Rice allelochemicals

RICE ALLELOCHEMICALS AND THEIR EFFECTS ON RELATED PATHOGENS... 

Rimando, A.M., Olofsdotter, M., Dayan, F.E., Duke, S.O. Searching for rice allelochemicals An example of bioassay-guided isolation. Agron J 2001 93 16-20. 

Abstract All plants produce compounds that are phytotoxic to another plant species at some concentration. In some cases, these compounds function, at least in part, in plant/plant interactions, where a phytotoxin donor plant adversely affects a target plant, resulting in an advantage for the donor plant. This review discusses how such an allelochemical role of a phytotoxin can be proven and provides examples of some of the more studied phytochemicals that have been implicated in allelopathy. These include artemisinin, cineoles, P-triketones, catechin, sorgoleone, juglone and related quinones, rice allelochemicals, benzoxazinoids, common phenolic acids, l-DOPA, and m-tyrosine. Mechanisms of avoiding autotoxicity in the donor species are also discussed. 

FIg.l Chemical structures of the natural phytotoxins mentioned in the text from Sections Artemisinin to Rice Allelochemicals... 

Additionally, the allelopathic potential of rice plants can be stimulated in the presence of other plants by increasing the production of allelochemicals that may help them repress the growth of competitors. The synthesis of some rice allelochemicals, such as flavone and cyclohexenone, can be stimulated in the presence of bamyardgrass nearby [117]. This has not been demonstrated for alkyl resorcinols however, our interest in other lipid resorcinol-derived allelochemicals, such as those produced in sorghum [85], has led our research group to identify, clone and characterize the substrate specificity of the key enzymes involved in the synthesis of the rice lipid resorcinols (unpublished data). 

Non-protein amino acids have previously been implicated in allelopathy. For example, mimosine has been associated with allelopathy of the legume tree Leucaena leucocephala [168]. l-DOPA, a compound structurally related to uj-tyrosine (Fig. 5), has been implicated in allelopathy of Mucuna pruriens ([158] and Section Rice Allelochemicals). Roots of pea (Pisum sativa) exude j3-(3-isoxazolin-5-on-2yl)-alanine which inhibits root growth on nonlegume plant species [169], although this non-protein amino acid is much less phytotoxic than m-tyrosine. 

One of the features of allelopathy, as pointed out by Rice (18). is that it is specific. Certain species, but not others, are affected by the allelochemics produced by a plant. This suggests that allelopathy by a weed must be positively demonstrated for each crop. References to specific reports of demonstrated allelopathy by a weed on a crop are given in Table 1. In some cases [crabgrass (Digitaria sanguinalis (L.) Scop.), spurge (Euphorbia spp.), etc.], no crop is listed in the allelopathy column. 

Several secondary metabolites are dithiolanes, as for instance the allelochem-icals found in the tropical weed, Sphenoclea zeylanica. Zeylanoxide A is present in two stereoisomeric forms depending on whether the chirality at sulfur is (R) or (5) the latter form is 44 (Scheme 15). Zeylanoxides B are similar but in these cases the oxide function is on the sulfur at the 2 position. These molecules inhibited root growth in rice seedlings as well as the germination of letuce seeds.95... 

Allelopathy is defined as biochemical interactions between one plant or microorganism (alga, bacteria, or virus) and another plant through the production of chemical compounds - secondary metabolites (allelochemicals), which influence, direct or indirect, harmful or beneficial, plant growth and development (Rice 1984). Allelochemicals are present in almost all plants and in many tissues, like leaves, stems, flowers, fruits, seeds, roots, or pollen and may be released from plants into the environment by volatilization, leaching, root exudation, and decomposition of plant residues (Chou 1990). 

Phenolic acids can be allelopathic but their presence in soil is ephemeral due to rapid degradation and/or sorption by soil particles (Inderjit 2004). Sorption of benzoic acid onto soil particles increased with concentration and it may explain the reason for the limited allelopathic effect of benzoic acid at concentrations often recorded in natural soil (Inderjit 2004). Microorganisms help to generate allelochemicals, but they may also modify toxic compounds into nontoxic compounds (Khanh et al. 2005). Allelochemicals are changed in composition and quantity during the residue decomposition. Allelopathy plays an important function in nutrient recycling (Rice 1984). 

