing apoplasmic iron

Iron (Fe) is an essential micronutrient for plants whose deficiency presents a major worldwide agricultural problem. Moreover, Fe is not easily available in neutral to alkaline soils, rendering plants deficient in Fe despite its abundance. Plants secrete phenolics, such as protocatechuic acid (PCA) and caffeic acid (CA), to take up and utilize apoplasmic precipitated Fe, but despite the rapid progress in understanding cellular and subcellular Fe transport, the molecular mechanisms of phenolics synthesis and secretion are not clear. Recently, we isolated and characterized a phenolics efflux transporter in rice by characterizing a mutant in which the amount of PCA and CA in the xylem sap was dramatically reduced, which we hence named phenolics efflux zero 1 (pez1). PEZ1 is a plasma membrane protein that transports PCA when expressed in Xenopus laevis oocytes, and characterization of PEZ1 knockdown and overexpressing plants revealed that it plays an essential role in solubilizing precipitated apoplasmic Fe. The identification of PEZ1 will increase our understanding of apoplasmic Fe solubilization as well as promote research on phenolics efflux mechanisms in different organisms.Key words: iron, Oryza sativa, phenolics transport, protocatechuic acid, xylem sapAlthough mineral soils contain over 6% iron (Fe),¹ it predominantly exists as Fe(III) chelates, and plants ultimately cannot absorb Fe under various physiological conditions such as high soil pH in alkaline soils.² Thus, plants growing in high-pH soils are not very efficient in developing and stabilizing chlorophyll, resulting in the yellowing of leaves, poor growth and reduced yield. Plants, however, have developed sophisticated mechanisms to take up the small amount of soluble Fe. Non-graminaceous plants release protons, secrete phenolics, reduce Fe(III), and finally, take up Fe²⁺.^3–5 Once Fe is solubilized, Fe(III) is reduced to Fe²⁺ by a membrane-bound Fe(III) reductase oxidase.⁶ Then Fe²⁺ is transported into the root by an iron-regulated transporter (IRT1). In contrast, graminaceous plants rely on an Fe(III) chelation system through the secretion of mugineic acid (MA) family phytosiderophores.^3,7,8 The MAs are secreted to the rhizosphere through TOM1 ⁹ and then they chelate Fe(III); the resulting Fe-MA complex is transported by the Yellow Stripe family transporters (OsYSL15 in the case of rice¹⁰). Rice plants also have the ability to take up Fe²⁺ through the OsIRT1 transporter.¹¹In plants, Fe uptake from the apoplasm is well documented at the molecular level, with the exception of phenolics synthesis and efflux. Phenolics, such as protocatechuic acid (PCA), are reported to chelate Fe(III) solubilization and reduce it to Fe²⁺ in vitro.¹² Moreover, removing the secreted phenolics in hydroponic culture solution triggers Fe deficiency responses in roots by inhibiting the solubilization and utilization of apoplasmic Fe.¹³ In this manner, phenolics play a major role in Fe solubilization, besides which PCA and other phenolics play a diverse role in biological systems, such as acting as antioxidants and free radical scavengers, and in nitric oxide synthase.^14–17 Phenolics are also converted to lignin and suberin through the action of peroxidases.² The activity of peroxidases, as well as the formation of lignin, decreases under Fe deficient conditions.^2,18 As suberin plays an important role in controlling the movement of solutes,¹⁹ the role of phenolics in controlling water and mineral transport cannot be overlooked. Thus, understanding the molecular mechanism of phenolics efflux transport is crucial for developing strategies to mitigate widespread Fe deficiency.PEZ1 was isolated in an effort to characterize T-DNA mutants for genes regulated by cadmium (Cd). PEZ1 belongs to the multidrug and toxic compound extrusion transporter family whose members transport small organic compounds.