化感水稻抑草作用的根际生物学特性与研究展望

当前水稻化感作用研究主要集中在其遗传生理与分子生态特性和水稻化感物质的分离鉴定及其抑草作用的根际生物学过程与机制两方面。水稻化感作用是个可遗传的数量性状，控制该性状的QTL主要定位在第2、第3、第8、第9、第10染色体上，并存在显著的QTL上位性作用及其与环境的互作效应，但未见控制化感作用性状的QTL遗传信息与何种化感物质的产生紧密相关的研究报道。从现有水稻化感物质的分离鉴定结果看，水稻化感物质可分为三大类，即酚酸类、萜类和黄酮类物质。这三类物质对靶标植物(稗草)均有抑制作用，但酚酸类物质起化感抑草作用的有效浓度较另两类物质的高，且从土壤中检测到的浓度比室内测定的化感抑草作用有效浓度低得多。因此，酚酸类物质是否是一类化感物质经常遭到一些学者的质疑。然而，也有研究结果表明，逆境引起的水稻化感作用潜力增强与其合成酚酸类物质的基因表达增强以及所合成的该类物质分泌释放到根际土壤中的量增多有关。当抑制化感水稻的PAL-2-1基因表达后，其酚酸类物质含量降低，根际微生物数量也随之减少，其中黏细菌属的细菌丰度明显降低，化感抑草效果下降，因而认为在田间条件下化感水稻PAL-2-1基因调控其酚酸类物质合成，经根系分泌进入根际土壤后引起根际特异微生物的趋化性聚集，在这一过程中，释放的根系分泌物可能被土壤中存在的多样性微生物所降解，从而降低其在土壤中的浓度，但正是通过土壤微生物的降解与转化作用，引发了化感物质与根际微生物的藕合效应，并由此产生了水稻化感抑草现象。因此，深入研究这一根际生物学过程对于最终揭示水稻化感作用机理有着极其重要的理论与实际意义。

Rhizobiological Properties of Allelopathic Rice in Suppression of Weeds and Its Research Prospect

Two reviewed areas of the research include the genetic physiological and molecular ecological characteristics, and the allelochemical identification, rhizospheric biological process and mechanism of rice allelopathy. As a quantitative trait, the allelopathy is mediated by both genetic and environmental factors. QTLs for allelopathy are found mainly on chromosomes 2, 3, 8, 9, and 10 significantly interacting with the additive×additive epistatic effects and the environment. However, no relevant study has been reported concerning the correlation between the QTL genetics and allelochemical synthesis. Phenolic acids, terpenoids and flavonoids have been identified in laboratory to be the metabolites showing allelopathic potentials on the target weed (barnyardgrass). Since a concentration higher than what is normally required to exhibit the allelopathic effect in rice rhizospheric soils for phenolic acids, some researchers questioned its association with the allelopathy. On the other hand, our studies indicated that the rice allelopathic potential was enhanced under stress from the increased phenolic acids in soil. Furthermore, the gene expression of phenylalanine ammonia-lyase (PAL-2-1) in the allelopathic rice, PI312777, was inhibited by the RNA interference (RNAi). The transgenic rice showed decreases in the phenolic acid concentration and the rhizospheric bacterial diversity as compared with its wild type (WT), especially for myxobacterium, whose population was significantly lowered. The results suggested that the phenolic acids might be regulated by PAL-2-1 gene, and then be released into the soil resulting in the chemotactic aggregation of the rhizosphere characteristic microbes. Some species of microorganisms with vast diversity existing in soil could conceivably degrade plant root exudates leaving little allelochemicals to be detected. The degradation and transformation by the rhizosphere microorganisms might have a coupling effect with the allelochemicals to result in the crop’s allelopathic effect on weeds as observed. Consequently, studying the rhizospheric biological process could eventually reveal the detailed mechanism of the rice allelopathy.