根瘤菌與豆科宿主的關係

Abstract

根瘤菌與豆科宿主形成具固氮能力之根瘤，有關此一反應於1888年確立至今己逾百年，根瘤菌感染豆科宿主的過程已具大致的形式，但其細部的反應仍為假說的階段。有關二者間的關係早期係經由交互接種的方式確定菌種的分類地位，進而予以命名，也於此結果得知專一感染的現象。1974年確定學名者計有六種，此後關於根瘤菌命名方法重新修改目前關係較接近者計有三屬。對於專一感染的機轉曾有“凝集素－多醣體辨任假說”解釋此一現象，此一假說中認為豆科宿主的根毛表面分泌的凝集素與菌體分泌之細胞外多醣體間具專一辨認的反應，藉此反應建立菌體與宿主間的橋樑，達到專一感染的目的，惟此反應無法獲得直接的證據且有許多相異之結果被發表，因此該假說無法支持說明專一感染的現象。近年發現根瘤菌會受到植物根分泌物中屬於黃素的化合物誘導，使菌體分泌出一些寡醣，目前咸信類黃素及寡醣為訊號化合物，藉這些化合物間的差異誘使結瘤基因開啟，使能進行感染的反應，所以，現今根瘤菌與宿主間關係的研究，除了以交互感染確立最佳感染結果外，並需經由更多的反應及不同的技術，如免疫化學、基因工程及其他方法，方能更進一步推測二者間可能的專一關係。 Rhizobia can infect legume plants and form nodules in which rhizobia fix nitrogen gas. This reaction has been established over 100 years ago, in 1888. Although the infectious process has been interpreted, its detailed steps are not well established. The relationship between rhizobia and its host has been obtained we established the specificity of infectious relationship, and gave Rhizobium a species name. Until 1974, there were six species of Rhizobium, which subsequently were modified to three genera. Based on the mechanism of specific infection, some researchers suggested a ¡§lectin-polysaccharide recognition hypothesis¡¨for interpreting the phenomenon of infection. They postulated that leguminous lectins can recognize and bind bacterial extra cellular polysaccharides forming a bridge connecting rhizobia and host. The specificity of the infectious process is carried out through this bridge reaction. However, many contrary data were published, so that this hypothesis cannot be sustained for explaining the host-parasite relation. Recently, some investigations revealed that flavonoid compound, which are secreted from plant roots, can induce the nod genes of rhizobia. Thereafter, rhizobia releases oligosaccharides as signal compounds into the rhizophere, which in turn would decide the relationship between host and symbiont. Thus, we require more precise techniques and better results, ie. immunochemistry, genetic engineering, etc. to establish the infectious relationship between rhizobia and legume host.