²⁰ The substrates of PEZ1 were identified by analyzing liquid chromatography/mass spectrometry data profiles of the xylem sap of pez1-1 and pez1-2 mutants. The data indicated that PEZ1 transports PCA and caffeic acid (CA). Furthermore, PEZ1 transported radiolabeled PCA when expressed in Xenopus laevis oocytes. PEZ1 localizes to the plasma membrane in rice root cells, as well as in rice root hairs and onion epidermal cells. The pez1-2 mutant accumulated more Fe in the roots, but not in the leaves, compared to wild-type (WT) plants; the differences were greater in the presence of Cd, while no difference was observed in the accumulation of other metals. No significant difference was observed in zinc, manganese (Mn), and copper concentration between WT and pez1-2, in both the roots and shoots, with or without Cd. Fe concentration in the xylem sap was lower than in the WT, while no difference was observed for xylem Cd and Mn. Significant differences in the localization of insoluble Fe were observed when leaf samples were stained with Perl''s solution to examine the localization of Fe. These results suggested a clear role of PEZ1 in solubilizing precipitated apoplasmic Fe.²¹Secretion of excess PCA strongly solubilizes Fe precipitated in the stele, leading to symptoms of Fe excess. The analysis of PEZ1 overexpression lines confirmed this hypothesis. PEZ1 overexpression lines accumulated higher amounts of Fe in roots and leaves owing to the high solubilization of precipitated apoplasmic stele Fe, and as a result, the growth of these lines was severely restricted. In contrast, PEZ1 overexpression lines grew better than the WT in calcareous soil, showing that in these lines, PCA-solubilized Fe is available under Fe-limiting conditions.The expression of PEZ1 is regulated by Cd, and both of the PEZ1 knockdown mutants accumulated higher Cd amounts in leaves and seeds when grown in soil, without compromising morphological or physiological characteristics, like the SPAD value, leaf dry weight, yield, and the concentration of other metals in seeds. Why pez1 accumulates Cd is not clear. PCA has a lower affinity for Cd compared to glutathione, and PEZ1 does not transport Cd.²¹ Cd is partly transported through the Fe uptake system in plants.^22–26 Thus, in pez1, Cd accumulation seems to be triggered by the upregulation of OsIRT1. OsIRT1 localization in the phloem, its substrate specificity, and increased expression in pez1 mutants suggests that Fe and Cd uptake and translocation in pez1 mutants could be enhanced through OsIRT1,¹¹ and that an increased Cd accumulation in pez1 mutants may be due to the increase in OsIRT activity in a decreased Fe environment in which Cd will have reduced competition. PEZ1 localizes to the stele in root cells. The localization of different genes involved in Fe transport is summarized in Figure 1.Open in a separate windowFigure 1Tissue-specific expression of Fe homeostasis-related genes in rice root.In short, phenolics secretion affects Fe acquisition in rice. Reduced secretion of PCA in the pez1-2 mutant impairs the solubilization of precipitated apoplasmic Fe in the stele, and thus, the low availability of Fe leads to the induction of OsIRT1. As PEZ1 and OsIRT1 co-localize in the stele, the PCA secretion may complement Fe²⁺ uptake by OsIRT1 and seems to be an integral part of the Fe²⁺ uptake system in rice (Fig. 2). In contrast, the increase in phenolics secretion in PEZ1-overexpressing plants increases the solubilization of apoplasmic Fe, and plants showed an increased tolerance to Fe deficiency in alkaline soils. The identification of PEZ1 is an important step that helps in better understanding the solubilization of apoplasmic Fe and will generate research on phenolics efflux mechanisms in other plants.Open in a separate windowFigure 2Model of Fe and Cd uptake mechanisms in rice xylem. P.M., plasma membrane.     