Kong CH, Li HB, Hu F, Xu XH, Wang P (2006) Allelochemicals released by rice roots and residues in soil. Plant Soil 288 47-56... 

Mattice J, Lavy T, Skulman B, Dilday R (1998) Searching for allelochemicals in rice that control ducksalad. In Olofsdotter M (ed) Allelopathy in Rice, Proceedings of workshop on Allelopathy in Rice, IRRI, Manila, pp 81-98... 

Olofsdotter M, Rebulanan M, Madrid A, Wang DL, Navarez D, Oik DC. 2002. Why phenolic acids are unlikely primary allelochemicals in rice. Journal of Chemical Ecology 28 229-242. 

The unharvested parts of rice plants are generally mixed with the soil because this has been thought to be beneficial. It has been observed however, that productivity of the second crop of rice in a paddy is less than that of the first crop. Chou and Lin (45) found that aqueous extracts of decomposing rice residues in soil retarded radicle growth of rice seedlings and growth of rice plants. Maximum toxicity occurred in the first month of decomposition and declined thereafter. Some toxicity persisted for four months in the paddies. Five inhibitory phenolic acids were identified from decaying rice residues and several unidentified allelochemicals were isolated. 

Rice, E. L. "Pest Control with Nature s Chemicals Allelochemicals and Pheromones in Gardening and Agriculture" University of Oklahoma Press Norman, 1983. 

Such plants as Crambe are not numerous - they may constitute less than 7% of the general flora. However, allelochemicals can be not only harmful, but favorable, particularly at low concentration. Chemobrivenko (5) and other Soviet scientists assumed the possibility of positive chemical influence of adjacent plants. American authors. Rice (IQ) among them, took this position much later. 

In plants, biosynthesis and exudation of allelochemicals follows developmental, diurnal, and abiotic/biotic stress-dependent dynamics. Compounds from 14 different chemical classes have been linked to allelopathic interactions, including several simple phenolic acids (e.g., benzoic and hydroxycinnamic acids) and flavonoids [Rice, 1984 Macias et al., 2007]. The existence of several soil biophysical processes that can reduce the effective concentration and bioactivity of these compounds casts doubts on their actual relevance in allelopathic interactions [Olofsdotter et al., 2002]. However, there are well-documented examples of phenylpropanoid-mediated incompatible interactions among plants. Several Gramineae mediate allelopathic interactions by means of... 

Kong C, Liang W, Xu X, Hu F, Wang P, Jiang Y. 2004. Release and activity of allelochemicals from allelopathic rice seedlings. J Agric Food Chem 52 2861-2865. 

Any chemical produced by a plant (donor) that stimulates or inhibits the growth of a neighbour (receiver or receptor) is broadly termed an allelochemical. Typically, allelochemicals are secondary metabolites (Whittaker and Feeney 1971 Rice, 1984 Rizvi et al., 1992), produced as by-products of the acetate and shikimic acid pathways. They may also form as degradation products from the action of microbial enzymes... 

Many compounds with phytoalexin activity are also implicated in allelopathy between plants (Rice, 1984). For example, isoflavonoids are important phytoalexins (Ingham, 1982 Paxton, 1981 Dakora and Phillips, 1996) and allelochemicals (Tamura et al. 1967 1969) from the Leguminosae. Parbery et al. (1984) found that the isoflavonoids biochanin A, formononetin and genistein increased in subterranean clover by 62%, 123% and 75% respectively following infection by pepper spot (Leptosphaerulina trifolii). In comparison, Tamura et al. (1967 1969) isolated a succession of isoflavonoids (including biochanin A, formononetin and genistein) from the shoots of red clover (Trifolium pratense) that inhibited its own germination by 50% at concentrations of 50 ppm. 

Mattice J., Lavy T., Skulman B., Dilday R. Searching for allelochemicals in rice that control duck salad. In Allelopathy in Rice. M. Olofsdotter, ed. International Rice Research Institute, Manila, Philippines, 1998 pp. 81-98. 

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