Jitacksorn Siriluck, Sadowsky Michael J
University of Minnesota, Department of Soil, Water, and Climate, University of Minnesota, St. Paul, MN 55108, USA.
Appl Environ Microbiol. 2008 Jun;74(12):3749-56. doi: 10.1128/AEM.02939-07. Epub 2008 Apr 25.
The nodulation of Glycine max cv. Lambert and the nodulation-restricting plant introduction (PI) genotype PI 417566 by wild-type Bradyrhizobium japonicum USDA110 is regulated in a population-density-dependent manner. Nodulation on both plant genotypes was suppressed (inhibited) when plants received a high-density inoculum (10(9) cells/ml) of strain USDA110 grown in complex medium, and more nodules were produced on plants receiving a low-cell-density inoculum (10(5) cells/ml). Since cell-free supernatants from strain USDA110 grown to high cell density in complex medium decreased the expression of an nodY-lacZ fusion, this phenomenon was attributed to bradyoxetin-induced repression of nod gene expression. Inoculation of either the permissive soybean genotype (cv. Lambert) or PI 417566 with 10(9) cells/ml of the nodD2, nolA, nodW, and nwsB mutants of USDA110 enhanced nodulation (up to 24%) relative to that seen with inoculations done with 10(5) cells/ml of the mutants or the wild-type strain, indicating that these genes are involved in population-density-dependent nodulation of soybeans. In contrast, the number of nodules produced by an nodD1 mutant on either soybean genotype was less than those seen with the wild-type strain inoculated at a low inoculum density. The nodD2 mutant outcompeted B. japonicum strain USDA123 for nodulation of G. max cv. Lambert at a high or low inoculum density, and the results of root-tip-marking and time-to-nodulate studies indicated that the nolA and nodD2 mutants nodulated this soybean genotype faster than wild-type USDA110. Taken together, the results from these studies indicate that the nodD2 mutant of B. japonicum may be useful to enhance soybean nodulation at high inoculum densities and that NodD2 is a key repressor influencing host-controlled restriction of nodulation, density-dependent suppression of nodulation, perception of bradyoxetin, and competitiveness in the soybean-B. japonicum symbiosis.
野生型日本慢生根瘤菌USDA110对大豆品种Lambert以及具有结瘤限制作用的植物引进（PI）基因型PI 417566的结瘤作用，是以群体密度依赖的方式进行调控的。当植株接种在复合培养基中生长的高密度USDA110菌株菌液（10⁹个细胞/毫升）时，这两种植物基因型的结瘤均受到抑制（被抑制），而接种低细胞密度菌液（10⁵个细胞/毫升）的植株产生的根瘤更多。由于在复合培养基中生长至高密度的USDA110菌株的无细胞上清液降低了nodY - lacZ融合基因的表达，这种现象被归因于缓激肽诱导的nod基因表达抑制。用10⁹个细胞/毫升的USDA110的nodD2、nolA、nodW和nwsB突变体接种允许结瘤的大豆基因型（品种Lambert）或PI 417566，相对于用10⁵个细胞/毫升的突变体或野生型菌株接种而言，根瘤数量增加（高达24%），这表明这些基因参与了大豆的群体密度依赖型结瘤过程。相比之下，nodD1突变体在这两种大豆基因型上产生的根瘤数量少于以低接种密度接种野生型菌株时的根瘤数量。在高或低接种密度下，nodD2突变体在与大豆品种Lambert的结瘤竞争中胜过日本慢生根瘤菌USDA123菌株，根尖标记和结瘤时间研究结果表明，nolA和nodD2突变体对这种大豆基因型的结瘤速度比野生型USDA110更快。综合来看，这些研究结果表明，日本慢生根瘤菌的nodD2突变体可能有助于在高接种密度下增强大豆结瘤，并且NodD2是影响宿主控制的结瘤限制、结瘤的密度依赖性抑制、缓激肽感知以及大豆 - 日本慢生根瘤菌共生体系中竞争力的关键阻遏物。