Plant defensins: Defense,development and application

Plant defensins are small, highly stable, cysteine-rich peptides that constitute a part of the innate immune system primarily directed against fungal pathogens. Biological activities reported for plant defensins include antifungal activity, antibacterial activity, proteinase inhibitory activity and insect amylase inhibitory activity. Plant defensins have been shown to inhibit infectious diseases of humans and to induce apoptosis in a human pathogen. Transgenic plants overexpressing defensins are strongly resistant to fungal pathogens. Based on recent studies, some plant defensins are not merely toxic to microbes but also have roles in regulating plant growth and development.Key words: defensin, antifungal, antimicrobial peptide, development, innate immunityDefensins are diverse members of a large family of cationic host defence peptides (HDP), widely distributed throughout the plant and animal kingdoms.^1–3 Defensins and defensin-like peptides are functionally diverse, disrupting microbial membranes and acting as ligands for cellular recognition and signaling.⁴ In the early 1990s, the first members of the family of plant defensins were isolated from wheat and barley grains.^5,6 Those proteins were originally called γ-thionins because their size (∼5 kDa, 45 to 54 amino acids) and cysteine content (typically 4, 6 or 8 cysteine residues) were found to be similar to the thionins.⁷ Subsequent “γ-thionins” homologous proteins were indentified and cDNAs were cloned from various monocot or dicot seeds.⁸ Terras and his colleagues⁹ isolated two antifungal peptides, Rs-AFP1 and Rs-AFP2, noticed that the plant peptides'' structural and functional properties resemble those of insect and mammalian defensins, and therefore termed the family of peptides “plant defensins” in 1995. Sequences of more than 80 different plant defensin genes from different plant species were analyzed.¹⁰ A query of the UniProt database (www.uniprot.org/) currently reveals publications of 371 plant defensins available for review. The Arabidopsis genome alone contains more than 300 defensin-like (DEFL) peptides, 78% of which have a cysteine-stabilized α-helix β-sheet (CSαβ) motif common to plant and invertebrate defensins.¹¹ In addition, over 1,000 DEFL genes have been identified from plant EST projects.¹²Unlike the insect and mammalian defensins, which are mainly active against bacteria,^2,3,10,13 plant defensins, with a few exceptions, do not have antibacterial activity.¹⁴ Most plant defensins are involved in defense against a broad range of fungi.^2,3,10,15 They are not only active against phytopathogenic fungi (such as Fusarium culmorum and Botrytis cinerea), but also against baker''s yeast and human pathogenic fungi (such as Candida albicans).² Plant defensins have also been shown to inhibit the growth of roots and root hairs in Arabidopsis thaliana¹⁶ and alter growth of various tomato organs which can assume multiple functions related to defense and development.⁴     

RALFs: Peptide regulators of plant growth

Peptide signaling regulates a variety of developmental processes and environmental responses in plants.^1–6 For example, the peptide systemin induces the systemic defense response in tomato⁷ and defensins are small cysteine-rich proteins that are involved in the innate immune system of plants.^8,9 The CLAVATA3 peptide regulates meristem size¹⁰ and the SCR peptide is the pollen self-incompatibility recognition factor in the Brassicaceae.^11,12 LURE peptides produced by synergid cells attract pollen tubes to the embryo sac.⁹ RALFs are a recently discovered family of plant peptides that play a role in plant cell growth.Key words: peptide, growth factor, alkalinization     

Strigolactones as mediators of plant growth responses to environmental conditions

Strigolactones (SLs) have been recently identified as a new group of plant hormones or their derivatives thereof, shown to play a role in plant development. Evolutionary forces have driven the development of mechanisms in plants that allow adaptive adjustments to a variety of different habitats by employing plasticity in shoot and root growth and development. The ability of SLs to regulate both shoot and root development suggests a role in the plant''s response to its growth environment. To play this role, SL pathways need to be responsive to plant growth conditions, and affect plant growth toward increased adaptive adjustment. Here, the effects of SLs on shoot and root development are presented, and possible feedback loops between SLs and two environmental cues, light and nutrient status, are discussed; these might suggest a role for SLs in plants'' adaptive adjustment to growth conditions.Key words: strigolactones, light, nutrient status, root, shoot, branching, lateral roots, root hairsStrigolactones (SLs) are carotenoid-derived terpenoid lactones suggested to stem from the carotenoid pathway¹ via the activity of various oxygenases.^2,3 SLs production has been demonstrated in both monocotyledons and eudicotyledons (reviewed in ref. ⁴), suggesting their presence in many plant species.⁵ SLs are synthesized mainly in the roots and in some parts of the stem and then move towards the shoot apex (reviewed ref. ⁷).^6,8,9SLs were first characterized more than 40 years ago as germination stimulants of the parasitic plants Striga and Orobanche and later, as stimulants of arbuscular mycorrhiza hyphal branching as well (reviewed in ref. ^{4, 10–13}). Recently, SLs or derivatives thereof, have been identified as a new group of plant hormones, shown to play a role in inhibition of shoot branching,^2,3,8,9 thereby affecting shoot architecture; more recently they have also been shown to affect root growth by affecting auxin efflux.¹⁴Plants have developed mechanisms that allow adaptive adjustments to a variety of different habitats by employing plasticity in their growth and development.¹⁵ Shoot architecture is affected by environmental cues, such as light quality and quantity and nutrient status.^16–19 Root-system architecture and development are affected by environmental conditions such as nutrient availability (reviewed in ref. ^{20, 21}). At the same time, plant hormones are known to be involved in the regulation of plant growth, development and architecture (reviewed in ref. ^22–24) and to be mediators of the effects of environmental cues on plant development; one classic example is auxin''s role in the plant''s shade-avoidance response (reviewed in ref. ²⁵).The ability of SLs to regulate shoot and root development suggests that these phytohormones also have a role in the plant''s growth response to its environment. To play this putative role, SL pathways need to be responsive to plant growth conditions, and affect plant growth toward enhancing its adaptive adjustment. The present review examines the SLs'' possible role in adaptive adjustment of the plant''s response to growth conditions, by discussing their effect on plant development and the possible associations and feedback loops between SLs and two environmental cues: light and nutrient status.     

