Bradyrhizobium Rhizobitoxine Mutants with Altered Host-Range

缓生根瘤菌（blyyrhizobium japonicum）在大豆根瘤中产生的植物毒素根瘤素（rhizobitoxine）通过引起短暂的叶面褪绿，赋予了通常互利共生的致病特性，从而模糊了互惠共生和寄生共生的区别。毒素合成取决于寄主植物和细菌的基因型。通过选择在培养中不再合成根霉毒素的突变体，已经确定了几种具有改变的共生表型的突变体。一组突变体在培养物或植物中不产生毒素。这些包括根瘤菌毒素生物合成中结构基因的突变。有趣的是，其中一个基因需要翻译移码来表达。在共生关系中，最有趣的突变体是那些在培养中不能合成根瘤菌毒素，但在植物中产生根瘤菌毒素的水平高于野生型的突变体。因此，这些突变体产生更严重的黄化，植物无法恢复。此外，这组突变体改变了互惠共生关系。亲本菌株对携带Rj4基因的大豆结瘤的能力受到限制。这些突变体能够以与非限制菌株相似的速率结瘤Rj4大豆。这组突变体证明了根霉毒素的生物合成与慢生根瘤菌的宿主范围功能之间的关系。本提案的目的是研究控制这些寄生和互惠共生功能的生化和遗传机制。

The production of the phytotoxin rhizobitoxine by Bradyrhizobium japonicum in soybean root nodules confers a pathogenic character to the normally mutually beneficial symbiosis by causing a transient foliar chlorosis, thus blurring the distinction between mutualistic and parasitic symbiosis. Toxin synthesis is dependent upon the genotype of both the host plant and bacteria. By selecting for mutants that no longer synthesize rhizobitoxine in culture, several mutants with altered symbiotic phenotypes have been identified. One group of mutants does not make toxin in culture or in planta. These include mutations in structural genes in rhizobitoxine biosynthesis. Interestingly, one of these genes requires a translation frameshift for expression. Symbiotically, the most interesting group of mutants are those that fail to synthesize rhizobitoxine in cluture, but do produce rhizobitoxine in planta at levels greater than wild type. Consequently, these mutants produce more severe chlorosis from which the plant does not recover. In addition, this group of mutants has altered mutualistic symbiosis. The parental strain is restricted in its ability to nodulate soybeans that carry the Rj4 gene. These mutants are able to nodulate Rj4 soybeans at rates similar to unrestricted strains. This group of mutants demonstrates a relationship between rhizobitoxine biosynthesis and host range functions of Bradyrhizobium. The goals of this proposal are to examine the biochemical and genetic mechanisms controlling these parasitic and mutualistic symbiotic functions.