Bradyrhizobium Rhizobitoxine Mutants with Altered Host-Range

慢生根瘤菌产生植物毒素根瘤毒素 大豆根瘤中的大豆根瘤菌赋予致病性， 通常互惠互利的共生关系， 叶片失绿，从而模糊了互利 和寄生共生 毒素的合成依赖于 宿主植物和细菌的基因型。 通过选择 在培养物中不再合成根瘤菌毒素的突变体， 已经鉴定了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的结构基因突变 生物合成 有趣的是，其中一个基因需要一个 翻译移码表达。 共生，最 一组有趣的突变体是那些不能合成 根瘤菌毒素，但在植物中产生根瘤菌毒素， 水平高于野生型。 因此，这些突变体 植物不能恢复的更严重的失绿病。 在 此外，这群突变体改变了互利共生关系。 亲本菌株的繁殖能力受到限制 携带Rj4基因的大豆。 这些变种人能够 以与非限制性菌株相似的速率接种Rj4大豆。 这组突变体证明了 根瘤菌毒素的生物合成和寄主范围功能 慢生根瘤菌 本提案的目标是审查 控制这些寄生虫的生化和遗传机制， 互惠共生功能

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.