Strigolactones: Internal and external signals in plant symbioses?

As the newest plant hormone, strigolactone research is undergoing an exciting expansion. In less than five years, roles for strigolactones have been defined in shoot branching, secondary growth, root growth and nodulation, to add to the growing understanding of their role in arbuscular mycorrhizae and parasitic weed interactions.¹ Strigolactones are particularly fascinating as signaling molecules as they can act both inside the plant as an endogenous hormone and in the soil as a rhizosphere signal.^2-4 Our recent research has highlighted such a dual role for strigolactones, potentially acting as both an endogenous and exogenous signal for arbuscular mycorrhizal development.⁵ There is also significant interest in examining strigolactones as putative regulators of responses to environmental stimuli, especially the response to nutrient availability, given the strong regulation of strigolactone production by nitrate and phosphate observed in many species.^5,6 In particular, the potential for strigolactones to mediate the ecologically important response of mycorrhizal colonization to phosphate has been widely discussed. However, using a mutant approach we found that strigolactones are not essential for phosphate regulation of mycorrhizal colonization or nodulation.⁵ This is consistent with the relatively mild impairment of phosphate control of seedling root growth observed in Arabidopsis strigolactone mutants.⁷ This contrasts with the major role for strigolactones in phosphate control of shoot branching of rice and Arabidopsis^8,9 and indicates that the integration of strigolactones into our understanding of nutrient response will be complex. New data presented here, along with the recent discovery of phosphate specific CLE peptides,¹⁰ indicates a potential role for PsNARK, a component of the autoregulation of nodulation pathway, in phosphate control of nodulation.     

Plant defense pathways subverted by Agrobacterium for genetic transformation

The soil phytopathogen Agrobacterium has the unique ability to introduce single-stranded transferred DNA (T-DNA) from its tumor-inducing (Ti) plasmid into the host cell in a process known as horizontal gene transfer. Following its entry into the host cell cytoplasm, the T-DNA associates with the bacterial virulence (Vir) E2 protein, also exported from Agrobacterium, creating the T-DNA nucleoprotein complex (T-complex), which is then translocated into the nucleus where the DNA is integrated into the host chromatin. VirE2 protects the T-DNA from the host DNase activities, packages it into a helical filament and interacts with the host proteins, one of which, VIP1, facilitates nuclear import of the T-complex and its subsequent targeting to the host chromatin. Although the VirE2 and VIP1 protein components of the T-complex are vital for its intracellular transport, they must be removed to expose the T-DNA for integration. Our recent work demonstrated that this task is aided by an host defense-related F-box protein VBF that is induced by Agrobacterium infection and that recognizes and binds VIP1. VBF destabilizes VirE2 and VIP1 in yeast and plant cells, presumably via SCF-mediated proteasomal degradation. VBF expression in and export from the Agrobacterium cell lead to increased tumorigenesis. Here, we discuss these findings in the context of the “arms race” between Agrobacterium infectivity and plant defense.Key words: Arabidopsis, defense response, proteasomal degradation, bacterial infection, F-box proteinAgrobacterium infection of plants consists of a chain of events that usually starts in physically wounded tissue which produces Plant defense pathways subverted by Agrobacterium for genetic transformation small phenolic molecules, such as acetosyringone (AS).¹ These phenolics serve as chemotactic agents and activating signals for the virulence (vir) gene region of the Ti plasmid.^2,3 The vir gene products then process the T-DNA region of the Ti plasmid to a single-stranded DNA molecule that is exported with several Vir proteins into the host cell cytoplasm, in which it forms a the T-DNA nucleoprotein complex (T-complex).^4,5 The plant responds to the coming invasion by expressing and activating several defense-related proteins,⁵ such as VBF⁶ and VIP1,⁷ aimed at suppressing the pathogen. However, the Agrobacterium has evolved mechanisms to take advantage of these host defense proteins.⁸ Some of the unique strategies for achieving this goal include (1) the use of VIP1 to bind the T-complex—via the VIP1 interaction with the T-DNA packaging protein VirE2,^9,10—and assist its nuclear import⁷ and chromatin targeting,¹¹ and (2) the use of VBF to mark VIP1 and its associated VirE2 for proteasomal degradation, presumably for uncoating the T-complex prior to the T-DNA integration into the plant genome.^6,12 Here, we examine these subversion strategies in the context of “arms race” between Agrobacterium and plants.     

Why have chloroplasts developed a unique motility system?

Organelle movement in plants is dependent on actin filaments with most of the organelles being transported along the actin cables by class XI myosins. Although chloroplast movement is also actin filament-dependent, a potential role of myosin motors in this process is poorly understood. Interestingly, chloroplasts can move in any direction and change the direction within short time periods, suggesting that chloroplasts use the newly formed actin filaments rather than preexisting actin cables. Furthermore, the data on myosin gene knockouts and knockdowns in Arabidopsis and tobacco do not support myosins'' XI role in chloroplast movement. Our recent studies revealed that chloroplast movement and positioning are mediated by the short actin filaments localized at chloroplast periphery (cp-actin filaments) rather than cytoplasmic actin cables. The accumulation of cp-actin filaments depends on kinesin-like proteins, KAC1 and KAC2, as well as on a chloroplast outer membrane protein CHUP1. We propose that plants evolved a myosin XI-independent mechanism of the actin-based chloroplast movement that is distinct from the mechanism used by other organelles.Key words: actin, Arabidopsis, blue light, kinesin, myosin, organelle movement, phototropinOrganelle movement and positioning are pivotal aspects of the intracellular dynamics in most eukaryotes. Although plants are sessile organisms, their organelles are quickly repositioned in response to fluctuating environmental conditions and certain endogenous signals. By and large, plant organelle movements and positioning are dependent on actin filaments, although microtubules play certain accessory roles in organelle dynamics.^1,2 Actin inhibitors effectively retard the movements of mitochondria,^3–6 peroxisomes,^5,7–11 Golgi stacks,^12,13 endoplasmic reticulum (ER),^14,15 and nuclei.^16–18 These organelles are co-aligned and associated with actin filaments.^{5,7,8,10–12,15,18} Recent progress in this field started to reveal the molecular motility system responsible for the organelle transport in plants.¹⁹Chloroplast movement is among the most fascinating models of organelle movement in plants because it is precisely controlled by ambient light conditions.^20,21 Weak light induces chloroplast accumulation response so that chloroplasts can capture photosynthetic light efficiently (Fig. 1A). Strong light induces chloroplast avoidance response to escape from photodamage (Fig. 1B).²² The blue light-induced chloroplast movement is mediated by the blue light receptor phototropin (phot). In some cryptogam plants, the red light-induced chloroplast movement is regulated by a chimeric phytochrome/phototropin photoreceptor neochrome.^23–25 In a model plant Arabidopsis, phot1 and phot2 function redundantly to regulate the accumulation response,²⁶ whereas phot2 alone is essential for the avoidance response.^27,28 Several additional factors regulating chloroplast movement were identified by analyses of Arabidopsis mutants deficient in chloroplast photorelocation.^29–32 In particular, identification of CHUP1 (chloroplast unusual positioning 1) revealed the connection between chloroplasts and actin filaments at the molecular level.²⁹ CHUP1 is a chloroplast outer membrane protein capable of interacting with F-actin, G-actin and profilin in vitro.^29,33,34 The chup1 mutant plants are defective in both the chloroplast movement and chloroplast anchorage to the plasma membrane,^22,29,33 suggesting that CHUP1 plays an important role in linking chloroplasts to the plasma membrane through the actin filaments. However, how chloroplasts move using the actin filaments and whether chloroplast movement utilizes the actin-based motility system similar to other organelle movements remained to be determined.Open in a separate windowFigure 1Schematic distribution patterns of chloroplasts in a palisade cell under different light conditions, weak (A) and strong (B) lights. Shown as a side view of mid-part of the cell and a top view with three different levels (i.e., top, middle and bottom of the cell). The cell was irradiated from the leaf surface shown as arrows. Weak light induces chloroplast accumulation response (A) and strong light induces the avoidance response (B).Here, we review the recent findings pointing to existence of a novel actin-based mechanisms for chloroplast movement and discuss the differences between the mechanism responsible for movement of chloroplasts and other organelles.     

Roles of calcineurin B-like protein-interacting protein kinases in innate immunity in rice

Cytosolic free Ca²⁺ mobilization induced by microbe/pathogen-asssociated molecular patterns (MAMPs/PAMPs) plays key roles in plant innate immunity. However, components involved in Ca²⁺ signaling pathways still remain to be identified and possible involvement of the CBL (calcineurin B-like proteins)-CIPK (CBL-interacting protein kinases) system in biotic defense signaling have yet to be clarified. Recently we identified two CIPKs, OsCIPK14 and OsCIPK15, which are rapidly induced by MAMPs, involved in various MAMP-induced immune responses including defense-related gene expression, phytoalexin biosynthesis and hypersensitive cell death. MAMP-induced production of reactive oxygen species as well as cell browning were also suppressed in OsCIPK14/15-RNAi transgenic cell lines. Possible molecular mechanisms and physiological functions of the CIPKs in plant innate immunity are discussed.Key words: PAMPs/MAMPs, calcium signaling, CBL-CIPK, hypersensitive cell death, reactive oxygen speciesCa²⁺ plays an essential role as an intracellular second messenger in plants as well as in animals. Several families of Ca²⁺ sensor proteins have been identified in higher plants, which decode spatiotemporal patterns of intracellular Ca²⁺ concentration.^1,2 Calcineurin B-Like Proteins (CBLs) comprise a family of Ca²⁺ sensor proteins similar to both the regulatory β-subunit of calcineurin and neuronal Ca²⁺ sensors of animals.^3,4 Unlike calcineurin B that regulates protein phosphatases, CBLs specifically target a family of protein kinases referred to as CIPKs (CBL-Interacting Protein Kinases).⁵ The CBL-CIPK system has been shown to be involved in a wide range of signaling pathways, including abiotic stress responses such as drought and salt, plant hormone responses and K+ channel regulation.^6,7Following the recognition of pathogenic signals, plant cells initiate the activation of a widespread signal transduction network that trigger inducible defense responses, including the production of reactive oxygen species (ROS), biosynthesis of phytoalexins, expression of pathogenesis-related (PR) genes and reorganization of cytoskeletons and the vacuole,⁸ followed by a form of programmed cell death known as hypersensitive response (HR).^9,10 Because complexed spatiotemporal patterns of cytosolic free Ca²⁺ concentration ([Ca²⁺]_cyt) have been suggested to play pivotal roles in defense signaling,^1,9 multiple Ca²⁺ sensor proteins and their effectors should function in defense signaling pathways. Although possible involvement of some calmodulin isoforms^11–13 and the calmodulin-domain/calcium-dependent protein kinases (CDPKs)^14–19 has been suggested, other Ca²⁺-regulated signaling components still remain to be identified. No CBLs or CIPKs had so far been implicated as signaling components in innate immunity.     

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.